Wet-Weather Flow in the Urban Watershed: Technology and Management

Wet-Weather Flow in the Urban Watershed Technology and Management Edited by

Richard Field and Daniel Sullivan

LEWIS PUBLISHERS A CRC Press Company Boca Raton London New York Washington, D.C.

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Library of Congress Cataloging-in-Publication Data Wet-weather flow in the urban watershed : technology and management / edited by Richard Field and Daniel Sullivan. p. cm. Includes bibliographical references. ISBN 1-56676-916-7 (alk. paper) 1. Urban runoff--Management. I. Field, Richard, 1939- II. Sullivan, Daniel, 1948TD657 .W48 2002 628′.231—dc21

2002067138

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Preface It is an honor for us to have this opportunity to present this book on management of urban wetweather flow (WWF) in the watershed. This book is based principally on seminars presented by international experts to those professionals interested in urban WWF management. These seminars took place at the U.S. Environmental Protection Agency (U.S. EPA) in Edison, New Jersey and were organized by the Urban Watershed Management Branch (UWMB), Water Supply and Water Resources Division, National Risk Management Research Laboratory, Office of Research and Development, U.S. EPA. Seminar attendees mainly comprised staff members of the UWMB but also included representatives of U.S. EPA Region 2, U.S. EPA Office of Water, New Jersey Department of Environmental Protection, New York City Environmental Protection Administration, academia, and the consulting engineering sector. Chapter 1 (Management of Wet Weather Flow in the Urban Watershed), Chapter 8 (Management of Urban Wet Weather Flow Solids), and Chapter 9 (Beneficial Use of Urban Stormwater) were coauthored by Richard Field and Daniel Sullivan (Chapter 1), Chi-Yuan Fan (Chapter 8), and ChiYuan Fan and Richard Field (Chapter 9) who are members of the UWMB. The significance of urban WWF upsets and storm-generated pollution was recognized by the predecessor organization of the U.S. EPA, The U.S. Department of Health, Education, and Welfare, Public Health Service in its report “Pollutional Effects of Stormwater and Overflows from Combined Sewer Systems — A Preliminary Appraisal,” No. 1246, published in November 1964. The most notable developments since have been The National Pollution Discharge Elimination System Permit Application Regulations for Stormwater Discharges, 55 Federal Register 47990, November 16, 1990, and The National Combined Sewer Overflow Control Policy, 59 Federal Register 18688, April 19, 1994. When the U.S. EPA predecessor began its research work in urban storm-generated pollution control and stormwater management in 1965, the field was in its infancy. It is gratifying to see this field growing and gaining the international recognition it deserves. Abatement or prevention of pollution from storm-generated flow is one of the most challenging areas in the environmental engineering field. The facts of life — from an engineering standpoint — are difficult to face in terms of design and cost. Operational problems can be just as foreboding. The full impacts of “marginal” pollution, particularly that caused by uncontrolled overflows, must be recognized now and planning initiated to improve sewerage system efficiencies and bring all wastewater flows under control. Municipal programs with this objective cannot begin too soon because corrective action is time-consuming. Efforts devoted to improved sewerage systems will pay significant dividends in complete control of metropolitan wastewater problems and pollution abatement. Research and development are making available important answers on the most efficient and least costly methods needed to restore and maintain water resources for maximum usefulness to humans. It is clear that abatement requirements for storm-flow pollution are forthcoming. Already, federal and local governments have promulgated WWF treatment and control rules and standards. Now developed and developing regions can take advantage of a crucial opportunity and assess what has transpired around the globe and determine their own best water management strategy. To exemplify this, we can consider the water pollution control efforts in the United States. Historically, this nation has always approached water pollution control in a serial and segmented manner with respect to time and pollutant sources, respectively. The result is that the nation is still fighting the problem after more than 70 years of effort and tens of billions of dollars of expenditures.

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Initially, it abated sanitary sewage; first with primary treatment, and later, only after a long time, with secondary treatment. Somewhere between attempts to control sanitary sewage, industrial wastewater control became a requirement; however, pretreated industrial wastewaters are still released during an overflow event. Only recently was the nation forced to control combined sewer overflow (CSO), and now it is faced with requirements to abate separate stormwater pollution. The aforementioned historical approach to water pollution control has taken a very long time, and only after trial and error of each individualized and fragmented approach was it learned that receivingwater pollution problems remain. If, instead, an entire watershed or multidrainage area analysis had been conducted earlier, a determination could have been made of the overall pollution problem in the receiving water bodies; the pollution sources (or culprits) contributing to the problem; and an optimized, integrated, areawide program to correct the problem. After the macro- (or large-scale watershed) analysis is conducted, an optimized determination of what sources to be abated (or where to spend the monies) will be made. Then, with the resulting information, a micro- (or drainage area/pollutant source and control) analysis can be performed. Toxicants must also be part of the study. Past research has shown that storm-flow toxicants and resultant toxicity can significantly affect health and the environment. Regulations for toxicant control have been promulgated and will become more demanding. The control system designs should at least be made flexible to treat toxicants once toxicant control requirements are enforced. Prevention of toxic substance pollution must also be addressed. There is one other important factor that must be considered, i.e., the use and reclamation of stormwater for such beneficial purposes as aesthetic and recreational ponds, groundwater recharge, irrigation, fire protection, and industrial water supply. An optimal approach to integrated stormwater management is a total watershed or basinwide analysis including a macro- or large-basin-scale evaluation interfaced with a discretized micro- or small-catchment-scale evaluation involving the integration of (1) all catchments or drainage areas, tributaries, surrounding water bodies, and groundwater; (2) all pollutant source areas, land uses, and flows, i.e., combined sewer drainage areas, separate storm drainage areas including their dry weather discharges containing unauthorized or inappropriate cross-connections, existing water pollution control plant effluents, industrial wastewater discharges, discharges from other land uses, and air pollution fallout; and (3) added storm-flow sludge and residual solids handling and disposal. Flood and erosion control along with beneficial use and reclamation technology must also be integrated with pollution control, so that the retention and drainage facilities required for flood and erosion control can be simultaneously designed or retrofitted for pollution control and stormwater reclamation. In conclusion, knowledge of interconnecting basinwide waters and pollutant loads affecting the receiving water body and the subsurface and groundwater will result in knowing how to attain the optimum water resource and pollution abatement and a much more expedient and cost-effective water management program. This book covers a broad spectrum of urban WWF management and pollution abatement topics that will assist municipal engineers and consultants and be an important reference for academia. It includes WWF characteristics and a database, a Source Loading and Management Model (SLAMM), urban stormwater pollution abatement technologies and sediment management, lowimpact development, and stream protection and restoration. Richard Field Daniel Sullivan

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Acknowledgments A book of this nature results from the experiences and expertise of many and, in particular, from the volunteer efforts of a dedicated few to whom we are grateful. To the U.S. EPA, Water Supply and Water Resources Division, National Risk Management Research Laboratory, Office of Research and Development, who provided for this extremely important Urban Wet Weather Flow Management Research Program without which this book would not have been written. To our colleagues, who over the last four decades have given us the opportunity to learn from them. To our co-authors, for their dedication and perseverance, who took the time from their active schedules to communicate and share their experiences. To Marie Casserly of the U.S. EPA Urban Watershed Management Branch (UWMB), who as Branch Secretary made outstanding contributions with her dedicated word processing. To Carl Carco and Jaime Marin of Computer Sciences Corporation, who provide computer technical support to the UWMB, for their dedicated efforts to convert book chapters to the publisher’s format. And, finally, and most important, to Richard Field’s wife, Joan, and his children, Robyn, Stacie, and Shawn, and to Daniel Sullivan’s dear friend, Lynette Hamara, and his son, Robert Michael, for their constant support by being there.

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About the Editors Richard Field received a bachelor of civil engineering degree from the City College of New York (CCNY) in 1962 and a master of civil engineering degree (sanitary engineering option) from New York University (NYU) in 1963, graduating first in his class. He has since taken many postgraduate courses related to environmental engineering, construction technology, advanced mathematics, computer technology, etc. Mr. Field has worked in the environmental engineering field for 39 years. He is a registered professional engineer (P.E.) in the States of New York and New Jersey; a member of Chi-Epsilon National Civil Engineering Honor Fraternity; a member of the American Society of Civil Engineers (ASCE) and an executive committee member of its Urban Water Resources Research Council. He has been a member of the following committees: The Water Environment Federation (WEF) Water Environment Research Foundation Wet Weather Advisory Panel and Research Committee covering urban wet weather flows, the Environment Canada (the Canadian federal environmental agency) Steering Committee on CSO High-Rate Treatment, and the U.S. Environmental Protection Agency (U.S. EPA) Sanitary Sewer Overflow (SSO) Advisory Committee and Urban Wet Weather Flow Subcommittee. Since May 1970, he has been in charge of the U.S. EPA National Storm and CSO (combined sewer overflow) Technology Research and Development Program located at the National Risk Management Research Laboratory in Edison, New Jersey. Mr. Field has received numerous outstanding achievement awards and citations for on-the-job performance and technological contribution including two U.S. EPA bronze medals, the ASCE State-of-the-Art of Civil Engineering Award, two New York Water Pollution Control Association awards for excellence in technological advancement, three U.S. EPA Scientific and Technological Achievement awards including a first level award, and a first place U.S. EPA National Award in the CSO category. He has authored and coauthored, presented, and/or published a combination of more than 800 peer-reviewed articles/conference proceedings/papers/other papers/U.S.EPA reports/books and book chapters, some of which are internationally recognized publications in his field. He has been invited to lecture and present seminars throughout the world and has presented more than 300 times. Mr. Field is a U.S. EPA expert and an internationally recognized expert in urban wet weather discharge impacts and control technology including the areas of CSO, SSO, and infiltration/inflow (I/I), urban stormwater, diffuse or nonpoint sources, and watershed management. He is listed in Who’s Who in Engineering, Who’s Who in Technology Today, Who’s Who in Science and Engineering, Who’s Who in Finance and Industry, Who’s Who in the East, International Who’s Who of Professionals, and Who’s Who in the World.

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Daniel Sullivan received a bachelor of science in civil engineering from the Polytechnic Institute of Brooklyn in 1968 and a master of science in environmental sciences and engineering from the University of North Carolina in 1970. He has worked in the environmental engineering field for 32 years, is a registered professional engineer in the states of New York and New Jersey; a professional planner in New Jersey; and a member of Chi Epsilon, the National Civil Engineering Honor Fraternity. Since 1995 he has been Chief of the Urban Watershed Management Branch, which conducts the U.S. EPA national wet weather flow and watershed management research program. He began his U.S. EPA career in 1972 and has authored and coauthored, presented, and/or published a combination of more that 100 peer-reviewed articles/conference proceedings/papers, U.S. EPA reports/book chapters in topics of hazardous waste control and wet weather flow. He has received two U.S. EPA bronze medals for work in the U.S. EPA wet weather flow research program and environmental technology verification program.

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About the Contributors Richard M. Ashley is professor of environmental engineering and leads the Environmental Engineering Research Group at the University of Bradford, U.K. He is an accredited engineer, who has produced more than 150 publications. He is co-director of the Pennine Water Group (with the University of Sheffield) and an expert in sewer processes. He has researched all aspects of sewer solids over the past 20 years, and more recently has become involved in the development of wastewater systems that are more sustainable. He is currently working with Professor HvitvedJacobsen to produce a scientific and technical report on “Sewer Solids — State of the Art” for the International Water Association. Kelly A. Cave is the Director of the Watershed Management Division at the Wayne County Department of Environment, Detroit, Michigan. Ms. Cave manages the Rouge River National Wet Weather Demonstration Project (Rouge Project), a U.S. EPA-funded demonstration of a watershed approach to water pollution control in a major urban area. Ms. Cave has been involved with the Rouge Project since 1994, and has participated in design, implementation, and analysis of variety of watershed management and assessment techniques including combined sewer overflow control, stormwater management, water quality modeling and monitoring, public education and involvement, and habitat restoration. She has written various papers and given presentations about the Rouge Project. Prior to joining Wayne County, Ms. Cave worked on a variety of watershed and stormwater management projects nationwide during her 10-year employment by Camp Dresser & McKee. Ms. Cave received a B.S. and M.S. in civil engineering from Virginia Polytechnic Institute and State University in 1984 and 1986, respectively. Ms. Cave is a licensed professional engineer in Michigan and Virginia. Mow-Soung Cheng is the chief of the Technical Support Section in the Department of Environmental Resources, Prince George’s County, Maryland, and is responsible for promoting, enhancing, and advancing technologies for the county’s stormwater management programs. He is one of the major developers of the low-impact development concept being used in the county and being fostered by the U.S. EPA. He is a registered professional engineer in several states in the United States and is a graduate of Cheng-Kung University with a bachelor’s degree in hydraulic engineering, University of Pittsburgh with a master’s degree in civil engineering, and of the University of Iowa with a Ph.D. in water resources system engineering. He has published many technical papers in professional journals and conference proceedings and has made numerous technical presentations. Michael L. Clar, president of Ecosile, Inc., Ellicott City, Maryland, received his B.S. in civil engineering in 1971 from the University of Maryland and his M.S. in mining engineering in 1978 from the Pennsylvania State University. He is a registered professional engineer in Maryland and Pennsylvania, a member of the American Society of Civil Engineers (ASCE) serving as chairperson of ASCE Urban Water Resources Research Council, past president of Suburban Maryland Engineers Society, and President of the Maryland Society of Professional Engineers. Mr. Clar is a nationally recognized expert in stormwater management technology with over 30 years’ experience in the field. He is a key contributor to the development and implementation of low-impact development design, stream protection and restoration, and stormwater bioretention.

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Shirley Clark received her Ph.D. in environmental health engineering and M.S.C.E. from the University of Alabama at Birmingham (UAB). She also holds a B.S. in chemical engineering from Washington University in St. Louis. She is a registered professional engineer in Alabama. She is currently an assistant professor in the Department of Civil and Environmental Engineering at UAB. Prior to returning to UAB as a faculty member, she completed a 1-year post-doctoral appointment as a research engineer at the U.S. EPA Urban Watershed Management Branch in Edison, New Jersey. Between getting her bachelor’s and master’s degrees, she was associated with two environmental consulting firms in the New England area. She is a member of the American Society of Civil Engineers, the American Water Works Association, the American Water Resources Association, and the Water Environment Federation. At UAB, she teaches classes in hydrology, water supply and drainage design, and water and wastewater treatment. She has also taught units on water quality in several School of Public Health classes at UAB. Her current research focus is on the treatment of urban stormwater runoff and on pollution prevention through construction material substitution. She has published more than 20 major journal articles, conference papers, and reports on stormwater runoff treatment. Larry S. Coffman is the associate director of the Programs and Planning Division within the Prince George’s County, Maryland Department of Environmental Resources. Currently, he is responsible for oversight of many of the county’s environmental programs, including water and sewer planning, comprehensive watershed planning activities, natural resources conservation/restoration, NPDES stormwater management program, capital improvement programs for flood control, environmental restoration, and urban retrofit programs. He receive a bachelor of science in biology and chemistry from Lehigh University and has over 28 years of experience in the planning, development, and administration of the Prince George’s County stormwater management and water quality protection program. Mr. Coffman pioneered the development of the bioretention technology or “rain gardens” and his work on the development of the county’s low-impact development design approach for ecologically based and environmentally sensitive site designs resulted in Prince George’s County winning the U.S. EPA 1998 First Place National Excellence Award for Municipal Stormwater Programs. Chi-Yuan (Evan) Fan has been working for the U.S. EPA for the last 30 years and has held several positions as an environmental engineer in the U.S. EPA Region II office and in the ORD National Risk Management Research Laboratory (NRMRL and predecessor organizations). His current primary research interests are the development and demonstration of methodologies for designing integrated wet weather flow collection, control, and treatment for urban watershed. From 1988 to 1995, he was a researcher with the Superfund Technology Demonstration Division, and involved in the development of a series of in situ soil vapor extraction–based systems for removing volatile organic chemicals in the unsaturated zone. Prior to this position, he was an environmental engineer in the U.S. EPA Region II Water Division, in the U.S. EPA ORD/MERL Storm and Combined Sewer Technology Program, and with a number of consulting engineering firms in New York City. He has received three U.S. EPA bronze medals and has published over 70 articles, book chapters, and reports on wet weather flow control and treatment, the assessment of organic-contaminated sites, and evaluation of technologies for cleaning up these sites. He received his master’s degree in civil engineering with a sanitary engineering major from New Mexico State University and his B.S. in civil engineering from Chung-Yuan College of Science and Engineering in Taiwan, Republic of China. He is a registered professional engineer and a diplomate in the American Academy of Environmental Engineers. James W. Gracie is founder and president of Brightwater, Inc., Ellicott City, Maryland. Mr. Gracie, during the past 30 years, has developed expertise in overall watershed management techniques for stream and wetland protection, stream restoration, and fisheries management. His experience

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includes developing water quality monitoring efforts throughout the country and advancing the science of stream restoration in conjunction with Maryland State Highway projects and private development plans. Mr. Gracie has trained hundreds of individuals in short courses on “Applied Fluvial Geomorphology and Stream Restoration.” Mr. Gracie’s participation and leadership in state, local, regional, and national natural resource conservation efforts has provided him with extensive experience in water resources management. As twice chairman of the Board of Trout Unlimited, he provided leadership for an international conservation organization of 50,000 members and 350 local chapters in the United States interested in stream and fisheries management. Mr. Gracie organized and managed the Trout Unlimited chapter in Maryland and the Middle Atlantic council. He also assisted in the development of the Maryland citizens’ organization, Save Our Streams, and served as vice chairman of its Steering Committee. During this period, Mr. Gracie formed and managed the Maryland Cold Water Coalition, a coordinating body for groups concerned with the water quality of Maryland streams. He also served as director and executive committee member of the Maryland Wildlife Federation from 1979 through 1981. Jonathan Hird is a water resources engineer with FTN Associates, Ltd and received his M.S. degree in civil engineering from Louisiana State University in 2001 and B.S. from the University of East Anglia, England in 1993. While at Louisiana State University he worked diligently with his mentor, John Sansalone, on the subject of urban stormwater pollution control technology. T. Hvitved-Jacobsen is professor of environmental engineering, Aalborg University, Institute of Life Sciences, Department of Environmental Engineering, Denmark. He is an eminent engineer, with a substantial reputation in the field of wastewater system processes. He has undertaken research in drainage systems for several decades, and is currently chairman of the Sewer Systems and Processes Working Group of the Joint Urban Drainage Committee of the International Water Association (IWA)/International Association of Hydraulic Research (IAHR). He has produced more than 160 publications, many of which have been refereed. His latest publication is a book for CRC Press: Sewer Processes — Microbial and Chemical Process Engineering of Sewer Networks. Robert Pitt is currently a professor in the Department of Civil and Environmental Engineering at the University of Alabama, Tuscaloosa. He had previously served on the School of Engineering faculty at the University of Alabama at Birmingham, since 1987. Prior to that, he was a senior engineer for 16 years in industry and government, and continues to consult to many municipalities and engineering firms. He received his Ph.D. in civil and environmental engineering from the University of Wisconsin — Madison, his M.S.C.E. in environmental engineering/hydraulic engineering from San Jose State University, California and his B.S. in engineering science, from Humboldt State University, Arcata, California. He is a registered professional engineer (Wisconsin) and a diplomate of the American Academy of Environmental Engineers. During the past 30 years, Dr. Pitt has been the principal investigator for many water resources research projects conducted for the U.S. EPA, Environment Canada, Ontario Ministry of the Environment, and state and local governments concerning the effects, sources, and control of urban runoff. Bob has published more than 100 chapters, books, journal articles, and major research reports. He is a member of the ASCE, the WEF, the North American Lake Management Society, the AWRA, and the Society for Environmental Toxicology and Chemistry. John J. Sansalone is the Louisiana Land and Exploration Assistant Professor at Louisiana State University and has held faculty positions at the University of Cincinnati and the University of Calabria, Cosenza, Italy. He received his Ph.D. in environmental engineering from the University of Cincinnati in 1996 and is a professional engineer in Ohio. His research areas include physical and chemical treatment operations/processes for urban stormwater and snowmelt, and innovative treatment/reuse of wastewater, stormwater, residuals, environmental hydrology, high-rate anaerobic

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digestion, geoenvironmental engineering, and development of multipurpose urban infrastructure. Dr. Sansalone has experience as an academic, a consulting engineer, and a design/build general contractor. He has written more than 25 peer-reviewed publications and given over 100 conference/symposium presentations. Dr. Sansalone is active on many national and international professional committees. James T. Smullen is the National Hydraulics and Hydrology Discipline Leader and a senior vice president with Camp Dresser & McKee, Edison, New Jersey. His 20 years of experience in surface water management and water resource program planning include combined sewer overflow (CSO), sanitary sewer overflow (SSO), and stormwater planning and permitting; evaluation of point and nonpoint pollution control strategies; stormwater best management practice (BMP) development and alternative screening; quantitative environmental analyses and assessments of aquatic and marine systems; modeling and assessments of watershed hydrology and water quality; field instrumentation, data collection, and reporting for urban and agricultural, rainfall/runoff/water quality monitoring. Dr. Smullen holds a bachelor of science in civil and environmental engineering, a bachelor of arts in economics, and a master of science in civil and environmental engineering, from Rutgers University, and a doctor of philosophy in marine studies from the University of Delaware. He is a diplomate of the American Academy of Environmental Engineers and is licensed as a professional engineer in Delaware, New Jersey, and Pennsylvania. John G. Voorhees III has more than 13 years experience on water resources–related projects. These include stormwater modeling, floodplain modeling, and groundwater flow and contamination. He is a codeveloper of the urban stormwater quality model WinSLAMM and WinDETPOND, a detention pond water quality model. He is currently employed at the Wisconsin Department of Transportation as a stormwater engineer, where he is updating the department’s stormwater and erosion control rule as well as preparing the statewide technical design guidelines and conducting training for stormwater and erosion control. He has a B.S and M.S. in civil and environmental engineering from the University of Wisconsin — Madison. John is a professional engineer in Wisconsin. He has taught seminars and short courses for the University of Wisconsin Extension, the Minnesota Department of Pollution Control, the Wisconsin Department of Transportation, and the New York Department of Transportation.

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Table of Contents Chapter 1

Management of Wet Weather Flow in the Urban Watershed.....................................1

Richard Field and Daniel Sullivan Chapter 2

The Physical and Chemical Nature of Urban Stormwater Runoff Pollutants .........43

John Sansalone Chapter 3

National Stormwater Runoff Pollution Database .....................................................67

James T. Smullen and Kelly A. Cave Chapter 4

SLAMM: The Source Loading and Management Model ........................................79

Robert Pitt and John G. Voorhees III Chapter 5

Emerging Stormwater Controls for Critical Source Areas.....................................103

Robert Pitt and Shirley Clark Chapter 6

Treatment of Stormwater Runoff from Urban Pavement and Roadways..............141

John J. Sansalone and Jonathan Hird Chapter 7

Management of Sewer Sediments...........................................................................187

Richard M. Ashley and T. Hvitved-Jacobsen Chapter 8

Management of Wet Weather Flow Solids .............................................................225

Chi-Yuan Fan Chapter 9

Beneficial Use of Urban Stormwater......................................................................257

Chi-Yuan Fan and Richard Field Chapter 10 Low-Impact Development: An Ecologically Sensitive Alternative for Stormwater Management ........................................................................................271 Larry S. Coffman and Michael L. Clar Chapter 11 Low Impact Development: Hydrologic Analysis ...................................................295 Mow-Soung Cheng, Larry S. Coffman, and Michael L. Clar

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Chapter 12 Geomorphic Considerations in Stream Protection .................................................315 Michael L. Clar Chapter 13 Geomorphic Considerations in Stream Restoration ...............................................343 James W. Gracie Index ..............................................................................................................................................369

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1

Management of Wet Weather Flow in the Urban Watershed Richard Field and Daniel Sullivan

CONTENTS Introduction ........................................................................................................................................2 General Approach and Strategy .........................................................................................................2 Small Storm Hydrology..............................................................................................................2 Strategy .......................................................................................................................................3 Watershed Area Technologies and Practices .....................................................................................6 Regulations, Local Ordinances, and Public Education............................................................10 Source Control of Pollutants ....................................................................................................11 Source Treatment, Flow Attenuation, and Storm Runoff Infiltration .............................................14 Vegetative BMPs.......................................................................................................................14 Swales..............................................................................................................................14 Filter Strips......................................................................................................................14 Stormwater Wetlands.......................................................................................................14 Detention Facilities ...................................................................................................................15 Extended Detention Dry Ponds ......................................................................................15 Wet Ponds........................................................................................................................15 Infiltration Practices ..................................................................................................................16 Infiltration Trenches ........................................................................................................16 Infiltration Basins ............................................................................................................16 Porous Pavement .............................................................................................................16 Installed Drainage System ...............................................................................................................17 Illicit or Inappropriate Cross-Connections ...............................................................................18 Catchbasin Cleaning .................................................................................................................18 Critical Source Area Treatment Devices ..................................................................................19 Sand Filters......................................................................................................................19 Oil–Grit Separators .........................................................................................................20 Enhanced Treatment Device ...........................................................................................20 Infiltration..................................................................................................................................21 In-Line Storage .........................................................................................................................22 Off-Line Storage .......................................................................................................................23 Flow Balance Method .....................................................................................................23 Maintenance ..............................................................................................................................25 End-of-Pipe Treatment.....................................................................................................................25 Biological Treatment.................................................................................................................25 Use of Existing Treatment Facilities ........................................................................................25 Physical/Chemical Treatment ...................................................................................................25

1

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2

Wet-Weather Flow in the Urban Watershed: Technology and Management

Screening .........................................................................................................................26 Filtration ..........................................................................................................................27 Dissolved Air Flotation ...................................................................................................30 High-Gradient Magnetic Separation ...............................................................................31 Powdered Activated Carbon-Alum Coagulation.............................................................32 Disinfection .....................................................................................................................32 Swirl Regulators/Concentrators ......................................................................................35 Storage and Treatment Optimization...............................................................................................35 Beneficial Reuse of Stormwater ......................................................................................................36 References ........................................................................................................................................37

INTRODUCTION This chapter covers the control and treatment of stormwater in relation to the removal or reduction of stormwater pollutant loads. Although the control of stormwater to prevent flooding is not the emphasis of this chapter; the pollution abatement technologies discussed will help attenuate stormwater flows. However, as they are generally designed for small storm events, they will not provide sufficient capacity for the large events. Although prevention of stormwater flooding is not discussed in this chapter, a drainage system design should consider both pollutant and flooding aspects of stormwater.

GENERAL APPROACH AND STRATEGY SMALL STORM HYDROLOGY The selection of suitable abatement technologies requires an understanding of the size and distribution of storm events. These contribute to total volume of storm runoff and, with knowledge of the pollutant concentrations, provide the total pollutant load. Generally the smaller storm events are the critical storms to consider because for many parts of the United States, 85% of all the rains are less than 0.6 in. (15 mm) in depth and can generate about 70% of the total annual storm runoff (Pitt, 1987). The characteristics of small and large storm events can be very different in terms of the storm runoff generated, pollutant load, and receiving water impacts. However, the frequent small storms will have a more persistent impact, and less frequent large storms will have a larger impact but allow time for recovery between events. For small storm events, any inaccuracy in the estimation of the initial abstractions and the soil infiltration rates can significantly change the calculated storm runoff pollutant load. The initial abstractions include the rainfall depth required to satisfy surface wetting, surface depression storage, interception by hanging vegetation, and evaporation. Together with soil infiltration rates, the initial abstractions need to be accurately estimated to calculate the storm runoff volume. Initial abstractions for relatively impervious urban surfaces have been found to account for the first 0.2 to 0.4 in. (5 to 10 mm) of a storm event (Pitt, 1987). Others (Pecher, 1969; Viessman et al., 1977) have reported initial abstractions of between 0.02 and 0.14 in. (0.5 and 3.5 mm) for pavement areas depending on whether the areas are flat or sloping steeply. Figure 1.1 illustrates the runoff capture volume rates in Cincinnati, Ohio. Note that 95% of the runoff will be captured for the first 0.5 watershed in. (12.7 mm) (as stated above, 85% of all storms are less than 0.6 in., or 15 mm). This indicates that small precipitation events need to be considered when designing stormwater quality treatment facilities. Increases in design detention volume above these values will not significantly affect the percent capture (Urbonas and Stahre, 1993). Traditional stormwater flood control is concerned with the peak storm runoff flow rates from relatively infrequent large storm events and their conveyance to prevent flooding. This is a different

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Management of Wet Weather Flow in the Urban Watershed

Storage volume in millimeters* 5 10 15 20

100

3

25

Percent capture

90 80

Percent of runoff captured by various levels of storage

70

Runoff coefficient C = 0.5 Runoff = 15.8 in./yr (401 mm/yr) Storm events = 80/yr

60 50 0.0

0.2 0.4 0.6 0.8 1.0 Storage volume in inches* *Storage is the equivalent depth of water over entire tributary watershed.

FIGURE 1.1 Runoff capture volume rates in Cincinnati, Ohio. (From Urbonas, B. and Stahre, P., Stormwater: Best Management Practices and Detention, Prentice-Hall, Englewood Cliffs, NJ, 1993. With permission.)

set of criteria from that needed for storm runoff pollution control. Therefore, the data, storm runoff coefficients, models, etc. intended or developed to meet stormwater flood control requirements should be used with caution. This is illustrated by initial abstractions that can be a major portion of a small storm but will be a relatively insignificant portion of a large storm. In other words, just because a model for an area has been verified as providing accurate information for large storm events does not mean it will predict small events with the same level of confidence. A model developed and at present being updated for the calculation of urban stormwater runoff pollutant loads from small storms is Source Loading and Management Model: An Urban Nonpoint Source Water Quality Model (SLAMM) (Pitt, 1988). This model concentrates on the parameters discussed above to estimate better the urban storm runoff pollutant loads before and after application of best management practices (BMPs). However, this is mainly applicable to small areas and does not give a continuous time analysis. There are, however, a number of other models such as the U.S. EPA Storm Water Management Model (SWMM), which will allow a continuous time analysis for large drainage areas. Continuous time analysis will provide an optimum design for storage and treatment facilities based on long-term historical weather patterns. It should not be assumed from the above that the large, infrequent storm events do not cause polluted urban storm runoff or significant impacts on receiving waters but that their infrequency makes them a less significant factor than the smaller, frequent storms. Communities must design control systems that meet applicable regulations, and these systems may include large systems. There are several other factors that will affect the stormwater runoff pollutants and their concentrations, as discussed elsewhere, and these will also need to be taken into consideration when estimates are made of the urban storm runoff pollutant load.

STRATEGY The intermittent, widespread, and variable nature of urban stormwater runoff will require a flexible and creative approach to achieve the optimal control and treatment solution. This approach is likely to be a response to regulations and may not include BMPs and treatment processes. Traditional wastewater treatment methods, particularly secondary treatment processes that tend to operate under conditions closer to steady state, will not necessarily be suitable for the fluctuating loads of stormwater runoff. On the other hand, technologies used to control and treat combined sewer overflows (CSOs) are more likely to be applicable for the stormwater runoff and advantage should

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be taken of any experience or facilities of CSO origin that have application for separate stormwater runoff. Successful stormwater management to control urban storm runoff pollution will require an areawide approach combining prevention, reduction, and treatment practices/technologies. It is highly unlikely that one method will provide the best solution to control the widespread, diffuse nature of stormwater runoff and achieve the water quality required. Establishing an urban storm runoff pollution prevention and control plan requires a structured strategy, which will include the following steps: • • • • • • • • •

Define existing conditions. Set site-specific goals. Collect and analyze data. Refine site-specific goals. Assess and rank problems. Screen BMPs and treatment technologies. Select BMPs and treatment technologies. Implement plan. Monitor and reevaluate.

It is very likely that advantage can be taken of previous studies for either stormwater or CSO to get a head start. The above strategy is described in “Handbook: Urban Runoff Pollution Prevention and Control Planning” (U.S. Environmental Protection Agency, 1993a). Additional references that describe planning approaches for urban storm runoff pollution prevention and control are contained in Table 1.1. The above strategy will provide the control goals that are then used as the basis for selection of suitable technologies or approaches. The goals should initially be broad because the process of reviewing the technologies or approaches available will in itself generate information to focus and refine the goals to meet cost, level of control, public opinion, feasibility, and other restraints. A flexible approach, which through an iterative process of review and adjustment is focused to a specific action plan, is the only real way the complexity of urban stormwater can be managed. The specific action plan will also need to be subject to reassessment once feedback on its implementation is available. The above is only a very brief indication of the extensive work that will be required before the actual abatement technologies are implemented, and more detail is given in the above reference (U.S. Environmental Protection Agency, 1993a). The remainder of this chapter is concerned with an overview of the abatement technologies available. The chapter reviews the technologies by separating the drainage system into three physical areas: 1. Watershed area (i.e., storm runoff generation/collection area) 2. Installed and/or modified/natural drainage system (i.e., conveyance pipes, channels, storage, etc.) 3. End-of-pipe (i.e., point source) Technologies applicable to each of these areas are discussed and can be divided into structural and nonstructural. The nonstructural will cover approaches such as public education, regulations, and local ordinances which will have their main application to the upstream collection area. The structural approaches will be the main options for the drainage system and end-of-pipe areas and tend to be the more expensive items. The technologies and approaches for stormwater management referred to as BMPs generally cover the nonstructural or low-structural stormwater runoff controls. The point at which a stormwater management technology changes from a BMP to a unit treatment process (i.e., “highstructural” control) is often unclear; therefore, in this chapter BMPs refer to only the upstream watershed area prevention and/or control measures.

Developing and implementing the recommended plan

Analyze data and prepare forecasts

Quantifying pollution sources and effects Assessing alternatives

Formulate alternatives Compare alternatives and select recommended plan Prepare plan implementation program Implement plan

Establish objectives and standards Conduct inventory

Determining existing conditions

Urban Surface Water Management (Walesh, 1989)

Select alternative and record decision

Interpret, analyze, and evaluate data and forecasts Formulate and evaluate alternatives Evaluate and compare alternatives

Identify problems and opportunities and determine objectives Develop resource data

Developing the Watershed Plan (U.S. EPA, 1991a)

Select best alternatives and record decision

Identify problems Develop goals or objectives Formulate alternatives Evaluate alternatives

Inventory resources and forecast conditions

Developing Goals for Nonpoint Source Water Quality Projects (U.S. EPA, 1991b)

TABLE 1.1 Planning Approaches Suggested in Various Literature References

Identify NPS control measures Evaluate control measures Develop evaluation criteria Examine and screen measures Select measures Recommend control measures and implementation program

Define and describe problems

Initiate public participation Define existing conditions Review regulatory problems Define goals and objectives

Santa Clara Valley Nonpoint Source Study, Vol. II, NPS Control Program (SCVWD, 1990)

Implement near-term program Assess program effectiveness

Select near-term BMPs

Set priorities

Define goals Assess existing conditions

State of California Storm Water Best Management Practice Handbooks (Camp Dresser & McKee, 1993)

Determine attainable improvements

Assess existing data Compare conditions vs. objectives Determine extent of runoff problem Conduct selective field monitoring Refine problem estimates Assess alternatives

Urban Storm Water Management and Technology: Update and User’s Guide (U.S. EPA, 1977a)

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As stated previously, the optimal solution is likely to be an integrated approach using several practices and technologies. The management of the watershed using BMPs to prevent or control pollution at the source is likely to offer the most cost-effective solution and tend to be the basis of many stormwater management plans. However, although BMPs will be the preferred option, they will not always be feasible or by themselves sufficient to achieve the control objectives. For older and more heavily urbanized areas, BMPs are likely to have a limited application and some form of treatment prior to discharge may be required. There are a number of publications cited in Table 1.2 that cover the present state of the art on stormwater management using BMPs but do not generally review the end-of-pipe treatments that could be applied to stormwater as a final line of control. This chapter therefore includes treatment options available for stormwater pollution control that appear to be ignored in many stormwater management manuals. It should, however, be emphasized that it will be more cost effective to prevent potential urban storm runoff pollution problems and protect existing resources than to construct pollution controls once a problem exists. Unfortunately, for many areas the problems exist and retrospective prevention is not a feasible solution. The implementation of any stormwater management program will need to meet financial and probably schedule restraints; therefore, an early review and improved utilization of existing facilities can offer several advantages. These options are likely to be the quickest and least costly to be implemented, but they should also meet the objectives developed from the earlier stormwater management planning process. Examples might include the enforcement of existing regulations to control soil erosion during construction activities and adaptation of existing stormwater storage intended for flood control to also provide quality control for small storm events. New installations should consider design for both flood control and pollutant removals. The public does not generally perceive stormwater to be an environmental pollution problem. Furthermore, it does not appreciate the direct connection between some of its actions and the pollution consequences (e.g., disposal of engine oil and household toxic liquids down a storm drain or throwing litter on the street, which is transported by the storm runoff into the receiving water). Gaining public support to cooperate in the implementation and to pay for a stormwater management plan will be a major challenge. A strategy of concentrating efforts and resources on high-priority areas where results are likely to be achieved, or have been achieved, will help generate public support.

WATERSHED AREA TECHNOLOGIES AND PRACTICES There are many BMPs, but all BMPs are not suitable in every situation. It is important to understand which BMPs are suitable for the site conditions and can also achieve the required goals. This will assist in the realistic evaluation of the technical feasibility, implementation costs, and long-term maintenance requirements and costs. It is also important to appreciate that the reliability and performance of many BMPs have not been well established, with most BMPs still in the development stage. This is not to say that BMPs cannot be effective, in spite of not having a large bank of historical data on which to base design to enable confidence that the performance criteria will be met under site-specific conditions. The most-promising and best-understood BMPs are detention and extended detention basins and ponds. Less reliable in terms of predicting performance, but showing promise, are sand filter beds, wetlands, and infiltration basins (Roesner et al., 1989). A study of 11 types of water quality and quantity BMPs in use in Prince George’s County, Maryland (Metropolitan Washington Council of Governments, 1992a) was conducted to examine their performance and longevity. The report concluded that several of the BMPs had either failed or were not satisfying the designed performance. Generally, wet ponds, artificial marshes, sand filters, and infiltration trenches achieved moderate to high levels of removal for both particulate and soluble pollutants. Only wet ponds and artificial marshes demonstrated an ability to function

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TABLE 1.2 Urban Runoff and CSO BMP References Document Title

Author

BMPs Included

Controlling Urban Runoff. A Practical Manual for Planning and Designing Urban BMPS, 1987

Schueler

Detention Infiltration Vegetative Filtration Quality inlets

Protecting Water Quality in Urban Areas, 1989

MPCA

Housekeeping Detention Infiltration Vegetative Quality inlets

Guide to NPS Control, 1987

U.S. EPA

Water Resource Protection Technology: A Handbook of Measures to Protect Water Resources in Land Development, 1981

Urban Land Institute

Housekeeping Detention Infiltration Housekeeping Detention Infiltration Vegetative Quality inlets

Urban Targeting and Urban BMP Selection, 1990

Woodward-Clyde

Combined Sewer Overflow Pollution Abatement, 1989

WPCF

Urban Stormwater Management and Technology: An Assessment, 1974

U.S. EPA

Decision Maker’s Storm Water Handbook: A Primer, 1992

Phillips–U.S. EPA Region V

Urban Storm Water Management and Technology: Update and User Guide, 1977

U.S. EPA

Control and Treatment of Combined Sewer Overflows, 1993

Moffa

Housekeeping Detention Infiltration Vegetative Housekeeping Collection system Storage Treatment Housekeeping Collection system Storage Treatment Housekeeping Detention Infiltration Vegetative Filtration Quality inlets Source control Collection system Storage Treatment Source Control Collections system Storage Treatment

Information Available General description Effectiveness Design Use limitations Maintenance Cost Examples General description Effectiveness Use limitations Maintenance Cost Examples General description Effectiveness Cost General description Effectiveness Design Use limitations Maintenance Cost General description Effectiveness Design Use limitations General description Design Effectiveness Maintenance Cost General description Design Maintenance Use limitations General description Effectiveness Design Use limitations

General description Design Maintenance Use limitations General description Design Maintenance Use limitation

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TABLE 1.2 (continued) Urban Runoff and CSO BMP References Document Title

Author

BMPs Included

Coastal Nonpoint Source Control Program: Management Measures Guidance, 1993

U.S. EPA

Housekeeping Infiltration Vegetative Filtration Quality inlets

The Florida Development Manual: A Guide to Sound Land and Water Management, 1992

Livingston et al.

Housekeeping Infiltration Vegetative Quality inlets

Storm Water Management Manual for the Puget Sound Basin, 1991

WA DOE

Housekeeping Infiltration Vegetative Quality inlets

Stormwater Management, 1992

Wanielista and Yousef

Water quality Infiltration Detention

Stormwater: Best Management Practices and Detention for Water Quality, Drainage, and CSO Management, 1993

Urbonas and Stahre

Integrated Stormwater Management

Field, O’Shea, and Chin

Storage Source control Detention Treatment Water quality Detention Management Vegetative Infiltration Flood control Reclamation Collection systems

Information Available General description Effectiveness Design Use limitations Maintenance Cost Examples General description Effectiveness Design Use limitations Maintenance Cost Examples General description Effectiveness Design Use limitations Maintenance Cost Examples General description Effectiveness Examples Cost General description Effectiveness Design Use limitations General description Effectiveness Design Use limitations

for a relatively long time without routine maintenance. BMPs with poor performance ratings were infiltration basins, porous pavement, grass filters, swales, smaller “pocket” wetlands, extended detention dry ponds, and oil–grit separators. Infiltration BMPs had high failure rates, which could often be attributed to poor initial site selection and/or lack of proper maintenance. The above report contains many more details and recommendations on the use of BMPs. It is important to note that the reported poor performance of some of the BMPs is likely to be a function of one or more of the following: the design, installation, maintenance, or suitability of the area. Greater attention to these details is likely to reduce significantly the failure rate of BMPs. Other important design considerations include safety for maintenance access and operations, hazards to

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the general public through safety (e.g., drowning) or nuisance (e.g., mosquito breeding area), acceptance by the public (e.g., enhanced area aesthetics), and to assume conservative performances in the design until the historical data can justify a higher reliable performance. For any BMP involving soil infiltration of the storm runoff, it is important to consider the possible effects this could have on the groundwater. These could range from a relatively minor local raising of the water table resulting in reduced infiltration rates to more serious pollution of the groundwater, particularly if the groundwater is also used as a water source. Stormwater runoff is likely to have very low levels of pollution when compared with chemical and gasoline leaks/discharges and the soil will have some natural capacity to hold pollutants. However, the long-term buildup of pollutants in the soil or groundwater from storm runoff infiltration is not well known. Therefore, infiltration of urban storm runoff, especially from industrial and commercial areas that are likely to have higher levels of pollution, should be treated with caution. Infiltration of storm runoff can offer significant advantages of controlling storm runoff at the source, reduced risk of downstream flooding, and recharge of groundwater and groundwater supply to streams (i.e., low-flow augmentation or maintaining stream flow during dry weather periods). All of these and possibly other advantages can be offered at a relatively low cost by infiltration, and therefore the advantages will need to be judged against any pollution risks from urban runoff. The majority of treatment processes that can be readily applied to urban storm runoff are only effective for removal of the settleable solids. Removal of dissolved or colloidal pollutants will be minimal and therefore pollution prevention or control at the source offers an effective way to control the dissolved pollutants. Fortunately, however, many pollutants in the form of heavy metals and organic chemicals show significant association with the suspended solids (SS) (Pitt and Field, 1990; Pitt et al., 1991; 1993; 1994). Consequently, removal of the solids will also remove the associated pollutants. The previously mentioned goals for a stormwater management plan can be achieved in the watershed area via three basic avenues: 1. Regulations, Local Ordinances, and Public Education. This avenue should be the primary objective because it is likely to be the most cost-effective. Mainly nonstructural practices will be involved and application to new developments should be particularly effective. 2. Source Control of Pollutants. This will be closely related to the above. Both nonstructural and structural practices can be used to prevent pollutants from coming into contact with the stormwater and hence storm runoff. Management and structural practices will include flow diversion practices that keep uncontaminated stormwater from contacting contaminated surfaces or keep contaminated stormwater from contacting uncontaminated stormwater by a variety of structural means; exposure minimization practices that minimize the possibility of stormwater contacting pollutants by structural (diking, curbs, etc.) and management (coverings, loading and unloading practices) practices; mitigative practices that include plans to recover released or spilled pollutants in the advent of a release; preventative practices that include a variety of monitoring techniques intended to prevent releases; controlling sediment and erosion by vegetative and structural means; and infiltration practices that provide for infiltration of stormwater into the groundwater (structural and vegetative means) thereby reducing the total runoff. 3. Source Treatment, Flow Attenuation, and Storm Runoff Infiltration. These are mainly structural practices to provide upstream pollutant removal at the source, controlled stormwater release to the downstream conveyance system, and ground infiltration or reuse of the stormwater. Upstream pollutant removal enables treatment of stormwater runoff at high pollution source locations or “hot spots” before the pollutants enter the stormwater conveyance system. Areas of this type include but are not limited to vehicular parking areas, vehicular service stations, bus depots, industrial loading areas, etc.

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The following provides brief details of BMPs. Many of these BMPs can be combined and/or modified to best suit the conditions of the watershed under consideration. More information on BMPs can be found in the references listed in Table 1.2.

REGULATIONS, LOCAL ORDINANCES,

AND

PUBLIC EDUCATION

The regulatory approach can address a wide variety of stormwater management aspects, some of which are listed below. For any regulations to work there will need to be an existing framework within which to place the regulations (e.g., local ordinances, zoning, planning regulations, etc.) together with dedicated resources to enforce them. Without the institutional systems to set regulations in place and enforce them, regulations will not be effective. Regulations can be an important pollution prevention BMP with particular application to new developments to ensure that the pollution is prevented or controlled at the source and any implementation and maintenance costs are included in the development costs. New York State has compiled a manual on BMPs for new developments (New York State, 1992). Some typical regulations include the following: • Land use regulations • Zoning ordinances • Subdivision regulations • Site plan review procedures • Natural resource protection • Comprehensive storm runoff control regulations • Land acquisition Further details on a regulatory approach are contained in “Handbook: Urban Runoff Pollution Prevention and Control Planning” (U.S. Environmental Protection Agency, 1993a) and Urban Stormwater Management and Technology: Update and Users’ Guide (U.S. Environmental Protection Agency, 1977). Public education can have a significant role to play because an aroused and concerned public has the power to alter behavior at all levels. However, if the stormwater management plans are not adequately communicated and public opinion responded to, this power of the public can work against the implementation of a stormwater plan if it is viewed as an unnecessary extra cost and restriction on freedom. Gaining the public support as with all education does not stop but is a continuous process and applies to all sectors of the public. These sectors are listed below and discussed in the following paragraphs: • • • •

Residential Commercial Industrial Governmental

The residential sector is composed of all persons living in a drainage area and therefore education should focus on large groups. Long-range education goals can be tackled through school programs and shorter-range goals may be achieved through community groups. Advantage should be taken of working with groups looking for community improvement projects and opportunities arising from news media coverage and the associated publicity. The commercial sector is a fairly large and often diffuse group with which to communicate. The owners/managers and their staff will need to be included in any communication together with new businesses that are opening; existing businesses that are moving, expanding, and closing; and

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personnel that is changing. Methods of communication may include news announcements in the local press, mailed news items, individual contact by a public official, and follow-up repeated contacts to answer questions and cope with employee turnover. Public education can benefit from failures, such as violations of regulations that result in a fine and are reported in the local press. This not only informs the public about regulations, but also provides an incentive for the regulations to be followed because regulations are shown to be enforceable. The industrial sector is a smaller group and can be educated by direct contact with public officials, education of the consultants from whom industry seeks advice, and by education of trade associations. Indirect education opportunities are provided by speaking to meetings of professional organizations and by writing in professional newsletters and journals. Industrial decision makers are a relatively small group, which when informed or made aware of their obligations are likely to respond. Public officials should also communicate with other public officials and governmental institutions to ensure that they are aware of a stormwater management program and its implications. Examples include road, sanitation, and parks departments and workers at public institutions such as hospitals and prisons. A multilevel, multitarget public education program can help to avoid problems in implementing a stormwater management program. Further information on communicating a stormwater management program to the public can be found in “Designing an Effective Communication Program: A Blueprint for Success” (U.S. Environmental Protection Agency, 1992a), and “Urban Runoff Management Information/Education Products” (U.S. Environmental Protection Agency, 1993b). The latter reference is a catalog of available material and publications.

SOURCE CONTROL

OF

POLLUTANTS

Source controls are usually nonstructural practices, many of which can be termed “good housekeeping” practices. They are pollution prevention options that can be very effective. Some source controls are as follows: • • • • • • • • •

Cross-connection identification and removal Controlled construction activities Street sweeping Solid waste management Animal waste removal Toxic and hazardous waste management Reduced use of fertilizer, pesticide, and herbicide Reduced roadway sanding and salting Material and chemical substitution

Research on illicit or inappropriate cross-connections into separate stormwater drainage systems has shown that these can add a significant pollutant loading (Pitt and McLean, 1986; Schmidt and Spencer, 1986; Montoya, 1987; Washtenaw Co., 1988). This is also recognized in the National Pollution Discharge Elimination Permits System (NPDES) for stormwater discharges that require investigation of dry weather flows (DWFs) at stormwater outfalls. This will involve inspecting outfalls for DWFs, identifying illicit discharges from analysis of DWF samples, tracing the discharge source, and corrective action. DWF can originate from many sources; the most important sources may include sanitary wastewater (from sewer lines or septic tank systems), industrial and commercial pollutant entries, and vehicle service activities. It should be recognized that not every DWF will be a pollutant source and they may be caused by infiltrating potable water supply and clean groundwater. A full illicit connections investigation is likely to be time-consuming and costly. A methodology for identifying illicit discharges in the DWF and tracing the source using distinct

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Street surface Solids load (lb/curb-mi)

700 600 Total solids

500 400 300 200 100 50

100 150 200 250 Number of passed per year

300

FIGURE 1.2 Street sweeping: annual amount removed as a function of the number of passes per year at San Jose test site. (From U.S. EPA, EPA-600-79-161, 1979.)

characteristics of potential sources is described in “Investigation of Inappropriate Pollutant Entries into Storm Drainage Systems: A User’s Guide” (U.S. Environmental Protection Agency, 1993c) and “Investigation of Dry-Weather Pollutant Entries into Storm Drainage Systems” (Field et al., 1994). The User’s Guide concentrates on procedures that are relatively simple and that do not require sophisticated equipment or training. At a minimum the most severely contaminated outfalls must identified to assist in prioritizing areas to be investigated first, and at best the pollutant source should be identified. A stormwater management plan that ignores investigation of DWF is very likely to find that goals set to improve receiving water quality will not be achieved because of pollutants discharged in DWF. Soil erosion from construction sites together with wash off from stockpiled material and readymix concrete trucks can be a major source of pollutants (SS) for the relatively short construction duration. Requirements for phased removal of vegetative cover and early reestablishment of ground cover combined with detention of stormwater for sedimentation and filtering will help reduce the pollution from construction site stormwater runoff. It is important also to consider the period following construction when vegetative ground cover still needs to be fully established and occupants of new buildings may undertake landscaping. Further information can be found in “Reducing the Impacts of Stormwater Runoff from New Development” (New York State, 1992) and “Storm Water Pollution Prevention for Construction Activities” (U.S. Environmental Protection Agency, 1992b). Street sweeping studies (U.S. Environmental Protection Agency, 1979d; 1985) concluded that typical reduction in storm runoff pollutant loadings can be between 5 and 10% for street sweeping carried out every 2 days (sweeping more frequently than 2 days per week does not significantly reduce the solids loading any further, as illustrated in Figure 1.2); street cleaners did not significantly remove the smallest particulates (<100 µm) that the rain washes off; street cleaners were able to remove large fractions of large particulates (>200 µm); the reduction in storm runoff pollutant load is much less than the pollutant load removed by sweeping (as street surfaces only contribute ≤0.5 the total pollutant load), which can lead to a false sense of effectiveness; pavement type and condition have a pronounced effect on performance (as illustrated in Figure 1.3); and street sweeping results are highly variable such that the results from one city cannot be applied to another city. The above comments together with the fact that street storm runoff is only a part of the outfall discharge would imply that street cleaning is not particularly effective on its own but should be part of an overall program. Street cleaning is likely to be more effective for removal of heavy metals from vehicle emissions, which tend to associate with the particulates. Sweeping of parking areas, storage, and loading/transfer areas should be included in a cleaning program. Concentrated cleaning during

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Total solids removed (lb/curb-ml/yr)

Management of Wet Weather Flow in the Urban Watershed

50,000

13

Oil and screens surfaced streets or asphalt streets in poor condition

40,000 30,000

Asphalt streets in good condition

20,000 10,000 0

0

10

100

1,000

Number of passes per year

FIGURE 1.3 Street cleaner productivity in Bellevue, Washington. (From U.S. EPA, EPA-600-2-85/038, 1985.)

certain seasons is likely to be effective, e.g., during early spring in the snowbelt, when leaves accumulate in the fall, and prior to rainy seasons. Although the effectiveness of the above has not been shown, street cleaning does offer aesthetic improvements in the removal of large items from the streets and receiving water. Fugitive emissions from street sweeping will lead to increased air pollution and may need to be considered if an intensive street sweeping program is part of a stormwater management plan. Solid waste management involves the collection and proper disposal of solid waste to maintain clean streets, residences, and businesses. It can also be extended to the collection of items such as leaves during the fall. A study of stormwater runoff into Minneapolis lakes found that phosphorus levels were reduced by 30 to 40% when street gutters were kept free of leaves and lawn clippings (Minnesota Pollution Control Agency, 1989). Wastes from domesticated and wild animals represent a source of bacteria and other pollutants such as nitrogen that can be washed into the receiving waters. A study in San Francisco, California (Colt et al., 1977) estimated that the dogs, cats, and pigeons produced 54,500, 9000, and 2200 lb (247, 4, and 1 metric tons), respectively, of nitrogen a year for an area of 30,480 acres (12,343 ha). On an annual basis, bulk precipitation, dog wastes, and fertilizer accounted for 49, 23, and 22%, respectively, of the total nitrogen runoff. Controls through regulation and public education, if successful, could therefore have a major impact based on these figures. Toxic and hazardous waste management should review methods to prevent the dumping of household and automotive toxic and hazardous wastes into municipal stormwater inlets, catchbasins, and other storm drainage system entry points. Public education, special collection days for toxic materials, and posting of labels on stormwater inlets to warn of the pollution problems of dumping wastes are possible management options. Fertilizers, pesticides, and herbicides washed off the ground during storms can contribute to water pollution. Agriculture, recreation parks, and gardens can be sources of these pollutants. Controlling the use of these chemicals on municipal lands and educating gardeners and farmers to use the minimum amounts required and appropriate application methods can help reduce nutrient and toxic pollutants washed off by storm runoff. Sand and salt are applied as deicing agents to roads in many areas of the United States that experience freezing conditions and are then washed off by the meltwater and stormwater runoff. Effects of highway deicing appear most significant in causing contamination and damage of groundwater, public water supplies, roadside wells, farm supply ponds, and roadside soils, vegetation, and trees (U.S. Environmental Protection Agency, 1971). Deicers also contribute to deterioration of highway structures and pavements, and to accelerated corrosion of vehicles. Studies (U.S. Environmental Protection Agency, 1971) indicate that major problems in the control of deicing chemicals were the excessive application, misdirected spreading, poor storage practices, inaccurate weather forecasting, and the logistics of setting up the deicing operation. To address these problems

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two manuals of practice on the application and storage of deicing chemicals (U.S. Environmental Protection Agency, 1974a, c) were produced to give recommendations and improvements. They provide comprehensive details on storage management, layout, handling, application for various storm and temperature conditions, and use and calibration of equipment to minimize the amount of chemicals used. Studies were conducted on alternative deicing methods (U.S. Environmental Protection Agency, 1972a; 1976a; 1978) but these were more costly than the use of rock salt and therefore would be unlikely to have general economic application.

SOURCE TREATMENT, FLOW ATTENUATION, AND STORM RUNOFF INFILTRATION VEGETATIVE BMPS These developing practices have been the subject of many publications in the last 20 years, a few of which are listed in Table 1.2. Readers are directed to these or similar publications for more detailed information. Knowledge of the performance of these systems is limited, but the cited publications do contain lessons learned from their implementation and in some cases failure. Existing urbanized areas are unlikely to have the land space available for installation of many of these practices and in these situations their application will be restricted. Swales These are generally grassed stormwater conveyance channels that remove pollutants by filtration through the grass and infiltration through the soil. A slow velocity of flow, <1.5 ft/s (<46 cm/s), nearly flat longitudinal slope, <5%, and a vertical stand of dense vegetation higher than the water surface, ≈6 in. (15 cm) total height are important for effective operation (Metropolitan Washington Council of Governments, 1992b). Swales can be enhanced by the addition of check dams and wide depressions to increase storm runoff storage and promote greater settling of pollutants. A further enhancement would be in the development of a wetland channel (Urbonas and Stahre, 1993), but good design would be necessary to minimize the disadvantages of difficult maintenance access, mosquito breeding, and aesthetics to maximize the benefits of greater treatment potential. Filter Strips These are vegetated strips of land that act as “buffers” by accepting storm runoff as overland sheet flow from upstream developments and providing similar treatment potential mechanisms to that of swales, prior to discharge of the storm runoff to the storm drainage system. Low-velocity flows, installation of a level spreader and/or land grading to ensure sheet flow over the filter strip, and dense vegetative cover will enhance the filter strip performance (Metropolitan Washington Council of Governments, 1992b; Yu et al., 1993). Stormwater Wetlands These can be natural, modified natural, or constructed wetlands, and they remove pollutants through sedimentation, plant uptake, microbial decomposition, sorption, filtration, and exchange capacity. It is important to note that natural wetlands will be covered by regulations that will limit what can be discharged to the wetland and any modifications to enhance the wetland performance. Constructed stormwater wetlands can be designed for more effective pollutant removal with elements, such as a forebay for solids capture; meandering flow for extended detention of low flows; benching of bottom for different water depths and associated plants; and pondscaping with multiple species of wetland trees, shrubs, and plants (Metropolitan Washington Council of Governments, 1992b).

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Constructed wetland systems are increasingly being used and developed for wastewater treatment and this area could be a source of information (e.g., Water Environment Federation, 1990; U.S. Environmental Protection Agency, 1988) in addition to information in stormwater BMP publications.

DETENTION FACILITIES One of the most common structural controls for urban storm runoff and pollution loading is the construction of local ponds (including wetlands) to collect storm runoff, hold it long enough to improve its quality, and release it to receiving waters in a controlled manner. The basic removal mechanism for detention ponds is through settling of the solids with any associated pollutants, but controlled release will also attenuate the stormwater flows, which can be a benefit to receiving water streams that suffer from erosion and disturbance of aquatic habitat during peak flow conditions. It should be realized that a detention facility designed to provide pollution control for a particular size of storm is not likely to provide the same level of treatment for smaller or larger storms. For example, a detention facility designed to capture and release at a 10-year storm event over a certain time period may, in addition, need to have the discharge control orifice designed for a 2-year storm event to provide discharge control and hence treatment over a spread of storm events (Urbonas and Stahre, 1993). Detention ponds are in effect small dams, and the safety aspects associated with failure and overtopping should also be considered in the design. In a heavily urbanized landscape there is likely to be limited opportunities to use the types of detention facilities mentioned below, but use can be made of flat roof storage and temporary flooding of recreational areas, such as parks, and paved precinct areas, and automobile parking areas. Use of these facilities will obviously cause user inconvenience and possible hazard, which will need to be assessed along with the frequency and duration of flooding. Also the users and people responsible for maintenance should be aware of the designed function of these detention facilities so that they do not take measures to prevent the flooding. Extended Detention Dry Ponds These temporarily detain a portion of stormwater runoff for up to 48 h (a 24 h limit is more common) using an outlet control. They provide moderate but variable removal of particulate pollutants, negligible soluble pollutant removal, and quick accumulation of debris and sediment (Metropolitan Washington Council of Governments, 1992b). The performance can be enhanced by use of a forebay to allow sedimentation and easier removal from one area. Many dry ponds, which were originally intended for flood control, can be modified or retrofitted to serve as wet ponds, thereby providing the additional benefit of removing pollutants as well. Wet Ponds These have a permanent pool of water for treating incoming stormwater runoff. Wet ponds have a capacity greater than the permanent pond volume, which permits storage of the influent stormwater runoff and controlled release of the mixed influent and permanent pond water. They can provide moderate to high removal of particulate pollutants and reliable removal rates with pool sizes ranging from 0.5 to 1.0 in. (12.7 to 25.4 mm) of storm runoff per impervious acre (Metropolitan Washington Council of Governments, 1992b). Wet ponds offer better removal and less maintenance than dry ponds but need to be well designed to ensure they are a benefit to an area and do not cause aesthetic, safety, or mosquito breeding problems. The performance and maintenance requirements can be helped by installing a forebay to trap sediments and allow easier removal, and through use of a fringe wetland on a shallow water bench around the pond perimeter. There are several variations and combinations that can be used for the above detention systems to enhance the stormwater treatment or to suit local conditions better. Further details on the design,

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Wet-Weather Flow in the Urban Watershed: Technology and Management

performance, maintenance, and any special requirements and problems are available (Metropolitan Washington Council of Governments, 1987; 1992b; Wanielista and Yousef, 1992; Urbonas and Stahre, 1993).

INFILTRATION PRACTICES Infiltration practices have a high potential of controlling stormwater runoff by disposal at a local site level. However, the soil and water table conditions have to be suitable, a sufficiently conservative design has to be used, and adequate maintenance has to be undertaken to minimize the possibility of system failure. The importance of using only suitable sites together with adequate design and maintenance cannot be overstressed for this BMP. Another important aspect is the potential for groundwater pollution. Dissolved pollutants, which show little association with solids, would be the immediate concern, but other pollutants could be more of a problem in the long term. Sandy soils generally have high infiltration rates and a potential to filter the stormwater well. However, they are unlikely to provide good removals through sorption or ion exchange. Soil with a high organic content is likely to offer better capacity to absorb pollutants but at a slower infiltration rate. Infiltration in its simplest form involves maximizing the pervious area of ground available to allow infiltration of stormwater and minimize the storm runoff. This can be enhanced by directing storm runoff from impervious paved and roof areas to pervious areas, assuming sufficient infiltration capacity exists. Regulations that encourage the incorporation of a high proportion of pervious areas, particularly for new developments, can be effective. Infiltration Trenches These are shallow, excavated trenches that have been backfilled with stone to create an underground reservoir. Stormwater runoff that is diverted into the trench gradually exfiltrates from the trench into the surrounding soil and in many cases eventually to the water table. There are no real performance data on infiltration trench removals, but they are believed to have a good capacity to remove particulate pollutants and a moderate ability to remove soluble pollutants. Variations on this system include the use of perforated pipes to allow exfiltration and conveyance or storage of stormwater in excess of the filtration rate. Clogging of infiltration trenches is the most common cause of their failure. It is important to protect them from sediment loads during and after construction until the surrounding runoff area has developed ground cover to minimize erosion and sediment transport. The system can be enhanced and clogging reduced by providing pretreatment in the form of grass filter strips to filter particulates out of the storm runoff before reaching the infiltration trench (Metropolitan Washington Council of Governments, 1992b). Infiltration Basins These are similar to dry ponds (unlined), except that infiltration basins have an emergency spillway only and no standard outlet structure. The incoming stormwater runoff is stored until it gradually exfiltrates through the soil of the basin floor. The comments made about infiltration trenches will also apply to infiltration basins. Additionally, unlined detention ponds will allow some degree of infiltration. Porous Pavement This is a permeable, specially designed, asphalt-concrete mix that provides an alternative to conventional pavement, allowing stormwater to percolate through the porous pavement into a deep gravel storage base area that also acts as a subsurface foundation (Figure 1.4). Another type of design uses modular interlocking blocks with open cells placed over a deep stone storage base similar to the aforementioned porous pavement. The stored storm runoff then

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Porous concrete or asphalt Granular graded filter Uniform graded rock base Geotextile filter Native soil subbase

Coarse sand or sandy loam turf in all spaces Interlocking concrete blocks with vertical holes Fine gravel filter layer Granular graded filter Uniform graded rock base Geotextile filter Native soil subbase

FIGURE 1.4 Cross-section of porous pavement and cellular porous pavement. (From Urbonas, B. and Stahre, P., Stormwater: Best Management Practices and Detention, Prentice-Hall, Englewood Cliffs, NJ, 1993. With permission.)

gradually exfiltrates into the surrounding soil. In areas where soil has a slow infiltration rate subsurface piping may be installed to direct the stormwater away. Field studies have shown that porous pavement systems can remove significant levels of both soluble and particulate pollutants (U.S. Environmental Protection Agency, 1980; 1981b; Metropolitan Washington Council of Governments, 1992b). The previous caution about infiltration BMPs affecting the groundwater also applies here. In cases where infiltration is to be avoided, an impermeable membrane can be placed between the reservoir layer and existing soil or a geotextile filter can also be applied to increase solids capture. This system tends to be used in areas such as parking lots with gentle slopes and relatively light traffic. Sites that lack areas to form detention ponds or provide sufficient pervious areas can find this an attractive alternative. Sediment loads will clog the surface and should be avoided; this is particularly important during construction. Also a regular schedule of maintenance and cleaning of the porous pavement surface should be accomplished. On some installations the gravel bed/storage layer has been extended beyond the plan limits of the pavement and returned up at the edge of the pavement. This can enable, with suitable design, any excess storm runoff to be collected by the perimeter gravel. Construction costs of a porous pavement parking lot will be approximately equal to that of a conventional pavement parking lot requiring stormwater inlets and subsurface piping (Field et al., 1993).

INSTALLED DRAINAGE SYSTEM The goal of upstream BMPs will be to provide sufficient stormwater control to ensure that further downstream treatment is not needed. However, particularly for urban areas, it is highly unlikely that this goal will be totally achieved and further drainage system and end-of-pipe controls will need to be considered. Control practices that can be applied to the drainage system are relatively limited, especially for existing systems, and involve the items listed below: • Removal of illicit or inappropriate cross-connections • Catchbasin cleaning • Critical source area treatment devices

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• Infiltration • In-line storage • Off-line storage Many of the control options are similar to those used for CSO control, and in the case of new developments there is the option to install either separate or combined sewer systems. A combined sewer system with treatment is likely to provide the most effective solution in an urban/commercial environment where BMPs are unlikely to provide sufficient reduction of urban storm runoff pollution. Whether a combined sewer would be the most effective choice would depend on many factors including the required degree of treatment of separate stormwater discharges. In less urbanized areas with strongly enforced BMPs and public support there is a much greater possibility of the downstream stormwater in a separate system needing no further treatment. For new systems, advantage can be taken of increasing the pipe size and gradient to provide in-line storage and self-cleaning, respectively. This will incur an additional cost, which should be relatively small, but the feasibility will be subject to site conditions and available hydraulic head. Existing separate (and combined) drainage systems can be modified for in-line storage by the addition of flow control devices (weirs, flow regulators, etc.). Established urban areas with separate stormwater drainage systems are most likely to have an existing stormwater pollution problem that needs to be rectified. The following covers some of the options available.

ILLICIT

OR INAPPROPRIATE

CROSS-CONNECTIONS

This control was discussed under source control but also appears here because of its close relation to the drainage system. Identification and removal of illicit or inappropriate connections may provide a partial or complete solution but will be time-consuming and costly with no guarantee of success. Depending on the likely magnitude of the cross-connection problem, it is worth considering the alternative of accepting the pollution problem and providing treatment. If this decision is made early in the investigation, there is the potential to maximize the use of resources on the treatment option.

CATCHBASIN CLEANING A catchbasin has a sump below its outlet orifice invert to capture settleable solids, usually has a baffle or inverted pipe over its outlet to capture floatables, and is distinctly different from a stormwater inlet that has no sump. Pollution control performance is variable, with the trapped liquid generally having a high dissolved pollutant content, which is purged from catchbasins during a storm event contributing to intensification of the stormwater runoff pollutant loading. Countering this negative impact is the removal of pollutants associated with the settled solids and floatables (e.g., heavy metals and organics) retained in and subsequently cleaned from the basin (U.S. Environmental Protection Agency, 1977b). A regular cleaning schedule is important to maintain the catchbasin performance with a frequency such that sediment buildup is limited to 40 to 50% of the sump capacity (U.S. Environmental Protection Agency, 1977b) or at least twice a year depending upon conditions. A study (U.S. Environmental Protection Agency, 1983a) conducted in West Roxbury, Boston, Massachusetts took three catchbasins, cleaned them, and monitored four runoff events at each catchbasin. The average pollutant removals per storm are shown in Table 1.3. The same study also looked at the effectiveness of screening the stormwater runoff through U.S. standard number 8 brass mesh installed in the three catchbasins. The results indicated screens offered a slight gain in overall pollutant removal efficiency for catchbasins. The screens were effective for the removal of coarse material that could cause aesthetic problems in the receiving water, but the potential for

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TABLE 1.3 Pollutants Retained in Catchbasin Constituent

% Retained

SS Volatile SS COD BOD5

60–97 48–97 10–56 54–88

clogging and decomposition of trapped material reduced their value unless weekly cleaning was carried out. The present increased emphasis on stormwater management has resulted in a review of the role that screening at inlets and catchbasins can play. The City of Austin, Texas has developed its own form of inlet filter (Captur™), which is a relatively coarse screen for removal of larger stormwater debris, and others (Emcon North West) have developed screens utilizing filter material (5 to 100 µm) for removal of SS. The Storm and Combined Sewer Pollution Control Research Program of the U.S. EPA through the University of Alabama at Birmingham is at present evaluating a number of inlet or catchbasin screening/filtering devices (U.S. Environmental Protection Agency, 1992c).

CRITICAL SOURCE AREA TREATMENT DEVICES Research into the source of stormwater pollutants has shown that certain critical source areas can contribute a significant portion of the total urban storm runoff pollutant load (Pitt et al., 1991; 1994). Treatment of the critical source areas can therefore offer the potential for a greater benefit to reduce downstream pollutant loads. Potential critical source areas include vehicle service, garage, or parking areas; storage and transfer yards; and industrial materials-handling areas exposed to precipitation. Sand Filters These use a bed of sand through which the storm runoff is filtered prior to discharge to the drainage system or ground infiltration. Sand filters can offer high removal rates for sediment and trace metals, and moderate removals for nutrients, biological oxygen demand (BOD), and fecal coliform (FC) (Metropolitan Washington Council of Governments, 1992a). The arrangement of the sand filter bed can vary from an open pit with perforated pipes under the sand bed, as shown in Figure 1.5, to a more-sophisticated trench stormwater inlet, as shown in Figure 1.6, which includes a sediment chamber, weir, and sand filter chamber. Washington, D.C. has installed a few sand filters in chambers Cleanout pipe

Geotextile fabric

8" Perforated pipe

Geomembrane

FIGURE 1.5 Conceptual design of a sand filter system. (Metropolitan Washington Council of Governments, 1992b.)

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Wet-Weather Flow in the Urban Watershed: Technology and Management

Weir flow (one weir for each inlet grate)

Cover lids

Overland flow

10 in. (25 cm)

Water level

Sedimentation chamber (heavy sediments, organics, debris)

30 in. (0.76 m) 2 in. (5 cm)

Inlet grates Trapped solids 18 in. (46 cm) of sand Outfall pipe Filtration chamber

Screen covered with filter fabric

FIGURE 1.6 Sand filter stormwater inlet. (From Urbonas, B. and Stahre, P., Stormwater: Best Management Practices and Detention, Prentice-Hall, Englewood Cliffs, NJ, 1993. With permission.)

in the line of the drainage pipes for treatment of urban storm runoff. The storm runoff passes along the drainage pipe, enters the chamber, passes through the sand filter bed, and returns to the drainage pipe. An overflow bypass is incorporated in the chamber to handle flows in excess of the filter bed capacity. Maintenance of sand filter beds involves removal of debris from the surface, replacement of the top layer of sand, and raking of the surface. The frequency of this maintenance will be controlled by the rate of accumulation of filtered material. Oil–Grit Separators These are usually three stage underground chambers designed to retain storm runoff, remove heavy particulate by settling, and remove hydrocarbons by trapping floating material or adsorption onto settled solids. They have limited pollutant removal capability and only appear to trap coarse-grained solids and some hydrocarbons. Removal of silt and clay, nutrients, trace metals, and organic matter is expected to be slight. Without regular clean-out maintenance (e.g., every 3 months), resuspension is likely to limit any long-term removal. Enhanced Treatment Device Research is at present being conducted to develop a treatment device for runoff generated by small but critical toxicant source areas. This will consist of, first, a relatively small chamber filled with plastic, hollow slotted media to promote cascading and aeration of the inflow and volatilization of volatile compounds, together with a sump to collect any heavier solids that settle out. The first chamber will then feed the runoff into a second, sedimentation chamber incorporating tube or plate settling for enhancing sedimentation with floating sorption pillows to remove floating oil and grease. This chamber may also be fitted with aeration facilities depending on the results of the demonstration. The final chamber will contain a sand filter bed that may also be enhanced with either a homogeneously mixed layer of sand and peat or a granular activated carbon layer to improve removals (U.S. Environmental Protection Agency, 1992c). This treatment device is shown in Figure 1.7. The above research study will also install and monitor a filtering and preinfiltration device (SAGES), shown in Figure 1.8. The intention of this device is to provide a high level of filtration treatment to the storm runoff prior to local infiltration into the ground.

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Catchbasin – Packed column aerators Q1

Main Settling Chamber – Sorbent pillows – Fine bubble aerators – Tube settlers

21

Filtering chamber – Sorbent filter fabric – Mixed media filter layer (sand and peat) – Filter fabric – Gravel packed underdrain

Q0

FIGURE 1.7 Multichambered enhanced treatment device.

Ground surface

Surface in flood “Dirty” Water from upstream “Dirty”

New sages body

Diverted flow

Existing outflow “Dirty” Cable support Drill through floor of catch basin

Removable filters

Top of known aquifer Support grate

Gravel filter (oversize removal) Sand filter (silt removal) carbon filter (solutes removal) New infiltration zone for purified discharge to aquifer

FIGURE 1.8 SAGES Unit. (Courtesy of 931026 Ontario Limited, J. Van Egmond.)

INFILTRATION New installations offer the possibility of using porous conveyance pipes to promote infiltration, but this can only be recommended where the soil and water table conditions are suitable and stormwater pollutants will not cause a problem.

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Wet-Weather Flow in the Urban Watershed: Technology and Management

IN-LINE STORAGE This is the use of the unused volume in the drainage system network of pipes and channels to store storm runoff. In-line storage capacity can also be provided by storage tanks, basins, tunnels, or surface ponds that are connected in-line to the conveyance network. To gain maximum benefit from in-line storage, it should be combined with some form of treatment; otherwise, only flow attenuation will be achieved. The in-line storage is unlikely to offer any treatment in itself through settling, as the intent will be to make the system self-cleaning to reduce maintenance requirements. However, if the storage is combined with an end-of-pipe treatment the flow attenuation will help equalize the load to the treatment process, hence optimizing the size of the treatment plant and costs. The concept of combining storage and treatment to minimize the storage and treatment capacity required and hence to optimize the cost to control polluted stormwater is an important relationship. Further cost-effective solutions might be found if existing treatment facilities can be used, such as connection to an existing wastewater system. This is discussed in more detail later in the chapter as the storage does not necessarily need to be provided by in-line storage. Even without treatment, flow attenuation will help equalize the pollutant loading to be assimilated by the receiving water and reduce the peak flows and consequent erosion in the receiving stream. This can have a major contribution toward reduced disturbance of the aquatic ecosystem. The degree to which the existing conveyance system can be used for storage will be a function of the pipe sizes that will provide the storage volume; the pipe or channel gradient (relatively flat lines are likely to provide the most storage capacity without susceptibility to flooding of low areas); suitable locations for installation of control devices such as weirs; and the reliability of the installed control. It will be essential that accurate details of the existing system be collected from field surveys and as-built drawings. This will allow the storage capacity, numbers and locations of controls, and risk of upstream flooding to be assessed. This will also be invaluable in new drainage system design where conveyance pipes and channels can be up-sized and hydraulic controls can be designed into the system for added system storage and routing. Controls used to restrict flow causing a backup and storage in the system fall into two categories: either fixed or adjustable. Fixed systems are likely to be cheaper and to require less maintenance but do not offer the flexibility and the ability to maximize the storage potential. Adjustable systems can offer the advantage of being connected to a real-time control (RTC) system that, via a system of rainfall measurements and forecasts, monitoring of stormwater levels in critical sections of the drainage system, and input of these data into a computer system, can be adjusted to hold back or release stormwater to maximize storage capacity of the whole drainage system. RTC systems have been installed and are being further developed to control complex sewerage systems in the CSO field. The sophistication offered by an RTC system is unlikely to offer a cost-effective solution for a separate storm drainage system unless there is a large in-line storage capacity and the stored runoff is to be treated. Typical examples of fixed and adjustable flow regulators are as follows: Fixed Regulators Orifices Weirs (lateral and longitudinal) Steinscrew Hydrobrake Wirbeldrossel Swirl Stilling-pond weir

Adjustable Regulators Inflatable dams Tilting plate regulators Reverse-tainter gates Float-controlled gates Motor-operated or hydraulic gates

Some of the above are relatively inexpensive, quick to install, and an effective means of increasing storage. Several publications (U.S. Environmental Protection Agency, 1970a, b; 1977a;

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Urbonas and Stahre, 1993) on CSO control can provide more information on the above regulators. However, as stated earlier, without treatment the advantage of storage is only in flow attenuation. It should also be noted that some of the above regulators will concentrate the heavier solids in the stored storm runoff for a more-concentrated later release.

OFF-LINE STORAGE This refers to storage that is not in-line to the drainage conveyance system. Storage is achieved by diverting flow from the drainage system when a certain flow rate is exceeded. The diverted water is stored until sufficient capacity is available downstream. The off-line storage can be provided by any arrangement of basins, tanks, tunnels, etc. and, if gravity filling and emptying is not possible, it will involve pumping the water into or out of storage. Off-line storage, similar to in-line storage, can be designed to be relatively self-cleaning or have facilities to resuspend the settleable solids. Examples can be found in books on stormwater (Metcalf & Eddy, 1981; Field, 1990; Urbonas and Stahre, 1993). Off-line storage can also be used to provide treatment by sedimentation with the sludge either collected or diverted to a wastewater treatment plant. Many of the regulators listed under in-line storage can be used to divert the flow once the predetermined flow rate has been exceeded. In addition to the above-listed regulators, vortex and helical bend regulators/concentrators can be used. As their name suggests, they will concentrate the heavier solids into the underflow, which will continue to be conveyed along the drainage pipes. Therefore, end-of-pipe treatment is required if this concentrated pollutant load is to be prevented from reaching the receiving water. The regulator/concentrator can offer advantages for end-of-pipe treatment when the flow needs to be regulated to prevent the treatment capacity from being exceeded. End-of-pipe treatment can be satellite or central treatment. This is discussed further in the end-of-pipe treatment section. Flow Balance Method The flow balance method (FBM) system provides a means of storing discharged urban storm runoff in the receiving water. This allows either pump-back for treatment, when capacity is available, or treatment of the runoff by sedimentation until the next storm runoff event displaces the stored volume. The method was first developed in Sweden (Soderlund, 1988) as a means of protecting lakes against pollution from stormwater runoff and has since been demonstrated for control of CSO in a marine receiving water in Jamaica Bay, New York (Field et al., 1990; Forndran et al., 1991). Storage in the receiving water is achieved by forming a tank using flexible plastic curtains suspended from pontoons. The curtains are anchored to the receiving water bottom by concrete weights, and the base of the tank is formed by the receiving water bed. The relatively low cost of the materials and construction gives this system cost advantages over conventional concrete and steel tank systems (estimated to be one fifth to one tenth the cost), requires only a minimal amount of land space for controls and access, and has flexibility to expand the volume if required at a later date. The Swedish freshwater lake installations use a connected system of bays with openings between adjacent/sequential tanks to facilitate movement of the stormwater and lake water between tanks. Lake water can enter and leave these FBMs via the last tank in the series, which has an opening to the lake. Plug flow set up by the discharging stormwater displaces the lake water from the first to the second bay and on down the line until the discharge finishes or each bay is filled with stormwater (i.e., stormwater has to pass through all of the bays to gain access to the lake). A reverse-flow sequence occurs during pumpback of the stormwater to the wastewater treatment plant (WWTP). Figure 1.9a shows the FBM freshwater system. Sweden has invested in three of these installations, which have all been in operation for a number of years. The systems have withstood wave action up to 3 ft (0.9 m) as well as severe icing

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Flow Balancing Method Stormwater pump

Outfall

Tank side Concrete Weights

Outfall

Stormwater pump

Pontoon

Pontoon Lake side Plastic curtain

Plastic curtain Concrete Weights

Pontoons Stormwater

Boundary Layer

Concrete Weights

Seawater

End curtain Window Opening for flow in/out

b. Seawater Configuration

FIGURE 1.9 (a) Flow balance Method (FBM) — freshwater configuration. (b) Flow balance Method (FBM) — seawater configuration.

conditions. If a wall is punctured, patching is easily accomplished and general maintenance has been found to be inexpensive. The FBM has been successfully demonstrated in these lakes resulting in improved water quality in the lakes (Soderlund, 1988; Pitt and Dunkers, 1993). The marine FBM demonstration (Figure 1.9b) utilizes a different operating principle of density difference for displacement instead of plug flow. One tank is used, and the seawater is displaced vertically by the lower-density CSO influent floating on the higher-density seawater, and hence forming a stratified layer of CSO above the lower seawater layer. The demonstration project was in two phases. The first phase concentrated on proving the feasibility of the system concept to displace seawater, to form a stable CSO layer, to pump the CSO back to the WWTP, and for the system structure to withstand a marine environment including tidal exchange, freezing, and coastal storms. The second phase (at present in progress) expands the system capacity from 0.41 Mgal (1550 m3) to 2 Mgal (7570 m3), and concentrates on monitoring the system performance (U.S. Environmental Protection Agency, 1990). During the 2-year demonstration project the system withstood the marine environment (the FBM was located in a relatively sheltered seawater creek) with no structural damage or material degradation observed. The system was exposed to tidal ranges up to 7 ft (2 m), winds gusting to 40 mph (64 km/h), and icing conditions. The system was shown to retain CSO in a stratified layer, which remained relatively stable and could be pumped back to the WWTP. The FBM proved effective in trapping floatable material and a means of floatable material removal is part of the next phase. Pumpback of the settled solids from the FBM bed has been incorporated into both phases. It is important to note that, although an FBM can offer a cost-effective and quick way to construct a storage facility, it requires a suitable location and does have limits on its performance. There will be a certain amount of mixing with the receiving water. Not all of the stored volume will be pumped back, and any settleable solids will settle out of the stored storm runoff (regular pumpback of the accumulated sediment would help overcome this problem). The low cost and quick construction potential of the FBM could favor the use of this system as a temporary measure in cases of a severe problem that needs attention. Because the FBM uses the existing natural receiving water, it will require all the necessary permits involved in these situations.

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MAINTENANCE In order for the drainage system and the controls to work efficiently they should all be regularly maintained. This will generally consist of removing sediments from control devices, flushing drainage lines, and general inspections to identify any problems. Regular maintenance will also minimize any buildup of material that could be flushed out by a surge from a large storm event, and thereby minimize the shock loading caused by intermittent storm events.

END-OF-PIPE TREATMENT BIOLOGICAL TREATMENT Biological treatment provides a means of removing organic pollutants from the storm runoff either aerobically or anaerobically. For this treatment to be effective, the systems must be operated continuously to maintain an active biomass or be able to borrow the biomass from a system that does operate continuously. Biological processes are relatively sensitive and can be affected by the variable flow conditions and the relatively high concentration of nonbiodegradable solids in storm runoff. These factors tend to make high-rate physical treatment processes more suitable for stormwater applications with their ability to handle high and variable flow rates and solids concentrations. Partial exceptions to the above are biological systems that include attached growth, e.g., the trickling filter (with honeycomb plastic medium) and the rotating biological contactor (RBC), which are less susceptible to overloading shock loads compared with other biological systems, e.g., activated sludge processes. RBCs have achieved high removals at flows 8 to 10 times their base flow for CSO treatment (U.S. Environmental Protection Agency, 1974d). RBCs, however, like all biological processes need a food source to keep the microbes alive during extended dry periods and therefore have their limitations. The remainder of this section will therefore concentrate on the physical/chemical treatment processes that tend to be more suitable for treatment of stormwater.

USE

OF

EXISTING TREATMENT FACILITIES

As stated earlier any use of existing facilities is likely to provide cost-effective treatment, as long as an economic means of connecting the stormwater drainage system to the facility is possible. Use of spare capacity at wastewater treatment plants is one option, particularly if storage can be provided to equalize the storm runoff load. Even if the biological system has very little capacity, the primary treatment systems can often function well at somewhat higher overflow rates, which if combined with disinfection of the discharged storm runoff will offer significant treatment. Stormwater also tends to have a higher percentage of heavier solids than sanitary sewage, which will benefit removals at higher overflow rates. An alternative could be to construct additional primary treatment at a WWTP to run in series with existing facilities during DWF for improved treatment of DWF and run in parallel during wetweather flow (WWF) for some control over the total flow. Use of any storage facilities, either at an end-of-pipe or an upstream location, could provide treatment by sedimentation or storage to be released when treatment capacity is available.

PHYSICAL/CHEMICAL TREATMENT These processes generally offer good resistance to shock loads, ability to produce a low SS effluent consistently, and adaptability to automatic operation. Those described below are, with the exception of high-gradient magnetic separation and powdered activated carbon, only suitable for removal of SS and associated pollutants. The extent of removals will depend on the SS characteristics and the level of treatment applied. The physical/chemical systems discussed are the following:

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• • • • • • •

Screening Filtration Dissolved air flotation (DAF) High-gradient magnetic separation (HGMS) Powdered activated carbon-alum coagulation Disinfection Swirl concentrators/regulators

Screening Screens can be divided into four categories with the size of the SS removed directly related to the screen aperture size: Screen Type

Opening Size

Bar screen Coarse screen Fine screen Microscreen

>1 in. (>25.4 mm) 3/16–1 in. (4.8–25.4 mm) 1/250–3/16 in. (0.1–4.8 mm) <1/250 in. (<0.1 mm)

Bar and coarse screens have been used extensively in WWTP at the headworks to remove large objects. Depending on the level of treatment required for the storm runoff, the smaller aperture sized coarse screens may be sufficient; however, a higher level of treatment can be achieved using the bar and coarse screens in conjunction with fine screens or microscreens. The design of screens can be similar to that for WWTP and CSO, but with consideration for stormwater characteristics of intermittent operation and possible very high initial loads, which may not reflect WWTP operation characteristics. A self-cleaning system should be included for static screens to save manual cleaning during storm events together with automatic startup and shutdown. Catenary screens fall into the coarse screen category; they are rugged and reliable and commonly used for CSO facilities. Therefore, they are likely to be a good screen for use with storm runoff. Table 1.4 lists screening devices that fall into the fine screen and microscreen category, and were developed and used for SS removal from CSO. With no information on screening of separate stormwater, the information on screening CSO is a good starting point and the information given below is from CSO studies. Design parameters for static screens, microstrainers, drum screens, disk screens, and rotary screens are presented in Tables 1.5 through 1.7. The removal efficiency of screening devices is adjustable by changing the aperture (size of opening) of the screen placed on the unit, making these devices very versatile. In other words, the efficiencies of a screen treating a waste with a typical distribution of particle sizes will increase as the screen aperture decreases. Solids removal efficiencies are affected by two mechanisms: straining by the screen and filtering of smaller particles by the mat deposited by the initial straining. Suspended matter removal will increase with increasing thickness of the filter mat because of the filtering action of the mat itself, which is especially true for microstrainers. This will also increase the headloss across the screen. A study in Philadelphia, Pennsylvania (Field and Struzeski, 1972) showed (on a 23-µm aperture microscreen, (Microstrainer) that with a large variation in the influent SS, the effluent SS stayed relatively constant (e.g., if a 1000 mg/l influent SS gave a 10 mg/l effluent SS, then a 20 mg/l influent SS would still give a 10 mg/l effluent SS). Accordingly, treatment efficiencies vary with influent concentration. Microscreens and fine screens remove 25 to 90% of the SS, and 10 to 70% of the BOD5, depending on the screen aperture used and the wastewater being treated. The above Philadelphia study showed that improved removals and increased flux densities (hydraulic loadings) are possible using polyelectrolyte addition. This is also likely to be the case with storm runoff, but laboratory coagulation studies are needed to find the best polyelectrolyte and dosage for the particular storm

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TABLE 1.4 Description of Screening Devices Used in CSO Treatment Drum screen

Microstrainers

Rotostrainer

Disk strainer

Rotary screen

Static screen

Horizontally mounted cylinder with screen fabric aperture in the range of 100–843 µm; operates at 2–7 r/min Horizontally mounted cylinder with screen fabric aperture 23–100 µm; operates at 2–7 r/min Horizontally mounted cylinder made of parallel bars perpendicular to axis of drum; slot spacing in the range of 250–2500 µm; operates at 1–10 r/min Series of horizontally mounted woven wire disks mounted on a center shaft; screen aperture in the range of 45–500 µm; operates at 5–15 r/min Vertically aligned drum with screen fabric aperture in the range of 74–167 µm; operates at 30–65 r/min

Stationary inclined screening surface with slot spacing in the range of 250–1600 µm

Pretreatment

Solids are trapped on inside of drum and are backwashed to a collection trough Solids are trapped on inside of drum and are backwashed to a collection trough Solids are retained on surface of drum and are removed by a scraper blade

Main treatment

Pretreatment

Pretreatment, main treatment or post treatment of concentrated effluents Main treatment

Pretreatment

Unit achieves a 12–15% solids cake

Splits flow into two distinct streams: unit effluent and concentrate flow, in the proportion of approximately 85:15 No moving parts; used for removal of large suspended and settleable solids

TABLE 1.5 Design Parameters for Static Screens Hydraulic loading, gal/min/ft of width Incline of screens, degrees from vertical Slot space, µm Automatic controls

100–180 35a 250–1600 None

a

Bauer Hydrasieves™ have three-stage slopes on each screen: 25°, 35°, 45°. gal/min/ft × 0.207 = l/m/s. Source: EPA-600/8-77/014.

runoff characteristics. The optimum dosage will change with changes in the storm runoff characteristics requiring some form of automated monitoring (e.g., SS monitoring) for adjustment of dosage or setting of an average effective dosage. More detailed descriptions of the various screening devices are available in the literature (U.S. Environmental Protection Agency, 1977a; Metcalf & Eddy, 1981; Field, 1990; Water Environment Federation, 1992). Filtration Dual-media high-rate filtration (DMHRF) (>8 gal/ft2/min, or 20 m3/m2/h) removes small particulates that remain after screening and floc remaining after polyelectrolyte and/or coagulant addition. As implied, this provides a high level of treatment that can be applied after screening together with automated operation and limited space requirements. To be most effective, filtration through media that are graded from coarse to fine in the direction of flow is desirable. A single filter material with

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TABLE 1.6 Design Parameters for Microstrainers, Drum Screens, and Disk Screens Parameter Screen aperture, µm Screen material Drum speed, r/min Speed range Recommended speed Submergence of drum, % Flux density, gal/ft2/min of submergence screen Headloss, in. Backwash Volume, % of inflow Pressure, lb/in.2

Microstrainers

Drum Screen

23–100 Stainless steel or plastic

100–420 Stainless steel or plastic

Disk Screen 45–500 Wire cloth

2–7 5 60–80 10–45

2–7 5 60–70 20–50

5–15 — 50 20–25

10–24

6–24

18–24

0.5–3 30–50

0.5–3 30–50

Note: Unit’s waste product is a solids cake of 12–15% solids content. Conversions: gal/min/ft2 × 2.445 = m3/h/m2; in × 2.54 = cm; ft × 0.305 = cm; lb/in.2 × 0.0703 = kg/cm2 Source: EPA-600/8-77/014.

TABLE 1.7 Design Parameters for Rotary Screens Screen aperture, µm Range Recommended aperture Screen material Peripheral speed of screen, ft/s Drum speed, r/min Range Recommended speed Flux density, gal/ft2/min Hydraulic efficiency, % of inflow Backwash Volume, % of inflow Pressure, lb/in.2

74–167 105 Stainless steel or plastic 14–16 30–65 55 70–150 75–90 0.02–2.5 50

Conversions: ft/s × 0.305 = m/s; gal/ft2/min × 2.445 = m3/m2/h; lb/in.2 × 0.0703 = kg/cm2. Source: EPA-600/8-77/014.

constant specific gravity cannot conform to this principle because backwashing of the bed automatically grades the bed from coarse to fine in the direction of washing; however, the concept can be approached by using a two-layer bed. A typical case is the use of coarse anthracite particles on top of less coarse sand. As anthracite is less dense than sand, it can be coarse and still remain on top of the bed after the backwash operation. Typically, a unit comprises 5 ft of No. 3 anthracite (effective size 0.16 in., or 4.0 mm) placed over 3 ft of No. 612 sand (effective size 0.08 in., or 2.0 mm). This arrangement was shown superior to both coarser and finer media tested separately

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(U.S. Environmental Protection Agency, 1972b). Another alternative would be an upflow filter, but these units have limitations in that they cannot accept high hydraulic loadings (filtration rates). The principal parameters to be evaluated in selecting a DMHRF system are the media size, media depth, and filtration rate. Because much of the removal of solids from the water takes place within the filter media, their structure and composition is of major importance. Too fine a medium may produce a high-quality effluent but also may cause excessive headlosses and extremely short filter runs. On the other hand, media that are too coarse may fail to produce the desired clarity of the effluent. Therefore, the media for DMHRF should be selected by pilot testing using various materials in different proportions and at different flow rates. Depth of media is limited by headloss and backwash considerations. The deeper the bed, the greater the headloss and the harder to clean. However, there should be sufficient bed depth to retain the removed solids without breakthrough during the filter run period at the design hydraulic loading. Information is available on the use and design of DMHRF for treatment of drinking water, but a number of pilot studies have also been done using CSO that should provide more relevant information. The studies (U.S. Environmental Protection Agency, 1972b; 1979a, b) used 6, 12, and 30 in. (15, 30, and 76 cm) diameter filter columns, with anthracite and sand media with and without various dosages of coagulants and/or polyelectrolytes. A preliminary (420 µm) screening process was used upstream of the DMHRF to extend the treatment run time before backwashing. It was found that SS removal increased as influent SS concentration increased and decreased as hydraulic loading increased. Removal efficiency for the filter unit was about 65% for SS, 40% for BOD5, and 60% for chemical oxygen demand (COD). The addition of polyelectrolyte increased the SS removal to 94%, the BOD5 removal to 65%, and the COD removal to 65%. The length of filtration run averaged 6 h at a hydraulic loading of 24 gal/ft2/min (59 m3/m2/h). Tables 1.8 through 1.10 show removals of SS, BOD5, and heavy metals for a study in New York (U.S. Environmental Protection Agency, 1979a). Design parameters for DMHRF are presented in Table 1.11 (U.S. Environmental Protection Agency, 1977a).

TABLE 1.8 CSO-DMHRF Average SS Removals (New York, NY)

No chemicals Poly only Poly and alum

Plan Influent (mg/l)

Filter Influent (mg/l)

Filter Effluent (mg/l)

Filter Removals (%)

System Removals (%)

175 209 152

150 183 142

67 68 47

55 63 67

62 67 69

Source: EPA-600/2-79/015.

TABLE 1.9 CSO-DMHRF Average BOD5 Removals (New York, NY)

No chemicals Poly only Poly and alum

Plan Influent (mg/l)

Filter Influent (mg/l)

Filter Effluent (mg/l)

Filter Removals (%)

System Removals (%)

164 143 92

131 129 85

96 84 53

27 35 38

41 41 43

Source: EPA-600/2-79/015.

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TABLE 1.10 Removal of Heavy Metals by DMHRF (New York, NY) Cadmium

Chromium

Copper

Mercury

Nickel

Lead

Zinc

56

50

39

0

13

65

48

Average removal, % a aConcentration basis. Source: EPA-600/2-79/015.

TABLE 1.11 Design Parameters for DMHRF Filter media depth (ft) No. 3 anthracite No. 612 sand Effective size (mm) Anthracite Sand Flux density (gal/ft2/min) Range Design

4–5 2–3 4 2 840 24

Headloss (ft) Backwash volume (% of flow) Air Rate (standard (ft3/min/ft2) Time Water Rate (gal/ft2/min) Time (min)

5–30 4–10 10 10 60 15–20

Conversions: ft × 0.305 = m; gal/ft2/min × 2.445 m3/m2/h; standard ft3/min/ft2 × 0.305 = m3/m2/min.

Dissolved Air Flotation DAF is a unit operation used to separate solid particles or liquid droplets from a liquid phase. Separation is brought about by introducing fine air bubbles into the liquid phase. As the bubbles attach to the solid particles, the buoyant force of the combined particle and air bubbles is great enough to cause the particle to rise. Once the particles have floated to the surface, they are removed by skimming. The most common process for forming the air bubbles is to dissolve air into the waste stream under pressure and then release the pressure to allow the air to come out of solution. The pressurized flow carrying the dissolved air to the flotation tank is (1) the entire stormwater flow, (2) a portion of the stormwater flow (split flow pressurization), or (3) recycled DAF effluent. Higher overflow rates (1.3 to 10.0 gal/ft2/min, or 3.2 to 25 m3/m2/h) and shorter detention times (0.2 to 1.0 h) can be used for DAF when compared to conventional settling (0.2 to 0.7 gal/ft2/min, or 0.5 to 1.7 m3/m2/h); 1.0 to 3.0 h). Studies for CSO have shown that a treatment system consisting of screening (using a 297-µm aperture with a hydraulic loading rate of 50 gal/ft2/min, or 122.3 m3/m2/h)) followed by DAF can offer an effective level of treatment (U.S. Environmental Protection Agency, 1977c; 1979c). The basis of the system is that the screening removes the particles that are too heavy for the air bubbles to carry, and the DAF system removes the floating, neutral buoyancy, and remaining negative buoyancy particles. The addition of chemical flocculent in the form of ferric chloride and cationic polyelectrolyte was shown in the above two references to improve the removals. Table 1.12 shows the screening-DAF system design parameters (U.S. Environmental Protection Agency, 1977a). As with the other treatment processes discussed, there are no data available for treatment of separate storm runoff; however, from the CSO data it would appear that, except for sedimentation, screening-DAF is likely to be the most expensive treatment system.

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TABLE 1.12 Screening and DAF Design Parameters Overflow rate (gal/ft2/min) Low rate High rate Horizontal velocity (ft/min) Detention time (min) Flotation cell range Flotation cell average Saturation tank Mixing chamber Pressurized flow (% of total flow) Split-flow pressurization Effluent recycle pressurization Air-to-pressurized-flow ratio (standard ft3/min/10 gal) Air-to-solids ratio Pressure in saturation tank (lb/in.2) Float Volume (% of total volume) Solids concentration (% dry weight basis)

1.3–4.0 4–10.0 1.3–3.8 10–60 25 1–3 1 20–30 25–45 1.0 0.05–0.35 40–70 0.75–1.4 1–2

Conversions: gal/ft2/min × 2.445 = m3/m2/h; ft/min × 0.00508 = m/s; standard ft3/min/100 gal × 0.00747 = m3/min/100 l; lb/in.2 × 0.0703 = kg/m2. Source: EPA-600/8-77/014.

High-Gradient Magnetic Separation HGMS is a relatively new treatment technology for treatment of storm runoff or CSO but has been used successfully for a number of years in the treatment of water to or from industrial processes. A high degree of treatment is possible with this process, which will probably be greater than required to meet permitting requirements alone. In its simplest form, the high-gradient magnetic separator consists of a canister packed with a fibrous ferromagnetic material that is magnetized by a strong external magnetic field (coils surround the canister). The water to be treated is passed through the canister and the fibrous ferromagnetic matrix causes only a small hydraulic resistance because it occupies less than 5% of the canister volume. Upstream of the canister the water is prepared by binding finely divided magnetic seed particles, such as magnetic iron oxide (magnetite), to the nonmagnetic contaminants. Binding the magnetic seed is accomplished in two general ways: adsorption of the contaminant to the magnetic seed and chemical coagulation (alum). The magnetic particles are trapped on the edges of the magnetized fibers in the canister as the water passes through. When the matrix has become loaded with magnetic particles, they are easily washed off by turning off the magnetic field and backflushing. Particles ranging in size from soluble through settleable (>0.001 µm) may be removed with this process; design parameters for HGMS are presented in Table 1.13. HGMS can offer rapid filtration for many pollutants with greater efficiency than for sedimentation because the magnetic forces on the fine particles may be many times greater than gravitational forces. “Urban Stormwater Management and Technology: Update and User’s Guide” (U.S. Environmental Protection Agency 1977a) provides details of bench- and pilot-scale studies that have been conducted using HGMS to treat CSO (U.S. Environmental Protection Agency, 1977a).

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TABLE 1.13 Preliminary Design Parameters for High Gradient Magnetic Separators Magnetic field strength, kG a Maximum flux density, gas/ft2/min Maximum detention time, min Matrix loading, g solids/g of matrix fiber Magnetic addition, mg/l Magnetite-to-SS ratio Alum reduction mg/l Range Average Polyelectrolyte additon, mg/l

0.5–1.5 100 3 0.1/0.5 100–500 0.4–3.0 90–120 100 0.5–1.0

a

Kilogauss Convsersion: gal/ft2/min × 2.445 = m3/m2/h. Source: EPA-600/8-77/014.

For HGMS and all other treatments that involve an additive to enhance the solids removal, there is a need to accommodate the variation in storm runoff SS concentrations. This will require automatic monitoring and adjustment of the additive dosage for efficient operation. Powdered Activated Carbon-Alum Coagulation A treatment option that has the potential to remove dissolved organics is the use of powdered activated carbon with alum added to aid in subsequent clarification. This was demonstrated at a 100,000 gal/day (379 m3/day) pilot unit in Albany, New York (U.S. Environmental Protection Agency, 1973c; Field, 1990); using municipal sewage and CSO. A short flocculation period followed the addition of alum with settling of solids by gravity and disinfection of the effluent or filtering (tri-media) and disinfection prior to discharge. Carbon regeneration in a fluidized-bed furnace and alum recovery from the calcined sludge were also demonstrated, as was reuse of the reclaimed chemicals. Average carbon losses per regeneration cycle were 9.7%. Average removals were in excess of 94% for COD, 94% for BOD5, and 99% for SS with no filtration. Disinfection Disinfection is generally practiced at WWTPs to control pathogenic microorganisms. The development of disinfection techniques and measurement of their effectiveness to kill pathogens has been mainly derived from the sanitary wastewater field, where the concern has been to measure the presence of fecal contamination and ability to kill any pathogens and viruses of human origin. Because it is both difficult and expensive to isolate and measure specific pathogens in water, methods were developed to monitor certain indicator organisms, i.e., microorganisms indicative of the presence of fecal contamination. Bacteria of the total coliform (TC) group became the generally accepted indicator for fecal pollution, but includes different genera that do not all originate from fecal wastes (e.g., Citrobacter, Klebsiella, and Enterobacter). An improvement over the TC test is the more selective fecal coliform (FC) test, which selects primarily for Klebsiella and Escherichia coli bacteria. E. coli is the bacteria of interest because it is a consistent inhabitant of the intestinal tract of humans and other warm-blooded animals. The FC test is still not specific to enteric bacteria, and human-enteric bacteria in particular. In 1986, a

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U.S. EPA publication (U.S. Environmental Protection Agency, 1986) recommended that states “begin the transition process to the new (E. coli and enterococci) indicators.” However, many states still retain the TC and FC criterion and the most widely used bacteriological criterion in the United States is the maximum of 200 FC/100 ml. For discharges of separate storm runoff, the above criterion is unlikely to give a true indication of the potential risk of infection, as many of these indicator bacteria also originate from soils, vegetation, and animal feces. Stormwater runoff can contain high densities of the nonhuman indicator bacteria and epidemiological studies of recreational waters receiving stormwater runoff have found little correlation between indicator densities and swimming-related illnesses (U.S. Environmental Protection Agency, 1983b; 1984a; Calderon et al., 1991). In addition, a number of non-enteric pathogens found in stormwater runoff have been linked to respiratory illnesses and skin infections, a risk that is not assessed by the present fecal indicators. Although the present standards and indicators are unlikely to reflect the actual human disease contraction potential, i.e., pathogenicity of a storm flow and its receiving water, they are the only practical standards available. Also, urban storm runoff has a high potential to be contaminated by sanitary cross-connections that would make the standards more relevant. Therefore, until other more relevant indicators are developed and proved, the present standards should be used but with the caution that they may over- or underestimate the true risk. The paper entitled “The Detection and Disinfection of Pathogens in Storm-Generated Flows” (O’Shea and Field, 1992) covers this subject in more detail. Conventional municipal sewage disinfection generally involves the use of chlorine gas or sodium hypochlorite as the disinfectant. To be effective for disinfection purposes, a contact time of not less than 15 min at peak flow rate and a chlorine residual of 0.2 to 2.0 mg/l are commonly recommended. However, a different approach is required for storm runoff, because the flows have characteristics of intermittency, high flow rate, high SS content, wide temperature variation, and variable bacterial quality. Further aspects of disinfection practices that require consideration for storm runoff follow: 1. A residual disinfecting capability may not be permitted, because chlorine residuals and compounds discharged to natural waters may be harmful to human and aquatic life (i.e., formation of carcinogens, e.g., tri-halomethanes). 2. The coliform count is increased by surface runoff in quantities unrelated to pathogenic organism concentration. TC or FC levels may not be the most useful indication of disinfection requirements and efficiencies. 3. Discharge points requiring disinfection are often at outlying points on the drainage system and require unmanned, automated installations. The disinfectant used to treat storm runoff should be adaptable to intermittent use, be effective, and be safe and easy to dose the effluent. Table 1.14 shows disinfectants that might be used for storm flow disinfection. Chlorine and hypochlorite will react with ammonia to form chloramines and with phenols to form chlorophenols. These are toxic to aquatic life and the latter also produce taste and odor in the water. Chlorine dioxide (ClO2) does not react with ammonia and completely oxidizes phenols. Ozone has a more rapid disinfecting rate than chlorine, is effective in oxidizing phenols, and has the further advantage of supplying additional oxygen to the effluent. The increased disinfecting rate of ozone requires shorter contact times and results in a lower capital cost for a contactor, as compared with that for a chlorine contact tank. Ozone does not produce chlorinated hydrocarbons or a long-lasting residual as chlorine does, but it is unstable and must be generated on site just prior to application. Therefore, capital investment in a generating plant is required along with the operation and maintenance.

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TABLE 1.14 Characteristics of Principal Storm Flow Disinfection Agents Characteristics

Chlorine

Stability Reacts with ammonia to form chloramines Destroys phenols Produces a residual Affected by pH Hazards a

Hypochlorite

Clorine Dioxide

Ozone

Stable Yes

6-month half-life Yes

Unstable No

Unstable No

At high concentrations Yes More effective at pH < 7.5 Toxic

At high concentrations Yes More effective at pH < 7.5 Slight

Yes Short-lived a Slightly Toxic; explosive

Yes No No Toxic

Chlorine dioxide dissociates rapidly.

Source: EPA-600/8-77/014.

Another disinfection technique that promises short detention times and the absence of toxic reaction products is the use of ultraviolet (UV) light irradiation. The effectiveness of the early systems was limited for water with high concentrations of solids that tended to attenuate the UV energy. Later systems emit higher-intensity radiation for more effective treatment. More recently, modulated UV light has been reported to reduce viable bacteria by approximately 100-fold compared to populations observed after similar exposure to UV light that lacked modulation (Bank et al., 1990). The characteristics of storm runoff (i.e., intermittent and often high flows) together with the need to minimize capital costs for a treatment operation, lend themselves favorably to use of highrate disinfection. This refers to achieving either a given percent or a given bacterial count reduction through the use of decreased disinfectant contact time, increased mixing intensity, increased disinfectant concentration, chemicals having higher oxidizing rates, or various combinations of these. Where contact times are less than 10 min (usually in the range 1 to 5 min), adequate mixing is a critical parameter, providing complete dispersion of the disinfectant and forcing disinfectant contact with the maximum number of microorganisms. The more physical collisions high-intensity mixing causes, the lower the contact time requirements. Mixing can be accomplished by mechanical flash mixers at the point of disinfectant addition and at intermittent points, or by specially designed plug flow contact chambers containing closely spaced, corrugated parallel baffles that create a meandering path for the wastewater (U.S. Environmental Protection Agency, 1973b). High-rate disinfection was shown (for CSO) to be enhanced beyond the expected additive effect by sequential addition of Cl2 followed by ClO2 at intervals of 15 to 30 s (U.S. Environmental Protection Agency, 1975a, 1976b). A minimum effective combination of 8 mg/l of Cl2 followed by 2 mg/l of ClO2 was effective in reducing TC, FC, fecal streptococci, and viruses to acceptable target levels and compared to 25 mg/l Cl2 or 12 mg/l ClO2. It was surmised that the presence of free Cl2 in solution with chlorite ions (ClO –2 , the reduced state of ClO2) may cause the oxidation of ClO –2 back to its original state. This process would prolong the existence of ClO2, the more potent disinfectant. An equation and concept to enable the effect of high rate mixing to be taken into account in the disinfection process are provided (U.S. Environmental Protection Agency, 1973b). A velocity gradient (G), as defined in the equation below is used as a measure of the mixing intensity, but is also a measure of the opportunities for microorganism and disinfectant matter collisions per unit time per unit volume.

(

G = 1730

viscosity(centipoise)

)(

velocity(ft s) × channel slope (ft ft )

)

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The product of velocity gradient and contact time (GT) is the number of opportunities for collisions per unit volume during the contact time. It is important to note that if high-rate mixing is to be relied upon to provide effective disinfection, the velocity gradient should not reduce if the flow rate reduces; i.e., if the mixing intensity depends on the velocity of flow and not mechanical mixing, then the level of disinfection will be reduced at low flow rates. There will be some offset due to the longer detention times at lower flow rates, but the intensity of mixing will be the more significant parameter. Use of a Sutro weir for the influent and effluent will help maintain the peak rate velocity at all flow rates. Swirl Regulators/Concentrators These are compact flow-throttling and solids separation devices that also collect floatable material. The performance of the swirl device is very dependent on the settling characteristics of the solids in the stormwater. The U.S. EPA swirl is most effective at removing solids with characteristics similar to grit (≈0.008 in., or 0.2 mm, effective diameter, 2.65 specific gravity). It is important to appreciate this aspect of swirl devices and not expect significant removals of fine and low-specificgravity solids. The three most common configurations are the U.S. EPA swirl concentrator, the Fluidsep™ vortex separator, and the Storm King™ hydrodynamic separator. Although each separator is configured differently, the operation of each unit and the mechanism for solids separation are similar. The flow enters the unit tangentially and follows the perimeter wall of the cylindrical shell, creating a swirling, quiescent vortex flow pattern. The swirling action throttles the influent flow, and causes solids to be concentrated at the bottom of the unit. The throttled underflow containing the concentrated solids passes out through an outlet in the bottom of the unit, while the clarified supernatant passes out through the top of the unit. Various baffle arrangements are provided to capture floatables in the supernatant, which are then usually carried out in the underflow as the storm subsides and the water level in the swirl unit falls. During low-flow conditions all of the flow passes out via the bottom outlet and only when the flow increases does the throttling effect and buildup of water in the swirl occur. The solids separation is helped by the flow patterns, with the influent deflected into a slowermoving inner swirl pattern after one revolution around the perimeter of the swirl unit. Gravity separation occurs as particles follow a “long path” through the outer and inner swirl. Solids separation is also assisted by the shear forces set up between the inner and outer swirls, along the perimeter walls and the bottom. A U.S. EPA swirl regulator/concentrator is shown in Figure 1.10. The swirl device can offer a compact unit that functions as both a regulator for flow control and a solids concentrator, and when combined with treatment of the relatively heavy settleable solids can provide an effective treatment system. There are a number of references (U.S. Environmental Protection Agency, 1973a; 1974b; 1977d; 1982; 1984b) that provide performance and design information for the U.S. EPA swirl regulator/concentrator. A degritter version of the U.S. EPA swirl has also been developed (U.S. Environmental Protection Agency, 1977d; 1981a), which has no underflow and only removes the grit detritus portion.

STORAGE AND TREATMENT OPTIMIZATION As stated previously, storage alone will only offer flow attenuation, and treatment alone will only treat a fraction of the stormwater flow, or have such a large capacity to handle peak flows that the costs will be prohibitive. Therefore, combining the storage/treatment, finding the best balance, and if possible using existing facilities are likely to provide the most cost-effective solution for treatment of urban storm runoff. No two situations are likely to be the same, but a cost analysis to produce curves of storage alone, treatment alone, and the combined cost will produce an optimized cost curve as shown in

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J

Inflow

C E

D F

Foul sewer G

B

A

H K

Overflow

A B C D E F G H I J K

Inlet ramp Flow deflector Scum ring Overflow weir and weir plate Spoilers Floatables trap Foul sewer outlet Floor gutters Downshaft Secondary overflow weir Secondary gutter

FIGURE 1.10 Isometric view of a swirl combined sewer overflow regulator/separator. (From U.S. EPA, EPA600/8-82/013, 1982).

Figure 1.11. Factors such as the number of storms that are likely to exceed the capacity of the combined system need to be taken into account, and this approach will provide useful information on which to base a decision (U.S. Environmental Protection Agency, 1972b; Field et al., 1994a). Because of the variable nature of storm events, there will always be some storm events that generate runoff in excess of the storage/treatment capacity. The excess runoff will be treated by gravity settling in the storage basin prior to being discharged to the receiving water. Use of the swirl regulator/concentrator described above can provide some treatment to the runoff, which is either diverted to storage (alleviating bottom solids accumulation problems) or the receiving water.

BENEFICIAL REUSE OF STORMWATER The reuse of municipal wastewater for industry, nonpotable domestic usages, and groundwater recharge has been practiced for many years. In 1971, a U.S. EPA nationwide survey estimated that current reuse of treated municipal wastewater for industrial water supply, irrigation, and groundwater recharge was 53.5 billion, 77 billion, and 12 billion gal/year (200 million, 290 million, and 45 million m3/year), respectively (Environmental Protection Agency, 1975b). It is reasonable to expect that reuse of treated wastewater and/or stormwater for industrial cooling, nonpotable domestic water supply, and park and golf course irrigation will increase in the future. Many of the treatments discussed above are likely to produce an effluent quality that is of a higher standard than that required to meet a stormwater permit. Where there are suitable circumstances, an opportunity exists to take advantage of this higher effluent quality for reuse of the storm runoff. The intended reuse will govern the level of treatment required, but careful selection, design, and use of pilot studies should result in the required combination of the above technologies to achieve required effluent quality.

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NOTES 1. Total system costs include storage and treatment costs. 2. Treatment costs include cost of influent pump station and high rate filtration plant. 3. Storage cost is based on concrete tank costing $700 per 1000 gallons of storaged volume.

8.0

7.0

6.0 Capital Costs - ($ Mil.)

37

Total system costs

5.0

4.0

Storage facilities costs

3.0 Treatment plant costs 2.0

1.0 0

0

50 100 150 200 Treatment Plant Capacity - (MGD) Estimated Capital Costs of Storage and Treatment for 200 MGD Overflow

FIGURE 1.11 Estimated capital costs of storage and treatment for 200 MGD overflow. (From U.S. EPA, EPA-110EYI04/72, 1972b.)

The additional cost to provide treatment above that required to satisfy a discharge permit will need to be less than the cost of water from other sources for economic viability. With increasing demands on potable water supplies, the concept of reuse, in particular where a nonpotable water quality standard is required, will make this an increasingly more viable option. The chapter “Reclamation of Urban Stormwater” from Integrated Stormwater Management provides details and a hypothetical case study (Field et al., 1993).

REFERENCES Bank, H.L., John, J., Schmehl, M.K., and Dratch, R.J., 1990. Bactericidal effectiveness of modulated UV light, Appl. Environ. Microbiol., 56 (12), 3888–3889. Calderon, R.L., Mood, E.W., and Dufour, A.P., 1991. Health effects of swimmers and non-point sources of contaminated water, Int. J. Environ. Health, 1, 21. Camp Dresser & McKee, 1993. State of California Storm Water Best Management Practice Handbooks, The California State Water Quality Control Board. Colt, J., Tanji, K., and Tchobanoglous, G., 1977. Impact of Dog, Cat, and Pigeon Wastes on the Nitrogen Budget of San Francisco Storm Runoff, Department of Water Science and Engineering University of California, Davis, No. 4015. Field, R., 1990. Combined sewer overflows: control and treatment, in Control and Treatment of Combined Sewer Overflows, P.E. Moffa, Ed., Van Nostrand Rheinhold, New York, pp. 119–190. Field, R. and Struzeski, E.J., Jr., 1972. Management and control of combined sewer overflows, J. Water Pollut. Control Fed., 44(7).

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Field, R., Forndran, A., and Dunkers, K., 1990. Demonstration of in-receiving water storage of combined sewer overflows: in a marine/estuarine environment by the flow balance method, drainage systems and runoff reduction, in Proceedings of the Fifth International Conference on Urban Storm Drainage, Osaka, Japan, Vol. 2, Y. Iwasa and T. Sueishi, Eds., 759–764. Field, R., O’Shea, M.L., and Chin, K.K., Eds., 1993. Integrated Stormwater Management, Lewis, Boca Raton, FL. Field, R., O’Connor, T.P., and Pitt, R., 1994a. Optimization of CSO storage-treatment systems, in Proceeding of the Water Environment Fed. Specialty Conference Series: A Global Perspective for Reducing CSOs: Balancing Technologies, Costs and Water Quality, Louisville, KY, July 10–13. Field, R., Pitt, R., Lalor, M., Brown, M.P., and Vilkelis, W.V., 1994b. Investigation of dry-weather pollutant entries into storm drainage systems, ASCE J. Environ. Eng., October. Forndran, A., Field, R., Dunkers, K., and Moran, D., 1991. Balancing flow for CSO abatement, Water Environ. Technol., Water Environ. Fed., 3(5), pp. 54–58. Metcalf & Eddy, Inc., 1981. Wastewater Engineering: Collection and Pumping of Wastewater, McGraw-Hill, New York. Minnesota Pollution Control Agency, 1989. Protecting Water Quality in Urban Areas. Metropolitan Washington Council of Governments, 1987. Controlling Urban Runoff: Practical Manual for Planning and Designing Urban BMPs, T.R. Schueler, MWCG for Washington Metropolitan Water Resources Planning Board. Metropolitan Washington Council of Governments, 1992a. Analysis of Urban BMP Performance and Longevity. Prince George’s County, Department of Environmental Resources. Metropolitan Washington Council of Governments, 1992b. A Current Assessment of Urban Best Management Practices: Techniques for Reducing Non-Point Source Pollution in the Coastal Zone, T.A. Schueler, P.A. Kumble, M.A. Heraty, MWCG, for U.S. EPA. Metropolitan Washington Council of Governments, 1992c. Design of Stormwater Wetland Systems: Guidelines for Creating Diverse and Effective Stormwater Wetlands in the Mid-Atlantic Region, T.R. Schueler. Montoya, B.L. 1987. Urban Runoff Discharges from Sacramento, California. CVRWQCB No. 87-ISPSS, prepared for California Regional Water Quality Control Board, Central Valley Region. New York State Department of Environmental Conservation, 1992. Reducing the Impacts of Stormwater Runoff from New Development. O’Shea, M.L. and Field, R., 1992. The detection and disinfection of pathogens in storm-generated flows, Adv. Appl. Microbiol., 37. Pecher, R., 1969. The runoff coefficient and its dependance on rain duration, Berichte Int. Wasserwirtschaft und Gesundheitsingenieurwesen, No. 2, TU Munich [in German]. Pitt, R., 1987. Small Storm Urban Flow and Particulate Washoff Contributions to Outfall Discharges, Ph.D. dissertation, Department of Civil and Environmental Engineering, University of Wisconsin, Madison. Pitt, R., 1988. Source Loading And Management Model: An Urban Non Point Source Water Quality Model (SLAMM), University of Alabama at Birmingham. Pitt, R. and Dunkers, K., 1993. Lake water quality improvements from treatment of stormwater using the flow balancing method, presented at Water Environment Federation, 66th Annual Conference & Exposition, Anaheim, CA, October, AC93-019-001. Pitt, R. and Field, R., 1990. Hazardous and toxic wastes associated with urban stormwater runoff, in Proceedings 16th Annual RREL Hazardous Waste Research Symposium: Remedial Action, Treatment, and Disposal of Hazardous Waste, U.S. EPA, Office of Research and Development, Cincinnati, OH, EPA/600/9-90-037 (NTIS PB 91 148 379). Pitt, R. and McLean, J., 1986. Toronto Area Watershed Management Strategy Study: Humber River Pilot Watershed Project, Ontario Ministry of the Environment, Toronto, Ontario, Canada. Pitt, R., Barron, P., Ayyoubi, A., and Field, R., 1991. The treatability of urban stormwater toxicants, in Proceedings 17th Annual RREL Hazardous Waste Research Symposium: Remedial Action, Treatment, and Disposal of Hazardous Waste, U.S. EPA, Risk Reduction Engineering Laboratory, Office of Research and Development, Cincinnati, OH, EPA/600/9-91/002 (NTIS PB 91 233 627). Pitt, R., Ayyoubi, A., Field, R., and O’Shea, M.L., 1993. The treatability of urban stormwater toxicants, in Integrated Stormwater Management, R. Field, M.L. O’Shea, and K.K. Chin, Eds., Lewis Publishers, Boca Raton, FL, p. 121.

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Pitt, R., Field, R., Lalor, M., and Brown, M., 1995. Urban stormwater toxic pollutants: assessment, sources and treatability, Water Environ. Res., 67(3). Roesner, L.A., Urbonas, B.R., and Sonnen, M.B., Eds., 1989. Design of urban runoff quality controls, in Proceedings of an Engineering Foundation Conference on Current Practice and Design Criteria for Urban Quality Control, American Society of Civil Engineers, New York. Roesner, L.A., Burgess, E.H., and Aldrich, J.A., 1991. The hydrology of urban runoff quality management, in Proceedings of a Water Resources Planning and Management Conference, American Society of Civil Engineers, New Orleans, LA. Schmidt, S.D. and Spencer, D.R., 1986. The magnitude of improper waste discharges in an urban stormwater system, J. Water Pollut. Control Feder., 58(7). SCVWD, 1990. Santa Clara Valley Nonpoint Source Study, Vol. II: NPS Control Program, Santa Clara Valley Water District, San Jose, CA. Soderlund, H., 1988. Recovery of the Lake Ronningesjon in Taby, Sweden; results of storm and lake water treatment over the years 1981–1987, Vatten, 4(44). Urbonas, B. and Stahre, P., 1993. Stormwater: Best Management Practices and Detention, Prentice-Hall, Englewood Cliffs, NJ. U.S. Environmental Protection Agency, 1970a. Combined Sewer Regulator Overflow Facilities, American Public Works Association, Chicago, IL, 11022DMU07/70 (NTIS PB 215 902). U.S. Environmental Protection Agency, 1970b. Combined Sewer Regulation and Management — A Manual of Practice, American Public Works Association, Chicago, IL, 11022DMU08/70 (NTIS PB 195 676). U.S. Environmental Protection Agency, 1971. Environmental Impact of Highway Deicing, Edison Water Quality Research Laboratory, Edison, NJ, EPA11040GKK06/71 (NTIS PB 203 493). U.S. Environmental Protection Agency, 1972a. A Search: New Technology for Pavement Snow and Ice Control, D.M. Murray and M.R. Eigermann, ABT Associates, Inc., Cambridge, MA, EPA-R2-72-125 (NTIS PB 221 250). U.S. Environmental Protection Agency, 1972b. High-Rate Filtration of Combined Sewer Overflows (Cleveland), R. Nebolsine et al., Hydrotechnic Corp., New York, EPA-110EYI04/72 (NTIS PB 211 144). U.S. Environmental Protection Agency, 1973a. The Dual-Function Swirl Combined Sewer Overflow Regulator/Concentrator, R. Field, U.S. EPA, Edison, NJ, EPA-670/2-73-059 (NTIS PB 227 182/3). U.S. Environmental Protection Agency, 1973b. Combined Sewer Overflow Seminar Papers, U.S. EPA Storm and Combined Sewer Research Program and NYS-DEC, EPA-670/2-3-077 (NTIS PB 235 771). U.S. Environmental Protection Agency, 1973c. Physical-Chemical Treatment of Combined and Municipal Sewage, A.J. Shuckrow et al., Pacific NW Lab, Battelle Memorial Institute, Richland, WA, EPA-R273-149 (NTIS PB 219 668). U.S. Environmental Protection Agency, 1974a. Manual for Deicing Chemicals: Storage and Handling, D.L. Richardson et al., Arthur D. Little, Inc., Cambridge, MA, EPA-670/2-74-033 (NTIS PB 236 152). U.S. Environmental Protection Agency, 1974b. Relationship between Diameter and Height for Design of a Swirl Concentrator as a Combined Sewer Overflow Regulator, R.H. Sullivan et al., American Public Works Association, Chicago, IL, EPA-670/2-74-039 (NTIS PB 234 646). U.S. Environmental Protection Agency, 1974c. Manual for Deicing Chemicals: Application Practices, D.L. Richardson et al., Arthur D. Little, Inc., Cambridge, MA, EPA-670/2-74-045 (NTIS PB 239 694). U.S. Environmental Protection Agency, 1974d. Combined Sewer Overflow Treatment by the Rotating Biological Contactor Process, F.L. Welsh and D.J. Stucky, Autotrol Corp., Milwaukee, WI, EPA-670/274-050 (NTIS PB 231 892). U.S. Environmental Protection Agency, 1975a. Bench-Scale High-Rate Disinfection of Combined Sewer Overflows with Chlorine and Chlorine Dioxide, P.E. Moffa et al., O’Brien & Gere Engineers, Inc., Syracuse, NY, EPA-670/2-75-021 (NTIS PB 242 296). U.S. Environmental Protection Agency, 1975b. Current Municipal Wastewater Reuse Practices — Research Needs for the Potable Reuse of Municipal Wastewater, C.J. Schmidt, EPA-600/9-75-007. U.S. Environmental Protection Agency, 1976a. Development of a Hydrophobic Substance to Mitigate Pavement Ice Adhesion, B.H. Alborn and H.C. Poehlmann, Jr., Ball Brothers Research Corp., Boulder, CO, EPA-600/2-76-242 (NTIS PB 263 653). U.S. Environmental Protection Agency, 1976b. Proceedings of Workshop on Microorganisms in Urban Stormwater, R. Field et al., U.S. EPA, Edison, NJ, EPA-600/2-76-244 (NTIS PB 263 030).

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U.S. Environmental Protection Agency, 1977a. Urban Stormwater Management and Technology: Update and Users’ Guide, J. Lager et al., Metcalf & Eddy, Inc., Palo Alto, CA, EPA-600/8-77-014 (NTIS PB 275 654). U.S. Environmental Protection Agency, 1977b. Catchbasin Technology Overview and Assessment, J. Lager et al., Metcalf and Eddy, Inc., Palo Alto, CA, in association with Hydro-Research Science, Santa Clara, CA, EPA-600/2-77-051 (NTIS PB 270 092). U.S. Environmental Protection Agency, 1977c. Screening/Flotation Treatment of Combined Sewer Overflows, Vol I, Bench-Scale and Pilot Plant Investigations, M.K. Gupta et al., Envirex Environmental Science Division, Milwaukee, WI, EPA-600/2-77-69a (NTIS PB 272 834). U.S. Environmental Protection Agency, 1977d. Swirl Device for Regulation and Treating Combined Sewer Overflows, EPA Technology Transfer Capsule Report, R. Field and H. Masters, U.S. EPA, Edison, NJ, EPA-625/2-77-012 (ERIC 2012, Cincinnati). U.S. Environmental Protection Agency, 1978. Optimization and Testing of Highway Materials to Mitigate Ice Adhesion — Interim Report, M. Kruker and J.C. Cook, Washington State University, Pullman, EPA600/2-78-56 (NTIS PB 280927/5). U.S. Environmental Protection Agency, 1979a. Dual Process High-Rate Filtration of Raw Sanitary Sewage and Combined Sewer Overflows, H. Innerfield et al., NYC Department of Water Resources, EPA600/2-79-015 (NTIS PB 80-159 626/AS). U.S. Environmental Protection Agency, 1979b. Combined Sewer Overflow Abatement Program, Rochester, NY, Vol. I, Pilot Plant Evaluations, F.J. Drehwing et al., O’Brien & Gere Engineers, Inc., Syracuse, NY, EPA-600/2-79-031b (NTIS PB 80-159 262). U.S. Environmental Protection Agency, 1979c. Screening/Flotation Treatment of Combined Sewer Overflows, Vol. II, Full-Scale Operation, Racine, WI, T.L. Meinholz, Envirex, Inc., Milwaukee, WI, EPA-600/279-106a (NTIS PB 80-130 693). U.S. Environmental Protection Agency, 1979d. Demonstration of Nonpoint Pollution Abatement through Improved Street Cleaning Practices, R.E. Pitt, Woodward-Clyde Consultants, San Francisco, CA, EPA-600/2-79-161 (NTIS PB 80-108988). U.S. Environmental Protection Agency, 1980. Porous Pavement: Phase 1 Design and Operational Criteria, E.V. Diniz, Espey, Hustn & Associates, Inc., Albuquerque, NM, EPA-600/2-80-135 (NTIS PB 81 104 138). U.S. Environmental Protection Agency, 1981a. Field Evaluation of a Swirl Degritter at Tamworth, New South Wales, Australia, G.J. Shelly et al. G.J. Shelly Consulting Engrs., Tamworth, NSW, Australia, EPA600/2-81-063 (NTIS PB 81-219 602). U.S. Environmental Protection Agency, 1981b. Best Management Practices Implementation, Rochester, New York, C.B. Murphy et al., O’Brien & Gere Engineers, Inc., Syracuse, NY, EPA-905/9-81-002 (NTIS PB 82 169 210). U.S. Environmental Protection Agency, 1982. Design Manual — Swirl and Helical Bend Pollution Control Devices, R.H. Sullivan et al., American Public Works Association, Chicago, IL, EPA-600/8-82/013 (NTIS PB 82-266 172). U.S. Environmental Protection Agency, 1983a. Evaluation of Catchbasin Performance for Urban Stormwater Pollution Control, G.L. Aronson et al., Environmental Design & Planning, Inc., Boston, MA, EPA600/2-83-043 (NTIS PB 83-217745). U.S. Environmental Protection Agency, 1983b. Health Effects Criteria for Marine Recreational Waters, V.J. Cabelli, U.S. EPA, EPA-600/1-80-01. U.S. Environmental Protection Agency, 1984a. Health Effects Criteria for Fresh Recreational Waters, A.P. Dufour, U.S. EPA, EPA-600/1-84-004. U.S. Environmental Protection Agency, 1984b. Swirl and Helical Bend Regulator/Concentrator for Storm and Combined Sewer Overflow Control, W.C. Pisano et al., Environmental Design & Planning, Inc., Boston, MA, EPA-600/2-8/116 (NTIS PB 85-102 523/Reb.). U.S. Environmental Protection Agency, 1985. Characterization, Sources, and Control of Urban Runoff by Street and Sewerage Cleaning, R.E. Pitt, Consulting Engineer, Blue Mounds, WI, EPA-600/2-85/038 (NTIS PB 85-186500/Reb.). U.S. Environmental Protection Agency, 1986. Ambient Water Quality Criteria for Bacteria, EPA-440/5-84-002. U.S. Environmental Protection Agency, 1988. Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment, CERI, Cincinnati, OH, EPA/625/1-88/022.

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U.S. Environmental Protection Agency, 1990. Construction of a Flow Balancing Facility for Combined Sewer Overflow Retention, Project Enhancement and Floatables Removal. EPA Region II, New York, Cooperative Agreement C361320. U.S. Environmental Protection Agency, 1991a. Developing the watershed plan, in Nonpoint Source Watershed Workshop, Cincinnati, OH, EPA/625/4-91/027 (NTIS PB92-137504). U.S. Environmental Protection Agency, 1991b. Developing Goals for Nonpoint Source Water Quality Projects, in Nonpoint Source Watershed Workshop, Cincinnati, OH, EPA/625/4-91/027 (NTIS PB92-137504). U.S. Environmental Protection Agency, 1992a. Designing an Effective Communication Program: A Blueprint for Success, R.M. Beech and A.F. Dake, University of Michigan, School of Natural Resources and Environment for U.S. EPA Region 5. U.S. Environmental Protection Agency, 1992b. Storm Water Pollution Prevention for Construction Activities, EPA-832-R-92-005. U.S. Environmental Protection Agency, 1992c. Effects and Treatment of Stormwater Toxicants. Storm & Combined Sewer Pollution Control Research Program, Edison, NJ, Cooperative Agreement CR819573. U.S. Environmental Protection Agency, 1993a. Handbook: Urban Runoff Pollution Prevention and Control Planning, Metcalf & Eddy, Inc., Office of Research and Development, Cincinnati, OH, EPA/625/R-93/004. U.S. Environmental Protection Agency, 1993b. Urban Runoff Management Information/Education Products. Developed for U.S. EPA Region 5, Water Division, Wetlands and Watershed Section Watershed Management Unit and U.S. EPA Office of Wastewater Enforcement and Compliance Permits Division, NPDES Program Branch, Stormwater Section, Version 1. U.S. Environmental Protection Agency, 1993c. Investigation of Inappropriate Pollutant Entries into Storm Drainage Systems: A User’s Guide, R.E. Pitt, M. Lalor, R. Field, D. D. Adrian, D. Barbé, EPA/600/R-92/238. Viessman, W., Jr., Knapp, J., and Lewis, G.L., 1977. Introduction to Hydrology, 2nd ed., Harper & Row, New York, 69. Walesh, S.G., 1989. Urban Surface Water Management, John Wiley & Sons, New York. Wanielista, M.P. and Yousef, Y.A., 1992. Stormwater Management, John Wiley & Sons, New York. Washtenaw County Drain Commissioner, 1988. Huron River Pollution Abatement Project, Summary, Washtenaw County, MI. Water Environment Federation, 1990. Natural Systems for Wastewater Treatment: Manual of Practice, FD-16, WEF, Alexandria, VA. Water Environment Federation, 1992. Design of Municipal Wastewater Treatment Plants, Vol. I and II, WEF Manual of Practice 8, Alexandria, VA, ASCE Manual and Report on Engineering Practice 76, 2nd ed., American Society of Civil Engineers, New York. Yu, S.L., Kasnick, M.A., and Byrne, M.R., 1993. A level spreader/vegetative buffer strip system for urban stormwater management, in Integrated Stormwater Management, R. Field, M.L. O’Shea, and K.K. Chin, Eds., Lewis Publishers, Boca Raton, FL, p. 93.

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2

John J. Sansalone CONTENTS Influences on the Hydrologic Cycle by the Built Urban Environment ..........................................43 Introduction ...............................................................................................................................43 Impervious Surfaces and the Hydrologic Cycle ......................................................................44 Modification to the Runoff Hydrograph by the Urban Environment......................................44 Transport of Particulate Matter Due to Urban Anthropogenic Activities ...............................45 Acidic Deposition and the Urban Environment.......................................................................45 The Urban Environment as a Heat Island for Climate Modification ......................................46 Sources and Magnitude of Anthropogenic Urban Constituent Loadings .......................................47 The Role of Urban Environmental Hydrology................................................................................51 Heavy Metal Partitioning.................................................................................................................53 Treatment Design Implications for Dissolved Heavy Metals..................................................55 Physical and Chemical Characteristics of Stormwater Particulate Matter .....................................56 Measurement of Stormwater Particulate Matter ......................................................................57 Treatment Design Implications for Particulates and Particulate Heavy Metals......................63 References ........................................................................................................................................64

INFLUENCES ON THE HYDROLOGIC CYCLE BY THE BUILT URBAN ENVIRONMENT INTRODUCTION Changes in the physical and chemical characteristics of urban stormwater are, to a significant degree, related to the hydrologic and hydraulic modifications of the built urban environment. If one had to describe what single attribute or index of the built urban environment contributes significantly to modification of both water quality and quantity, it would be degree of imperviousness. This index directly quantifies the degree of hydrologic and hydraulic modification and indirectly provides an index for the delivery of pollutant mass generated in the urban environment. Examples of these impervious surfaces include roadway pavement, parking lots, rooftops, sidewalks, driveways, and compacted or altered urban surficial soils. Although the term impervious suggests that these urban surfaces infiltrate no rainfall, this is not absolutely correct. Depending on the condition of the surface, there can be a measurable amount of infiltration, in particular if the surface deteriorates or is not maintained in an impervious state. Data from 47 small urban

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watersheds across the United States indicate that an approximately linear relationship exists when the volume-based runoff coefficient is regressed against watershed imperviousness (Schueler, 1987). Examples of urban hydraulic modifications include catchbasins, inlets, curb and gutter, gutter and downspouts, storm sewers, ditches, lined channels, culverts, and pavement. These hydraulic modifications will generally reduce travel time and increase flow velocity as compared to the original natural condition. Urbanization profoundly alters the water quality and quantity balances of the localized urban hydrologic cycle.

IMPERVIOUS SURFACES

AND THE

HYDROLOGIC CYCLE

Impervious surfaces modify many components of the hydrologic cycle. The construction of impervious surfaces modifies surficial soils, and eliminates surficial soil as a pervious storage interface between the subsurface and the atmosphere. Impervious surfaces eliminate a significant degree of infiltration abstractions. In metropolitan areas that depend on groundwater, expansive impervious areas have a significant impact on reduction in groundwater recharge and a corresponding increase in surface water discharge. Depressional storage in urban areas can be reduced by a factor of 5 to 10 depending on the original natural state of the watershed and the degree of imperviousness generated from the urbanization (Viessman and Lewis, 1996). Storage abstractions are also significantly reduced or eliminated unless anthropogenic forms of such abstractions are designed into the urban hydrologic system such as through the use of detention or retention basins. In temperate climates, the built urban environment of highly impervious surfaces reduces evaporation and evapotranspiration. Commensurate with the expansion of built urban environments are modification to the ambient temperature regimes in these urban environments. As a result of urban design and construction of impervious surfaces, the rainfall runoff process can be significantly modified. In addition to hydrologic modifications there are hydraulic modifications that result in effective conveyance of stormwater (by design) and the associated modification of transport for soluble and solid constituents.

MODIFICATION

TO THE

RUNOFF HYDROGRAPH

BY THE

URBAN ENVIRONMENT

Compared to the hydrograph from the predeveloped natural environment, the urban runoff hydrograph from the same drainage area has the following characteristic modifications: (1) a greater peak discharge, (2) increased runoff volume, and (3) a decreased time to peak or lag time. It has been observed that urbanization increases the rate of peak discharge more rapidly than the increased volume generated (Viessman and Lewis, 1996). Each of these hydrologic characteristics and the urban hydraulic characteristics impact stormwater quantity and quality. These land use or urbanization effects are well documented in the literature and are typically categorized as (1) changes in peak flow characteristics, (2) changes in total runoff, (3) changes in water quality, and (4) changes in hydrologic morphology (Leopold, 1968). There are a number of typical “rules of thumb” one can expect for moderately developed watersheds (Schueler, 1987). Increased peak discharges are two to five times higher than preurbanization conditions. A moderately developed watershed can produce over twice the runoff volume as compared to preurbanization conditions. If extensive drainage “improvements” are made in an urbanizing watershed, time of concentration can be decreased by a factor of 2. Land-use modifications can either reduce lag time (as in urbanization) or increase lag time (increased infiltration) or reduce lag time by reducing flow travel time (urbanization) or increasing flow travel time (as for retention/detention or infiltration). An example illustrates the impact of both hydrologic and hydraulic modification in urban areas. If the percentage of an urban area served by storm sewers (hydraulic modification) is plotted against percentage of impervious area (hydrologic and hydraulic modification), it can be shown that the ratio of discharges after and before urbanization increase as both percentages increase (Leopold, 1968).

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The Physical and Chemical Nature of Urban Stormwater Runoff Pollutants

TRANSPORT

OF

PARTICULATE MATTER DUE

TO

45

URBAN ANTHROPOGENIC ACTIVITIES

Because urban surfaces and drainage systems are designed to provide more effective and rapid conveyance than natural systems, transport of suspended solids can be an order of magnitude greater than transport in the original natural environment (Viessman and Lewis, 1996). If particulate transport is not mass-limited, higher peak discharges and improved hydraulic characteristics of urban surfaces result in increased entrainment and transport of particulate matter from impervious surfaces (Sansalone et al., 1998). Construction of urban infrastructure is also a significant source of suspended material. On a flux basis, construction site runoff can transport more suspended material than that generated by urban anthropogenic activities and washed off the completed urban infrastructure. Once the original natural environment’s vegetative surface cover is removed through clearing and grubbing and without adequate erosion control, quantities of eroded material can be significant during the relatively short construction phase (Pitt, 1985). Short-term eroded sediment loads exceeding 40 tons/acre-year have been reported in the literature for loads transported during the construction of urban infrastructure (Novotny and Chesters, 1981). In comparison, for urbanized Cincinnati, long-term steady-state loads of anthropogenic solids washed off the urban surface are 1 to 2 tons/acre-year (Sansalone et al., 1998). These results are based on 2 years of urban stormwater analyses of total suspended solids (TSS) in urban Cincinnati. Particulate transport in urban drainage systems is important not only because of environmental and drainage issues related to particulate matter itself, but also because many constituents are transported on the surface of particulate matter entrained for sufficient time in stormwater (Malcom, 1989; Sansalone and Tribouillard, 1999).

ACIDIC DEPOSITION

AND THE

URBAN ENVIRONMENT

Because the impervious urban surface acts as the interface between the atmosphere and the subsurface, the urban surface is loaded by wet and dry anthropogenic atmospheric deposition. The burning of fossil fuels, generation of gaseous industrial waste products, and increasing traffic activities (despite significant decreases in emissions per vehicle) in urban areas generate hydrocarbons, oxides of nitrogen and sulfur, a variety of cations and anions, and particulate matter. Acidic atmospheric deposition as a result of these urban activities can occur in the wet form as rain, snow, aquasols (for example, fog or clouds) or in the dry form as aerosol particles or gas. Wet deposition processes that lead to acid precipitation will typically include ions of H+, SO42– , NO3– , NOx, NH 4+, Na+, Cl–, Mg2+, and Ca2+. In acid rain the predominant ions are H+, NH4+, Ca2+, SO4–, and NO3– . In accord with overall aqueous charge balance, the sum of cationic and anionic charges must balance. Dry deposition processes that lead to acid precipitation will typically include SO2, NO2, HNO3, NH3, HCl, and O3 gases and aerosol particles of SO42– , NO3– , and NH4+ . The term acid rain implies removal of acidic constituents or acid-causing constituents by wet deposition. However, dry deposition of acidic substances also occurs and the effects commonly attributed to acid rain are a combination of wet and dry acidic deposition. Because of the partial pressure of CO2 in the natural atmosphere, with a concentration of 330 ppm, the “natural” acidity of rainwater can be calculated and measured to be approximately 5.6. This value (as opposed to a neutral pH of 7) can be used as the baseline for acid precipitation. Rainwater below this pH is poorly buffered and in the absence of common basic buffering compounds such as NH3 and CaCO3, rainwater pH values due to natural sulfur compounds can be expected to be in the range of 5.0 (Seinfeld, 1986). Therefore, for rainfall pH greater than 5.6 it is generally concluded either that there have been little anthropogenic influences to promote acidification or that sufficient buffering capacity was available to control acidification. Rainfall with pH values between 5.0 and 5.6 may have anthropogenic influences but not to an extent that greatly exceeds the acidification potential of natural background sulfur compounds. If rainfall pH is less than 5.0, anthropogenic influences toward acidification have occurred (Seinfeld, 1986). Figure 2.1 illustrates rainfall pH as a function of previous dry days for urban Cincinnati.

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data (n=14) R2 = 0.80 trendline

7 6

CO2

5 pH

SO2

4 3 acid fog

2 1 0 0

2

4

6

8

10

12

14

previous dry days (pdd)

FIGURE 2.1 Illustration of rainfall pH depression in urban Cincinnati during 1995 and 1996.

This process of acid deposition either wet or dry exacerbates the corrosion, flaking, decay, leaching, or dissolution of urban infrastructure surfaces. These surfaces of urban infrastructure are an important source of many pollutants. In urban areas, a significant source of heavy metals is traffic activities (Sansalone et al., 1997). These heavy metals are a result of vehicular component abrasion or abrasion of tires against pavement and are dry-deposited on the urban surface. In addition, metal components are a common component of many urban surfaces, such as structural infrastructure, metal flashing, roofing materials, downspouts, galvanized pipes, metal plating, paints, wood preservatives, vehicular components, galvanized vehicular and infrastructure components, tires, highway infrastructure, and catalytic converters. Over time, these infrastructure materials become sources of heavy metals through the processes described above. Low pH levels in rainfall accelerate such processes. For example, rainfall pH in urban Cincinnati typically ranges from 3.5 to 4.5 and urban Washington, D.C. rainfall pH ranges from 3.9 to 4.5 (Schueler, 1987; Sansalone et al., 1997). Both of these urban areas exert anthropogenic influences and inputs to the urban atmosphere leading to wet and dry acid deposition on the surface of the urban environment. During a wet-weather event, previous dry acid deposition that accumulates on the pavement surface is rapidly washed off. Up to 90% of this atmospheric deposition is washed off by rainfall runoff events (Oberts, 1985). Increasing degree of imperviousness alters the sensitivity of a catchment or watershed to this acid precipitation. The natural environment with surficial soil and bedrock of higher leachable calcium content, as compared to most urban surface, has a greater ability to neutralize the effects of acidic deposition. Acidic deposition on impervious urban surfaces alters the acid–base chemistry in urban runoff and can have a significant impact on phenomena such as stormwater heavy metal partitioning. This is particularly important at the upper end of urban catchments where abraded and dry-deposited heavy metals impacted by low rainfall pH are dissolved rapidly. For local receiving waters and surficial soils at the upper end of these urban catchments, the impact of low rainfall pH on the preferential dissolution of heavy metals in the urban environment can be significant (Sansalone and Buchberger, 1997). Additionally, increased alkalinity generated though rainfall infiltration and percolation processes has been cut off by impervious surfaces, and the resulting surface water alkalinity and therefore buffering capacity is significantly reduced. The urban surface therefore not only alters the quantity of runoff, but also the quality of runoff impacting that surface.

THE URBAN ENVIRONMENT

AS A

HEAT ISLAND

FOR

CLIMATE MODIFICATION

The constructed urban environment can have a significant effect on the urban and regional climate. Since the late 19th century, European scientists were aware from the analyses of weather records that large European cities such as Paris, Berlin, Vienna, and London acted as “heat islands” (Landsberg, 1956). Urban areas modify their heat transfer mechanisms (altering their radiation heat transfer and add sensible heat) to the point that urban areas are warmer than surrounding rural

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47

areas. Temperature data from North America indicates that there is approximately a 2°C temperature rise per order of magnitude population increase in the population range of 103 to 107 (Landsberg, 1981). Data taken from Landsberg indicate that urban-induced temperature increases were greatest for increasing summer temperatures. The degree of imperviousness can play a significant role in modification of evaporation and temperature in the built urban environment. Urban areas such as St. Louis, which rapidly developed after the end of World War II, with a population of 2.6 million in 1996, have precipitation levels in the urban area that over the last 40 years have consistently demonstrated a 5- to 10-cm increase over the surrounding nonurbanized areas (Changnon, 1992). Intensive field studies of 6-year duration in St. Louis and 4-year duration in Chicago demonstrated that urban areas have a significant influence on heavy (>2.5 cm) convective rainfalls in humid continental climates when the atmosphere was highly unstable (Changnon, 1979). Large metropolitan areas such as St. Louis, Chicago, Washington, D.C., and Cincinnati are now home to approximately 70% of the North American population. The localized and regional climate effects of these urban heat islands in North America will likely be irreversible at least within the next century. There is little chance that the localized climates of these urban centers will return to their preurbanized climates of 100 years or more ago. Research has shown that urban–industrial complexes such as the St. Louis and Chicago areas can have a local and regional influence on clouds and precipitation. The mechanisms for such climate influences include the following: 1. Modified in-cloud processes by urban additions of cloud condensation nuclei 2. Changes to sensible and latent heat fluxes for the urban area, creating and enhancing vertical air movements 3. Increased updrafts and convergence of air currents both from momentum shifts due to urban infrastructure, including pavement and buildings, and from differential heating that influences the boundary layer around the urban environment and modifies the surrounding atmosphere 4. Enhanced moisture due to discharge of water vapor from industrial sources such as cooling towers (Changnon, 1992) Evaluation of summer rainfall in Washington, D.C. found that the urban area caused showers to develop, the urban heat island and the urban area heating led to preferential mesoscale forcing and localized cloud development and enhancement (Harnack and Landsberg, 1975).

SOURCES AND MAGNITUDE OF ANTHROPOGENIC URBAN CONSTITUENT LOADINGS Stormwater runoff from urban areas and roadways transports significant event and annual loads of heavy metals and a wide gradation of particulate matter to receiving waters (Ball et al., 1991). Particulate matter is ubiquitous in the urban environment because so many of our urban activities generate particulate matter, in particular traffic and maintenance activities. The increase in traffic and number of vehicles worldwide has increased at a greater rate in the last 50 years, as shown in Table 2.1. This trend is expected to continue at least through the first quarter of the 21st century. Statistics regarding the “average” vehicle are provided in Table 2.2. Stormwater runoff transports significant loads of dissolved, colloidal, and suspended solids in a complex heterogeneous mixture that includes heavy metals and inorganic and organic compounds. From urban interstate highway pavement alone, annual heavy metal, TSS, chemical oxygen demand (COD) loadings, and stormwater flows have been shown to equal or exceed annual loadings and flows from untreated domestic wastewater for a given urban area (Sansalone et al., 1998). Stormwater transports a wide gradation of particulate matter ranging in size from smaller than 1 µm to greater than 10,000 µm (Sansalone et al., 1998). From a water quality and treatment perspective,

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TABLE 2.1 Worldwide Vehicle and Human Population Growth Year

Vehicles

Population

1950 1995 2025

0.05 × 109 0.5 × 109 1.0 × 109

2.6 × 109 5.5 × 109 8.0 × 109

Note: As of the year 2000, the U.S. population was 0.28 × 109, total number of vehicles was 0.22 × 109, total roadway mileage was 6.3 × 106, and annual vehiclemiles traveled was 2.7 × 1012.

TABLE 2.2 Statistics for the “Average” Vehicle in the Year 2000 Average vehicle mass Average service life Distance traveled Fuel consumed Oil consumed Carbon discharged Total Pb, Zn, and Cu component mass

1200 10 160,000 12,000 200 3200 36

kg years km l l kg kg

Note: Consumption and discharge values are summed over the total average service life of 10 years.

FUEL SYSTEM - VOCs - organic compounds BODY/FRAME - Zn, Cr, Sn ENGINE - Zn, Cu, Cr, Mn, Ni - Oil, grease BRAKES TIRES1 - Cu, Pb -Zn (3.0 mg/vehicle-km) -Cd (0.02 mg/vehicle-km) EXHAUST - particulates -solids (mean dia. = 20µm)

1: (FHWA, 1984)

PAVEMENT -solids, particulates -PAHs (asphalt) -phenols (asphalt) -thermal (asphalt)

FIGURE 2.2 Vehicular and pavement sources of constituents generated by traffic activities.

entrained solids having reactive sites and large surface-to-volume ratios are capable of mediating transport of heavy metals. Sources of these heavy metals are illustrated in Figure 2.2. Figure 2.3 illustrates the relative contributions of particulate matter for an urban Cincinnati case study. With respect to brake wear, contributing 15% of the total particulate mass, the relative contribution of selected heavy metals for a typical 130-g brake pad is given in Figure 2.4. For urban conditions with high levels of traffic, there are a number of primary sources of anthropogenic pollutants. Traffic characteristics, including traffic volume, measured as average daily traffic (ADT) or vehicles during storm (VDS), vehicular speed, traffic level duration, and mix of vehicles and trucks, are a primary source. Urban design, including infrastructure material, for example, pavement,

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49

tire wear 28 -31% engine/brake wear 15% settleable exhaust 6%

pavement wear

atmospheric deposition

44 - 49%

3%

Concentration [µg/g]

FIGURE 2.3 Sources of traffic-generated particulates for urban Cincinnati transportation land use. For the experimental site on I-75 on Millcreek Expressway with an ADT = 150,000, approximately 13,500 mg/m2/day of particulate matter was generated in 1995. 40,000 30,000

30,600

20,000 7,454

10,000

2,200

1,050

124

Cr

Pb

Zn

0 Cu

Ni

FIGURE 2.4 Selected heavy metal content of typical 130-g brake pad. (Data from Armstrong, 1994.)

galvanized metal or painted metal, drainage design, and degree of impervious area, can result in significant sources. Urban maintenance activities can create significant sources, for example, construction, urban repairs, sanding/salting, and application of herbicides or fertilizer. Climate parameters can play an important role, for example, precipitation type, intensity/duration/frequency of precipitation events, winds, temperature, and urban microclimates. Finally, there is a whole range of other sources that include accidents, spills, manufacturing, industries, commerce, littering, animals, and garbage disposal. Urban infrastructure is a source of anthropogenic constituents, for example, heavy metals generated from corrosion, flaking, decay, leaching, or dissolution of urban infrastructure surfaces. Much of modern infrastructure constructed of or containing exposed metal is galvanized, for example, with a zinc-based galvanizing process. The most common process is hot-dipped galvanizing (HDG) with continuous galvanizing also used for infrastructure such as roadway guardrails. Typical specifications for zinc coatings range from 1.5 to 4.0 oz/ft2 of guardrail surface. However, over time in the aggressive urban environment, the galvanized surface is leached from this infrastructure during rainfall runoff events, or during condensation of water vapor on these surfaces, finding its way into the local environment. One common example is the formation of Cu2CO3(OH)2 (malachite, a greenish-colored hydrous copper carbonate) from leaching of copper from copper roofing and precipitation of the leachate in the surficial layer of concrete pavement surrounding a building. Each of these heavy metals has various acute and chronic toxicity effects in receiving waters and or surficial soils. Toxicity impacts for species of these metals are illustrated in Figure 2.5. For urban stormwater, zinc, cadmium, copper, and lead are commonly cited and examined for comparison to discharge regulations. Although some of these heavy metals are nutrients or essential minerals at trace levels, for example, copper or zinc, these metals can be very toxic to particular living organisms at elevated levels. Depending on factors such as pH, redox conditions, alkalinity, residence time, complexing agents, and suspended solids these heavy metals can be preferentially dissolved in ionic form where they can exert an immediate toxicity impact. As ionic species, each of these metals exists primarily as cations with a +2 charge.

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Wet-Weather Flow in the Urban Watershed: Technology and Management

6.7 +2 Cu 29 63.55

high toxicity for: • aquatic plants • invertebrates • fish (Cu2+) (CuOH+) (Cu2OH22+)

+2 • cytotoxin • carcinogen Cd • toxicity enhanced 48 112.41 w/ Cu & Zn present (Cd2+) 8.7

+2 • substantial gill damage in fish Zn • increased toxicity 30 65.38 for fish under high Temp. and low D.O. (Zn2+)

7.6

7.3 +2 • neurotoxin • hemoglogin Pb inhibitor for fish and 82 207.2 humans (anemia) (Pb2+) (RnPb)

FIGURE 2.5 Selected toxicity impacts for various species of Cu, Zn, Cd, and Pb. The pKa of each element is listed in the upper left of each box. The primary charge on each element in ionic form is shown in the upper right-hand corner. The atomic number is shown in the bottom left and atomic weight is shown in the bottom right of each box.

In urban transportation corridors and roadway environments, heavy metals are generated primarily from the abrasion of metal-containing vehicular parts, including the abrasive interaction of tires against pavement and oil and grease leakage (Lygren et al., 1984; Muschack, 1990; Ball et al., 1991; Armstrong, 1994). Roadway stormwater levels of zinc, copper, cadmium, lead, chromium, and nickel are significantly above ambient background levels, and for many heavily traveled roadways, zinc, copper, lead, and cadmium often exceed U.S. EPA and state EPA surface water discharge criteria on an event basis (Sansalone et al., 1997). Stormwater from urban and transportation land uses is a complex physicochemical heterogeneous mixture of heavy metals, particulate matter, inorganic and organic compounds with variations in flow, concentrations, and mass loadings that sometimes vary by orders of magnitude during a single hydrologic event. This complexity has made stormwater very difficult to treat. For example, 2 years of research results generated from data collected at instrumented urban roadway sites in urban Cincinnati demonstrate the variation in magnitude of event mean concentration (EMC) values between discrete hydrologic events. For total zinc, EMCs ranged from 15,244 to 459 µg/l, total copper from 325 to 43 µg/l, lead from 88 to 33 µg/l, chromium from 35 to 13 µg/l, and cadmium from 11 to 5 µg/l (Sansalone et al., 1997). Figure 2.6 illustrates such an example for the urban metropolitan Cincinnati area based on data collected from 1995 and 1996. The urban interstate and major arterial pavement area typically constitutes less than 10 to 15% of the total pavement area for an urban area while generating a significantly disproportionate pollutant load especially with respect to heavy metals. In fact, it has been reported in the literature as early as 1974 that stormwater runoff from urban pavement represented a greater pollutant loading to receiving water than point source wastewater discharges from that same urban area (Klein et al., 1974). 40 km2 as a % of

Urban Cincinnati data utilized: total pavement area • 800,000 Population (280 Lpd per capita) for urban Cincinnati 15% Mean annual rainfall (C = 0.7)1 •1050 mm Interstate and arterial road area • 40 km2 RUNOFF WASTEWATER Flow (liters) COD [mg/L] TSS [mg/L] ZnT2 [µg/L] CuT [µg/L] PbT [µg/L] CdT [µg/L]

5 x 1010 350 200 3 4500 (232.0tonsm ) 150 ( 7.7 tonsm ) 90 ( 3.6 tonsm ) 12 ( 0.7 tonsm )

8 × 1010 400 (Metcalf and Eddy) 200 75 (USEPA, 1993) 35 10 1

1 C: 2 T: 3

Volume-based runoff coefficient Total = dissolved + particulate-bound TSS: 180 mg/L for 81 Urban commercial and residential areas (NURP,1983)

FIGURE 2.6 Urban Cincinnati storm water runoff compared to untreated domestic wastewater.

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The Physical and Chemical Nature of Urban Stormwater Runoff Pollutants

Elapsed time (min) 0

5

10 15 20 25 30 35 40

0

0

Elapsed time (min) 10 15 20 25 30 35 40 16000

200 160 120

24

80

32

40

40

0

LOG 10 NT

8000 4000 0 16000 TSS VSS

12000 8000 4000

Mass (mg)

L NV

16 14 12 10 8 6 4 2 0

12000

Mass (mg)

16

TDS VDS

Number volume size, L

Intensity (mm/hr) LOG 10 NT [cm-3]

Rain

Flow (L/min)nv (mm )

Flow

8

16 14 12 10 8 6 4 2 0

5

51

0

FIGURE 2.7 The hydrology and transport of 12 June 1997 from a 300-m2 impervious urban surface.

THE ROLE OF URBAN ENVIRONMENTAL HYDROLOGY Our constructed urban environments are designed for rapid and efficient transport of stormwater flows. The highly impervious nature, relative lack of roughness, and reduced hydraulic resistance of our urban pavement and drainage systems have resulted in increased urban stormwater peak flows and increased peak flow volumes promoting effective transport of particulate and dissolved contaminants generated by our urban activities. When the urban surface is loaded by a hydrologic event, even a complex, high-intensity, high-volume runoff event, the transport of contaminant mass is directly coupled to the hydrology and hydraulics of the urban surface (Singh, 1997; Sansalone et al., 1998). A simple example illustrates these interactions. Figure 2.7 demonstrates the rapid unsteady hydrology and coupled mass transport response of an urban pavement watershed for the dissolved and particulate mass fractions. Dissolved fractions (TDS, VDS) and suspended fractions (TSS, VSS) show a direct response as does particle counts per milliliter (NT). Although not contributing significantly to suspended mass, finely abraded particulate matter with a mean LNV of approximately 4 µm generated by morning rush hour traffic passing over the paved watershed produced NT values that remained high across the entire falling limb of the hydrograph. Both the stochastic nature of rainfall and the rapid, unsteady runoff response across the urban environment are significant challenges to the design and efficacy of unit operations and processes for stormwater. Because the concentration of solids transported in rainfall runoff can vary by orders of magnitude during a storm event, a single index, designated an EMC is often used to characterize concentrations (Huber, 1993). EMCs represent a flow average concentration for the event and are computed as

EMC =

M =C = V

∫

tr

0

c(t )q(t )dt

∫

tr

0

(2.1)

q(t )dt

where M = total mass of a constituent over entire event duration V = total volume of flow over entire event duration

(M) (l3)

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Wet-Weather Flow in the Urban Watershed: Technology and Management —

C c(t) q(t) tr t

= = = = =

flow-weighted average concentration for entire event time-variable solid or dissolved fraction time-variable flow duration of the event time

(M/l3) (M/l3) (l3/T) (T) (T)

The EMC is a useful comparative index; however, it does not provide insight into the temporal washoff of constituents when considering concepts such as the “first flush.” The original concept of a first flush refers to the delivery of a disproportionately large mass of a constituent during the early portion of the runoff hydrograph. Suppose runoff starts at a designated time 0. Then, a first flush exists at time t if the normalized cumulative mass load m(t) exceeds the normalized runoff volume v(t) at all instants during the interval (0,t) (Sansalone and Buchberger, 1997). Normalized cumulative mass and runoff volume are often plotted as a function of dimensionless time (t/tr). For any time, t, a disproportionate delivery of mass with respect to runoff volume would satisfy this criterion. The criterion is defined as m(t ) >1 v( t )

(2.2)

In terms of transport from the urban pavement surface, the concept of a “first flush” has been used and misused to describe such transport induced by a runoff hydrograph. Quite simply, the first flush refers to the disproportionate delivery of a large pollutant load during the early part of the runoff hydrograph. As a result, various rules of thumb have been mistakenly used to identify the first flush and thereby selectively capture and treat only the early part of the runoff hydrograph. However, when the measured transport of a contaminant is analytically represented as the timedependent ratio of the cumulative normalized mass to the cumulative normalized flow volume, one can demonstrate that the majority of a pollutant load is not necessarily delivered during the early part of the runoff hydrograph, although contaminant concentration may mimic an exponential-type decline during the passing of the hydrograph. Figure 2.8 illustrates this point and also demonstrates that suspended particulate fraction and dissolved fraction exhibit distinct responses for another single hydrograph event loading the same 300-m2 impervious urban surface. In Figure 2.8, normalized mass points of TSS or TDS that are located above the solid flow volume curve indicate that a disproportionate amount of incremental mass is transported at that incremental time with respect to incremental flow volume. Clearly, the ability to capture and treat the latter half of this hydrograph is as critical as the first half. Elapsed Time (min) 0

10

20

30

40

Normalized Time 50

0.00

50

800

100

600

150

400

200

200

250

0

0.40

0.60

0.80

1.00 1.0 0.8 0.6 0.4

TSS TDS

0.2

Normalized mass

1000

Flow (L/min)

Intensity (mm/hr)

0

0.20

0.0

FIGURE 2.8 Lack of a mass first flush for 8 August 1996 from a 300-m2 impervious urban surface.

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53

In fact, it can be shown that delivery of a contaminant mass is a function of the dissolved or particulate phase of the contaminant, the rainfall and flow intensity, and the traffic flow (Sansalone et al., 1998). Therefore, the selection and potential efficacy of a unit operation or process technology may in fact require the ability to treat the entire duration of the runoff from a mass basis and not just the early part of the runoff hydrograph from a concentration basis.

HEAVY METAL PARTITIONING Heavy metal partitioning between the dissolved and particulate-bound fractions in stormwater is a dynamic process. Whether in pavement runoff, urban stormwater, or any aqueous system, there is a temporal partitioning between heavy metals in solution and solids whether these solids are in suspension (TSS, VSS) or as settleable solids that may be part of a fixed or mobile bed load. This partitioning includes specific mass transfer mechanisms of sorption, ion exchange, and surface complexation with both organic and inorganic sites on the solid matter. These partitioning reactions are generally nonlinearly reversible between the solid phase and soluble phase concentrations. Total concentration of a heavy metal is therefore the sum of the dissolved (cd) and the particulate-bound concentrations (cp), where cT = cd + c p

(2.3)

Operationally, the soluble or dissolved fraction is that fraction that passes the 0.45-µm membrane filter and therefore contains both the dissolved and part of the colloidal-bound heavy metals. The solid phase concentration, cp, is defined as the product of the heavy metal concentration on the solid phase, cs, in terms of mass/mass of solids and the concentration of the adsorbing solid material in the aqueous system, m, typically measured as TSS in terms of mass/volume of aqueous solution: c p = (cs )( m)

(2.4)

Under equilibrium conditions, when the rates of sorption and desorption are equal, concentration equilibrium exists between the dissolved and solid phase concentrations of a heavy metal (Sansalone et al., 1998). The ratio of these phases at equilibrium is referred to as the partitioning coefficient, Kd, for a particular heavy metal at a particular pH and redox level: Kd = c p cd

(2.5)

Substitution of Equation 2.3 and 2.4 into Equation 2.5, and rearranging, yields the dissolved fraction ( fd ) and the particulate-bound fraction ( fp), defined as

[

]

fd = D ( D + P) = cd cT = 1 1 + Kd ( m)

[

] [1 + K (m)]

f p = P ( D + P) = c p cT = ( Kd )( m)

d

(2.6) (2.7)

where D is the dissolved mass of a heavy metal (mg) and P is the particulate-bound mass of a heavy metal (mg). For fd > 0.5, the heavy metal mass is mainly in dissolved form. The product of (Kd)(m) is dimensionless and Kd is usually expressed as liters per kilogram (l/kg). The larger the Kd value, the greater the partitioning to the solid phase. Heavy metals in pavement runoff have Kd values that range from 102 to over 106 (Sansalone and Buchberger, 1997).

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Wet-Weather Flow in the Urban Watershed: Technology and Management

For each discrete sample, dissolved and particulate heavy metal concentrations were obtained after sample preparation and digestion through inductively coupled plasma spectroscopy (ICP). Each respective sample concentration (ci) (dissolved or particulate) was multiplied by the qsf volume (vi ) representative of the discrete time increment to determine heavy metal or solids mass (mi ) as shown in equation 2.8. mi = (ci )(vi )

(2.8)

To evaluate the temporal partitioning, these (mi) results can be plotted for both dissolved and particulate fractions as a function of elapsed time. To illustrate the temporal variation in partitioning, sample statistics can be calculated to describe the characteristics of the dissolved fraction for an event. This partitioning, which varies throughout a rainfall runoff event, is a function of pH, alkalinity, residence time, and solids characteristics, each of which varies significantly between hydrologic events and traffic patterns (Sansalone and Buchberger, 1997). As a result of very low rainfall alkalinity, low rainfall pH trends (3.5 to 5.5) as shown in Figure 2.1 and low pavement residence time, urban roadway runoff can be of low alkalinity (<50 mg/l as CaCO3) and low runoff pH, especially for asphalt pavement as compared to Portland cement concrete (PCC) pavement runoff. This results in dissolution of finely abraded metallic particles generated from traffic activities, and this metal mass partitions predominately to the dissolved fraction for short residence times. Understanding the kinetics of this nonequilibrium partitioning is critical for proper monitoring, conceptual design, and viability of unit operations and processes that may be applied as in situ or source control treatment. Figure 2.9 provides an illustration of the time-dependent partitioning of copper in stormwater samples for a fixed set of stormwater chemistry parameters (Sansalone and Buchberger, 1997). For sampling and monitoring, this analysis indicates that after 6 h for the given stormwater chemistry, the copper mass is partitioning to the particulate-bound fraction. Additionally, a consistent increase in the partitioning coefficient, Kd, as a function of time for lead, copper, cadmium, and zinc can be observed for the 8 August 1996 event despite the inverse trend in TSS. Knowledge of the partitioning kinetics and the relative fractions of dissolved ( fd ) and particulate-bound ( fp ) mass delivered for treatment is of fundamental importance for in situ treatments where residence times on the urban surface or in the urban drainage system in the presence of entrained particulate matter are less than several hours. An example of the relative fractions of dissolved and particulate-bound heavy metals is provided in Figure 2.10. Results in this figure illustrate adsorption or precipitation unit processes need consideration in treatment design and that a knowledge of partitioning for a given set of residence time and stormwater chemistry parameters is a necessary first step for conceptual and detailed treatment design. 300

particulate-bound

100 50

dissolved

0

[KdL/kg]

Cu [µg/L]

150

pH = 6.5 Alkalinity = 30 mg/L TSS = 150 mg/L

1e+4

600

1e+3

Pb 400 Cu Cd 200 Zn TSS

1e+2 1e+1

0

1e+0

0 4 8 12 16 20 24 Time from sampling (hours)

TSS [mg/L]

200

800

1e+5

250

0

20 40 60 80 Elapsed Time (min.)

FIGURE 2.9 Time-dependent partitioning for copper in stormwater runoff samples for 15 July 1995 runoff event and the consistent temporal increase in heavy metal partitioning coefficient, Kd, for an 8 August 1996 event.

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The Physical and Chemical Nature of Urban Stormwater Runoff Pollutants

0

Elapsed time (min.) 20 40 60

225 1

Dissolved Particulate

150 75

0

0

40

fd = 0.71 (n=27)

30 20 10 0

Cumulative Pb (mg)

Cumulative Cu (mg)

35 30 f d = 0.80 (n=27) 25 20 15 10 5 0

80 2

f d = 0.76 (n=27)

Cumulative Cd (mg)

Cumulative Zn (mg)

300 f d = 0.83 (n=27)

Elapsed time (min.) 20 40 60

0

80

55

FIGURE 2.10 Dissolved fraction mass (fd) partitioning for an 18 June 1996 runoff event (fd + fp = 1.0).

5000 3987

Zn

4481

f d = 0.89

2500 0 150

10 5

110

120 147. 4

100 93. 4 50 17 0 dissolved EMC

12

9.2 3.7

fd= 0.77 5.6

0

Cu

f d = 0.63 18

Cd

15

147 Pb

150 82 89.6 100 51.5 50

0 USEPA criterion

total EMC

fd= 0.57

State criterion

• n = 13 rain events • Site EMC: TSS = 204 mg/L • Site EMC: pH = 6.8

FIGURE 2.11 Site mean dissolved concentrations, total concentrations, and fd for Cincinnati highway runoff. The vertical axis is concentration [µg/l]. Criteria are for modified warmwater surface water (Mill Creek).

The dominance of the dissolved mass for all metals, including relatively insoluble lead for short urban surface residence times (initial pavement residence time <15 min), was typical with fd values for zinc and cadmium of approximately 0.8 or greater, copper between 0.60 to 0.80, and lead between 0.5 to 0.7 at this urban site (Sansalone, 1999). Note that the dissolved fraction increase dominates throughout the rainfall runoff event for all metals. These results are typical of results from all 13 rainfall runoff events characterized over 2 years at the urban Cincinnati highway site. These results illustrate the need for treatment of the dissolved fraction when considering in situ solutions. A summary of equilibrium dissolved fraction results at the upper end of an urban Cincinnati watershed is shown in Figure 2.11. The site mean results summarized in Figure 2.11 indicate that for all metals except lead that dissolved and total concentrations are significantly greater than the criteria. Results indicate that treatment of the dissolved fraction when considering in situ control is critical for the success of any treatment strategy.

TREATMENT DESIGN IMPLICATIONS

FOR

DISSOLVED HEAVY METALS

These results, which indicate preferential partitioning to the dissolved phase at the upper end of an urban watershed for residence times of less than several hours, have important implications for design of in situ treatment or best management practices (BMPs). Each of these results reported have an important effect on initial selection and then on the details of treatment design for the selected in situ BMPs.

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First, partitioning results for all the heavy metals (including lead) indicate that partitioning is dominated by the dissolved fraction of the heavy metal mass for all but the early portion of each rainfall runoff event. This conclusion is consistent across all events despite a random selection of hydrologic events characterized over a 2-year period and despite the variation in runoff event mean pH values of 6.20 to 7.56. These partitioning results are consistent with the fact that τsf is typically less than 15 min for pavement sheet flow, qsf (Sansalone et al., 1998). In addition, the runoff leaving the pavement is very poorly buffered with a site event mean alkalinity of 25 mg/l (as CaCO3). Therefore, despite variations in event hydrology and some variation in event mean pH, the partitioning results demonstrated that toward the upper end of the urban catchment, which is the location for in situ source control BMPs, heavy metals can predominately partition toward the dissolved phase. In terms of implications for BMPs, these results indicate that in situ treatment BMPs, placed near the upper end of an urban pavement watershed with similar pH, alkalinity, TSS, and residence time, can be effective if they are selected and then designed to provide capture (a function of hydrology and hydraulics) and then treatment of the dissolved fraction (either through sorption or precipitation). If the BMP is not designed with a treatment mechanism for the dissolved fraction, the heavy metal trap efficiency of the BMP can at best be equal to only the (1 – fd)mi fraction for a given heavy metal, i, assuming full runoff capture by the BMP. For example, consider zinc for the 18 June 1996 event. If a BMP was designed to treat only the particulate-bound fraction of zinc and was not designed with a mechanism to remove the dissolved fraction (sorption, precipitation, etc.), the removal efficiency (in percent) for the total mass of zinc would be (1 – 0.83)mi or 17% of mi . This assumes that through filtration or sedimentation mechanisms, the BMP captures the entire particulate-bound mass. At the upper end of the watershed there is relatively little time for heavy metal contact with entrained TSS, another reason for the predominant dissolved fraction, fd , for each event. Any process at environmental pH levels (5.5 to 8.5) that promotes increased residence time and interfacial contact will increase the probability of heavy metal solutes partitioning to solid matter, either as entrained TSS as in a detention basin or in a fixed media device. This partitioning of solute to solid is by definition adsorption, whether in a detention basin or a fixed media device. In fact, the only mechanisms available to remove nonvolatile heavy metal solutes from the water column in a detention basin are partitioning to solids and then sedimentation; an increase in pH to promote precipitation; or through various biological uptake vectors. Many basins, particularly retention basins, can have residence times in terms of many hours or days as opposed to minutes, thus promoting partitioning at environmental pH levels. However, once heavy metals become part of the basin sediments, the potential exists for the sediment in the benthic zone to become anaerobic, releasing the heavy metals and permitting advection and diffusion of the heavy metals back into the water column. Many current BMP designs utilize mechanisms of filtration and sedimentation, therefore promoting removal of particles, solids (TSS), and particulate-bound constituents including heavy metals. Although these mechanisms are important and must be utilized, the only way that such mechanisms can effectively remove the dissolved fraction of a heavy metal is if that metal is first partitioned to entrained or suspended solids and then filtered or removed through sedimentation. In addition, if a filter medium is to be utilized to remove dissolved heavy metals, it must be designed to sorb and retain the dissolved heavy metal and have sufficient surface area for a reasonable design life.

PHYSICAL AND CHEMICAL CHARACTERISTICS OF STORMWATER PARTICULATE MATTER Urban stormwater mobilizes a wide gradation of anthropogenic particulate matter that is capable of mediating the equilibrium partitioning of heavy metals discharged in the urban environment. For residence times conditions that result in partitioning equilibration (12 to 24 h), heavy metal

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Snow (n = 20 events)

Rainfall-runoff (n = 13 events) 100

100

80

80

60

60

40

40

20

20

57

% passingby mass

The Physical and Chemical Nature of Urban Stormwater Runoff Pollutants

0 0 10,000 1,000 100 10 1 10,000 1,000 100 10 1 Particle diameter (µm) Particle diameter (µm)

FIGURE 2.12 Range of particle gradations for snowmelt (shaded area in left plot) and rainfall runoff (shaded area in right plot).

partitioning to stormwater particulate matter can be significant. This is in contrast to partitioning at the upper end of the urban watershed with residence times less than several hours, where most heavy metal mass is in dissolved form. Anthropogenic activities and infrastructure, in particular, transportation, are significant sources of particulate matter ranging from submicron particles to gravel-size material. A primary source of particles is vehicular-infrastructure abrasion, including tire–pavement interaction (Kobriger and Geinopolos, 1984). Abraded tires particles have a mean diameter less than 20 µm and a specific gravity between 1.5 and 1.7, while abraded vehicular material and infrastructure range in size from less than 1 µm to over 104 µm with measured specific gravities from 1.6 to over 5.0 (Sansalone and Tribouillard, 1999). Typical particle gradations are illustrated in Figure 2.12 for snowmelt and rainfall runoff at the urban Cincinnati sites. Data were measured gravimetrically using mechanical sieve analyses.

MEASUREMENT

OF

STORMWATER PARTICULATE MATTER

Granulometric characteristics and loading of suspended particulate matter in urban stormwater play a significant role in water quality and consequently in the operation and viability of treatment systems designed to mediate such water quality. Granulometry parameters that are important from a treatment perspective include mass loading, particle gradation, particle diameter indices such as number volume mean size, lnv, particle density, ρs, specific surface area (SSA), surface area (SA), particle number (Nt), and particulate mass concentration (M) (as measured by gravimetric indices such as TSS). Particle number, or counts, is the measure of the number of particles of a given number volume mean size (lnv) fraction per volume of aqueous suspension. This can be carried out on a volume basis (such as µl of solids/l of aqueous suspension) or gravimetrically for each increment of particle size with the particle density as the conversion parameter between the two methods. Gravimetrically, this is carried out in combination with mechanical sieves and a hydrometer. Volumetrically, this is carried out with techniques such as laser-based particle analysis instrumentation. For particles where the gradation is above approximately 25 µm, these analyses are most accurately and routinely carried out gravimetrically as laser-based instrumentation can be expensive compared to gravimetric methods. However, analyses of particulate gradation smaller than 25 µm requires more sophisticated experimentation and equipment with particle analyses in the 1 to 25 µm range. For either the gradation larger or smaller than 25 µm, particle number, Nt, and statistical parameters such as lnv can be used to characterize the gradation. The number-volume mean size, lnv, is a statistically weighted value of particle diameters based on both particle number in a given size increment and volume of a spherical particle of the same size. The lnv can be related to mass concentration, M, of a particle gradation through particle density for a particular lnv as shown in Figure 2.13. A particle size distribution (PSD) or gradation for stormwater or snowmelt for particles larger than 25 µm can be carried out using mechanical sieve analysis for each of the recovered particulate

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Wet-Weather Flow in the Urban Watershed: Technology and Management

M

M =ñsC3l3nvNt Ni =

mi ñsiVsiVl 1

 ∑ Nil3nv,i 3  lnv =  i  ∑ N  i  i 

ρs C3 lnv Nt Ni mi ρsi Vsi Vl

= Mass concentration of particulates, gm/cm3 = Particle density, gm/cm3 = π/6 (spherical geometry) = Number-volume mean size, µm = Particle count, cm-3 = Particle count for interval i, cm-3 = Mass recovered for interval i, gm = Particle density for interval i, gm/cm3 = Particle volume for interval i, cm3 = Volume of aqueous suspension, cm3

FIGURE 2.13 Relationships between granulometry parameters and parameter definitions.

log Nt (cm3)

log Nt (cm3)

matter samples. This determination utilized ASTM D 421 for sample preparation and ASTM D 422 for mechanical sieve analysis (American Society for Testing and Materials, 1990). The mechanical sieve analysis deviates from ASTM D 422 in that many additional sieve sizes are utilized, from the #4 (4750 µm) through the #500 (25 µm). The sieve sizes utilized include the #4, #10, #14, #20, #30, #40, #50, #60, #80, #100, #140, #200, #270, #325, #400, #500, and the pan. The small increments chosen between the #10 through #140 are based on previous work indicating the majority of the particulate matter is located in this range (American Society for Testing and Materials, 1990). Typically, a hydrometer analysis would be performed for particles less than 75 µm, separating the sand-size particles greater than 75 µm from the silt-size particles less than 75 µm (Sansalone et al., 1998). However, for sample recovery, SSA determination, and chemical composition analysis, dry mechanical sieve analysis has been shown to be the best protocol. Upon completion of the mechanical sieve analysis a mass balance is carried out as a check. Once the particulate matter is separated through mechanical sieving each separate size increment is stored for additional analysis. Granulometric analyses of the particle data, for particle gradations both larger and smaller than 75 µm, and down to 1 µm in size could be modeled with a two-parameter power law function. This was true for both snowmelt and rainfall runoff. Figure 2.14 illustrates two typical fits for snowmelt particles from two of ten snow sampling sites across urban Cincinnati. Note that for all sites the fit was excellent as indicated by the r2 values for each site. The regressions were performed on the untransformed data. 5 4 3 2 1 0 5 4 3 2 1 0

3SW

4NE

0

1 2 3 log lnv,i (µm) Observed Nt = αlnv,i-β Predicted

4

Site 1SE 1SW 2ES 3SW

α

β

2.0E+08 3.0E+08 7.0E+08 1.0E+09

2.4181 2.4947 2.6462 2.4998

r2 0.973 0.990 0.991 0.992

4NE 5NE 6SW 7SW

9.0E+07 2.0E+08 8.0E+07 8.0E+08

2.3993 2.3315 2.2239 2.5454

0.986 0.977 0.988 0.989

8SW 9NE

6.0E+07 2.4015 1.0E+09 2.5394

0.987 0.983

–β ) to snowmelt particle data from ten urban Cincinnati sites. α is FIGURE 2.14 Fit of power law (Nt = αlnv,i an index for particle concentration, and β is the slope of the line and an index to assess the physicochemical particle interactions.

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59

Particle counts/mL

TSS 80

10 3

60

1 10 Particle dia. (µm)

40 2000 µm gravel

75 µm

20 silt

sand

Particle mass finer ( % )

100 10 6

0 10000

1000 100 Particle diameter (µm)

10

FIGURE 2.15 Superposition of gravimetric-based particle analysis and particle number-based analysis for the same stormwater gradation.

80

Measured SSA EGME method

60 40 20 0 9500 25 Particle dia. (µm)

100 Calculated SSA 80 - Spherical particles - Solid particles 60 - Constant ρs 40

SSA (m 2/g) X 1000

SSA (m 2/g)

100

20 0 9500

Particle dia. (µm)

25

FIGURE 2.16 Difference between measured SSA and calculated SSA for stormwater particles. Note that the SSA scale for the calculated SSA is multiplied by 1000 to generate the same nominal scale on both plots.

Because stormwater and snowmelt have such a large gradation of particles ranging from less than 1 µm to greater than 10,000 µm, it is important to realize that both gravimetric and particle number analyses are required to examine fully the entrained particulate characteristics of stormwater. For example, if one relies on only mass gradations, as, for example, those shown in Figure 2.12, one might have the impression that particles in the finer gradation, for example, below 10 µm do not play an important role in water quality issues. Correspondingly, if one relies only on particle numbers to characterize stormwater particulate matter, one may have the impression that particles larger than 10 µm do not play an important role in water quality. Therefore, both methodologies are required to fully characterize stormwater particles. An example of both methods superimposed on the same plot is illustrated in Figure 2.15. From a water quality perspective, particulate matter SSA provides a measure of the surface area per unit mass, and is a useful index when considering the partitioning of constituents on the surface of particulate matter. Because of the additional work involved with measurement of SSA, particle size is commonly used as a surrogate measure of SSA. However, SSA results based on particle size and the assumption of spherical particles can grossly underestimate actual SSA values (Sansalone et al., 1998). An example of this is provided in Figure 2.16. The measurement of SSA for each separate particle size increment was based on the EGME method (Carter et al., 1986), and modified for roadway particulate matter (Sansalone et al., 1998). The EGME method determines the amount of EGME (HOCH2CH2OCH2CH3), a polar liquid with a relatively high vapor pressure adsorbed at a constant vapor pressure, by measuring the increase in sample weight, at equilibrium, due to the adsorbed monolayer of EGME. One EGME molecule has a cross-sectional coverage of 0.52 nm2 and a molecular weight of 90.12 g/mol. An EGME molecular monolayer of 1 m2 consists of 0.286 mg of EGME. Based on a monolayer surface coverage and molecular weight of the EGME molecule, SSA is calculated using the expression:

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Wet-Weather Flow in the Urban Watershed: Technology and Management

SSA =

Wa (0.000286) Ws

(2.9)

where SSA = specific surface area (m2/g) Wa = measured weight of EGME retained by the sample (g) Ws = measured weight of dried particulate matter sample (g) Each SSA batch of samples contained separate control samples of known SSA that were utilized to evaluate the accuracy of the EGME measurements at equilibrium. These control materials of known SSA included research-grade hectorite and granular activated carbon. The research-grade hectorite has a reported SSA value of 461.5 m2/g (Carter et al., 1986). The granular activated carbon was a Filtersorb F400 with a reported SSA of 1000 to 1100 m2/g (Calgon, 1995). Measured SSA results for these calibration materials were within ±10% of reported values using this method. Because of the gradation of particle sizes and SSA variation across the gradation, a measure of the total surface area was needed. SSA results were summed over each particle size distribution (PSD) to yield surface area distribution as a function of particle diameter, using the following formulation for each particle diameter range: SAi = ( mi )(SSAi )

(2.10)

where SAi = surface area of particulate matter having particle diameter i (m2) mi = mass fraction of particulate matter having particle diameter i (g) SSAi = specific surface area of particulate matter having particle diameter i (m2/g) Once particles are separated into discrete size ranges, metal element analysis can be conducted on each separate particle size increment. From each separate particle size increment a representative portion, approximately 0.5 to 1 g of dry particulate matter is measured to ±0.1 mg and sampled for digestion and subsequent metal element analysis. Dry particulate matter samples were aciddigested using a microwave-assisted procedure based on SW-846 Method 3015 (U.S. EPA, 1990). Once digested and filtered metal element analyses is conducted on an ICP spectrometer or an atomic absorption spectrometer. Resulting concentration results are converted from µg/l to µg/g based using acid-digested volume dilutions and dry particle mass digested. Metal element concentration results in terms of µg/g are converted to total metal element mass using Mei = ( mi )(ci )

(2.11)

where Mei = mass of metal element associated with particulate matter having particle diameter i (g) mi = dry mass of particulate matter solids having particle diameter i (g) ci = representative concentration of particulate-bound metal element for particle diameter i (µg/g) Measurement of particle density or specific gravity (particle density normalized to the density of water) is important from both a water quality and drainage perspective. Physical processes such as advective transport, sedimentation, filtration, and reentrainment of particles are all influenced by particle specific gravity. Many design procedures, such as the design of sedimentation basins,

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61

Probability

utilize the concept of minimum trapping efficiency based on a chosen design particle and design storm or design surface overflow rate theory (Malcom, 1989). However, designers of drainage systems, construction sedimentation basins, water quality basins, or filters rarely have access to specific gravity as a function of particle diameter. Measurement of particle specific gravity is carried out using an inert gas pycnometer. The measurement of specific gravity followed ASTM D 5550–94 (American Society for Testing and Materials, 1994). The gas utilized in this procedure is ultrahigh pure (UHP) helium. Helium gas was chosen for inertness and ability to enter pore space approaching 1 Å (10–10 m) in diameter. An inert gas pycnometer determines the true density of particles through measurement of the pressure difference when a known volume of helium gas is introduced into a test cell of known volume and dry mass of solids. The basis for the procedure is the ideal gas equation. The gas pycnometer procedure is utilized in lieu of the more common liquid pycnometer procedure, for several reasons (Blake and Hartge, 1986). First, sufficient material (mass of material) in the finer particle sizes is not available to utilize the liquid pycnometer technique. Second, because of the sorbed surface constituents, such as metal elements, the liquid pycnometer procedure is potentially a destructive procedure with respect to analysis of surface constituents because of solid–solute partitioning in the liquid pycnometer. The procedure involves obtaining three separate and independent subsamples of sufficient mass from each particle size to provide replicated specific gravity measurements. This material was dried at 60°C before cooling in a dessicator before and after measurement of the dry solid mass in the pycnometer test cell. Individual subsamples from the 4750- and 2000-µm particle sizes utilized 17 to 19 dry grams of particles and a 25.644-cm3 test cell. Subsamples from the 1400 µm through pan particle sizes (<25 µm) utilized 3 to 5 dry grams of particles and a 12.561-cm3 test cell. Ambient laboratory temperature was held constant between 21 and 23°C. In stormwater and snowmelt there is a wide range of particle gradation, with particulate matter generated from a wide range of sources in the urban environment. However, a constant specific gravity is assumed across the entire gradation. Typically, this specific gravity is assumed to be 2.65, representative of a quartz sand particle for calculations such as settling velocity. For particles in the range of 10,000 to 25 µm, analyses were conducted to examine the distribution of particle density across the gradation. Particle density data from ten sites around urban Cincinnati were analyzed for particle density and fit to normal distributions. Particle densities were separated between sand-size material (>75 µm) designated as “coarse” and silt-size material (<75 µm) designated as “fine.” Results were compared to the assumption of a specific gravity of 2.65. The fit of the real data to normal distributions is illustrated in Figure 2.17. 0.25 0.20 0.15 0.10 0.05 0.00 2.4 0.25 0.20 0.15 0.10 0.05 0.00 2.4

Observed Predicted

Coarse

Total

2.6 2.8 3.0 3.2 2.4 2.6 2.8 3.0 Density, gm/cm3 Density, gm/cm3 µ, σ, Fine Class gm/cm3 gm/cm3

2.6 2.8 3.0 Density, gm/cm3

3.2

3.2 r2

Total

2.83

0.10

0.96

Sand

2.86

0.08

0.92

Silt

2.75

0.08

0.82

FIGURE 2.17 Frequency distributions fit to normal distributions for total, coarse (>75 µm) and fine gradation (<75 µm) for snowmelt particulate data from ten urban Cincinnati sites.

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50 (measured SSAi) data 40 30 20 10 i 1 0 4000 Particle diameter (µm)

in

15

mass fraction (%)

Wet-Weather Flow in the Urban Watershed: Technology and Management

SSA (m 2 /g)

62

(measured mi) data from PSD

30 20 10

i1

in

0 4000

300

15

calculated (SAi) results from

SA i (m 2)

SAi = Σ{(SSAi)(mi)} SAi: Incremental particle surface area (m2) SSAi: Incremental specific surface area (m2/g) mi : Incremental particle mass (g)

6000 discrete mof SSA with m i i 4000 2000 0 4000

15

Particle diameter (µm)

40

9000

Cd mass (mg)

0 240

Zn mass R2= 0.94

0 60

0 10000

Cu mass R2= 0.81

850

SA

SA (m2)

SSA

250

106

63

45

Particle diameter (µm)

Pb mass (mg)

Cu mass (mg) Zn mass (mg) SSA (m2/g)

FIGURE 2.18 Relationship between PSD, SSA, and SA for stormwater particles.

15

0 2.0

Cd mass R2= 0.90

0 40

0 10000

Pb mass R2= 0.97

850

250

106

63

45

15

Particle diameter (µm)

FIGURE 2.19 Distribution of heavy metal mass as a function of stormwater particle SA.

With respect to water quality, the granulometry characteristics and loading of particulate matter can play a significant role in mediating the partitioning and transport of anthropogenic constituents. Particles, especially those that are organic in nature such as tire particles less than 20 µm in size, can be difficult to separate from the aqueous solution. Granulometry parameters that are important from a treatment perspective include mass loading, PSD, specific gravity, SSA, total SA, and particulate-bound constituent concentration constituent mass. A typical relationship between PSD, SSA, and SA is illustrated in Figure 2.18 for settleable and suspended stormwater particulate matter (Sansalone and Tribouillard, 1999). SSA and SA results illustrated in Figure 2.18 demonstrate that although SSA does increase with decreasing particle diameter, the total SA is associated with the midrange particle size. The increase in SSA with decreasing particle size is not monotonically increasing as would be expected for spherical particles of constant specific gravity. Although SSA increases with decreasing particle diameter, calculations using the assumption of solid spherical particles grossly underestimate actual SSA values shown. Whether associated with anthropogenic urban particles or engineered sorptive media for the separation of dissolved heavy metals from stormwater, solid–water interfacial surface area is an important parameter. This is shown in Figure 2.19 for equilibrium conditions (>24 h). Figure 2.19 illustrates coefficients of determination for particulate-bound heavy metal mass regressed against total particle SA for stormwater particles. Although SSA does increase with decreasing particle diameter, the distribution of heavy metal mass and particle surface area is strongly correlated.

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The Physical and Chemical Nature of Urban Stormwater Runoff Pollutants

15 25 38 45 53 75 106 150 181 250 300 425 600 850 1400 2000 4750

16

Particle diameter (µm)

15 25 38 45 53 75 106 150 181 250 300 425 600 850 1400 2000 4750

Particle diameter (µm)

63

50 40

12

Incremental Cumulative

10

Surface area (m/g)SSA (m22)

Mass retained (%)3

14

8 6 4 2

Particle density (g/cm)

20 10 0 2500 2000 1500 1000 500 0

1000

100

Particle diameter (µm)

10

15 25 38 45 53 75 106 150 181 250 300 425 600 850 1400 2000 4750

0 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 10000

30

Particle diameter (µm)

FIGURE 2.20 Variation in urban snowmelt particle granulometry as a function of particle diameter.

Particle density trend results, as a function of particle diameter for the settleable solid granulometry shown in Figure 2.20, as measured by helium gas pycnometry, provides parameters for unit operations design for particle separations. Results of a February 4, 1998 snowmelt event indicate that particle density decreases across the settleable solids gradation. Although these results indicate particles are mainly inorganic (including iron) down to 25 µm, suspended particles less than 25 µm have a density less than 2, a much lower mass as TSS, and significantly higher particle numbers (Sansalone and Tribouillard, 1999).

TREATMENT DESIGN IMPLICATIONS FOR PARTICULATES AND PARTICULATE HEAVY METALS This section has focused on the physical and chemical characteristics of urban particulate matter in rainfall runoff and snowmelt. This particulate matter was generated from and subsequently exposed to urban and transportation activities. Exposure of this particulate matter varied from durations less than 1 h to durations that were measured in terms of days. Both the physical and chemical characteristics presented in this section facilitate decision making with respect to management strategies, regulations, treatment, and potential disposal of these residuals. For all ten urban sites examined the predominance of cumulative residual mass is associated with the very coarse fraction of particles, on average those particles larger than 600 to 1000 µm. Therefore, if treatment efficiency were based on mass, unit operations such as grit separation or management strategies would focus on these particles first. A wide gradation of particulate residuals exists for all urban Cincinnati sites for both rainfall runoff and snowmelt and ranged from greater than 10,000 µm to less than 25 µm. For all sites, incremental SSA results increase with decreasing particle diameter. Although this trend is not monotonically increasing as would be expected for spherical particles and with some variability in the data, the incremental trend of increasing SSA with decreasing particle diameter is consistent for all sites. Also, the overall range of SSA values typically remains within an order of magnitude between 5 and 50 m2/g for all sites. This range is approximately three orders of magnitude greater than SSA values calculated based on the assumptions of solid, spherical particles of constant specific gravity. For all sites, and in contrast to incremental SSA values, incremental

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total SA values decreased with decreasing particle diameter. Incremental total SA values closely followed those of incremental residual mass retained for each site. These results are similar to results for stormwater particles transported during rainfall runoff events, where SSA and SA demonstrated the same trends as reported here (Sansalone et al., 1998). Incremental measurement of ρs demonstrated that, in general, there is a decrease in ρs with decreasing particle diameter, although a number of sites illustrate variability in this trend above 100 µm. Below 100 µm, a consistently decreasing gradation of ρs is observed. Values of ρs range from 3.2 to 2.5 illustrating the inorganic and organic variability of the particulate residuals. In general, the higher ρs values are associated with the larger particles indicating their mainly inorganic content, and the lower ρs values are associated with smaller particles indicating a trend toward a higher relative organic content. It is expected that if enough particulate material less than 20 µm was recovered and separated for pycnometry analysis, ρs results would demonstrate a continued decline to less than 2.0, reflecting the influence of abraded tire material with a mean diameter of 20 µm and a ρs of 1.6 to 1.7 (Sansalone and Tribouillard, 1999). Results of cumulative heavy metal mass trends across the particle size gradations for each site indicate that the predominance of heavy metal mass is associated with the coarse size particle fraction (>250 µm). This trend is similar to that for residual mass retained and total SA where the predominance of both are associated with the coarse fraction of particles. These general trends are consistent for all sites, and while both the residual particulate mass and heavy metal mass may vary from site to site, the trends are consistent and similar despite a physical separation of up to 27 km across urban Cincinnati and a wide range of conditions such as traffic levels, drainage area, pavement slopes, pavement type, and localized wind patterns. Heavy metal mass trend results as a function of particle diameter provide information in the decision-making process for residual treatment, management strategies, regulations, and disposal. For example, because the predominance of heavy metal mass is associated with the coarse size fraction, these particles are most readily removed by sedimentation as opposed to finer particles. These results provides designers and decision makers with information needed to assess the stormwater quality control strategies.

REFERENCES American Society for Testing and Materials, 1990. D 422-63, Standard Test Method for Particle-Size Analysis of Soils, Annual Book of Standards, Vol. 04.08. American Society for Testing and Materials, 1994. D 5550-94, Standard Test Method for Specific Gravity of Soil Solids by Gas Pycnometer, Annual Book of Standards, Vol. 04.08. Armstrong, L.J., 1994. Contribution of heavy metals to stormwater from automotive disc brake pad wear, Santa Clara Nonpoint Source Pollution Control Program, Woodward-Clyde. Ball, D., Hamilton, R., and Harrison, R., 1991. The influence of highway-related pollutants on environmental quality, in Highway Pollution, R. Hamilton and R. Harrison, Eds., Elsevier Science, New York, 1–47. Blake, G.R. and Hartge, K.H., 1986. Particle Density in Methods of Soil Analysis, Part 1 — Physical and Mineralogical Methods, American Society of Agronomy Soil Science Society of America, Madison, WI. Calgon Carbon Corporation, 1995. F400 Specification Sheet, P.O. Box 717, Pittsburgh, PA, 15230-0717. Carter, D.L., Mortland, M.M, and Kemper, W.D., 1986. Specific surface, in Methods of Soil Analysis, Part 1 — Physical and Mineralogical Methods, American Society of Agronomy Soil Science Society of America, Madison, WI. Changnon, S.A., 1979. Rainfall changes in summer caused by St. Louis, Science, 205, 402–404. Changnon, S.A., 1992. Inadvertent weather modification in urban areas: lessons for global climate change, Bull. Am. Meteorol. Soc., 73(5), 619–627. Harnack, R.P. and Landberg, H.E., 1975. Selected cases of convective precipitation caused by the metropolitan area of Washington D.C., J. Appl. Meteorol., 14, 1050–1060. Huber, W., 1993. Contaminant transport in surface water, in Handbook of Hydrology, D.R. Maidment, Ed., McGraw-Hill, New York, 14.1–14.50.

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Klein, L.A., Lang, M., Nash, N., and Kirschner, S.L., 1974. Sources of metals in New York City wastewater, J. Water Pollut. Control Fed., 46(12), 2653–2662. Kobriger, N.P., and Geinopolos, A., 1984. Sources and Migration of Highway Runoff Pollutants — Research Report, Vol. III. Report FHWA/RD-84/059 (PB86-227915) FHWA, U.S. Department of Transportation. Landsberg, H.E., 1956. The climates of towns, in Man’s Role in Changing the Face of the Earth, University of Chicago Press, Chicago, 584–603. Landsberg, H.E., 1981. The Urban Climate, Int. Geophys. Ser., 28, 275. Leopold, L.B., 1968. Hydrology for Urban Land Planning, United States Geological Circular No. 554, U.S. Government Printing Office, Washington, D.C. Lygren, E., Gjessing, E., and Berglind, L., 1984. Pollution transport from a highway, Sci. Total Environ., 33, 147–159. Malcom, H.R., 1989. Elements of Urban Stormwater Design, North Carolina State University, Raleigh. Muschack, W., 1990. Pollution of street run-off by traffic and local conditions, Sci. Total Environ., 93, 419–431. Novotny, V. and Chester, G., 1981. Handbook of Nonpoint Pollution: Sources and Management, Van Nostrand Reinhold, New York. Oberts, G., 1985. Magnitude and problems of nonpoint pollution from urban and urbanizing areas, in Proceedings of Nonpoint Pollution Abatement Symposium, Milwaukee, WI, 20–23 April, 1–12. O’Connor, D.J., 1988. Models of sorptive toxic substances in freshwater systems. II: Lakes and Reservoirs, ASCE J. Environ. Eng., 114(3), 533–551. Pitt, R., 1985. Characterizing and Controlling Urban Runoff through Street and Sewerage Cleaning, U.S. Environmental Protection Agency, U.S. EPA Document 600-52-85-038. Sansalone, J.J., 1999. Adsorptive-infiltration of metals in urban drainage — media characteristics, Sci. Total Environ., 235, 179–188. Sansalone, J.J. and Buchberger, S.G., 1997. Partitioning and first flush of metals in urban roadway stormwater, ASCE J. Environ. Eng., 123(2), 134–143. Sansalone, J.J. and Tribouillard, T., 1999. Variation in characteristics of abraded roadway particles as a function of particle size — implications for water quality and drainage, Trans. Res. Rec., 1690, 153–163. Sansalone, J.J., Koran, J., Buchberger, S.G. and Smithson J., 1998. Physical characteristics of highway solids transported during rainfall, ASCE J. Environ. Eng., 124(5), 427–440. Schueler, T.R., 1987. Controlling Urban Runoff — A Practical Manual for Planning and Designing Urban BMPs, Metropolitan Washington Council of Governments, Washington, D.C. Seinfeld, J.H., 1986. Atmospheric Chemistry and Physics of Air Pollution, John Wiley & Sons, New York. Singh, V. P., 1997. Kinematic Wave Modeling in Water Resources: Environmental Hydrology, John Wiley & Sons, New York. U.S. EPA, 1990. Test methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846, 3rd ed., Final Update. Washington, D.C. Viessman, W. and Lewis, L., 1996. Introduction to Hydrology, 4th ed., HarperCollins College Publishers, New York.

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National Stormwater Runoff Pollution Database James T. Smullen and Kelly A. Cave

CONTENTS Introduction ......................................................................................................................................67 The Nationwide Urban Runoff Program..................................................................................67 Major National Urban Runoff Data Collection Efforts Conducted since NURP ...................68 The CDM National Urban Stormwater Quality Research Grant ............................................69 Methods ............................................................................................................................................69 Gathering the Urban Stormwater Data.....................................................................................69 NURP Data......................................................................................................................69 USGS National Urban Storm Runoff Database .............................................................70 Stormwater NPDES Permit Data....................................................................................70 Other Stormwater Monitoring Data................................................................................71 Database Compilation...............................................................................................................71 Analytical Approach .................................................................................................................72 Investigation of the Statistical Probability Distribution of the EMCs at the Site Level ................................................................................................................................72 Combining Data from the Different Studies ..................................................................73 Results ..............................................................................................................................................75 Conclusions ......................................................................................................................................76 Acknowledgments ............................................................................................................................77 References ........................................................................................................................................77

INTRODUCTION By the mid-1960s, government agencies in the United States had identified stormwater runoff as a major pollution source to the nation’s waterways and had begun to organize research programs to investigate the problem. By the mid-1970s, stormwater runoff generally was considered to contribute as much as half the total pollutant loading discharged to the nation’s surface waters from all sources. However, recognition that the nature, causes, severity, and opportunities for control of runoff pollution were not well understood led the U.S. Congress to include the establishment of the Nationwide Urban Runoff Program (NURP) in the 1977 Amendments to the Clean Water Act (PL 95-217). The U.S. Environmental Protection Agency (U.S. EPA) developed NURP to expand the state of knowledge of urban runoff pollution by instituting data collection and applied research projects in selected urban areas throughout the country.

THE NATIONWIDE URBAN RUNOFF PROGRAM A central result of the NURP studies was the development of urban runoff pollution loading factors in the form of event mean concentrations (EMCs) (U.S. EPA, 1983). EMC is defined as the total 0-56676-916-7/03/$0.00+$1.50 © 2003 by CRC Press LLC

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mass load of a chemical parameter yielded from a site during a storm divided by the total runoff water volume discharged during the storm. For sampling programs that are based on flow-weighted techniques, the EMC simply is taken as the flow-weighted mean concentration. In studies employing sequential discrete sampling approaches, the EMC is taken as the area under the loading rate curve (loadograph) divided by the area under the flow rate curve (hydrograph). The NURP studies relied upon ten standard water quality constituents to characterize urban runoff. The program developed EMCs for these and other pollutants, drawing upon data collected from over 2300 station-storms at more than 81 urban sites located in 28 different metropolitan areas. Among the conclusions reached by the NURP investigators are two of the more important: 1. The variance of the EMCs when data from sites are grouped by land use type or geographic region is so great that differences in measures of central tendency among groups statistically are not significant. 2. Statistically, the entire sample of EMCs and the medians of all EMCs among sites are lognormally distributed. These results were derived from investigations of data for the ten indicator chemical species analyzed during the program. In the United States, the NURP EMCs are used by stormwater management planners for estimating pollutant loadings, the effectiveness of management measures, and water quality impacts. The most common use of the EMCs is in screening models where pollutant loads (Li ) are estimated as the product of the area of urban land (AU), the rainfall runoff depth as estimated by a modified rational formula approach (dr), and a constant pollutant concentration (Ci ), usually estimated from the EMCs reported by NURP. This framework (i.e., Li = AU dr Ci ), for example, is the basis for the U.S. Department of Commerce National Oceanic and Atmospheric Administration (NOAA) National Coastal Pollution Discharge Inventory (NCPDI) urban nonpoint-source methodology, which is used to estimate loadings to all the estuaries in the United States. This same approach to estimating runoff loadings was employed widely in planning-level characterizations performed recently by most major cities across the United States as part of their stormwater and combined sewer overflow discharge permitting requirements. In applications of the NCPDI, in keeping with the NURP findings, NOAA has not attempted to differentiate pollutant concentrations among land uses, allowing only land use–derived differences in rainfall runoff coefficients to account for different estimated loading rates among land use types. However, in many other applications of this methodology, investigators have attempted to differentiate among land use loadings by applying the NURP EMCs computed by land use, despite NURP results and conclusions to the contrary.

MAJOR NATIONAL URBAN RUNOFF DATA COLLECTION EFFORTS CONDUCTED NURP

SINCE

The NURP EMCs continue to be used widely in the United States for a variety of stormwater management planning purposes. However, a considerable amount of urban runoff water quality monitoring data has been collected in the United States since the completion of the NURP studies in the early 1980s. The most comprehensive of these was a nationwide urban runoff monitoring effort, conducted by the U.S. Geological Survey (USGS), that continued after the cessation of the NURP activities. The resulting USGS urban storm runoff database includes data collected through the mid-1980s for over 1100 station-storms at more than 97 urban sites located in 21 metropolitan areas, with 5 stations common to the NURP data set (Driver et al., 1985; Mustard et al., 1987; Driver and Tasker, 1990). Additionally, many major cities in the United States recently collected urban runoff quality data as part of the application requirements for stormwater discharge permits under the National Pollutant Discharge Elimination System (NPDES). Information from more than 30 cities and from several regional programs has been gathered, which includes data for more than

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800 station-storms for over 150 parameters. The City of Austin, Texas also collected stormwater data that includes information from an additional 1200 station-storms. These data, along with that from several other programs, also were acquired.

THE CDM NATIONAL URBAN STORMWATER QUALITY RESEARCH GRANT Although data from the four sources noted, and others, have been available in various forms in the public domain for some time, there was no apparent attempt to gather and analyze all these data for comparing measures of central tendency of EMCs. In 1993, the Technical Council of Camp Dresser & McKee (CDM) provided the authors with a research grant to acquire these data for developing and analyzing an Urban Stormwater Quality DataBase for use by CDM in its various projects conducted for clients throughout the world. Specifically, the CDM Technical Council research grant provided resources to accomplish the following: • Incorporate the data into a single database and subject it to uniform quality assurance and quality control efforts • Calculate EMCs for data collected since NURP and perform comparisons among the various sources • Stratify data from watersheds of similar land uses, geographic, climatic, and physiographic regions, and management practices for analyses • Test the new data set to see if the increase in the number of observations over the NURP data set reduces EMC variance among the stratified groupings sufficiently to allow the identification of statistically significant differences by season, by types of land uses, or by geographic or physiographic regions • Develop statistics to project stormwater quality and efficiencies of select management practices (e.g., detention ponds, street sweeping) This chapter presents the results of the data gathering efforts and compares the results of EMC determinations for the original ten NURP indicator chemical constituents among the sources of the data described above. Proposed future development of the database and of analyses to be performed also are discussed.

METHODS GATHERING

THE

URBAN STORMWATER DATA

To date, data were gathered from four major and several minor sources to develop the CDM National Stormwater Quality Database, including the original NURP data, USGS data, NPDES stormwater permit data, City of Austin data, and others. Descriptions of the data-gathering efforts are provided below. NURP Data Original data from the NURP studies were available in digital format only for 16 of the nationwide study areas. These data, acquired from the Illinois State Water Survey, were acquired in a variety of text formats, all slightly different for each study area, and sometimes differing among the sampling locations of each study area. In addition, some of these data were reported as EMCs and some were reported as stormwater loads and runoff volume. Therefore, for comparison of urban runoff data among the sources discussed, the EMC estimates available in the NURP final report (U.S. EPA, 1983) were used, including data from 22 of the 28 urban stormwater monitoring projects from across the United States. These 22 projects were focused on runoff pollution characterization, whereas the remaining six were intended to characterize the operation of runoff pollutant control

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management practices. The data from that report are available as the number of observations, means, medians, and coefficients of variation for sampling sites, but not for individual station storms. They represent the results of analyses for as many as 2000 storms at 77 sites. A system of logical and numerical quality assurance and quality control (QA/QC) checks was developed and incorporated with the data management and quality evaluation concepts in use for the water quality data collection efforts of the Rouge River Program (Morea et al., 1994). This system was implemented using Statistical Analysis System (SAS, 1989) code and, as the data were brought into SAS data sets, these procedures were applied as part of a QA/QC screening process. The pollutant constituents used in NURP as stormwater runoff indicator parameters are as follows: Total suspended solids (TSS) Five-day biochemical oxygen demand (BOD5) Chemical oxygen demand (COD) Total phosphorus (TP) Total soluble phosphorus (TSP) Total Kjeldahl nitrogen (TKN) Nitrate plus nitrite (NO2,3) Extractable copper (ECU) Extractable lead (EPB) Extractable zinc (EZN) Analytical methods for the determination of these parameters are described in the U.S. EPA Methods for Chemical Analysis of Water and Wastes (1979). These ten are the parameters used for comparative purposes in this chapter. USGS National Urban Storm Runoff Database In 1987, the USGS compiled data from its network of urban rainfall, runoff, and water quality gauging stations. This compilation resulted in a database of rainfall characteristics, discharge, water quality analyses, and drainage basin characteristics. The USGS urban stormwater database contained storm load and characteristics data for 1144 storms at 97 stations in 21 metropolitan areas. The USGS water quality field data collection techniques were generally comparable to those used by most NURP investigators, and the laboratory analytical techniques for the ten indicator parameters were directly comparable to those used by the NURP study participants. The storm load files were obtained in a digital format that was supplied with the associated water resources investigations report (Mustard et al., 1987). However, because the original database developed by USGS that had included EMCs was no longer available, computer programs were developed using SAS to import the text-formatted storm load data into SAS data sets. Only data from monitoring locations that exhibited little or no base flow did not include stormwater management control practices, and were not included in the NURP study data set, were allowed to remain in the USGS portion of the database. This process resulted in as many as 599 and as few as 287 EMC estimates from among the available station-storms for the nine parameters included (there were no BOD5 data in the USGS data set). As the USGS data were incorporated into SAS data sets, the same QA/QC procedures that were used to screen the NURP data were applied. Stormwater NPDES Permit Data Urban runoff data collected by cities in the United States as part of their stormwater NPDES discharge permit applications were gathered for this project from over 30 cities and regional programs, most of which were collected under CDM-managed permit compliance projects. The

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data generally included land use information, rainfall data, and analytical sampling data for over 150 parameters. A single value, the EMC, was reported for each parameter for each storm event, for a total of 816 station-storms (soluble phosphorous was not available in the NPDES data set). The water quality field data collection techniques generally were comparable to those used by most NURP and USGS investigators, but some likely were not as well controlled as those of the two prior data collection efforts. The field laboratory analytical techniques required to be used by all NPDES permittees either were the U.S. EPA methods or, in some cases, the comparable USGS methods. The NPDES permit application data initially were processed by CDM staff located in the Rouge River National Wet Weather Demonstration Program office in Detroit. Considerable difficulties were overcome in loading the data into the Rouge River Program Oracle database, including handling data that were submitted in many different spreadsheet formats and with differing detection limit levels. As the NPDES data were incorporated into the Rouge River Program Oracle database, data management and evaluation quality control procedures established for the Rouge River National Wet Weather Demonstration Program were implemented before the data were transferred to SAS data sets (Morea et al., 1994). Other Stormwater Monitoring Data Data from other major stormwater monitoring programs also were collected by the Rouge River Program staff and incorporated into the Oracle database. For example, the City of Austin provided its stormwater monitoring database, including stormwater monitoring data from over 1200 stationstorms. The Austin data collection efforts complied with the U.S. EPA field and laboratory analytical methods. The Austin data also included inflow and outflow data from sites with stormwater runoff control best management practices (BMPs). As with the NPDES data, the Rouge River Program required that the data be incorporated into the Oracle database for quality control prior to transfer to SAS data sets. Data from discrete samples taken during storm events were supplied by the Austin staff in addition to flow data. These data will be used in the future to estimate EMCs for the Austin data for eventual inclusion in the CDM stormwater database. However, these estimates have not been finalized and the Austin data have not yet been incorporated in the EMC analysis presented in this chapter. Data collected for other large runoff pollution studies have been identified and targeted for future acquisition and inclusion in the database. These are the runoff pollution survey data from the Occoquan and Four Mile Run Basins studies conducted in northern Virginia (Hartigan et al., 1977; Smullen et al., 1978; Smullen, 1979); the U.S. EPA Urban Rainfall-Runoff-Quality Data Base (Huber et al., 1979); and the U.S. EPA Chesapeake Bay Program runoff studies (Smullen et al., 1982). Some of these data already have been gathered and will be included in future updates of the database.

DATABASE COMPILATION The process of compiling the urban runoff data gathered from the various sources into a single digital database was complicated by the format of the available information from the NURP study. For the USGS and NPDES data, the EMCs could either be acquired or computed for each stationstorm for which data had been collected. However, as discussed previously, the station-storm EMCs were not readily available for the NURP data. Rather, the available NURP EMC statistics were estimated for the data grouped by sampling station. If the NURP EMC values were re-created from the remaining collection of digital data, the available number of observations would be reduced significantly from that of the original NURP work, and that reduced data set would have produced estimates different from those found in the original NURP documentation. Because one of the principal goals was to compare new estimates of EMCs to those previously made by the NURP

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investigators, it was considered important to be able to reproduce the measures of central tendency and statistical distributions from the previous purposes of this study. Therefore, it was decided to proceed by working with statistics produced for data grouped to the sampling station level (i.e., stratified by sampling site) for all three of the independent data sets. Initially, data from the NURP and USGS studies and the NPDES stormwater permit application programs were incorporated into SAS data sets from text files. NURP data were stored as EMCs and their associated statistics, by site, as discussed above. USGS and NPDES data were stored as EMCs initially by event. At that point, once again, another series of logical and numerical quality assurance and quality control checks was applied. Then, EMCs and the required statistics were estimated for the USGS and NPDES data, grouped by site, and the data sets were combined at the site-grouped level. Again, USGS stations that were part of the original NURP report were not included as part of the USGS portion of the combined data set. The EMCs for the conventional parameters are stored in units of milligrams per liter (mg/l), and the metals are stored in units of micrograms per liter (µg/L).

ANALYTICAL APPROACH The analyses performed in support of the results discussed in this chapter were intended to investigate the statistical distribution of the stormwater runoff EMC data at the site-grouped level, to provide a basis to combine all the data and to infer measures of central tendency and associated statistics at the event level, and to use those results to compare the newly estimated EMC statistics with those of the various individual sources. It is asserted that, if the site-grouped data exhibit lognormal characteristics, then the individual storm-level statistics most likely also are distributed lognormally. This result would be consistent with the findings of the NURP investigators and a number of others who have explored this problem since (see, for example, NCTCOG, 1994, or Quasebarth et al., 1994). Note that, for these initial analyses of the ten indicator parameters, “below detection limit” data points were set equal to either the laboratory-identified or the method-detection limit. Investigation of the Statistical Probability Distribution of the EMCs at the Site Level The distribution of the data is an important determination because statistics used to characterize the data are distribution based, and, therefore, if the distribution is defined properly, the more likely it is that the statistics will describe the data properly. The NURP investigators noted that on a visual and a statistical basis, with relatively few and isolated exceptions, the EMCs at individual sites were characterized by lognormal distributions. This determination leads to the convenient result that the central tendency and the variability of EMCs at individual sites are well characterized by the estimates of the mean, median, and coefficient of variation (i.e., ratio of the standard deviation to the mean). For this study, the probability distributions of the data was investigated both on a graphical and statistical basis. First, a plot was generated for each pollutant to inspect visually the basis for an assumption of lognormality. Probability plots were used to graphically evaluate the appropriateness of an assumption of a lognormal distribution by plotting the estimated site-specific EMCs vs. their exceedence probability. For study purposes, the exceedence probability was calculated using Blom’s plotting positions (i.e., {I – 3/8}/{n + 1/4}, where I is the observation rank) converted to a standard normal variable (standard normal deviate, z, with mean = 0 and variance = 1). Figure 3.1 shows site-grouped EMCs for several parameters, with three different plotting characters used to identify the data sources of each site EMC. Shown on the graph is a solid line depicting an estimate of the lognormal distribution with the same mean and variance of the data vs. its z-scores (or equivalent

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SITE EVENT MEAN CONCENTRATIONS Lognormal Probability Plot – P

Phosphorus mg/L

10.00

1.00

0.10

0.01 -3

-2

-1 0 1 2 Standard Normal Random Variable z + + + NURP    USGS • • • NPDES

3

FIGURE 3.1 Cumulative distribution for site-grouped TP EMCs.

exceedence probability). By observation, the probability distribution of the TP, site-grouped EMCs would appear to be approximated quite well by a lognormal distribution. In fact, similar results were found for the other nine parameters, with the greatest observable differences occurring for the metals. However, in no case was the agreement judged to be so poor as to lead to rejection of the distribution. This result was reinforced by application of the Shapiro–Wilk test, with estimates for the Shapiro–Wilk statistic W, calculated for the log-transformed site EMCs, generally supporting the choice of a lognormal distribution. The probability plot correlation coefficient (PPCC) test for lognormality (Vogel, 1986; 1987; Stedinger et al., 1993) was used as an alternative indicator of the likelihood that the site-grouped EMCs exhibit lognormal distribution behavior. Table 3.1 shows the PPCC for each parameter and the 5% significance level based on the number of observations. The PPCC statistics for four of the constituent EMCs (TSS, NO2+3, ECU, and EZN) were regarded as good indicators that these constituent EMCs are well described by lognormal distributions, with acceptance at the 5% confidence level. For the other five constituents where data exists (COD, TP, TSP, TKN, and EPB), the significance levels must be relaxed to 10% to accept lognormality. This result is mirrored in the Type I error statistics, which indicate that the greatest chance of accepting non-lognormal distributions occurs when they actually are lognormal for these five parameters. Despite the influx of data from new sources and the increase in the number of observations over that of the NURP data set, investigations led to no compelling arguments to reject the conclusions of the NURP study regarding lognormality of EMCs. Therefore, analyses proceeded, accepting the mean or the median and coefficient of variation as fully characterizing the central tendency and variability of the urban runoff EMCs among sites or groups of sites. Combining Data from the Different Studies Data were combined from the three sources (NURP, USGS, and NPDES) to compute new estimates of EMC population means and medians with many more degrees of freedom than were available to the NURP investigators. Given the mean (or E[(ln x)] for lognormal data), a “pooled” mean can be calculated that represents the mean of the total population of sample data. The population of data, in this case, is the measurements of EMCs. The following formulae (from Taylor, 1990) can be used to calculate the pooled, or grand mean, and the standard deviation:

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TABLE 3.1 Test for Lognormal Distribution Using Probability Plot Correlation Coefficient at 5% Confidence

Constituent Total suspended solids Biochemical oxygen demand Chemical oxygen demand Phosphorus Soluble phosphorus Total Kjeldhal nitrogen Nitrite and nitrate Copper Lead Zinc

N

PPCC

P(5%)

Type I Error for Assuming Logonormal Distribution, %

111 — 111 124 74 123 90 70 110 82

0.9886 — 0.9798 0.9825 0.9769 0.9749 0.9889 0.9947 0.9791 0.996

0.9887 — 0.9887 0.9902 0.9833 0.9901 0.9859 0.98 0.9881 0.9847

4.70 — <1 <1 <1.30 <1 >10 >10 <1 >10

Note: N is the number of observations; PPCC is the probability plot correlation coefficient = correlation of observations with z-scores; P(5%) is critical value of the PPCC test statistic at the 5% confidence level; PPCC > P(5%) indicates acceptance of lognormal distribution; Type I error is the error associated with rejecting the distribution as lognormal when it is lognormal.

X=

x1W1 + x2W2 + … + xk Wk W + W +…+ W 1

sX =

2

k

[1 (W + W + … + W )] 1

= Wi =

2

k

ni sx2i

where xI is the mean—from data set I, Wi is the weight given to that mean, as the inverse of the sample variance sx2i , X is an estimate of the grand mean, and s x– is the standard deviation of the grand mean. A pooled standard deviation can be calculated from the standard deviations of the individual data sets to obtain a better estimate of the standard deviation of the population of data. The estimate of the pooled standard deviation is obtained using the following equation:

sp =

(s d f + s d f 2 1

+ … + sksup 2 d fk (d f1 + d f2 + … + d fk ) 1

2 2

2

)

where sp is the pooled standard deviation and sk2 and dfk are the sample variance and degrees of freedom for data set k. Both the grand mean and standard deviation of EMCs are calculated in log space since the data are regarded as lognormally distributed. The transforms shown in Table 3.2 are used to report the resulting statistics in arithmetic space (U.S. EPA, 1984).

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TABLE 3.2 Relationship among Statistical Properties for Arithmetic and Lognormal Transforms Arithmetic

Definition of Term

Logarithmic

x ux νx2 νx CV or v x~

Random variable Mean Variance Standard deviation Coefficient of variation Median

ln x ulnx νln2 x νln x Not used Not used

µx = e

(µ ln x +1 2 σ 2ln x )

(

µ x = x˜ 1 + ν2x

)

σ ln x =

(ln(1 + ν )) 2 x

µ ln x = ln( x˜ )

µ x˜ = d ln x

µ ln x = ln µ x e 

σ 2 −1 ν x =  e ln x   

µ ln x = ln µ x 

1 2 σ ln2 x

 

(1 + ν )  2 x

σ x = µ x νx

RESULTS Grand means and medians for the ten parameters were calculated using the methods described above and the data from the three national data sets (NURP, USGS, and NPDES). The means and medians and number of observations used in the estimates for both the pooled data and for the NURP data are included in Table 3.3. Note that the NURP means and medians shown in Table 3.3 are those that were computed with the data set generated in the study. They differ slightly from those reported by the NURP investigators because of the difference in degrees of freedom. The differences between the pooled means and those estimated from the NURP data range from a 79% lower estimate for Cu to a 36% higher estimate for BOD5. For TSS, the individual NPDES EMC mean was 70% lower than those from the NURP or the USGS estimates, which were quite close. The NPDES estimate was mostly responsible for the 55% reduction in the pooled estimate over the NURP mean. However, the NURP and NPDES individual estimates of mean EMCs BOD5 were quite close (means of 10.4 and 14.4, respectively). The differences varied among the rest of the conventional pollutants, with the USGS data influencing the pooled results most for COD and NO2+3 and the NPDES data having more influence on the pooled results for TP and TKN. For Pb and Cu, the individual means and medians for the NPDES EMCs are approximately five and ten times less, respectively, than that of the estimates from the USGS or NURP data. However, for Zn, the NURP and NPDES estimates are close and the USGS mean is quite a bit higher, but there are more observations in the NPDES data set, resulting in a modest decrease in the pooled estimate compared to NURP. The lower metal concentrations in the NPDES data could be the result of sampling problems in those studies (e.g., poor solids recovery). However, they also may reflect the use of cleaner techniques that are more prevalent in the 1990s for sampling, chain of custody transfers, and laboratory methods for metals. These cleaner techniques have evolved over the past decade in response to the criticism of metals determinations from previous decades.

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TABLE 3.3 Pooled and NURP EMC Estimates

Constituent

Units

Total Suspended Solids

mg/l

Biochemical oxygen demand5

mg/l

Chemical oxygen demand

mg/l

Total phosphorus

mg/l

Soluble phosphorus

mg/l

Total Kjeldhal nitrogen

mg/l

Nitrite and nitrate

mg/l

Copper

µg/l

Lead

µg/l

Zinc

µg/l

EMCs

Source of Dataa

Mean

Median

Number of Events

Pooledb NURP Pooled NURP Pooled NURP Pooled NURP Pooledc NURP Pooled NURP Pooled NURP Pooled NURP Pooled NURP Pooled NURP

78.4 174 14.1 10.4 52.8 66.1 0.315 0.337 0.129 0.1 1.73 1.67 0.658 0.837 13.5 66.6 67.5 175 162 176

54.5 113 11.5 8.39 44.7 55 0.259 0.266 0.103 0.078 1.47 1.41 0.533 0.666 11.1 54.8 50.7 131 129 140

3047 2000 1035 474 2639 1538 3094 1902 1091 767 2693 1601 2016 1234 1657 849 2713 1579 2234 1281

Note: Statistics computed using formulae for pooled mean and standard deviations. a b c

Pooled data sources include NURP, USGS, and NPDES. No BOD5 data available in the USGS data set — pooled includes NURP + NPDES. No TSP data included in the NPDES data set — pooled includes NURP + USGS.

Source: Smullen et al., 1999.

CONCLUSIONS The variation between the NURP results and those developed here from the pooling of the three national databases is important. Runoff pollution assessments based upon the new EMC numbers would yield lower loading estimates for seven of the ten indicator parameters (TSS, COD, TP, NO2+3, Cu, Pb, and Zn) and higher loading estimates for three parameters (BOD5, TSP, and TKN). Suspended sediment loads based on the NOAA NCPDI approach would be over 50% less using the new estimates. The NiP ratio, an important consideration in algal management, is over 20% greater when computed from the pooled medians than when it is computed from the published NURP values. While initially these differences may not appear to be so dramatic, they could in fact be large enough to merit reevaluating selected management strategies in existing control programs. Work is continuing with the database, adding the newly acquired data and investigating further the differences among the various sources of data. In the future, the lognormal distributions for variables with detection limit observations will be exploited to compute statistics. The data stratified by land use, geography, and seasons will be evaluated to see if the increased number of observations will reduce the variance in the stratified groups enough to expose statistically significant differences.

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The database will also be exploited to understand better the effects of runoff control practices and to refine its predictive capabilities. The overall goals are to improve urban runoff characterization and evaluation techniques and to provide the most up-to-date and functional tools for use in satisfying stormwater management project needs.

ACKNOWLEDGMENTS This work was funded by a 1993 grant from the Technical Council of Camp Dresser & McKee, Inc., made possible in large part by the encouragement and support of the CDM Chief Technical Officer, Dr. Larry Roesner, and the CDM National Practice Leader for Water Resources, Nilo Priede. Thomas Quasebarth of CDM, Dr. Nancy Driver of the USGS, and Dr. Colleen Hughes, a CDM engineer located in Rouge River National Wet Weather Demonstration Program Office, provided valuable help in our data acquisition efforts. Dr. Hughes performed the data conditioning and quality assurance for the NPDES data under the direction of Kelly Cave. Amy Shallcross performed similar data conditioning for the NURP and USGS data, and performed database and statistical analysis support. Dr. Richard Vogel of Tufts University offered valuable insight into the tests for statistical distributions.

REFERENCES Driver, N.E. and Tasker, G.D., 1990. Techniques for Estimation of Storm-Runoff Loads,Volumes, and Selected Constituent Concentrations in Urban Watersheds in the United States, U.S. Geological Survey WaterSupply Paper 2363, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225. Driver, N.E., Mustard, M.H., Rhinesmith, R.B., and Middelburg, R.F., 1985. U.S. Geological Survey UrbanStormwater Data Base for 22 Metropolitan Areas throughout the United States, U.S. Geological Survey Open-File Report 85-337, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225. Hartigan, J.P., Grizzard, T.J., Randall, C.W., Smullen, J.T., and Derewianka, M., 1977. Urban land use characteristics and runoff pollution loadings, timing and solubility, Eos, 59(5): 394. Huber, W.C., Heaney, J.P., Smolenyak, K.J., and Aggidis, D.A., 1979. Urban Rainfall-Runoff-Quality Data Base: Update with Statistical Analyses, EPA-600/8-79-004, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH 45268. Methods for Chemical Analysis of Water and Wastes, 1979. EPA-600/4-79-020, U.S. Environmental Protection Agency, Environmental Monitoring and Support Laboratory, Cincinnati, OH 45268. Morea, S., Hummel, G., Little, J.P., and Taylor, E., 1994. Data Management and Evaluation Guide, Rouge River National Wet Weather Demonstration Program, Wayne County, MI 48226. Mustard, M. H., Driver, N.E., Chyr, J., and Hansen, B.G., 1987. U.S. Geological Survey Urban-Stormwater Data Base of Constituent Storm Loads; Characteristics of Rainfall, Runoff, and Antecedent Conditions; and Basin Characteristics, U.S. Geological Survey Open-File Report 87-4036, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225. NCTCOG, 1994. Storm Water Discharge Characterization: Final Summary Report, North Central Texas Council of Governments, Dallas, TX. Quasebarth, T.F., Cave, K.A., Wagner, R.A., Denison, D., Mikesell, M.D., and Sidhu, A., 1994. Nonpoint Source Data Assessment and Field Investigation, Technical Report, Rouge River National Wet Weather Demonstration Program, Wayne County, MI 48226. Smullen, J.T., 1979. A Simple Empirical Model of Runoff Pollution for Environmental Planning, MS thesis, Department of Civil and Environmental Engineering, Rutgers University, New Brunswick, NJ. Smullen, J.T., Hartigan, J.P., and Grizzard, T.J., 1978. Assessment of runoff pollution in coastal watersheds, in Coastal Zone 78, American Society of Civil Engineers, New York, 840–857. Smullen, J.T., Shallcross, A.L., and Cave, K.A., 1999. Updating the U.S. nationwide urban runoff quality data base, Water Sci. Technol., 39(12), 9–16.

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Smullen, J.T, Taft, J.L., and Macknis, J., 1982. Nutrient and Sediment Loads to the Tidal Chesapeake Bay System, in Chesapeake Bay Program Technical Studies: A Synthesis, E.G. Macalaster, D.A. Barker, and M.E. Kasper, Eds., U.S. Environmental Protection Agency, Washington, D.C., 147–261. Stedinger, J.R., Vogel, R.M., and Foufoula-Georgiou, E., 1993. Frequency analysis of extreme events, in Handbook of Hydrology, D.R. Maidment, Ed., McGraw-Hill, New York, 18.1–18.66. Taylor, K.T., 1990. Statistical Techniques for Data Analysis, Lewis Publishers/CRC Press, Boca Raton, FL. U.S. EPA, 1983. Results of the Nationwide Urban Runoff Program: Volume 1 — Final Report, U.S. Environmental Protection Agency, NTIS Assession Number: PB84-185552, National Technical Information Service, U.S. Department of Commerce, Springfield, VA. U.S. EPA, 1984. A Probabilistic Methodology for Analyzing Water Quality Effects of Urban Runoff on Rivers and Streams, Office of Water, U.S. Environmental Protection Agency, Washington, D.C. Vogel, R.M., 1886. The probability plot correlation coefficient test for normal, lognormal, and Gumbel distributional hypotheses, Water Resourc. Res., 22(4), 587–590. Vogel, R.M., 1987. Correction to “The probability plot correlation coefficient test for normal, lognormal, and Gumbel distributional hypotheses,” Water Resourc. Res., 23(10), 2013.

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4

SLAMM, the Source Loading and Management Model Robert Pitt and John Voorhees

CONTENTS Introduction ......................................................................................................................................79 History of Slamm and Typical Uses................................................................................................80 SLAMM Process Descriptions ........................................................................................................81 Unique Attributes of SLAMM.........................................................................................................82 Small Storm Hydrology............................................................................................................84 Particulate Washoff ...................................................................................................................89 SLAMM Computational Processes .................................................................................................91 Use of Slamm to Identify Pollutant Sources and to Evaluate Different Control Programs ..........92 Future Directions for SLAMM........................................................................................................96 References ........................................................................................................................................99

INTRODUCTION The Source Loading and Management Model (SLAMM) was originally developed to better understand the relationships between sources of urban runoff pollutants and runoff quality. It has been continually expanded since the late 1970s and now includes a wide variety of source area and outfall control practices (infiltration practices, wet detention ponds, porous pavement, street cleaning, catchbasin cleaning, and grass swales). SLAMM is strongly based on actual field observations, with minimal reliance on pure theoretical processes that have not been adequately documented or confirmed in the field. SLAMM is mostly used as a planning tool, to better understand sources of urban runoff pollutants and their control. Special emphasis has been placed on small storm hydrology and particulate washoff in SLAMM, common areas of misuse in many stormwater quality models. Many currently available urban runoff models have their roots in drainage design where the emphasis is on very large and rare rains. In contrast, stormwater quality problems are mostly associated with common and relatively small rains. The assumptions and simplifications that are legitimately used with drainage design models are not appropriate for water quality models. SLAMM therefore incorporates unique process descriptions to predict more accurately the sources of runoff pollutants and flows for the storms of most interest in stormwater quality analyses. However, SLAMM can be effectively used in conjunction with drainage design models to incorporate the mutual benefits of water quality controls on drainage design. SLAMM has been used in many areas of North America and has been shown to predict stormwater flows and pollutant characteristics accurately for a broad range of rains, development characteristics, and control practices. As with all stormwater models, SLAMM needs to be accurately calibrated and then tested (verified) as part of any local stormwater management effort.

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SLAMM is unique in many aspects. One of its most important features is its ability to consider many stormwater controls (affecting source areas, drainage systems, and outfalls) together, for a long series of rains. Another is its ability to describe accurately a drainage area in sufficient detail for water quality investigations, but without requiring a great deal of superfluous information that field studies have shown to be of little value in accurately predicting discharge results. SLAMM also applies stochastic analysis procedures to more accurately represent actual uncertainty in model input parameters to better predict the actual range of outfall conditions (especially pollutant concentrations). However, the main reason SLAMM was developed was because of errors contained in many existing urban runoff models. These errors were obvious when comparing actual field measurements to the solutions obtained from model algorithms.

HISTORY OF SLAMM AND TYPICAL USES SLAMM was initially developed to evaluate stormwater control practices more efficiently. It soon became evident that in order to accurately evaluate the effectiveness of stormwater controls at an outfall, the sources of the pollutants or problem water flows must be known. SLAMM has evolved to include a variety of source area and end-of-pipe controls and the ability to predict the concentrations and loadings of many different pollutants from a large number of potential source areas. SLAMM calculates mass balances for both particulate and dissolved pollutants and runoff flow volumes for different development characteristics and rainfalls. It was designed to give relatively simple answers (pollutant mass discharges and control measure effects for a very large variety of potential conditions). SLAMM was developed primarily as a planning-level tool, that is, to generate information needed to make planning-level decisions, while not generating or requiring superfluous information. Its primary capabilities include predicting flow and pollutant discharges that reflect a broad variety of development conditions and the use of many combinations of common urban runoff control practices. Control practices evaluated by SLAMM include disconnections of pavements and roofs, rain gardens, amended soils, detention ponds, infiltration devices, porous pavements, rain barrels and cisterns for on-site re-use, grass swales, catchbasin cleaning, and street cleaning. These controls can be evaluated in many combinations and at many source areas as well as the outfall location. SLAMM also predicts the relative contributions of different source areas (roofs, streets, parking areas, landscaped areas, undeveloped areas, etc.) for each land use investigated. As an aid in designing urban drainage systems, SLAMM also calculates correct NRCS curve numbers that reflect specific development and control characteristics. These curve numbers can then be used in conjunction with available urban drainage procedures to reflect the water quantity reduction benefits of stormwater quality controls. SLAMM is normally used to predict source area contributions and outfall discharges. However, SLAMM has been used in conjunction with a receiving water model (HSPF) to examine the ultimate receiving water effects of urban runoff (Ontario, 1986), and has recently been modified to be integrated with SWMM (Pitt et al., 1999c) to more accurately consider the joint benefits of source area controls on drainage design. The development of SLAMM began in the mid-1970s, primarily as a data reduction tool for use in early street cleaning and pollutant source identification projects sponsored by the U.S. EPA Storm and Combined Sewer Pollution Control Program (Pitt, 1979; 1984; Pitt and Bozeman, 1982). Additional information contained in SLAMM was obtained during the U.S. EPA Nationwide Urban Runoff Program (NURP) (U.S. EPA, 1983), especially the early Alameda County, California (Pitt and Shawley, 1982), and the Bellevue, Washington (Pitt and Bissonnette, 1984) projects. The completion of the model was made possible by the remainder of the NURP projects and additional field studies and programming support sponsored by the Ontario Ministry of the Environment (Pitt and McLean, 1986), the Wisconsin Department of Natural Resources (Pitt, 1986), and Region V of the U.S. EPA. Early users of SLAMM included the Ontario Ministry of the Environment Toronto Area Watershed Management Strategy (TAWMS) study (Pitt and McLean, 1986) and the Wisconsin

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Department of Natural Resources Priority Watershed Program (Pitt, 1986). SLAMM can now be effectively used as a tool to enable watershed planners to obtain a better understanding of the effectiveness of different control practice programs. Some of the major users of SLAMM have been associated with the Nonpoint Source Pollution Control Program of the Wisconsin Department of Natural Resources, where SLAMM has been used for a number of years to support its extensive urban stormwater planning and cost-sharing program (Thum et al., 1990; Kim et al., 1993a, b; Ventura and Kim, 1993; Bachhuber, 1996; Bannerman et al., 1996; Haubner and Joeres, 1996; Legg et al., 1996). Many of these applications have included the integrated use of SLAMM with GIS models. A logical approach to stormwater management requires knowledge of the problems that are to be solved, the sources of the problem pollutants, and the effectiveness of stormwater management practices that can control the problem pollutants at their sources and at outfalls. SLAMM is designed to provide information on these last two aspects of this approach.

SLAMM PROCESS DESCRIPTIONS Linsley (1982), in a paper summarizing urban runoff models, defined a model as a mathematical or physical system obeying certain conditions. The behavior of a model must be analogous to the system under study. Linsley felt that a comprehensive literature search would uncover at least several hundred, if not several thousand, models that have been used to predict runoff from rainfall information. He included in his review paper an interesting set of definitions for the many adjectives that have been used to describe hydraulic models: • Deterministic — Based on the assumption that the process can be defined in physical terms without a random component. • Stochastic — Based on the assumption that the flow at any time is a function of the antecedent flows and a random component. • Conceptual — Model is designed according to a conceptual understanding of the hydraulic cycle with empirically determined functions to describe the various subprocesses. • Theoretical — Model is written as a series of mathematical functions describing a theoretical concept of the hydologic cycle. • Black box — Model uses an appropriate mathematical function or functions which is fitted to the data without regard to the processes it represents. • Continuous — Model is designed to simulate long periods of time without being reset to the observed data. Such models require some form of moisture storage accounting. • Event — Designed to simulate a single runoff event given the initial conditions. • Complete — Includes algorithms for computing the volume of runoff from rainfall and distributing this volume into the form of a hydrograph. • Routing — Model contains no algorithms for rainfall-runoff but simply distributes a given volume of runoff in time by routing or unit-hydrograph computations. • Simplified — Uses algorithms which have been deliberately simplified, or uses large time increments to minimize computer running time.

These labels may create more confusion than insight. Many relatively simple models not only have numerous descriptions for different model elements, but they also have conflicting descriptions as well. As an example, theoretical process descriptions are commonly coupled with conceptual and statistical (black box) descriptions. This is much more common with water quality models that have been constructed based on older hydraulic models (such as the development of HSPF from HSP from SWM). Each process contained in a model should have its own unique set of descriptors (deterministic or stochastic; and conceptual, theoretical, or black box), while the overall model design also dictates another set of descriptors (continuous or event; plus possibly complete, routing, and simplified). A complete set of descriptors would therefore become very confusing. It would be much better if the processes and the model design were well documented.

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Troutman (1985) described the preconceived differences between deterministic models or black box models. He concluded that the distinction between these two seemingly conflicting categories of models was not at all clear, or important, when analyzing errors. He found that some of the confusion in these model categories was because some users categorized statistical models as black box models (such as defined above by Linsley in 1982). He gives as an example the general assumption of runoff that tends to vary proportionally with rainfall. This conceptual relationship is typically reflected by a very simple statistical black box model. He further shows that many of the most complex physically based conceptual hydrologic models currently used contain many process descriptions where some of the variables are simply statistically related to other variables. Because these models are large and complex, these relationships are commonly overlooked. His major conclusion is that any rainfall–runoff model can be defined as a conceptual model, and that the distinctions between black box and physically based (conceptual) models are not clear or useful. He states that every model becomes a statistical model when the errors are rigorously and objectively examined by representing the errors as random variables having a probabilistic structure. Like many models, SLAMM has attributes that fit many of Linsley’s descriptors. Table 4.1 is a matrix showing these different attributes for different processes in the model. All components and processes in SLAMM have residual errors that cannot be completely explained through calibration. SLAMM therefore includes Monte Carlo simulation techniques and batch processing to consider this residual so model results reflect these uncertainties. Some of the model input parameters are directly measured, such as the areas and characteristics of the contributing areas in the watershed, and the pollutant associations with particulate solids from these areas. The rainfall–runoff components, particulate accumulation rates, and street cleaning effects are based on conceptual models, and have been extensively verified through many prior studies and do not require local measurements. Infiltration, grass swale, and detention pond effects are based on standard theoretical approaches that have also been verified under many conditions. Particulate washoff and catchbasin cleaning are based on statistical curve-fits, based on measured parameters (street dirt loading, street texture, flow rate, prior accumulation, etc.). Many of the processes are continuous in that variations in runoff, particulate loadings, water in ponds, water in infiltration devices, etc. are continuously modeled throughout the study period, with interevent effects on the device performance considered during subsequent wet-weather events. Other processes are only event based, in that field measurements in urban areas have not shown important or significant benefits of continuous simulations. Interestingly, rainfall–runoff processes are not continuously modeled in SLAMM, but are only based on conditions present at the time of rainfall initiation. Antecedent soil moisture has little effect on disturbed urban soils, compared to soil compaction, and the large amount of pavement dominating runoff processes for the common small and mediumsized rains that SLAMM was designed to simulate. SLAMM has been shown to predict runoff volumes very accurately for many rain types throughout the United States with this simplification. Runoff is converted to hydrograph representations where rate of flow changes have important effects on performance of control devices, such as detention ponds, swales, and infiltration devices. Use of SLAMM requires careful measurements of contributing areas and characteristics, from watershed surveys and aerial photographs. Calibrations of the rainfall–runoff, particulate accumulation and washoff processes, and pollutant associations are based on regional data. Model verification is based on a set of observed outfall events.

UNIQUE ATTRIBUTES OF SLAMM The following discusses two important aspects included in SLAMM that are incorrectly considered in most currently used stormwater models: the runoff predictions associated with small and moderate-sized events associated with the majority of receiving water problems, and the washoff of particulate pollutants from urban surfaces.

Source areas Development characteristics Rainfall-runoff Particulate accumulation Particulate washoff Pollutant associations Street cleaning Catchbasin cleaning Infiltration Grass swales Detention

Process or Input Parameters

7

9 9

Deterministic 1 1 2 3 2 3 3 2 2 2 1

Stochastic

7

8 7

Conceptual

8 8 9

Theoretical

8

8

Statistical

TABLE 4.1 Major Process Descriptions in SLAMM (attributes total 10 for each process)

Yes

Yes Yes Yes

Yes

N/A N/A

Continuous

Yes

Yes Yes

Yes

Event

Yes Yes Yes

Yes

Complete

Yes Yes

Yes

Simplified

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Percent Associated with Rain, or less

100

80

Accumulative Rain Count

60

Accumulative Runoff Quantity

40

20

0

0.1

1 Rain (inches)

FIGURE 4.1 Accumulative rain count and associated runoff volumes for medium density residential areas monitored in Milwaukee, WI. (Data from Bannerman, R. et al., PB84-114164, 1983.)

Percent Associated with Rain, or less

100 Pb 80

PO4 COD

60

SS

40

20

0

0.1

1 Rain (inches)

FIGURE 4.2 Accumulative pollutant loadings for medium density residential areas monitored in Milwaukee, WI. (Data from Bannerman, R. et al., PB84-114164, 1983.)

SMALL STORM HYDROLOGY One of the major problems with conventional stormwater models concerns runoff volume estimates associated with small and moderate-sized storms. Figures 4.1 and 4.2 show the importance of common small storms when considering total annual pollutant discharges. Figure 4.1 shows the accumulative rain count and the associated accumulative runoff volume for a medium density residential area in Milwaukee, Wisconsin, based on 1983 monitored data (Bannerman et al., 1983). This figure shows that the median rain, by count, was about 0.3 in. (7.5 mm), while the rain associated with the median runoff quantity is about 0.75 in. (20 mm). Therefore, more than half

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80 75 70 65 60 Rain

Discharge (cts)

55 50

Total Area

45 40 35 30 Other impervious surfaces 25 20 15 Street surfaces

10 5 0

Previous surfaces 0

25

50

75 100 Time (minutes)

125

150

FIGURE 4.3 Variable contributing areas in urban watersheds.

of the runoff from this common medium density residential area was associated with rain events that were smaller that 0.75 in. (20 mm). The 1983 rains (which were monitored during the Milwaukee NURP project) included several very large storms, which are also shown on Figure 4.1. These large storms (of 3 to 5 in., or 75 to 125 mm in depth) distort Figure 4.1 because, on average, the Milwaukee area only can expect one 3.5 in. (90 mm) storm every 5 years. In most years, these large rains would not occur and the significance of the smaller rains would be even greater Figure 4.2 shows the accumulative loadings of different pollutants (suspended solids, chemical oxygen demand, phosphates, and lead) monitored during 1983 in Milwaukee at the same site as the rain and runoff data shown in Figure 4.1 (Bannerman et al., 1983). When Figure 4.2 is compared with Figure 4.1, it is seen that the runoff and discharge distributions are very similar. This is a simple way of indicating that there were no significant trends of stormwater concentrations for different size events. There were substantial variations in pollutant concentrations observed, but they were random and not related to storm size. Similar conclusions were noted when all of the NURP data was evaluated (U.S. EPA, 1983). Therefore, accurately knowing the runoff volume is most important when studying pollutant discharges, not runoff flow rates. By better understanding the significance and runoff generation potential of these small rains, runoff problems would be better understood. By knowing the relative contributions of water and pollutants from each source area, it is possible to evaluate potential source area runoff controls for different rains. Figure 4.3 illustrates the concept of variable contributing areas as applied to urban watersheds. This figure indicates the relative significance of three major source areas (street surfaces, other impervious surfaces, and pervious surfaces) in an urban area. The individual flow rates associated with each of these source areas increase until their time of concentrations are met. The flow rate then remains constant for

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25 Maximum total losses 6.7 mm Initial losses 0.9 mm

Runoff (mm)

20

Maximum variable losses 5.8 mm 15 All losses satisfied at 13.5 mm runoff 10 5 0

0

5

10 15 Rain (mm)

20

25

FIGURE 4.4 Measured rainfall–runoff from a typical city street. (From Pitt, R., Ph.D. thesis, University of Wisconsin, Madison, 1987.)

each source area until the rain event ends. When the rain stops, runoff recession curves occur, draining the individual source areas. The three component hydrographs are then added together to form the complete hydrograph for the area. Calculating the percentage of the total hydrograph associated with each individual source area enables estimates of the relative importance of each source area to be quantified. The relative pollutant discharges from each area can then be calculated from the runoff pollutant strengths associated with each area. When the time of concentration and the rain duration are equal for an area, the maximum runoff rate for that rain intensity is reached. The time of concentration occurs when the complete drainage area is contributing runoff to the point of concern. If the rain duration exceeds the time of concentration, then the maximum runoff rate is maintained until the rain ends. When the rain ends, the runoff rate decreases according to a recession curve for that surface. The example shown in Figure 4.3 is for a rain duration greater than the times of concentrations for the street surfaces and other impervious areas, but shorter than the time of concentration for the pervious areas. Similar runoff quantities originated from each of the three source areas for this example. If the same rain intensity occurs, but lasts for twice the duration (a less frequent storm), the runoff rates for the street surfaces and other impervious surfaces will be the same until the end of the rain, when their recession curves would begin. However, the relative runoff contribution from the pervious surfaces would increase substantially. If the same rain intensity occurs, but only for half of the original duration, the street surfaces time of concentration is barely met, and the other impervious surfaces would not have reached their time of concentration. In this last example, the pervious surfaces would barely begin to cause runoff, and the street surfaces are the dominant source of runoff water. Figure 4.4 shows monitored rainfall–runoff results from one of a series of tests conducted to investigate runoff losses associated with common small rains on pavement (Pitt, 1987). This figure indicates that initial abstractions (measured to be detention storage associated with street texture and pavement slope) for this pavement totaled about 0.04 in. (1 mm), while the total rainfall losses were about 0.25 in. (6 mm). The other losses after the initial abstractions were mostly associated with infiltration through the relatively thin and porous pavement material and through cracks and seams. These maximum losses occurred after about 0.8 in. (20 mm) of rain. For a relatively small rain of about 0.3 in. (7 mm), almost one half of the rain falling on this pavement did not contribute to runoff. During smaller storms, the majority of the rainfall did not contribute to runoff. These rainfall losses for pavement are similar for most city streets and are substantially greater than commonly considered in stormwater models. Runoff yields from large expanses of pavement (such as parking areas) and for high-use roadways (highways) are much greater than for most roadways. Large parking areas have minimal infiltration losses because of the long horizontal flow distances

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to the edge of the pavement, while the thicker and denser pavements of high-use roadways allow only minimal amounts of water infiltration. Only special pavement base materials are capable of allowing significant water infiltration. Normally, the pavement bases therefore typically act as the “aquaclude” for pavement structures. The water entering a pavement is therefore restricted to the storage volume in the pavement, plus the effects of the drainage of water from the pavement. Inpavement storage volume is usually very small. For relatively narrow streets, pavement drainage through the pavement edges (following Darcy’s law) allows more rainfall losses than for the longer flow paths associated with parking lots, for example. Most stormwater models use rainfall–runoff relationships that have been developed and used for many years for drainage design. Drainage design is concerned with rain depths of at least several inches (hundreds of millimeters). When these same procedures are used to estimate the runoff associated with common small storms (which are the most important in water quality investigations), the runoff predictions can be highly inaccurate. As an example, the volumetric runoff coefficient (the ratio of the runoff to the rain depth) observed at outfalls varies for each rain depth. This ratio can be about 0.1 for storms of about 0.5 in. (12 mm) but may approach about 0.4 for a moderate size storm of 2.5 in. (65 mm) or greater, which is typically associated with drainage events for medium density residential areas. However, the NURP study (U.S. EPA, 1983) recommended the use of constant (average) volumetric runoff coefficients for the stormwater permit process. Therefore, common small storms would likely have their runoff volumes overpredicted. During recent research on the infiltration rates of disturbed urban soils, it was found that compaction was much more significant than moisture for many conditions (Pitt et al., 1999b). Figures 4.5 and 4.6 are three-dimensional plots of the observed infiltration data, illustrating effects

8

Infiltration Rate (in/

hr)

10

6 4

d

ompa

ct

Comp

act

tur ate

Nonc

Sa

0

Dr

y

2

FIGURE 4.5 Effects of compaction and moisture on clayey urban soils. (From Pitt, R. et al., 1999b.)

20

hr) Infiltration Rate (in/

15 10

ompa

ct

Comp

tur ate

Nonc

act

Sa

0

d

Dr

y

5

FIGURE 4.6 Effects of compaction and moisture on sandy urban soils. (From Pitt, R. et al., 1999b.)

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125 ter en c ) ys ing al nti alle pp e o h d i t sh es l (wi yr sit ntia n n i e a de h d esi =r hig ity r off ys) n d s alle n ru out l a den h t a i i l (w erc dium ntia mm me reside co y sit den ium d e m

Runoff, mm

100 75 50 25 0

0

100

50

125

medium density residential (with alleys)

90 med

ium

85 80 75

75 100 Rain, mm

shopping center commercial and high den sity residential

95 CN

25

0

25

den

sity

resid

entia

l (wit

50 75 Rain, mm

hou

t alle ys)

100

125

FIGURE 4.7 Actual NRCS curve numbers from monitored Milwaukee, WI. (From Bannerman, R. et al., PB84-114164, 1983.)

of soil-water levels and compaction, for both sand and clay. Four general conditions were observed to be statistically unique. Compaction has the greatest effect on infiltration rates in sandy soils, with little detrimental effects associated with higher soil-water content. Clay soils, however, are affected by both compaction and soil-water content. Compaction was seen to have about the same effect as saturation on these soils, with saturated and compacted clayey soils having very little effective infiltration. Therefore, if commonly occurring compaction was ignored, runoff from pervious areas could be overpredicted. Figure 4.7 shows the actual calculated Natural Resources Conservation Service (NRCS) curve numbers (CN) associated with different storms at a medium density residential site in Milwaukee (SCS, 1986). This figure shows that the actual CN values vary dramatically for the different rain depths that actually occurred at this site. The actual CN values approach the CN values that would be selected for this type of site only for rains greater than several inches (hundreds of millimeters) in depth. The actual CN values are substantially greater for the smaller common storms, especially for rains less than the 1 in. (25 mm) minimum rain criteria given by NRCS (SCS, 1986) for the use of this procedure. These results are similar to those obtained at many other sites. In almost all cases, the CN values for storms of less than a 0.5 in. (12 mm) are 90, or greater. Therefore, the smaller storms actually contribute much more runoff than would typically be assumed if using NRCS procedures. The curve number method was initially developed, and is most appropriate, for use in the design of drainage systems associated with storms of much greater size than those of interest in stormwater quality investigations. SLAMM makes runoff predictions using the small storm hydrology methods developed by Pitt (1987). Figure 4.8 shows the verification of the small storm hydrology method used in SLAMM for storms from a commercial area in Milwaukee. This figure shows that the calculated runoff for many storms over a wide range of conditions was very close to the actual observed runoff. Figure 4.9 shows a similar plot of the predicted vs. observed runoff for a Milwaukee medium density residential

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Calculated Runoff [in]

10.00

1.00

.10

.01 .01

.1 1 Observed Runoff [in]

10

FIGURE 4.8 Verification of the small storm hydrology components of SLAMM for a commercial site in Milwaukee, WI.

Predicted Total Runoff [in]

10

1

.1

.01 .01

.1 1 Observed Runoff [in]

10

FIGURE 4.9 Verification of the small storm hydrology components of SLAMM for a medium residential area in Milwaukee, WI.

area. These two sites were substantially different from each other in the amount of impervious surfaces and how these areas were connected to the drainage system. Similar satisfactory comparisons using these small storm hydrology models for a wide range of rain events have been made for other locations, including Portland, Oregon (Sutherland, 1993) and Toronto, Canada (Pitt and McLean, 1986).

PARTICULATE WASHOFF Another unique feature of SLAMM is its correct use of a washoff model to predict the losses of suspended solids from different surfaces. SLAMM calculates suspended solids washoff based on individual first-flush (exponential) relationships for each surface. These relationships were derived from observations during both controlled tests and during actual rains for individual homogeneous surfaces (Pitt and McLean, 1986; Pitt, 1987). These washoff relationships have been verified during runoff observations from large and complex drainages (Pitt, 1987). Figure 4.10 shows washoff plots for total solids, suspended solids (>0.45 µm), and dissolved solids (<0.45 µm) during an example

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13.8 g/m2

5.0 3.0 2.0 1.5 1.0 0.8 0.5 0.4 0

5

10

15

20

1.3 12.6 g/m2

10.0 8.0

Filterable residue washoff (g/m2)

Total residue washoff (g/m2)

15.0 10.0 8.0

Non-filterable residue washoff (g/m2)

90

5.0 3.0 2.0 1.5 1.0 0.8 0.5 0.4

1.2 g/m2

1.0 0.8

0.5

0.3 0.2 0.15 0.1

0

5

10

15

20

0

5

10

15

20

Rain (mm)

FIGURE 4.10 Washoff plots for HDS test (high rain intensity, dirty, and smooth street). (From Pitt, R., Ph.D. thesis, University of Wisconsin, Madison, 1987.)

controlled street surface washoff test (Pitt, 1987). These plots indicate the accumulative (gram per square meter, g/m2) washoff as a function of rain depth. Also shown on these plots are the total street dirt loadings. As an example, 13.8 g/m2 of total solids were on the street surfaces before the controlled rain event. After about 15 mm of rain fell on the test sites, almost 90% of the particulates that would wash off (about 3 g/m2) did wash off, similar to the rain depth needed for “complete” washoff as reported by earlier studies by Sartor and Boyd (1972). However, the total quantity of material that could possibly wash off (about 3 g/m2) is a small fraction of the total loading that was on the street (13.8 g/m2). If the relationship between total available loading and total loading of particulates is not considered (as in many stormwater models), then the predicted washoff would be greatly in error. Figure 4.10 also shows washoff of the smallest particle sizes (“dissolved solids,” <0.45 µm) as a function of total rain. Here the total loading of the filterable solids on the streets was only about 1 g/m2 and almost all of these small particles were available for washoff during these rains. Figure 4.10 also shows the washoff of the largest particles (“suspended solids,” >0.45 µm) on the street. Here, the street loading was 12.6 g/m2, with only about 1.8 g/m2 available for washoff. The predicted washoff of suspended solids could be in error by 700% if the total loading on the street was assumed to be removable by rains. SLAMM uses test results from Pitt (1987) that measured the washoff and street dirt loading availability relationships for many street surfaces, rain intensities, and street dirt loadings to predict the amount of washoff more accurately. Another common problem with stormwater models is the use of incorrect particulate accumulation rates for different surfaces. Figure 4.11 shows an example of the accumulation and deposition of street surface particulates for two residential areas monitored in San Jose, California (Pitt, 1979). The two areas were very similar in land use, but the street textures were quite different. The goodcondition asphalt streets were quite smooth, while the oil and screens overlaid streets were very rough. Immediately after intensive street cleaning, the rough streets still had substantial particulate loadings, while the smooth streets had substantially less. The accumulation of debris on the streets also increased the street dirt loadings over time. The accumulation rates were very similar for these two different streets having the same land uses. However, the loadings on the streets at any time were quite different because of the greatly different initial loading values (permanent storage loadings). If infrequent street dirt loading observations are made, the true shape of the accumulation rate curve may not be accurately known. As an example, the early Sartor and Boyd (1972) test results that have been used in many stormwater models assumed that the initial loading values after rains were close to zero, instead of the actual substantial initial loadings. The accumulation rates

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2,500 Lost to Air

Total Solids Street Loading (lb/curb - mi)

Deposition 2,000 Accumulation

OIL/SCREENS 1,500

1,000 Lost to Air Deposition

500

Accumulation 0

0

5

10 15 20 25 Days Since Last Cleaned

30

FIGURE 4.11 Deposition and accumulation of street dirt.

were calculated by using the slope between each individual loading value and the origin (zero time and zero loading), rather than between loadings from adjacent sampling times. This can easily result in accumulation rates many times greater than actually occurred. The street dirt deposition rates were found to only be a function of the land uses, but the street dirt loadings were a function of the land use and street texture. The accumulation rates slowly decreased as a function of time and eventually became zero, with the loading remaining constant, after a period of about 1 month of either no street cleaning or no rains. Figure 4.11 shows that the deposition and accumulation rates on the streets were about the same until about 1 or 2 weeks after a rain. If the streets were not cleaned for longer periods, then the accumulation rate decreased because of fugitive dust losses of street dirt to surrounding areas by winds or vehicle turbulence. In most areas of the United States (having rains at least every week or two), the actual accumulation of material on street surfaces is likely constant, with little fugitive dust losses (Pitt, 1979). SLAMM includes a large number of street dirt accumulation and deposition rate relationships that have been obtained for many monitoring sites throughout the United States and in Canada. The accumulation rates are a function of the land uses, whereas the initial loadings on the streets are a function of street texture. The decreasing accumulation rate is also a function of the time after a street cleaning or large rain event.

SLAMM COMPUTATIONAL PROCESSES In most urban areas, there are a wide variety of drainage systems from concrete curb and gutters to grass swales, along with directly connected roof drainage systems and drainage systems that drain to pervious areas. Development characteristics define the magnitude of these drainage efficiency attributes, along with the areas associated with each surface type (road surfaces, roofs, landscaped areas, etc.). The use of SLAMM shows that these characteristics greatly affect runoff quality and quantity. Land use alone is usually not sufficient to describe these characteristics. The types of the drainage system (curbs and gutters or grass swales) and roof connections (directly

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connected or draining to pervious area) are probably the most important attributes affecting runoff characteristics. These attributes are not directly related to land use, but some trends are obvious: most roofs in strip commercial and shopping center areas are directly connected, and the roadside is most likely drained by curbs and gutters, for example. Different land uses, of course, are also associated with different levels of pollutant generation. For example, industrial areas usually have the greatest pollutant accumulations due to material transfer and storage, and heavy truck traffic. SLAMM uses the water volume and suspended solids concentrations at the outfall to calculate the other pollutant concentrations and loadings. SLAMM keeps track of the portion of the total outfall suspended solids loading and runoff volume that originated from each source area. The suspended solids fractions are then used to develop weighted loading factors associated with each pollutant. In a similar manner, dissolved pollutant concentrations and loadings are calculated based on the percentage of water volume that originates from each of the source areas within the drainage system. SLAMM predicts urban runoff discharge parameters (total storm runoff flow volume, flowweighted pollutant concentrations, and total storm pollutant yields) for many individual storms and for the complete study period. It has built-in Monte Carlo sampling procedures to consider many of the uncertainties common in model input values. This enables the model output to be expressed in probabilistic terms that more accurately represent the likely range of results expected. Early versions of SLAMM only used average concentration factors for different land use areas and source areas. This was satisfactory for predicting the event mean concentrations (EMCs), as used by NURP (U.S. EPA, 1983) for an extended period of time and in calculating the unit area loadings for different land uses. However, to predict the probability distributions of the concentrations, it was necessary to include probability information for the concentrations found in the different source areas. Statistical analyses of concentration data (attempting to relate concentration trends to rain depths and season, for example) from these different source areas have not been able to explain all of the variation in concentrations that have been observed (Pitt et al., 1999c). The statistical analyses also indicate that most pollutant concentration values from individual source areas are distributed lognormally (U.S. EPA, 1983). Therefore, lognormally distributed random concentration values are used in SLAMM for these different areas. The result is much more reasonable predictions for concentration distributions at the outfall when compared to actual observed conditions. This provides more accurate estimates of criteria violations for different stormwater pollutants at an outfall for long continuous simulations.

USE OF SLAMM TO IDENTIFY POLLUTANT SOURCES AND TO EVALUATE DIFFERENT CONTROL PROGRAMS Table 4.2 is a field sheet that has been developed to assist users of SLAMM describe test watershed areas. This sheet is mostly used to evaluate stormwater control retrofit practices in existing developed areas, and to examine how different new development standards affect runoff conditions. Much of the information on the sheet is not actually required to operate SLAMM, but can be important when considering additional control programs (such as public education and good housekeeping practices) that are not quantified by SLAMM. The most important information shown on this sheet is the land use, the type of the gutter or drainage system, and the method of drainage from roofs and large paved areas to the drainage system. The efficiency of drainage in an area, specifically if roof runoff or parking runoff drains across grass surfaces, can be very important when determining the amount of water and pollutants that enter the outfall system. Similarly, the presence of grass swales in an area may substantially reduce the amount of pollutants and water discharged. This information is therefore required to use SLAMM. The areas of the different surfaces in each land use is also very important for SLAMM. Figure 4.12 is an example showing the areas of different surfaces for a medium density residential

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TABLE 4.2 Study Area Description Field Sheet Location: Site number: Date: Time: Photo numbers: Roll number: Land-use and industrial activity: Residential: low medium high density single family multiple family trailer parks high rise apartments Income level: low medium high Age of development: <1930 ‘30-’50 ‘51-’70 ‘71-’80 new Institutional: school hospital other (type): Commercial: strip shop. center downtown hotel offices Industrial: light medium heavy (manufacturing) describe: Open space: undeveloped park golf cemetery Other: freeway utility ROW railroad ROW other: Maintenance of building: excellent moderate poor 1 2 3 4+ stories Heights of buildings: Roof drains: underground gutter impervious pervious Roof types: flat comp. shingle wood shingle other: Sediment source nearby? No Yes (describe): Treated wood near street? No telephone poles fence other: Landscaping near road: quantity: None some much type: deciduous evergreen lawn maintenance: excessive adequate poor leafs on street: none some much Topography: street slope: flat (<2%) medium (2-5%) steep (>5%) land slope: flat (<2%) medium (2-5%) steep (>5%) Traffic speed: <25 mph 25-40 mph >40 mph Traffic density: Light moderate heavy Parking density: none light moderate heavy Width of street: number of parking lanes: number of driving lanes: Condition of street: good fair poor asphalt concrete unpaved Texture of street: Driveways: paved unpaved condition: good fair poor texture: smooth intermediate rough Gutter material: grass swale lined ditch concrete asphalt condition: good fair poor street/gutter interface: smooth fair uneven Litter loadings near street: clean fair dirty Parking/storage areas (describe): condition of pavement: good fair poor texture of pavement: smooth intermediate rough unpaved Other paved area (such as alleys and playgrounds), describe: condition: good fair poor texture: smooth intermediate rough Notes:

93

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6000 5000

m2/ha

4000 3000 2000 1000

Rooftops

Decks/Sheds

Pools

Backyard

Walkways

Driveways

Frontyard

Sidewalk

Grass Strip

Street

0

Percentage Flow Contribution

FIGURE 4.12 Source areas — Milwaukee medium density residential areas (without alleys). (From Pitt, R., Ph.D. thesis, University of Wisconsin, Madison, 1987.) 100 90 80 70 60 50 40 30 20 10 0 0.01

Roofs Landscaped Areas Driveways

Streets

0.1

1.0

4.0

Rain Depth (inches)

FIGURE 4.13 Flow sources for example medium density residential area having clayey soils. (From Pitt, R. and Voorhees, J., EPA/625/R-95/003, 1995.)

area in Milwaukee. As shown in this example, streets make up between 10 and 20% of the total area, while landscaped areas can make up about half of the drainage area. The variation of these different surfaces can be very large within a designated area. The analysis of many candidate areas may therefore be necessary to understand how effective or how consistent the model results may be for a general land use classification. One of the first problems in evaluating an urban area for stormwater controls is the need to understand where the pollutants of concern are originating under different rain conditions. Figure 4.13 is an example for a typical medium density residential area showing the percentage of runoff originating from different major sources, as a function of rain depth. For storms of up to about 0.1 in. in depth, street surfaces contribute about one half to the total runoff to the outfall. This contribution decreased to about 20% for storms greater than about 0.25 in. in depth. This decrease in the significance of streets as a source of water is associated with an increase of water contributions from landscaped areas (which make up more than 75% of the area and have clayey soils). Similarly, the significance of runoff from driveways and roofs also starts off relatively high

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95

and then decreases with increasing storm depth. Obviously, this is just an example plot and the source contributions would vary greatly for different land uses/development conditions, rainfall patterns, and the use of different source area controls. A major use of SLAMM is to promote better understanding of the role of different sources of pollutants. As an example, to control suspended solids, street cleaning (or any other method to reduce the washoff of particulates from streets) may be very effective for the smallest storms, but would have very little benefit for storms greater than about 0.25 in. in depth. However, erosion control from landscaped surfaces may be effective over a wider range of storms. The following list shows the different control programs that were investigated in this hypothetical medium density residential area: • • • • • • • • •

Base level (as built in 1961 to 1980 with no additional controls) Catchbasin cleaning Street cleaning Grass swales Roof disconnections Wet detention pond Catchbasin and street cleaning combined Roof disconnections and grass swales combined All of the controls combined

This residential area, which was based upon actual Birmingham, Alabama field observations for homes built between 1961 and 1980, has no controls, including no street cleaning or catchbasin cleaning. The use of catchbasin cleaning in the area, in addition to street cleaning was evaluated. Grass swale use was also evaluated, but swales are an unlikely retrofit option, and would only be appropriate for newly developing areas. However, it is possible to disconnect some of the roof drainages and divert the roof runoff away from the drainage system and onto grass surfaces for infiltration in existing developments. In addition, wet detention ponds can be retrofitted in different areas and at outfalls. Besides those controls examined individually, catchbasin and street cleaning controls combined were also evaluated, in addition to the combination of disconnecting some of the rooftops and the use of grass swales. Finally, all of the controls together were also examined. The following list shows a general description of this area: • • • • • •

All curb and gutter drainage (in fair condition) 70% of roofs drain to landscaped areas 50% of driveways drain to lawns 90% of streets are intermediate texture (remaining are rough) No street cleaning No catchbasins

About one half of the driveways currently drain to landscaped areas, while the other half drain directly to the pavement or the drainage system. Almost all of the streets are of intermediate texture, and about 10% are rough textured. As noted earlier, there currently is no street cleaning or catchbasin cleaning. The level of catchbasin use that was investigated for this site included 950 ft3 of total sump volume per 100 acres (typical for this land use), with a cost of about $50 per catchbasin cleaning. Typically, catchbasins in this area could be cleaned about twice a year for a total annual cost of about $85 per acre of the watershed. Street cleaning could also be used with a monthly cleaning effort for about $30 per year per watershed acre. Light parking and no parking restrictions during cleaning are assumed, and the cleaning cost is estimated to be $80 per curb mile.

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Grass swale drainage was also investigated, assuming that swales could be used throughout the area, there could be 350 ft of swales per acre (typical for this land use), and the swales were 3.5 ft wide. Because of the clayey soil conditions, an average infiltration rate of about 0.5 in./h was used in this analysis, based on many different double ring infiltrometer tests of typical soil conditions. Swales cost much less than conventional curb and gutter systems, but have an increased maintenance frequency. Again, the use of grass swales is appropriate for new development, but not for retrofitting in this area. Roof disconnections could also be utilized as a control measure by directing all roof drains to landscaped areas. The objective would be to direct all the roof drains to landscaped areas. Because 70% of the roofs already drain to the landscaped areas, only 30% could be further disconnected, at a cost of about $125 per household. The estimated total annual cost would be about $10 per watershed acre. An outfall wet detention pond suitable for 100 acres of this medium density residential area would have a wet pond surface of 0.5% of the drainage area to provide about 90% suspended solids control. It would need 3 ft of dead storage and live storage equal to runoff from a 1.25-in. rain. A 90° V notch weir and a 5-ft-wide emergency spillway could be used. No seepage or evaporation was assumed. The total annual cost was estimated to be about $130 per watershed acre. Table 4.3 summarizes the SLAMM results for runoff volume, suspended solids, filterable phosphate, and total lead for 100 acres of this medium density residential area. The only control practices evaluated that would reduce runoff volume are the grass swales and roof disconnections. All of the other control practices evaluated do not infiltrate stormwater. Table 4.3 also shows the total annual average volumetric runoff coefficient (Rv) for these different options. The base level condition has an annual flow-weighted Rv of about 0.3, while the use of swales would reduce the Rv to about 0.1. Only a small reduction of Rv (less than 10%) would be associated with complete roof disconnections compared to the existing situation because of the large number of roof disconnections that already occur. The suspended solids analyses shows that catchbasin cleaning alone could result in about 14% suspended solids reductions. Street cleaning would have very little benefit, while the use of grass swales would reduce the suspended solids discharges by about 60%. Grass swales would have minimal effect on the reduction of suspended solids concentrations at the outfall (they are primarily an infiltration device, having very little filtering benefits). Wet detention ponds would remove about 90% of the mass and concentrations of suspended solids. Similar observations can be made for filterable phosphates and lead. Figures 4.14 through 4.17 show the maximum percentage reductions in runoff volume and pollutants, along with associated unit removal costs. As an example, Figure 4.14 shows that roof disconnections would have a very small potential maximum benefit for runoff volume reduction and at a very high unit cost compared to the other practices. The use of grass swales could have about a 60% reduction at minimal cost. The use of roof disconnection plus swales would slightly increase the maximum benefit to about 65%, at a small unit cost. Obviously, the use of roof disconnections alone, or all controlled practices combined, is very inefficient for this example. For suspended solids control, catchbasin cleaning and street cleaning would have minimal benefit at high cost, while the use of grass swales would produce a substantial benefit at very small cost. However, if additional control is necessary, the use of wet detention ponds may be necessary at a higher cost. If close to 95% reduction of suspended solids were required, then all of the controls investigated could be used together, but at substantial cost.

FUTURE DIRECTIONS FOR SLAMM Recent U.S. EPA-funded research has developed a framework for future modifications to the SLAMM model. Emerging control technologies (especially for critical source area controls in ultraurban areas) have included inlets and inlet inserts (Pitt and Field, 1998), stormwater filtration

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TABLE 4.3 SLAMM Predicted Runoff and Pollutant Discharge Conditions for Examplea Birmingham 1976 rains: (112 rains, 55 in. total 0.01–3.84 in. each)

Runoff Volume Annual Flow-wtg. ft3/acre Rv

Base (no controls) 59800 59800 Catchbasin cleaning 0 Reduction (lb or ft3) 0 Reduction (%) N/A 3 Cost ($/lb or $/ft ) ($85/acre/yr) Street cleaning 59800 0 Reduction (lb or ft3) 0 Reduction (%) N/A Cost ($/lb or $/ft3) ($30/acre/yr) Grass swales 23300 36500 Reduction (lb or ft3) 61 Reduction (%) Minimal Cost ($/lb or $/ft3) ($minimal/acre/yr) 56000 Roof disconnections 3800 Reduction (lb or ft3) 6 Reduction (%) 0 Cost ($/lb or $/ft3) ($10/acre/yr) Wet detention pond 59800 0 Reduction (lb or ft3) 0 Reduction (%) N/A 3 Cost ($/lb or $/ft ) ($130/acre/yr) CB and street cleaning 59800 0 Reduction (lb or ft3) 0 Reduction (%) N/A Cost ($/lb or $/ft3) ($115/acre/yr) 20900 Roof dis. and swales 38900 Reduction (lb or ft3) 65 Reduction (%) 0.00026 Cost ($/lb or $/ft3) ($10/acre/yr) All above controls 20900 38900 Reduction (lb or ft3) 65 Reduction (%) 0.0066 Cost ($/lb or $/ft3) ($225/acre/yr)

0.3 0.3

Suspended Solids CN Range

Flow-wtg. Annual mg/l lb/acre

77–100 77–100

385 331 14

0.3

77–100

385 0

0.12

63–100

380 1

0.28

76–100

410 –6

0.3

77–100

49 87

0.3

77–100

331 14

0.1

63–100

403 –5

0.1

63–100

a

42 89

Filterable Phosphate

Total Lead

Flow-wtg. µg/l

Annual lb/acre

1430 1230 200 14 0.43

157 157

0.58 0.58 0 0 N/A

543 468

1430 0 0 N/A

157

0.58 0 0 N/A

543

554 876 61 Minimal

151

0.22 0.36 62 Minimal

513

1430 0 0 N/A

156

0.55 0.03 5 333

443

185 1250 87 0.10

157

0.58 0 0 N/A

69

1230 200 14 0.58

157

0.58 0 0 N/A

468

526 904 63 0.01

139

0.18 0.40 69 25

352

55 1375 96 0.19

139

0.18 0.40 69 638

36

0

0

4

1

0

0

11

11

Flow-wtg. Annual µg/l lb/acre

14

0

6

18

87

14

35

93

2.0 1.7 0.29 14 293 2.0 0.01 0.49 3000 0.75 1.28 63 Minimal 1.6 0.48 24 21 0.26 1.8 87 73 1.7 0.29 14 397 0.46 1.6 77 6.4 0.05 1.98 97 129

Medium density residential area, developed in 1961–1980, with clayey soils (curbs and gutters); new development controls (not retrofit). Source: Pitt, R. and Voorhees, J., EPA/625/R-95/003, 1995.

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$/cubic foot reduction

0.007 all controls

0.006 0.005 0.004 0.003 0.002

roof disconnections

0.001 0.000 0

grass swales 10

20

30

40

50

roof disc. + swales 60

70

80

90

100

Maximum percentage runoff volume reduction

$/pound suspended solids reduction

FIGURE 4.14 Cost-effectiveness data for runoff volume reduction benefits. (From Pitt, R. and Voorhees, J., EPA/625/R-95/003, 1995.)

0.7 0.6

CB and street cleaning

0.5 0.4

catchbasin cleaning

0.3

all controls

0.2 roof disc. and grass grass swales swales

0.1 0.0 0

wet detention

10 20 30 40 50 60 70 80 90 Maximum percentage runoff volume reduction

100

$/pound dissolved phosphate reduction

FIGURE 4.15 Cost-effectiveness data for suspended solids reduction benefits. (From Pitt, R. and Voorhees, J., EPA/625/R-95/003, 1995.)

700 all controls

600 500 400 300

roof disconnections

200 100 0 0

grass swales

roof disc. and grass swales

10 20 30 40 50 60 70 80 90 100 Maximum percentage dissolved phosphate reduction

FIGURE 4.16 Cost-effectiveness data for dissolved phosphate reduction benefits. (From Pitt, R. and Voorhees, J., EPA/625/R-95/003, 1995.)

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$/pound total lead reduction

SLAMM, the Source Loading and Management Model

99

500 400

CB and street cleaning

300

catchbasin cleaning

200 100 0 0

all controls roof disconnections

grass swales

roof disc. wet and grass swales detention

10 20 30 40 50 60 70 80 90 Maximum percentage total lead reduction

100

FIGURE 4.17 Cost-effectiveness data for total lead reduction benefits. (From Pitt, R. and Voorhees, J., EPA/625/R-95/003, 1995.)

(Clark and Pitt, 1999), and treatment trains (Pitt et al., 1999a). The information obtained during these projects is being used to modify SLAMM to include these control technologies. In addition, U.S. EPA-funded research on infiltration in disturbed urban soils and demonstrations of infiltration benefits through soil amendments (Pitt et al., 1999b) is being used to further advance the urban hydrology aspects of the model. Finally, SLAMM is being modified to enable its integration with SWMM to more accurately consider the benefits of source area controls for stormwater quality objectives on drainage objectives (Pitt et al., publication pending). This project will basically substitute the RUNOFF Block in SWMM with SLAMM to account better for small storm processes and to take advantage of its greater flexibility in evaluating source area flow and pollutant controls. The SWMM EXTRAN and TRANSPORT blocks will be used to simulate the performance of the drainage system. The resulting model will enable more efficient and effective evaluations than either alone. Overall, Pitt et al. (publication pending) developed an improved methodology to design wet weather flow drainage systems that considers both water quality and drainage benefits. A review of past, present, and emerging control technologies was conducted to present suitable combinations of practices that may be most suitable for many different conditions. Additional information concerning SLAMM is available at www.WINSLAMM.com.

REFERENCES Bachhuber, J.A., 1996. A decision making approach for stormwater management measures: a case example in the City of Waukesha, Wisconsin, in North American Water and Environment Congress, American Society of Civil Engineers. Reston, VA, C-184-1. Bannerman, R., Baun, K., Bohn, M., Hughes, P.E., and Graczyk, D.A., 1983. Evaluation of Urban Nonpoint Source Pollution Management in Milwaukee County, Wisconsin, Vol. I, Grant P005432-01-5, PB 84114164. U.S. Environmental Protection Agency, Water Planning Division, November. Bannerman, R.T., Legg, A.D., and Greb, S.R., 1996. Quality of Wisconsin Stormwater, 1989–94. U.S. Geological Survey. Open-file report 96-458. Madison, WI, 26. Clark, S. and Pitt, R., Stormwater Treatment at Critical Areas: Evaluation of Filtration Media for Stormwater Treatment, U.S. Environmental Protection Agency, Water Supply and Water Resources Division, National Risk Management Research Laboratory. Cincinnati, OH, 442 pp. Haubner, S.M. and Joeres, E.F., 1996. Using a GIS for estimating input parameters in urban stormwater quality modeling, Water Resour. Bull., 32(6), 1341–1351, December. Kim, K. and Ventura, S., Large-scale modeling of urban nonpoint source pollution using a geographical information system, Photogramm. Eng. Remote Sensing, 59(10), 1539–1544. Kim, K., Thum, P.G., and Prey, J., 1993. Urban non-point source pollution assessment using a geographical information system, J. Environ. Manage., 39(39), 157–170.

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Legg, A.D., Bannerman, R.T., and Panuska, J., 1996. Variation in the Relation of Rainfall to Runoff from Residential Lawns in Madison, Wisconsin, July and August 1995, U.S. Geological Survey, WaterResources Investigations Report 96-4194, Madison, WI, 11 pp. Linsley, R.K., 1982. Rainfall-runoff models — an overview, in Rainfall-Runoff Relationships, V.P. Singh, Ed., Water Resources Publications, Highlands Ranch, CO. Ontario Ministry of the Environment, 1986. Humber River Water Quality Management Plan, Toronto Area Watershed Management Strategy, Toronto, Ontario. Pitt, R., 1979. Demonstration of Nonpoint Pollution Abatement through Improved Street Cleaning Practices, EPA-600/2-79-161, U.S. Environmental Protection Agency, Cincinnati, OH. Pitt, R., 1984. Characterization, Sources, and Control of Urban Runoff by Street and Sewerage Cleaning, Contract R-80597012, U.S. Environmental Protection Agency, Office of Research and Development, Cincinnati, OH. Pitt, R., 1986. Runoff controls in Wisconsin’s priority watersheds, in Proceedings, Conference on Urban Runoff Quality — Impact and Quality Enhancement Technology, Henniker, NH, B. Urbonas and L.A. Roesner, Eds., American Society of Civil Engineering, New York, June. Pitt, R., 1987. Small Storm Flow and Particulate Washoff Contributions to Outfall Discharges, Ph.D. dissertation, Department of Civil and Environmental Engineering, the University of Wisconsin–Madison, November. Pitt, R. and Bissonnette, P., 1984. Bellevue Urban Runoff Program, Summary Report, Storm and Surface Water Utility, Bellevue, WA, November. Pitt, R. and Bozeman, M., 1982. Sources of Urban Runoff Pollution and Its Effects on an Urban Creek, EPA600/S2-82-090, U.S. Environmental Protection Agency, Cincinnati, OH, December. Pitt, R. and Field, R., 1998. An evaluation of storm drainage inlet devices for stormwater quality treatment, presented at Water Environment Federation 71st Annual Conference & Exposition, WEFTEC Technology Forum, Orlando, FL, October. Pitt, R. and McLean, J., 1986. Toronto Area Watershed Management Strategy Study — Humber River Pilot Watershed Project, Ontario Ministry of the Environment, Toronto, Ontario, June. Pitt, R. and Shawley, G., 1982. A Demonstration of Non-Point Source Pollution Management on Castro Valley Creek, Alameda County Flood Control and Water Conservation District (Hayward, CA) for the Nationwide Urban Runoff Program, U.S. Environmental Protection Agency, Water Planning Division, Washington, D.C., June. Pitt, R. and Voorhees, J., 1995. Source loading and management model (SLAMM), Seminar Publication: National Conference on Urban Runoff Management: Enhancing Urban Watershed Management at the Local, County, and State Levels. March 30–April 2, 1993, Center for Environmental Research Information, U.S. Environmental Protection Agency, EPA/625/R-95/003, Cincinnati, OH, 225–243, April. Pitt, R., Robertson, B., Barron, P., Ayyoubi, A., and Clark, S., 1999a. Stormwater Treatment at Critical Areas: The Multi-Chambered Treatment Train (MCTT), U.S. Environmental Protection Agency, Wet Weather Flow Management Program, National Risk Management Research Laboratory, EPA/600/R-99/017, Cincinnati, OH, 505 pp. Pitt, R., Lantrip, J., Harrison, R., Henry, C., and Hue, D., 1999b. Infiltration through Disturbed Urban Soils and Compost-Amended Soil Effects on Runoff Quality and Quantity, U.S. Environmental Protection Agency, Water Supply and Water Resources Division, National Risk Management Research Laboratory, Cincinnati, OH, 338 pp. Pitt, R., Lilburn, M., Nix, S., Durrans, S.R., Burian, S., Voorhees, J., and Martinson, J., publication pending. Guidance Manual for Integrated Wet Weather Flow (WWF) Collection and Treatment Systems for Newly Urbanized Areas (New WWF Systems), U.S. Environmental Protection Agency, 612 pp. Sartor, J.D. and Boyd, G.B., 1972. Water Pollution Aspects of Street Surface Contaminants, EPA-R2-72-081, U.S. Environmental Protection Agency, November. SCS (U.S. Soil Conservation Service, now Natural Resources Conservation Service), 1986. Urban Hydrology for Small Watersheds, U.S. Department of Agriculture Technical Release 55 (revised), June. Sutherland, R., 1993. Portland Stormwater Quality using SIMPTM, Draft report, OTAK, Inc. Lake Oswego, OR. Thum, P.G., Pickett, S.R., Niemann, B.J., Jr., and Ventura, S.J., 1990. LIS/GIS: integrating nonpoint pollution assessment with land development planning, Wisc. Land Inf. Newsl., University of Wisconsin, Madison. 2, 1–11.

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Troutman, B.M., 1985. Errors and parameter estimation in precipitation-runoff modeling: 2-case study, Water Resour. Res., 21(8), 1195–1213. U.S. Environmental Protection Agency, 1983. Final Report for the Nationwide Urban Runoff Program, Water Planning Division, Washington, D.C., December. Ventura, S.J. and Kim, K., Modeling urban nonpoint source pollution with a geographical information system, Water Resour. Bull., 29(2), 189–198.

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5

Emerging Stormwater Controls for Critical Source Areas Robert Pitt and Shirley Clark

CONTENTS Overview of Design Objectives and General Approach in the Selection of Stormwater Controls ..........................................................................................................................................104 Introduction .............................................................................................................................104 Design Approach Affected by Runoff and Pollutant Yields for Different Rain Categories................................................................................................................................105 Candidate Scenarios for Urban Drainage...............................................................................106 Stormwater Controls Suitable for Retrofitting in Existing Areas ................................107 Recommended Controls for New Developments .........................................................108 Public Works Activities Historically Used for Stormwater Control at Critical Areas .................108 Catchbasins and Other Floatable and Grit Traps ...................................................................109 Suggestions for Optimal Storm Drainage Inlet Use.....................................................112 Grit Traps.......................................................................................................................113 Oil/Water Separators ...............................................................................................................113 Factors Relevant to Oil/Water Separator Performance.................................................114 Gravity Separation.........................................................................................................114 Maintenance of Oil/Water Separators...........................................................................115 Performance of Oil/Water Separators for Treating Stormwater...................................115 Street Cleaning........................................................................................................................117 Prevention of Dry Weather Pollutant Entries into Sewerage Systems ..................................120 Investigative Procedures................................................................................................121 Drainage Area Mapping ......................................................................................121 Tracer Selection ...................................................................................................121 Field Surveys .......................................................................................................122 Analyses of Data/Samples...................................................................................123 Investigation and Remediation ............................................................................123 Emerging Critical Source Area Controls.......................................................................................123 Filtration of Stormwater .........................................................................................................124 Sand ...............................................................................................................................124 Composted Leaves ........................................................................................................125 Peat Moss ......................................................................................................................126 Design of Stormwater Filters........................................................................................127 Selection of Filtration Media for Pollutant Removal Capabilities .....................127 Design of Filters for Specified Filtration Durations ...........................................128

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Chemical-Assisted Sedimentation ..........................................................................................130 Combination Practices ............................................................................................................132 Example of Combination Practices Using Filtration: The Multichambered Treatment Train (MCTT) ..............................................................................................132 Summary ........................................................................................................................................135 References ......................................................................................................................................136

OVERVIEW OF DESIGN OBJECTIVES AND GENERAL APPROACH IN THE SELECTION OF STORMWATER CONTROLS INTRODUCTION An extensive literature review and survey of past and current drainage design practices during a recent U.S. EPA-funded research project found that design standards have not changed significantly during the past 25 years, but that there has been a shift toward the use of more sophisticated design tools (Pitt et al., publication pending). Unfortunately, current practices were identified as inadequately addressing water quality issues, even though almost all survey respondents recognized the significance of wet weather flow impacts. The use of long-term continuous simulation and addressing small storms that can be responsible for important receiving water quality problems is a recommended improvement in current design practices. Important changes in urban water management will also be needed in coming years to balance the needs for both water quality and quantity control in developing areas. This recent U.S. EPA research, along with many other current literature sources, found that it is possible and best to develop stormwater management design guidelines based on local rain conditions. Small events, making up the majority of rain events, commonly exceed bacteria and metal criteria, but are relatively easy to control through simple infiltration or on-site reuse of the stormwater. Moderate-sized rains, however, are responsible for the majority of the runoff volumes and pollutant discharges. The runoff from these events can also be significantly reduced, but certainly not eliminated, through infiltration, but larger flows will have to be treated to reduce pollutant concentrations and excessive discharge rates. Large rains that approach and may exceed the capacities of the drainage system produce little of the annual flows and are rare. In addition, significant pollutant concentration reductions during these large events would be difficult and very expensive because of the very large flows involved. However, runoff flow rates should be reduced to produce in-stream flow rate distributions less than critical values in order to protect in-stream habitat. Numerous researchers have found that receiving waters degrade sharply after the impervious area in the watershed exceeds about 5 to 10%. It may be possible in many cases, especially for newly developing areas, to determine the appropriate level of stormwater control to compensate for impervious areas greater than these critical values. In addition, researchers have found that critical flow rates may be identified that define stable streambed conditions. If excessive flows are discharged at levels below this critical value, much less damage may occur than if the frequency of flows above this critical value is increased. Many stormwater problems can be reduced or eliminated using relatively simple changes in development practices. Stormwater from numerous, small events can be effectively eliminated from surface runoff. Runoff from moderate-sized storms can be significantly reduced and larger flows that cannot be eliminated can be treated. Runoff from large storms needs to be safely conveyed to minimize property damage and safety issues, and to have the flow rate distribution modified to minimize habitat problems in the receiving water. However, these benefits can only be realized with significant runoff volume reductions that are much easier to implement at the time of development. Implementing appropriate stormwater controls at the time of development is much more cost-effective and will provide much more effective levels of control than if only retrofitting is

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used. Extensive retrofitting may only result in low to moderate levels of stormwater control (generally about 25% reductions over the whole watershed), whereas implementation of a broad set of appropriate controls (using both sedimentation and infiltration) at the time of development could result in control levels approaching 75%, or more. Different drainage design criteria and receiving water use objectives often require the examination of different types of rains for the design of urban drainage systems. These different (and often conflicting) objectives of a stormwater drainage system can be addressed by using distinct portions of the long-term rainfall record. Several historical examinations (including Heaney et al., 1977) have also considered the need for the examination of a wide range of rain events for drainage design. However, the lack of efficient computer resources severely restricted long-term analyses in the past. Currently, computer resources are much more available and are capable of much more comprehensive investigations (Gregory and James, 1996). In addition to having more efficient computational resources, it is also necessary to re-examine some of the fundamental urban hydrology modeling assumptions (Pitt, 1987). Most of the urban hydrology methods currently used for drainage design have been successfully used for large “design” storms. Obviously, this approach (providing urban areas safe from excessive flooding and associated flood-related damages) is the most critical objective of urban drainage. However, it is now possible (and legally required in many areas) to provide urban drainage systems that also minimize other problems associated with urban stormwater. This broader set of urban drainage objectives requires a broader approach to drainage design, and the use of hydrology methods with different assumptions and simplifications. Stormwater treatment at critical source areas is usually quite different from treatment at other areas. The control of small critical area contributions to urban runoff may be the most cost-effective approach for treatment/reduction of stormwater toxicants. The general features of the critical source areas appear to be large paved areas, heavy vehicular traffic (especially frequent and large numbers of vehicle starts, such as at convenience stores), and outdoor use or storage of problem pollutants. Most of these areas are quite small, as they are small commercial establishments. However, large areas of continuous pavement (such as at shopping malls) are also critical sources. Runoff from these larger areas can usually be controlled through the use of preferred wet detention ponds, but runoff control for small areas usually requires special applications.

DESIGN APPROACH AFFECTED RAIN CATEGORIES

BY

RUNOFF

AND

POLLUTANT YIELDS

FOR

DIFFERENT

The basics for developing appropriate stormwater controls is understanding that specific receiving water problems are associated with specific rain depth categories. To identify which rain categories are important for which receiving water problems, long-term evaluations are needed. Long-term continuous simulations using Atlanta, Georgia, rain data were made using SLAMM, the Source Loading and Management Model (Pitt, 1986; Pitt and Voorhees, 1995; www. WINSLAMM.com). These simulations were based on 8 years of rainfall records for the years from 1985 through 1992, containing about 1000 individual rains. The rainfall records were from certified NOAA (National Oceanographic and Atmospheric Administration) weather stations and were obtained from CD-ROMs distributed by EarthInfo (Boulder, CO). Hourly rainfall depths for the indicated periods were downloaded from the CD-ROMs into an Excel spreadsheet. This file was then read by an utility program included in the SLAMM software package. This rainfall file utility combined adjacent hourly rainfall values into individual rains, based on user selections (at least 6 h of no rain was used to separate adjacent rain events and all rain depths were used, with the exception of the “trace” values; similar analyses were made using interevent definitions ranging from 3 to 24 h, with little differences in the conclusions). These rain files were then used in SLAMM for typical medium density and strip commercial developments. The outputs of these computer runs for Atlanta are plotted on Figure 5.1. The following summarizes these evaluations:

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Percent Associated with Rain, or Less

Atlanta, GA Rain and Runoff Distributions ('85-'92) 100

80

60

40

20

0 0.01

0.1 1 Rain (inches)

10

FIGURE 5.1 Modeled rainfall and runoff cumulative probability density functions (CDFs).

• <0.3 in. These rains account for most of the events (about 60%), but little of the runoff volume (5 to 9%), and are therefore easiest to control. They produce much less pollutant mass discharge and probably have less receiving water effects than other rains. However, the runoff pollutant concentrations likely exceed regulatory standards for several categories of critical pollutants, especially bacteria and some total recoverable metals. They also cause large numbers of overflow events in uncontrolled combined sewers. These rains are very common, occurring once or twice a week. Rains less than about 0.1 in. would not produce noticeable runoff. • 0.3 to 4 in. These rains account for the majority of the runoff volume (about 85%) and produce moderate to high flows. They account for about 40% of the annual rain events. These rains occur on the average about every 2 weeks and subject the receiving waters to frequent high pollutant loads and moderate to high flows. • >4 in. These rains probably produce the most damaging flows, from a habitat destruction standpoint, and occur every several months (at least once or twice a year). These recurring high flows establish the energy gradient of the stream and cause unstable stream banks. Less than 2% of the rains are in this category, and they are responsible for about 5 to 9% of the annual runoff and pollutant discharges. • Very large rains. This category is rarely represented in field studies due to the rarity of these extreme events and the typically short duration of most field observations. These rains occur only rarely (every several decades, or less frequently) and produce extremely large flows. For example, during the 8-year monitoring period (1985 through 1992), the largest rain was about 7 in. (less than a 1% probability of occurring in any 1 year, if it had a duration of about 1 day). These extreme rains produce only a very small fraction of the annual average discharges. However, when they do occur, great property and receiving water damage results. The receiving water damage (mostly associated with habitat destruction, sediment scouring, and the flushing of organisms great distances downstream and out of the system) can conceivably naturally recover to before-storm conditions within a few years.

CANDIDATE SCENARIOS

FOR

URBAN DRAINAGE

It is much more difficult to achieve significant stormwater improvements by retrofitting controls in existing areas compared to establishing controls in newly developing areas. However, it is unlikely

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that stormwater controls can ever maintain predevelopment receiving water conditions except for the most optimal conditions. Stormwater management should therefore be considered necessary to reduce the rate and extent of degradation of receiving waters, but not to preserve predevelopment conditions. From a retrofit perspective, the control of gross litter and garbage, plus combined sewer overflow (CSO) control, is of primary concern. Control of toxicants is unreasonable and not very critical in the presence of gross contamination of waterways. Even acceptable litter and floatable control may likely be beyond reasonable financial means in many existing areas. However, many stormwater controls can be implemented at the time of development in a cost-effective manner that can have significant benefits to the receiving waters. Stormwater Controls Suitable for Retrofitting in Existing Areas The ability to construct new stormwater controls in existing areas is severely restricted to both a limited number of suitable controls and to the extent of control that may be accomplished. In addition, retrofitted controls are always much more costly than if the same control had been used at the time of development. Retrofitting stormwater controls is generally limited to the practices shown on the following list. In general, items near the top of this list are more popular for implementation, although they may not be the least expensive or the most effective. • Public Works Practices (may be implemented by Municipal Streets Department, Parks and Recreation Department, Engineering Department, Sanitation District, etc.): • Enhanced street cleaning • Increased catchbasin cleaning • Repairs of trash hoods in catchbasins • Installation of catchbasin inserts • Rebuilding of inlets to create catchbasins • Litter control campaign, with increased availability/pickup of trash receptacles • Public environmental education campaign • Dog waste control enforcement • Household toxicant collection • Enhanced enforcement of erosion control requirements • Modifications to public place landscaping maintenance • Infiltration Practices at Source Areas • Orientation of rooftop drains toward pervious areas (disconnections) • Amendment of soils with compost • Residential and commercial landscaping modifications (such as constructing rain gardens and other small bioretention areas) • French drains for roof runoff in areas with little pervious areas • Replacement of pavement with porous paver blocks • Outfall Wet Detention Ponds: • Modification of existing dry detention ponds for enhanced pollution control • Enhanced performance at existing wet ponds (modify outlet structure, construct forebay, provide post-treatment using media filtration or wetland treatment) • Construction of new wet detention ponds in available areas • Control Runoff at Critical Source Areas (such as vehicle service facilities, scrap yards, etc.): • Sand perimeter filters • Underground sedimentation/filtration units • Other Controls: • Replacement of exposed galvanized metal with nonpolluting material • Provision of new regional stormwater controls downstream of existing developed areas

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Recommended Controls for New Developments Stormwater controls established at the time of development can take advantage of simple grading and site layout options that can have significant stormwater benefits with little cost. In addition, the design of the storm drainage system can be easily modified for pollution control objectives, while still meeting drainage objectives. Most importantly, regional stormwater control facilities can be effectively located and sized with little interference with existing development. The following list indicates some likely effective wastewater collection scenarios for several different conditions for new developments (Pitt et al., publication pending): • Low and very low density residential developments (> acre lot sizes). Sanitary wastewater should be treated on site using septic tanks and advanced on-site treatment options. Domestic water conservation to reduce sanitary wastewater flows should be an important component of these systems. Most stormwater should be infiltrated on site by directing runoff from paved and roof areas to small bioretention areas. Disturbed soil areas should use compost-amended soils and should otherwise be constructed to minimize soil compaction. Roads should have grass swale drainage to accommodate moderate to large storms. • Medium density developments (1/4 to 2 acre lot sizes). Separate sanitary wastewater and stormwater drainage systems should be used. Sanitary wastewater collection systems must be constructed and maintained to eliminate inflow and infiltration (I/I), or use vacuum or pressurized conveyance systems. Again, most stormwater should be infiltrated on site by directing runoff from paved and roof areas to small bioretention areas. Paved areas should be minimized and the use of porous pavements and paver blocks should be used for walkways, driveways, overflow parking areas, etc. Disturbed soil areas should use compostamended soils and should otherwise be constructed to minimize soil compaction. Grass swale drainages should be encouraged to accommodate moderate to large storms for the excess runoff in residential areas, depending on slope, soil types, and other features affecting swale stability. Commercial and industrial areas should also use grass swales, depending on groundwater contamination potential and available space. Wet detention ponds should be used for controlling runoff from commercial and industrial areas. Special controls should be used at critical source areas that have excessive pollution-generating potential. • High density developments. Combined sewer systems could be effectively used in these areas. On-site infiltration of the least-contaminated stormwater (such as from roofs and landscaped areas) is needed to minimize wet weather flows. On-site storage of sanitary wastewaters during wet weather (Preul, 1996), plus extensive use of in-line and off-line storage, and the use of effective high-rate treatment systems would minimize the damage associated with any CSOs. The treatment of the wet weather flows at the wastewater treatment facility would likely result in less pollutant discharge in these areas than if conventional separate wastewater collection systems were used. The following sections review selected control practices that can be used at critical source areas to control runoff from these areas typically having the highest unit area pollutant discharges. As such, they are important supplements to the general stormwater controls outlined above. These critical source area controls are represented by some traditional “public works” practices commonly carried out by municipalities, in addition to other specialized controls that are more likely to produce significant reductions in pollutant discharges.

PUBLIC WORKS ACTIVITIES HISTORICALLY USED FOR STORMWATER CONTROL AT CRITICAL AREAS Public works control practices are commonly used in critical source areas and at “ultra-urban” locations, because of the difficulty and cost of retrofitting alternative stormwater controls in these

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small and intensively built-up areas. This section describes typical expected performance of catchbasins, oil and water separators, street cleaning, and describes investigations to identify and correct inappropriate discharges to storm drainage systems. Most of these public works stormwater control practices have been used for some time and can be effective in controlling litter and other floatable material as well as other gross contamination. Although these practices have limited pollutant removal capabilities for most cases, the control of floatable materials in wet weather flows is a fundamental goal that should be achieved in all areas.

CATCHBASINS

AND

OTHER FLOATABLE

AND

GRIT TRAPS

Storm drainage system inlet structures can be separated into three general categories. The first category is a simple inlet that comprises a grating at the curb and a box, with the discharge located at the bottom of the box, which connects directly to the main storm drainage or combined sewerage. This inlet simply directs the runoff to the drainage system and contains no attributes that would improve water quality. However, large debris (several centimeters in size) may accumulate (if present in the stormwater, which is unlikely). The second type of inlet is similar to the simple inlet, but it contains a sump that typically extends 0.5 to 1 m below the bottom of the outlet. This is termed a catchbasin in the United States, or a gully pot in the United Kingdom, and has been shown to trap appreciable portions of the coarse sediment. The third category is also similar to the simple inlet, but contains some type of screening to trap debris. These include small cast-iron perforated buckets placed under the street grating, as used in Germany, large perforated and lipped stainless steel plates placed under the street grating, as used in Austin, Texas, and a number of proprietary devices incorporating filter fabric or other types of screening placed to intercept the stormwater flow. Over the past 85 years, there has been extensive use of catchbasins for coarse material removal from stormwater runoff (Lager et al., 1977), mainly to reduce sedimentation problems in the storm drainage system. Catchbasins have also been utilized in Europe for over a century. The purpose of catchbasins historically has been to prevent the clogging of sewer lines with sediment and organic debris, and to prevent odors from escaping from the sewers by creating a water seal. Over the years, many different styles of catchbasins have been used, and many different enhancement devices have been added to increase their effectiveness. According to Lager et al. (1977), catchbasins were considered marginal in performance as early as the turn of the century. They felt that the use of catchbasins may be more of a tradition for most municipalities rather than a practice based on performance. Sartor and Boyd (1972) suggested that all catchbasins should be filled in, citing their ineffectiveness at removal of pollutants and the threat of slug pollution of the scoured material. Grottker (1990) was more positive. He reports of an inlet design in Germany that is modified with sumps and a primary filter to screen out the larger debris. He recommended the modified device as a cost-saving practice that improves water quality. Catchbasin performance has been investigated for some time in the United States. Sartor and Boyd (1972) conducted controlled field tests of a catchbasin in San Francisco, using simulated sediment in fire hydrant water flows. They sampled water flowing into and out of a catchbasin for sediment and basic pollutant analyses. Lager et al. (1977) was the first U.S. EPA-funded research effort that included a theoretical laboratory investigation to evaluate sedimentation in catchbasins and to develop effective designs. The mobility of catchbasin sediments was investigated by Pitt (1979). Long-duration tests were conducted using an “idealized” catchbasin (based on the Lager et al. 1977 design), retrofitted in San Jose, California. The research focused on resuspension of sediment from a full catchbasin over an extended time period. It was concluded that the amount of catchbasin and sewerage sediment was very large in comparison with storm runoff yields, but was not very mobile. Cleaning catchbasins would enable them to continue to trap sediment, instead of reaching a steady-state loading and allowing subsequent stormwater flows to pass through untreated. Pitt (1985) statistically compared catchbasin supernatant with outfall water quality and did not detect any significant differences. However, Butler et al. (1995) investigated gully pot supernatant

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water and found that it may contribute to the more greatly polluted first flush of stormwater reported for some locations. Specific problems have been associated with the anaerobic conditions that rapidly form in the supernatant water during dry weather, causing the release of oxygen-demanding material, ammonium, and possible sulfides. These anaerobic conditions also affect the bioavailability of the heavy metals in the flushed water. Catchbasins, simple inlets, manholes, and sewerage sediment accumulations were monitored at more than 200 locations in Bellevue, Washington, in two mixed residential and commercial study areas (Pitt, 1985). These locations were studied over 3 years to monitor accumulation of sediment and sediment quality. The sediment in the catchbasins and the sewerage was found to be the largest particles that were washed from the streets. The sewerage and catchbasin sediments had a much smaller median particle size than the street dirt and were therefore more potentially polluting than the particulates that can be removed by street cleaning. Cleaning catchbasins twice a year was found to allow the catchbasins to capture particulates most effectively. This cleaning schedule was found to reduce the total residue and lead urban runoff yields by between 10 and 25%, and chemical oxygen demand (COD), total Kjeldahl nitrogen, total phosphorus, and zinc by between 5 and 10% (Pitt and Shawley, 1982). Catchbasins have been found to be effective in removing pollutants associated with coarser runoff solids (Pitt, 1985). Moderate reductions in total and suspended solids (up to 45% reduction for low gutter flows) were indicated by a number of prior studies (such as Pitt, 1979; Aronson et al., 1983; and Pitt, 1985). However, relatively few pollutants are associated with these coarser solids (Pitt, 1979; 1985). Pitt (1985) found that catchbasins will accumulate sediments until the sediments reach about 60% of the total sump capacity (or to about 0.3 m under the catchbasin outlet). After that level, the sediment is at an equilibrium, with scour balancing new deposition. Earlier U.S. EPA research (Lager et al., 1977) found that an optimal catchbasin design should have the following dimensions: if the outlet pipe is D in diameter, its bottom should be located about 2.5D below the street level and 4D from the bottom of the catchbasin sump. The overall height of the catchbasin should therefore be 6.5D, with a diameter of 4D. Butler et al. (1995) found that the median particle size of the sump particles was between about 300 and 3000 µm, with less than 10% of the particles smaller than 100 µm, near the typical upper limit of particles found in stormwater. Catchbasin sumps therefore trap the largest particles that are flowing in the water, and allow the more contaminated finer particles to flow through the inlet structure. Butler et al. (1995) and Butler and Karunaratne (1995) present sediment-trapping equations for sediment in gully pots, based on detailed laboratory tests. The sediment-trapping performance was found to be dependent on the flow rate passing through the gully pot, and on the particle sizes of the sediment. The depth of sediment in the gully pot had a lesser effect on the capture performance. In all cases, decreased flows substantially increased the trapping efficiency and larger particles had substantially greater trapping efficiency than smaller particles, as expected. Three storm drain inlet devices were evaluated in Stafford Township, New Jersey, by Pitt and Field (1998) as part of a U.S. EPA-funded study: an optimally designed catchbasin with a sump and two representative designs that used filter material. The inlet devices were located in a residential area. The monitoring program included 12 inlet and effluent samples from these devices over several different storms. Complete organic and metallic toxicant analyses, in addition to conventional pollutants, were included in the analytical program. In addition to these field tests, controlled tests were also conducted in the laboratory to further evaluate filter fabrics used in some inlet devices. Samples were analyzed for a wide range of toxicants using very low detection limits (about 1 to 10 µg/L). The constituents analyzed include heavy metals and organics (phenols, PAHs, phthalate esters, and chlorinated pesticides). Particle size distributions, using a Coulter Multi-Sizer II, were also made, in addition to conventional analyses for COD, major ions, nutrients, suspended and dissolved solids, turbidity, color, pH, and conductivity. All samples were also partitioned into filterable and nonfilterable components before COD and toxicant analyses to better estimate fate

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and treatability. All samples were also screened using the Microtox toxicity test to measure relative reductions in toxicity associated with the inlet devices. Conventional Catchbasin with Sump. A sump was installed in the bottom of an existing storm drain inlet by digging out the bottom and placing a section of 36-in. concrete pipe on end. The outlet pipe was reduced to 8 in. and the sump depth was 36 in. Inlet water was sampled before entering the catchbasin, while outlet water was sampled after passing through the unit. Filter Fabric Unit. A filter fabric unit, with a set of dual horizontal trays, each containing about 0.1 m2 of filter fabric, was retrofitted into one of the existing inlets for testing. When the filter fabric clogs on the upper tray, the stormwater overflows a small rectangular weir, onto another similar tray located beneath the upper tray. Again, paired samples were obtained above and under the unit for analyses. According to the manufacturer, this system can handle up to 300 gallons per minute. The unit tested has been replaced by the manufacture with a new type of catchbasin filter that also includes a selection of filtering media. Coarse Filter Unit. A coarse filter was also retrofitted into an existing storm drain inlet. This unit uses a relatively coarse foam material (about 1 mm cell diameter and 8 mm thick) that is sandwiched between two pieces of galvanized screening for support. This unit was fitted in the inlet, sealed along the bottom and sides on the outlet side, forcing any water through the unit before it is discharged. The filter was placed in front of the catchbasin outlet in a near vertical position. Its main purpose is to filter debris, including leaves and grass clippings, from stormwater. As with the other units, the inlet and outlet water was simultaneously sampled for analyses. The catchbasin with the sump was the only device that showed important and significant removals for several pollutants: Total solids (0 to 50%, average 22%) Suspended solids (0 to 55%, average 32%) Turbidity (0 to 65%, average 38%) Color (0 to 50%, average 24%) The coarse screen unit showed consistent washout of material, while both the coarse screen unit and the catchbasin showed slight increases for several major ions, most likely associated with contact with concrete and other drainage system materials. None of the other parameters or inlet devices demonstrated significant differences between the influent and effluent water (at the 95% confidence level, or better), except for the filter fabric unit, which showed a small removal for nitrate. Several significant and large increases in major ion concentrations were noted for the catchbasin (bicarbonate, magnesium, and calcium) and for the coarse screen unit (bicarbonate and potassium). These increases, which are not believed to be very important, may have been due to the runoff water being affected by the concrete in the inlet devices. These increases are likely part of the general process where runoff water increases its alkalinity and buffer capacity as it flows through urban areas. The significant and large increases in total solids, suspended solids, volatile solids, and conductivity for the coarse screen unit imply washout of decomposing collected organic solids (mostly leaves). The coarse screen unit traps large debris, including decomposable organic material, behind the screen. Stormwater then flows through this material as it passes through the screen, as in most inlet screening/filtering devices. If not frequently removed, this organic material may decompose and wash through the screen in subsequent storms. The large debris was not represented in the influent water samples, but after partial decomposition, this material could have added to the solids concentrations in the effluent samples. The catchbasin did not exhibit this increase in solids concentrations likely because the collected material is trapped in the sump and not subjected to water passing through the material. Previous catchbasin tests found that catchbasin supernatant water quality is not significantly different from runoff water quality, nor is the collected debris

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easily or commonly scoured from the sump. The filter fabric unit did not exhibit this increase in solids, possibly because it trapped relatively small amounts of debris, and the overflow weirs allowed the subsequent stormwater to flow over the trapped debris instead of being forced through the debris. Suggestions for Optimal Storm Drainage Inlet Use The best catchbasin configuration for a specific location would be dependent on site conditions and would probably incorporate a combination of features from several different inlet designs. The primary design should incorporate a catchbasin with a sump, as described by Lager et al. (1977), with an inverted (hooded) outlet. If large enough, catchbasins with sumps have been shown to provide a moderate level of suspended solids reductions in stormwater under a wide range of conditions in many studies in the United States and Europe. The use of filter fabrics in catchbasins is not likely to be beneficial because of their rapid clogging from retained sediment and trash. The use of coarser screens in catchbasin inlets is also not likely to result in water quality improvements, based on conventional water pollutant analyses. However, well-designed and maintained screens can result in substantial trash and litter reductions. It is important that the screen not trap organic material in the flow path of the stormwater. Prior research (Pitt, 1979; 1985) has shown that if most of the trapped material is contained in the catchbasin sump, it is out of the direct flow path and unlikely to be scoured during high flows, or to degrade overlying supernatant water. Storm drainage inlet devices also should not be considered as leaf control options, or used in areas having very heavy trash loadings, unless they can be cleaned after practically every storm. The goal is a storm drainage inlet device that: • • • • •

does not cause flooding when it clogs with debris, does not force stormwater through the captured material, does not have adverse hydraulic head loss properties, maximizes pollutant reductions, and requires inexpensive and only infrequent maintenance.

The following suggestions and design guidelines should meet some of these criteria. Catchbasins in newly developing areas could be more optimally designed than the suggestions below, especially by enlarging the sumps and by providing large and separate offset litter traps. The basic catchbasin (having an appropriately sized sump with a hooded outlet) should be used in most areas. This is the most robust configuration. In almost all full-scale field investigations, this design has been shown to withstand extreme flows with little scouring losses, no significant differences between supernatant water quality and runoff quality, and minimal insect problems. It will trap the bed load from the stormwater (especially important in areas using sand for traction control) and will trap a low to moderate amount of suspended solids (about 30 to 45% of the annual loadings). The largest fraction of the sediment in the flowing stormwater will be trapped, in preference to the finer material that has greater amounts of associated pollutants. Their hydraulic capacities are designed using conventional procedures (grating and outlet dimensions), while the sump is designed based on the desired cleaning frequency. Figure 5.2 is this basic recommended configuration, based on Lager et al. (1977). The use of filter fabrics or other fine screens as an integral part of a storm drain inlet is not recommended unless shown to have minimal problem potential. Pitt and Field (1998) showed that filter fabric screens may provide important reductions (about 50%) in suspended solids and COD. However, the filter fabrics can only withstand about 1 to 2 mm accumulation of sediment before they clog. This is about 4 kg of sediment per square meter of fabric. If runoff had a suspended solids concentration of 100 mg/L, the maximum loading of stormwater tolerated would be about 40 m. For a typical application (1 ha paved drainage area to a 1 m2 filter fabric in an inlet box), only about 5 to 10 mm of runoff could be filtered before absolute clogging.

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grating cover

1.5D

normal water elevation

hooded outlet

D

scour depth (about 0.3 m)

sump depth depending on cleaning freq. (at least 1 m, or 4D) captured sediment and debris

maximum sediment depth

4D (at least 0.75 m)

FIGURE 5.2 Basic catchbasin design. (Based on Lager et al., 1977.)

Grit Traps Several proprietary stormwater treatment devices are being marketed throughout North America. These devices can be located underground. Unfortunately, comprehensive testing with actual stormwater is not available for most of these devices. The designs and demonstrations are mostly based on reduction of relatively large particles that rarely occur in stormwater. The suspended solids in stormwater is mostly in the range of 1 to 100 µm, with only a small fraction of the mass (usually <10%) associated with particles greater than 100 µm. These devices are designed to capture particle sizes that have typically been found on streets, not in the runoff water (Pitt, 1987). These devices are excellent grit chambers (and can probably capture floating oils) and can be used to prevent sand-sized particles from accumulating in sewerage. Very little scour of the captured grit material is also likely with these devices. However, they are not likely to provide important reductions of most stormwater pollutants, especially the toxicants. If a site is grossly contaminated with oils or grit, then a proprietary oil/water separator or grit chamber is needed, but further treatment will also likely be necessary, such as with a media filter.

OIL/WATER SEPARATORS This section (summarized from Pitt et al., 1999) briefly examines oil/water separators and their expected ability to treat stormwater. These devices are extensively used to treat industrial wastewaters

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and have been shown to be effective in those applications for which they were designed. These units perform best at very high levels of oil contamination, such as may be found at some industrial locations (API, 1990). Generally, they obtain about 90% reductions in oil if the influent oil concentrations are greater than about 10,000 mg/L. Reductions of about 50% would occur at influent oil concentrations of about 200 mg/L. Very little reduction is expected at levels less than about 100 mg/L. Little information is available demonstrating their effectiveness in treating stormwater, which usually has oil contamination levels of much less than 100 mg/L. Factors Relevant to Oil/Water Separator Performance Many factors affect separator performance, including the quantity of oil, oil density, water temperature, and other waste stream characteristics. The most important characteristic affecting oil removal performance is oil droplet size, from which the critical rise rate can be determined. After determining the rise rate, design flow rate, and effective horizontal separation area, the separator can be appropriately sized. Oil/water mixtures are usually divided into four categories: 1. Free-floating oil, with oil droplet sizes of 250 µm or more, is evidenced by an oil slick or film on the water surface. In this case, the oil has separated from the water. 2. Oil droplets and globules ranging in size from 10 to 300 µm. This range is the most important range when dealing with oil/water separation. 3. Emulsions, which have sizes in the 1 to 30 µm range. 4. “Dissolved” oil with diameters of less than 10 µm. The largest oil droplets are easily separated from water using a basic spill trap or separation device. Smaller droplets cause wide-ranging differences in performance from different separation devices. Emulsions are of two types: stable and unstable. Stable emulsions are usually the result of surfactants (i.e., soaps and detergents), which hold the droplets in solution. This type of emulsion is often present in cleaning operations and can often be very difficult to remove. Unstable emulsions are created by shearing forces present in mixing: the oil is held in suspension when the interfacial tension of the surface of the drops is equal to the force acting on the drops. These will generally separate by physical methods such as extended settling times or filtration methods. Oil/water separators are not able to treat stable emulsions or dissolved oil. Gravity Separation Gravity separation relies on the density differences between oil and water. Oil will rise to the water surface unless some other contributing factor such as a solvent or detergent interferes with the process. For gravity units, this density difference is the only mechanism by which separation occurs. Other technologies, such as air flotation, coalescing plates, and impingement coalescing filters, enhance the separation process by mechanical means. Gravity separators are the most basic type of separator and are the most widely used. They have few, if any, moving parts and require little maintenance with regard to the structure or operation of the device. Usually, separators are designed to meet the criteria of the American Petroleum Institute (API), and are fitted with other devices such as coalescing plate interceptors (CPI) and filters. Even though these separators are effective in removing free and unstable oil emulsions, they are ineffective in removing most emulsions and soluble oil fractions (Ford, 1978). Furthermore, it is important to remember that no gravity oil/water separation device will have a significant impact on many of the other important stormwater pollutants, requiring additional treatment (Highland Tank). The conventional API oil/water separator consists of a large chamber divided by baffles into three sections. The first chamber acts as an equalization chamber where grit and larger solids settle

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and turbulent flow slows before entering the main separation chamber. Often, manufacturers suggest the use of a catchbasin or interceptor tank as a pretreatment device so that coarse material will be kept from entering the oil/water separation tank. After entering the main chamber, solids settle to the bottom and oil rises to the top, according to Stokes’ law. Larger API oil/water separators contain a sludge scraper that continually removes the captured settled solids into a sludge pit. The oil is also removed by an oil skimmer operating on the water surface. At the end of the separation chamber, all oil particles having a diameter of larger than the critical size have theoretically risen to the surface and have been removed by an oil skimmer. Small API units usually do not contain an oil skimmer, sludge scrapper, or sludge pit. Although they are less costly due to the absence of moving parts, they require more frequent cleaning and maintenance. These smaller units have been shown to be as effective as the larger more expensive units, if they receive proper maintenance at regular intervals. The API (1990) stipulates that if its design criteria are met, then the separator will remove all oil droplets greater than about 150 µm in diameter. The API reports that retention times are usually greater than the actual design values because actual flows are usually smaller than design flows; hence smaller droplets are removed most of the time. This finding is confirmed by Rupperd (1993) in a study of an oil/water separator treatment device in the community of Velizy, France. Also, API tanks are known to remove large amounts of oil effectively, including slugs of pure oil, and will not be overwhelmed (Tramier, 1983). Studies have also shown that these separators can produce effluents down to 30 ppm (Delaine, 1995), routinely at 30 to 150 ppm, with occasional concentrations above 150 ppm, depending on the flow rate, and hence the retention times (Ford, 1978). The API has stated that very few separators with ratios of surface area to flow within the API design range achieved effluent oil concentrations lower that 100 ppm (API, 1990). Therefore, the API separator is a recommended system for the removal of solids and gross oil as a pretreatment device upstream of another treatment system, if additional pollutants of concern are present, or if more stringent effluent standards are to be met. Maintenance of Oil/Water Separators Problems with oil/water separators can be attributed largely to poor maintenance by allowing waste materials to accumulate in the system to levels that hinder performance and to levels that can be readily scoured during intermittent high flows. When excess oil accumulates, it will be forced around the oil retention baffle and make its way into the discharge stream. Also, sludge buildup is a major reason for failure. As waste builds up, the volume in the chamber above the sludge layer is reduced and therefore the retention time is also reduced, allowing oil to be discharged. Therefore, the efficiency of oil/water separators in trapping and retaining solids and hydrocarbons depends largely upon how they are maintained. They must be designed for ease of maintenance and be frequently maintained. Apparently, few oil/water separators built for stormwater control are adequately maintained. Ease of maintenance must be considered when designing separators, including providing easy access. Maintenance on these devices is accomplished by using suction equipment, such as a truckmounted vacuum utilized by personnel trained to handle potentially hazardous waste. The vacuum is used to skim off the top oil layer and the device is then drained. In larger devices, the corrugated plates are left in place, but, otherwise, they are lifted out along with any other filter devices that are present. The sludge is then vacuumed out or shoveled out and any remaining solids are loosened by spraying hot water at normal pressure. Performance of Oil/Water Separators for Treating Stormwater Manufacturers state that efficiencies observed during testing of oil/water separators are on the order of 97 to 99% for the removal of oil from wastewater. The test method typically applies oil to a

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paved washpad, with water added via a sprinkler system to simulate rainfall. Oil is of a specified density (typically, 0.72 to 0.95). These synthetic events are necessary to evaluate the performance of a separator but do not necessarily reflect the processes that occur during actual rainfall conditions where rapidly changing flow rates, unknown oil mixtures, and other pollutants are present. Published research is difficult to find on how these units actually perform once placed in operation. Interception of solid particles through settling and flotation of oils and other floatables are processes occurring within an oil/water separator. French studies have shown that the average suspended solids removal efficiency of separators is about 50% (Aires and Tabuchi, 1995). Oil/water separation requires an ascending speed of about 8 m/h, while the settling velocity of solids require descending velocities on the order of 1 to 3 m/h. At rates of 20% of the design flow rate, about 80% of the solids are removed; at 30% of the design flow rate, about 50% of the solids are removed. Negative removals also occur as the result of resuspension of previously settled material (Legrand et al., 1994). When the concentration of the oil in the wastewater is high, the oil removal efficiency increases. In Velizy, France, Rupperd (1993) found that oil/water separators fitted with cross-current separators had removal efficiencies ranging from zero to 90%, with an average of 47%. Low efficiencies were associated with low influent levels and greater efficiencies were associated with higher influent levels. This finding supports those of Tramier (1983), stated earlier, that separators are effective in removing large amounts of oil when the oil concentrations are elevated. The Metropolitan Washington Council of Governments (Washington, D.C.) has conducted a survey of 109 separator vaults in suburban Maryland and subsequently examined 17 in detail to determine their long-term effectiveness (Schueler and Shepp, 1993). These separators were used for controlling runoff from areas associated with automobile usage. These separators were either precast or poured-in-place concrete structures consisting of one, two, or three chambers. The results of this study revealed that the amount of trapped sediments within separators varied from month to month and that the contained waters were commonly completely displaced during even minor storms (Shepp and Cole, 1992). Of the original 109 separators that were observed in the survey, devices younger than 1 year old were effective in trapping sediments. Devices older than 1 year appeared to lose as much sediment as they retained. Not one of these separators had received maintenance since its installation. Survey observations suggested no net accumulation of sediment over time, in part because they received strong variations in flow. Of the 109 separators surveyed in this suburban Maryland study, 100% had received no maintenance, 1% needed structural repair, 6% were observed to have clogged trash racks, 84% contained high oil concentrations in the sediments trapped in their first chamber, 77% contained high oil concentrations in the sediments trapped in their second chambers, 27% contained high oil and floatables loading in their first chambers, and 23% contained high oil and floatables loading in their second chambers, all therefore requiring maintenance. Numerous manufacturers have developed small prefabricated separators to remove oils and solids from runoff. These separators are rarely specifically designed and sized for stormwater discharges, but usually consist of modified oil/water separators. Solids are intended to settle and oils are intended to rise within these separators, either by free fall/rise or by countercurrent or cross-current lamella separation. Many of these separators have been installed in France, especially along highways (Rupperd, 1993). Despite the number of installations, few studies have been carried out to assess their efficiency (Aires and Tabuchi, 1995). The historical use of oil/water separators to treat stormwater has been shown to be ineffective for various reasons, especially lack of maintenance and poor design for the relatively low levels of oils present in most stormwaters (Schueler 1994). Stormwater treatment test results from Fourage (1992), Rupperd (1993), and Legrand et al. (1994) show that these devices are usually greatly undersized. They may possibly work reasonably well at flow rates between 20 and 30% of their published design hydraulic capacities. For higher flow rates, the flow is very turbulent (the Reynolds numbers can be higher than 6000), and improvement in settling by using lamella plates is very poor. These devices need to be cleaned very frequently. If they are not cleaned, the deposits are

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scoured during storm events, with negative efficiencies. However, the cleaning is usually manually conducted, and expensive. In addition, the maintenance job is not very easy because the separators are very small. Some new devices are equipped with automatic sediment extraction pumps, which should be a significant improvement. Currently, these researchers have found that the cleaning frequencies are very insufficient and the stormwater quality benefits from using oil/water separators are very limited. The multichambered treatment train (MCTT), described later in this chapter, was developed to specifically address many of the stated problems found for oil/water separators used for stormwater treatment at critical source areas (Pitt et al., 1999). It was developed and tested with specific stormwater conditions in mind, plus it has been tested at several sizes for the reduction of stormwater pollutants of concern. The MCTT is intended to reduce organic and metallic toxicants, plus suspended solids, in the stormwater. Oil/water separators are intended to reduce very large concentrations of floating oils that may be present in industrial wastewaters. The extremely high concentrations of oils that the oil/water separators are most effective in removing are very rare in stormwater, even from critical source areas. If a site has these high levels, then an oil/water separator may be needed, in addition to other controls to reduce the other critical pollutants likely present. The MCTT can remove the typically highest levels of oils that may be present in stormwater from most critical source areas, plus also provide control of the trace toxicants present.

STREET CLEANING Street cleaning was extensively studied during early U.S. EPA-funded research projects. It was thought to be an effective runoff water quality control practice because of the large quantities of pollutants found on streets during early stormwater research in the United States (Sartor and Boyd, 1972). Because streets were assumed to contribute most of the urban runoff flows and pollutants, street cleaning was assumed to be a potentially effective practice. Unfortunately, few data have shown street cleaning to be effective because of the different sized particles that street cleaners remove compared to the particles that are mostly removed by rains. Furthermore, in many areas, rains are relatively frequent and keep the streets cleaner than typical threshold values that most street cleaners can remove. However, in the arid west of the United States, rains are very infrequent, allowing streets to become quite dirty during the late summer and fall. Extensive street cleaning during this time has been shown to result in important suspended solids and heavy metal reductions in runoff (Pitt, 1979; Pitt and Shawley, 1982). In other areas of the United States, especially in the wet southeast where large and frequent rains occur, street cleaning is likely to have much less direct water quality benefit, beyond possible important litter and floatable control. Obviously, street cleaning is most effective in areas having large fractions of pavement in good condition that can be accessed by street cleaners. Many critical source areas (especially large parking areas, paved equipment storage yards, etc.) could likely benefit with more frequent cleaning, especially with new equipment designed for better removal of fine particulates. Street cleaning plays an important role in most public works departments as an aesthetic and safety control measure. Street cleaning is also important to reduce massive dirt and debris buildups present in the spring in northern regions. Leaf cleanup by street cleaning is also necessary in most areas in the fall. Factors significantly affecting street cleaning performance include street dirt loadings, street texture, litter and moisture, parked car conditions, and equipment operating conditions (Pitt, 1979). If the 500 to 1000 µm particle loadings are less than about 75 kg/curb-km for smooth asphalt streets, conventional street cleaning does little good. As the loadings increase, the removals also increase: with loadings of about 10 kg/curb-km, less than 25% removals can be expected, while removals of up to about 50% can be expected if the initial loadings are as high as 40 kg/curb-km for this particle size. The removal performance decreases substantially for smaller particles, including those that are most readily washed off the street during rains and contribute to stormwater pollution.

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Particles of different sizes “behave” quite differently on streets. Typical street dirt total solids loadings show a “sawtooth” pattern with time between street cleaning or rain washoff events. The patterns for the separate particle sizes vary considerably for different particle sizes. Typical mechanical street cleaners remove a good deal (about 70%) of the coarse particles in the path of the street cleaner, but they remove very little of the finer particles (Sartor and Boyd, 1972; Pitt, 1979). Rains, however, remove very little of the large particles, but can remove large amounts (about 50%) of the fine particles (Bannerman et al., 1983; Pitt, 1985; 1987). The intermediate particle sizes show reduced removals by both street cleaners and rain. Conventional street cleaning therefore does not have a very positive effect on stormwater quality because conventional street cleaners preferentially remove the large particles, and the smaller particles from the street that are most effectively removed during rains. Valiron (1992) confirmed the earlier U.S. studies by showing that street cleaners only remove about 15% of the finest particles (less than 40 µm), while close to 80% of the largest particles (>2000 µm) are removed. Enhanced street cleaner performance was obtained with a modified regenerative-air street cleaner, especially at low loadings during tests in Bellevue, Washington (Pitt, 1985). The improved performance was much greater for the fine particle sizes, where the mechanical street cleaner did not remove any significant quantities of material. The larger particles were removed with about the same effectiveness by both street cleaner types. Other tests of vacuum street cleaners (Pitt, 1979) and regenerative-air street cleaners (Pitt and Shawley, 1982) showed very few differences in performance when compared to more standard mechanical street cleaners. These earlier tests were conducted in areas with much higher street loadings, especially for the larger particle sizes, than in Bellevue. It is expected that the high loadings of the large particles armored the small particles, so they could not be removed. For high loadings, it may be best to use a tandem operation, where the streets are first cleaned with a mechanical street cleaner to remove the large particles, followed by a regenerative-air street cleaner to remove the finer particles. Ellis (1986) concluded that street cleaning is most efficient if conventional street sweeping (using broom-operated equipment) is conducted in a tandem operation with vacuuming, and if it is done three times per week. He did find that conventional tandem sweeping-vacuum machines are very sensitive to the clogging of their filters and to street moisture levels that cause particles to adhere to the street surface, preventing their efficient removal. General street cleaning efficiency depends on the speed of the machines, the number of passes, the street loading and street texture, and interference from parked vehicles (Pitt, 1979). Much information concerning street cleaning productivity has been collected in many areas. The early tests (Clark and Cobbin, 1963; Sartor and Boyd, 1972) were conducted in controlled strips using heavy loadings of simulates instead of natural street dirt at typical loadings. Later tests, from the mid-1970s to mid-1980s, were conducted in large study areas (20 to 200 ha) by measuring actual street dirt loadings on many street segments immediately before and after typical street cleaning. These large-scale tests are of most interest, as they monitored both street surface phenomena and runoff characteristics. The following list briefly describes these large-scale street cleaning performance tests that have been conducted in the United States: • San Jose, California tests during 1976 and 1977 (Pitt, 1979) considered different street textures and conditions; multiple passes, vacuum-assisted, and two types of mechanical street cleaners; a wide range of cleaning frequencies; and effects of parking densities and parking controls. • Castro Valley, California tests during 1979 and 1980 (Pitt and Shawley, 1982) considered street slopes, mechanical and regenerative-air street cleaners, and several cleaning frequencies. This was an early Nationwide Urban Runoff Program (NURP) project of the U.S. EPA (1983). • Reno/Sparks, Nevada tests during 1981 (Pitt and Sutherland, 1982) considered different land uses, street textures, equipment speeds, multiple passes, full-width cleaning, and vacuum and mechanical street cleaners in an arid and dusty area.

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• Bellevue, Washington tests from 1980 through 1982 (Pitt, 1985) considered mechanical, regenerative-air, and modified regenerative-air street cleaners, different land uses, different cleaning frequencies, and different street textures in a humid and clean area. This was also a NURP project (U.S. EPA, 1983). • Champaign-Urbana, Illinois tests from 1980 and 1981 (Terstriep et al., 1982) examined spring cleanup, different cleaning frequencies and land uses, and used a three-wheel mechanical street cleaner. This was also a NURP project (U.S. EPA, 1983). • Milwaukee, Wisconsin tests from 1979 to 1983 (Bannerman et al., 1983) examined various street cleaning frequencies at five study sites, including residential and commercial land uses and large parking lots. This was also a NURP project (U.S. EPA, 1983). • Winston-Salem, North Carolina tests during their NURP (U.S. EPA, 1983) project examined different land uses and cleaning frequencies. Sutherland (1996, and with Jelen, 1996) conducted tests using a new style street cleaner that shows promise in removing large fractions of most of the street dirt particulates, even the small particles that are most heavily contaminated. The Enviro Whirl I, from Enviro Whirl Technologies, Inc., is capable of much improved removal of fine particles from the streets compared to any other street cleaner ever tested. This machine was also able to remove large fractions of the fine particles even in the presence of heavy loadings of large particles. This is a built-in tandem machine, incorporating rotating sweeper brooms within a powerful vacuum head. Model analyses for Portland, Oregon, indicate that monthly cleaning in a residential area may reduce the suspended solids discharges by about 50%, compared to only about 15% when using the older mechanical street cleaners that were tested during the early 1980s. This equipment is currently being evaluated in large-scale tests by the Wisconsin Department of Natural Resources and the Wisconsin Department of Transportation (Bannerman, personal communication). The pollutant removal benefits of street cleaning is directly dependent on the contributions of pollutants from the streets. In the Pacific Northwest region of the United States, the large number of mild rains results in much of the runoff pollutants originating from the streets. In the Southeast, in contrast, where the rains are much larger, with greater rain intensities, the streets contribute a much smaller fraction of the annual pollutant loads for the same residential land uses. However, in heavily paved areas, such as on freeways, large parking lots, or paved storage areas, street cleaning of these surfaces, especially with an effective machine like the Enviro Whirl, should result in significant runoff improvements. These many tests have examined a comprehensive selection of alternative street cleaning programs. Not all alternatives have been examined under all conditions, but sufficient information has been collectively obtained to examine many alternative street cleaning control options. Few instances of significant and important reductions in runoff pollutant discharges have been reported during these large-scale tests. The primary and historical role of street cleaning is for litter control. Litter is also an important water pollutant in receiving waters. Litter affects the aesthetic attributes and recreation uses of waters, plus it may have direct negative biological and water quality effects. Litter has not received much attention as a water pollutant, possibly because it is not routinely monitored during stormwater research efforts. The City of New York conducted a special study to investigate the role of enhanced street cleaning (using intensive manual street sweeping) to reduce floatable litter entering the city waterways (Newman et al., 1996). During the summer of 1993, the city hired temporary workers to sweep manually near-curb street areas and sidewalks in a pilot watershed area having 240 km of curb face. Two levels of manual sweeping supplemented the twice-per-week mechanical street cleaning the area normally receives. Continuous litter monitoring was also conducted to quantify the differences in floatable litter loadings found on the streets and sidewalks. An additional four manual sweepings each week to the two mechanical cleanings reduced the litter loadings by about 64% (on a weight basis) and by about 51% (on a surface area basis). Litter loading analyses were

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also conducted in areas where almost continuous manual sweeping (8 to 12 daily sweeps, 7 days per week) was conducted by special business organizations. In these special areas, the litter loadings were between 73 and 82% cleaner than comparable areas only receiving the twice weekly mechanical cleaning. They concluded that manual sweeping could be an important tool in reducing floatable pollution, especially in heavily congested areas such as Manhattan. New York City is also investigating catchbasin modifications and outfall netting for the control of floatable litter. Normal street cleaning operations for aesthetics and traffic safety purposes are not very satisfactory from a stormwater quality perspective. These objectives are different and the removal efficiency for fine and highly polluted particles is very low. Unless the street cleaning operations can remove the fine particles, they will always be limited in their pollutant removal effectiveness. Some efficient machines are now available to clean porous pavements and infiltration structures, and new tandem machines that incorporate both brooms and vacuums have recently been shown to be very efficient, even for the smaller particles. Conventional street cleaning operations preferentially remove the largest particles, while rain preferentially remove the smallest particles. In addition, street cleaners are very inefficient when the street dirt loadings are low, when the street texture is coarse, and when parked cars interfere. However, it should also be noted that streets are not the major source of stormwater pollutants for all rains in all areas. Pavement is the major source of pollutants for the smallest rains, but other areas contribute significant pollutants for moderate and large rains. Therefore, the ability of street cleaning to improve runoff quality is dependent on many issues, including the local rain patterns and other sources of runoff pollutants. More research is needed to investigate newer pavement cleaning technologies in areas such as industrial storage areas and commercial parking areas, which are critical pollutant sources.

PREVENTION

OF

DRY WEATHER POLLUTANT ENTRIES

INTO

SEWERAGE SYSTEMS

Inappropriate discharges to separate storm drainage systems can be a significant source of the pollutants being discharged to an urban receiving water. It is important that these sources be identified and corrected. Interest in these sources is an outgrowth of investigations into the larger problem of determining the role urban stormwater runoff plays as a contributor to receiving water quality problems. The U.S. EPA Storm and Combined Sewer Overflow Pollution Control Research and Nationwide Urban Runoff Programs helped highlight the problem with data confirming pollution found in urban storm drainage systems. Regulations, such as the National Pollution Discharge Elimination System (NPDES), require that certain industries and municipalities conduct investigations to determine the locations of inappropriate dry weather entries into storm drainage systems. One example of the magnitude of the problem associated with inappropriate discharges follows. A study in Sacramento, California (Montoya, 1987) found that slightly less than half the volume of water discharged from a stormwater drainage system was not directly attributable to rainfallinduced runoff. Illicit and/or inappropriate entries to the storm drainage system are likely sources of the additional discharges and can account for a significant amount of the pollutants discharged from storm drainage systems. The methods described in the following discussion were developed through U.S. EPA funding and can be applied to detection of inappropriate discharges associated with dry weather flows (Pitt et al., 1993). Common non-stormwater entries include sanitary wastewater; automobile maintenance and operation waste products; laundry wastewater; household toxic substances and pollutants; accident and spill waste streams; runoff from excessive irrigation; and industrial cooling water, rinse water, and other process wastewater. Although these sources can enter the storm drainage system a variety of ways, they generally result from (1) direct connections, such as wastewater piping either mistakenly or deliberately connected to the storm drains, or (2) indirect connections, which include infiltration into the storm drainage system and spills received by drain inlets. Sources of contamination can be divided into those discharging continuously and those discharging intermittently.

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Investigative Procedures The procedures described here provide an investigative procedure that will allow a user to first determine whether significant non-stormwater entries are present in a storm drain, and then identify the potential source responsible (e.g., industrial, residential, or commercial) as an aid to ultimately locating the source. Drainage Area Mapping The mapping exercise is carried out as a desktop operation using existing data/information and field visits to collect additional data/information and/or confirm existing information. It must contain complete descriptions of the drainage areas, including outfall locations, drainage system layout, subcatchment boundaries for each outfall, critical land use areas, permitted discharges to the storm drainage system, city limits, major streets, streams, etc. Tracer Selection To detect and identify non-stormwater entries, the dry weather outfall discharge is analyzed for selected tracers. The selected tracers are relatively unique components of the potential contaminating sources and hence provide a means to identify them. An ideal tracer should exhibit the following properties: • Significant difference in concentrations between polluting and nonpolluting sources • Small variations in concentrations within each likely pollutant source category • A conservative behavior (i.e., no significant concentration change due to physical, chemical, and/or biological processes) • Ease of measurement with safety, adequate detection limits, good sensitivity, and repeatability A review of case studies and literature characterizing potential inappropriate entries led to the recommended tracers (listed below) to identify common pollutant sources (e.g., sanitary wastewater, septic tank effluent, laundry wastewater, vehicle wash wastewater, potable water, and natural waters): • Specific Conductivity. Specific conductivity can be used as an indicator of dissolved solids. The variation between water and wastewater sources can be substantial enough to indicate the source of a dry weather flow, and because the measurement is easy, quick, and inexpensive, it is a suggested tracer. • Fluoride. Fluoride concentrations were shown to be a reliable indicator of potable water where fluoride levels in the raw water supply are adjusted to consistent levels and where groundwater has low to nonmeasurable natural fluoride levels. Fluoride can often be used to separate treated potable water from untreated water sources. Untreated water sources can include local springs, groundwater, regional surface flows, or nonpotable industrial waters. If the treated potable water has no fluoride added, or if the natural water has fluoride concentrations close to potable water fluoride concentrations, then fluoride may not be an appropriate indicator. Some industrial and commercial wastewaters may contain large concentrations of fluorides, making quantitative analyses difficult, however. • Hardness. Hardness is useful in distinguishing between natural and treated waters (like fluoride), as well as between clean treated waters and waters that have been subjected to domestic use. It should be noted that hardness of waters varies considerably from place to place, with groundwaters generally harder than surface waters. • Ammonia/Ammonium. The presence or absence of ammonia (NH3), or ammonium ion (NH4+), has been commonly used as a chemical indicator for prioritizing sanitary wastewater cross-connection drainage problems. Ammonia should be useful in identifying sanitary wastes and distinguishing them from commercial water usage.

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• Potassium. Greater potassium concentrations have been noted for sanitary wastewater compared to potable water. These potassium increases following domestic water usage reveal its potential as a tracer parameter. • Surfactants. Surfactants from detergents used in household and industrial laundering and other cleaning operations result in its high concentrations in wastewater. Anionic surfactants account for approximately two thirds of the total surfactants used in detergents, and are commonly measured as methylene blue active substances (MBAS). Some researchers (Alhajjar et al., 1989) have not found surfactants in septic tank effluent suggesting that surfactants can be totally degraded in the septic tanks. Surfactants can be used to identify sanitary or laundry wastewater sources and distinguish between infiltrating septic tank effluent and other washwaters. Surfactants were the most useful tracer to identify problematic waters. • pH. The pH of most dry weather flow sources is close to neutral (pH = 7). However, pH values may be extreme (below 6 and above 9) in certain inappropriate commercial and industrial flows or where groundwaters contain dissolved minerals. If unusual pH values are observed, then the drainage system needs to be carefully evaluated. Note that pH values are log-transformed values and therefore flow contributions cannot be proportioned using pH directly in the same way “linear” concentration values can. • Temperature. An elevated temperature of a receiving water can indicate contamination, particularly in cold weather. Sanitary wastewater and cooling water are examples of causes to temperature elevation and a rough heat balance may be conducted to identify a grossly contaminated outfall. It is essential that the investigation have adequate local tracer data for all the potential sources in a study area. Local tracer data are obtained by sampling discharges for specific desired tracers at potential pollution sources that produce specific process wastewaters, regardless of whether or not an illicit entry to the storm drainage system is present. This becomes the database of “local” characteristics of those tracers of that local area for comparison to background flows and storm drainage characteristics of that local area. For each tracer, the concentration means and standard deviations for all the potential source flows, including the natural waters or background waters (e.g., groundwaters). The data are necessary to confirm the source and the proportion of the outfall dry weather flow contributed by the source (example given later). Without this information the likelihood of identifying the pollutant sources is greatly reduced. It is important to note that the tracer data should not be built up from data obtained for other area investigations. A number of exotic tracers have also been proposed (cholesterol compounds, caffeine, pharmaceuticals, DNA characteristics of Escherichia coli bacteria, stable ion ratios, etc.), but the analytical methods are usually very expensive and the detection sensitivities are inadequate for many of these potential tracers. However, it is likely that some of these, or others, may become very useful through further research and method development. Field Surveys Field investigations are used to locate and record all outfalls, and involve physically wading, boating, etc. the receiving waters in search of all known and unknown outfalls. At each outfall the inspection and sampling should at least include the following: • • • •

Accurate location of outfall and assignment of ID number Photographs of outfall Outfall discharge flow rate estimate (and note whether continuous or intermittent discharge) Physical inspection and record of outfall characteristics including odor, color, turbidity, floatable matter (fecal matter, sanitary discards, solids, oil sheen, etc.), deposits, stains,

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vegetation affected by pollutants, damage to outfall structure, and discharge water temperature • Collection of dry weather discharge samples for tracer analyses in the laboratory (specific conductivity and temperature can be field measured) Intermittent flows will be more difficult to confirm and sample. Additional field visits, use of automatic samplers, and/or flow damming or screening techniques must be utilized for indicating and obtaining samples of intermittent flows. Analyses of Data/Samples The recommended analytical procedures and associated equipment in Pitt et al. (1993) have been selected based on laboratory and field testing of analytical methods using the following criteria: • • • • • •

Operator safety and absence of hazardous test components Appropriate detection limits Freedom from interferences Good analytical precision (repeatability) Low cost and good durability Minimal operator training

For consistent results the analyses should be carried out in the laboratory and not in the field, except for temperature and specific conductivity. Field analyses may be conducted for pH by using portable pH meters or litmus paper depending upon the degree of accuracy required and time constraints. Note that pH is a support tracer and not a primary parameter. The analysis method must provide adequate detection limits (i.e., measurement of the lowest required concentration) and precision (i.e., consistent results). To estimate the required detection limit it is necessary to know or estimate the tracer mean concentration and standard deviation. Investigation and Remediation The investigation of pollutant sources are divided into two major areas: 1. Analysis of outfall dry weather data/observations to correlate with potential sources: • Observable parameters • Simple checklist for major flow component identification • Flowchart for most significant flow component identification • Matrix algebra solution of simultaneous equations • Matrix algebra considering probability distributions of library data using Monte Carlo statistical modeling 2. Upstream surveys to progressively narrow the drainage area(s) of concern and locate the pollutant source(s) Observable parameters are items covered by physical inspection, consisting of odor, color, turbidity, floatables, stains, vegetation, etc. These parameters will be clearly visible and indicate gross contamination at outfalls and may be indicators of intermittent flows. Observable parameters cannot be relied upon as a sole method for the evaluation of outfalls. A contaminated discharge may not be visible and can only be determined by other methods (Lalor, 1993; Pitt et al., 1993).

EMERGING CRITICAL SOURCE AREA CONTROLS The following discussion presents a few specialized options that can be used at small critical source areas, or at existing stormwater controls where enhanced control is needed. These options cause minimal interference with the site use. Some of these devices have been in use for some time, but

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have not been widely implemented, while others are relatively new control devices with limited, but promising, performance information. There are obviously additional controls that may be suitable for this use, but the following controls (filtration, combination practices, and chemical assisted settling) have substantial available performance data and have been shown to be broadly applicable. These controls are used when significant stormwater quality improvement is needed, beyond the litter and grit capture capabilities of the above-described public works practices.

FILTRATION

OF

STORMWATER

Small source area stormwater runoff treatment devices using various forms of filtration have been developed and used at many locations, especially in areas having poor soils where infiltration (too clayey) or wet detention ponds (too sandy) cannot be used. The following paragraphs describe the different filtering media that have been evaluated for stormwater control, summarized from Clark and Pitt (1999). Sand The use of sand filtration is common throughout the United States. Water supply treatment plants have successfully used sand filtration for many years and wastewater treatment plants often use sand filtration to polish their effluent before release. Sand filtration of stormwater began in earnest in Austin, Texas, as a replacement to wet detention ponds in arid areas having sandy soils where it would be difficult to maintain a suitable wet pond. The Austin sand filters are used both for single sites and for drainage areas less than 20 ha. The filters are designed to hold and treat the first 13 mm of runoff and the pollutant removal ability of the sand filters has been found to be very good. According to the City of Austin design guidelines, the minimum depth of sand should be 0.5 m. If the city’s design guidelines are followed, the expected pollutant removal efficiencies, which are based upon the results of the City of Austin’s stormwater monitoring program, are as follows: Pollutant Fecal coliform bacteria Total suspended solids (TSS) Total nitrogen Total kjeldahl nitrogen Nitrate–nitrogen Total phosphorus BOD Total organic carbon Iron Lead Zinc

Removal Efficiency (%) 76 70 21 46 0 33 70 48 45 45 45

Source: City of Austin, 1988.

In Washington, D.C., sand filters are used both to improve water quality and to delay the entrance of large slug inputs of runoff into the combined sewer system. Water quality filters are designed to retain and treat 8 to 13 mm of runoff with the final design based on the amount of imperviousness in the watershed. The State of Delaware considers the sand filter to be an acceptable method for achieving the 80% reduction requirement of suspended solids. Sand filters in Delaware are intended for sites that have impervious areas that will drain directly to the filter. The purpose of the sand filter is to help prevent or postpone clogging of an infiltration device. According to the State of Delaware guidelines, sand filtration is “intended for use on small sites where overall site imperviousness is maximized.

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Examples of these sites would be fast food restaurants, gas stations or industrial sites where space for retrofitting with other infiltration devices, such as detention ponds, is not available” (Shaver, undated). According to Delaware’s recommendations, the sand filter will adequately remove particulates (TSS removal efficiency 75 to 85%) but will not remove soluble compounds. Studies of a sand filter in Maryland show that it is just now becoming clogged after 6 years of use in a heavily used parking lot. Inspection of the sand below the surface of the filter has shown that oil, grease, and finer sediments have migrated into the filter, but only to a depth of approximately 50 to 75 mm (Shaver, undated). It has been generally expected that sand would retain any particles that it trapped. However, during controlled tests Clark and Pitt (1999) found that fresh sand (without aging and associated biological growths) by itself did not retain stormwater toxicants (which are mostly associated with very fine particles). This lack of ability to retain stormwater toxicants prompted the investigation of other filtration media during their research. Combinations of filtration media, especially those using organic materials (activated carbon, peat moss, composted leaves, and ion-exchange resins) along with sand, were investigated for their ability to more permanently retain stormwater pollutants. Composted Leaves Composts made from yard waste, primarily leaves, have been found to have a very high capacity for adsorbing heavy metals, oils, greases, nutrients, and organic toxins due to the humic content of the compost. These humic compounds are stable, insoluble, and have a high molecular weight. The humics act like polyelectrolytes and adsorb the toxicants. The composted leaf filter was developed by W&H Pacific (now Stormwater Management, Inc.) for Washington County (Washington), the Unified Sewer Agency, and the Metropolitan Service District of Washington County (W&H Pacific, 1992). The initial filter design consists of a bottom impermeable membrane with a drainage layer above. Above the drainage layer is a geotextile fabric above which is the compost material. A new design, the CSF II includes a concrete vault, with a flow spreader and a main tank area. The tank includes modular units containing the compost, and the stormwater flows horizontally through the compost. These modular units can be easily removed for maintenance and be used for a variety or mixture of media. The actual pollutant removal occurs in the compost material. The removal processes that occur in the compost are filtration, adsorption, ion exchange, and biodegradation of organics. Testing of a prototype of the initial design has shown the following pollutant removal rates:

Pollutant

Removal Rate (%)

Turbidity Suspended solids Total volatile suspended solids COD Settleable solids Total phosphorus Total Kjeldahl nitrogen Cooper Zinc Aluminum Iron Petroleum hydrocarbons

84 95 89 67 96 40 56 67 88 87 89 87

Source: W&H Pacific, 1992.

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Peat Moss Peat is partially decomposed organic material, excluding coal, that is formed from dead plant remains in water in the absence of air. The physical structure and chemical composition of peat is determined by the types of plants (mosses, sedges, and other wetland plants) from which it is formed. Peat is physically and chemically complex and is highly organic. The main components of peat are humic and fulvic acids and cellulose. The permeability of peat varies greatly and is determined by its degree of decomposition and the plants from which it came. Generally, the more decomposed the peat, the lower its hydraulic conductivity. Peats are generally lightweight when dry and are highly adsorptive of water. Because of the lignins, cellulosic compounds, and humic and fulvic acids in peat, peat is highly colloidal and has a high cation-exchange capacity. Peat also is polar and has a high specific adsorption for dissolved solids such as transition metals and polar organic compounds. Peat has an excellent natural capacity for ion exchange with copper, zinc, lead, and mercury, especially at pH levels between 3.0 and 8.5. This adsorption, complexing, and exchange of various metal cations occur principally through the carboxyl, phenolic, and hydroxyl groups in the humic and fulvic acids. This capacity to bind and retain cations, however, is finite and reversible and is determined mostly by the pH of the solution. Peat is an excellent substrate for microbial growth and assimilation of nutrients and organic waste materials because of its high C:N:P ratio, which often approaches 100:10:1. Nitrifying and denitrifying bacteria are typically present in large numbers in natural peat. The ability of peat to retain phosphorus in the long term is related to its calcium, aluminum, iron, and ash content with the higher the content of each of the above constituents, the higher the retention capability. Peat moss (sphagnum moss) is a fibric peat. It has easily identifiable undecomposed fibrous organic materials and its bulk density is generally less than 0.1 g/cc. Because of its highly porous structure, peat moss can have a high hydraulic conductivity, up to 140 cm/h. It is typically brown and/or yellow in color and has a high water holding capacity. For filtration devices, peat generally has been combined with sand to create a peat–sand filter (PSF). The PSF is an anthropogenic filtration system, unlike the sand or peat filtration systems that were first used as wastewater treatment systems in areas where these soils naturally occur. The PSF removes most of the phosphorus, biological oxygen demand (BOD), and pathogens, and with a good grass cover additional nutrient removal occurs. The PSF system developed by the Metropolitan Washington Council of Governments (Washington, D.C.) has a good grass cover on top underlain by 300 to 500 mm of peat. The peat layer is supported by a 100-mm mixture of sand and peat, which is supported by a 500- to 600-mm layer of fine to medium grain sand. Under the sand is gravel and the drainage pipe. The mixture layer is required because it provides the necessary continuous contact between the peat and the sand layers, ensuring a uniform water flow. Because this is a biological filtration system, it works best during the growing season when the grass cover can provide the additional nutrient removal that will not occur in the peat–sand regimes of the system (Galli, 1990). The PSF is usually an aerobic system. However, modifications to the original design by the Metropolitan Washington Council have been made to account for atypical site conditions or removal requirements. The estimated pollutant removal efficiency for the PSF system for stormwater runoff is given below: Pollutant

Removal Efficiency (%)

Suspended solids Total phosphorus Total nitrogen BOD Trace metals Bacteria

90 70 50 90 80 90

Source: Galli, 1990.

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Design of Stormwater Filters The information obtained by Clark and Pitt (1999) can be used to develop design guidelines for stormwater filtration, especially in conjunction with additional reported information in the literature. The design of a stormwater filter needs to be divided into two phases. The first phase is the selection of the media to achieve the desired pollutant removal goals. The second phase is the sizing of the filter to achieve the desired run time before replacement of the media. The main objective of this research was to monitor a variety of filtration media to determine their pollutant removal capabilities, as noted previously. However, it soon became apparent that the filters were more limited by clogging caused by suspended solids in the stormwater, long before reductions in their pollutant removal capabilities could be identified. Pretreatment of the stormwater so the suspended solids content is about 10 mg/L, or less, is probably necessary to take greater advantage of the pollutant retention capabilities of most media. This level of pretreatment, however, may make further stormwater control unnecessary, except for unusual conditions. Of course, it may be more cost-effective to consider shortened filter run times, without pretreatment, and not utilize all of the pollutant retention capabilities of the media. Urbonas (1999) monitored the performance of full-scale stormwater filters in Denver and reported serious problems with untreated stormwater bypassing the filters due to clogging. Therefore, much care needs to be taken when designing stormwater filtration to ensure acceptable performance over relatively long periods. Selection of Filtration Media for Pollutant Removal Capabilities The selection of the filter media needs to be based on the desired pollutant removal performance and the associated site conditions. If based on suspended solids alone for untreated stormwater (a likely common and useful criteria, but resulting in shortened service life), then the filtration media would be ranked according to the following: 1. >90% control of suspended solids: compost/sand, activated carbon/sand, Zeolite/sand, Enretech/sand 2. 80 to 90% control of suspended solids: sand, peat/sand 3. Very little control of suspended solids: filter fabrics If based on a wider range of pollutants for untreated stormwater, then the ranking would be as follows: 1. Sand, activated carbon/sand, Enretech/sand (no pollutant degradation, but sand by itself may not offer “permanent” pollutant retention until it is aged and has biological growths and/or deposition of silts and oils 2. Zeolite/sand (no degradation) 3. Compost/sand (color degradation) 4. Peat moss/sand (turbidity and pH degradation) 5. Filter fabrics alone (very little pollutant removal benefit) Presettling of the stormwater was conducted to reduce the solids loadings on the filters to increase the run times before clogging to take better advantage of the pollutant retention capabilities of the media. Settling reduced the stormwater suspended solids to about 10 mg/L, with about 90% of the particles (by volume) less than 10 µm in size. The untreated stormwater had a suspended solids concentration of about 30 to 50 mg/L, but many of the particles were larger, with about 90% of the particles less than 50 µm. The presettling also reduced the other stormwater pollutants (color and turbidity by about 50%, and COD by about 90%, for example). This presettling was similar to what would occur with a well-designed and well-operated wet detention pond. This presettling had a significant effect on the filter performance, as noted, and the rankings would be as follows, considering a wide range of stormwater pollutants (suspended solids removal by itself would not be a suitable criterion, as it is not likely to be reduced any further by the filters after the presettling):

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TABLE 5.1 Expected Full-Scale Media Flow Capacities

1. 2. 3. 4.

Capacity to <1 m/day

Capacity to 10 m/day

Filtration Media Category

5,000 gSS/m2 5,000 10,000 15,000

1,250 gSS/m2 2,500 5,000 7,500

Enretech/sand; forest/sand Compost/sand; peat/sand Zeolite/sand; act. carbon/sand Sand

Peat moss/sand (with degradation in color, turbidity, and pH) Activated carbon/sand (no degradation, but fewer benefits) Enretech/sand, forest/sand, sand (few changes, either good or bad) Compost/sand (many negative changes)

Obviously, knowing the stormwater control objectives and options will significantly affect the selection of the treatment media. This is most evident with the compost material. If suspended solids removal is the sole criterion, with minimal stormwater pretreatment, then it is the recommended choice (if one can live with a slight color increase in the stormwater, which is probably not too serious). However, if a filter is to be used after significant pretreatment to obtain a longer filter life, a compost filter would be the last choice (not considering economics). Design of Filters for Specified Filtration Durations The filtration durations measured during these tests can be used to develop filter designs. It is recommended that allowable suspended solids loadings be used as the primary controlling factor in filtration design. Clogging is assumed to occur when the filtration rate becomes less than about 1 m/day. Obviously, the filter would still function at smaller filtration flow rates, especially for the smallest rains in arid areas, but an excessive amount of filter bypassing would likely occur for moderate rains in humid areas. The wide ranges in filter run times as a function of water are mostly dependent on the suspended solids content of the water, especially when the water is pretreated. Therefore, the suspended solids loading capacities are recommended for design purposes. The filter capacity ranges may be grouped into the following approximate categories, as shown on Table 5.1. Filter designs can be made based on the predicted annual discharge of suspended solids to the filtration device and the desired filter replacement interval. As an example, Table 5.2 shows the

TABLE 5.2 Volumetric Runoff Coefficients (Rv) for Different Urban Areas Area Low density residential land use Medium density residential land use High density residential land use Commercial land use Industrial land use Paved areas Sandy soils Clayey soils

Volumetric Runoff Coefficient (Rv ) 0.15–0.35 0.3–0.5 0.5–0.75 0.8 0.6 0.85 0.1 0.3

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TABLE 5.3 Typical Suspended Solids Concentrations in Runoff from Various Urban Surfaces Suspended Solids Concentration (mg/L)

Source Area Roof runoff Paved parking, storage, driveway, streets, and walk areas Unpaved parking and storage areas Landscaped areas Construction site runoff Combined sewer overflows Detention pond water Mixed stormwater Effluent after high level of pretreatment of stormwater

10 50 250 500 10,000 100 20 150 5

volumetric runoff coefficients (Rv) that can be used to approximate the fraction of the annual rainfall that would occur as runoff for various land uses and surface conditions, based on small storm hydrology concepts (Pitt, 1987). In addition, Table 5.3 summarizes likely suspended solids concentrations associated with different urban areas and waters. Using the information in the above two tables and the local annual rain depth, it is possible to estimate the annual suspended solids loading from an area. The following three examples illustrate these simple calculations. EXAMPLE 5.1 A 1.0 ha paved parking area, in an area receiving 1.0 m of rain per year:

(50 mg

(

)(

)

SS l) (0.85) (1 m year ) (1 ha ) 10, 000 m 2 ha 1000 l m 3 (g 1000 mg)

= 425, 000 g SS year Therefore, if a peat–sand filter is to be used having an expected suspended solids capacity of 5000 g/m2 before clogging, then 85 m2 of this filter will be needed for each year of desired operation for this 1.0-ha site. This is about 0.9% of the paved area per year of operation. If this water is pretreated so the effluent has about 5 mg/L suspended solids, then only about 0.2% of the contributing paved area would be needed for the filter. A sand filter would only be about 1/3 of this size.

EXAMPLE 5.2 A 100 ha medium density residential area having sandy soils and 1.0 m of rain per year:

(150 mg

(

)(

)

SS l) (0.3) (1 m year ) (100 ha ) 10, 000 m 2 ha 1000 l m 3 (g 1000 mg)

= 45, 000, 000 g SS year The unit area loading of suspended solids for this residential area (450 kg SS/ha-year) is about the same as in the previous example (425 kg SS/ha-year), requiring about the same unit area dedicated for the filter. The reduced amount of runoff is balanced by the increased suspended solids concentration.

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EXAMPLE 5.3 A 1.0 ha rooftop in an area having 1.0 m of rain per year:

(10 mg

(

)(

)

SS l) (0.85) (1 m year ) (1 ha ) 10, 000 m 2 ha 1000 l m 3 (g 1000 mg)

= 85, 000 g SS year The unit area loading of suspended solids from this area (85 kg SS/ha-year) is much less than for the other areas and would only require a filter about 0.2% of the roofed drainage area per year of operation.

It is recommended that the filter media be about 50 cm in depth and that a surface grass cover be used, with roots not extending beyond half of the filter depth. This should enable a filtration life of about five times the basic life observed during these tests. In addition, it is highly recommended that significant pretreatment of the water be used to reduce the suspended solids concentrations to about 10 mg/L before filtration for pollutant removal. This pretreatment can be accomplished using grass filters, wet detention ponds, or other specialized treatment (such as the sedimentation chamber in the MCTT). The selection of the specific filtration media should be based on the desired pollutant reductions, but should in all cases include amendments to plain sand if immediate and permanent pollutant reductions are desired.

CHEMICAL-ASSISTED SEDIMENTATION Chemical addition has been used for many years in water treatment, and in lake management. More recently, full-scale implementations of chemical-assisted settling has been used for the treatment of stormwater in wet detention ponds or at outfalls into small urban lakes. The chemicals tested and used include alum (generally a complex of aluminum and sulfate), ferric chloride, and aluminum chloride compounds, plus various coagulant aids. Gietz (1983), in a series of laboratory tests in Ontario, found that an alum dosage of 4 to 6 mg/L was the most effective for highly polluted stormwater runoff. Overdosages of alum and ferric chloride generally gave poor results. He found that it was difficult to add the correct dosage of coagulant because of the changing pollutant concentrations in the runoff. Low flow velocities also reduced mixing effectiveness and may require mechanical assistance. The flocs that were formed with the coagulants were easily disturbed by runoff turbulence. Kronis (1982), in a series of Ontario bench- and pilot-scale tests, found that disinfection of stormwater with NaOCl at 5 mg/L available chlorine reduced fecal coliform populations to less than 10 organisms per 100 ml. He identified alum dosages of 30 mg/L as a preferred coagulant, with 10 to 30% increases in removals of particulate residue, BOD5, COD, and total phosphorus as compared to plain sedimentation. However, chemical-assisted settling generally produced moderate and erratic reductions in bacteria populations. Disinfection in wet detention ponds may be expensive, but it may be the only feasible method of significantly reducing bacteria populations in areas with serious bacteria problems. Heinzmann (1993) described the development of a coagulation and flocculation treatment procedure for stormwater in Berlin, Germany. He found that because the stormwater was weakly buffered and was very soft, a polyaluminum chloride, with a cationic coagulant aid (polyacrylamid), was most suitable. A constant dosage of 0.06 mmol/L (as Al) was used, resulting in pH levels always greater than 6. The constant dosage was possible because the pH and buffering capacity of the stormwater was relatively constant during storms. He found that the best enhanced stormwater treatment process used coagulation and flocculation in a pipe designed for both microfloc and macrofloc formation, and final separation by filtration. The filtration was much better than the 1 h sedimentation typically used in Berlin sedimentation tanks. He did find that a 6 min flocculation

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time was sufficient before filtration. He found significant removals of phosphorus (to <0.2 mg/L), organic compounds (including PCB and PAHs), solids (to <5 mg/L), lead, and copper. However, very poor removal of zinc was noted, and pollution prevention (decreased use of galvanized metals) was recommended. In the 1-h sedimentation tanks, without any chemical addition, the phosphorus (about 0.5 mg/L) and solids (about 50 mg/L) effluent concentrations were not nearly as low. The costs for this enhanced treatment (7 to 10 DM/m3 in 1990) was about 10 to 40% higher than with the ordinary 1-h sedimentation tanks alone. Pitt et al. (1994) described a full-scale stormwater treatment plant, using the Karl Dunkers system for treatment of separate stormwater and lake water. This system has been operating since 1981 in Lake Rönningesjön, near Stockholm, Sweden. The treatment facility uses ferric chloride and polymer precipitation and crossflow lamella clarifiers for the removal of phosphorus. The overall phosphorus removal rate for the 11 years from 1981 through 1991 was about 17 kg/year. About 40% of the phosphorus removal occurred from sedimentation processes, while the remaining removal occurred in the chemical treatment facility. This phosphorus removal would theoretically cause a reduction in phosphorus concentrations of about 10 µg/L per year in the lake, or a total phosphorus reduction of about 100 µg/L during the data period since the treatment system began operation. About 70% of this phosphorus removal was associated with the treatment of stormwater, while about 30% was associated with the treatment of lake water. The lake phosphorus concentration improvements averaged about 50 µg/L, only about one half the theoretical improvement, probably because of sediment–water interchange of phosphorus, or other unmeasured phosphorus sources. The 1996 NALMS (North American Lakes Management Society) conference included several presentations describing the use of alum for stormwater treatment. Harper and Herr (1996) described the historical use of alum to treat stormwater entering Lake Ella in Tallahassee, Florida, which began in 1986. A liquid slurry of alum is injected into the major storm drainage entering the lake, on a flow-weighted basis during rains. The alum forms precipitates with phosphorus, suspended solids, and heavy metals, which then settle in the lake. This treatment system resulted in immediate and substantial improvements to Lake Ella water quality. There are currently 23 alum stormwater treatment systems in Florida. Harper and Herr (1996) report that alum treatment of stormwater has consistently achieved 90% reductions in total phosphorus, 50 to 70% reductions in total nitrogen, 50 to 90% reductions in heavy metals, and >99% reductions in fecal coliform bacteria. The precipitates of the phosphorus and heavy metals have been shown to be extremely stable over a wide range of dissolved oxygen and pH conditions. Harper and Herr (1996) also reported on a very large alum project at Lake Maggiore in St. Petersburg. This 156-ha lake receives stormwater from a 927-ha watershed. Water quality problems were noted as early as the 1950s; they included fish kills, algal blooms, nuisance macrophyte algal growths, and high bacteria levels. An environmental assessment determined that an 80% reduction in the annual phosphorus discharges from the stormwater and base flow would result in an acceptable trophic status for the lake. Five alum treatment plants were then designed and were put in operation in August 1997, comprising the largest alum stormwater treatment system ever built. An alum pilot-scale treatment system for stormwater, located in Minnesota, was described by Kloiber and Brezonik (1996). This system injected 1 mg/L (as Al) alum into a storm sewer at a pumping station just upstream of a 1.2-acre wet detention pond. The few minutes travel time between injection and the pond allowed 75 to 80% reductions in soluble reactive phosphorus. However, the pond retained only 40% of the added aluminum, increasing to 70% when a coagulant aid was used. The lowest total aluminum concentration in the pond effluent was 0.26 mg/L, still exceeding the water quality standard. They concluded that closer evaluations of the toxicity and bioavailability of the aluminum associated with alum stormwater treatment are needed. During treatability tests of stormwater from critical source areas, Pitt et al. (1995) found that alum addition significantly increased the toxicity of the water (as indicated using the Microtox screening procedure). Pitt (preliminary findings) recently conducted a series of chemical addition treatability tests for stormwater. He examined alum, ferric chloride, and ferric sulfate (all with and without organic

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polymers), and organic polymers alone. He also tested the benefits of adding a microsand (75 to 150 µm) as a coagulant aid. Preliminary findings indicate that ferric chloride with the microsand is the most effective chemical for treating stormwater. The concentrations of the ferric chloride are in the range of 30 to 80 mg/L, and the microsand is added to produce a turbidity of about 200 NTU. Heavy metals (copper, lead, and zinc, in both particulate and filterable forms) and toxicants (as indicated by the Microtox screening test) removals have been greater than 80%, with many tests greater than 95%. Phosphates are also significantly reduced (by about 50%). Alum actually added toxicity (possibly through zinc contamination in the alum, or by the dissolved aluminum) and many of the polymers also added COD and toxicity. It was not clear how sensitive dosage control would have to be to provide acceptable levels of heavy metal control by chemical treatment in stormwater.

COMBINATION PRACTICES Example of Combination Practices using Filtration: The Multichambered Treatment Train (MCTT) The MCTT is an example of a stormwater device that utilizes a combination of processes, especially pretreatment of stormwater using sedimentation, followed by media filtration. It was developed based on early U.S. EPA-sponsored research on treatability of stormwater at critical source areas (Pitt et al., 1995). The MCTT contains aeration, sedimentation, sorption, and sand–peat filtration and was developed by Pitt et al. (1999). The MCTT is most suitable for use at relatively small and isolated paved critical source areas, from about 0.1 to 1 ha (0.25 to 2.5 acre) in area, where surface land is not available for stormwater controls. Typical locations include gas stations, junk yards, bus barns, public works yards, car washes, fast-food restaurants, convenience stores, etc., and other areas where the stormwater has a high probability of containing high concentrations of oils and filterable toxic pollutants that are difficult to treat by other means. A typical MCTT requires between 0.5 and 1.5% of the paved drainage area, which is about 1/3 of the area required for a well-designed wet detention pond, and is generally installed below ground. A pilot-scale MCTT was constructed in Birmingham, Alabama, at a large parking area at the University of Alabama at Birmingham campus, and tested over a 6-month monitoring period. Two additional full-scale MCTT units have also been constructed and were monitored as part of the Wisconsin 319 grant from the U.S. EPA. Complete organic and metallic toxicant analyses, in addition to conventional pollutants, are included in the evaluation of these units. Figure 5.3 shows a general cross-sectional view of the MCTT. It includes a special catchbasin followed by a two-chambered tank that is intended to reduce a broad range of toxicants (volatile,

Catchbasin – Packed column aerators Q1

Main Settling Chamber – Sorbent pillows – Fine bubble aerators – Tube settlers

Filtering Chamber – Sorbent filter fabric, – Mixed media filter layer (sand and peat) – Filter fabric – Gravel packed underdrain

Q0

FIGURE 5.3 General schematic of MCTT. (From Pitt, R. et al., EPA/600/R-99/017, 1999.)

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TABLE 5.4 Median Toxicity Reductions for Different Treatment Holding Times Holding Period for 2.1 m depth (h)

Median Toxicity Reduction (%) per Individual Rain

6 12 24 36 48 72

46 60 75 84 92 100

TABLE 5.5 Effects of Storage Volume and Holding Periods on Annual Runoff Treated and on Total Annual Toxicity Reduction (Birmingham, AL, rains) Storage Volume Corresponding to 12.7 mm Rain with 10.2 mm Runoff (0.50 in. rain with 0.40 in. runoff)

Storage Volume Corresponding to 38.1 mm Rain with 33.5 mm Runoff (1.50 in. rain with 1.32 in. runoff)

Holding Period (h)

% Annual Runoff Treated

% Annual Toxicity Reduction

% Annual Runoff Treated

% Annual Toxicity Reduction

6 12 24 36 48 72

84 62 52 48 46 44

36 37 39 41 42 44

100 100 98 91 88 84

46 60 73 77 81 84

particulate, and dissolved). The MCTT includes a special catchbasin (based on the Lager et al. 1977 design) followed by two tank chambers that are intended to reduce a broad range of suspended solids and stormwater toxicants (volatile, particulate, and dissolved). The runoff enters the catchbasin chamber by passing over a flash aerator (small column packing balls with countercurrent airflow) to remove any highly volatile components present in the runoff (unlikely). This catchbasin also serves as a grit chamber to remove the largest (fastest settling) particles. The second chamber serves as an enhanced settling chamber to remove smaller particles and has inclined tube settlers to enhance sedimentation. The settling time in this main settling chamber usually ranges from 20 to 70 h. This chamber also contains fine bubble diffusers and sorbent pads to further enhance the removal of floatable hydrocarbons and additional volatile compounds. The water is then pumped to the final chamber at a slow rate to maximize pollutant reductions. The final chamber contains a mixed media (sand and peat) slow filter, with a filter fabric layer. The MCTT is typically sized to totally contain all of the runoff from a 6 to 20 mm (0.25 to 0.8 in.) rain, depending on treatment objectives, interevent time, typical rain size, and rain intensity for an area. Table 5.4 shows the median toxicity reductions for various holding times for a 2.1-m-deep main settling chamber, based on laboratory bench-scale treatability tests. Table 5.5 shows how this device would operate for Birmingham, Alabama, rains. Short holding times result in much of the annual rainfall being treated (the unit is empty before most of the rains begin, because it rains about every 3 to 5 days), but each rain is not treated very well, because of the short settling periods. Therefore,

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the annual treatment level approaches a constant level with long holding periods. In this example, a relatively large main settling chamber is needed to contain large fractions of most of the rains. Long-term continuous analyses have been conducted to identify the most cost-effective MCTT sizes (and holding times) for different treatment objectives for many U.S. locations (Pitt, 1996). During monitoring of 13 storms at the Birmingham pilot-scale MCTT facility (designed for 90% toxicity reductions), the following overall median removal rates were observed: 96% for total toxicity (as measured using the Microtox screening test), 98% for filtered toxicity, 83% for suspended solids, 60% for COD, 40% for turbidity, 100% for lead, 91% for zinc, 100% for n-nitrodi-n-proplamine, 100% for pyrene, and 99% for bis(2-ethyl hexyl)phthalate. The color was increased by about 50% due to staining from the peat and the pH decreased by about one half pH unit, also from the peat media. Ammonia nitrogen was increased by several times, and nitrate nitrogen had low removals (about 14%). The MCTT performed better than intended because of the additional treatment provided by the final ion-exchange/filtration chamber. It had very effective removal rates for both filtered and particulate stormwater toxicants and suspended solids. Increased filterable toxicant removals were obtained in the peat–sand mixed media filter/ion-exchange chamber, at the expense of increased color, lowered pH, and depressed COD and nitrate removal rates. Increases in color and a slight decrease in pH occurred during the filtration step at the pilotscale unit. The main settling chamber resulted in substantial reductions in total and dissolved toxicity, lead, zinc, certain organic toxicants, suspended solids, COD, turbidity, and color. The filter/ion-exchange unit is also responsible for additional filterable toxicant reductions. However, the catchbasin/grit chamber did not indicate any significant improvements in water quality, although it is an important element in reducing maintenance problems by trapping bulk material. Results from the full-scale tests of the MCTT in Wisconsin (Corsi et al., 1999) are encouraging and corroborate the high levels of treatment observed during the Birmingham pilot-scale tests. Table 5.6 shows the treatment levels that have been observed during seven tests in Minocqua (during 1 year of operation) and 15 tests in Milwaukee (also during 1 year of operation), compared to the pilot-scale Birmingham test results (13 events). These data indicate high reductions for suspended solids (83 to 98%), COD (60 to 86%), turbidity (40 to 94%), phosphorus (80 to 88%), lead (93 to 96%), zinc (90 to 91%), and for many organic toxicants (generally, 65 to 100%). The reductions of dissolved heavy metals (filtered through 0.45-µm filters) were also all greater than 65% during the full-scale tests. None of the organic toxicants was ever observed in effluent water from either full-scale MCTT, even considering the excellent detection limits available at the Wisconsin State Department of Hygiene Laboratories that conducted the analyses. The influent organic toxicant concentrations were all less than 5 µg/L and were only found in the unfiltered sample fractions. The Wisconsin MCTT effluent concentrations were also very low for all of the other constituents monitored: <10 mg/L for suspended solids, <0.1 mg/L for phosphorus, <5 µg/L for cadmium and lead, and <20 µg/L for copper and zinc. The pH changes in the Milwaukee MCTT were much less than those observed during the Birmingham pilot-scale tests, possibly because of the added activated carbon in the final chamber in Milwaukee. Color was also much better controlled in the full-scale Milwaukee MCTT. The Milwaukee installation is at a public works garage and serves about 0.1 ha (0.25 acre) of pavement. This MCTT was designed to withstand very heavy vehicles driving over the unit. The estimated cost was $54,000 (including a $16,000 engineering cost), but the actual total capital cost was $72,000. The high cost was likely due to uncertainties associated with construction of an unknown device by the contractors and because it was a retrofit installation. As an example, installation problems occurred due to sanitary sewerage not being accurately located as mapped. The Minocqua site is at a 1-ha (2.5-acre) newly paved parking area serving a state park and commercial area. It was located in a grassed area and was also a retrofit installation, designed to fit within an existing storm drainage system. The installed capital cost of this MCTT was about $95,000 and included the installation of the 3.0 × 4.6 m (10 × 15 ft) box culverts used for the main

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TABLE 5.6 Performance Data for Full-Scale MCTT Tests, Compared to Birmingham Pilot-Scale MCTT Results (median reductions and median effluent quality) Milwaukee MCTT (15 events) Suspended solids Volatile suspended solids COD Turbidity pH Ammonia Nitrates Phosphorus (total) Phosphorus (filtered) Microtox toxicity (total) Microtox toxicity (filtered) Cadmium (total) Cadmium (filtered) Copper (total) Copper (filtered) Lead (total) Lead (filtered) Zinc (total) Zinc (filtered) Benzo(a)anthracene Benzo(b)fluoranthene Dibenzo(a,h)anthracene Fluoranthene Indeno(1,2,3-cd)pyrene Phenanthrene Pentachlorophenol Phenol Pyrene

98 (<5 mg/L) 94 (<5 mg/L) 86 (13 mg/L) 94 (3 NTU) –7 (7.9 pH) 47 (0.06 mg/L) 33 (0.3 mg/L) 88 (0.02 mg/L) 78 (0.002 mg/L) na na 91 (0.1 µg/L) 66 (0.05 µg/L) 90 (3 µg/L) 73 (1.4 µg/L) 96 (1.8 µg/L) 78 (<0.4 µg/L) 91 (<20 µg/L) 68 (<8 µg/L) >45 (<0.05 µg/L) >95 (<0.1 µg/L) 89 (<0.02 µg/L) 98 (<0.1 µg/L) >90 (<0.1 µg/L) 99 (<0.05 µg/L) na na 98 (<0.05 µg/L)

Minocqua MCTT (7 events) 85 na na na na na na 80 na na na na na 65 na nd na 90 na >65 >75 >90 >90 >95 >65 na na >75

(10 mg/L)

(<0.1 mg/L)

(15 µg/L) (<3 µg/L) (15 µg/L) (<0.2 (<0.1 (<0.1 (<0.1 (<0.1 (<0.2

µg/L) µg/L) µg/L) µg/L) µg/L) µg/L)

(<0.2 µg/L)

Birmingham MCTT (13 events) 83 66 60 40 8 -210 24 nd nd 100 87 18 16 15 17 93 42 91 54 nd nd nd 100 nd nd 100 99 100

(5.5 mg/L) (6 mg/L) (17 mg/L) (4.4 NTU) (6.4 pH) (0.31 mg/L) (1.5 mg/L)

(0%) (3%) (0.6 µg/L) (0.5 µg/L) (15 µg/L) (21 µg/L) (<2 µg/L) (<2 µg/L) (18 µg/L) (6 µg/L)

(<0.6 µg/L)

(<1 µg/L) (<0.4 µg/L) (<0.5 µg/L)

Note: na = not analyzed; nd = not detected in most of the samples.

settling chamber (13 m, or 42 ft, long) and the filtering chamber (7.3 m, or 24 ft, long). In perspective, these costs are about equal to the costs of installation of porous pavement (about $40,000 per acre of pavement).

SUMMARY There are numerous stormwater treatment options available. General approaches for stormwater control can be described based on site-specific rainfall and runoff distributions and a knowledge of local receiving water problems and use goals. The following general strategy could be reasonably followed, with numerous exceptions and substitutions available. On-site infiltration can be utilized to completely control (and eliminate) runoff from the smallest, and most common storms. This would significantly reduce the number of events occurring (and decrease the violations of bacteria and some heavy metals), while helping to match the predevelopment hydrology of the area (protecting in-stream habitat). Moderate to large events are most effectively controlled through wet

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detention ponds where the water is treated before discharge. The large events are the basis for storm drainage design to prevent flooding damage. The largest events that may occur in an area will exceed the capacity of the storm drainage system. Therefore, site development must provide for these failures by directing excess water through safe secondary drainage systems and away from homes and primary transportation corridors. This general approach is most suitable in developing areas. Retrofitting stormwater controls is much more challenging, less effective, and more costly. In addition to the general approach outlined above, special consideration needs to be applied to critical source areas. These areas have higher than normal unit area pollutant loadings, especially for toxicants. Typical critical source areas include areas having some of the following characteristics: large amounts of pavement, storage of equipment or materials, scrap yards, frequent automobile starts, vehicle maintenance areas, etc. Public works practices (catchbasin inlets, oil/water separators, and street cleaning) have been used in these areas to control stormwater. However, these practices are mostly limited to litter and gross pollution control (obvious needed objectives), but have limited pollutant control for most pollutants of interest (such as nutrients, bacteria, solids, and toxicants). Emerging critical source area controls are being developed to provide a much greater level of treatment for stormwater from these areas. These controls must be applicable to small, isolated areas and have minimal disturbance to site activities. Some of the most commonly used controls suitable for this use are stormwater filters. Newly developed information for different types of stormwater filters (and combination practices) indicate that moderate to high levels of treatment are possible for runoff from these critical areas. Therefore, a successful stormwater management program for an area must be driven on site knowledge and objectives and may include numerous and different stormwater control practices.

REFERENCES Aires, N. and Tabuchi, J.P., 1995. Hydrocarbon separators and stormwater treatment [in French], TSM, Spec. Iss. Stormwater, 11, 862–864. Alhajjar, B.J., Harkin, J.M., and Chesters, G., 1989. Detergent formula and characteristics of wastewater in septic tanks, J. Water Pollut. Control Fed., 61(5). API (American Petroleum Institute), 1990. Monographs of Refinery Environmental Control — Management of Water Discharges. Design and Operation of Oil-Water Separators, American Petroleum Institute, Washington, D.C. Aronson, G., Watson, D., and Pisano, W., 1983. Evaluation of Catchbasin Performance for Urban Stormwater Pollution Control, U.S. Environmental Protection Agency, Grant R-804578, EPA-600/2-83-043, 78 pp., Cincinnati, June. Bannerman, R., Baun, K., Bohn, M., Hughes, P.E., and Graczyk, D.A., 1983. Evaluation of Urban Nonpoint Source Pollution Management in Milwaukee County, Wisconsin, PB 84-114164, U.S. Environmental Protection Agency, Chicago, IL. Butler, D. and Karunaratne, S.H.P.G., 1995. The suspended solids trap efficiency of the roadside gully pot, Water Res., 29(2), 719–729. Butler, D., Xiao, Y., Karunaratne, S.H.P.G., and Thedchanamoorthy, S., 1995. The gully pot as a physical and biological reactor, Water Sci. Technol., 31(7), 219–228. City of Austin, Texas, 1988. Design Guidelines for Water Quality Control Basins, Environmental DCM, City of Austin Transportation and Public Services Department. Clark, D.E., Jr., and Cobbin, W.C., 1963. Removal Effectiveness of Simulated Dry Fallout from Paved Areas by Motorized and Vacuumized Street Sweepers, USNRDL-TR-746, U.S. Naval Radiological Defense Laboratory, Alexandria, VA. Clark, S. and Pitt, R., 1999. Stormwater Treatment at Critical Areas, Vol. 3: Evaluation of Filtration Media for Stormwater Treatment, U.S. Environmental Protection Agency, Water Supply and Water Resources Division, National Risk Management Research Laboratory, Cincinnati, OH, 442 pp.

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Corsi, S., Greb, S., Bannerman, R., and Pitt, R., 1999. Evaluation of the Multi-Chambered Treatment Train, a Retrofit Water Quality Management Practice, U.S. Geological Survey Open-File Report, Middleton, WI. Delaine, J., 1995. Separating oil from water offshore, Chem. Eng., 419, 31–34. Ellis, J.B., 1986. Pollutional aspects of urban runoff, in Urban Runoff Pollution, NATO ASI Series, Vol. G10, Springer Verlag, Berlin, 1–38. Ford, D., 1978. Technologies for removal of hydrocarbons from surface and groundwater sources, in Oil in Freshwater: Chemistry, Biology, Countermeasure Technology, Vandermeulen, J.G. and S.E. Hruey, Eds., Pergamon Press, New York, 413–430. Fourage, M., 1992. Assessment of the efficiency of a prefabricated separator for stormwater treatment. Thoughts to and tests of materials to trap hydrocarbons (in French), unpublished DESS student report, Universities of Nancy and Metz, September. Galli, J., 1990. Peat-Sand Filters: A Proposed Stormwater Management Practice for Urbanized Areas, prepared for the Coordinated Anacostia Retrofit Programme and Office of Policy and Planning, D.C. Department of Public Works, December. Gietz, R.J., 1983. Urban Runoff Treatment in the Kennedy-Burnett Settling Pond, prepared for the Rideau River Stormwater Management Study, Pollution Control Division, Works Department, Regional Municipality of Ottawa-Carleton, Ottawa, Ontario, March. Gregory, M. and James, W., 1996. Management of time-series data for long-term, continuous modeling, Advances in Modeling the Management of Stormwater Impacts, Computational Hydraulics International, Guelph, Ontario, 115–151. Grottker, M., 1990. Pollutant removal by gully pots in different catchment areas, Sci. Total Environ., 93, 515–522. Harper, H.H. and Herr, J.L., 1996. Alum treatment of stormwater — the first ten years: what have we learned and where do we go from here? presented at North American Lake Management Society (NALMS) Conference, Minneapolis/St. Paul, October. Heaney, J.P., Huber, W.C., Medina, M.A., Jr, Murphy, M.P., Nix, S.J., and Hasan, S.M., 1977. Nationwide Evaluation of Combined Sewer Overflows and Urban Stormwater Discharges, Volume II: Cost Assessment and Impacts, EPA-600/2-77-064b, NTIS PB-266005, U.S. Environmental Protection Agency, Cincinnati, OH, March, 363 pp. Heinzmann, B., 1993. Coagulation and flocculation of stormwater from a separated storm sewer — a new possibility for enhanced treatment, in 2nd International Conference on Upgrading of Wastewater Treatment Plants, Berlin, September 21–24, 1993, sponsored by IAWQ and EWPCA, 304–313. Kloiber, S.M. and Brezonik, P.L., 1996. Effectiveness of continuous alum treatment for phosphorus removal from stormwater, presented at North American Lake Management Society (NALMS) Conference, Minneapolis/St. Paul, October. Kronis, H., 1982. Physical-Chemical Treatment and Disinfection of Stormwater, Research Report 88, Wastewater Treatment Section, Ontario Ministry of the Environment, Toronto, March. Lager, J.A., Smith, W.G., and Tchobanoglous, G., 1977. Catchbasin Technology Overview and Assessment, U.S. EPA. Contract 68-03-0274, EPA-600/2-77-051, Cincinnati, OH, 129 pp. Lalor, M. M., 1993. Assessment of Non-Stormwater Discharges to Storm Drainage Systems in Residential and Commercial Land-Use Areas, Ph.D. dissertation, Vanderbilt University, Nashville, TN. Legrand, J., Maillot, H., Nougarède, F., and Defontaine, S., 1994. A device for stormwater treatment in the urban development zone of Annœullin [in French], TSM, 11, 639–643. Montoya, B.L., 1987. Urban Runoff Discharges from Sacramento, California, CVRWQCB Report Number 87-1SPSS, prepared for the California Regional Water Quality Control Board, Central Valley Region. Newman, T.L., Leo, W.M., Muekker, J.A., and Gaffoglio, R., 1996. Effectiveness of street cleaning for floatables control, in Proceedings: Urban Wet Weather Pollution: Controlling Sewer Overflows and Stormwater Pollution, June 16–19, 1996, Quebec City, Quebec, Water Environment Federation, Alexandria, VA. Pitt, R., 1979. Demonstration of Nonpoint Pollution Abatement through Improved Street Cleaning Practices, EPA-600/2-79-161, U.S. Environmental Protection Agency, Cincinnati, OH. Pitt, R., 1985. Characterizing and Controlling Urban Runoff through Street and Sewerage Cleaning. U.S. Environmental Protection Agency, Contract R-805929012, EPA/2-85/038, PB 85-186500/AS, Cincinnati, OH, 467 pp.

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Pitt, R., 1986. The incorporation of urban runoff controls in the Wisconsin Priority Watershed Program, in Advanced Topics in Urban Runoff Research, B. Urbonas and L.A. Roesner, Eds., Engineering Foundation and ASCE, New York, 290–313. Pitt, R., 1987. Small Storm Flow and Particulate Washoff Contributions to Outfall Discharges, Ph.D. dissertation, Department of Civil and Environmental Engineering, the University of Wisconsin, Madison. Pitt, R., 1996. New critical source area controls in the SLAMM stormwater quality models, paper presented at the Assessing the Cumulative Impacts of Watershed Development on Aquatic Ecosystems and Water Quality, Chicago, IL, March 18–21. Pitt, R. and Field, R., 1998. An evaluation of storm drainage inlet devices for stormwater quality treatment, presented at Water Environment Federation 71st Annual Conference & Exposition, WEFTEC Technology Forum, Orlando, FL, October. Pitt, R. and Shawley, G., 1982. A Demonstration of Non-Point Source Pollution Management on Castro Valley Creek, Alameda County Flood Control and Water Conservation District (Hayward, CA) for the Nationwide Urban Runoff Program, U.S. Environmental Protection Agency, Water Planning Division, Washington, D.C. Pitt, R. and Sutherland, R., 1982. Washoe County Urban Stormwater Management Program; Volume 2, Street Particulate Data Collection and Analyses, Washoe Council of Governments, Reno, NV. Pitt, R. and Voorhees, J., 1995. Source loading and management model (SLAMM), Seminar Publication: National Conference on Urban Runoff Management: Enhancing Urban Watershed Management at the Local, County, and State Levels, March 30–April 2, 1993, Center for Environmental Research Information, U.S. Environmental Protection Agency, EPA/625/R-95/003, Cincinnati. OH, 225–243. Pitt, R., Lalor, M., Field, R., Adrian, D.D., and Barbé, D., 1993. Investigation of Inappropriate Pollutant Entries into Storm Drainage Systems, A User’s Guide, EPA/600/R-92/238, U.S. Environmental Protection Agency, Cincinnati, OH. Pitt, R., Field, R., and Dunkers, K., 1994. Combined sewer overflow control through in-receiving water storage: an efficiency evaluation, presented at Water Environment Federation Specialty Conference on a Global Perspective for Reducing CSOs: Balancing Technologies, Costs, and Water Quality, Louisville, KY. Pitt, R.E., Field, R., Lalor, M., and Brown, M., 1995. Urban stormwater toxic pollutants: assessment, sources, and treatability, Water Environ. Res., 67(3), 260–275. Pitt, R., Robertson, B., Barron, P., Ayyoubi, A., and Clark, S., 1999. Stormwater Treatment at Critical Areas: The Multi-Chambered Treatment Train (MCTT), U.S. Environmental Protection Agency, Wet Weather Flow Management Program, National Risk Management Research Laboratory, EPA/600/R-99/017, Cincinnati, OH, 505 pp. Pitt, R., Nix, S., Voorhees, J., Durrans, S.R., and Burian, S., publication pending. Guidance Manual for Integrated Wet Weather Flow (WWF) Collection and Treatment Systems for Newly Urbanized Areas (New WWF Systems), Second year project report: model integration and use, Wet Weather Research Program, U.S. Environmental Protection Agency, Cooperative agreement CX 824933-01-0, February. Preul, H.C., 1996. Combined sewage prevention system (CSCP) for domestic wastewater control, presented at the 7th International Conference on Urban Storm Drainage, Hannover, Germany, 193–198. Rupperd, Y., 1993. A lamellar separator for urban street runoff treatment [in French], Bull. Liaison Lab. Ponts Chaussees, 183, 85–90. Sartor, J. and Boyd, G., 1972. Water Pollution Aspects of Street Surface Contaminants, Contract 14-12-921, EPA-R2-72-081, U.S. Environmental Protection Agency, Washington, D.C., November, 236 pp. Schueler, T., Ed., 1994. Hydrocarbon hotspots in the urban landscape: can they be controlled? Watershed Prot. Tech., 1(1), 3–5. Schueler, T., and Shepp, D., 1993. The Quality of Trapped Sediments and Pool Water within Oil Grit Separators in Suburban Maryland, Metropolitan Washington Council of Governments, Washington, D.C., 48 pp. Shepp, D., and Cole, D., 1992. A Field Survey of Oil-Grit Separators in Suburban Maryland, Metropolitan Washington Council of Governments, Washington, D.C., 51 pp. Sutherland, R.C., 1996. Studies show sweeping has beneficial impact on stormwater quality, APWA Rep., 8–23. Sutherland, R.C. and Jelen, S.L., 1996. Sophisticated stormwater quality modeling is worth the effort, in Advances in Modeling the Management of Stormwater Impacts, W. James, Ed., Computational Hydraulics International, Guelph, Ontario.

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Terstriep, M.L., Bender, G.M., and Noel, D.C., 1982. Final Report — NURP Project, Champaign, Illinois: Evaluation of the Effectiveness of Municipal Street Sweeping in the Control of Urban Storm Runoff Pollution, State Water Survey Division, Illinois Department of Energy and Natural Resources, Champaign-Urbana, IL, December. Tramier, B., 1983. Water Treatment Technology — IP 84-011, Institute of Petroleum, London, England. Urbonas, B.R., 1999. Design of a sand filter for stormwater quality enhancement, Water Environ. Res., 71(1), 102–113. U.S. EPA (U.S. Environmental Protection Agency), 1983. Final Report for the Nationwide Urban Runoff Program, Water Planning Division, Washington, D.C., December. Valiron, F., 1992. Usual techniques for stormwater pollutant removal in urban areas [in French], Provisory Report for the Seine-Normandie Water Agency, February, 61 pp. W&H Pacific, 1992. Methods and Results Summary: Compost Storm Water Filter System, W&H Pacific, Portland, OR.

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6

Treatment of Stormwater Runoff from Urban Pavement and Roadways John J. Sansalone and Jonathan Hird

CONTENTS The Complicated Nature of Stormwater with Respect to Treatment Potential............................143 Hydrologic Factors Affecting Treatment Selection and Effectiveness ..................................143 Environmental Factors Affecting Treatment Selection and Effectiveness.............................143 Physicochemical Aspects of Stormwater Affecting Treatment Selection and Effectiveness....144 Physical, Chemical, and Biological Unit Operations and Processes for Stormwater ..................145 Sedimentation and Settling.....................................................................................................145 Filtration..................................................................................................................................146 Adsorption...............................................................................................................................146 Precipitation ............................................................................................................................146 Ion Exchange ..........................................................................................................................147 Disinfection .............................................................................................................................147 Biological Processes ...............................................................................................................147 Historical Development of Controls or Best Management Practices ...........................................147 Introduction to Best Management Practices ..........................................................................147 Development of BMPs in the United States ..........................................................................149 Development of BMPs in Europe, Japan, and Australia .......................................................150 Effectiveness of Single Unit Operation In Situ Control BMP......................................................151 Settling Basins................................................................................................................................151 Dry Detention Basins..............................................................................................................151 Design Guidance and Construction Considerations .....................................................152 Operation and Maintenance Considerations .................................................................153 Hazardous Waste Disposal ............................................................................................153 Cost and Design Life ....................................................................................................153 Wet Retention Ponds...............................................................................................................154 Design Guidance and Construction Considerations .....................................................155 Operation and Maintenance Considerations .................................................................155 Hazardous Waste Disposal ............................................................................................156 Cost and Design Life ....................................................................................................156 Filtration Systems ..........................................................................................................................156 Vegetated Swales ....................................................................................................................157 Design and Site Considerations ....................................................................................157 Operation and Maintenance ..........................................................................................158 Residuals Management .................................................................................................158 Cost and Design Life ....................................................................................................158 0-56676-916-7/03/$0.00+$1.50 © 2003 by CRC Press LLC

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Filter Strips .............................................................................................................................158 Design and Site Considerations ....................................................................................159 Operation and Maintenance ..........................................................................................160 Residual Management ...................................................................................................160 Cost Considerations.......................................................................................................160 Sand Filters .............................................................................................................................160 Design Guidance Considerations ..................................................................................161 Operation and Maintenance Considerations .................................................................161 Cost and Design Life ....................................................................................................162 Porous Pavement.....................................................................................................................162 Design and Site Considerations ....................................................................................163 Operation and Maintenance ..........................................................................................163 Cost and Design Life ....................................................................................................164 Infrastructure Modification and Appurtenances ............................................................................164 Vortex/Swirl Separators ..........................................................................................................165 Oil and Water Separators........................................................................................................165 Design and Sizing Considerations ................................................................................166 Operation and Maintenance ..........................................................................................166 Cost and Design Life ....................................................................................................166 Effectiveness of Multiple Unit Operations, Process Controls, and BMPs ...................................167 Adsorptive–Filtration Systems................................................................................................167 Partial Exfiltration Systems ....................................................................................................167 Modified Partial Exfiltration Trench Reactors .......................................................................167 Design and Site Considerations ....................................................................................168 Operation and Maintenance ..........................................................................................169 Cost and Design Life ....................................................................................................169 Treatment Trains .....................................................................................................................170 Design and Site Considerations ....................................................................................170 Operation and Maintenance ..........................................................................................170 Cost and Design Life ....................................................................................................170 Below-Grade Treatment Storage Systems..............................................................................171 Design and Sizing Considerations ................................................................................172 Operation and Maintenance Considerations .................................................................172 Constructed Wetlands .............................................................................................................172 Design and Site Considerations ....................................................................................174 Operation and Maintenance ..........................................................................................175 Cost and Design Life ....................................................................................................175 Centralized Treatment Plants..................................................................................................175 Design Considerations...................................................................................................177 Assessment of Treatment Effectiveness ........................................................................................177 Event Focus Basis...................................................................................................................177 Wet Weather Sampling and Treatment Assessment .....................................................177 Baseline Parameters ......................................................................................................178 Seasonal or Annual Focus ......................................................................................................178 Wet and Dry Weather Treatment and Assessment........................................................178 Baseline Parameters ......................................................................................................178 Analytical Techniques.............................................................................................................178 Can BMPs Be Evaluated As Black Boxes? ...........................................................................179 Quality Control and Quality Assurance .................................................................................179 Development and Operational Aspects of Treatment....................................................................180

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Cost and Design Life..............................................................................................................180 Construction ............................................................................................................................180 Operation and Maintenance....................................................................................................181 Cleaning ..................................................................................................................................181 Hazardous Waste Disposal......................................................................................................181 Postconstruction Considerations.............................................................................................182 References ......................................................................................................................................183

THE COMPLICATED NATURE OF STORMWATER WITH RESPECT TO TREATMENT POTENTIAL If one compares the potential efficacy of stormwater treatment with that of conventional domestic wastewater treatment, there are a number of significant differences. Each of these differences poses a unique challenge for the efficacy of stormwater treatment. These differences include hydrologic factors, environmental considerations, and the physical and chemical characteristics of stormwater.

HYDROLOGIC FACTORS AFFECTING TREATMENT SELECTION

AND

EFFECTIVENESS

Rainfall runoff is a stochastic process. Therefore, there is significant uncertainty associated with the temporal occurrence and forecasting of stormwater loadings generated by the rainfall runoff process. Quantification of critical stormwater parameters includes parameters such as intensity, duration, and frequency. These factors are linked using design storm concepts, such as intensityduration-frequency (IDF) formulations or probabilistic methods. As a consequence the stochastic nature of rainfall runoff is a fundamental constraint when considering stormwater treatment design. In addition to the stochastic nature of rainfall runoff is the unsteady nature of flow process associated with it. Flow rate can vary by several orders of magnitude during a discrete rainfall runoff event. Also, at a given location, peak flows can vary by over an order of magnitude between events with significantly different IDF parameters. Depending on locality, season, and event conditions, the unsteady pattern of flow and the disposition of peak flow during the storm are quite variable. Both the unsteady flow patterns and the magnitude of peak flow profoundly influence the delivery and transport of stormwater pollutants. The unsteady pattern and large variability of peak flow rates are both fundamental constraints when considering stormwater treatment design. When considering any treatment scenario for a water or wastewater stream, two fundamental characteristics to be considered are the volume and duration of flow. This is also the case for stormwater. However, as with other characteristics of stormwater, both the volume and duration of flow are also stochastic and variable quantities. Both quantities can vary by orders of magnitude between rainfall runoff events for a given locality. The stochastic nature and variability of both flow volume and duration are also both fundamental constraints when considering stormwater treatment design. In summary, hydrologic factors have a profound influence on the selection of stormwater treatment alternatives and on the potential efficacy of such treatment.

ENVIRONMENTAL FACTORS AFFECTING TREATMENT SELECTION

AND

EFFECTIVENESS

Environmental factors, for example, regional and microclimate characteristics, as well as demographics vary both spatially and temporally. Although urban sites may share similar characteristics or indices, no two urban sites are the same. In fact, the urban and roadway environment are dynamic and therefore environmental factors change over time. Demographic changes and the commensurate anthropogenic activities associated with them can significantly modify many of these environmental and site factors. For example, the degree of imperviousness for an urban or roadway site has a major influence on site hydrology, pollutant loadings, and the feasibility and efficacy stormwater treatment.

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In urban and roadway areas, pollutant loadings are predominantly anthropogenic. As a result, changing demographics and those activities associated with urban land use (such as transportation and traffic) result in pollutant loadings that are temporally and spatially variable. Combining the variability of pollutant loadings with the variability of hydrologic loadings results in pollutant delivery rates that are highly nonlinear and variable. As with hydrologic characteristics the factors associated with the variability of pollutant loadings generates constraints when considering stormwater treatment. In addition to the variability of pollutant loadings, site geometry, cover conditions, degree of imperviousness, and topography are all factors that have an influence on treatment design. For example, the linearly extended nature of highway systems suggests in situ treatment of similar geometry. In contrast, a parking area with an aspect ratio close to 1 suggests a type of treatment in a single location with stormwater directed toward that single location. Topography has a significant influence on flow regime, pollutant mixing, concentration of flow, and the hydraulics of treatment. Cover conditions and degree of imperviousness influence the availability, detachment, transport, delivery, and magnitude of runoff quantity and quality. Degree of imperviousness is considered a primary index when modeling both urban water quantity and quality. All of these site conditions have a major influence on stormwater treatment. The surface of the urban environment is bounded above by the atmosphere. Other than regional climate and microclimate issues, atmospheric conditions can play an important role in modifying the nature of urban stormwater and therefore the treatment of stormwater. The modification of local wind patterns in the urban environment alters the entrainment and deposition of dustfall and entrained anthropogenic matter. This is especially true in transportation corridors where trafficinduced wind is an important vector for such anthropogenic matter. In the urban environment, acid rainfall is a classic example of the interaction between anthropogenic activities that generate pHreducing aerosols, compounds, and entrained particulate matter, and the resulting poorly buffered acidic rainfall. These constituents, including sulfur compounds and oxides of nitrogen, can be retained in the urban atmosphere for days and in some cases weeks until a rainfall event washes these constituents from the atmosphere. Deposition of these acidic constituents occurs not only in wet form as in a rainfall runoff event, but also as dry acidic deposition between precipitation events. The pH depression of rainfall and runoff has a significant impact on the partitioning of heavy metals at the upper end of urban catchments and therefore on the design and potential efficacy of in situ stormwater treatment. The surface of the urban environment is also bounded below by a wide range of soil and subsurface conditions. These conditions are significantly modified by the urban surface above and the anthropogenic activities that occur there. Soil conditions are modified through extensive placement of impervious pavement in urban areas, infrastructure construction, and urban environmental phenomena that alter the physical, chemical, and biological characteristics of the soil conditions. Of fundamental importance are the long-term decrease in porosity and saturated hydraulic conductivity of urban soils as well as the loss of infiltration potential due to the impervious nature of the surface placed on these soils. The resultant alterations of soil characteristics significantly impact the quantity and quality of stormwater and the required capacity for both in situ and central treatment of stormwater.

PHYSICOCHEMICAL ASPECTS AND EFFECTIVENESS

OF

STORMWATER AFFECTING TREATMENT SELECTION

This section is a succinct review of Chapter 2, which investigates these aspects in depth. Nonetheless these aspects are so fundamental for treatment design that they are very briefly reviewed again here before the in-depth best management practices (BMPs) and treatment analysis that follows. The physicochemical characteristics of urban stormwater, especially that generated directly from pavement areas, appear to dominate the biology of stormwater. Therefore, the focus of both treatment and characterization is on physicochemical aspects.

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Redox and pH are master variables that mediate partitioning, surface complexation, and speciation of inorganic and organic species in stormwater. In stormwater surface flows, redox conditions are typically oxidizing, although in subsurface treatment, retention basins, or wetland the redox potential can change to reducing conditions. Reducing conditions can promote the dissolution and availability of heavy metals and modify the efficacy of treatment. The acidic pH of urban rainfall and runoff favors dissolution and influences speciation of heavy metals. Therefore, BMPs must address these pH conditions and utilize treatment mechanisms that can modify the pH, partitioning, or surface complexation of stormwater species. Stormwater transports solid matter with a wide range of loadings, gradations, densities, and surface characteristics. Individually, each of these solid matter characteristics has an impact on treatment. While there will be a continued need for gross solid indices such as settleable (SS), total suspended (TSS), and total volatile (VSS) solids, effective treatment will require a much more fundamental consideration of solid matter and its loadings in stormwater. Residence time distribution (RTD) is a critical concept when considering stormwater characterization and when designing for treatment. For characterization, the RTD from when rainfall generates flow from the urban surface to when this flow reaches the point of treatment influences characteristics such as partitioning, pH, and flocculation. For BMP design RTD is critical for the efficacy of the chosen treatment and to identify issues such as short-circuiting. Precipitation type has a critical impact on treatment selection. Specifically, treatment of urban snow and runoff requires an approach vastly different from that for rainfall runoff. Pollutant levels in snow are orders of magnitude higher than in rainfall runoff. Also, partitioning is altered, RTDs in the presence of urban and pollutants are much greater, and the volume of runoff is much lower. Because of these characteristics treatment requirements for snow are vastly different.

PHYSICAL, CHEMICAL, AND BIOLOGICAL UNIT OPERATIONS AND PROCESSES FOR STORMWATER Once the water quantity and quality characteristics of the urban stormwater stream to be treated are determined, treatment mechanisms can be selected. Historically, there has not been a widespread effort to consider stormwater treatment in terms of unit operations and processes, in contrast to conventional domestic wastewater or drinking water treatment. In part, this may be due to the complicated nature of stormwater as described earlier when compared with other water or wastewater streams. However, unit operations and process concepts, especially when used in combination will have to be applied to stormwater to effectuate in situ or centralized treatment. For all these systems residual management will need significant consideration. Residuals management is a critical issue and will eventually assume a parallel effort in stormwater treatment as biosolids management has in wastewater treatment.

SEDIMENTATION

AND

SETTLING

Historically, sedimentation as a single unit operation has been one of the most common primary unit operations applied to nearly all forms of water and wastewater. More recently, this has also been the case for stormwater treatment. Settling design theory as developed from water and wastewater treatment can be effectively applied to stormwater. However, the characteristics of stormwater that make it unique when compared to water and wastewater must be accommodated in any design. These characteristics include unsteady flow, a range of solids gradations from over 10,000 to less than 1 µm, a range of particle densities, and temporally variable loadings and flocculant characteristics. These characteristics must be quantified in both the design and analysis of treatment. Simple models such as overflow rate theory and analytical models such as Hazen’s settling efficiency model can be applied with varying degrees of success to stormwater settling. Numerical models can also be applied to stormwater. Future numerical models designed specifically

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for stormwater characteristics could offer important guidance when considering fundamental aspects of stormwater settling. In terms of existing practice, through design experience and “rules of thumb,” wet and dry basins employing sedimentation as a primary unit operation are a common form of stormwater treatment. Although settling is clearly a necessary primary unit operation, it is by no means sufficient if the water quality objective is rigorous and comprehensive stormwater treatment.

FILTRATION Along with sedimentation, filtration has historically been used for primary treatment of water and wastewater. Filtration can be very effective for stormwater clarification. As with water and wastewater treatment filtration is best combined with other unit operations in a treatment train. In this way filtration of stormwater can prove very effective as secondary treatment with primary unit operation treatment such as mixing–coagulation–flocculation and sedimentation preceding filtration. Such a series of unit operations can be facilitated easily with centralized or package-type treatment. Although it is often more difficult in their application, this still demonstrates the potential feasibility for in situ treatment. The major problem currently with in situ treatment using filtration as a sole primary unit operation is the issue of clogging and the current difficulty associated with backwashing or otherwise rehabilitating the clean bed characteristics of the filtration unit. Although potentially more effective than settling for smaller suspended solids (less than 100 µm) and for even smaller colloidal solids, filtration alone without partitioning or surface complexation will not be effective for treatment of dissolved constituents. Poor early designs in the United States during the 1970s left a lingering negative impression on the use of filtration for stormwater treatment. With proper design and application, filtration can be a very effective unit operation especially when used as secondary treatment.

ADSORPTION Dissolved solutes such as the soluble fraction of heavy metals or organics are removed from solution and are taken up (partitioned or surface-complexed) to an adsorbent. This is the unit process of adsorption. This adsorbent can either be material in a fixed-bed-type matrix or can be entrained in the water or waste stream as for example with powder activated carbon (PAC). Adsorption is a surface exchange phenomenon and therefore depends on factors such as adsorbent surface area and surface chemistry, flow regime, and RTD. In many cases, as with surface complexation of heavy metals, adsorption phenomena are also critically pH dependent. Adsorption phenomena can range from reversible physical-type adsorption to relatively irreversible chemical adsorption. For urban stormwater, typically containing elevated levels of heavy metals in a poorly buffered aqueous matrix, adsorption is a potentially important unit process. To date, it has unfortunately been rarely given sufficient consideration or application for treatment of dissolved stormwater constituents. Adsorption is an important unit process when evaluating infiltration treatment of stormwater in soils with even low clay content.

PRECIPITATION Precipitation is a unit process for the removal of ionic species (such as heavy metals) from solution in the form of insoluble chemical species. For many heavy metal ions this can be facilitated to produce insoluble hydroxide and carbonate species. Precipitation is used extensively in the drinking water industry to remove calcium and magnesium for water softening. Precipitation can also be applied for organic constituents. If one considers adsorption phenomena as driven by pH, precipitation represents the process end point for a wide range of adsorption phenomena. Precipitation can occur naturally in stormwater especially as a result of increased stormwater RTD in the presence of natural carbonate species and other species that promote precipitation at appropriate levels of pH. Precipitation has rarely been utilized as a unit process for stormwater treatment. However, it

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is a potentially viable process given the proper in situ conditions and is easily facilitated for centralized treatment.

ION EXCHANGE As with adsorption, ion exchange is a unit process for treatment of the dissolved fraction. Ion exchange is defined as a process where an insoluble substance removes ions of positive or negative charge from an electrolytic solution and releases other ions of like positive or negative charge into solution in a chemically equivalent manner. As with adsorption, this unit process has rarely been used for stormwater. However, ion exchange is an important unit process when considering treatment of stormwater infiltrated through clayey soils. Ion exchange could be used as a process to remove dissolved constituents more selectively from stormwater as compared to basic adsorption and should be considered as a secondary or tertiary treatment for stormwater. Ion exchange could be incorporated for a range of in situ type configurations or for centralized treatment preceded by primary unit operations.

DISINFECTION Disinfection is the destruction or removal of pathogenic microorganisms in water. Disinfection can be accomplished by a variety of unit processes including chlorination, ozonation, or the application of a host of other chemical oxidants. Ultraviolet (UV) and gamma irradiation have also been applied. Water and wastewater have been the waters commonly receiving disinfection during the latter half of the 20th century. If stormwater has been treated using disinfectants, it has been so only in the context of combined sewer overflows. Stormwater as a waste stream is rarely disinfected, although microbial counts can be several orders of magnitude greater than that of untreated domestic wastewater. Effective disinfection requires the removal of suspended matter and organic material first. Therefore, as part of a central treatment scheme, disinfection of stormwater may be feasible. Decentralized in situ treatment of stormwater will prove to be a far more difficult process.

BIOLOGICAL PROCESSES Natural systems, constructed wetlands, and phytoremediation/photo-oxidation have gained attention recently as alternative systems for wastewater treatment. Of these, constructed wetlands have been most commonly used for the treatment of stormwater. Although such systems are nominally called biological, the operations and processes that occur are mainly physical and chemical especially for urban stormwater that can be primarily inorganic in nature. In such systems, organic constituents such as conversion and degradation of oil and grease can still be facilitated biologically, but the predominant treatment mechanisms are physical and chemical. In many cases, stormwater is far more similar to a heterogeneous inorganic industrial waste stream and therefore strict aerobic biological processes such as activated sludge, trickling filters, and similar treatments may not be very effective for stormwater. This appears to be applicable for both in situ and central treatment. The exception may be for techniques such as biosorption, although in essence this is a physicochemical unit process.

HISTORICAL DEVELOPMENT OF CONTROLS OR BEST MANAGEMENT PRACTICES INTRODUCTION

TO

BEST MANAGEMENT PRACTICES

BMPs are structural or nonstructural practices, or combinations of both, designed to act as an effective and practical means of minimizing the impacts of infrastructure and anthropogenic activities on water quality (U.S. DOT, 1996). Structural BMPs operate by intercepting and detaining

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Control Classification

Technology examples BMP Type

Technology and Capitol Intensity

Source Controls

unleaded fuels P3

Treatment Controls Hydraulic Controls

porous pavement

PET’s

retention and detention

Source Control

In-Situ Control

End-Of-Pipe Control

Low Cost High Tech.

WWTP Centralized Treatment

High Cost Low Tech.

High Tech.

FIGURE 6.1 The continuum of BMP controls.

stormwater runoff for a sufficient period of time, in order that the target contaminants settle out of the water column or are filtered through the underlying soil. These trapped pollutants should then be managed accordingly. Nonstructural BMPs are typically “source control” mechanisms, designed to minimize the accumulation of pollutants and minimize their initial concentrations in the stormwater runoff. They are generally used in conjunction with structural controls to generate a more complete treatment system. BMPs can be divided into three major categories: 1. Source controls are typically nonstructural mechanisms intended to prevent the initial incidence of pollution or to intercept the pollutants before they enter the storm drainage system. 2. Treatment-based controls are controls that physically, chemically, or biologically treat the stormwater, using some structural device. 3. Hydraulic controls are structural controls that attenuate the urban hydrograph or divert flows away from source areas. This delineation of the different approaches to the control of stormwater pollution is by no means clear-cut and is essentially a continuum of classification according to the control regime to which they exert on the stormwater runoff. Figure 6.1 displays this continuum. By definition, the only true type of source control is the P3 (pollution prevention plans) approach. This management practice completely removes the constituent that can potentially become a pollutant from the life cycle. Examples of this approach are the removal of lead from gasoline or the removal of CFCs from aerosols. In removing them from the production process (i.e., discontinuing their use in the production process) the respective pathway of potential environmental contamination is eliminated. The remainder of the spectrum of source controls are simply degrees of source control, utilized farther down the pollutant transport pathway. Source controls are regarded as practices, or structures, that minimize the degree to which the contaminant propagates itself into the environment. The closer to the true source of the contamination the management practice is implemented, the greater the reduction in the magnitude of the detrimental impact on the environment and the nearer to the true definition of true source control the management practice becomes. It is the fundamental objective in the application of BMPs to stormwater management to minimize the detrimental impacts on a receiving body of water. Therefore, by definition, the entire spectrum of BMPs available can be effectively regarded as source controls or qualitative stormwater mitigation practices.

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Hydraulic controls quantitatively mitigate the impacts of reduced infiltration and increased runoff by means of intercepting and temporarily detaining the runoff surcharge. This grants either sufficient time to allow infiltration to the subsurface or the excess is later released as a controlled discharge as soon as downstream treatment units or conveyance structures have sufficient capacity. Such detention and retention effectively results in the following: • Reduction in the maximum discharge (Qp) • Reduction of the total discharge (Qt) • Translocation of the time to peak discharge (tp) Similarly, the entire spectrum of BMPs also displays some degree of hydraulic control. However, some BMPs are installed for their specific hydraulic control capabilities, for example, retention and detention basins. Other controls such as source or treatment controls also display limited hydraulic controls, but these are of secondary importance to their qualitative control capacities. In short, BMPs are classified as source, treatment, or hydraulic controls depending on their primary control function.

DEVELOPMENT

OF

BMPS

IN THE

UNITED STATES

The general historical concern in urban stormwater mitigation has been to reduce the incidence, magnitude, and frequency of downstream flooding. Recent legislative developments have increased the owner and operator responsibilities to qualitative controls on stormwater discharge prior to predevelopment levels. BMPs in the United States can be generally characterized as treatment-based or hydraulic structural controls. They are typically the results of proven empirical engineering practice and differ substantially from the highly developed and predictable engineered systems associated with conventional wastewater treatment facilities. For most stormwater pollutants, BMPs are nothing more than pollutant management containments and not truly treatment mechanisms. Treatment implies elimination or transformation of the pollutant into an environmentally compatible constituent. The role of BMPs in controlling the potential toxicity of stormwater runoff is continually evolving, generally in line with regulatory developments to control point and nonpoint discharges. The term BMPs in relation to urban runoff management in the United States was adopted in the 1970s to represent control measures and practices that could be applied to reduce runoff volumes and contaminant concentrations in urban runoff. Their initial development were in direct response to Federal Highway Administration (FHWA) research during the early 1970s that evaluated water quality impacts of highway runoff to receiving waters. The findings and recommendations were then directly applied to the broader field of urban stormwater runoff with the passing of Phase I of the National Pollution Discharge Elimination System (NPDES) (33 U.S.C §1362(14)) as part of the amendments made to the Clean Water Act in 1977(§402). Under Phase I of the NPDES, permits are required for municipal separate storm sewers serving medium to large populations (greater than 250,000 or 100,000 people, respectively) and for industrial stormwater discharges. Phase II, passed into law October 1999, extended all Phase I permit requirements to apply to the following: • “All point source discharges of stormwater from commercial retail and industrial facilities and from municipal separate storm sewer systems serving populations of 10,000 with satellite populations of 1,000 (40 C.F.R. §122) • “For those (Phase II) discharges that are contributing to a water quality impairment or are a significant contributor of pollutants to waters of the U.S. (40 C.F.R. §122-124) • Specifically sites of construction that disturb an area greater than 1 and less than 5 acres of land.

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Applicants for Phase I and II permits must be fully compliant by the end of 2004, to avoid noncompliance violations, punishable by fines. It is this final phase of permitting requirements that has provided the driving force for the increased impetus in development and implementation of BMPs in the management of urban stormwater runoff.

DEVELOPMENT

OF

BMPS

IN

EUROPE, JAPAN,

AND

AUSTRALIA

Current directions in BMPs in Europe, Japan, and Australia are typified by nonstructural source control BMPs that are designed to prevent the initial contamination of the stormwater. In comparison, trends in the United States focus on structural treatment-based or hydraulic controls that attempt to mitigate the effects of already detrimentally impacted stormwater runoff. These two schools of thought can be represented as preventative management practice vs. remediative action. The reason for this fundamental difference in approach is that regulatory controls governing the treatment and management of stormwater runoff have been in place for a much longer period of time in Europe and elsewhere. Throughout the majority of Europe, stormwater management falls under the jurisdiction of wastewater and its management is regulated accordingly. This difference is also depicted in the disparity in the infrastructure prevalent in the two approaches to stormwater management. Traditionally in Europe, Japan, and Australia municipal wastewater and stormwater are treated and managed together in a combined storm sewer system (CSS). However, throughout the majority of the United States, wastewater and stormwater are kept separate and are managed separately. It is only in the older Victorian communities of the Northeast, Midwest, and Mid-Atlantic where CSS is the prevalent management practice. This is a direct result of infrastructure installation under the guidance of engineers employing European stormwater management doctrines. More recent developments in BMPs across Europe have been driven by the European Community Urban Wastewater Treatment Directive (CEC, 1991). Promulgated in 1988, this transEuropean community directive requires at least stormwater collection and treatment (depending on population equivalent) for any population center generating stormwater runoff. Urban centers with more than 2000 population equivalents are mandated to provide collection and at least secondary treatment. Table 6.1 lists the compliance schedule. The difference in application of this directive (essentially equivalent to US. EPA NPDES Phase II) is not only in the compliance schedule but also the significantly smaller population centers to which

TABLE 6.1 The European Directive for Urban Stormwater Management Schedule of Compliance Population Equivalents

Receiving Body Classification Sensitivea

Normal

Less Sensitive (coastal)

a b c

Collection requirement Degree of treatment Compliance deadline Collection requirement Degree of treatment Compliance deadline Collection requirement Degree of treatment Compliance deadline

0–2000

2000–10,000

10,000–15,000

15,000–150,000

If collection Appropriateb 31-12-05 If collection Appropriate 31-12-05 If collection Appropriate 31-12-05

Collection Secondaryc 21-12-05 Collection Secondary 31-12-05 Collection Appropriate 31-12-05

Collection Advanced 13-12-98 Collection Secondary 31-12-05 Collection At least primary 31-12-05

Collection Advanced 31-12-98 Collection Secondary 31-12-00 Collection At least primary 31-12-00

Sensitive water bodies are defined as eutrophic or potentially eutrophic. Wastewater collection not mandatory for this population equivalent. Treatment is mandatory. Appropriate treatment if discharge to coastal waters.

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it applies. NPDES Phase II applies to population centers of over 10,000 people, whereas the European Community Directive 91/271/EEC applies to any source of urban stormwater runoff irrespective of population equivalents. This directive is even more specific in that it delineates on the grounds of population equivalents, in order to consider commercial and industrial generators. Not only is the scope of the European Community Directive more encompassing than NPDES Phases I and II, and more applicable to much smaller population center equivalents, but the degree of treatment required is directly related to the sensitivity to pollution of the receiving water body. This level of responsibility is not given any direct consideration in NPDES regulatory requirements. This disparity in the level of stringency between the United States and European Community Member States is a key indicator in the level of significance assigned by each regulatory body, upon the magnitude of urban stormwater runoff contributions to the water quality of receiving streams.

EFFECTIVENESS OF SINGLE UNIT OPERATION IN SITU CONTROL BMPs Proper design and operation of any BMP or treatment requires knowledge of hydraulic and biological characteristics of the influent runoff and the pertinent pollutant physical, chemical, and mechanistic or stochastic response of the engineered BMP or treatment system to a specific loading regime. Such knowledge is required for economic screening assessment, detailed design treatment performance analysis.

SETTLING BASINS Probably the most common BMP for urban stormwater quantity and quality mitigation is the use of settling basins or detention ponds. The net effect of detention basins is to store a significant fraction of the incoming storm runoff volume and then release that which does not percolate into the subsoil later through a controlled overflow in order that it does not significantly influence the uncontrolled postdevelopment storm hydrograph. With appropriate regional management, not only does the use of settling basins reduce downstream flooding and bank erosion, but the interception and detention of the stormwater runoff facilitates settling of particulate matter and therefore improves stormwater quality. However, the caveat here is appropriate design and management on a regional or watershed basis. Individual basin designs can in their own right exacerbate both water quantity (flooding) and water quality issues downstream. Open detention ponds or settling basins used to detain urban stormwater and reduce peak flows can be designed as one of the following: 1. Extended detention (dry) basins that only temporarily detain stormwater 2. Wet retention ponds that maintain a permanent body of water 3. An integrated combination of both of the above

DRY DETENTION BASINS Initially applied in the early 1970s in North America, Europe, and Australia to mitigate the postdevelopment runoff using design storm concepts, extended dry detention basins were rapidly implemented to control runoff from events for smaller-magnitude, higher-frequency events. Sedimentation is the primary pollutant removal mechanism associated with these settling basins, with phase partitioning operating as a secondary removal mechanism. As such, the pollutant removal efficiency is dependent on whether a given pollutant is in the particulate-bound or soluble phase. If the greater proportion of heavy metal pollutants present in stormwater are in the particulatebound phase (Sansalone and Buchberger, 1997; Malcolm, 1989), the fundamental rationale for

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treating stormwater in settling basins is to detain the water for a sufficient period of time to enable settling of the particulate matter and percolation into the subsoil. Measured field data indicate that 60 to 70% of settleable particulate urban sediments can be settled out within the first 6 h of detention, with the remainder settling out over the next 2 days (WEF, 1998). Maximum removal rates after 48 h of detention range from 80 to 90% (U.S. DOT, 1996). However, only limited maximum removal efficiencies (40 to 50%) can be expected for soluble-phase pollutants. Unfortunately, some of the pollutants of greatest concern in urban stormwater (such as heavy metals, nitrates, orthophosphates, and other organic compounds) can be predominantly in the soluble phase, depending on pH and redox conditions. Removal of the solublephase pollutants can be obtained if the whole basin, or a portion thereof, is managed as a shallow wetland (see section on “Wet Retention Basins”) to utilize natural biological removal processes. Dry detention basins may demonstrate feasibility as a BMP when the following pertain: • It is anticipated that particularly heavy particulate pollutant loadings are to be introduced to receiving waters. This typically occurs for urban roads with an ADT >30,000. • Mitigation of urban runoff is a required by regulation to reduce peak flow discharges and improve stormwater quality, but where vegetated strips are not feasible. • Issues of hazardous residuals management have been addressed. • Safety concerns of the temporary water surface can be assured. • End-of-pipe discharges, for example, from an outfall from an MS4, dictate primary treatment. • Surface overflow rates or more sophisticated sedimentation models indicate that the design suspended or particulate load can be removed for the design storm by the proposed design facility. • Land prices are not prohibitive. Design Guidance and Construction Considerations The analytical and numerical design tools are readily available and should always be utilized for effective basin design, whether such tools are simple overflow rate models (Q/A) or more sophisticated numeric models. For preliminary design practice, standard rules of thumb have been developed, based primarily on practical experience. The balance of these sections provides examples of some of these rules of thumb. Although an effective, low-cost means of stormwater management, dry detention basins do require a significant surface area per treated volume (Pettersson, 1996), and area requirements typically in the range of 0.5 to 2.0% of the total tributary development area (U.S. DOT, 1996). It is generally accepted that the upper range for contributing drainage area for non-arid regions is in the range of 20 to 30 ha (U.S. DOT, 1996). Estimating the appropriate geometry of a detention facility, although highly site specific, is largely based on empirical findings. To optimize the performance efficiency of detention ponds, the geometry must be sized accordingly, as simply sizing the facility for a required storage volume will not ensure maximum pollutant removal. This optimization is achieved by maximizing the primary flow axis of the pond by ensuring a minimum length-to-width ratio of at least 4:1. This ensures the establishment of low velocities and quiescent flow, thus promoting sedimentation. The design storage volume should have a capacity of 120% of the first-flush volume above the lowest outlet in the basin. The extra 20% is to account for sediment accumulation. The average storm runoff volume needs to be detained at least 24 h to achieve equivalent TSS removal after 6 h in a settling tube. The volume should be determined using a 24 to 48 h emptying time, with an outlet designed so that the full volume of the basin be drained in approximately 40 h, with no more than 50% released in the first 12 h.

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Ideally, a two-stage basin design should be implemented whenever feasible, with a lower portion with a capacity of 15 to 20% of the overall design capacity. A two-stage design ensures the detention of stormwater runoff from events smaller than the design storm. This is of fundamental importance as it is these small, high-frequency events that deliver the majority of the annual runoff volume to the basin (Helsel et al., 1979). This lower pool (0.5 to 0.9 m deep) will fill more frequently, reducing the periods of standing water and sediment deposition over the remainder of the basin. The upper stage should be 0.6 to 1.8 m deep with a base sloping at 2% to a low-flow channel. Operation and Maintenance Considerations Fundamental maintenance requirements include control of pond vegetation, removal of accumulated bottom sediments, and removal of debris from all inlet and outflow structures. Sediment removal is a vital requirement as continued accumulation gradually reduces the available storage volume and thus reduces the detention times below target levels. Best national estimates state that approximately 1% of the storage capacity associated with the 2-year design storm can be lost annually (U.S. DOT, 1996). This value explains the 20% safety margin added to the design storage capacity over the typical 20-year design life of the facility. It is estimated that the average dredging requirements will be approximately every 10 years, although local conditions could modify the specific removal cycles. If the pond buffer and upper stage are managed as a meadow rather than a lawn, then mowing requirements can be reduced from14 to 2 operations per year (late spring and late fall) (U.S. DOT, 1996). Routine inspections should be performed on a periodic basis, preferably during wet weather to ensure whether the structure operates in the manner for which it was originally intended and to determine whether the the facility is meeting its design target detention times. Hazardous Waste Disposal Accumulated sediments display a high affinity for metals, although only a small fraction leached out to toxicity characteristics leaching procedure (TCLP) extract solution (U.S. DOT, 1996). Depending on factors, which include heavy metal loading, sediment characteristics, loading duration, pH, and redox conditions, the bottom sediments can be classified as a hazardous waste and must be managed accordingly. Cost and Design Life Fixed capital costs of construction for a dry extended detention pond can be estimated from the following equation: C = 168.39 ∗ V 0.69 where C = construction cost (estimate) V = volume of storage (m3) of the pond up to the crest of the emergency spillway Originally developed by the Metropolitan Washington Council of Governments in 1985, the equation has been updated to current dollar value, assuming an increased cost of 3% per year. This cost estimate should only be used for planning purposes as there are many additional costs that can affect the actual fixed capital estimate. It is recommended that an additional 25% be added to this estimate to account for unforeseen contingencies (i.e., design, permits, construction oversights, etc.) The largest single cost of construction for the installation of a dry extended basin is the cost of excavation. Therefore, selecting and utilizing natural depressions in the topography wherever

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possible will reduce excavation requirements. In the situation where an extended detention pond already exists, retrofitting and modifying the outlet structure can increase the storage volume at a fraction of the cost of constructing a new facility. The estimated design life of these structure is 25 years and with the required maintenance regime listed above, the total annual maintenance cost for both routine and nonroutine maintenance is estimated at 3 to 5% of the base construction cost.

WET RETENTION PONDS Wet retention ponds can be an effective stormwater quality and quantity BMP and if correctly sized and managed can achieve high removal rates from stormwater runoff of both particulate-bound settleable solids as well as soluble-phase pollutants and nutrients. Wet retention ponds are similar to dry retention basins, except that a permanent body of water is incorporated into the design. This permanent feature enables biological and chemical processes to facilitate the reduction of soluble nutrients such as nitrates and orthophosphates, present in the water (U.S. DOT, 1996). A surcharge detention storage volume or water quality control volume (WQCV) can be readily incorporated in to the design for quantitative stormwater mitigation as well as qualitative. The primary pollutant removal processes are a combination of physical sedimentation, partitioning, and biological uptake and transformation depending on pH, redox conditions, and residence time. Influent stormwaters mix and partially or completely replace the existing water. Between events, biological activity and nutrient uptake by algae remove soluble-phase pollutants and nutrients from the water. Because soluble nutrients have negligible settling velocities, partitioning and biological uptake are the primary pollutant removal mechanisms. Resident fauna convert the soluble nutrients into biomass, which in turn settles out of the water column. Once nutrients and organic materials are trapped in the sediments, they are consumed by bacteria and removed from the system. As a result, wet retention ponds have superior pollutant removal efficiencies in comparison to dry detention basins. Heavy storm events may cause sediment resuspension and interstitial pore water mixing, possibly causing anoxic conditions, which in turn may remobilize soluble-phase contaminants in the sediments. Soluble nutrient removal efficiencies of wet retention ponds are directly related to the ratio of the volume of the permanent pool of water and the volume of runoff from the average design storm event (WEF, 1998). Empirically derived guidelines suggest that the volume of water in the permanent pool of water be at least three times the runoff volume. This volume is approximately estimated as 13 mm of runoff per impervious hectare (U.S. DOT, 1996). This sizing relationship is based on soluble nutrient target removal efficiencies, but is more than adequate for sediment removal goals. Wet retention ponds may demonstrate feasibility as a BMP when the following conditions exist: • Stormwater mitigation is required in a residential or commercial area, with a drainage area of at least 8 ha, possessing a large contribution of off-site drainage and with a guaranteed water source. • When eutrophication goals in downstream reservoirs or lakes is the primary objective of the BMP. • The aim is to reduce nutrient loadings into environmentally sensitive tidal estuaries or bays. • The stormwater runoff has a high concentration of soluble-phase contaminants of concern. • Issues of hazardous residuals management have been addressed. • Safety concerns of the temporary water surface can be assured. • End-of-pipe discharges, for example, from an outfall from an MS4, dictate primary treatment. • Surface overflow rates or more sophisticated sedimentation models indicate that the design suspended or particulate load can be removed for the design storm by the proposed design facility.

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Design Guidance and Construction Considerations Similarly to extended dry detention ponds, the readily available analytical and numerical design tools should always be utilized for effective basin design, irrespective of model complexity. As with dry detention basins, based on engineering design practice, standard rules of thumb have been developed, and are included here for assistance in preliminary design. The drainage area should be such that an adequate base flow is maintained in the pond to ensure a permanent pool of water throughout the entire year. If the wet pond is inappropriately maintained, or stagnation occurs, algal blooms, odor, and insect nuisances will occur. A drainage area of at least 8 ha is recommended to provide sufficient drainage to maintain the pond at a typical depth of between 1.0 and 2.5 m, in order to maintain aerobic conditions in the water. The minimum depth of the open water should be maintained at a depth greater than the depth of sunlight penetration (typically, 2 to 2.5 m) to prevent emergent plant growth across its entirety. This depth should also protect against wind-generated sediment resupension in all but the most intense storm events. The maximum drainage area for which a single wet retention pond can be considered as a BMP is 25 km2 and the area requirements for effective stormwater mitigation is typically 1 to 3% of the contributing drainage area. A 3-m-minimum-wide littoral zone and a wetland vegetation bench surrounding the pond with an area 25 to 50% of the water surface of the permanent pool provides for an aquatic habitat that enhances biological pollutant removal and reduces algal mat formation. Pond geometry should be configured to maximize time of travel between the inlet and outlet control structures to promote mixing between new runoff and with the permanent water body. Ideally, this optimization is achieved by maximizing the primary flow axis of the pond by ensuring a minimum length-to-width ratio of at least 4:1. If this is unattainable, similar effects can be achieved with the use of baffles, a curvilinear geometric configuration, or with strategically placed emergent plant growth. Operation and Maintenance Considerations Overall routine and nonroutine maintenance is essentially the same for both wet retention ponds and dry detention ponds. Sediment removal cycles are similar to that of dry detention basins. Not only can wet retention ponds serve as an effective stormwater BMP, but when applied in a residential or commercial setting, they can also provide an aesthetic enhancement to the development, particularly so if regular routine and nonroutine maintenance can be ensured. Nuisance control (odors, insects, and weeds) is the most frequent maintenance requirement, although if properly sized and operated these requirements should be minimal (Table 6.2).

TABLE 6.2 Comparison of Pollutant Removal Efficiencies by Extended Dry Basins and Wet Retention Ponds Type of Practice Extended dry detention Wet retention

Total Suspended Sediments 70–90 70–90

Nitrogen 0 20–30 50–70 30–40

(Diss) (Total) (Diss) (Total)

Phosphorus

Lead

Zinc

Biological Oxygen Demand

0 20–50 50–70 50–60

70–80

40–50

20–40

70–80

40–50

20–40

(Diss) (Total) (Diss) (Total)

Source: WEF, 1998. Urban runoff quality management, WEF Manual of Practice, No. 23, Water Environment Federation and the American Society of Civil Engineers.

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Hazardous Waste Disposal Residuals management for wet retention ponds are essentially identical for both wet and dry systems, although there are the additional costs incurred in pond drainage for these required management procedures (wet retention and dry detention). Cost and Design Life Wet retention ponds are a more cost-effective BMP in larger, more intensively developed areas and are not as cost-effective in smaller drainage areas, where a dry detention basin or infiltration pond may be more suitable. Therefore, a wet retention pond is a good treatment alternative for inclusion as a regional stormwater management facility, with runoff collected from numerous sites. Nutrient removal afforded by wet retention basins requires two to seven times more volume than an extended dry detention basin, depending on local meteorology. Larger volume requirements necessitate larger structures and more land. Consequently, costs of such facilities are typically 50 to 150% more than for extended dry detention basins. Cost of construction is directly related to the volume of the permanent pond and can be estimated using the formula developed by the Metropolitan Washington Council of Governments in 1985 and then adjusted for current conditions. C = 6.1 (V 0.02832)

0.75

where C = construction cost in U.S.$ V = volume of storage (m3) in the permanent pool up to the crest of the spillway This cost estimate should only be used for planning purposes as there are many additional costs that can affect the actual fixed capital estimate. It is recommended that an additional 25% be added to this estimate to account for unforeseen contingencies (i.e., design, permits, construction oversights, etc.). Although the above equation does not consider land costs, it is a reasonable assumption to consider these as negligible, as most zoning boards require that a minimum area be reserved for open space. The overall cost of construction can be reduced by selecting and utilizing natural depressions in the topography. The estimated design life of these structure is 25 years and with the required maintenance regime listed above, the total annual maintenance cost for both routine and nonroutine maintenance is estimated at 3 to 5% of the base construction cost.

FILTRATION SYSTEMS The fundamental operation rationale of vegetated grass swales and filter strips systems as stormwater BMPs are twofold. Primarily they are used for stormwater conveyance while operating as a pretreatment measure for other downstream BMPs. Their principal treatment objective is particulate load removal from the runoff that would otherwise clog or reduce performance efficiencies in subsequent downstream BMPs. In utilizing this two-phase approach, higher pollutant removal efficiencies are obtainable and maintenance requirements reduced, thereby increasing the design life of the control facility. Although the primary pollutant removal mechanisms are through physical vegetative filtration and soil infiltration, dissolved constituents may also be removed, albeit to a lesser extent, through chemical or biological processes mediated by the vegetation and the soil. The selection rationale of vegetated swales and filter strips as BMPs is for pretreatment measures and as a unit component of an integrated stormwater management system.

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VEGETATED SWALES Vegetated grassed swales, or biofilters, are shallow vegetated channels designed primarily as stormwater conveyance systems, with flow depths rarely above the height of the vegetation that grows within them. Generally less expensive to construct and more aesthetically pleasing than concrete or rock-lined drainage channels, vegetated swales can be configured in various geometries. Their control of peak discharges is twofold. First, the vegetation significantly reduces runoff velocity (depending on the length and slope of the swale). As a result, the time of concentration for the contributing watershed is increased, and can in part partially attenuate the postdevelopment discharge rate. Second, a portion of the stormwater passing through the swales infiltrates into the soil and does not appear at the downstream control point. The significance of this infiltration is dependent on the soil characteristics, although, typically, this does not exceed a few tenths of an inch of the runoff depth (Schueler, 1987). The primary pollutant removal mechanisms are by physical filtration through grass and infiltration through the soil, although runoff waters are typically not detained for long enough to remove fine suspended sediments effectively. Potential pollutant removal capabilities are related to channel dimensions, longitudinal slope (which controls retention times and volumes) and vegetation type. A well-designed, well-maintained swale system can be expected to remove 70% TSS, 30% total phosphorus, 25% total nitrogen, and 50 to 90% trace metals (U.S. DOT, 1996). Although they can provide sufficient control under light to moderate runoff conditions, vegetated swales have a limited capacity for stormwater quality and quantity mitigation during large storm events. However, their performance efficiencies can be enhanced with the installation of check dams or wide depressions to increase runoff storage capacity, increase detention times, and promote greater settling of pollutants (U.S. DOT, 1996). There also may appear seasonal variations in pollutant removal efficiencies. Seasonal dormancy of the plant coverage can lead to reductions in pollutant uptake and reductions in flow velocity reduction capacities. Vegetation decomposition in the fall and the possible winter absence of grass can lead to potential outwelling of nutrients and expose the swale to erosion. Vegetated swales alone or in combination with a vegetated filter system may demonstrate feasibility as a stormwater mitigation BMP in the following applications: • In single family residential developments and highway medians as an alternative to curb and gutter drainage systems. • For small catchments, typically serving less than 4 ha, with slopes no greater than 5%. • As a pretreatment measure for other downstream BMPs, particularly infiltration devices. • When anticipated peak runoff discharges are less than 5 cfs and if runoff velocities are less than 3 fps. • Where flow is diffuse or can be readily distributed accordingly. • Where the potential for groundwater contamination for soils with high infiltration rates has been addressed • Where its characteristic linear geometry lends itself to linear system applications, such as alongside highways or property lines. Design and Site Considerations The suitability and removal performances depend on the size of the area serviced, the soil type, the slope, and geometry of the swale system. Swales that have long, wide channels with gentle slopes (less than 5%) perform better than a short narrow configuration. A minimum length of 200 ft is recommended, although some instances indicate effectiveness at lengths of 100 to 125 ft (U.S. DOT, 1996). Typically, width varies from 2 to 8 ft, with a recommended maximum of 10 ft. For maximum performance efficiencies, underlying soil type should be dry, with good drainage and

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TABLE 6.3 Criteria for Optimum Vegetated Swale Performance Parameter

Optimum Criteria

Hydraulic residence time Average flow velocity Swale width Swale length Swale slope Side slope ratio (h:v)

9 min ≤27 m3/s (0.9 ft/s) 2.4 m (8 ft) 61 m (200 ft) ~2–6% 4:1

Minimum Criteriaa ≥5 min 0.6 m (2 ft) 30 m (100 ft) ~1% 2:1

a

Criteria at or below minimum values can be used when compensatory adjustments are made to the standard design. Source: U.S. DOT, 1996. Evaluation and management of highway runoff water quality, FWHA-PD-96-032, Washington, D.C.

high infiltration rates (minimum effective infiltration rate = 4.3 mm/h). Heavy clayey soils that impair infiltration and promote ponding should be avoided. Grass is typically the preferred vegetative cover, due to its ability to grow through thin sediment deposits, thereby stabilizing deposited sediment and preventing resuspension. Also, few herbaceous plant species can provide the same density and surface per unit area. Operation and Maintenance System maintenance is typically minimal and is restricted to keeping the grass cover dense and vigorous, with grass height maintained at 5 mm above design water depth. Grass clippings should be collected and deposited off site or a mulcher mower used. Herbicide use should be kept to a minimum and fertilizer requirements are typically obsolete, as the runoff typically contains sufficient nutrients. Sediment accumulations need to be removed when they exceed 100 mm in depth or cover vegetation. This removal regime should be performed periodically or as inspection dictates. Typically sediment removal is required from 3 to 10% of the length of the swale per year (Table 6.3). Residuals Management Depending on the type of pollutants accumulated, some sediments may be considered hazardous waste or toxic material and are therefore subject to standard management requirements and disposal restrictions. Cost and Design Life Generally, grassed swales cost less to construct than curbs, gutters, or underground pipes. The most variable cost is in the price of the vegetative cover applied, depending on the species chosen and method of application. Structural enhancements will also add to the cost. Typical costs of construction range from $16 to $49 per linear meter, depending on site conditions, swale geometry, and degree of internal storage.

FILTER STRIPS Filter strips or vegetated buffer strips are vegetated sections of land similar to vegetated swales, except they are essentially flat, or with low slopes, and are designed to only accept stormwater

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runoff as overland sheet flow. They are designed and operated to intercept flow, lower flow velocities, and spread the runoff as sheet flow. They are typically viewed as one component of an integrated stormwater management system. The types of vegetation used covers a broad spectrum of plant types, from grass to large stands of trees. The typically dense vegetative cover facilitates conventional pollutant removal through detention, filtration by vegetation, and infiltration into the soils. Forested filter strips have a greater pollutant removal capability than grassed strips, due to the greater uptake and longer-term retention of nutrients in the forest biomass. However, forested strips typically need to be at least double the length of grassed strips to compensate for the less dense vegetative cover in forested strips. Also, trees and other woody vegetation have a propensity to disrupt uniform sheet flow. Removal of soluble pollutants is accomplished when pollutants infiltrate into the soil and are subsequently sorbed to the soil or taken up by the plant roots. Consequently, soluble pollutant removal depends on the infiltration rates of the soils, although soluble pollutant removal efficiencies are typically low, as only a modest proportion of the runoff becomes infiltration. Measured field data have shown typical removal rates of 70% for TSS, 40% for phosphorus (particulate), 40% for zinc (particulate), 25% for lead, and 10% for nitrite/nitrate. Filter strips can not treat high-velocity flows and do not provide for enough storage to effectively reduce peak discharges to predevelopment levels. This poor performance effectiveness in stormwater quantity mitigation typically confines their use to rural or low-density development applications. Their primary application is along rural roads, where runoff that would otherwise pass untreated directly into a receiving stream passes through the filter strip before entering a conveyance system or a stormwater quality mitigation facility, such as an infiltration trench. Filter strips may demonstrate feasibility as stormwater mitigation BMP in the following instances: • Rural, low-density developments • Highway runoff control (two lanes with maximum ADT < 30,000), with maximum pavement width of 335 to 1000 ft (WEF, 1998) • Highway runoff such that would otherwise discharge directly to a receiving water body • Small contributing runoff areas, with low peak discharges and low peak runoff velocities • Pretreatment contingency for the maintenance of subsequent downstream BMPs • Where flow is diffuse or can be readily distributed accordingly • Where the potential for groundwater contamination for soils with high infiltration rates has been addressed • Where its characteristic linear geometry lends itself to linear system applications, such as alongside highways or property lines Design and Site Considerations Pollutant removal rates are a function of length, slope, soil permeability, size of contributing runoff, and runoff velocity. Although the minimum length in the direction of flow is generally accepted to be 6 m, lengths of the order of 30 to 90 m are required for the removal of smaller particulates. Successful performance of filter strips is contingent on maintaining shallow, unconcentrated sheet flow. Concentrated or channeled flow can substantially reduce the typical design life of 10 to 20 years. Flat slopes (<5%) and low to fair permeability (0.15 to 4.3 mm/h) of natural subsoil are required for the effective operation of filter strips. Organic rich soils improve pollutant removal efficiencies. This can be improved further if the water table is within 1 m of the surface, i.e., within the rhyzosphere. Filter strips should be constructed of dense, soil-binding, deep-rooted water-resistant plants. For grass filter strips, dense turf is needed to promote sedimentation and entrapment and to protect against erosion, with turf grass maintained to a height of 50 to 60 mm.Typically, each filter strip should serve no more than 2 ha, or less, as this reduces the potential for flow concentration.

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TABLE 6.4 Criteria for Optimum Vegetated Filter Strip Performance Parameter Tranverse slope Hydraulic residence time Average flow velocity Strip width Average depth of flow Manning’s friction factor (n)

Optimum Criteria <5% (6–15% range) 5–9 min ≤27 m/sec (0.9 ft/s) No greater than that where uniform flow can be guaranteed <12.5 mm 0.2 (grassed) 0.24 (if infrequently mowed)

Source: U.S. DOT, 1996. Evaluation and management of highway runoff water quality, FWHA-PD-96-032, Washington, D.C.

To work effectively and avoid flow channeling, a filter strip must exhibit the following characteristics: • Be equipped with a water distribution device, e.g., porous pavement strips, slotted curbstones, rock-filled trench • Be densely vegetated with a mix of erosion resistant plant species that effectively binds the soil • Be graded to an even, uniform, and relatively low slope • Be as least as long (or wide) as the contributing runoff area Operation and Maintenance Required maintenance is dependent on whether the filter strip is allowed to follow a natural vegetative succession or not, as is often the case. Maintenance requirements and costs are substantially lower for these “natural” strips. Grass should be mowed to maintain dense, vigorous growth, with stem heights maintained at 50 to 60 mm, and should be managed as a lawn or short-grass meadow, with two to three mowing cycles per year to prevent natural succession if desired. Annual inspection of the filter strips is preferred to ensure continued uniform flow distribution into and across the filter strip. Similarly, sediment removal may be necessary to maintain uniform sheet flow and the original slope grade (Table 6.4). Residual Management Appropriate residual management is essentially the same for filter strips as for vegetated swales. Cost Considerations Filter strip establishment costs are very low and almost negligible when an existing meadow area is reserved at the site prior to development. The main variable in the cost is in the type of vegetation planted (grass or trees) and the planting method used (sod or seeding). For grassed filter strips, typically under 2 ha, construction (planting) costs range from ~$1650/ha, for conventional grass seeding, up to $10,200/ha for sodding. For forested filter strips, planting costs range from $250/ha for deciduous trees, to $500/ha for coniferous up to $2500 to $12,350 for mixed variable tree species. All costs are rated at 1996 dollar amounts (U.S. DOT, 1996).

SAND FILTERS Although in use for the improvement of water quality since the early 1820s in France and occasionally England, the use of sand filters for the treatment of stormwater runoff is a somewhat recent

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innovation. Declining steadily in popularity for water/wastewater treatment applications throughout the latter part of this century, the technology experienced a revival in use in the early 1980s, albeit for the treatment of stormwater for which it had not previously been applied. The objective of sand filters is to remove sediment particulates and the pollutants from the first flush of pavement or impervious area runoff (U.S. DOT, 1996). This filtration of both the soluble and particulate phase nutrients, organics, and coliform bacteria is facilitated by the mat of bacterial slime that develops under normal operating conditions. The primary pollutant removal mechanisms are pollutant dependent. Large-grained suspended sediments are removed by sedimentation in a presettling basin, with the finer particles and particulate-bound heavy metals physically strained in the filter media. The interstitial matrix of sand can effectively remove particles 1/60 the diameter of particles removed in settling basins, and 1 m2 of typical sand filter media has an equivalent surface area to that of 400 m2 of settling basin (U.S. DOT, 1996). Soluble-phase nutrients, such as phosphorus, nitrogen, and biological oxygen demand (BOD), are removed principally by chemical adsorption onto the surface of the sand grains and chemical precipitation of the resultant complexation reactions, often involving heavy metals. Biochemical transformation by naturally present heterotrophic and chemoautotrophic bacteria facilitates surface adsorption phenomena. Under the anaerobic microenvironments within the interstitial matrix, denitrification can result in nitrogen removal efficiencies greater than 50%. One of the major advantages of sand filters is their adaptability and applicability to a whole suite of site-specific conditions. Often they are the default stormwater BMP, where conventional BMPs fail or cannot be applied at all. Well-designed and operated sand filters have very low failure rates. As a consequence, sand filters may demonstrate feasibility as a stormwater BMP, under the following conditions: • Areas of thin soils and low soil infiltration rates. • Arid regions with high evaporation rates. Limited rainfall and high evaporation rates otherwise preclude the use of retention ponds or wetlands. The typically hot, dry climate of these regions, particularly during the summer, limits the use of vegetative systems. • When there is limited space availability, sand filters can be placed beneath pavement to maximize land use potential. • Where its characteristic linear geometry lends itself to linear system applications, such as alongside highways or curbsides. • When the groundwater is to be protected. Design Guidance Considerations There exist many design permutations of this technology, as engineers have adapted the original concept to meet site-specific needs. A particular development is the peat–sand filter, which is adapted to provide a sorption layer and vegetative cover to provide for a greater degree of pollutant removal. The fundamental operational concern of sand filters is premature clogging when the filters receive excessive sediment-laden runoff from bare soil or construction activities. Consequently, it is recommended that these units only be used to treat runoff from small, relatively impervious watersheds, e.g., parking lots and roadways. The inclusion of a presettling basin to remove settleable solids will greatly improve the effective design life of the filter system. Caution should be used in their application in colder climates, as there has been only a limited demonstration of their use under these conditions. In general, the biological processes by which the filters improve the water quality are substantially retarded in colder conditions. In particular, a bypass system should be available under the possibility of ground freezing during extreme weather conditions. Operation and Maintenance Considerations To ensure continued and long-lasting effective performance efficiencies, sand filters typically necessitate a higher degree of routine maintenance in comparison to other BMPs. Insufficient

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maintenance and inappropriate operation can rapidly lead to clogging, a reduction in performance effectiveness, and ultimately complete system failure, leading to costly remediative action. During the first year of operation, an intensive regime of quarterly inspections are recommended, as well as inspections after large storm events, to ensure effective operational guidelines for maintenance personnel. Cost and Design Life Although a relatively costly alternative in comparison with other conventional BMPs, with typical installation costs ranging from $25,000 to $50,000 per impervious hectare served, there are several other economic factors that when considered make sand filters a highly attractive alternative. Because sand filters can be installed underneath infrastructures (for example, parking lots or roads) they can offer considerable savings in land requirements and associated costs, particularly in areas where real estate is expensive. However, sand filters offer only stormwater quality mitigation and these savings in land purchases must be set against the costs of stormwater quantity control structures. Current costs of installation are somewhat inflated due to the infancy of the technology. These costs of installation and operation have continued to fall (by as much as 50% in some applications) as engineers, architects, and contractors become more familiar with the development and implementation practices. Because of the recency of their successful implementation, their exist little data regarding design life and extended performance efficiencies over this period. This will increase as the technology meets with increasing favor.

POROUS PAVEMENT Constructed from either porous concrete or modular porous concrete blocks, porous pavements are an innovative yet effective stormwater mitigation BMP in both the in situ storage and disposal of runoff and the consequent attenuation of the stormwater hydrograph. They are also capable of significant reductions in target pollutants in runoff from intensely impervious urban areas. This is achieved by allowing stormwater to infiltrate the surface pavement layer rapidly to enter a highvoid aggregate subgrade layer. As much as 90% of the runoff is stored in this “reservoir” layer until it either infiltrates into the underlying soil strata or is routed to a conventional stormwater conveyance system. The inclusion of porous pavement in the infrastructure effectively reduces the amount of directly connected impervious surface within a catchment. With the majority of the potential runoff retained on site, the need for conventional stormwater conveyances, such as curbs, gutters, storm channels, and detention ponds, can be significantly reduced, if not eliminated. The majority of pollutant removal occurs in the underlying soil strata. As a consequence the degree of pollutant removal is directly proportional to the degree to which stormwater runoff is exfiltrated to the underlying soil. The principal mechanism of pollutant removal is through sorption of soluble-phase pollutants to the surface of soil particles, with organically rich soils displaying higher pollutant removal performance efficiencies than sandy soils. Resident bacteria in the soil matrix are capable of biotransformation and attenuation of many problematic organic pollutants, including hydrocarbons, that are otherwise very persistent contaminants in stormwater and are typically very difficult to remove. Field measurements have demonstrated removal efficiencies upward of 98% of potentially carcinogenic compounds such as benzopyrene and benzanthracene (Pratt et al., 1999). Readily applicable in extensively impermeable urban areas, porous pavements are typically only practical for low traffic volume applications. Offering a safer driving surface, with better skid resistance and reduced hydroplaning, porous pavements have been successfully demonstrated on roads with much greater ADT counts (Colandini et al., 1999), although the higher traffic volumes

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tend to lead to premature clogging. As a consequence, porous pavements may demonstrate feasibility as a stormwater BMP for contributing low traffic volume watersheds less than 4 ha, in the following applications: • • • • •

Parking lots Airports Access roads Driveways Emergency and parking lanes on highways

However, results from experimental porous pavement sections along the Paris Beltway indicate that even with an ADT of 200,000, a high porous pavement infiltration capacity can be maintained. Design and Site Considerations Porous and asphalt pavements are constructed in the same way as conventional pavement, except that the sand and finer fractions are left out of the pavement mix, thereby effectively increasing the void space ratio within the matrix. The modular concrete block surface is created by tessellating the individual blocks over a coarse gravel, which in turn is located on a porous geotextile fabric layer. There are three fundamental system designs for porous pavements: A full exfiltration system is designed to retain 100% of the captured runoff until it exfiltrates into the soil. Consequently, the reservoir section must be sized to accommodate the entire increase of runoff from the design storm and provide total peak discharge, volume, and water quality controls for all rain events less than or equal to the design storm. Partial exfiltration systems are installed when it is not feasible to exfiltrate the entire volume of runoff from a design event or there are concerns regarding clogging and the long-term permability of the subsoils. To accommodate this, underground perforated pipes are placed within the reservoir to collect excess runoff for discharge from a central outlet. Typically, these underdrains are sized for the excess runoff from the 2-year design event, with runoff from smaller storms exfiltrated before it can be collected and thus receiving maximum pollutant removal afforded by full exfiltration. Water quality exfiltration systems are designed to handle only the first flush of runoff volume (typically regarded as the volume generated by 1/2 in. of runoff per contributing impervious acre), with runoff in excess of the so-call first-flush volumes collected and conveyed to a conventional stormwater treatment facility. Although they do not necessarily satisfy stormwater storage requirements, such facilities enable smaller capacity and therefore less costly treatment systems downstream. The reservoir section should be designed to drain completely within a maximum of 3 days after the maximum storm event. This is essential in order that the soil beneath is allowed to dry out and maintain the aerobic conditions favored by the beneficial bacteria. At the other extreme, poor pollutant removal efficiencies have been reported for systems that drain in less than 6 h (Schueler, 1987). Operation and Maintenance Fundamental to the successful application of porous pavement technology is the fact that it is primarily a quantitative stormwater mitigation BMP. It must be emphasized that porous pavements are not designed to remove coarse-grained sediments as the porous surfaces have become clogged within 1 to 3 years when exposed to sediment-rich runoff (WEF, 1999). To prevent premature

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TABLE 6.5 Typical Demonstrated Pollutant Removal Efficiencies for Porous Pavements Pollutant Sediment Total phosphorus Total nitrogen Chemical oxygen demand Zinc Cadmium Lead Petroleum derived hydrocarbons

Removal Rate % 80–95a 60–65 80–85 82b 99b 85b 98b 98c

a

It is essential to emphasize that porous pavements are not intentionally designed to remove coarse particulates, as large grained sediments rapidly clog the porous surface. b Maximum reported removal efficiency. c Only measured at experimental site.

clogging of the porous surface, the porous pavement should be vacuum-swept and then highpressure jet-washed at least four times per year. In temperate climates the use of abrasive snow removal and deicers should be minimized. In fact, it has been reported that snow and ice melt is more rapid on porous pavement, which suggests that prohibiting these materials may not be a major inconvenience. Porous pavement rehabilitation is a difficult and costly procedure, as vacuuming has little effect. To increase permeability locally on clogged surfaces, regularly spaced small holes (13 mm) can be drilled through the pavement course. If the clogging is more widespread or if the subsoil becomes clogged, only complete replacement of the paved surface is effective (Table 6.5). Cost and Design Life Currently, there exists only a small amount of data on the effective life of a porous pavement BMP, although reasonable estimates are in the range of 5 to 10 years with correct design, operation, and maintenance. However, the majority of documented premature system failures have primarily been due to poor construction and maintenance practices that has led to widespread complete or partial clogging of the porous surface within 5 years. The oldest porous pavement BMP has been successfully operated for 10 years. The economics of porous pavement installation is very site specific and an individual economic evaluation should be performed for each system installation. Typically, porous asphalt/concrete is 10 to 15% higher than its conventional counterparts. When considering the costs of porous pavement installation, only the incremental costs above the costs of the installation of a conventional pavement may demonstrate feasibility vs. the added benefits of improved water quality and runoff volume attenuation.

INFRASTRUCTURE MODIFICATION AND APPURTENANCES The removal of settleable solids, floatable debris, and colloidal pollutants is not only essential in meeting target effluent qualities, but is also essential in maintaining the design utility of downstream BMPs. Without their removal, whether it be through settling, floatation, or coalescence, the design

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life (before system failure) and performance efficiencies of subsequent treatment units are drastically reduced, because these BMPs either become overloaded beyond their design treatment capacity or are prematurely clogged, resulting in short-circuiting. Therefore, it is desirable to include these separators and concentrators that effectively reduce the strength of the runoff, ahead of a stormwaterquality mitigation device rather than to have to design a treatment unit with a much greater and more costly treatment capacity or perform complicated and costly rehabilitation to clogged systems. The removal of both settleable solids and colloidal pollutants is additionally important because target pollutants such as nutrients, synthetic organics, and heavy metals are often in both the soluble and suspended particulate load. Swirl or vortex separators and solid concentrators are designed to remove petroleum compounds (such as oil and grease), grit, and other floatable debris from stormwater that could otherwise foul downstream stormwater mitigation devices. They are not designed to function as stormwater mitigation devices in their own right, but as a pretreatment device. Their primary utility is for the general reduction in concentration of stormwater through the separation of both solids and buoyant pollutants from the runoff in order that the overflow can be discharged to a downstream stormwater treatment unit without any detriment to their utility. Initially applied to combined stormwater sewers in Europe to reduce the influent strength of this combined waste stream into wastewater treatment plants, their use has seen increasing application in urban stormwater treatment. Their use as a pretreatment device effectively removes excessive solids loadings from runoff so that they do not prematurely clog or otherwise foul subsequent treatment units. Their application is of particular significance prior to BMPs that experience high failure rates when exposed to high solids loadings.

VORTEX/SWIRL SEPARATORS The design concept of vortex separators again originated in the realm of CSOs, where the solids are separated from the wastewater with particularly high solids loadings, with the concentrated solids passed back into the interceptor sewer and ultimately to the wastewater treatment plants. The overflow, cleansed of the bulk of its settleable solids, can then be discharged into the receiving waters. This design rationale can be directly applied to urban stormwater treatment, where highstrength runoff can have its solids loading significantly reduced and the pretreated water can then be conveyed to a subsequent BMP. Without this type of pretreatment, not only do downstream BMPs become easily overloaded with solids concentrations beyond their treatment capacity, but also they become increasingly prone to premature clogging. Vortex or swirl separators achieve the separation process by routing flow through a circular path to create a vortex. Heavier particles are drawn toward the outer diameters and drawn down through even smaller diameter paths and ultimately out of a bottom drain or collection device. Flow containing smaller particles (<30 µm) discharges over the top of the device thus achieving the desired separation. Performance efficiencies can achieve total solids removal of up to 80%, with similar removal efficiencies reported for chemical oxygen demand (COD) removal (Brombach et al., 1993). Therefore, the inclusion of swirl/vortex separators into the arsenal of BMP technologies not only improves performance efficiencies of downstream units, but also increases their design life by reducing the potential for premature clogging.

OIL

AND

WATER SEPARATORS

A particular application of specific target pollutant removal from the waste stream are oil/water separators. The removal of petroleum and other problematic products separates potentially carcinogenic compounds from the waste stream. Situations for which the mitigation of such wastewater characteristics is of specific benefit are businesses such as gas stations, vehicular maintenance facilities, and other commercial and industrial facilities that generate high levels of oil products in runoff wastes.

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Public facilities where oil separators and solid concentrators include the following: • • • •

Marine ports Airfields Fleet vehicle maintenance washing facilities Mass transit park-and-ride lots

Design and Sizing Considerations Sizing criteria for vortex separators are typically in the range of 5 to 10 m3/ha of contributing watershed, depending on the design storm on which the units are based. To achieve similar pollutant removal efficiencies with conventional storm overflow tanks would require a separator volume of two to three times the volume of that of a vortex separator. A single separator with a specific volume of only 7.2 m3/ha, for an 11-ha watershed, removed 1000 kg/year of grit that would otherwise have been conveyed to subsequent BMPs (Brombach et al., 1997). In events of magnitude below that of the design storm, the vortex mechanism is not initiated and the system essentially acts as a simple retention basin, with settled solids accumulating in the base. During larger events, separation will occur through the commencement of the vortex separator mechanism. The basic configuration of oil/water separators somewhat resembles that of a septic tank, but with a greater length-to-width ratio to facilitate the separation process. Larger facilities resemble municipal wastewater primary sedimentation basins. Conventional separators are capable of reducing oil and grease concentrations to 10 mg/l or less, when the droplets diameter is greater than 150 µm. Sizing criteria are based on the rising velocity of the droplets, using oil density and droplet size to calculate this criteria, if direct measurement is not possible. Design sizing for a target efficiency goal requires information regarding droplet size distributions and the specific gravity of petroleum products in stormwater for which there are little or no data, with the exception of runoff from oil refineries. A significant portion of the target pollutants is also attached to fine particular matter and is therefore removed by settling, not floatation. As a consequence, the performance of oil/water separator application to urban stormwater mitigation is uncertain. Manufactured package separator units can be used up to flows of several cubic meters per second. For flows greater than this, the unit must be independently designed. Given the paucity of data pertaining to petroleum products in stormwater and the variability of separator technology among commercial vendors, it is recommended that design engineers or planners consult directly with the vendors for a package unit that will meet the target removal efficiencies. Operation and Maintenance Swirl separators and solid concentrators typically require substantially less maintenance than conventional separators, due to their self-cleaning abilities and no reported incidents of becoming clogged (Brombach et al., 1997). The cleaning regime for the routine removal of the concentrated solids will be determined through initial operational experience. Overflow sieves typically need cleaning approximately four times per year, depending on the loading rates. Separators designed exclusively for petroleum product removal should be preceded by a solids removal unit operation or trash rack, as settleable solids can rapidly lead to system fouling that can considerably complicate maintenance procedures. Monthly checks should be performed during the wet season with cleaning requirements of several times per year. Cost and Design Life Because the infancy of the application of vortex separators to stormwater BMPs, there are little or no reliable data regarding their design lives, although typical engineering practice assumes an operational design life on the order of 30 years.

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EFFECTIVENESS OF MULTIPLE UNIT OPERATIONS, PROCESS CONTROLS, AND BMPs ADSORPTIVE–FILTRATION SYSTEMS Whenever site conditions permit, a substantial percentage (60 to 90%) of the total annual urban runoff of the urban stormwater can be diverted to surrounding soils and disposed of on site through a combination of infiltration and exfiltration systems. Through a complex combination of infiltration, adsorption, precipitation, and exfiltration, adsorptive–filtration systems, such systems generally offer the highest removal performance efficiencies for the vast majority of target pollutants (generally no less than 70% and as high as 99% for heavy metals). However, they are typically limited by their ability to serve only a small service area watershed and a propensity to premature clogging.

PARTIAL EXFILTRATION SYSTEMS Infiltration systems are an adaptable BMP that can provide qualitative controls on both soluble and particulate pollutants. They can also provide quantitative stormwater mitigation, capable of reducing stormwater volume and peak runoff rate that, if unchecked, can readily inundate drainage appurtenances and overload subsequent treatment units. Runoff is diverted into the trench and either fully exfiltrates into the soil beneath (complete exfiltration system) or enters a perforated pipe underdrain and is routed to an outflow facility (partial exfiltration system, PER). Suspended solids and dissolved metals transported with the runoff flow are immobilized in the PER by filtration and sorption/precipitation, respectively, with the potential for bacterial uptake or conversion upon exfiltration through the underlying soil. In addition, temporary storage of stormwater in the pore spaces attenuates the runoff hydrograph. Consequently, the PER serves as a dual-purpose stormwater BMP. Not only does it serve as a reactor to improve stormwater quality, but it also acts as a reservoir to reduce peak runoff rates (Sansalone et al., 1999). PER configurations can be applied in urban areas as an infiltration trench accepting either a runoff line loading or as a packed-bed reactor accepting point loading (Sansalone, 1999c).

MODIFIED PARTIAL EXFILTRATION TRENCH REACTORS PERs are a hybrid design consisting of an excavated trench that has been lined with a geotextile filter fabric and then backfilled with an aggregate mixture (porosity range 30 to 40%) to form an underground detention basin. The sorptive properties of the PER can be enhanced by modified or oxide-coated media backfill (Sansalone, 1997). This effectively promotes sorption of dissolved metals and filtration of particulate-bound metals. A perforated underdrain is placed near the top of the trench, so that all runoff from events below the design storm will be completely exfiltrated, and runoff will only be conveyed off site when runoff volumes are equivalent to or greater than the detention capacity of the subgrade layer (WEF, 1998). As with other infiltration systems, infiltration trenches are not intended to either trap coarse sediments or be exposed to runoff with excessive sediment loads. Such applications would lead to premature clogging (within 5 years) and, ultimately, system failure. PERs are frequently used in combination with vegetated swales and it is recommended that grass buffers (minimum width 6 m) be installed around the trench to trap coarse sediments. PERs may demonstrate feasibility as a BMP when complete exfiltration to the subsoil is either inappropriate or cannot be assured due to the possibility of clogging or concerns regarding the long-term permeability of the matrix. Combination with a modular porous pavement capping layer provides a buffering capacity for high runoff volumes that would otherwise result in ponding above the PER. Depending on the degree of subgrade storage/exfiltration afforded, PERs can provide groundwater recharge and low-flow augmentation of base flow in streams by diverting

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TABLE 6.6 Pollutant Removal Efficiencies for the Interstate-75 Site Partial Exfiltration Trench Urban Pollutant TSSa VSSa Total phosphorus Total nitrogen Heavy metals (dissolved)a Heavy metals (particulate bound)a BOD Bacteria a

Removal Rate, % >85 >90 65–75 60–70 85–99 >75–95 90 98

Limiting Factor Should be trapped before reaching trench Leaching of remineralized organic phosphorus Leaching of soluble nitrate Behavior similar to soils of high surface area and charge Leaching of dissolved organic matter Straining

Interstate-75 Cincinnati experimental site.

Source: Sansalone, J.J. and Buchberger, S.G., J. Environ. Eng. Div. ASCE, 123(2), 134, 1997. With permission.

60 to 90% of annual urban runoff back into the soil (Li et al., 1999). Hydrologic flow through the PER typically propagates as a two-dimensional variably saturated flow, characterized by skewed residence time distributions, long travel times, high exfiltration rates to the surrounding soils, and low tracer mass recovery at the PER underdrain (Sansalone et al., 1999). These characteristics of the flow in PERs are of particular significance in the control of urban stormwater quality, because the first-flush runoff often contains a disproportionately high pollutant load (Sansalone et al., 1999). A full-scale PER with iron-oxide-coated sand backfill was installed at an experimental site along the hard shoulder of Interstate-75 in Cincinnati, Ohio and was loaded with a design infiltration capacity of 50 l/min-m2. After a year of full operation, pollutant removal efficiencies still remained high. Dissolved heavy metal mass removal exceeded 90% (Table 6.6). The particulate-bound fraction of heavy metal removal rate exceeded 80%. TSS and volatile suspended solid removal exceeded 85 and 90%, respectively (Sansalone, 1999). Because PERs are a modification to underdrainage, they can be installed as a retrofit to existing underdrain applications, whereas other BMPs designed to handle concentrated flows would be constrained in many of these locations. Typical applications include the following: • • • • •

Residential lots and pavements Commercial developments Parking lots Highway median strips Highway and pavement shoulders

Design and Site Considerations Drainage area and soil type are the two most important siting considerations. Individual trenches are primarily on-site control devices that require only a small area and are seldom practical or economic with contributing watersheds larger than 4 ha. They should only be considered as the BMP when placed above permeable soils with infiltration rates greater than 7 mm/h; 13 mm/h is a recommended design minimum (Schueler, 1987). This rate is associated with sand, loamy sand, loam, and loamy silt soil groups. Typically, the PERs are sized for the 2-year storm event, although it is advised to design for larger events to compensate for loss of infiltration capacity due to the typical reduction of hydraulic conductivity of the soils over the design life. They should not be

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sited on soils with clay contents greater than 50% or with a combined silt/clay content of greater than 60% (U.S. DOT, 1996). There is a positive correlation between organic content of the soil and pollutant removal efficiencies. The seasonally high water table is at least 0.6 to 1.2 m below the base of the trench and the depth to bedrock is at least 1.2 m below the base. However, there is some contention regarding the use of PERs directly on bedrock or artificially created hardpans. Typically, long and deep trenches (optimum recommended design geometry of 1 m wide and 1 to 2 m deep) require the least amount of porous media and are most effective, as increasing the surface area of the bottom of the trench improves pollutant removal. Also, broader-based trenches reduce the potential for clogging by spreading the exfiltration over a wider area. The drain time should be between 2 and 3 days, with the total volume draining in 72 h to ensure the maintenance of aerobic conditions in both the subgrade layer and underlying soils. Minimum drain time should be 24 h, as drain times below this display poor pollutant removal efficiencies. Operation and Maintenance The fundamental maintenance concern is to prevent clogging, and site management should be directed at reducing (or regularly maintaining) coarse sediment loads to the PER. A well-designed and operated PER that has been appropriately sited requires only minimal maintenance. However, it may be difficult to ensure maintenance needs comply with maintenance regimes. Because they are located subgrade they are inconspicuous and not easily visible or accessed. The PER should be inspected several times during the first few months of operation and at least annually thereafter. The PER should be examined after large storms to check for surface ponding, a symptom indicative of potential clogging of the trench. Peripheral vegetated strips should be maintained to ensure complete vegetative covering and reseeded if bare soil patches develop. The other major concerns for the maintenance of an effective PER are desorption due to deicing salts and the maintenance of positive redox conditions. Cost and Design Life Field data indicate that dissolved or particulate-bound breakthrough may not occur for at least 15 years. A 90% breakthrough indicates that the adsorptive capacity has been exhausted (Sansalone and Hird, 1999). With correct design and appropriate management, the trench will function for 10 to 15 years before clogging occurs. The degree of rehabilitation depends on the depth and extent of the clogged zones. Individual PERs are most cost-effective for small areas where the required storage is less than 280 m3 and are the only economical BMP employed in this size range. Above this limit, detention basins or wet retention ponds are more cost-effective. Construction costs can be estimated using the following equation (U.S. DOT, 1996):

(

C = 32.7 ∗ 35.3 V 0.63

)

where C = construction costs ($) V = storage volume (m3) of the void space in the trench (~40% of the excavated trench volume) This does not include special inlets or filter strips for the pretreatment of the runoff. Routine maintenance costs are typically 10 to 15% of the installation costs, over the design life of the PER. Nonroutine maintenance costs are dominated by the need for rehabilitation of a clogged PER should the situation arise and can easily exceed the initial construction cost. As a modification to the current practice of underdrainage already installed along highways, additional costs to provide water quality improvement are reduced (Sansalone, 1999).

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TREATMENT TRAINS If pollutant removal objectives are not being met by a particular single unit operation type process, whether it be due to overloading beyond the treatment capacity, or a change in influent characteristics, it may be prudent to consider combining BMPs into a sequential treatment train to obtain the desired results, site limitations permitting. Often BMPs are combined to offer quantitative controls in a BMP otherwise providing only qualitative controls. Treatment trains are an on-site integrated combination of unit operations and processes specifically configured in response to the specific target pollutant removal goals and to optimize pollutant removal efficiencies. Any number of process permutations exist, but typically consist of a pretreatment process to reduce influent strength and perform detention functions to prevent inundation and overloading of subsequent unit operations. For example, exfiltration trenches can be used with improved performance efficiencies when placed under vegetated swales, or extended detention ponds can be used in combination with emergent wetlands. Generally, the fundamental objective of these BMPs is to maximize process utility and pollutant removal efficiencies. As a result of this, their major advantage over other individual BMPs is the opportunity for process optimization tailored specifically to meet influent characteristics and site constraints. The modular configuration treatment trains are adaptable to almost any range of influent characteristics, site constraints, and watershed sizes. There will be many projects where extenuating circumstances exist that prevent the maximum utility of a selected individual standard BMP. Modifications of standard designs are often necessary to meet treatment objectives and an integrated design approach to the combination of various unit operations can effectively meet these objectives when specific restrictions or limitations would otherwise preclude the use of a particular BMP. Each stormwater mitigation facility is very site specific and treatment potential is governed by the runoff composition and by the storm hydrograph characteristics, which in turn are affected by the watershed. Site considerations and contaminants of concern may dictate the suggestion of a particular BMP. However, not all target pollutants may be removed to desired levels and in fact certain runoff characteristics (such as excessive sediment loadings) may be detrimental to the long-term functionality of the BMP. Consequently, some sort of pretreatment (sedimentation, oil/water separation) is recommended so that the entire suite of target pollutants can be removed to desired levels. Design and Site Considerations It is the design and site considerations that will dictate the optimum configuration capable of achieving the desired pollutant removal performances. It is often site constraints, particularly land requirements, that typically preclude a specific BMP from meeting its design performance criteria. Appropriate combinations can significantly reduce total land requirements by reducing influent strength to facilities that are typically spatially demanding and, in doing so, it is often quite possible to negate site-limiting constraints, which in turn enables the implementation of a desired BMP that otherwise could not be considered. Operation and Maintenance Operation and maintenance requirements for specific treatment train configurations are operation dependent in accordance with the selected BMPs being combined. Cost and Design Life Similarly, fixed capital and operation and maintenance costs will depend on the exact configuration and combination of the BMPs used. To derive an overall cost, the facility should be itemized, with each unit evaluated separately and the costs summed for a total cost. Integrated treatment trains

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are often much less expensive than a single BMP due to the reduced land requirements needed for a target removal efficiency.

BELOW-GRADE TREATMENT STORAGE SYSTEMS For technical reasons, stormwater or CSO conveyance systems are designed much larger than required to handle the design flow. As a consequence, such storm sewers provide substantial inline storage capacity, a capacity that has been widely exploited in the reduction of peak stormwater discharges and in the attenuation of the stormwater hydrograph. Most typical CSOs are designed with a capacity for 5 to 10 year storm events. However, interceptors that convey the water to the treatment plant normally have a capacity four to six times the maximum dry weather flow (Novotny and Olem, 1994). Consequently, sewers often display significant volumetric redundancy that can be used as in-line or piped storage. By increasing the time of travel to the treatment facility through runoff detention, there is greater opportunity for stormwater quality improvements, through the sedimentation of suspended solids and particulatebound pollutants. However, detention times are rarely sufficient for substantial removal of soluble pollutants, such as nutrients. Stormwater detention within the conveyance system can be in a variety of forms. In-line storage is where the extra volume of the oversized conduits provides flow attenuation and temporary detention of the stormwater. In-line storage facilities also provide for maximum utility of existing dry weather flow treatment facilities, minimize their overload, and allow for subsequent treatment of the stored excess flow that would otherwise have exceeded the treatment capacity of the treatment facility, leading to washout and system failure. Off-line storage relieves an overloaded sewer by diverting the excess flows to a sewer or other temporary detention facility that is off line to the main sewer. This excess is stored in these offline systems until the runoff rates have subsided sufficiently so that flows to the treatment facility have returned to within its treatment capacity and the facility can now safely accept the diverted flow. Detention tunnels are where stormwater is detained in a localized expansion in the conduit, behind a flow restriction device. Typically, dry weather flows pass the flow restrictor undetained. Detention basins can also be used as sedimentation basins, by detaining the runoff sufficiently behind a weir structure for sedimentation to occur. Until the recent promulgation of stormwater quality control regulations, this application has only been applied to the quantitative mitigation of stormwater runoff as a method for peak flow reductions, with less regard for the benefits offered in qualitative stormwater control. Instead of detaining excess flows for the sole purpose of peak flow attenuation, by increasing the detention time of the stormwater within the system, further increases in the potential for water quality improvements can be made. Below-grade treatment and storage practices are particularly effective when designed to capture and detain the first flush from urban runoff, which has a disproportionately high contaminant loading. Recent studies have shown that an 85% reduction in the BOD load can be realized by capturing the first 0.8 to 2.5 cm of the runoff (Schueler, 1987). Field data indicate that stormwater is clarified within the first 24 h of detention, during which suspended solids (including particulate-bound pollutants) were reduced to 10 to 20 mg/l, with sedimentation accumulation rates of 0.15 cm/year. Below this level, sedimentation rates are very slow (Novotny and Olem, 1994), (Table 6.7). Below-grade tunnels that operate as sedimentation tunnels are normally kept full of water between storm events, with the use of weirs and flow restrictors. When new runoff enters the tunnel, it displaces the existing stored water. The degree of improvement in the water quality is dependent on the holding time in the tunnel, the velocity of the runoff within the tunnel, and particle size distributions. Accurate prediction of pollutant removal efficiencies is difficult because of the complexities of the drainage networks and only approximations can be inferred. Parallel laboratory studies required

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TABLE 6.7 Separation of Pollutants in a Stormwater Detention/Settling Tunnel Pollutant TSS NH3–N NO–3 and NO–2 Total N PO4–P Total P BOD5 COD Pb Cd Cu Oil and grease

Percent Removal, % 70–80 60–70 <25 <30 55–65 50–65 58–68 20–40 76–82 74–82 48–60 97–98

a 4 to 7 day detention to achieve comparable detention. However, this can be significant during intermittent rainfall events. Design and Sizing Considerations For an experimental Cincinnati site, field data suggest that over 75% of the total annual rainfall is contained in storms with a rainfall depth of less than 2.5 cm (Novonty and Olem, 1994). The capture of the first 2.5 cm of rainfall would not only capture the total volume of the majority of the storms (94% of the storms in Cincinnati are less than 2.5 cm) but also 2/3 of the volume of larger than 2.5 cm storms. Hence, 91% of the runoff that falls on the watershed would be captured and subsequently treated. Operation and Maintenance Considerations Routine cleaning and removal of accumulated sludge deposits in off-line storage tunnels is necessary to control odors and corrosive gases. It is very expensive and often very dangerous requiring highly skilled personnel to operate under very confined entry conditions; noxious gas buildup can also lead to very serious safety concerns. Poorly ventilated chambers can lead to the generation of anaerobic conditions and the production of hydrogen sulfide. Not only is this a toxic gas in very low concentrations, but it can also increase corrosion of the chamber walls. This in turn can rapidly reduce the long-term integrity of the storage system infrastructure due to leaking and concerns regarding infiltration and interception. In-line storage tunnels are typically self-flushing. Sediment removal is very expensive and so such tunnels are sized with a capacity that is sufficiently large that sediment removal operations only have to be performed every 5 to 15 years.

CONSTRUCTED WETLANDS Wetlands specifically designed to capture pollutants from urban stormwater are receiving increased application as a quantitative and qualitative urban stormwater BMP when the price and availability of land is not prohibitive. Capable of substantial attenuation of the urban storm hydrograph, they are also highly effective in reducing nearly all contaminants of concern in urban runoff to low levels. Several hundred

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constructed wetlands have been applied to the treatment of municipal and industrial waste, although the application of this technology to stormwater management is less well documented. The vast diversity in their application design and operation offers the potential for a high degree of control of an equally diverse range of target pollutants, at a high rate of reliability and offering the greatest ancillary benefits of any other stormwater BMP. However, land requirements are extensive, particularly if peak discharge controls as well as pollutant removal are desired and design pollutant removal performance efficiencies are substantially delayed until the system has become well established. The lack of accepted standard design, construction, and operating criteria has limited the competitiveness of constructed wetlands as stormwater BMP. Wetlands can be designed as follows: • Free surface (FW) with a lined soil base emergent vegetation and the water surface exposed to the air • Subsurface (SF) or vegetated submerged bed (VSB) with a lined soil base overlain by media, emergent vegetation, and with a water level below the surface Even though SF or VSB applications have lower land requirements, due to the greater surface area for biological growth, it is FW systems that are generally more suited to stormwater applications, where nuisance problems associated with municipal wastewater FW treatment are much less significant. Odor problems rarely exist and the wide range of flow rates and the highly unpredictable changes in hydraulic conductivity make SF systems very sensitive to system failure. Also, there is typically a desire to integrate the treatment system with the landscape and provide ancillary benefits. Demonstrated pollutant removal efficiencies are higher constructed wetlands than their natural equivalents, due in part to the greater process control and more intensive management practices (Strecker, 1995). Even though the utilization of natural wetlands is more cost-effective as a stormwater BMP, permitting and regulatory issues typically preclude their application. Fundamental pollutant removal is through a wide range of physical, chemical, and biological processes. A summary of the primary pollutant mechanisms is listed in Table 6.8. Hydrology is one of the most influential controls on pollutant removal due to its effects on sedimentation, aeration, biological transformation, and adsorption onto bottom sediments. The large surface area of the sediment layer of the wetland encourages much greater levels of absorption, adsorption, filtration, microbial transformation, and biological utilization than might normally occur in conventional channelized water courses (Strecker, 1995). Developing standard removal rates is difficult due to the wide variety in their design, although reported ranges are listed in Table 6.9. Generally increasing the detention time and vegetative

TABLE 6.8 Projected Long-Term Pollutant Removal Rates for Constructed Wetlands Pollutant TSS Total P Total N NO3– –N BOD, COD, TOC Pb Zn FC

Removal Rate, % 75–80 45–65 25–40 45 15 75–80 50 Two orders of magnitude

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TABLE 6.9 Constructed Wetland Pollutant Removal Mechanisms Mechanism

Pollutants Affected

Soil Incorporation

Physical TSS, soluble and particulate BOD and COD, pathogens, P, N, heavy metals, synthetic organics TSS, soluble and particulate BOD and COD, pathogens, P, N, heavy metals, synthetic organics All

Precipitation Adsorption

Chemical Dissolved P, heavy metals Dissolved P, heavy metals, synthetic organics

Ion exchange Oxidation Photolysis Volatilization

Dissolved metals COD, petroleum hydrocarbons, synthetic organics COD, petroleum hydrocarbons, synthetic organics Volatile petroleum hydrocarbons, synthetic organics

Microbial decomposition Plant uptake Natural die-off Nitrification

Biological BOD, COD, petroleum hydrocarbons, synthetic organics P,N, metals Pathogens NH3–N

Denitrification

NO3, NO2–N

Sedimentation Filtration

Promoted By

Low turbulence Fine, dense herbaceous plants Medium-fine textured soil

High alkalinity High soil Al, Fe (P), high soil organic concentration, circumneutral pH High soil cation exchange capacity Aerobic conditions High light penetration High temperature and air turbulence

High plant surface area and soil organics High plant metabolic activity and surface area Plant excretions Dissolved O2 > 2mg/l, low toxics, temp. > 5–7oC, circumneutral pH Anaerobic, low toxics, temp., > 15oC

Source: Strecker, E.W., EPA-625-R-95-003, 1995.

concentration will increase pollutant removal as the potential for sedimentation and biological processing of target pollutants increases with increasing detention time. It is essential to bear in mind that even though the wetland is designed as a nutrient sink, the wetland may periodically act as a nutrient source, although this phenomenon is only poorly understood. Design and Site Considerations Although surface overflow or more numerical models used for the design of retention basins can be applied to FW constructed wetlands, more fundamental data and models are needed for the more complex water quality chemistry. However, with ever-increasingly successful applications, more standardized design practice and rules of thumb have been generated for application to preliminary design practices. These rules of thumb have been based on practical experience of numerical models in constructed wetland design. The wetland may be operated as an individual BMP, as an integrated as part of a larger nonpointsource treatment facility in conjunction with other units such as wet pond, sediment forebay, or infiltration basin, or as a conveyance structure to a downstream treatment facility or to centralized treatment. The volumetric wet weather sizing procedure is identical to that of extended wet retention ponds with open water volume equal to 50% of the total wetland area. To maximize contact time and reduce short-circuiting potential a minimum length-to-width ratio of 3:1 is recommended. The

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wetland pond should provide a minimum permanent storage equal to 75% of the water quality control volume, the size of which can be determined in the same way as for wet retention ponds (Kadlec and Knight, 1996). Peak flow reductions and stormwater volume discharges can be reduced by providing a surcharge storage volume above the water quality capture volume. Drain time for this flood storage volume should be less than 24 h to prevent detriment to the vegetation. This is easily achieved with the inclusion of peripheral elevated berms. A minimum depth of water should be maintained for the emergent plant life but should be shallow enough for aerobic conditions to extend to the sediment–water interface. Increasing the surface area of the wetland increases the nutrient removal capabilities, although nutrient loading to a wetland used for stormwater treatment should not exceed 50 kg/ha of phosphorus and 262 kg/ha of nitrogen per year (Kadlec and Knight, 1996). The pond should be sized to meet this minimum size requirement if the annual nutrient loads are known. Operation and Maintenance Maintenance requirements for constructed wetlands are typically among the lowest of all the stormwater BMPs, although swales generally require an even more passive operation and maintenance regime (Kadlec and Knight, 1996). Other maintenance obligations are limited to vegetation harvesting (every 5 years) and accumulated sediment removal (every 5 to 15 years depending on inspection). Periodic drawdown to expose surficial sediment will promote the oxidation of accumulated organics. Water levels and detention times required to meet pollutant target goals can be controlled by manipulating the adjustable outlet control devices. Obviously, routine maintenance schedules will be facility specific and will be determined during initial operational inspection procedures. Harvesting has little effect on chemical or nutrient removal unless performed several times during the growing season, which maximizes plant metabolic activity and ultimately nutrient removal. Inlet devices should be inspected regularly to ensure even distribution of the influent wastewater. Cost and Design Life Accurate cost estimation is difficult due to the variability in the aspects of wetland design. Typically, approximations are performed by itemizing each facet of the project and sum the costs accordingly. The largest cost considerations are excavation, grading, liner installation, inlet and outlet structures, and vegetation costs. Reliable estimates can be derived by applying cost criteria similar to that for wet retention ponds.

CENTRALIZED TREATMENT PLANTS For urban stormwater runoff to be treated at a wastewater treatment facility, currently the centralized treatment plant must be served by a combined sewer system. Combined sewer systems are designed to provide collection, transport, and treatment of the dry weather wastewater flow plus the stormwater runoff that is diverted into the sewer system. Combined sewer systems are predominant in the older communities of the Northeast, Mid-Atlantic, and Midwest regions of the United States as well as in large European cities (Metcalf & Eddy, 1991) because the cotreatment of both municipal wastewater and stormwater runoff is a legacy of engineering practice at the time of infrastructure installation. More recent engineering practice is for separate collection systems for municipal and stormwater runoff, although the emphasis is shifting back to combined systems in accordance with regulatory developments in the realm of stormwater management. Such centralized treatment processes typically consist of the standard facilities utilized at conventional wastewater plants, but typically with increased grit removal and handling capacities capable of dealing with the greater solids loadings associated with stormwater runoff.

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Conventional physical treatment units applied to municipal wastewater can be readily applied to combined sewer systems, with a demonstrated degree of success and reliability. However, biological and physiochemical process applications to combined wastewater have substantial limitations. The biomass used to assimilate the nutrients in the combined wastewater must be maintained during dry weather, which is difficult unless at an existing municipal wastewater treatment facility. Also, the biological process in the stabilization of the waste stream are sensitive to load irregularities and certain contaminants in the stormwater can prove toxic to the microbial population. To overcome these limitations, the stormwater can be released gradually during dry weather conditions (if stormwater storage is available) such that peak flows are attenuated and the stormwater is diluted. A comprehensive discussion of pollutant removal mechanisms and efficiencies of conventional wastewater treatment facilities is beyond the scope of this chapter but can be found in such popular texts as Metcalf & Eddy (1991). However, a general discussion of their relation to the treatment of stormwater runoff as a BMP for stormwater mitigation is included here. Centralized treatment is generally considered the BMP when the following conditions are present: • The sheer volume of the stormwater runoff precludes effective on-site treatment and disposal • Site constraints and land availability considerations prevent decentralized management practices • The temporal variations and fluctuations in runoff volumes exceed both baseline and standard operational capacities Also, there is the realization that for most urban areas in the United States, stormwater delivers more pollutant mass to receiving waters than untreated domestic wastewater (Klein et al., 1974; Sansalone et al., 1998b). Although centralized treatment is an effective measure for the qualitative management of urban stormwater runoff, their application is limited by concerns regarding CSOs and problems for treatment capacity exceedence during wet weather flows. The costs of treatment facilities with sufficient hydraulic and treatment capacities capable of handling stormwater flow are typically regarded as prohibitive. As a consequence of this and to prevent CSOs, sewer systems for the collection and treatment of both municipal and stormwater runoff are equipped with in-line flow diversion devices to divert and detain the flow in excess of the treatment capacity to either in-line or off-line temporary detention facilities, until a time when it can flow (or be pumped) back into the sewer system during dry weather, or when sufficient capacity for downstream treatment becomes available. The reader is referred elsewhere in this chapter for a discussion of in-line flow detention and off-line storage. Such flow equalization has many advantages on the design and operation of centralized treatment facilities designed for combined sewer treatment: • Simplicity of design and operation requires only moderate modifications to conventional treatment facilities to accommodate the flow rate and quality characteristics of the combined waste stream. • Rapid response to changes in flow rate can be accommodated without hydraulic overloading, washout, and ultimately noncompliance violations. • The stored flow is fully treated during dry weather conditions, thus eliminating the discharge of the contaminants associated with stormwater runoff. However, this mode of operation requires large area requirements for in-line detention and off-line storage. Also increased maintenance costs of aeration equipment are necessary if long periods of flow detention are required.

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Design Considerations Combined storm sewer systems are sized to handle stormwater flows corresponding to a design storm event, as the wastewater contribution to the flow is only a small fraction of the design flow (Metcalf & Eddy, 1991). Based on current engineering practice, the ratios of wet to dry weather capacity for combined sewer design range from 1:1 to 8:1, with a median ratio of 4:1 (Metcalf & Eddy, 1991). Similarly, specific design guidelines and recommendations for unit operations and processes are beyond the scope of this chapter and the reader is referred to standard wastewater treatment facility design manuals for a comprehensive discussion of them, examples of which are the Water and Environment Federation Manuals of Practice (WEF, 1999).

ASSESSMENT OF TREATMENT EFFECTIVENESS EVENT FOCUS BASIS Wet Weather Sampling and Treatment Assessment The sampling program should provide representative and sufficient data to support planned analyses and design parameters. To effectively evaluate the performance effectiveness, sampling procedures should coincide with wet weather conditions. For any treatment unit, knowledge of the residence time distribution is required. To evaluate pollutant removal efficiencies, samples should be collected from the locations where the influent runoff enters the treatment unit and where the runoff leaves as treated effluent. If the treatment system contains multiple unit operations or processes, performance assessment will require sampling across each individual operation or process. If the treatment unit includes a permanent body of water, or at least residual water from a previous event, effluent sampling should be carried out no sooner than at least the hydraulic retention time of the unit. If sampling is carried out before this time, water quality analyses will be performed on displaced water from the previous event. This will overestimate pollution removal efficiencies due to the increased hydraulic retention time, i.e., equivalent to the intervals between storm events. For these analyses to be realistic, they must be performed over a representative hydraulic retention time, starting at the onset of the storm and continuing until cessation of effluent discharge, when the discharge levels return to base flow conditions where applicable. The sampling regime will be site and treatment specific and should be modified accordingly, the specifics of which will be the result of preliminary investigations. Sampling frequency should be tailored specifically such that the pollutant transport throughout the entire storm is captured. Site-specific investigations will reveal such important criteria as the time of concentration of the rainfall and intensity and duration of the hydraulic residence time (HRT). It is these indicators that will determine the logistics of the sampling regime. For each event, rainfall sampling must be conducted from a similar site, removed from the experimental location to establish background levels of constituents of concern. Typical methods for estimating pollutant loads include continuous-flow measurements and some form of automated sampling that is either timed or triggered by some feature of the runoff hydrograph. Grab sampling with continuous discharge gauging can be used to estimate load in very selected cases. Grab sampling is usually much less expensive than automated sampling methods and is typically much simpler to manage. However, information gained regarding treatment processes is also very limited unless flow and treatment processes are steady and continuous. These significant factors of cost and ease make grab sampling an attractive alternative to automated sampling and therefore worthy of consideration even for monitoring programs with the objective of estimating pollutant loads. Grab sampling should be carefully evaluated to determine its applicability for each

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monitoring situation. Grab sampling may be appropriate for systems in which the distribution of annual loading occurs over an extended period of several months, rather than a few events. In addition, grab sampling may be used to monitor low flows and background concentrations. For systems exhibiting any significant variability in discharge loading or where the majority of the pollutant load is transported by a few events (such as snowmelt in some northern temperate regions), however, grab sampling is not recommended. Pollution reduction efficiencies can be determined from direct comparisons of influent and effluent characteristics. However, the operation or process performance assessment will require a much more detailed analysis protocol as opposed to treating the BMP or treatment system as a “black box.” Baseline Parameters The delineation of baseline parameters is a twofold process. First, it is important (if possible) to obtain runoff characteristics prior to development. If this is not feasible, a control site with similar hydrologic characteristics is identified and selected. Comparison of this to postdevelopment runoff characteristics will quantify the magnitude of the impacts of anthropogenic activities on runoff characteristics and the storm hydrograph. Second, it is important to determine the background concentrations in systems exhibiting base flow characteristics (for example, wet retention ponds, constructed wetlands, etc.) for effluent between individual storm events. The delineation of these baseline parameters can then be used to separate background noise from “signal” data.

SEASONAL

OR

ANNUAL FOCUS

Wet and Dry Weather Treatment and Assessment Although BMPs for urban stormwater runoff mitigation are designed, by definition, for wet weather conditions, their performance effectiveness should be evaluated during both wet and dry events. It is essential to determine if the system performs well under both climatic regimes and to investigate whether the treatment capacity is exceeded if the system becomes inundated or if the return period frequency of each event is too short. To prevent noncompliance violations of the original treatment objectives, it is essential that the performance is evaluated under all extreme climatic conditions to which the system will typically be exposed. At the other climatic extreme, performance evaluations should be undertaken in situations where the climate has a distinct dry season. Baseline Parameters To prevent system short-circuiting or system failure, similar evaluations should be made when the system has been exposed to extended periods of “downtime” or drought to establish if performance efficiency is affected by such extended periods of inactivity and, if so, what baseline operational requirements must be met to prevent this. Obviously, in view of this certain BMPs have climatic constraints. For example, a constructed wetland would not be suitable either in an extreme arid environment or in an environment where frequent and extended freezing of the subsurface is a concern.

ANALYTICAL TECHNIQUES A significant challenge for effective monitoring is to isolate the changes in runoff loading and water quality caused by the implementation of management measures from those changes caused by the other sources of variability. In short, the task is to separate the effect, or “signal,” from the noise. Existing data may be used for problem definition, or for a preimplementation baseline data set if the collection protocol matches the monitoring objective, design, and quality assurance/quality control

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(QA/QC) required for the postimplementation data collection. Existing data may also be used for assessing parameter variability and estimating the number of samples or the time period for the monitoring survey based on the desired level of significance and error. The capability to plan for and use statistical analyses, therefore, is essential to the development and implementation of successful monitoring programs. A qualified statistician should be consulted to review the proposed monitoring design, the plan for statistical analyses, the application of statistical techniques, and the interpretation of the analytic results. For trend detection some of the appropriate tests include Student’s t-test, linear regression, time series, and nonparametric trend tests. For an assessment of impact and causality, a careful tracking of treatment is required and the two-sample Student’s t-test, linear regression, and intervention time series are appropriate statistical tests. Evidence from experimental pilot studies, pollutant runoff monitoring, and modeling studies may be used to support the conclusion of causality. A comparison of regression lines for data collected before BMPs were implemented (pre-BMP) and for data collected after BMPs were implemented (post-BMP) can be used to explore the presence of trends in a paired-watershed study. Failure to observe improvement may mean that the problem is not carefully documented, management action is not directed properly, the strength of the treatment is inadequate, or the monitoring program is not sensitive enough to detect change. A midcourse evaluation, if conducted early enough, provides an opportunity for modifications in project goals or monitoring design. Clear reporting of the results of statistical analyses is essential to effective communication with managers. Graphical techniques and simple narrative interpretations of statistical findings generally help managers obtain the level of detail they need to make decisions regarding subsequent actions.

CAN BMPS BE EVALUATED AS BLACK BOXES? To model a particular BMP as a black box, one accounts for mass (or concentration) entering and departing from a control volume (treatment unit) with no regard for the internal processes occurring within this control volume that are responsible for the differences in effluent and influent characteristics. Fundamental selection criteria for the optimization of the BMP selection process are largely governed by the target pollutant for removal. The effectiveness of this mitigation is inherently governed by the physical, biochemical, or chemical processes responsible for trapping or removing the target pollutants from the intercepted runoff. By considering the BMP as a black box, the planner is disregarding the fundamental mechanisms controlling the effectiveness of the treatment unit in stormwater mitigation. The neglect of such process criteria can directly lead to the selection of an inappropriate BMP with an inadequate pollutant removal capacity, due to the mismatch of the primary pollutant removal mechanism with the target pollutant. In doing so, the probability of nonattainment of treatment objectives is high. Historically, stormwater management practices have been regarded as black boxes, with planners solely concerned with influent and effluent characteristics. Consequently, there are widespread examples of systems not meeting their treatment objectives resulting in noncompliance violations. In learning from these mistakes, the planner has a greater control over treatment efficiencies and consequently a greater success in the application of Best Management Practice to the mitigation of stormwater runoff in the urban environment.

QUALITY CONTROL

AND

QUALITY ASSURANCE

Quality control refers to the routine application of procedures for obtaining prescribed standards of performance in the monitoring and measurement process. Quality assurance includes the quality control functions and involves a totally integrated program for ensuring the reliability of monitoring and measurement data.

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Under the U.S. EPA quality assurance and quality control (QA/QC) program, a full assessment of the data quality needed to meet the intended use must be made prior to specification of QA/QC controls. The determination of data quality is accomplished through the development of data quality objectives (DQOs), which are qualitative and quantitative statements developed by data users to specify the quality of data needed to support specific decisions or regulatory actions. Establishment of DQOs involves interaction of decision makers and the technical staff. Effective quality assurance and quality control (QA/QC) procedures and a clear delineation of QA/QC responsibilities are essential to ensure the utility of environmental monitoring data. The U.S. EPA QA/QC program requires that all U.S. EPA National Program Offices, Regional Offices, and laboratories participate in a centrally planned, directed, and coordinated agency-wide QA/QC program. This requirement also applies to efforts carried out by the states and interstate agencies that are supported by the U.S. EPA through grants, contracts, or other formalized agreements. Each office or laboratory that generates data under the U.S. EPA QA/QC program must implement, at a minimum, the prescribed procedures to ensure that precision, accuracy, completeness, comparability, and representativeness of data are known and documented. In addition, the U.S. EPA QA/QC procedures apply throughout the study design, sample collection, sample custody, laboratory analysis, data review (including data editing and storage), data analysis, and reporting phases. Specifically, the methods employed to assure quality assurance, data quality, and sample collection and preservation are Standard Methods 1020, 1030, and 1060, respectively (APHA, 1995).

DEVELOPMENT AND OPERATIONAL ASPECTS OF TREATMENT COST

AND

DESIGN LIFE

With all else equal, it is the level of capital outlay required to implement an appropriate BMP and the level of treatment required for a given application that will ultimately govern its selection. Fixed capital cost or cost of construction can be reasonably accurately estimated, although the omission of other financial considerations, such as operation and maintenance costs, can lead to very expensive errors in the economic analysis and even the selection of an inappropriate BMP. Although annual operational and maintenance costs typically form only a small percent of the budget, when considered over a typical 10-year design life, these costs can be substantial. It is essential that such “hidden” costs be considered in the full economic analysis. Design life is generally assigned as 10 years. Because of the infancy of the field of urban stormwater management, properly designed, operated, and maintained BMPs have yet to reach the conclusion of their design life. It is essential that systems be properly maintained to ensure their continued performance. For example, the frequently documented failures of porous pavements and other infiltration devices can be directly related to the inundation with excessively coarse sediment loads. Such systems are not designed to handle such loads and the omission of control measures to prevent the loads is directly responsible for the premature failure, often within 5 years of installation. Initial additional investment in control structures such as sediment traps ahead of infiltration systems would clearly reduce the potential for clogging and increase the design life and reduce the need for expensive rehabilitation procedures.

CONSTRUCTION Although the field of urban stormwater management is still somewhat in its infancy, reported results to date from the construction programs of new BMPs have greatly improved (Skupien, 1995b) as a result of the following:

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• The maturation of older flood control programs and natural treatment systems • Continued growth of hydrologic and hydraulic databases and improved computer modeling capabilities • Improved process design methods and understanding • The introduction of formal training, construction, and inspection programs As a result, the ability of program managers, designers, and construction contractors to meet their responsibilities for effective BMP performance has improved significantly in recent years. Such improvements have led to an increased interest and favor in the field of urban stormwater management. Constructability is a key concern when considering the selection of a specific BMP. Constructability is defined as a measure of the effort required to implement a structural BMP. A BMP that is highly “constructable” utilizes materials that are readily and locally available, relatively inexpensive, and that do not require specific handling measures. This will increase durability and improve the adaptability to site-specific requirements. In adopting this approach the planner can minimize the required initial capital investment and thereby increase favorability with state and local decision makers.

OPERATION

AND

MAINTENANCE

A critical step in ensuring the success of a management measure is proper operation and maintenance (O&M) of each practice. During the design process, an O&M plan that identifies continual procedures, schedules, and responsibility for operating and maintaining the practices should be drafted. Once designed and installed, it is crucial that the individual practices be operated and maintained to ensure that they function as intended. These procedures are generally applied by the landowner or operator responsible for implementing the management measures. States may wish to develop programs that ensure that O&M is performed by the responsible individuals or entities. Failure to operate and maintain a treatment system adequately can lead to ineffective performance, structural failure, and ultimately a failure to realize a return on the initial investment (Skupien, 1995a).

CLEANING Cleaning of the BMP structure, although often performed for aesthetic reasons, is executed for more than this alone. The removal of debris from trash racks is essential to maintain their performance integrity and to prevent stormwater backups and overflows. For the continued effective performance of porous pavements, routine pressure washing and vacuuming are necessary to remove potential material that could lead to premature clogging. Although clogging is an extreme consequence, without this routine cleaning of the porous surface, the porosity of the surface is reduced and the infiltration capacity retarded accordingly. This can lead to increased preferential runoff and the short-circuiting of the treatment system. Such shortcircuiting will lead to nonattainment of design goals and can therefore be regarded as system failure. Routine cleaning (the schedule of which is practice dependent) should therefore be included in standard O&M guidelines and should be budgeted for accordingly. Failure to do so can lead to rehabilitative processes that far exceed the cost of a routine cleaning regime.

HAZARDOUS WASTE DISPOSAL From a qualitative perspective, the primary objectives of the application of BMPs to urban stormwater management are threefold:

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1. Reduce, eliminate, or truncate pollutant transport pathways into stormwater runoff through source, in situ, and treatment control measures. 2. Divert these transport pathways to a control structure for the interception and removal of the pollutants, followed by the design-controlled release of the effluent. 3. Concentrate a highly diffuse source into a contained treatment unit for management and disposal. The removal of and concentration of the contaminants from the runoff can lead to significant accumulation of residuals that can be potentially hazardous depending on the target pollutants to be removed. Handling, disposal, and reuse of these residuals will become a critical aspect of stormwater treatment, parallel to the prominent role that biosolids management has become for wastewater treatment. Accumulated sediments display a high affinity to metals, although typically only a small fraction leach out to TCLP extract solution (U.S. DOT, 1996). Depending on factors that include heavy metal loading, sediment characteristics, loading duration, pH and redox conditions, the bottom sediments can be potentially classified as a hazardous waste and must be managed accordingly. The required management of such hazardous residuals can be expensive and lead to additional operation and management costs. However, the removal of potentially hazardous accumulated sediments generally operates on a 5 to 10 year schedule. Even though the removal and disposal of such hazardous residuals is an infrequent but necessary requirement, provisions for their appropriate management should be included in the design and operational guidelines.

POSTCONSTRUCTION CONSIDERATIONS Effective performance of BMPs is essential in the justification of the attainment of high initial treatment goals and the equally high anticipated costs of urban runoff management (Skupien, 1995a). To meet these needs it is fundamentally important for a regimented program to provide the following: • Inspection: Periodic yet routine observation and evaluation of a BMP and it components to determine maintenance requirements • Maintenance: Preventative and corrective measures taken to ensure the continued safe, effective, and reliable BMP performance • Performance monitoring: Extended analytical observation and evaluation to determine effectiveness and improvement needs Historically, these needs have been left wanting, a neglect that is typical of long-term programs. Failure to meet these needs will not only lead to a reduction in BMP treatment efficiency, but can also lead to the creation of additional environmental, health, and safety concerns, of which the BMP was initially designed to prevent. In doing so, additional expenditures and resource allocation will be incurred. Consistent and appropriate inspection and maintenance will ensure the continued effective operation of the BMP. As a result, monitoring the performance of the management practice implemented is a fundamental postconstruction responsibility. This is essential to evaluate treatment effectiveness of the system or systems to ascertain whether or not the selected BMP is meeting its design objectives. Obviously, because of the passive nature of some of the systems (for example, vegetated swales and constructed wetlands), a period of acclimation is necessary before the system will attain its optimum treatment efficiency. Typically, regulators (whether they be federal, state, or city officials) will grant a grace period to enable such periods of phase-in and acclimation. However, because of the random and often unpredictable nature of storm events and poor experimental

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design of analysis programs, it is often difficult to evaluate how well treatment goals are being met, regardless of the proficiency of design, construction, operation, and maintenance. Best management measures are systems of practices, technologies, processes, siting criteria, operating methods, or other alternatives. Pollution control programs generally consist of systems of management measures applied over well-defined geographic areas. Combinations of the measures described above are likely to be found in any given area to be monitored. Monitoring programs, therefore, must often be directed at measuring the cumulative effectiveness of a range of different measures applied in different areas at different times within a specified watershed. Under these conditions, the monitoring approaches for source-control and direct-impact-reduction measures are typically used, while process monitoring is not generally used other than to track the effectiveness of the specific in situ control measures implemented.

REFERENCES Ahmed, H.U., 1996. The Effect of Fluxrate and Solids Accumulation on Small Size Particle Accumulation in Expandable Granular Bead Filters, master’s thesis, Louisiana State University, Baton Rouge. Amrhein, C. and Strong, J., 1990. The effect of deicing chemicals on major ion and trace metal chemistry in roadside soils, in The Environmental Impact of Highway Deicing, Institute of Ecology Publ. 33, University of California, Davis, September. APHA (American Public Health Association), 1992. Standard Methods for the Examination of Water and Wastewater, 18th ed., Greenberg, A., Clesceri, L., and Eaton, A., Eds., APHA, Washington D.C. Brombach, H., Xanthopoulos, C., Hahn, H.H., and Pisano, W.C., 1993. Experience with vortex separators for combined sewer overflow control, Water Sci. Technol., 27(5-6), 93–104. CEC (Council of the European Communities), 1991. EC Urban Wastewater Treatment Directive, 91/271/EEC, Official Journal L135, 30 May. Colandi, V., 1999. Effects of a porous pavement with reservoir structure on runoff water: water quality and fate of heavy metals, Water Sci. Technol., 39(2), 111–117. Colandie, V., Legret, M., Brosseaud, Y., and Balades, J., 1999. Metallic pollution in clogging materials of urban porous pavements, Water Sci. Technol., 1–8. Colorado UDFCD, 1999. Urban Drainage and Flood Control District Drainage Criteria Manual, Vol. 3, September. Drennan, D., Golz, W., Ahmed, H., and Malone, R.F., 1995. Clarification abilities of floating bead filters used in recirculating aquaculture system, in Aquaculture Engineering and Waste Management, Proceedings from the Aquaculture Exposition VIII and Aquaculture Mid-Atlantic Conference, June 24–28, Washington, D.C., 256–267. Federal Highway Administration, 1980. Underground Disposal of Stormwater Runoff, FHWA-TS-80-218, Washington, D.C. Field, R., Masters, H., and Singer, M., 1982. An overview of porous pavement research, Water Resourc. Bull. AWRA, 18(2), 265–270. Hebrard, C. and Delolme, C., 1999. Role of the biotic compartment in the transfer of zinc through the Vadose zone — appliction to an infiltration system, Water Sci. Technol., 39(2), 209–215. Helsel, D., Kim, J., Grizzard, T., Randall, C., and Hoehn, R., 1979. Land use influences on metals in storm drainage, J. Water Pollution Control Fed., 51(54), 709–717. Jacobsen, P. and Mikkelsen, P., 1992. Urban Stormwater Infiltration, Technical University of Denmark, Lyngby. Kadlec, R.H. and Knight, R.L., 1996. Treatment Wetlands, Lewis, Boca Raton, FL. Klein, L.A., Lang, M., Nash, N., and Kirschner, S.L., 1974. Sources of metals in New York City wastewater, J. Water Pollution Control Fed., 46(12), 2653–2662. Koran, J., 1997. Effect of Particle Size on Distribution of Heavy Metals in Highway Runoff, University of Cincinnati, Cincinnati, OH. Li, Y., Buchbeger, S.G., and Sansalone, J.J., 1999. Variably saturated flow in storm-water partial exfiltration trench, J. Environ. Eng., June, 556–565.

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Maestri, B. and Lord, B.N., 1987. Guide for mitigation of highway stormwater runoff pollution, Sci. Total Environ., 59, 467–476. Malcom, H., 1989. Elements of Urban Stormwater Design, North Carolina State University, Raleigh. Malone, R.F, Beecher, L.E., and DeLosReyes, A.A., 1998. Sizing and management of floating bead bioclarifiers, in Proceedings of the Second International Conference on Recirculating Aquaculture, Aquaculture Engineering Society, 319–341. Metcalf & Eddy, Inc., 1991. Wastewater Engineering: Treatment, Disposal and Reuse, 3rd ed., Revised by Tchobanoglous and F.L. Burton. Michelbach, S. and Wohrle, C., 1993. Settleable solids in a combined sewer system, settling characteristics, heavy metals, efficiency of stormwater tanks, Water Sci. Technol., 27(5-6), 153–164. Novotny, V. and Olem, H., 1994. Water Quality: Prevention, Identification, and Management of Diffuse Pollution, Van Nostrand Reinhold, New York. Pettersson, T., 1996. Pollution Reduction in Stormwater Detention Ponds, Chalmers University of Technology, Goteborg, Sweden. Pratt, C.J., Newman, A.P., and Bond, P.C., 1999. Mineral oil bio-degradation within a permeable pavement: long term observations, Water Sci. Technol., 39(2), 103–109. Sansalone, J. J., 1996. Adsorptive Infiltration by Oxide Coated Sand Media for Immobilizing Metal Elements in Runoff, University of Cincinnati, Cincinnati, OH. Sansalone, J.J., 1997. Infiltration as an urban source control for heavy metals and solids, in XXVIIth IAHR Congress Proceedings, San Francisco, Aug. Sansalone, J.J., 1999a. Adsorptive infiltration of metals in urban drainage — media characteristics, Sci. Total Environ., 235, 179–188. Sansalone, J.J., 1999b. In-situ performance of a passive infiltration treatment system for metal elements in urban surface waters, Water Sci. Technol., 39(2), 193–200. Sansalone, J.J., 1999c. In-situ performance of a passive treatment system for metal source control, Water Sci. Technol., 39(2), 193–200. Sansalone, J.J., 2000. The role of water in ecologically sustainable transportation, Transp. Res. Rec., Millenium Paper Collection. Sansalone, J.J. and Buchberger, S.G. 1997. Partitioning and first flush of metals and solids in urban highway runoff, J. Environ. Eng. Div. ASCE, 123(2), 134–143. Sansalone, J.J., Buchberger, S.G., Koran, J.M., and Smithson, J.A., 1998a. Relationship between particle size distribution and specific surface area of urban roadway stormwater solids, Transp. Res. Rec. 1601, 95–108. Sansalone, J.J. and Hird, J.P., 1999. Treatment of stormwater runoff from urban pavements and roadways, Stormwater Management in the Watershed, Technomic Publishing Company, Lancaster, PA. Sansalone, J.J., Koran, J., Buchberger, S., and Smithson, J., 1998b. Physical characteristics of highway solids transported during rainfall, J. Environ. Eng. Div. ASCE, 124(5), May. Sansalone, J.J., Smithson, J.A., and Koran, J.M., 1998c. Development and testing of a partial exfiltration trench for in situ treatment of highway drainage, Transp. Res. Rec., 1647, 34–42. Schueler, T., 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMP, Metropolitan Washington Council of Governments, Washington, D.C., 275 pp. Shaver, E., 1995. Use of sand filters as an urban best management practice, in National Conference on Urban Run-off Management: Enhancing Urban Watershed Management at the Local, County and State Levels, EPA Seminar Publication, Chicago, IL, April, EPA-625-R-95-003. Sherard, J., Dunnigan, L., and Talbot, J. 1984. Basic properties of sand and gravel filters, J. Geotech. Eng. Am. Soc. Civil Eng. Am. Soc. Civil Eng., 110(6), 684–700. Skupien, J.J., 1995a. Post-construction responsibilities for effective performance of best management practices, in National Conference on Urban Run-off Management: Enhancing Urban Watershed Management at the Local, County and State Levels, EPA Seminar Publication, Chicago, IL, April, EPA-625-R-95-003. Skupien, J.J., 1995b. Design considerations for urban best management practices, in National Conference on Urban Run-off Management: Enhancing Urban Watershed Management at the Local, County and State Levels, EPA Seminar Publication, Chicago, IL, April, EPA-625-R-95-003. Stahre, P. and Urbonas, B., 1990. Stormwater Detention, Prentice-Hall, Englewood Cliffs, NJ, 338 pp.

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Strecker, E.W., 1995. The use of wetlands for stormwater pollution control, in National Conference on Urban Run-off Management: Enhancing Urban Watershed Management at the Local, County and State Levels, EPA Seminar Publication, Chicago, IL, April, EPA-625-R-95-003. Stumm, W., 1992. Chemistry of the Solid-Water Interface, John Wiley & Sons, New York. Tenney, S., 1995. An Evaluation of Highway Runoff Filtration Systems, CRWR 265, Austin, TX. U.S. Department of Transportation, 1996. Evaluation and Management of Highway Runoff Water Quality. FWHA-PD-96-032. Washington, D.C. U.S. Environmental Protection Agency, 1980. Phase 1: Design and Operational Criteria, EPA-600-2-80-135, Washington, D.C. U.S. Environmental Protection Agency, 1993. Urban Runoff Pollution Prevention and Control Planning, EPA625-R-93-004, Washington, D.C. U.S. Environmental Protection Agency, 1995. Combined Sewer Overflows: Guidance for Nine Minimum Controls, EPA-832-B-95-003, Washington, D.C. U.S. Environmental Protection Agency, 1996. National Conference on Urban Runoff Management: Enhancing Urban Watershed Management at the Local, County, and State Levels, EPA-625-R-95-003, Washington, D.C. Warnaars, E., Larsen, A.V., Jacobsen, P., and Mikkelson, P.S., 1999. Hydologic behaviour of stormwater infiltration trenches in a central urban area during 2½ years of operation, Water Sci. Technol., 39(2), 217–224. WEF, 1998. Urban runoff quality management, WEF Manual of Practice, No. 23, Water Environment Federation and the American Society of Civil Engineers. Wiess, K., 1993. Stormwater and the Clean Water Act: Municipal separate storm sewers in the moratorium, United States Environmental Protection Agency. National Conference on Urban Runoff Management: Enhancing Urban Watershed Management at the Local, County, and State Levels. EPA-625-R-95003, Washington, D.C.

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7

Management of Sewer Sediments Richard M. Ashley and T. Hvitved-Jacobsen

CONTENTS Introduction ....................................................................................................................................187 The Nature of Sewer Sediments ....................................................................................................188 Characteristics and Pollutant Properties of Sewer Solids .............................................................188 Transport .................................................................................................................................188 Suspended Solids and Colloids ..............................................................................................190 Sewer Sediments — Deposits and Near-Bed Material..........................................................190 Biofilm ....................................................................................................................................193 Effects on the Performance of Sewer Systems ......................................................................194 Sewer Solids Control at Source and at Inlets ...............................................................................195 Behavioral Aspects..................................................................................................................197 Gullies and Other Inlet Controls ............................................................................................200 Street Cleaning........................................................................................................................200 Design and Operation of Sewer Systems to Control Problems of Sewer Solids.........................201 Design of Sewers ....................................................................................................................201 Prediction of Sedimentation and Control of Sediments in Existing Sewers................................205 Sedimentation..........................................................................................................................209 Management and Disposal of Sewer Solids..................................................................................209 Sediment Management in Large Sewers................................................................................210 Sewer Cleaning in Smaller Sewers ........................................................................................214 Sustainability and Disposal of Sediment Removed from Sewers .........................................214 References ......................................................................................................................................219

INTRODUCTION A sewer is a collector and transporter of wastewater. The time taken by wastewater to enter and move through a sewer system provides sufficient retention for the wastewater and conveyed solids to be transformed by in-sewer processes. Hence, a sewer is a temporary system for accumulating solids and allowing physical, chemical, and biological processes to occur that affect the sewer itself as well as any successive treatment processes and receiving waters. No significant changes in sewer design and operation have been made for decades, and much of the developed world’s sewerage systems have been inherited from our forebears (Ashley et al., 1999b). Most new systems have been installed due to urban expansion on the edges of towns and cities, and new large interceptor sewers have also needed to be constructed to deal with the overloading problems caused by this outward expansion of the sewer network. The management of the transport and transformation processes that occur in sewers is now seen to be a very important aspect of sewer design and operation, and is particularly significant for the larger interceptor sewer 0-56676-916-7/03/$0.00+$1.50 © 2003 by CRC Press LLC

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systems, where flow ranges vary the most. As yet, most interest has been directed to how solids can be managed, with design and maintenance geared to encouraging any solids that enter a sewer to be transported to a point for which treatment or removal can be provided cost-effectively. Nonetheless, sediment deposition in sewers is ubiquitous, largely because of the wide range of types of solids, that enter systems and the intermittent and highly variable nature of in-sewer flow regimes. In the United States, estimates of dry weather flow deposition in combined sewers vary from 5 to 30% of the daily inputs of solids and pollutants (Pisano et al., 1998). In Europe, average deposition rates have been measured at between 30 and 500 g/m length of sewer in a day (Ashley et al., 1992). Even sewers designed to be supposedly “self-cleansing” will have transient sediment deposits and part of the load in transport will move near the bed (May et al., 1996). While there are generic aspects to the management of sewer solids, any approach needs first to acknowledge that there is no generally applicable solution, and each situation needs to be considered individually. For sewer sediment management purposes there are a number of aspects of interest, depending upon whether the sewerage system is existing or is being designed: • • • • • •

The nature of the solids in the wastewater The ambient hydraulic regimes Sediment deposition potential Occurrence of deposits, where and how much The nature of the deposited material Potential problems: • Hydraulic • Pollution/shock loading/H2S and methane generation

THE NATURE OF SEWER SEDIMENTS There are three types of occurrence of sewer solids: 1. Suspended solids (and colloids) 2. Sewer sediments, i.e., deposits and near-bed material 3. Biofilm These three types of sewer solids comprise, together with associated attached and dissolved pollutants, the solids constituents of the discharges that arrive at treatment plants and spill from combined sewer overflows (CSO). The characteristics and the physical, chemical, and biological processes associated with each of these solids are different. This is of fundamental importance for the definition of the characteristics and nature of sewer solids. Figure 7.1 shows diagrammatically the processes and occurrence of solids in sewers. Table 7.1 outlines a sediment taxonomy, originally suggested for use in U.K. sewers, which has become more widely used as an initial sediment classification system. In practice, these classes are not always found in a segregated form, with mixtures of types being common, particularly where flows vary in combined sewers, with alternating dry and wet weather periods (Ashley and Crabtree, 1992).

CHARACTERISTICS AND POLLUTANT PROPERTIES OF SEWER SOLIDS TRANSPORT Sewer solids are transported in a variety of ways, as illustrated in Figure 7.2. The modes of transport may be defined in classic hydraulic terms (e.g., Verbanck et al., 1994; Raudkivi, 1990) as the folllowing:

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Sewer

Sewer Atmosphere Reaeration

Wastewater Processes in suspension

Wastewater Biofilm (slimes) Class D Mobile solids Class C

Coarse granular sediments Class A

Sediment/Biofilm processes

Biofilm processes

FIGURE 7.1 Outline illustrating the processes and occurrence of solids in sewers (for explanation of the classes A, C, and D see Table 7.1).

TABLE 7.1 Sewer Sediment Taxonomy for U.K. Usage % by Granular Particle Size (mm) minimum–mean–maximum

Sediment Type

Description/Where Found

Wet Density x 103 kg/m3

<0.063

0.063–2.0

2.0–50

Organic Content (%)

A

Coarse, granular bed material widespread Mobile, fine grained found in slack zones, in isolation, and overlying Type A Organic pipe wall slimes Fine grained mineral and organic material found in CSO storage tanks

1.72

1–6–30

3–61–87

3–33–90

7

1.17

29–45–73

5–55–71

0

50

1.21 1.46

17–32–52 1–22–80

1–62–83 1–69–85

1–6–20 4–9–80

61 22

C

D E

Source: Adapted from Crabtree, 1989.

Wash load Total Sediment Load

V, Q

Suspended load Bed material load

Relative concentration of solids in transport

Bed load

FIGURE 7.2 Diagrammatic illustration of modes of transport of sediments in sewers.

1. Wash load (finest) 2. Suspended load 3. Bed load (largest and densest) In general, these classes relate to particles that are in ascending order of size and/or density.

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Suspended solids in wastewater may settle and accumulate as sewer deposits and biofilms may form on sewer walls (see Figure 7.1). During periods of increased flow, erosion of both sewer deposits and biofilm may occur, together with changes in other associated pollutants. Of most significance for the buildup in bed deposits are the solids that are transported mainly along the bed of the sewer, traditionally known as bed load, and where deposits exist, may roll, bounce, or jump (saltation) along the surface. These solids usually form the bulk of the bed deposits, as once movement is arrested due to a hydraulic discontinuity, such as a sewer junction or other flow deceleration, they become more difficult to move, even when flows accelerate again (Ashley and Verbanck, 1996). Inputs to sewers and in-sewer conditions vary both temporally and spatially. Hence there is significant variability in the solids characteristics during runoff events. Measurements in developed countries show wide variability in the characteristics of the solid particles and pollutants even at a single site (see other chapters).

SUSPENDED SOLIDS

AND

COLLOIDS

Major physical characteristics for suspended solids of importance for their behavior during urban wet weather discharges are particle size, density, and settling characteristics. Based largely on extensive French results, as indicated in Table 7.1, the size characteristics for suspended solids in wet weather discharges can be generalized (Chebbo, 1992; Chebbo and Bachoc, 1992; Chebbo et al., 1995) as follows: • Fine particles dominate in suspension, with a median diameter (D50) ranging from 25 to 44 µm. • These fine particles have a tendency to agglomerate, particularly when they are organic. • The results of grain size measurements are similar for a range of sewer systems and also for combined or separate systems. • The solids deposited in human-entry-size sewers (>1 m high) are generally coarser than the solids transported in suspension in storm or dry weather flows. • Large suspended solids particles have typically a lower specific gravity than the small particles. Chebbo et al. (1990), for example, found that the specific gravity was highest (2400 kg/m3) in the 100 to 250 µm range in combined sewers and this declined to 1300 kg/m3 for particles >1600 µm. • For all but a small fraction (less than 10%), settling velocities for the smaller particles conveyed in suspension in wet weather are generally as shown in Table 7.2. The mean velocity is generally between 4 and 11 m/h. • The smallest particles have the most variable settling velocity and are those that carry the highest associated pollutant loads. Caution should be observed when taking account of settling velocity data, as determining representative results for the settlement of sewage particulates is problematic (e.g., Pisano, 1996; Lucas-Aiguier, 1998). Figure 7.3, together with Table 7.2, illustrates that the sewer solids discharged from a CSO during rainfall (those with the lowest settling velocity) are likely to exert the most significant impact on receiving waters in terms of oxygen demand.

SEWER SEDIMENTS — DEPOSITS

AND

NEAR-BED MATERIAL

Bed deposits found in the invert of combined sewers predominantly consist of granular mineral particles. The accumulation of these deposits has been observed in discontinuities of sewers, in, for example, depressions and obstacles in pipe inverts, branches, connections, and poor pipe joints (Ashley et al., 2000). The bed load transport process is primarily responsible for the origin of these

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TABLE 7.2 Typical Composition of Organic Matter (expressed as % of chemical oxygen demand, COD) in Sewer Sediments Distributed within Different Groups of Settling Velocities Settling Velocity (mm/s)

Xs, Fasta

Xs, Mediuma

Xs, Slowlya

XBwb

>20 2–20 0.02–2

4 6 8

10 20 30

83 68 56

3 6 6

Note: Xs is hydrolizable substrate (usual units gCOD/m3). a

Corresponding to a fast, medium, and slow rate of hydrolysis of particulate substrate. b Heterotrophic bacterial biomass. Source: Vollertsen, J. 1998. Solids in combined sewers — characterization and transformation. Ph.D. dissertation. Aalborg University, Denmark. With permission.

% less than

100 80 60

Combined sewers

40 20

Separate storm sewers 0 0.01

0.1

1

10

100

1000

settling velocity (m/h)

FIGURE 7.3 Envelopes showing cumulative distributions of settling velocities for suspended solids in stormwater and combined sewer networks. (Adapted from French studies; Chebbo, 1992.)

sediments. The filling up of depressions leads to a smoothing and a steepening of the hydraulic grade line and therefore ultimately attempts to lead to a “correction” of any original hydraulic irregularities. However, “singularities,” which are blocking the bed-load transport, can cause a severe buildup of sediment bed deposits in the sewer length just upstream of any obstacles or hydraulic change such as a downstream junction. Results from the extensive range of European studies in the United Kingdom (Ashley and Crabtree, 1992; Gent et al., 1996), Belgium (Verbanck, 1990; Torfs, 1995), and France (Bachoc, 1992; Laplace et al., 1992) show that deposit buildup varies considerably, with storm flows just as likely to result in increases in amounts of deposits as decreases, as illustrated in Figure 7.4. Sewer deposits normally contain a certain amount of organic material (typically 1% to >20%), which may lead to an organic binding, creating a cohesive-like sediment bed. For this reason bed deposits often exhibit high (cohesive) shear resistance and require higher erosional shear stresses to erode than are required for noncohesive granular material such as sand. Once the cohesive-like deposits have been entrained and are in movement, they lose these bonding forces. Hence, for the deposition and transport processes, the cohesive properties may be ignored (Crabtree, 1989).

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100

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

Rainfall [mm/24h]

Sediment Volume [m3]

90

0 0

100 200 300 400 500 600 Number of Days After the Complete Dredging (13/07/88)

700

FIGURE 7.4 Deposit buildup in Marseille trunk sewer with time. (From Laplace, D. et al., Water Sci. Technol., 25(8), 91, 1992. With permission from the copyright holders, IWA.)

With the help of flushing tests, Stotz and Krauth (1986) found that the characteristics of sewer deposits were subject to changes during dry weather periods. The longer the bed deposits remained, the more kinetic energy had to be applied to cause erosion, and even after a residence time of only 12 h a greater resistance was observed. This was explained by the binding of organic bed particles and a consolidation of the sediment bed by dewatering. Recently, however, Vollertsen (1998) demonstrated that the development of biofilms on the surface of sewer sediment beds enhanced the resistance to erosion, and although biodegradation within the bed could lead to a reduction in bed strength, due to the formation of methane gas bubbles, the effects of bioprocesses are likely to generally increase bed strength. Understanding of the behavior of cohesive-like sediment beds requires the use of rheological parameters. Parameters such as yield strength are used for the description of the strength of resistance to movement of cohesive materials. Small stresses at the water–solid interface deform the sediments elastically, but when the stress exceeds the yield value, continuous deformation occurs with viscous flowing of the material (Wotherspoon and Ashley, 1992). Applied stress rheometry (Williams et al., 1989) has been used to investigate the elasticoviscous properties of sewer sediments, as illustrated in Figure 7.5. The physical characteristics of sewer deposits can be described in terms of both individual particulates and bulk properties. The hydraulic and structural conditions in the sewer, together with the nature of the inputs, will control the type of material that deposits at a given location. Crabtree (1989) proposed a working taxonomy based on four primary classes A, C, D, E, with a fifth class B, comprising agglutinated or cemented Class A material (see Table 7.1). The large organic fraction found in Classes C and D, the weak surficial layer and the wall slimes, respectively, shows that these sediments differ markedly from traditional “sediments” found in pipe inverts. Sediments are usually mixed between the class types (Ashley et al., 1989; 1992). Type A sewer sediment material is the most commonly found in combined sewer systems and comprises the bulk granular material usually originating from road surfaces. Numerous studies have been collated to demonstrate the deposit properties given in Table 7.3 (e.g., Ashley et al., 2000a).

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0.75

Creep Deformation of sediment is measured under a series of constant stress values τy = 86 1N/m2

Displacement (rad)

0.60

0.45 τ = 53 4N/m2 0.30

0.15

τ = 26 7N/m2 τ = 13 3N/m2

0

0

12

Stress Applied

24

36

48

60

Stress Removed Time (sec)

FIGURE 7.5 Creep analysis of typical combined sewer sediment deposit. (From Wotherspoon, D.J.J. and Ashley, R.M., Water Sci. Technol., 25(8), 165, 1992. With permission of the copyright holders, IWA.)

TABLE 7.3 Pollutant Properties of Sewer Sediment Bed Material (Type A) Parameter Total solids Volatile solids COD BOD5 BOD (4-hour) Organic N Ammonia (NH4-N) a

Units

Range

g/kg (% of total) (g/kg)a (g/kg)a (mg/kg)a (mg/kg)a (mg/kg)a

350–815 3–19 6–270 1–90 100–700 200–1500 10–300

Dry weight.

Heavy metal concentrations in the sediments may vary considerably depending on the quality of incoming wastewater and urban runoff. Examples of the heavy metals in sewer bed deposits in Germany are shown in Table 7.4.

BIOFILM Biofilms are produced in sewers at the surfaces exposed to the water phase (Hvitved-Jacobsen, 2001). These “slimes” consist mainly of microorganisms, extracellular polymeric substances (EPS) produced by the microorganisms, and adsorbed organic and inorganic compounds from the wastewater (Jahn and Nielsen, 1996). In gravity sewers, slime thickness is a few millimeters, and is a function of flows, whereas in pressure mains the higher flow velocity allows lower bacterial growth rates with thinner biofilms during anaerobic conditions. Very thick biofilms do occur, where a significant increase in the friction factor has led to a reduced hydraulic capacity of the sewer (e.g.,

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TABLE 7.4 Heavy Metals in Sewer Bed Deposits (each result is based on two to three random samples) Site

Metal

Bad Mergentheim, Samples not Fractioned (Brombach et al., 1992)

Hildesheim, Fraction D < 0.2 mm (Ristenpart et al., 1995)

Lead (mg/kg)* Copper (mg/kg)* Zinc (mg/kg)* Nickel (mg/kg)*

14 26 327 14

173 168 14 26

a

Dry weight.

Characklis and Marshall, 1990; U.S. EPA, 1985). The composition of sewer biofilms is poorly understood. Even though EPS makes up 90% of the slime, only limited studies of the organic composition constituents have been undertaken (Jahn and Nielsen, 1998). The protein fraction is the largest, making up approximately 50% of the organics, with carbohydrate and humic substances comprising 15 to 25%. Biomass detaches from biofilms to the bulk flow by erosion and sloughing. Single bacteria and parts of the biofilm erode continuously from the surface, whereas bigger parts of the biofilm may slough intermittently. Sloughing may take place when large changes in shear stress, e.g., during a runoff event, or changes in substrate conditions occur, but the mechanisms are not well understood (Norsker et al., 1995).

EFFECTS

ON THE

PERFORMANCE

OF

SEWER SYSTEMS

The presence of solids in sewers can cause a variety of problems, as outlined in Table 7.5 (Ashley et al., 2000; 2002b). Deposits cause blockages that may eventually cause undesirable surcharging and even flooding during both dry weather and storm conditions, while microbial processes contribute to the production of hydrogen sulfide and methane, causing odors, dangers to sewer workers, and corrosion as well as acute and chronic pollutant effects in receiving waters. Deposits in manholes and tanks and blinding and partial blockage of screens at CSOs and treatment plants cause more-localized problems. Large organic and inorganic gross solids also escape from sewers, even when screens are installed and cause both aesthetic (unsightly) and (potential) health risks when deposited in receiving environments. Hence, the problems may be summarized as follows: • Accumulation of sediments in sewers inhibits the designed conveyance of flow; this has two effects: • Surcharging of the sewer causing premature operation of overflows • Increases in the frequency of flooding • Sewer solids can overwhelm “control” systems such as screens or treatment plants when washed out. • Sediments act as stores/accumulators of pollutants due to two processes: • Gradual accumulation with time • Facilitating the opportunity for biochemical activity within the deposit

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TABLE 7.5 Outline of Effects Caused by Sewer Solids Effect Caused by Solids Reduction in hydraulic capacity and flooding Premature operation of CSOs Gases and explosions Odor problems Sewer corrosion Pump impeller abrasion Screen blockages and damage Shock loads to treatment plants Rodents (rats) Health risks Volatile organic compounds Enhanced pollutant washout from CSOs Fat and grease deposits — can reduce capacity or be washed out in “lumps”

Description Deposition of solids in inverts, permanent or semipermanent As above Generated from biological degradation in bed deposits and biofilm (methane, etc.) Hydrogen sulfide, organic sulfides, etc. emitted to atmosphere As above, at moist surfaces in atmosphere Inorganic solids in flow (typically washed through system in wet weather) Large solids (inorganic and organic) Foul flushes and bed erosion, releasing both solids and pollutants Source of food (organic solids) To, in particular, sewer workers from gases, rodents, and volatile organic compounds Typically only of significance in industrial areas All types of solids and associated pollutants during wet weather Buildup on sewer walls, particularly around ambient surface levels, can also develop into balls

In the last case, the problems occur due to acute in-sewer pollutant buildup, releasing gases, for example (Fan et al., 2001), and when re-eroded, by impact on either the environment via direct discharge or as a shock load at a treatment plant. Catchment studies of the mass balances for pollutants conveyed into sewers and those discharged from downstream outfalls show that more pollutants are emitted than enter the system due to the in-sewer process changes that occur in the deposits when they remain undisturbed for even relatively short periods between storms (Gromaire-Mertz et al., 1998). Sewers should be designed so that the flow velocities are sufficient either to convey sediments through the system or to re-erode any deposits. Because design and construction practices in the past were based on limited knowledge, existing sewers suffer extensively from deposition. Hence, there is also a need to define appropriate maintenance practices to minimize the blockage and pollutant buildup problems that occur (Ashley et al., 2000). Current knowledge, however, is also limited, and the design of “sediment-free” sewers is generally not possible. The treatment of sewer solids (in-sewer and at end of pipe), the impact of solids on wastewater treatment plants (WWTP), tanks, and overflows, and their disposal are thus of primary importance in sewage handling and sewerage design practices.

SEWER SOLIDS CONTROL AT SOURCE AND AT INLETS Management at the source is usually the most effective way of dealing with problems. Sewer solids originate from a variety of sources, as illustrated in Figure 7.6, from Butler and Clark (1995), and as described in Table 7.6. Much of the solids and associated pollutants arise from human activity, with domestic, construction, and transport sources comprising typically the largest amounts. Although industrial waste producers have been made to behave more environmentally responsibly by regulation and policing in the developed world, the “environmentally impacting” behavior of individuals, particularly in the home, has been an area largely uninvestigated and uncontrolled. Hence, in Europe, few concerted efforts have been made to encourage individuals to be more responsible in their actions. As a consequence, wastewater systems continue to receive and discharge into the environment a variety of biochemical cocktails, which, even where the wastewater receives

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Entry

Precipitation

Source More careful control of road gritting and construction site operation

Solids control at source: domestic, commercial and industrial behavior, BMPs, in-site treatment, resource recovery and/or waste routed into solid waste stream

Winter grit, road surface and construction materials

Industrial and domestic waste water

Paved areas

Exit

Street sweeping

Gully pots

Gully cleaning

Combined sewer

Sewer cleaning

Ingress of surrounding soil

Legend Combined sewer overflows

Liquid Solid

Sewage treatment

Grit, screenings and sludge removal

Receiving Waters

FIGURE 7.6 Sediment entering sewers and control/removal options. (Adapted from Butler and Clark, 1995. Reproduced by kind permission of CIRIA.)

conventional treatment, can still contain substances, such as endocrine disrupters, that impact drastically on the receiving waters. Any moves toward “more” sustainable wastewater systems, using, for example, greywater recycling and/or local wastewater management, will be considerably constrained by these careless chemical discharges into these localized systems. Controls thus need to relate to the following: • Education and behavior at the individual level regarding the type and amounts of solids (and other pollutants) introduced to wastewater systems (Ashley et al., 1999c) • Solids control via street sweeping and runoff management from construction sites (Gromaire-Mertz et al., 2000; Field et al., 2000; Masters-Williams et al., 2001) • Design and operation of devices and structures at inlets to sewer systems (Pitt and Field, 1998) A recent review of the sources during wet weather (Heaney et al., 1999; Field et al., 2000) has shown that directly connected impervious areas typically contribute the most pollutants in runoff, whereas for combined sewers the largest solids and pollutant loads are likely to originate from domestic sewage inputs. Hazardous waste emissions via the waterborne route from the home have not been extensively investigated, although Ainger et al. (1998) indicate that these emissions should not be a problem. Reported studies are mostly concerned with “characteristic” pollutants that are important for the maintenance of health, safety, and structural integrity of the receiving sewer systems, and the efficiency of performance of WWTPs. This has historically been because of the

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TABLE 7.6 Nature of Solids/Sediments from Different Sources Source

Particle Characteristics

Description

1. Winter gritting/salting

Salt <1.5 mm, sand 1.3 mm Finest particles only: <250 µm enter sewer

2. Road surfacing and road works 3. Ingress ofsurrounding ground

All sizes possible depending on gully All sizes possible

4. Industrial/commercial processes

Typical of domestic

5. Construction works 6. Flooding 7. Runoff from impervious areas

>1000 mm possible >1000 mm possible Typically solids <250 µm enter sewer

8. Domestic sewage

Up to 100 mm

Salt solubility <95%, sand also used Total may comprise up to 30% of total mass annually entering gully May mobilize other materials Primarily inorganic; finest particles only enter sewer. Depends upon sewer state of repair Organic and inorganic Trade effluent standards should preclude unusual or dangerous (toxic, bioaccumulative, etc.) solids All sizes organic, inorganic possible All sizes organic, inorganic possible These solids may be up to 40% by mass of total; roof surface can provide up to 30% of total; organic and inorganic Largest organic solids source — typically 97% of these solids; all enter sewer Organic and inorganic Entry via gullies; size reduced when discharged into sewer

9. Soil/erosion from pervious areas 10. Wind-blown sand/soil/litter

Typically <1 mm Large organics possible Inorganics <5 mm

ability to dilute and disperse the chemical pollutants, typically comprising the hazardous discharges, within large flow volumes. Unfortunately, this dilution (solution to pollution) cannot continue to work where “more sustainable” wastewater systems are introduced, as these invariably utilize lower discharged flow volumes, but with the same total masses of pollutants; hence, these are higher in concentration (Parkinson, 1999). It is therefore incumbent on all users of these systems to behave more responsibly if more sustainable systems are to be developed. A variety of alternative sewerage systems have been devised for developing countries and areas where main sewers are impractical, and which utilize solids removal systems prior to the sewer. For example, in the United States, small-diameter gravity sewers (SGDS) as small as 50 mm are used (Otis, 1986; 1986; Field et al., 2000). In Africa, settled sewerage systems are based on a similar idea using septic tanks to remove the solids prior to smaller-sized sewers (Mara, 1996). In developing countries, sewers may in fact be open channels, and also susceptible to the ingress of many more large gross waste and other solids (litter and debris) (e.g., Kolsky, 1998).

BEHAVIORAL ASPECTS Solids originating from domestic wastewater sources can be categorized into a number of types: • Fine fecal and other organic particles (sanitary solids) • Large fecal and other organic matter (gross and kitchen waste solids) • Paper, rags, and miscellaneous sewage litter (sanitary refuse) These categories also apply to commercial and other workplaces, where other substances may be added, subject to trade effluent controls. Because of the diversity of the inputs from industrial sources, they will not be considered further here.

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The use of the toilet in the developed world is virtually ubiquitous. However, not all countries use them in the same way. The concentration of sanitary solids in sewage is widely reported in the standard texts, although the location or representativeness of the sample is not normally specified (i.e., source, in-sewer, or at the sewage treatment works). Similarly, these are assumed to be mean values, representative of the whole flow, whereas samples are normally from one location only in the cross section. Approximately 1 g/capita/day of solid waste material (other than feces) is screened at U.K. wastewater treatment plants (Haddon, 1995). Sidwick (1984) reports a screening solids production rate of 0.01 to 0.03 m3/1000 capita/day, which can be interpreted as 0.05 to 0.15 sanitary items/capita/day. Data from Jefferies and Ashley’s (1994) study of gross solids discharge in combined sewers can be interpreted to give a rate of 0.05 visible items/capita/day. The average disposal rate reported by Friedler et al. (1996) was 0.15 refuse items/capita/day, 72% of which was due to female WC usage. The most common item of refuse (23% of those reported) was the tampon. In the United Kingdom, in 1999, some 2.5 million tampons, 1.4 million sanitary towels and 700,000 panty liners were found to be flushed into sewers via the water closet (WC) every day (Ashley et al., 2002c). It is not only in the United Kingdom that the WC is used as a rubbish bin. A limited questionnaire survey was undertaken of the items disposed of via the WC in 72 countries. Some 33% of respondents claimed that sanitary items, other than feces and toilet paper, were regularly flushed, and in some countries “disposable” diapers are also put into the WC (Ashley et al., 1999). Of concern is the increase in the amount of plastic used in these items and changes in public usage patterns, shifting to using more of the plastic-based products. While pressure can be applied to manufacturers to develop fully biodegradable products using less plastic, the numbers and weights of these items being disposed of is likely to increase in the foreseeable future. However, evidence from the study investigating public attitudes to flushing these and other items such as condoms and cotton swabs, suggests that given the right information, the public may be willing to change behavior. In many countries there is no culture of flushing these items into sewer systems from the WC as public behavior has historically utilized the solid waste disposal route, or alternatively the domestic plumbing systems effectively preclude these items due to blockage risks. However, in the United Kingdom and United States, it is unlikely that there will be any significant reduction in these items found in sewers in the near future, necessitating the continuing usage of expensive screens and transport systems for their control and disposal (Ashley et al., 2002). A study undertaken in the United Kingdom and Portugal (Almeida et al., 1999) has investigated the solids and associated (particulate and dissolved) pollutants originating from domestic sources, as illustrated in Figure 7.7, for a typical residential area. In the United States and certain other

COD_t/TSS/PO4 300 250

[g]

[g]

200 150 100 50 0 0:00

NH3/NO3 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 21:00 24:00 Time [h]

3:00

6:00

9:00

12:00

15:00

18:00

FIGURE 7.7 Total load in wastewater (g/100 inhabitants). (From Almeida, M.C. et al., Urban Water, 1(1), 1999. With permission from Elsevier Science.)

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TABLE 7.7 Pollutant Loads from Residential Sources Discharging to Sewer Garbage Grinders Parameter BOD5 Suspended solids Nitrogen Phosphorus

Toilets

Basins, Sinks, Appliances

(g/cap/day)

(mg/l)

(g/cap/day)

(mg/l)

(g/cap/day)

(mg/l)

11–31 16–44 0.2–0.9 0.1

2380 3500 79 13

7–24 13–37 4.1–16.8 0.6–1.6

260 450 140 20

25–39 11–23 1.1–2.0 2.2–3.4

260 160 17 26

From Ashley, R.M. et al., Urban Water, 2, 263, 2000. With permission from Elsevier Science.

countries, kitchen sink grinders allow the introduction of more organic solids into the sewer system than is usual in countries like Denmark, where these are not used. An example of the effects of garbage grinders is given in Table 7.7, as summarized from U.S. EPA (1992). Table 7.7 illustrates the importance of kitchen grinders, as these generate typically the highest solids loads per capita and the greatest concentration of solids in the flow due to the relatively low water volume discharged concurrently (8 l/capita/day). Garbage grinders are now being installed in a number of European countries, and supposedly will take a range of wastes, including plastics. In Europe, one discouragement to putting kitchen organic waste, and certain other sanitary solids into the solid waste route, the European Directive on Landfill, sets targets limiting the amount of “organic” waste that can be disposed of to landfill sites. It is possible that to reach the specified targets, waste operators may encourage the public to dispose of more of such waste into the sewerage system. Controls that help reduce the solids conveyed from construction sites are well established in the United States, but are not conventionally dealt with in Europe. For example, Colorado Urban Drainage and Flood Control District (1999) provides a detailed methodology for dealing with the problem, beginning with a sediment erosion and control plan. Temporary protection measures during construction should fit what is required postconstruction. In the United Kingdom, the Construction Industry Research and Information Association (CIRIA) recently published a report to assist with the control of water pollution from runoff from construction sites (Masters-Williams, 2001). Among the recommendations are the need to develop appropriate performance indicators for the selection of contracting organizations and the need for techniques to identify clearly the true costs of water pollution from construction sites. Separate sanitary sewers serve a large portion of the sewered population in the United States and in other countries, and have been generally believed to be preferable to combined sewers. The separate sanitary sewers are of smaller diameter than combined or storm sewers, and serve residential, commercial, and industrial areas. While sanitary systems are not specifically designed to carry stormwater per se, stormwater and groundwater do enter these systems. This is a common and complicated problem for sewer owners and the design of sanitary sewers in the United States must include capacity for inflow and infiltration (I/I), which may actually exceed sanitary flow rates (ASCE/WPCF, 1982; Heaney et al., 1999; Field et al., 2000). “Inflow” is defined as wrongly connected inputs, while infiltration comes from groundwater sources. Where wrong connections exist, or where there are significant infiltration flows, the possibility of solids being transferred into the sewer should be considered. The estimation of I/I is difficult, necessitating good sewer records and reliable flow and rainfall data (e.g., Crawford et al., 1999). Unfortunately little work has been undertaken to quantify the solids associated with these sources. Methods for tracking wrong connections are described by Lalor (1993) for the U.S. EPA for identifying these sources, and apply to detection in dry weather flows, but are also applicable to wet weather flows.

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GULLIES

AND

OTHER INLET CONTROLS

Surface sediment controls in most countries are directed to the management of highway drainage systems. The problematic bulk sediments in drainage systems are presumed to originate largely from road drainage inputs. It is clear that the optimum way to deal with potential problems caused by these sediments depends upon institutional and management arrangements; this is because sediments may be controlled either on the surface or below ground. In many countries such as the United Kingdom, different authorities have differing responsibilities for parts of this system. For example, highway authorities (agencies) are responsible for road drainage, but not for sewerage. Although it can generally be shown that the most cost-effective place to deal with sediments is at the end-of-pipe treatment works, the problem of how to ensure that the sediments arrive at a suitable treatment location remains. Because of this, it is also possible to conclude the following from U.K. studies (Butler and Clark, 1995): • A gully pot (catchbasin) is vital in controlling entry of sediments to drainage systems. • Street sweeping practices appear to have only a minor influence on overall solids control for what enters drainage systems and (in many cases) could be discontinued. In fact, street cleaning managers in the United States and Europe are usually interested only in the effectiveness in terms of the requirements to control litter and for road safety, and are not interested in the problem caused by sediment ingress to drainage systems.

STREET CLEANING The methods commonly used are manual or mechanical sweeping and or street washing. Manual sweeping efficiency is largely unknown, but U.K. data show that overall it is less than 48%. Conventional street cleaning has a low pollution control efficiency because the finest particles (smaller than 100 µm) are not very well removed (usually <20% efficiency). Mechanical sweepers are good at large particle removal, but overall achieve only about 50% solids removal, whereas vacuum sweepers can achieve up to 84%, although they are ineffective on wet surfaces. U.S. studies show that even with three times per week sweeps, the overall reduction in solids and pollutants entering the drainage system was only 33%. New street cleaners are emerging that appear to be more effective at removing large fractions of most of the street dirt particulates, including the smaller particles that are most heavily contaminated. For example, Sutherland (1996) showed, using one of these devices (Enviro Whirl I) that monthly cleaning in residential areas could reduce suspended solids discharges in stormwater by about 50%, compared with only about 15% when using conventional mechanical street cleaners. Street washing with low-pressure water is more efficient than sweeping for the removal of fine particles from surfaces. This flushing moves the particles toward the sewer system inlets. In separate systems, the overall water pollution benefits of street flushing are very low, except for litter reduction, but the effects are potentially to increase in-sewer accumulation of sediments. There may be some benefit in street flushing where sewers are combined, as a “controlled” flush could preempt the resulting flush that would occur in the sewer the next time it rained. The controlled flush would be into highly “diluting” dry weather flow. However, this option is not used due to the differing responsibilities for managing streets and sewers. Most authorities are unsure of the amounts of material removed from street sweeping or catchbasin emptying, which usually goes to landfill/tip. There are currently a limited number of models used for the assessment of sediment behavior in gully pots. Fletcher and Pratt (1981) developed a model based on a physical description of the gully pot processes. The model simulates erosion of the gully bed deposits, but the deposition is not modeled. Wada et al. (1987) attempted to verify this model in a laboratory investigation. Butler et al. (1996) developed a simple model where only the sediment deposition component was taken

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into account. This model was extended recently to incorporate erosion, and partially tested (Butler and Menon, 1999). Although there are now a number of models of sediment entry into drainage systems through gully pots, these are still difficult to calibrate and verify. The major problem is the large amount of data required for calibration of the substantial numbers of coefficients, and notwithstanding the use of these calibration coefficients, the accuracy of calibrated models is still not particularly good unless a great deal of data is available (Deletic et al., 2000). Vacuum tankers are widely used in the United Kingdom to empty gullies. Eventual disposal may discharge the liquor to a sewer or to an alternative waste disposal outlet. Gully arisings have to be disposed of carefully as they are “controlled waste” in terms of U.K. legislation. Little information is available about the efficiency of cleaning. Butler and Clark (1995) assessed more than 100 gullies undergoing cleaning. The overall efficiency was 72% in terms of the amount of sediment removed, with a range 20 to 90%. Residual sediments were usually the larger particles >25 mm. In fact, only some 79% of gullies were cleaned, resulting in an overall combined efficiency of only about 50%. Overall, some 10% of the sediment in the pot was found to be swept into the sewer or downstream watercourse during the cleaning process. More data are available for gully cleaning costs than for street sweeping, and a unit cost of £1.50 to £3 per gully (1988) was noted for the United Kingdom, with areas with a higher rate of cleaning being the cheapest.

DESIGN AND OPERATION OF SEWER SYSTEMS TO CONTROL PROBLEMS OF SEWER SOLIDS Significant progress has recently been made in understanding the nature and behavior of sewer sediments since the early work in the United States in the 1970s (e.g., Pisano et al., 1979). Translation of research results into practical use has largely concentrated on the development of computational models to represent the quality performance of sewer networks. Sewer flow quality models have been developed by a number of commercial and academic organizations (Ashley et al., 1999), and the commercial models are being utilized to design and improve the performance of sewer networks. Each of these computer models may (in principle) be used to simulate the hydraulic and quality effects of sediments in sewers. However, the primary objective of most of the models has been to develop simulation tools that can model the discharge of pollutants from overflows and into treatment plants. Table 7.8, from Ashley et al. (1999), illustrates the capabilities of the most commonly used models in relation to in-sewer processes. It should be noted that none of these deals properly with tanks and storage units, primarily because of the lack of knowledge about sediment and pollutant behavior in these ancillaries. It can be seen from Table 7.8 that sewer sediment processes should be amenable to modeling using the Danish Hydraulic Institute model known as MOUSETRAP, and the flexibility of the SWMM code and submodules would allow a user to produce an individual model. However, outstanding deficiencies in sewer process understanding relate to both the physical behavior of the solids and associated pollutants and the chemical and biological aspects. Hence, these comprehensive deterministic sewer flow quality models are inevitably forced to average or aggregate particle characteristics and transformations, and to model single event effects, rather than longer-term system performance, as is required when looking at sewer sedimentation. The various processes need to be brought together better if more effective deterministic models are to be developed. As a consequence, much simpler approaches are required for system design and management for the foreseeable future.

DESIGN

OF

SEWERS

It has been traditional to approach the design of surface water, foul, or combined sewers by applying a single design criterion (e.g., Hager, 1999). This may be a minimum velocity, a critical average boundary shear stress as is often used in Scandinavian countries, or a minimum gradient.

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TABLE 7.8 Principal Commercial and Semicommercial Sewer Flow Quality Models and Selected Capabilities

Model

Able to Predict Sediment Buildup in Pipes

Hydraulic Solution

Hydroworks

Full

Mousetrap Canoe

Full Full and simplified

Model couples hydraulics and sediment bed (from 2001) Yes In theorya

SWMM

Extran — full Transport — simplified Simplified

No In theorya In theorya

Flupol a

Multiple Sediment Fractions

Models Biochemical Processes

Bed Load Transport

2

No

Total load only

Yes Yes (based on settling velocity) Not modeled No Yes

Yes Yes

Yes No

No No No

No No No

Impossible without a bed load transport model except in steeper sewers.

Source: Adapted from Ashley et al., 1999b.

TABLE 7.9 Minimum Velocity and Bed Slope for Sewer Self-Cleansing for German ATV Procedure Diameter (mm) Minimum velocity (m/s) Minimum bed slope (%)

150 0.48 0.27

300 0.56 0.15

600 0.84 0.14

1000 1.12 0.13

1500 1.39 0.12

2000 1.62 0.11

2400 1.79 0.11

3000 2.03 0.11

Source: Adapted from Hagen, 1999.

Camp’s formula is used frequently in the United States for sewer design: minimum bed shear τ m = ρgK (s − 1)d where ρ = g = K= s = d =

fluid density gravitational acceleration a constant = 0.8 particle specific gravity particle size

This gives τm = 12.6 N/m2 for 1 mm sand particles (typical of what might be washed into a sewer from highways). In some countries a more-sophisticated approach is now used, which does take into account sediment transport. For example, in Germany, where ATV standards are used (Hager, 1999), the standard tables and relationships are based on characteristic sediment particles, rather than transport capacity. Table 7.9 illustrates the minimum slopes recommended for 50% part-full flow. The traditional criteria take no account of the characteristics of the sediment, of the suspended sediment concentration or transport mode, or of any cohesion between the sediment particles. It has been known for some time that these single criterion approaches are naive and that small sewers (typically <500 mm) are often overdesigned and large sewers (>1000 mm) may be underdesigned (e.g., Ackers et al., 1996; Heaney et al., 1999). Hence, smaller sized sewers are laid to too steep gradients

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10.0

Velocity (m/s)

Macke Ackers-White Novak and Nalturi

Coarse sediment Medium sediment

1.0

0.1 0.0

0.5 1.0 Diameter (m)

2.0

FIGURE 7.8 Theoretical self-cleansing velocity required for given sewer size. X is the sediment transport rate; y the depth of sediment; D depth of conduit; d50 the median particle size; s particle relative density; ko the overall effective boundary roughness. From CIRIA, 1986. (Based on a range of transport equations: Macke, 1983; Ackers and White, 1973; Novak and Nalluri, 1975.)

and the larger ones are not steep enough, as illustrated in Figure 7.8, from CIRIA (1986). The implications from this are that in order to ensure self-cleansing for larger sewers (>1 m), steeper gradients than given in the approaches above are needed, resulting in deeper and more expensive excavation. There are a number of other problems with the current approaches: • No account is taken of gross solids — most important in the smaller collector sewers near the heads of networks. • Sediment transport capacity of the flow is not considered explicitly. • There is no consideration of modes of sediment transport; and no acknowledgment that bed deposits exist (even in “self-cleansing” sewers), albeit they may be transitory in nature, linked to changes in bed roughness. Research using granular particles in circular and other shaped sections has led to new ideas for sewer design, which help to mitigate the known deficiencies of existing design approaches. Of greatest significance is the recognition that by allowing “limited” deposition (a small proportion of pipe diameter), the transport capacity of the flow can be increased and pipe gradients minimized, even for the larger sewers. However, application of these ideas to real sewer sediments has not been carried out, other than by using limited existing data, and this has so far proved inconclusive (May et al., 1996; Arthur et al., 1999).

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Studies looking at the effect of section shape on sediment transport capacity for rigid boundary sections (Loveless, 1991; Torfs et al., 1994) show that the ideal section shape to convey high concentrations of granular sediments is the rectangular conduit with the largest axis horizontal. It is clear that up to a limiting condition (typically <20% of the pipe diameter) the presence of a deposited sediment bed in a pipe can increase the sediment transport capacity. In view of this, and acknowledging the fact that there are rarely conditions in real sewer flows where there are not some sediments either on, or moving close to, the bed, new criteria have evolved in the United Kingdom for sediment-controlled design. This is based on the principle that the most efficient design is one in which the transport capacity provides a balance between deposition and erosion, resulting in temporally averaged sediment depths — a balance between deposition and erosion, i.e., for a specified proportional depth of flow or frequency of occurrence, flows should be sufficient to (Ackers et al., 1996) accomplish the following: • Erode deposits from a bed, which may exhibit cohesive-like properties • Transport a given concentration of fine-grained and/or low-density particles in suspension • Transport the coarser particles as bed load at a rate required to achieve a specified temporally averaged (acceptable) bed depth To ensure that each of the above criteria is achieved, as one or other will lead to the critical design requirement, different analytical approaches are needed encompassing the following: 1. A minimum bed shear stress condition to ensure erosion occurs 2. Application of a suspended sediment transport equation 3. A bed load sediment transport equation accounting for either • A limit of deposition condition • An allowable proportional depth of sediment deposit The allowance of a controlled depth of sediment enables more economic designs to be achieved for the larger pipes, in particular, as illustrated in Figure 7.9 (May, 1994). Figure 7.9 shows that, for the assumed design criteria, allowable proportional sediment depths of 1 to 2% can reduce the required gradient to transport a given sediment load by typically a factor of 2 to 3. Notwithstanding these results, which are based on laboratory studies, other research carrying out experiments using much deeper, loose beds than those of May (Nalluri et al., 1994) suggest that there is an optimum relative depth of sediment bed for part-full pipe channel flows. This optimum may be determined from the balance between the increased transport capacity allowed by the broader bed and the increased resistance caused thereby. It is important to note that the relative flow depth is a significant factor here, and Figure 7.9 shows that under pipe-full flows there are no optima. In practice, of course, relative sediment depths of up to 20% of pipe diameter may not be acceptable as a temporal norm for operational maintenance reasons. A detailed analysis is also required where limited deposition is allowable, to determine the hydraulic performance of the pipe — the overall friction factor must be determined to compute the flow capacity accurately. Clearly the correct evaluation of hydraulic performance interacts with the design criteria above, and any calculations will be iterative. The proposed approach thus requires the application of a three-stage analysis (Ackers et al., 1996): 1. Application of a suspended transport relationship 2. Application of a bed load transport relationship 3. Check that a critical bed shear is achieved to ensure that any (allowable) deposits will be eroded Recommended equations are provided for the first two stages, depending on whether or not transport is at limit of deposition (LOD) (no deposits), as illustrated in Figure 7.10.

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Minimum flow velocity: V m/s

5.0

X = ylD = d50 = s = ka =

50 mg/l 1.0 750 µm 2.6 0.6 mm

2.0

1.0

D = 3.0m D = 1.8m D = 1.2m D = 0.6m

0.5

D = 0.3m 0

2

4

6

8

10

Proportional sediment depth: y sID (%)

FIGURE 7.9 Required minimum flow velocity for an allowable sediment bed depth for a range of pipe sizes under pipe full flow conditions.

The equations due to Macke (1983), Ackers and White (1973), and Ackers (1991) are well established and straightforward to apply. The equations recommended by Ackers et al. (1996) for transport over a deposited bed, however, are a variant of earlier relationships and provide very little improvement compared with much simpler relationships developed by other researchers such as Nalluri et al. (1994). The characteristics of sewer sediment particles and those transported in suspension or near the bed are now fairly well documented (e.g., Crabtree et al., 1991; Ashley et al., 1994). The bed strength characteristics have been measured and related to the erosion potential with a highly concentrated flow over a deposited bed (e.g., Wotherspoon, 1994), although little is known about the complex bed processes (Vollertsen, 1998). The sediment criteria suggested in the CIRIA project (Ackers et al., 1996), in the absence of local data accord with the ranges given in Table 7.10. A typical set of results from application of the approach for surface water and combined sewers is shown in Figure 7.11. The criteria suggested in the figure are compared with the ATV procedure data in Table 7.9. This new comprehensive approach to sewer design is largely based on studies that have investigated only granular particles transported in (laboratory-scale) pipes. Despite their limitations, the methods have introduced a new scientific rigor and are expected to lead to a more rational and economic approach to sewer design, despite questions about the applicability to real sewer conditions (Arthur et al., 1999).

PREDICTION OF SEDIMENTATION AND CONTROL OF SEDIMENTS IN EXISTING SEWERS The deterministic computational models given in Table 7.8 have been used to predict sedimentation in sewers. These use an approach similar to that described in the last section, with full-solution hydraulic models linked with complex or more-simplified transport equations. Many of the packages

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Ciria Methodology Define sediment, and transport loads for each mode

Define topography range

Yes Specify % allowable deposition range (usually 1, 2%) Determine overall friction factor

Limited deposition OK?

Determine L, Q, v relationship

No Determine L, Q, v relationshipclean water Adjust capacity/ required hydraulic gradient to account for sediment transport

Suspended Bedload Transport Cohesion Sediment Criterion Criterion Criterion Macke-limit of For allowable % deposi- Check threshold shear deposition tion May or other eqn. stress reached Ackers-White where limited deposition acceptable Where shear velocity high transport as suspended loaduse Ackers-White

Assess critical velocity

Produce design table for range of diametersre-working hydraulics

FIGURE 7.10 Outline of CIRIA 141 sewer design method. Key: Macke (1983); May — from Ackers et al. (1996); Ackers and White (1975). i is hydraulic gradient; Q discharge; v flow velocity.

TABLE 7.10 Ranges of Sediment Characteristics Suggested for Use in CIRIA Sewer Design Method in the Absence of Local Measured Data Sediment “Sanitary” solids Stormwater solids Grit

Transported as

Conc. (mg/l)

µm) D50 (µ

Specific Gravity

Suspension Suspension Bed load

100–350–500 50–350–1000 10–50–200

10–40–60 20–60–100 300–750–1000

1.01–1.4–1.6 1.1–2.0–2.5 2.3–2.6–2.7

Source: Ackers, J.C. et al., CIRIA Report 141, CIRIA, London. Reproduced by kind permission of CIRIA.

do not couple the sediment bed temporal changes with the flow field and thus utilize a “static” bed for hydraulic computations. None of the models includes the effects of the near-bed organic solids believed to be principally responsible for foul flushes (Arthur et al., 1996). As yet, the prolonged computational times required to model annual and longer time series rainfall events make it difficult to use detailed models for the prediction of longer-term (even equilibrium) sedimentation in existing sewer systems. As a consequence, a number of simpler alternative approaches have been developed to predict sedimentation and produce control strategies.

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Medium, LOD Minimum design velocity: m/s

2.0

High, LOD

High, 2% ATV procedure–no deposition (Table 7.9)

1.6

Medium, 2%

1.2 0.8

Deposition criteria:LOD Limit of deposition 2% Allowable bed depth

0.4 0

0

500

1000

1500 2000 Pipe diameter: mm

2500

3000

FIGURE 7.11 Design chart for typical surface water and combined sewer conditions. (Adapted from Butler et al., Proc. of the Institution of Civil Engineers: Water Maritime and Engineering, 118(2), 1996. With permission of Thomas Telford Publishing.) High, medium, low = sediment transport loads; LOD = limit of deposition; no sediment; 2% = an allowable transient bed deposit of 2% of pipe diameter. (Adapted from Ackers et al., 1996.)

Deposition in sewers generally occurs during periods of dry weather and during decelerating flows when storm runoff is receding and corresponds with structural and hydraulic discontinuities, such as joints, gradient changes, and junctions (Bachoc, 1992). It is also known that the propensity for sediment deposition will be different depending upon a range of factors (adapted from Ashley et al., 1992; 2000): Sediment sources

Sewerage system

Sewage and sediment

Hydraulic regime

Land use Population connected Per capita waste generation Service area Infiltration Duration of dry period(s) Combined/separate Total length Slope-average (or equivalent) Sewer size range Conduit shapes Age of conduits Construction/joints/materials Roughness Storage capacity Maintenance programmes Particle sizes Particle density and settling characteristics Organic content Sediment bed depth Bed roughness Flow/velocity/depth range and rate of variation Shear stresses (from above) Sediment transport capacity Localized hydraulic discontinuity effects (connections, junctions, downstream controls)

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In most sewers, the dry weather flow patterns provide supplies of solids from upstream. Storms provide a more random source of material and disturbance to deposits, eroding, and redepositing. The deposits therefore become layered and mixed due to interacting processes and ongoing biochemical reactions. Current methods of predicting sediment deposition include the following: • • • •

Surveys — usually closed circuit TV (CCTV) Hydraulic analysis using characteristic criteria such as velocity thresholds Conceptual and risk modeling to produce a ranking for potential sediment deposition The use of sewer flow quality models, which include some or all of these

Application of these methods has usually been to particular sites or catchments, without demonstration of a generic approach. An expert system developed by Gérard and Chocat (1998) has been successfully applied to the catchment of Lyon, France. In this approach, risk of deposition was assigned to pipes and structures throughout the system based upon key structural characteristics. However, the approach was not considered sufficiently flexible. To predict deposition, it is first necessary to understand the nature of sediment transport in sewers. Conditions are significantly different in terms of flow boundaries, regimes, and nature of particles, compared with river and estuarine situations, and transfer of knowledge from these “classical” areas of hydraulics has so far achieved limited successful application (Ashley and Verbanck, 1996). The important aspects are as follows: • As in classical hydraulics, the main sewer deposits originate from the larger solids transported near the bed. • Recurrent flow regimes in sewers (dry weather flows) create diurnal patterns of transport, deposition, and erosion. • Sudden catastrophic (rain) events occur, which can cause large quantities of sediments to be eroded and/or deposited over short time periods. • There are a range of sizes of organic particles in sewers, which are responsible for two effects: cohesive-like gluing (smaller particles); larger material moving as “near-bed” load (quasi-stationary), which provides a source of readily erodible high-strength pollutants and/or becomes entrapped in the bed as an organic layer. Under certain circumstances the material moving near the bed of a sewer, and that mostly responsible for deposit buildup, either with or without an initial deposited bed, may be traditional (granular) bed load. Studies of the bed load transport in a number of sewers have shown that mostly granular particles (2 to 10 mm) are collected in traps located in the invert of the sewer in the steeper (>2%) sewer sections, whereas in the shallower slope (<0.1%) sections, the particles are more similar to those observed in suspension. The transition from “bed” to “bed load” transport (incipient motion) in the steeper Marseilles sewer was shown to be predictable using the traditional Shields’ criterion, and the evolution of the bed deposits including both erosion and deposition was shown to be predictable over a period of 1000 days with a “bed load” relationship derived from the MeyerPeter and Muller formula, taking into account a mixing layer and grain size classes (Lin and LeGuennec, 1996). A near bed granular solids transport relationship was obtained (Arthur et al., 1996) for transport in the main sewer in Dundee in Scotland, based on the modification of an existing relationship (Perrusquía and Nalluri, 1994) for transport over deposited beds. This is applicable where there is known bed load transport of granular particles, either in steep sewers or during storms (Ashley and Verbanck, 1996). An empirical equation was developed for the transport capacity in the sewer for estimating the total solids moving near the bed during dry weather.

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SEDIMENTATION It should be possible to estimate deposition by determining the difference between the flow capacity to transport sediment and that being transported. Any sediment load in excess of the capacity will deposit. The commercial sewer flow quality models operate on these principles (e.g., Mark et al., 1996). Unfortunately it is necessary to operate such models over lengthy time periods in order to establish “equilibrium” conditions within a sewer network in terms of where and how much deposit will build up. The constraint used, that the solids are represented in these models by limited ranges of particle sizes and densities and that these are generally uniform and time independent across a catchment, limits the current ability of these models to predict deposition. A simplified approach to estimating deposition was first proposed for small sanitary sewers from U.S. studies in the 1970s (Pisano et al., 1979): Z = 40 Z = 40

 τc   0.004 

−12

for τ > 0.004 lb ft 2 for τ ≤ 0.004 lb ft 2

where Z = the percentage of dry weather solids deposited if the wall shear stress τ is less than τc. This suggests no upper bound to deposition in any pipe length, and hence needs to be modified to reflect what is observed in practice: • Sediment deposits can become graded with time — coarser particles in the upper parts of sewer “reaches” • Sediment beds attempt to ameliorate the effects of the sewer that cause them to deposit initially; i.e., where there are gradients that are too shallow, the deposits will tend to form an artificial steeper bed, thus becoming “self-equilibrating.” • Bed roughness (particle and form) interact with the above, having the greatest effects when the bed is at half depth in a circular pipe. These factors have led to a modification of the above equation: τ  Z = 0.889 ×  0   τc 

−1.2

 W  ×  b   Wmax 

here Wb is the bed width and the maximum width of the pipe is Wmax. Application of this relationship has been used successfully to predict dry weather deposition in the sewers in Dundee (Ashley et al., 2000) as illustrated in Figure 7.12.

MANAGEMENT AND DISPOSAL OF SEWER SOLIDS Ideally, sediments and other solids should be kept out of wastewater systems. Failing this option, it is presumed that the solids transferred into the system are most economically managed by their conveyance to a downstream facility (Butler and Clark, 1995). However, this assumption has not been fully explored, and it is probable that local circumstances would determine which option for sediment management is the best. Currently, the approaches used are twofold: 1. Extract, transport, and dispose of solids from locations where they collect (either by design or by default). 2. Reentrain any deposits and encourage them to move “downstream” to some main collection point where they can be collected and dealt with.

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FIGURE 7.12 Predicted average sediment depths in main Dundee interceptor sewer using simplified combined erosion/deposition model. (From Ashley, R.M. et al., Urban Water, 2, 263, 2000. With permission from Elsevier Science.)

Another option, usually impractical, for in-sewer sediments, is to alter the ambient hydraulic regimes to minimize deposition. However, this usually necessitates large-scale infrastructural investment, which is usually presumed uneconomic. Solids (and attached pollutants) that remain in systems, even when being transported, have the potential to become modified as part of the ongoing sewer processes; thus, if the sewer is considered to be a “reactor,” these solids will experience “treatment” as they pass through the system. One disadvantage of this is the potential for anaerobic conditions to occur, with the generation of hydrogen sulfide, and attendant problems. A comparison of sewer flushing with sediment removal or treatment has recently been made (Pisano et al., 1998; Fan et al., 2001) and has concluded that flushing is a viable economic solution for in-sewer sediment management, notwithstanding the potential need to provide treatment to avoid development of anaerobic conditions. As yet, no definitive studies have looked at the relative merits of the various options for managing sediments. Regulations and options for ultimate disposal of arisings from extracted sediments are making handling more difficult. Ultimately, it is probable that in developed countries arisings will have to receive some form of treatment prior to reuse or landfill.

SEDIMENT MANAGEMENT

IN

LARGE SEWERS

Maintenance of larger human-entry sewers has not altered significantly from the methods developed at the end of the 19th century. Figure 7.13 shows the results from a recent survey of U.K. sewerage operators and their reported techniques used to clean all sizes of sewer. Frequent maintenance of these systems requires significant manual labour and is hence very expensive; consequently, many operators undertake only reactive rather than proactive maintenance, when blockages or flooding necessitates. Often operators are ignorant of the link between sewer deposits and an increased frequency of overflow discharges (e.g., Fraser et al., 1998; Ashley et al., 2000). Mechanical methods are often not very efficient for cleaning larger sewers. Routine maintenance removal of human-entry sewer sediments depends on location, although the ranges of sediment volume removed per capita or per hectare are fairly uniform, averaging some 140 to 300 l/ha/year,

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Methods of In-Pipe Sediment Control 100 80

(%)

60 40 20 0

Jetting Vactoring Winching Silt Traps Manual Flushing

FIGURE 7.13 Results from a U.K. survey of sewer cleaning techniques. (From Fraser, A. et al., Water Sci. Technol., 37(1), 1998. With permission of the copyright holder, IWA.)

representing up to 12 l/capita/year. A study in Bordeaux, France concluded that when including all sources of sediment deposits (catchbasins, tanks, grit chambers, and sewers), some 30 kg/capita or 440 kg/ha of sediments had to be collected and disposed per year (Delattre et al., 1998). For larger sewers, alternatives such as the Gottingen Cleaning Balls System (Lorenzen et al., 1996) are reportedly effective. The system relies on an appropriate method of ball introduction at an upstream manhole and subsequent capture. Hence, the technique is only appropriate for sewers of reasonably large diameter. The ball is approximately 25% of the sewer cross section, and has rubber “flutes” on the outer surface to encourage spinning and localized high velocities. According to studies in Germany (Dinkelacker, 1992) using balls in Hannover with five runs/week, in a 6.7-km sewer length (1400 to 1800 mm) showed that average sediment depths fell from about 300 mm to a few traces. These balls were released automatically by dropping from an upstream manhole, and subsequently collected automatically at the treatment plant. Only on one occasion in 3 years did a ball become stuck (it then caused the following ball to stick) and need to be removed manually. A much simpler (and older) example of a manual technique is that employed within the Brussels sewer system where a (Vane) wagon incorporating a flushing vane is physically maneuvered along the length of the cunette-shaped sewer system, largely via differences in hydrostatic head across the vane, which can be lowered into the flow. The vane disturbs the sediment, which is subsequently transported downstream with the sewer flow. In a number of mainland European countries, such “active” sediment control devices are used. The French systems, used in Belgium as well as France, have utilized these vane wagons, which are trolleys set to run on the side walkways (banquettes) in the larger sewers (Figure 7.14). These have large, hinged doors that may be lowered into the bottom part of the sewer (the cunette) and, using the head of water behind, they “shove” the sediment ahead of them as they are pushed down the sewer. Facilities for the removal of the sediment are required at strategic locations. Sewer flushing is one of the oldest methods in use, and entails the generation of a rapidly varying “dam-break” flushing wave traveling along the sewer. This resuspends and transports the deposits downstream. There are two approaches in use for larger sewers: an in-sewer installation and one that requires a storage tank to collect the volume of water required to effect the flush. There are a range of in-sewer devices that can be used to build up a head of water automatically and then, by hydrostatic imbalance, tip or otherwise allow a “flood” to translate downstream to flush sewer lengths of the order of 100 m. The HYDRASS gate is one such device, as illustrated in Figure 7.15 (Chebbo et al., 1996). Typically, such gates tip some 10 to 20 times per day, and should be installed at the upstream ends of shallow gradient pipes where the network downstream is much steeper. Modeling of these systems has been undertaken using both laboratory studies and computational fluid dynamics (e.g., Laine et al., 1998). The alternative, using chambers or tipping

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Ville de Bruxelles:

Collecteur de la Senne Wagon-vanne à vis oscillante:

2.190

Ech.: 0.025 m.p.m.

0.75

0.775

0.60

Coupe longitudinate

Coupe transversate

FIGURE 7.14 Brussels vane wagon used for “automatic” sewer cleaning.

Hydrass

FIGURE 7.15 Operation of HYDRASS gate for sewer flushing. (Adapted from Chebbo et al., 1996.)

flushers, have been used for many years in tanks, but are now proposed for greater application in sewers (Pisano et al., 1998). In-line sediment traps have been used extensively for the control of river sediments. Historically, such systems have also been used in sewers, but with limited extent. These traps are sumps in the bed of sewers as illustrated in Figure 7.16. There are now only a few traps in the United Kingdom, although there are many hundreds operating in France (Dartus and Alquier, 1985; Paitry et al., 1990). With modern suction methods of emptying such traps, this approach may be very effective in preventing problems caused by in-sewer deposition. The main operational problems arising from the use of silt traps are associated with emptying and include time taken, odor, disruption to traffic, and inconvenience to the general public. Over the years the frequency of cleaning decreased in the sewer network in Dundee, Scotland as it was so labor intensive. Although this was partially offset by the reduction in sediment ingress due to the provision of paved surfaces, the lack of maintenance resulted in an increase in deposits within the sewer network. At the start of the 1980s there was

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Type 1 Outlet

Inlet

1-3m

Outlet on certain types

2.44m 1m Sand

trap

Cesspool Trap

Flow

Type 2

1.4m

Gate

3-5m to 6m

1.5m

Gate

r Side D i ve r s i o n S e w e

FIGURE 7.16 Sediment traps in sewers in Dundee, Scotland.

real concern as almost all of the (12) traps were in need of emptying and there was a buildup of some 600 mm of sediment in the main interceptor sewer. In the 1980s, clearance work using a combination of labor and mechanical apparatus was undertaken, and nowadays specialist contractors have large jetter/vacuum machines, which run quietly, dewater, and reduce the need for human entry into the sewer system. A number of studies have been carried out to determine rates of trap filling; however, these have so far been largely site specific. Bertrand-Krajewski et al. (1996) were able to relate the rainfall volume to the mass of solids collected in French traps in Bordeaux, provided the rainfall did not exceed 50 mm in total. Most of the material collected was granular (~1 mm in size), and the traps were found to be 70 to 80% efficient at collecting the bed load. In Dundee, however, where the material is more organic, the masses collected in the main sewer could be determined using a combination of a near-bed (organic) solids empirical equation for dry weather and a separate bed load transport formula for granular sediment during wet weather (Fraser et al., 1998). Attempts have also been made to estimate trap filling rates using computational fluid dynamic techniques (Schmitt et al., 1998; Buxton et al., 2001) and from laboratory models. A strategy has recently been developed to locate traps cost-effectively using these various approaches (Ashley et al., 2002b). In 1996, the cost of cleaning some 5 km of the main (approximately 1500 mm high) Dundee interceptor sewer was approximately U.S.$50,000 using specialist contractors and the equivalent data from French studies indicate costs of some $1000 to $10,000/km of sewer, presumably depending upon the scale of the contract. In a survey carried out in ten French agglomerations, the costs for removing 1 m3 of deposit from human-entry sewers was found to be between $150 and $1000 (Sanchez, 1987).

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SEWER CLEANING

IN

SMALLER SEWERS

The maintenance of non-human-entry (smaller) sewers does not cause as many problems due to efficient equipment, with, for example, high-pressure water jets combined with vacuum systems downstream. Such techniques are inefficient in human-entry sewers for which the transport of solids stirred up by the water jets would require high conveyance flows, which are not available due to limited water supply rates. Common smaller sewer cleaning techniques may be manual or automated and include the following (Pisano et al., 1998): • Power rodding: The power equipment applies torque to the rod as it is pushed through the line, rotating the cleaning device attached to the lead end. Efficient in pipes up to 300 mm in size. • Balling: Pressure of a water head creates high-velocity water flow around a ball, such as an inflated rubber cleaning ball with an outside spiral thread and swivel, causing spin. Removes settled grit and grease. Useful for sewers up to 600 mm in size. • Jetting: Direction of high velocities of water against the pipe walls at various angles. Efficient for routine cleaning of small diameter, low flow sewers, and for the removal of large debris. • Poly pigs, kites, and bags: Used in a similar manner as balls with the rigid rims of bags and kites causing the scouring action. Poly pig is used for larger sanitary sewers without a restraining line. • The power bucket machine: Used mainly to remove debris from a break or an accumulation that cannot be cleared by hydraulic methods. Cannot be used where the pipe is so completely blocked that a cable cannot be threaded between manholes. The bucket has two opposing hinged jaws for collecting the debris. Buckets and tools range up to 900 mm in diameter. • Sewer flushing: As for larger sewers, but for smaller sewers, the water source has traditionally been from a surface input, most recently fire hydrants or direct mains connections. In Dundee the latter were used up until the 1950s before being discontinued due to the risk of cross-contamination. Recently, in-sewer systems have been used where an upstream chamber has been used to store the flush water and released using a gate.

SUSTAINABILITY

AND

DISPOSAL

OF

SEDIMENT REMOVED

FROM

SEWERS

The devising of sustainable wastewater systems necessitates an holistic approach that includes the following tripartite aspects, in addition to technical performance: 1. Social and cultural 2. Environmental 3. Economic Various approaches are in use to “quantify” sustainability, usually based on some form of indicators, and whole life considerations are essential if more effective and efficient systems are to be devised. The options for control and management of sewer solids (see Figure 7.6) include nonstructural measures, structural measures, and reactive maintenance: 1. Nonstructural measures: • Education of users of wastewater systems • Financial disincentives where education has failed or where usage is inefficient • Political or regulatory measures

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2. Structural measures: • New products (not requiring wastewater system disposal) • New technologies for input reduction (e.g., noneroding road materials) • Inlet controls • New sewer design and operation techniques to convey solids more effectively and/or to “treat” them within the sewer • Improved end-of-pipe treatment and removal (screens) 3. Reactive maintenance: • This may be considered as a failure to attain the most efficient system, comprising surface cleaning, inlet structure cleaning, sewer cleaning, etc. However, automatic systems such as flushing gates may be an option that proves to be part of a “most” sustainable solution. Devising more sustainable systems to manage wastewater solids requires a difficult multicriteria analysis to compare benefits and drawbacks of noncommensurate measures for the social, environmental, and economic components, even where simple approaches using indicators are used (Ashley et al., 1999; 2002a). Traditional approaches to wastewater management have suffered because they have tended to weight “economic” criteria the most highly in decision making, without considering the overall (whole life) picture and the broader impacts. This has led to the classical problem of transferability of, for example, environmental impacts from one area to another, e.g., cleaner effluent but more sludge at WWTPs. New studies are now attempting to look at sewerage systems within this whole life perspective, although few are considering all of the tripartite elements of sustainability. Much of the work relates to the application of tools, and a European project COST624 (COST Action 624, 2002) is reviewing which analytical tools may be best suited to assisting in these endeavors for the devising of optimum ways of operating wastewater systems. Examples of the types of tools being used to provide environmental information for wastewater systems are as follows (Wrisberg et al., 2002): Decision support tools Analytical tools

Procedural tools

Multicriteria decision support systems (MCDSS) Life cycle assessment (LCA) Material flow accounting (MFA) Material intensity per service unit (MIPS) Environmental risk assessment (ERA) Socioeconomic assessment Life cycle costing (LCC) Cost–benefit analysis (CBA) Multicriteria analysis (MCA) Environmental management systems (EMS) Environmental impact assessment (EIA) Environmental audit (EA)

Can be methodological, expert system based or other; should be transparent to all stakeholders Many of these tools have been developed for products rather than processes. Systems can be 1. Demand driven: To fulfill a need 2. Boundary driven: Processes occurring within a defined boundary (may be time) 3. Agreement driven: System defined by agreements between supply–demand chain; currently, there are no analytical tools suitable for these systems Agreed methodological approaches, used for the systems above

The application of cost–benefit analysis is now axiomatic in any development decision related to wastewater systems, although the boundaries set for these analyses are often parochial and do not truly consider whole life or whole system economics. The problems of accounting for environmental systems, including valuation of the environment and externalities, make proper economic considerations virtually impossible within contemporary accounting systems. A number of studies have been undertaken using life cycle analysis applied to wastewater treatment systems. Of note is the LCA and MIPS (material intensity per service unit) analysis used to justify the construction of a self-contained wastewater system for a 350-person community in Germany (Otterpohl et al., 1997; Otterpohl, 2001). Fewer studies have considered the inputs to

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systems, or the sewer network. A useful overview of relevant applications is given by Lundin (1999). Gigerl and Rosenwinkel (1998) present a background to LCA as applicable to a German sewer system, accounting for all materials and energy for sewer construction and final end-of-life budgets. Sewer operation included maintenance, repair, and cleaning. Swiss studies referred to in this work (Boller, 1997) revealed that in terms of sustainability, the effects of heavy metals in the sludge and sediments and ultimate disposal are paramount, although these problems may be localized due to the nature of Swiss inputs to wastewater systems. A comprehensive study has been carried out to look at the overall LCA of municipal wastewater systems (Tillman et al., 1998). Typically, the alternatives to conventional wastewater management were found to have lower environmental impacts. Other relevant studies by Cretaz et al. (1997) and Parkinson (1999) have considered the LCA of potable water management together with domestic use of rainwater, and strategies for the sustainable management of domestic wastewater, respectively. In the latter case, however, only limited consideration of the resource and energy implications was undertaken, with the emphasis on modeling of the hydraulic and water quality aspects. This study paid particular attention to the effect of lower wastewater discharges and changes in water quality from domestic sources for a range of behavioral and infrastructural changes in the home. Overall, the favored option was the use of stormwater for WC flushing as having the least effect on the performance of the downstream existing sewerage system, as it helped to maintain sediment transport, while reducing CSO spills. A multidisciplinary approach has been taken in a U.K. study where the relative sustainability of flushing (WC) or binning large sanitary waste items has been investigated (Ashley et al., 1999). In the first part of the study, mass balances for the solids, cost–benefit analyses linked with environmental valuation, and social acceptability of habit change were investigated, to show that it was more sustainable to dispose of these items via the solid rather than waterborne waste route. The MFA/LCA of the sewer solids has been investigated (Johnstone et al., 1999; Gouda et al., 2002). Figure 7.17 shows the sewer-related pathways for tampons, the most ubiquitous sanitary solids, used to develop the LCA. Figure 7.18 illustrates the application of LCA to all types of sewer solid. LCA is a structured and transparent technique that may be used to account for all physicochemical effects on environmental systems. The LCA study has been used to determine which of these effects are likely to be the most important for a range of options for the better management of sanitary waste currently flushed into WCs in a U.K. catchment with a population of about 1500. Use of the eco-indicator approach, provided in the SIMAPRO™ (Pre Consultants B.V., Netherlands) LCA software, suggests that the preferred option should be to encourage behavioral change to stop flushing. This case example demonstrates clearly the limitations of LCA in this type of application, as the preferred option requires major social behavioral change, an aspect that LCA cannot handle. Hence, where decisions are required, LCA can usefully provide baseline data, but is not recommended for consideration of options that have significant social or economic dimensions. Disposal of arisings from sewer sediment removal is also becoming more of a problem in the United Kingdom, in terms of the various legislative controls. The waste is difficult to classify as it may have a variety of constituents, making it a “special waste.” Usually, however, sewer sediment is not yet so classified as it is not flammable, toxic, or corrosive. Since 1992, any carrier of sediment arisings (other than the undertaker) must be registered with the local waste disposal authority. New initiatives by the U.K. government may redefine special wastes to include those that could be infectious to humans and other organisms, and that include substances with ecotoxic effects on animals, plants, soil, water, and via the food chain. The European Union directive for landfill (2000) also has significance. This is based on the leachate concentration from the waste, which should be assessed. It is clear that in view of the high variability in sewer sediment composition, both temporally and spatially, any attempts to classify arisings generally are impossible. This may necessitate the taking of samples and analysis prior to disposal of any material removed.

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Raw Material and Energy Raw Material and Energy Raw Material and Energy

217

applicator

Tampon

packaging

disposal

Raw material and energy

Toilet

Incinerator Landfill

Water Drain

Blockages

Labour

Manual Administration

Materials Acid, pressurized water, plunged

Transport Raw Material and Energy

Sewer

Raw Material and Energy

Combined sewer overflow (screen)

Raw Material and Energy

Pumping station

Raw Material and Energy

WTP (with screens)

Combined sewer overflow (no screen) Disposal to sea Ecological effects and aesthetic pollution

Incinerator

Repair and maintenance

WTP (no screens)

Material collected Raw Material and Energy

Vehicles, repairs fuel etc.

Repair and maintenance

Blockages Landfill Land space

Environmental Costs

Leachate Air emissions

Air Emissions

Solid Waste

FIGURE 7.17 Pathways for one sanitary solid item (tampon) used to develop a life cycle analysis. (Adapted from Ashley et al., 1999c.)

Alternatives to landfill disposal include incineration (with subsequent landfill), co-disposal with normal municipal wastes, co-disposal by conveying the solids to a WWTP, and cleaning the solids using hydrocyclones (very prevalent in the Netherlands for dredged wastes). A novel study in France has considered the recovery of sewer and storm inlet sediments for subsequent reuse (Delattre et al., 1998). The term sewerage solids by-products (SSBPs) has been used to promote the view that these solids are potential resources. The study considered specifically the nature and associated quality of the SSBPs at a range of locations in seven cities in southern France, and thence the appropriate degree of treatment for cleaning to an adequate condition for reuse. Table 7.11 shows some of the key characteristics of the sediments and attached pollutants. It is concluded that these solids have low organic and nutrient content, but high levels of metals and hydrocarbons, with wide differences between sites. Overall some 9000 to 13,000 T/year of solids are expected to be removed from sewers and associated infrastructure, including traps. More than 90% of the particles were found to be >0.2 mm in size. Solids treatment using four-stage sieving/washing/classification/dewatering was found to provide effectively clean sand suitable for road construction. Alternative, less rigorous treatment makes the sediment more amenable for landfill disposal. The wash water is highly contaminated and requires careful treatment. The overall costs are claimed to be better than the costs of current disposal options.

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Gross solids sanitary towels, panty liners, tampons, applicators, cotton buds, wipes, condoms etc.

Proposed options • Install 6 mm screens on overflows at WwTW • Run 'Think Before You Flush' )TBYF) public awareness campaigns • Install flow storage in sewer network • Retrofit stormwater source control • Sewer rehabilitation • Retrofit outlet flow constrictors on existing WCs

LCA

WC

InfoWorks

Energy Water

Sewer solids • particles from erosion of roofing material • grit from re-surfacing work • grit from de-icing • solids from construction sites • detritus and litter from road and paved areas • solids washed or blown from surrounding unpaved areas

DWF and rainfall data

Proposed options • do nothing (grit removal as present) • street sweeping and gully pots • settled sewerage LCA • sediment traps • sewer rehabilitation • flushing systems, gates and balls

Material

• Gully pot • Pipes/manholes due to leak or failure

Gross solids and sewer sediment mass balance for proposed options

LCA/SimaPro Raw material Manufacturing Use/Reuse/Recycling Waste Management

Products/ Water effluent/ Co-product Solids waste Air emission

Velocity and depth Sewer systems

FIGURE 7.18 Overall conceptual approach to application of LCA to the management of sewer solids. (From Gouda, H. et al., in Proc. Conf. on Sewer Systems and Processes, Paris, April 2002. With permission.) (Note: Infoworks is the copyright of Wallingford software, U.K.)

TABLE 7.11 Characteristics of Sediments from a Range of Sites from Southern France Location Catchbasins Detention tanks (surface) Streets Combined sewers Sanitary sewers Storm sewers

Mass of Solids Available/year

Organic Content (%)

TOC (g/100 g dry solids)

Zinc (mg/kg dry solids)

60 kg/basin Small <27,000 T — 0.7 T/km —

2–35 4–22 5–7 1–42 3–4 <1

2–23 3–13 2–3 1–15 1–3 <1

10–1480 100–970 410–600 400–600 10–380 <30

Source: Adapted from Delattre et al., 1998.

Other studies have considered the reuse of sand from highway deicing as part of an enhanced management strategy to minimize nonpoint pollution and improve traffic safety (Guo, 1999) and it is clear that conclusions regarding the benefit of doing this are very location specific. Any attempt to reuse materials that are potentially contaminated with metals and other toxic substances requires careful risk and cost assessments, within a multicriteria decision framework, as described above. The inherent uncertainties involved in these processes make it imperative that these are accounted for if the best option is to be selected (e.g., Stansbury et al., 1999). It is clear that there is still a long way to go in understanding the behavior of solids in sewers, and that options for management need to be developed that are robust and deal with whole life and sustainable perspectives. For the time being, the management of these solids requires the application of a variety of techniques, which need to be applied at different points and scales within a sewer catchment (Chebbo et al., 1996).

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Management of Wet Weather Flow Solids Chi-Yuan Fan

CONTENTS Introduction ....................................................................................................................................226 Regulatory Background .................................................................................................................226 Sewer Sediment Solids Origin, Impact, and Control....................................................................227 Sewer Sediment Solids Origin................................................................................................227 Solids from Runoff — Overland Surface Flow ...........................................................228 Sanitary Wastewater Input during Dry Weather Flow..................................................229 Sewer Solids Loading and Transport .....................................................................................229 Estimation of Dry Weather Pollutant Deposition Loading ..........................................229 Hydraulic Characteristics of Sewer Solids ...................................................................230 Impact of Sewer Sediment Solids ..........................................................................................231 Structural Deterioration of Sewer Pipe.........................................................................232 Surface Water Effects....................................................................................................232 Groundwater Effects......................................................................................................233 Control Methods .....................................................................................................................234 Better Design of New Sewers.......................................................................................234 Sewer Flushing ..............................................................................................................235 Treatment of Combined Sewer Overflow......................................................................................236 Background .............................................................................................................................236 High-Rate Treatment Processes..............................................................................................237 Flow Rate Considerations .............................................................................................238 Modify WWTP Treatment Train Mode of Operation ..................................................238 Physical Treatment..................................................................................................................239 Screening .......................................................................................................................239 Enhanced Solid–Liquid Separation...............................................................................240 High-Rate Processes......................................................................................................241 Biological Treatment .....................................................................................................243 Summary .................................................................................................................................245 Management of WWF Solids ........................................................................................................246 Characteristics and Implication ..............................................................................................246 Treatment Processes................................................................................................................247 Dewatering.....................................................................................................................248 Stabilization Processes ..................................................................................................248 Handling and Disposal Alternatives..............................................................................250 References ......................................................................................................................................251

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INTRODUCTION This chapter describes an integrated approach to manage solids generation in urban wet weather flows (WWFs), both in the combined sewer system and at the treatment plant, to better understand the fate and transport of these solids and to prevent their discharge to receiving waters during storm-flow events. The threats to receiving water quality by discharges from wastewater treatment plants (WWTPs) and urban storm-generated WWF (including combined sewer overflow, CSO, as well as polluted runoff from urban catchments) are well known. Recently, researchers reported that sewer sediment deposited from prior storms contributed a significant amount of pollutants to receiving waters (Butler, 1996). In most cases, CSO carries resuspended sewer sediment and generates a highly concentrated pollutant load sometimes associated with the “first flush” phenomenon (Saget et al., 1996; Arthur and Ashley, 1998; Krebs et al., 1999). During low-flow dry weather periods, sanitary wastewater solids deposit in combined sewers because flow velocity is usually below the particle settling velocity. Estimates of solids deposition from dry weather flow (DWF) in combined sewer systems range from 5 to 30% of the daily suspended solids (SS) pollution loading. The average dry period between storm events is about 4 days for many areas of the United States especially along the eastern seaboard. If 25% of the daily pollution loading accumulates in the collection system, an intense rainstorm causing a 2-h CSO, after 4 days of antecedent dry weather, will wash the equivalent of 1-day flow of raw sanitary wastewater to the receiving waters. Furthermore, a 1-day equivalent of raw sanitary wastewater, discharged within a 2-h period, is 12 times the rate at which raw sanitary wastewater enters the collection system. Sewer sediments contain a high pollutant concentration (80,000 mg/L of BOD5; 200,000 mg/L of COD; and 200 mg/L of NH3–N) (Arthur et al., 1996). As storm-flow intensities increase, resuspension of sewer sediment will occur. In combined sewers this occurs when they hydraulically overload and discharge as CSOs. Discharges of sewer sediment particles (also called near-bed sediment) can account for a significant amount of toxic contamination in receiving water sediments. Thus, researchers are starting to pay closer attention to sewer sediment, the major contributor of toxic substances in CSO. The sewer sediment layer contains organic materials and sulfides that can generate toxic, corrosive, and hazardous gases, e.g., hydrogen sulfide and methane, under anoxic conditions. Sulfates are reduced to hydrogen sulfide and then oxidized to sulfuric acid by biochemical transformation; the acid attacks the sewer, thereby weakening its structural integrity. Extensive corrosion of concrete results in cracks and infiltration and exfiltration of raw wastewater causing overflow and WWTP overloading and groundwater contamination, respectively. Thus, control of sewer sediment not only protects urban receiving water quality but also prevents hazardous conditions in sewer systems and maintains the structural integrity of the sewer. One of the challenges in protecting urban watersheds lies in effectively managing contaminated sediments in both the sewer system and the receiving water. To enable urban communities to develop better plans for reducing the risks associated with WWF, research is needed to develop tools for a better understanding and assessment of the fate and transport of sediment solids and associated pollutants. Research is also needed for the development of high-rate processes to treat WWF and to dispose of and/or recover residual solids generated from CSO control measures.

REGULATORY BACKGROUND On April 19, 1994 the U.S. Environmental Protection Agency (U.S. EPA) issued a national “Combined Sewer Overflow (CSO) Control Policy” (U.S. EPA, 1994). This policy establishes a consistent national approach for controlling CSOs to the nation’s waters through the National Pollutant Discharge Elimination System (NPDES) permit program. The main purposes of the CSO Control Policy are to elaborate on the U.S. EPA National CSO Control Strategy (published September 8, 1989) (U.S. EPA, 1989a) and to expedite compliance with the requirements of the Clean Water

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Act (CWA). The CSO Control Policy established four key principles to guide CSO planning decisions by municipalities, NPDES authorities, and water quality standards authorities: 1. Providing clear levels of control that would be presumed to meet appropriate health and environmental objectives. 2. Providing sufficient flexibility to municipalities, especially financially disadvantaged communities, in considering the site-specific nature of CSOs and determining the most costeffective means of reducing pollutants and meeting CWA objectives and requirements. 3. Allowing a phased approach to implementation of CSO controls considering a community’s financial capability. 4. Reviewing and revising, as appropriate, water quality standards and their implementation procedures when developing CSO control plans to reflect the site-specific wet weather impacts of CSOs. Although implementation of the 1989 strategy has resulted in progress toward controlling CSOs, significant public health and water quality risks remain. Under the NPDES program, all permits for CSO should require the nine minimum controls as a minimum best available technology economically achievable and best conventional technology (BAT/BCT) established on a best professional judgment (BPJ) basis by the permitting authority (40 CFR 125.3). The nine minimum controls are as follows (U.S. EPA, 1995a): 1. 2. 3. 4. 5. 6. 7. 8. 9.

Proper operation and regular maintenance programs for the sewer system and the CSOs Maximum use of the collection system for storage Review and modification of pretreatment requirements to assure CSO impacts are minimized Maximization of flow to the publicly owned treatment works (POTWs) for treatment Prohibition of CSOs during dry weather Control of solid and floatable materials in CSOs Pollution prevention Adequate public notification of CSO occurrences and CSO impacts Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls

In May 1995, the U.S. EPA Office of Water published guidance to municipalities regarding implementation of the nine minimum controls without extensive engineering studies or significant construction costs prior to the implementation of long-term control measures (U.S. EPA, 1995b). The technologies described in the guidance are mainly for controlling larger visible materials from CSO, such as trash solid and floatable materials, which do not reduce sewer sediment solids emissions. In 2001, the U.S. EPA identified previous progress made in implementation and enforcement of CSO controls prior to, and as a result of, the 1994 CSO Control Policy (U.S. EPA, 2001). All 32 states (including the District of Columbia) with combined sewer systems have developed CSO strategies, and most have adopted the key provisions of the CSO Control Policy. In spite of the progress that has been made, however, CSOs still present a potentially serious environmental and public health threat in some areas. For long-term control measures in reducing the severity of toxic contaminants and pollution in surface receiving water and receiving water sediment, an integrated sewer sediment solids management approach (i.e., reduction of sewer sediment and high-rate treatment) may be more effective than the conventional means of minimizing CSO impacts and maximizing POTW treatment capacity.

SEWER SEDIMENT SOLIDS ORIGIN, IMPACT, AND CONTROL SEWER SEDIMENT SOLIDS ORIGIN The sediment solids and associated pollutants found in combined sewers result from DWF sanitary wastewater solids deposition and washoff from land surfaces during storm events. A review of the

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sources (Heaney et al., 1999) shows that directly connected impervious areas contribute a high pollutant loading in separate storm sewers. For combined sewers, the largest solids and pollutant loads are likely to originate from sanitary wastewater input during dry weather. Solids from Runoff — Overland Surface Flow The particulates and associated pollutants in urban stormwater come mainly from atmospheric deposition, rooftops, parking lots, and streets/highways. Other sources include construction sites, commercial and industrial parking lots, automobile maintenance operations, leaking sewer infrastructure, accidental spills, and runoff from lawn irrigation. Atmospheric Deposition In the United States, each year over millions of tons of pollutants emit into the troposphere zone of the atmosphere; this has the potential to redeposit in the urban and terrestrial watershed and subsequently be transported downstream to receiving waters. The factors affecting atmospheric deposition include wind speed and direction, dry dust fall, site temperature and precipitation (snow and rainfall), elevation and slope of the land, land use, and sources of air pollution (automobile, industrial, and residential emission). Pollutants in the atmosphere contribute significantly in urban WWF contamination through dustfall and by wash out. As reported by Cotham and Bidleman (1995) and Hilts (1996), enormous amounts of certain toxic pollutants contained in urban storm runoff are associated with atmospheric deposition. Runoff from Roadways, Parking Lots, and Rooftops One of the major sources of pollutants in urban drainage catchments is runoff from urban streets (Sansalon,1996; Sansalone and Buchberger, 1996), highways (Shaheen, 1975; Montrejaud-Vignoles et al., 1996), building rooftops (Sakakibara, 1996; Förster, 1996), and parking areas (Pitt et al., 1995; Nowakowska-Blaszczyk and Zakrzewski, 1996). In some cases, treated wood has been identified as a potential source of arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), and zinc (Zn) in stormwater (Weis and Weis, 1996). Table 8.1 depicts relationships between toxic pollutants and land use. Distributions of heavy metals and hydrocarbons in urban stormwater are associated with their particulate fractions and the relative size of SS. Particles finer than 250 µm contain more heavy metals and total petroleum hydrocarbons (TPHs) than particles larger than 250 µm, and about 70% of the heavy metals are attached to particles finer than 100 µm (Ellis and Revitt, 1982). Vignoles

TABLE 8.1 Toxic Pollutant Concentrations vs. Land Use in France Toxic Pollutant Concentration, µg/L Land Use Residential area Commercial area Industrial area Agriculture Undeveloped land Highway

Cr

Cu

Pb

Zn

TPHs

1.0–2.1 2.5–6.5 7.0 <10 1.5–3.5 3.0

3–8 4–9 15 1.5 1.5–5.5 11.0

2.5–7.5 5–12 — — 1–9 —

11–40 45–130 245 140 1–2.5 60

1700 9000 3000 — — 400

Note: TPHs = total petroleum hydrocarbons. Source: U.S. Geological Survey (1999).

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TABLE 8.2 Particle Size vs. Metal Concentration Metal Concentration,

g/g (% Distribution)

Particle Size Range, m

Cd

Co

Cr

Cu

Mn

Ni

Pb

Zn

>100 50–100 40–50 32–40 20–32 10–20 <10

13(18) 11(11) 11(6) 6(5) 5(5) 6(9) 14(46)

18(9) 16(5) 25(4) 20(6) 18(6) 22(10) 53(60)

21(5) 25(4) 26(2) 50(6) 23(3) 39(9) 134(71)

42(7) 62(8) 57(3) 46(4) 42(4) 81(11) 171(63)

86(8) 59(4) 70(3) 53(3) 54(4) 85(7) 320(71)

31(8) 27(5) 31(4) 31(5) 27(5) 39(10) 99(63)

104(4) 129(4) 181(2) 163(4) 158(5) 247(8) 822(73)

272(5) 419(6) 469(3) 398(5) 331(5) 801(16) 1232(60)

Note: Co = cobalt, Mn = manganese.

and Herremans (1995) examined the heavy metal associations with different particle sizes in stormwater samples from Toulouse, France and discovered that the vast majority of the heavy metal loadings in stormwater were associated with particles less than 10 µm in size. These results are shown in Table 8.2. Snowmelt runoff is greater in volume than typically considered in drainage designs, resulting in greater winter flooding than during the summer; however, there is still a notable lack of information about urban runoff during the winter season (Thorolfsson and Brandt, 1996). Sansalone (1996) investigated the forms of stormwater and snowmelt heavy metals and reported that Zn, Cd, and Cu were mainly dissolved in stormwater, whereas only Cd was mainly dissolved in snowmelt. Sanitary Wastewater Input during Dry Weather Flow Solids originating from sanitary wastewater sources can be categorized into three types (see Chapter 7): 1. Fine fecal and other organic particles (sanitary solids) 2. Large fecal and other organic matter (gross and kitchen waste solids) 3. Paper, rags, and miscellaneous sewage litter (sanitary refuse) Categories 2 and 3 are most likely to be settled in the sewer during daily low flow period. Chapter 7, Management of Sewer Sediments, addresses detailed analyses of fate and transport of the sewer solid.

SEWER SOLIDS LOADING

AND

TRANSPORT

This section reviews past and current research efforts by the U.S. EPA and others on sewer solids loading and the solid hydraulic transport processes. Estimation of Dry Weather Pollutant Deposition Loading Development of sewer sediment control alternatives requires a good prediction of the location and quantity of deposited solids that build up during the DWF period. Based on regression analysis of field data, Pisano and Queiroz (1977; 1984) developed three empirical models (i.e., Simplest, Intermediate, and Elaborate) for estimating total solids (TS) deposition loading in combined sewer systems. The purposes of these “simplest,” “intermediate,” and “elaborate” models are, respectively,

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for assessment, analysis, and planning of sewer sediment control alternatives. Five independent variables were tested in the regression analyses, including service area (A), sewer diameter (D), sewer length (L), per capita flow rate (Q), and sewer slope (S). The Elaborate Model represents the highest multiple-correlation coefficient value (R2  0.90), and the R2 is  0.85 for the Simplest Model. Models calibrated with field data collected from Boston and Fitchburg, Massachusetts and Cleveland, Ohio are as follows: Fitchburg, MA (Pisano and Queiroz, 1977):

(

)(

)(

)

(

)(

)(

)(

(

)(

)(

[R ) [R

)

[R

Simplest Model:

TS = 0.0076 L1.063 S −0.436 Q −0.51

Intermediate Model:

TS = 0.0013 L1.18 D 0.604 A −0.178 S −0.418 Q −0.51

Elaborate Model:

− 0.819 − 0.108 TS = 0.0038 L0.814 SPD SPD Q −0.51 4

)(

)(

2

2

2

] = 0.85] = 0.85

]

= 0.95

Cleveland, OH (Pisano and Queiroz, 1984):

(

)(

)(

)

Simplest Model:

TS = 0.0088 L1.065 S −0.433 Q −0.539

Elaborate Model:

−0.519 −0.148 TS = 0.00108 L0.948 S −0.323 SPD SPD Q −0.518 4

where A D L LPD Q S SPD SPD/4 TS

(

)(

)(

)(

)(

)

[R

2

= 0.88

]

[R

2

= 0.94

]

= service area of collection sewer system (acres) = average sewer diameter (in.) = sewer length (ft) = sewer length corresponding to 80% of the solids deposited in the sewer system (ft) = flow rate per capita, including allowance for infiltration (gpcd) = average sewer slope (ft/ft) = sewer slope corresponding to LPD, (ft/ft) = sewer slope corresponding to 1/4 of the percentage of sewer length (LPD) below which 80% of the solids deposit (ft/ft) = daily total wastewater solids deposition loading in collection system (lb/day)

As shown, all R2 values of these regression models are ≥0.85. The differences of R2 values between Boston and Cleveland are <5% for the Simplest Model and <1% for the Elaborate Model. Hydraulic Characteristics of Sewer Solids Sewer-flow-carrying velocities for solid-phase matter were first evaluated at a sewer line pilot system in 1967 (FMC, 1967a). Results indicated that the flow velocity for developing incipient motion of the settled solids was much greater than the solids settling velocity (i.e., 0.44 m/s, or 1.44 ft/s, vs. 0.27 m/s, or 0.88 ft/s). Microscopic examination of the sewer sediment samples collected from the test pilot sewer line found that sandlike particles with a size range of 40 to 900 µm had a specific gravity range of 2.4 to 2.6. These particles required a minimum flow velocity of 1 m/s (3 ft/s) to resuspend sediment from the bottom of the test sewer line. Another investigation was conducted to determine the settling characteristics (including size and specific gravity distribution) of solids in sanitary wastewater, CSO, and stormwater runoff (Dalrymple et al., 1975). The settling velocity distributions, for road dust (10 to 20 µm) and wastewater solids (74 to 149 µm), appeared to follow Stokes’ law for spherical particles at these size ranges.

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According to Stokes, for Reynolds numbers (R) < 1, the settling velocity (Vs) can be expressed as R=

Vs = where Vs = D = γs = γw = µ =

Vs D µ

(8.1)

D2 ( γ s − γ w )

(8.2)

18 µ

settling velocity particle diameter specific gravity of particle specific gravity of fluid fluid viscosity

Sonnen (1977) studied solids-associated pollutant transport in sewer systems and evaluated alternative methods for alleviating sewer sediment problems through modifications of the quality model of the U.S. EPA Storm Water Management Model (SWMM). Four subroutines were built into the quality model program for simulating sewer solids deposition (SETVEL), scour, and transport processes (SCRDEP and SCROUT), and estimating the costs and performance of the sediment control and treatment facilities (TREAT). A wide range of R values were used in the SETVEL subroutine for different flow conditions, i.e., turbulent, transitional, or laminar (Stokes) flows, to determine settling velocity (Sonnen, 1977). The general equation used in the subroutine is  4 gD ( γ s − γ w )  Vs =    3 CD γ w  where Vs D γs γw CD g

= = = = = =

12

(8.3)

settling velocity particle diameter specific gravity of particle specific gravity of fluid drag coefficient acceleration due to gravity

For R > 3000, a CD value of 0.4 is assumed to start computation. If the calculated R is < 3000, turbulent conditions do not exist. The value of CD is recomputed by using the following equation: CD =

24 3 + + 0.34 R R

(8.4)

This leads to a new value of Vs. By using Newton’s method of iterative process with new values of R and CD, Equation 8.4 will eventually converge to a final solution.

IMPACT

OF

SEWER SEDIMENT SOLIDS

In general, sewers will not maintain self-cleansing velocities at all times. The diurnal pattern of DWF and the temporal distribution and nature of sediments found in sewer flows may result in the

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deposition of some “juvenile” sediments at times of low flow. The subsequent erosion and transport of these sediments at times of higher flow during a storm flow event, either as suspended load or bed load, contribute to the first-flush phenomena or polluted segment in CSO (Saget et al., 1996; Arthur and Ashley, 1998; Krebs et al., 1999). During low-flow dry weather periods, sanitary wastewater solids deposited in combined sewer systems can generate hydrogen sulfide and methane gases due to the anaerobic conditions. Sulfates are reduced to hydrogen sulfide gas that can then be oxidized to sulfuric acid on pipes and structure walls. Furthermore, these sediments are discharged to urban streams during storm flow events and can cause degradation of receiving water quality. Thus, dry weather sewer sedimentation not only creates hazardous conditions and sewer degradation but also contributes significant pollutant loads to the urban receiving waters during wet weather high-flow periods. Furthermore, broken sewer lines cause direct exfiltration of raw sanitary wastewater and sewer sediment leachate into subsurface groundwaters. Structural Deterioration of Sewer Pipe The primary cause of odor and corrosion in collection systems is the sulfide ion (S–), which is produced from sulfate (SO4– ) by biochemical transformation processes in a slime layer on the submerged portion of sewer pipes and structures. Once S– is released from the wastewater as hydrogen sulfide (H2S) gas, odor and corrosion problems begin. Bacteria utilize H2S gas and produce sulfuric acid (H2SO4) (Speller, 1951; Thistlethwayte,1972; Sawyer et al., 1994). For sanitary wastewater the main source of S– is SO4–. Sulfide generation is a bacterially mediated process occurring in the submerged portion of combined and sanitary sewers and force mains. Fresh sanitary wastewater entering a collection system is usually free of S–. However, a dissolved form of S– soon appears as a result of low dissolved oxygen content; high-strength wastewater; low flow velocity; long detention time in the collection system; elevated wastewater temperature; and extensive pumping (U.S. EPA, 1985). The effect of H2SO4 on concrete surfaces in the sewer environment can be devastating. Sections of collection interceptors and entire pump stations have been known to collapse because of loss of structural stability from corrosion. In severe instances, pipe failure, disruption of service, street surface cave ins, and uncontrolled releases of wastewater to surface streams and/or groundwater can occur. Surface Water Effects WWF causes 40 to 80% of the total annual organic loading entering receiving waters from a city. During a single storm event, WWF accounts for about 95% of the organic load as well as high loads of heavy metals and petroleum hydrocarbons (Field and Turkeltaub, 1981). CSO can have damaging impacts on receiving waters. The U.S. EPA evaluated the distribution and biological impacts of discharged particulates for selected CSO and storm drain points in the Seattle, Washington region (Tomlinson et al., 1980). The concentrations of SS and associated heavy metals and chlorinated hydrocarbons were greater for the CSO than for the storm drains. Particulate distributions were influenced by various dispersion processes, including water density layering, near-bottom offshore streaming, and advection along the shoreline. Human enteric viruses were also detected in the CSO, but were not found in storm drainage or in any near-outfall sediments. However, impacts of discharges on the freshwater benthos raised concern relative to the feeding success of sport fish due to polluted sediments. Saul et al. (1999) investigated the production of undesirable solids in CSO as it related to social, economic, and ethnic factors. The goals of the research were first to determine the differences in sewer solids characteristics that were ultimately discharged to the receiving water and then to use the characteristics of the solids to predict the efficiency of CSO treatment devices, especially CSO storage basins. St. Michelbach and Brombach (1999) showed that the nutrient content, especially

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of dissolved phosphorus, from CSO and existing WWTPs was endangering the health of Lake Constance. They proposed a simple methodology to estimate the nutrient loads from CSO to the lake, the results of which can be used to determine the cost-effectiveness of CSO improvement vs. WWTP improvement. Sanudo-Wilhelmy and Gill (1999) compared current pollutant concentrations in the Hudson River Estuary, New York with concentrations measured in the 1970s. The concentrations of Cu, Cd, nickel (Ni), and Zn have declined, while concentrations of dissolved nutrients (namely, PO4) have remained relatively constant during the same period of time. This suggests that WWTP improvements in the New York/New Jersey metropolitan area have not been as effective at reducing nutrient levels within the estuary as they have been in reducing heavy metals. Rather than inputs from point sources, the release of lead (Pb) and mercury (Hg) from watershed soils, and Ni and Cu from estuarine sediments, may represent the primary contemporary sources of these metals to the estuary. Mason et al. (1999) showed that the Chesapeake Bay was an efficient trap for Hg. However, in the estuary, methylation of the Hg occurred, the bay became a source of methylmercury, and on a watershed scale, only about 5% of the total atmospheric deposition of Hg was exported to the ocean. Venkatesan et al. (1999) investigated the potential for using sediment cores to determine the history of chlorinated pesticide and PCB application in a watershed. They found that the sediment cores accurately reflected the length of chemical use in the watershed, and that the surface sediment layer, after mixing and resuspension were accounted for, reflected the reduction in use that had occurred during the last few years. The long-term impacts of WWF toxic pollutants to stream habitat are dependent on bioavailability and accumulation of the substances by aquatic life. Herrmann et al. (1999) found that the concentration of ammonia plus urea in CSO was found to be a significant measure of a fish kill after an overflow event, more relevant than the concentration of ammonia alone. Groundwater Effects In 1999, the U.S. EPA conducted a nationwide study to quantify leakage of sanitary and industrial wastewater sewer systems based on groundwater table elevations. The study indicated low levels of wastewater exfiltration (less than groundwater infiltration) in much of the midwestern and eastern parts of the United States due to relatively high groundwater tables. However, problems of exfiltration in the western United States seem more widespread because of lower groundwater tables (U.S. EPA, 2000). Thus, under high groundwater level conditions, contamination of subsurface zones in the vicinity of a leaking sewer may not occur due to high-rate infiltration of groundwater into sanitary sewers than exfiltration of raw sewage into soil. However, groundwater contamination is likely to be more severe at locations where groundwater fluctuates. Possible groundwater contamination, resulting from sewers that have collapsed or catastrophically failed and from sewers which are believed to suffer from long-term deterioration, has been noted in groundwater contamination studies (U.S. EPA, 1989b). In those areas with a shallow depth of wells and a high permeability of soil, any surface contamination could easily migrate to the groundwater. Thus, a significant amount of groundwater contamination is as attributable to surface runoff as to leaky sewer exfiltration. Squillace et al. (1996) and Zogorski et al. (1996) investigated urban stormwater as a source of groundwater MTBE contamination. Mull (1996) stated that traffic areas are the third most important source of groundwater contamination in Germany (after abandoned industrial sites and leaky sewers). The most important contaminants are chlorinated hydrocarbons, sulfates, organic compounds, and nitrates. Heavy metals are generally not an important groundwater contaminant because of their affinity for soils. Trauth and Xanthopoulus (1996) examined the long-term trends in groundwater quality in Karlsruhe, Germany. Results indicated that the urban land use could cause a long-term adverse influence on the groundwater quality. The concentration of many pollutants has increased by about

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30 to 40% over 20 years. In Dortmund, Germany, an infiltration trench for stormwater disposal caused Zn problems that were associated with the low pH value (about 4) in the infiltration water (Hütter and Remmler, 1996).

CONTROL METHODS Management of solids to prevent their entering a sewer system is usually the most cost-effective way of reducing sewer sediment solids. Source control methods include application of best management practices (BMPs), such as improved street cleaning, improved street inlet design and operation, and other nonstructural BMPs. Elimination of kitchen garbage grinders and segregation of sanitary items from toilets can be more cost-effective than control of SS, floatable solids, and debris at CSO points. Evaluation of BMP cost-effectiveness is covered elsewhere. The following sections briefly discuss three methods to better manage sewer sediment solids: better design of new sewers, sediment flushing, and sediment traps. Better Design of New Sewers Although sewers are designed to be self-cleansing, this is not always achieved. There are methods being proposed in the U.K. that improve the design of sewers to better achieve self-cleansing flow. To meet this criterion, a modified design approach has been developed by the U.K. Construction Industry Research and Information Association (CIRIA) for achieving self-cleansing design velocity (Ackers et al., 1996). The CIRIA design guidelines include criteria for the transport of fine-grained material in suspension, the transport of coarser sediments as near-bed solids, and the erosion of cohesive sediment deposits as well as guidelines on the minimum-flow velocity and pipe gradient for different types and sizes of sewer. An important parameter to assure self-cleansing is average shear stress (τ), which is the amount of force the fluid exerts on the wetted perimeter of the pipe. It can be expressed as τ = γRS where τ = γ = R = S =

average shear stress, N/m2 unit weight of water, N/m3 hydraulic radius, m sewer slope, m/m

The hydraulic radius (R) is the cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing sewer (or wetted perimeter). The range of minimum shear stress criteria is 1.3 to 12.6 N/m2. For example, results of laboratory testing indicated that the design flow condition should produce a minimum value of bed-shear stress of 2.0 N/m2 (Ackers et al., 1996). Another important parameter is bed-shear stress, which is the amount of force the fluid exerts on the bed of sediment in the pipe. Bed-shear stress is related to bed load resuspension and movement. The deposited sediment will exhibit additional strength due to cohesion. If the peak DWF velocity or bed-shear stress is of sufficient magnitude to erode these sediments, the sewer will maintain self-cleansing operation at times of DWF. If this condition is not satisfied, then longterm “mature” sediment beds will form that will not ordinarily be resuspended and will be removed only during occasional periods of extreme flow conditions. May (1993) defined an efficient selfcleansing sewer as “one having a sediment-transporting capacity that is sufficient to maintain a balance between the amounts of deposition and erosion, with a time-averaged depth of sediment

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deposit that minimizes the combined costs of construction, operation and maintenance.” To achieve such self-cleansing performance, these criteria apply: 1. Flows equaling or exceeding a limit appropriate to the sewer must have the capacity to transport a minimum concentration of fine-grained particles in suspension (applicable for all types of sewerage systems). 2. The capacity of flows to transport coarser granular material such as bed load should be sufficient to limit the depth of deposition to a specified proportion of the pipe diameter. This criterion generally relates to combined and stormwater systems. Limit of deposition generally applies to sanitary sewer designs. In this context, shear forces in sanitary systems must be sufficient to avoid deposition of large particles. 3. Flows with a specified frequency of occurrence must have the ability to erode bed particles from a deposited granular bed that may have developed a certain degree of cohesive strength (applicable to all systems). Sewer Flushing Flushing of sewers has been a concern dating back to the time of the Roman Empire. Ogden (1898) described early historical efforts for cleaning sewers in Syracuse, New York at the turn of the century. The concept of sewer flushing is to induce an unsteady wave by either rapidly adding external water or creating a “dambreak” effect by the quick opening of a restraining gate. The aim is to resuspend, scour, and transport deposited pollutants to the WWTP during DWF periods or to displace solids deposited in the upper reaches of large collection systems closer to the system outlet. This control method is either to reduce depositing pollutants that may be resuspended and overflow during wet events and/or to decrease the time of solids transport within the collection system. During wet-weather events these accumulated loads may then be more quickly displaced to the treatment head works before overflows occur or be more efficiently captured by wet weather firstflush capture storage facilities. In 1966, the U.S. EPA predecessor agency initiated a series of research projects to demonstrate the feasibility of periodic flushing during dry weather. The first phase of work was performed by FMC Corporation at its Central Engineering Laboratories in Santa Clara, California to determine the feasibility of a periodic flushing system for combined sewer cleaning (FMC, 1967b). The study included a demonstration of the flushing concept, small-scale hydraulic modeling, and design and development of cost estimates for constructing test equipment (FMC, 1967b; 1972). In 1974, the Boston Metropolitan District Commission initiated a combined sewer management study for assessing alternative strategies for abating CSO discharges to portions of Boston Harbor, Massachusetts. During the study a number of equations based on the critical fluid shear stress theory for estimation of dry weather deposition and flushing criteria were developed. Although the model was crude, the agreement with visual-field observations was reasonable. The model was then used to analyze deposition problem segments within a service area of 1200 ha (2965 acres) entailing roughly 152,500 m (500,350 ft) of sewer. Roughly 3000 manhole-to-manhole segments were analyzed for deposition, and it was determined that roughly 17% of the segments contained about 75% of the estimated dry weather wastewater deposition. Most of these segments were small-diameter CS laterals. Flushing criteria were empirically developed using data generated during the earlier FMC research to estimate required flushing volumes (Process Research, Inc., 1976). In 1979, a 3-year research and development program sponsored by the U.S. EPA was conducted in Boston to determine the pollution-reduction potential of flushing combined sewer laterals using flush water from a water tanker. It was concluded that small-volume flushing would transport organics/nutrients and heavy metals sufficient distances [>310 m, or >1,000 ft) to make the sewer flushing option feasible and attractive (Pisano et al., 1979). Kaufman and Lai (1978; 1980) also

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reported that application of sewer flushing in the heavily solid deposited sewer line during dry weather periods to alleviate the WWF first-flush effect. Furthermore, the flushing method used in conjunction with additional structural control (e.g., storage facility) has proved to be a cost-effective method of urban runoff pollution abatement. During the last two decades over 13,000 CSO storage facilities have been constructed in Germany with over 500 comprising in-line storage in sewers ranging from 1.8 m (72 in.) to 2.1 m (84 in.) in diameter with lengths of 125 m (400 ft) to 180 m (600 ft). Discharge throttles control the outlet discharge to about twice the average DWF plus infiltration. Many different methods for cleaning these pipes have been tried over the years. The most popular has been the flushing gate (FG) system developed in Germany (Pisano et al., 1997). The FG system has been used to clean settled debris and sediment in sewers, interceptors, tunnels, and retention and detention tanks in Germany and Switzerland. This technology was first used in 1986 for cleaning a tank in Bad Marienberg (a small town with a population <10,000 people, about 100 km, or 62 mi, northeast of Frankfurt, Germany). In that same year the first two pipe storage projects using the FG technology were implemented. As of 1995, in Europe there are 284 installations with over 600 units in operation. Approximately 37% of the projects are designed to flush sewers, interceptors, and tunnels ranging from 0.25 m (10 in.) to 4.3 m (14 ft) in diameter and flushing lengths of up to 340 m (1120 ft) for large-diameter pipes. The balance of FG installations are for cleaning sediments from CSO tanks. The largest tank project is in Paris, France for an underground 120,000 m3 (32 Mgal) tank using 43 FGs. In the City of Cambridge, Massachusetts, grit deposition and debris accumulations were severe in the old system of combined sewers, storm drains, and sanitary trunk sewers due to the flat topography of the area. That condition was exacerbated by hydraulic constraints imposed on the system’s outlet by the Alewife Brook (shallow stream) and downstream sanitary siphons (again, because of the Alewife Brook). The use of pumping systems to lift flows from sewers and drains to permit self-scouring velocities would be prohibitively expensive. Accordingly, a series of passive automatic flushing systems along with grit control structures were constructed within the northeast portion of the City of Cambridge separated storm and sanitary sewer system tributary to the Alewife Brook (Pisano et al., 2000). The automated flushing systems use quick opening (hydraulic operated) gates, discharging collected stormwater in conjunction with downstream collector grit pits at four locations covering a distance of 1220 m for storm sewer. The new 1000-m sanitary trunk sewer line would be flushed daily using spent filtrate water from the City of Cambridge’s new water treatment plant located in the same vicinity.

TREATMENT OF COMBINED SEWER OVERFLOW BACKGROUND In accordance with the “Combined Sewer Overflow (CSO) — Guidance for Nine Minimum Controls” (U.S. EPA, 1995a), the U.S. EPA encourages POTWs to retrofit existing sewerage systems and encourages treatment facilities to convey and treat additional WWF by maximizing treatment hydraulic capacity with high-rate processes. Furthermore, the POTWs are also required to control coarse solid and floatable materials in CSOs. As a result, a tremendous amount of solids will be generated from the CSO control measures. Thus, development and evaluation of high-rate processes for WWF treatment and innovative technologies for the management of sewer sediment solids are two essential components of an integrated approach to urban WWF solids management. This section overviews a series of high-rate treatment systems that have been evaluated for treating WWF. CSO pollution can be controlled at its source by upland management; by storing flows during storms for subsequent treatment; by treatment, e.g., increasing the POTW capacity to provide treatment during a storm or satellite treatment at the overflow location; or by combinations of these methods. In a combined sewer system (CSS) catchment, no one method will provide the best

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solution; thus, this section addresses treatment system options to reduce pollutant mass loadings generated by CSOs. Under the U.S. EPA National CSO Control Policy for maximizing flow to the POTW, the WWF requires primary clarification (or its equivalent), with disinfection where necessary. To accomplish this task, several treatment plant parameters must be evaluated, including the ability of the existing treatment facilities to adequately handle increased hydraulic loads generated during rainstorm events and to process limitations that may result from the increased loading rates. The overall objective is to reduce the total pollutant load going into the stream from the complete CSS. Federal and state effluent standards need to be met regardless of the POTW influent flow rate. Construction of new end-of-pipe WWF treatment facilities without upstream storage is usually the most expensive and least desirable control alternative. Also, it is relatively difficult to treat WWF using conventional methods due to the adverse, intense, and intermittent flow conditions and unpredictable shock-loading effects. These adverse conditions are particularly detrimental to microorganism-dependent biological processes. Physicochemical treatment techniques have shown more promise than biological processes in overcoming storm shock-loading effects, but even these processes require modifications and demonstration of their effectiveness for WWF (Field, 1990). More research concerning the benefit–cost of WWF control treatment is needed. Many municipalities question whether the benefits of the program justify the costs. What would help in convincing skeptical municipalities would be “monetized” estimates of the benefits of the cleaner water stemming from the WWF pollution abatement program in comparison to the cost of the program. Further, because costs are extremely important in selecting the best option to improve water quality, not only will the efficiency of the treatment controls proposed be evaluated, but also the percentage of total stormwater runoff treated and the percentage of stormwater runoff bypassed. Providing this information will help in determining the cost–benefits of the proposed WWF treatment systems.

HIGH-RATE TREATMENT PROCESSES To reduce capital investments needed to retrofit and enlarge the existing WWTP as well as construct new ones, the search for suitable WWF treatment technologies is directed toward high-rate operations that can handle maximum loadings. A variety of high-rate treatment methods show a potential to handle WWF. A majority of them need to be demonstrated at full scale, including the following: • Physical separation with and without chemical addition (e.g., enhanced settling, finemesh screening, filtration, dissolved air flotation (DAF), activated carbon, continuous deflection separator, high-gradient magnetic separation) (Nebolsine et al., 1972; Maher, 1974; Gupta et al., 1977a; Drehwing et al., 1979; Innerfield et al., 1979; Meinhol et al., 1979; Shelley et al., 1981; Wong, 1997). • Biological processes (activated sludge, aerated lagoons, trickling filters, biological aerated filter, rotating biological contactor) (Homack et al., 1973; Welsh and Stucky, 1974; Agnew et al., 1975; Field and O’Connor, 1997). All treatment processes, or their combinations, can be adjuncts to the existing WWTP or can serve as remote satellite facilities at overflow points (upstream of the outfall). It may be costeffective and practical to use WWF treatment technologies at the source of pollution upstream or upland to prevent the pollutants from entering the drainage system and causing additional burden to the WWTP. An example of such an approach is upstream treatment of stormwater runoff from critical source areas, such as parking lots, storage areas, and especially vehicular service stations. As a final point, there may be alternatives to the conventional design of combined sanitary and storm sewerage systems, such as household and business plumbing to minimize WWF more costeffectively and to address new uses and potentially different configurations to the standard sewerage systems practiced throughout the United States today.

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Flow Rate Considerations Wastewater treatment plants are typically designed to treat municipal wastewater during DWF. Difficulties occur when WWF enters the plant due to higher flow rates and differing flow characteristics. Differences between WWF and DWF that should be considered in determining the applicability of WWTP design standards to WWF include (1) WWF is intermittent, where DWF is relatively constant; (2) CSO consists of particles from stormwater in addition to those from municipal wastewater, and these particles have different densities, sizes, toxicity, and biodegradability; and (3) WWF contains nonenteric microorganisms that cannot be detected by using the bacterial indicators of enteric origin used for analyzing municipal wastewater (Field and Struzeski, 1972). The WWTP design average flow rate is commonly based on the average flow during some maximum significant period of 4 to 16 h, depending on the circumstance. Peak design rate, usually two to four times the design average DWF, is used for hydraulic sizing to accommodate peak diurnal DWF. In New Providence, New Jersey, a trickling filter biological treatment plant was operated at an average DWF of 0.5 MGD into peak rate of 6.0 MGD during wet weather events (Homack et al., 1973). Modify WWTP Treatment Train Mode of Operation It is very important to maintain flexibility when operating any WWTP. This is especially true to accommodate WWFs. Influent flow controls should not be preset but should be adjustable in regulating hydraulic flow through the plant. During dry weather, all of the wastewater must enter the plant and receive treatment. During wet weather, when flow rate varies, control settings must change to allow the maximum treatment without upsetting any unit processes. This flexible operation of the controls is necessary because of the varying wastewater flow rates in the sewer system during a storm event. A modification useful during storms is regulating excess flow to different treatment unit processes to increase the hydraulic capacity of the treatment facility. An example of this mode of operation can be found at the Clatskanie, Oregon wastewater treatment facility. During dry weather and other low-flow conditions, the facility operates up to 0.5 MGD in a primary clarification and activated sludge mode. For higher flows, the facility can be run in a contact stabilization mode with the primary clarifier operating in the DAF mode. These two dual-use processes enable the city to increase flow through its facility by six to ten times the DWF (Benedict and Roelfs, 1981). Another possible operating mode, which can be considered if any of the treatment unit processes are severely upset as a result of the hydraulic overloading of the treatment facility, is a split-flow mode of operation. This operating mode involves the treatment of wastewaters by normal treatment unit processes up to the maximum capacity, while rerouting flows above this capacity to other unit processes. An example of this can be found at the Van Lare WWTP in Rochester, New York. During wet weather periods, the present process capacity of 100 MGD is often exceeded, thereby resulting in upsets of certain biological unit processes. Under the split-flow mode of operation, a maximum flow rate of 100 MGD is passed through the biological unit processes for treatment, while the amount of wastewater flows exceeding this amount (100 MGD) receives full primary settling and disinfection (Drehwing et al., 1979; Murphy et al., 1982). This mode of operation offers a number of advantages, namely, overall improvement in annual facility performance at higher flow rates, increased savings in operating and maintenance costs, and reduction in sludge age with resultant better dewaterability. Excess flows normally should as a minimum receive primary treatment and disinfection. Use of a dual media high-rate filtration system for treating WWF while polishing final effluent during dry weather has been evaluated at the City of New York WWTP (Innerfeld et al., 1979). Results indicated that the DMHR filter would be able to provide relative uniform final effluent throughout the year.

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PHYSICAL TREATMENT Physical treatment unit operations form the basic and most often used treatment processes for the treatment of CSO to separate solids from liquid, including screens, sedimentation, flotation, and filtration. These are distinguished from biological treatment methods. Efficiency of these processes may be enhanced with coagulation and coagulation-aid chemicals, as addressed below in “Enhanced Solid–Liquid Separation.” Screening Screens have been used to achieve various levels of SS removal. There are three screening process applications: 1. Pretreatment, where screening is used to remove floatable and coarse materials prior to protect downstream equipment 2. Main treatment, where screening is used as a preliminary treatment process to remove suspended solids before the secondary treatment processes 3. Dual treatment, where screening provides pretreatment of either DWF or WWF high-rate treatment processes, such as dissolved-air flotation and dual-media high-rate filtration. Screen units are divided into four categories: 1. 2. 3. 4.

Bar screens with openings greater than 1 in. (>2.54 mm) Coarse screens with openings from 0.1875 to 1.0 in. (4.76 to 25.4 mm) Fine screens with openings from 0.004 to 0.185 in. (0.105 to 4.76 mm) Microscreens with openings less than 0.004 in. (<0.105 mm or 105 µm)

No special studies have been made to evaluate bar and coarse screens in relation to CSO, so the basis for their design should be the same as for their uses in dry weather treatment facilities. Because CSO contains a significant amount of coarse debris, which is aesthetically undesirable, use of coarse screens as the minimum CSO treatment may be useful. Fine screens and microscreens (also known as microstrainer) are discussed together because in most cases they operate in a similar manner. Several distinct types of screening devices have been developed and tested for floatables/SS removal from CSO, including vibratory fine screen, rotary drum screen, and microstrainer. Screens in conjunction with dissolved-air flotation and dual-media high-rate filtration were evaluated for the treatment of CSOs at various technology demonstration sites (U.S. EPA, 1970a; 1972; Glover and Herbert, 1973; Bursztynsky et al., 1975; Gupta et al., 1977a; Meinhol et al., 1979). Floatables and SS removal efficiencies are affected by two mechanisms: (1) straining by the screen and (2) filtering of smaller particles by the mat deposited during the straining. Removal efficiency of screening devices is adjustable by changing the aperture size of the screen making these devices very versatile. The efficiencies of screens treating wastewater with a typical distribution of particle sizes will increase as the size of screen opening decreases. Fine-mesh screens and microstrainers remove from 25 to 90% of the SS and 10 to 70% of the BOD5, depending on the size of screens used and the type of wastewater being treated. At the Philadelphia demonstration, polyelectrolyte addition (0.25 to 1.5 mg/L) improved the operating efficiency of the microstrainer. Removal of SS increased from 70 to 78%, and the average effluent SS was reduced from 40 to 29 mg/L. The flux also increased from an average of 23 to 39 gpm/ft2. After an extensive laboratory coagulation study, moderately charged, high-molecular-weight cationic polyelectrolytes were found to be the most suitable for screening applications (Glover and Herbert, 1973).

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Enhanced Solid–Liquid Separation Conceptually, the simplest solid–liquid separation process is sedimentation, or gravity settling. Sedimentation is a process in which solid particles settle by gravity from water in a relatively quiescent flow field, with the output streams a sludge and a decantable supernatant liquid (i.e., effluent). Conventional sedimentation in a settling unit is commonly used for grit removal, particulate matter removal in the primary settling tank, and mixed liquid–solids removal in the secondary settling tank. Primary settling tanks are used to remove the readily settleable solids before subsequent treatment. In the design of a sedimentation process unit, the settling velocities are based on settling column measurements. However, because of the complexity of particle interaction and statistical errors in the measurements, engineers use simplified approximation values in terms of overflow rate for unit design. For domestic wastewater, primary settling tanks are usually designed for hydraulic loading/overflow rates of 800 to 1000 gpd/ft2 to obtain 50 to 60% SS removal. When combined with chemical treatment and flocculation, the sedimentation unit can increase the removal of 80 to 95% SS with higher overflow rates up to 1600 gpd/ft2. Sedimentation with Chemicals Addition Sedimentation can be enhanced by the addition of chemicals prior to settling. These treatment processes usually include coagulation, flocculation, sedimentation, and filtration. Coagulation (or destabilization of colloidal solids) is achieved by addition of coagulants (alum, ferric sulfate, or ferric chloride) and coagulation aids (i.e., polyelectrolyte) rapidly mixed with the water. Small floc particles form; further slow stirring allows the floc particles to agglomerate into larger particles in the process known as flocculation. The chemical enhances the settleability of the flocculated materials; however, further removal of floc particles by filtration is commonly used. Settling can also be enhanced using inclined settling plates. Inclined Plates/Tubes Settler SS settling can be significantly enhanced with the installation of packs of inclined parallel plates in sedimentation tanks. It consists of a pack or packs of parallel plates placed at 2-in. intervals and inclined at 45 and 55o angles from the horizontal. The inclined plate settler has been widely accepted for industrial and domestic wastewater treatment (Shiragami et al., 1988). The popularity of the process unit is mainly due to its capability of high removal efficiency, so that higher overflow rates could be used resulting in a smaller sedimentation basin (Demir, 1995; Daligault et al., 1999). Thus, significantly less space is required for this unit. Dissolved Air Flotation The primary flotation system used in the treatment of CSO is DAF. The DAF system involves saturating either a portion or all of the wastewater feed (or recycled flow) and pressurizing it (30 to 70 psi) in a tank. The pressurized effluent is then mixed with incoming CSO and subsequently released into a flotation tank. The excess dissolved air then separates from the solution, now under atmospheric pressure, and the minute rising gas bubbles attach themselves to particles in the CSO. Due to entrained air, the CSO particles have a greatly increased vertical rise rate (about 4.0 gpm/ft2 as compared to 0.3 gpm/ft2 without DAF). The floated materials rise to the surface to form a froth layer that is continuously skimmed off by specially designed flight scrapers. The retention time in the flotation chambers is usually about 20 to 60 min. The effectiveness of DAF depends upon the attachment of bubbles to the particles, which are removed from the CSO. The attraction between the air bubble and particle is primarily a result of the particle surface charges and bubble-size distribution. Polyelectrolytes are frequently used as flotation aids to enhance DAF performance and to create a thicker layer of sludge. DAF has been used for many years by industry to treat oily wastewaters and to thicken sludge, but it is not widely used to treat municipal wastewater and CSO. In the 1970s, the U.S. EPA extensively investigated the DAF system for treating CSO (U.S. EPA, 1970c; 1972; Bursztynsky

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et al., 1975; Gupta et al., 1977a; Meinhol et al., 1979). In 1970, a 5 MGD screening DAF demonstration facility was constructed in Milwaukee, Wisconsin to evaluate the effectiveness of the system for treating CSO (U.S. EPA, 1970c). The process units of the treatment system included a drum screen (50 mesh; 297 µm openings), mixing tank, and flotation and chlorine contact tanks. The design hydraulic loading for the screen was 50 gpm/ft2, and the drum rotation speed varied from 0.5 to 5.0 rpm. The treated effluent was discharged to the Menomonee River. The average SS, COD, BOD5, and Kjeldahl nitrogen (TKN) removal efficiencies for the combined screening–DAF system were 43, 41, 35, and 29% without chemicals and 71, 57, 60, and 24% with chemicals, respectively (Gupta et al., 1977a). This process has a definite advantage over gravity sedimentation when used on CSO since particles with densities both higher and lower than the liquid can be removed in one operation. DAF also aids in the removal of oil and grease, which are not as readily removed during sedimentation. The principal parameters that affect removal efficiencies are (1) overflow rate, (2) amount of air dissolved in the flows, and (3) chemical addition. Chemical addition has been used to improve removals; ferric chloride has been the chemical most commonly added. High-Rate Processes Dual-Media High-Rate Filtration Dual-media high-rate filtration (DMHRF) has been used for removing finer particles in industrial water supply and wastewater treatment. The U.S. EPA investigated the DMHRF pilot system for CSO treatment in the early 1970s (Nebolsine et al., 1972; Innerfeld et al., 1979) and found that the DMHRF system (with filtration rate of >16 gpm/ft2) removes small size particulates that remain after screening, including floc formed after polyelectrolyte and/or coagulant addition. The principal advantages of this system are high treatment efficiencies, automated operation, and limited space requirements. To be most effective, filtration through media that are graded from coarse to fine in the direction of filtration is desirable. A typical case is the use of coarse anthracite particles on top of less coarse sand. Because anthracite is less dense than sand, it can be coarse and still remain on top of the bed after the backwash operation. The principal parameters to be evaluated in selecting a DMHRF system are media size, media depth, and filtration rate. As much of the removal of solids from the water takes place within filter media, their structure and composition is of major importance. If the medium is too fine, it may produce a high-quality effluent but also may cause excessive head losses and extremely short filter runs. On the other hand, media that is too coarse may fail to produce the desired clarity of the effluent. Therefore, the selection of media for DMHRF should be made by pilot-testing using various materials in different proportions and at different flow rates. Depth of media is limited by headloss and backwash considerations. The deeper the bed, the greater the headloss and the harder it is to clean. On the other hand, the filter bed should be of sufficient depth to retain the removed SS within the pore space of the media for the duration of the filter run without SS breakthrough. The design filtration flux must be such that the effluent will be of a desired quality without causing excessive headloss through the filter, which in turn requires frequent backwashing. At high flux, shear forces seem to have significant effects on solids retention and removal. Several DMHRF pilot study installations have been demonstrated for control of CSO pollution. These facilities have used 6, 12, and 30 in. diameter filter columns with anthracite and sand media, together with various dosages of coagulants and/or polyelectrolytes. The removal of SS by DMHRF was found to vary directly with influent SS concentrations and inversely with flux or hydraulic loading rate. In 1976, the U.S. EPA and the City of New York Department of Environmental Protection conducted a pilot study to demonstrate the dual use of DMHRF for treating CSO and polishing DWF plant effluent at New York City (Innerfeld et al., 1979). Results indicated the system provided overall average SS removals of 61% across the filter and 66% across the system with an average influent SS concentration of 182 mg/L. BOD5 removals from CSO averaged 32% across

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TABLE 8.3 Removal of Heavy Metals by DMHRF System Constituent

Cd

Cr

Cu

Hg

Ni

Pb

Zn

Average removal, % a

56

50

39

0

13

65

48

a

Concentration basis

Source: Innerfeld, H. et al., EPA-600/2-79-015, 1979.

the filter and 41% across the system with an average influent BOD5 of 136 mg/L. Limited tests were also run in the New York City WWTP (Innerfeld et al., 1979) to determine heavy metals reduction. These results, shown in Table 8.3, represent composite samples. Comparisons with alternative treatment systems show that DMHRF is cost-competitive with conventional sedimentation facilities for dual processes (treatment of DWF and CSO flows), and yet DMHRF has only 5 to 7% of the area requirements. If only CSO is being treated, DMHRF is competitive with DAF and microstraining processes. High-Rate Sedimentation (Microcarrier) Many treatment alternatives are available to control WWF pollution. In recent years, a new highrate chemical-clarification process, using microsand (or a microcarrier, MC) as a weighted carrier of colloidal and larger particles, has been developed and applied for treating drinking water supply and has been tested for treating combined WWF/DWF wastewater. This process consists of the addition of an MC and coagulant into the influent in a mixing chamber followed by flocculation and sedimentation tanks. Accordingly, the addition and recirculation of an MC will result in higher particle settling velocities and tank overflow rates. It has been reported that an MC unit could achieve SS removal over 80% at a range of overflow rates of 50 m3/h/m2 (20 gpm/ft2), which is about ten times the conventional sedimentation process overflow rate. In this process, WWF, which consists of particles stretching the range from small colloids (0.02 µm) to coarse colloids (>2.0 mm), is mixed with an MC. With addition of coagulants in a mixing tank, the MC aggregates with colloidal particles and results in a fast settling velocity (>60 m/h). The initiation of the reaction of coagulation–flocculation processes is improved by the presence of the MC and polyelectrolytes, a coagulation aid that increases the bonding of the floc to the MC, resulting in an even higher settling velocity. The MC plays a crucial role in enhancing settling properties and, in particular, the removal of colloidal particles and associated contaminants. However, the basic process mechanisms are not available to the public. The U.S. EPA modified the conventional jar test procedure to enable study of the MC process for treating urban WWF (Ding et al., 1999). The operational parameters include type and dosage of coagulant and coagulant aid, mixing rate and duration, and MC type, size, and concentration. The experimental results revealed that MC-weighted coagulation dramatically reduced coagulation–flocculation duration (to <3 min) and settling time (to <8 min), producing flocs with a high settling velocity and high-quality supernatant. After the MC coagulation, the particles in the supernatant of the jar were found to be <2 µm (indicating the removal capability of the process for fine colloidal particles) and turbidity reduced from >80 NTU to <2 NTU. The U.S. EPA has investigated the effects of different types and dosages of coagulant and coagulant aid in conjunction with MC as well as the effect of MC on particle size distributions and zeta potential of colloids in urban WWF by using modified jar test procedure. However, operational control parameters are not yet verified by a large-scale pilot or full-scale treatment facility. Thus, methods are needed for evaluating the process parameters, which enable engineers to develop process design criteria, equipment specifications, and tools for plant operators to effectively control the mixing and flocculation processes for treating WWF.

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Biological Treatment Biological treatment is a means for stabilizing dissolved organic matter and removing nonsettleable colloidal solids. This can be accomplished either aerobically or anaerobically. Two major categories of aerobic biological systems include suspended growth processes and attached film growth processes. Biological treatment processes are generally classified as secondary treatment capable of removing between 70 and 95% of the BOD5 and SS from waste flows at dry weather design flow rates and loadings. When biological treatment processes are used for combined wastewater treatment, removal efficiencies are lower (the percent organic matter is smaller for CSO solids than for DWF solids) and are controlled to a large degree by hydraulic and organic loading rates. Most biological systems are susceptible to overloading conditions and shock loads as compared to physical treatment processes. Aerobic Suspended Growth Processes The principal aerobic suspended growth biological treatment processes include the activated sludge process, aerated lagoons, oxidation ponds, and aerobic digestion process. Of these four processes, the contact stabilization activated sludge processes (with aerobic sludge digestion) and aerated lagoons have been demonstrated for combined DWF/WWF treatment. Activated Sludge

Activated sludge systems are the most extensively used in treating municipal and industrial wastewater. The process is a continuous system that involves an aeration reactor, followed by solids–liquid separation, with a portion of the biomass collected from the bottom of final settling tanks being recycled. In the aerator, flocculated biological growths are mixed with wastewater and aerated. Aqueous organic waste streams having <5000 mg/L MLSS of biological growths are continuously circulated and contacted in the presence of oxygen. Microorganisms in activated sludge systems serve to perform hydrolysis and oxidation reactions. Many modifications of this process have been developed, but the two basic process variations are the conventional system and contact stabilization. In the conventional system, absorption, flocculation, and synthesis are all accomplished in a single process step; the contact stabilization system oxidizes and synthesizes removed organic pollutants in a separate aeration tank. The main advantage of the contact stabilization process is the small amount of aeration capacity needed; the contact tank volume is only 5 to 20% of the normal aeration tanks. These processes were fieldevaluated for joint dry/wet weather treatment of municipal wastewater at Kenosha, Wisconsin (Agnew et al., 1975) and Clatsknite, Oregon (Benedict and Roelfs, 1981). The City of Kenosha constructed a 20-MGD modified contact stabilization process for demonstrating the treatment of both DWF and WWF (Agnew et al., 1975). The system consisted of a pumping station, a grit basin, a contact tank, a stabilization tank, and a final clarifier. The main difference between normal contact stabilization processes and the modified application was the periodic usage of the system for treating CSO. In conventional contact stabilization, the returnactivated sludge is continually transferred from the underflow in the final clarifier to the stabilization tank where it is aerated for a period of 2 to 3 h and then transferred to the contact tanks. However, as the demonstration system was only to be used periodically during wet weather, it was necessary to provide a viable stabilized sludge ready for use. Thus, the waste activated sludge from the existing dry weather plant was reserved to be used as a source of biological solids in the stabilization tank. The system was operated and evaluated during 49 runs in which 180 million gallons of potential CSO was treated. Based on these tests, expected removal efficiencies for total SS (TSS), BOD5, and TOC are 90, 85, and 76%, respectively. Furthermore, the dry weather treatment plant efficiency was improved by use of a final clarifier during periods when the demonstration system was not in use. After the demonstration system was installed, the performance of DWF treatment system increased from 82 to 94% and 64 to 88% for BOD5 and TSS, respectively. The modification of the contact stabilization process used in the Kenosha demonstration project is a cost-effective method for the

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treatment of CSO in locations having an adequate wastewater conveyance system and an existing dry weather biological treatment process. In future applications, the expected treatment efficiencies will be directly proportional to the quality of treatment already achieved by the existing dry weather plant adjoining the potential CSO treatment system. This is a direct result of the potential overflow treatment system using the waste activated sludge generated by the adjoining dry weather treatment plant. In Clatsknie, Oregon, the city conducted a 2-year full-scale evaluation of the performance capability of physical and biological processes for joint treatment of DWF/WWF at its wastewater treatment plant (Benedict and Roelfs, 1981). The plant consists of primary clarification and conventional activated sludge processes for treating an average flow of 0.2 MGD and peak flow of 0.5 MGD for DWF. However, the secondary process has the capability of being run in a contact stabilization mode, and the primary clarifier can be operated in a DAF mode. With these process modifications, flows up to 1.25 MGD, more than six times the DWF, can be effectively treated. Results of the evaluation program showed that based on mean values, the DAF-contact stabilization treatment system configuration maintained effluent TSS and BOD5 concentrations of 2 to 24 mg/L and 6 to 11 mg/L, respectively, for average flows up to 1.25 MGD (or six times the DWF). During DWF, plant removal efficiencies for TSS and BOD5 were 95%. During WWF, removal efficiencies for storms with peak flows ranging from 0.5 to 2.3 MGD (or 3 to 12 times DWF) were 71 and 73% for TSS and BOD5, respectively. Seven heavy metals (Cd, Cr, Cu, Pb, Hg, Ni, and Zn) were monitored during the study. Three constituents (Cd, Cr, and Hg) were not present in detectable quantities (i.e., < detection limits). The four remaining (Cu, Pb, Ni, and Zn) were present in CSOs ranging from 20 to 40 µg/L, except for Zn which ranged from 140 to 260 µg/L. Generally, the heavy metals experienced very little reduction in the primary settling tanks, but all heavy metals except Ni were significantly removed in the secondary process (i.e., contact stabilization process). The average TKN concentrations were 22 and 15 mg/L for DWF and WWF, respectively, and the average total phosphorus (TP) concentrations were 6 and 2 mg/L for DWF and WWF, respectively. Generally, very little nutrient removal occurred in the primary process, but significant removal occurred in the secondary process. Overall plant removal of TKN and TP averaged 67 and 30%, respectively. The capital cost of the DAF-contact stabilization capability was estimated to be 14% over the cost of a standard DWF plant. The total cost including operation and maintenance was estimated to be 10% over the cost of a standard DWF plant. A cost-effective analysis comparison of joint DWF/WWF treatment, sewer system rehabilitation, and flow equalization storage for the City of Clatskanie showed that joint treatment was considerably less expensive and was nearly as effective in controlling inflow and infiltration (I/I) discharges. Aerated Lagoons

The aerated lagoon process developed from the addition of mechanical aeration to waste stabilization ponds. Usually, the lagoon is an earthen basin with sloping sides, about 6 to 17 ft deep. For the treatment of CSO, it may be necessary to line the basin with an impermeable material, i.e., geomembrane liner or asphalt. Because CSOs in aerated lagoons are generally not as well mixed as in activated sludge basins, a small amount of SS settles to the bottom and undergoes anaerobic microbial decomposition. Retention times are much longer than for activated sludge, and where anaerobic decomposition has taken place, the detention time is even longer. Pollutant removal efficiencies by treatment lagoons have varied from highs of 85 to 95% to negative values due to excessive algae production and carryover. In addition to the type of lagoon and the number of cells in series (stages), several major factors that influence removal efficiencies include (1) detention time, (2) source of oxygen supply, (3) mixing efficiency, (4) organic and hydraulic loading rates, and (5) algae removal mechanisms. Attached Film Growth Processes The attached film growth processes include the trickling filter, the rotating biological contactor (RBC), and the fixed-bed nitrification reactor. They also achieve nitrification conversion of ammonia

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to nitrate. The trickling filter and RBC systems have been evaluated for combined DWF/WWF treatment. Trickling Filters

In the trickling filter unit, CSO is sprayed by two to four arms rotating through the air on top of fixed media (i.e., crushed stone or plastic media) and is allowed to trickle through the bed. A microbial slime is formed on the media, able to decompose organic matter in the CSO stream. The microorganisms that grow on the media are aerobic. Their activities are similar to those of activated sludge processes, but they are more tolerant of variable hydraulic and organic loadings than the activated sludge system. The microbial slime remains aerobic primarily at its surface where air and water interface with the cells. A combination of series-parallel high-rate trickling filters to treat both the DWF and the higher flows encountered during wet weather was evaluated at the New Providence wastewater treatment plant in New Jersey (Homack et al., 1973). The plant was designed to alleviate hydraulic overloading and the resultant loss of treatment efficiency caused by excessive infiltration. It uses two high-rate trickling filters, one with rock media and the other with plastic media, operating in parallel to treat WWF (6.0 MGD) and in series during DWF. During dry weather periods the plant is operated with an average flow of 0.5 MGD. A study of plant efficiency for 1 year indicated that during dry weather controlled flow operation, the BOD5 and TSS removal efficiency varied from 85 to 90%. When operated during wet weather, the BOD5 and TSS removal efficiency varied from 56 to 74%. The study has shown that it is both economical and technically feasible to design, construct, and operate a combination of seriesparallel high-rate trickling filters to treat both the DWF and the WWF encountered during storm events. However, a drawback of this type of operation is the tendency for colloidal suspended material to be discharged from the filters during high flow rates. Unless chemical precipitation and low secondary clarifier overflow rates or supplemental biological flocculation in the form of a pond are provided, high SS and BOD5 removal will not be obtained. Rotating Biological Contactor The RBC consists of a series of flat, parallel disks that are rotated while partially immersed in the wastewater. Biological organisms that are attached to the surfaces of the disks form a slime layer that covers the surface of the disks. The biological organism layer both adsorbs and absorbs colloidal and dissolved organic matter present in the waste stream. Disk rotation enhances oxygen transfer and maintains the biomass in an aerobic condition. Rotation also is the mechanism for removing excess slime that is generated by synthesis of the waste materials and is sloughed off gradually into the mixed liquor and removed by the subsequent sedimentation tanks. During WWF, the RBC is capable to treat at eight to ten times dry weather design flows.

SUMMARY An operational problem common to all stormwater biological systems is that of maintaining a viable biomass to treat flows during wet weather conditions. At New Providence, trickling filters are operated in series during dry weather and in parallel during wet weather. This type of operation maintains a viable microorganism population during dry weather and also provides greater capacity for the wet weather flows. For processes that borrow biomass from dry weather facilities or allow the biomass to develop, a lag in process efficiency may be experienced as the biomass becomes acclimated to the changing waste strength and flow rate. Also, because of the limited ability of biological systems to handle fluctuating and high hydraulic shock loads, storage/detention facilities placed before the biological processes may be required. General maintenance problems experienced by wet weather biological facilities are similar to those experienced at conventional biological installations. Winter operation of mechanical surface aerators have had some serious drawbacks, including icing, tipping, or sinking. Other methods of providing the required oxygen that show promise and have been demonstrated at many dry weather facilities include diffused air systems and submerged tube aerators.

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MANAGEMENT OF WWF SOLIDS Historically, wastewater sludge has been considered to be a waste material and has been dumped in municipal and commercial landfills or the ocean, as these practices were easy and cheap. Over the years, there has been an increasing recognition that wastewater residuals are a part of the natural life cycle and hence have nutrient and soil-enhancing properties, as well as increased recognition of the polluting effects of poor sludge management. This has made them a practical choice for a variety of land applications, thus moving the biosolids management processes from disposal focused to recycling oriented. Thus, biosolids is the term now used to emphasize the beneficial nature of this recyclable sludge material. The U.S. EPA estimated the annual WWTP biosolids production to be approximately 6.9 million tons (dry solids) in 1998 and predicted that 7 million tons of biosolids would be generated for use or disposal in 2000, growing to 8 million tons in 2005 and to 8.5 million tons in 2010 (U.S. EPA, 1999). Furthermore, biosolids generation is on the increase as the development of new and advanced wastewater treatment plants continues. In addition, CSO control and treatment facilities generate a wide variety of waste products, known as residuals, including organic and inorganic compounds in liquid and solid forms. Field and O’Shea (1994) reported that the estimated annual sludge volume generated in the United States from CSO and urban storm runoff control facilities would add an additional volume ranging from 11 to 228 million m3 and 27 to 547 million m3, respectively. Thus, a tremendous load will be added to the existing POTW sludge treatment facilities handling WWF. With increasing waste disposal costs and limited landfill capacity in the some parts of the country, the demand for biosolids handling and processing is greater than ever before. Biosolids have been applied to agricultural lands, forests, or reclaimed lands. More recently, there has been greater emphasis on applications such as landscaping and nurseries for recreational uses. However, the beneficial use of biosolids will continue to be hindered by public opposition in some areas of the country. The public acceptance issues include concerns about pollutants in the biosolids, risk of disease, and odors. Over time, these concerns can be effectively addressed through a combination of approaches, including assessment of public attitudes, modifications to biosolids management programs, outreach and education, and marketing of biosolids products. A complete management program for a CSO treatment facility should include the development of a plan to remove and dispose of these residuals in a cost-effective method that meets all regulatory requirements.

CHARACTERISTICS

AND IMPLICATION

In 1973, the U.S. EPA recognized the need for defining the problems of and establishing treatment procedures for handling and disposing of biosolids from CSO that resulted in a three-phased research project: 1. To characterize the residual sludges arising from the physical, physicochemical, and biological treatment of CSO (Gupta et al., 1977b) 2. To assess the effort that the United States will have to exert in the area of sludge handling and disposal resulting from the treatment of CSO (Huibregtse et al., 1977) 3. To evaluate the handling and disposal of CSO treatment residuals including the development and demonstration of process treatment systems for handling and disposing of the sludges that arise from treatment of CSO (Osantowski et al., 1977) The volume and the characteristics of biosolids generated from the CSO treatment facilities vary widely, depending on the type of treatment process used. The major constituent of sewer sediment contains various types of organic substances mixed with variety of soils and inorganic pollutants. Thus, the most notable differences from WWTP biosolids were the high grit, high toxic

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TABLE 8.4 Heavy Metals in CSO Biosolids Constituents

Concentration (mg/kg dry solids)

Cr Cu Pb Hg Ni Zn

52–2471 200–2454 164–2448 0.01–100.5 83–995 697–7154

TABLE 8.5 Pesticide and PCB in CSO Biosolids Constituents Dieldrin PCB pp = DDD pp = DDT

Concentration (g/kg dry solids) Nondetectable Nondetectable Nondetectable Nondetectable

to to to to

192 6570 225 170

pollutants, and low volatile solids content in CSO residuals plus their intermittent generation. A nationwide analysis indicates an average yearly sludge volume of 156 million m3 (41.5 billion gal) could be expected from CSO if complete CSO treatment were implemented. This compares to a raw primary sludge volume of 61 million m3 (16 billion gal) from WWTP (Huibregtse et al., 1977). However, the average solids concentration in CSO sludge is about l% compared to 2 to 7% in raw primary sludges. This is due to the high-volume, low-solids residuals generated by treatment processes employing screens. The heavy metal and pesticide concentrations of the various CSO residuals were observed to be significant. Heavy metal (Cr, Cu, Pb, Hg, Ni, and Zn) concentrations in the CSO biosolids were significant and varied widely for the biosolids investigated. The range of heavy metal concentrations for the various sites investigated were indicated in Table 8.4 (Gupta et al., 1977b). Pesticide and PCB concentrations in the CSO biosolids investigated were also observed to be significant. Generally, the PCB concentrations were higher than those for pp′DDD, ppDDT, and dieldrin. The range of PCB and pesticide values for the various sites investigated is indicated in Table 8.5 (Gupta et al., 1977b). Ultimate disposal of WWF biosolids that are contaminated with toxic substances requires careful risk and cost assessments. Stansbury et al. (1999) developed a risk–cost analysis methodology that evaluates uncertainty directly in the decision framework. Due to the uncertainties involved in the risk–cost analysis processes, examine all risk factors for all disposal alternatives for selecting the best option.

TREATMENT PROCESSES Most biosolids undergo complete treatment at the WWTP site before they are used or disposed of to meet regulatory requirements that protect public health and the environment, facilitate handling, and reduce costs. Biosolids characteristics can determine a municipality’s choice of use or disposal methods. Only biosolids that meet certain regulatory requirements for pathogens, vector attraction

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reduction, and metal content, for example, can be land-applied or used as compost. Even those biosolids that are disposed of rather than land-applied must meet regulatory requirements. Also, with regard to handling and cost, the water content of biosolids can affect many aspects of biosolids management, such as transportation and the size of treatment and use or disposal operations. Some biosolids treatment processes reduce the volume or mass of the biosolids (such as digestion processes), whereas others increase biosolids mass (for example, when lime is added to control pathogens). The two essential biosolids treatment processes are stabilization and dewatering. Dewatering Dewatering removes excess water from biosolids. It decreases biosolids volume by reducing the water content of biosolids and increasing the solids concentration. Dewatering often is a necessary process before treatment or use, such as before composting, heat drying, or biosolids preparation for land application, although liquid biosolids can be land-applied using common or specialized application methods. Dewatering is also necessary for biosolids destined for incineration to prevent damage to boilers and decrease the energy required for biosolids combustion. Additionally, landfilled biosolids require dewatering because disposal of liquids in landfills is prohibited. Dewatering makes handling of the biosolids easier by converting liquid biosolids to a damp cake, and it reduces transportation costs, although cost savings should be weighed against the cost of dewatering. Dewatering might be undesirable for land application of biosolids in regions where water itself is a valuable agricultural resource. Prior to dewatering, biosolids are usually conditioned and thickened. In conditioning, chemicals, such as ferric chloride, lime, or polymers, are added to facilitate the separation of solids by aggregating small particles into larger masses or “flocs.” In thickening, part of the water bound to biosolids particles is removed to concentrate the solid materials. Gravity thickening is a common practice. A number of dewatering processes can be used, including vacuum filters, belt filter, plateand-frame press, and centrifuges. Vacuum filter: Typically achieves 12 to 22% solids content and involves rotating a drum submerged in a vat of biosolids, applying a vacuum from within the drum, drawing water into the drum, and leaving the solids or “filter cake” on the outer drum filter medium. The dewatered biosolids are scraped off the filter. Belt filter: Can achieve 20 to 32% solids content. They work by exerting pressure on biosolids placed between two filter belts, which are passed through a series of rollers. The pressure forces water out of the biosolids, and the dried biosolids cake is retained on the belt filter press. Plate-and-frame press: The most efficient dewatering process, which can produce 35 to 45% solids content. It works by squeezing the biosolids between two porous plates or diaphragms. The pressure forces water out of the biosolids, and the dried biosolids cake is retained on the plates. Centrifuge: Spins biosolids in a horizontal, cylindrical vessel at high speeds, with the solids concentrating on the outside of the vessel. These solids are then scraped off. Centrifuging can result in a 25 to 35% solids content. Stabilization Processes Stabilization processes refer to a number of processes that reduce pathogen levels, odor, and volatile solids content. Biosolids must be stabilized to some extent before most types of use or disposal. Major methods of stabilization include alkali (lime) stabilization, anaerobic digestion (decomposition of organics by microorganisms in the absence of oxygen), aerobic digestion (decomposition of organics by microorganisms in the presence of oxygen), composting, and heat drying. Alkaline Stabilization Alkaline stabilization has also been implemented using either quicklime (CaO) or hydrated lime [Ca(OH)2], which is added to either liquid biosolids before dewatering or dewatered biosolids in

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a contained mechanical mixer. Traditional lime stabilization processes are capable of producing biosolids that meet the minimum pathogen and vector attraction reduction requirements found in the 40 CFR Part 503 rules governing land application of biosolids; sufficient lime is added so that the pH of the biosolids/lime mixture is raised to 12.0 or above for a period of 2 h. The elevated pH helps to reduce biological action and odors. Bioreactors Bioreactors are self-contained, aboveground units used for the treatment of contaminated sediment slurries. Bioreactors are most effective for treating small volumes of soils and sludges with high organic contaminant concentrations. Although system designs are based on site and contaminant requirements, most bioreactors consist of a treatment vessel in which the concentration and distribution of oxygen and nutrients are highly controlled. Environmental parameters such as soil permeability, temperature, moisture content, mixing, and hydraulic retention time can also be controlled in bioreactors. Operational control of the bioreactor system greatly enhances the microbial growth and petroleum degradation rates. However, because bioreactors are controlled environments, there are construction size limitations, which in turn limit the volume of soil or sediment that can be treated. Composting Composting is an aboveground process that biologically degrades organic waste matter into a humuslike end product. Composting systems are used to degrade and stabilize organics such as manure, municipal sewage sludge, municipal refuse, yard waste, and food processing wastes. Composting systems have traditionally used a consortium of bacteria that thrive in different temperature ranges to stabilize municipal wastewater sludge and minimize yard waste. Composting principles have also been applied to treat soils contaminated with petroleum wastes and other toxic materials. The process relies on the interaction of a variety of microorganisms including bacteria, protozoa, actinomycetes, and fungi. In composting operations, mesophilic (thrive in temperatures between 25 and 40°C) and thermophilic (thrive in temperatures between 40 and 70°C) bacteria are chiefly responsible for the decomposition of proteins, lipids, and fats (McKinney, 1962). Protozoa consume these same categories of organic compounds as well as preying on the bacteria themselves. However, the abundance of protozoa in a composting operation is highly reduced when the temperatures are elevated into the thermic temperature range. Actinomycetes and fungi are prevalent during both the mesic and thermic stages of composting. They are considered to be responsible for the degradation of complex organic compounds such as carbohydrates and cellulose. One of the features of composting is the variant temperature range stages induced during the degradation of organic compounds. In the early stages of composting, the temperature increases from ambient temperatures to approximately 40°C and as degradation of the organic matter proceeds, evolved heat and temperatures rise into the thermophilic range. Aerated static pile windrow composting and in-vessel composting methods include the following operations: amendment of the organic matter with a bulking agent, aeration of the compost pile, recovery of the bulking agent, further curing of the compost, and product utilization or disposal. Most composting methods attempt to maintain an aerated environment to stimulate the growth of fast-growing aerobic bacteria with differences in methods mainly associated with construction and operation. Biomounds Biomound bio-oxidation systems are similar in construction and operation to aerated static pile composting systems. Because the technology has only recently been applied to contaminated soils, a plethora of names have been used to describe these systems including ex situ bio-oxidation, aboveground bioventing, soil-heap venting, augmented vacuum heap, aboveground bioaugmented soil venting, soil composting (a misnomer), bio-burritos, and biomounds. All of these variations of aboveground noncontainerized bioremediation systems for solid materials are referred to as biomounds. Biomound remediation can be enhanced by active aeration and the addition of nutrients

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or augmented by the addition of microbial cultures. Active aeration refers to the use of a soil vapor extraction system placed in the soil pile to enhance the transfer of oxygen to the native microbes degrading the contaminants and to simultaneously remove volatile constituents. It has been widely used for cleaning up petroleum-contaminated soils. Fan and Tafuri (1994) reported on seven case studies of biomound system applications for cleaning up petroleum fuel- and oil-contaminated soil. The parameter that was consistently applied at all of the sites is the addition of nutrients to enhance the growth of the natural microbial population. Active biomound soil venting was applied at half of the sites, the addition of cultured microorganisms to the biomound was performed at two of the sites, and compost with indigenous bacteria was added to soil at two sites. Manifolded biomound venting systems are often installed to enhance the aeration of low-permeability soils. In addition, cultured microorganisms are most often added to soils with very low indigenous bacterial population densities. Furthermore, the biomound performance will be enhanced by addition of extracellular enzymes or augmentation with “engineered” intracellular microorganisms, because enzymes are known for their high degree of efficiency in converting substrates to end products (Fan and Krishnamurthy, 1995). Handling and Disposal Alternatives Options for handling of biosolids generated from CSO treatment facilities include dewatering at on-site satellite locations in a separate parallel facility at the WWTP, or bleedback into sewer and treatment along with the DWF. The following general guidelines should be considered for selection of options: 1. Evaluation of sludge handling processes from the standpoint that the high grit, high toxic substance content and low volatile organics of CSO biosolids, along with the variable and intermittent generation, reduces the number of treatment processes applicable for CSO sludge handling and disposal. 2. Based on untreated CSO biosolids characteristics and known information about the processes, the following processes may be generally applicable: chemical conditioning, gravity thickening, lime stabilization, dewatering by vacuum filtration or centrifugation, and final disposal to land application and/or landfill. Environmental impact should be evaluated before adopting recovery of biosolids for land application and soil enhancement due to the high concentration of toxic heavy metals in the CSO biosolids. The following sections discuss pertinent information relating to the above three alternatives, especially the satellite facility and bleed/pumpback of the biosolids to the DWF sludge handling/treatment and disposal facilities. Bleed/Pumpback of CSO Biosolids to WWTP This option involves the bleed/pumpback of biosolids generated from satellite CSO treatment to the WWTP facilities. The DWF plant may have excess capacity available to handle the CSO biosolids, and it may have the lowest-cost biosolids handling option due to reduced transportation and use of existing DWF facilities for treatment. However, this alternative has inherent disadvantages that make the procedure inapplicable. For example, the solids loadings (assuming complete transport and no solids settling in the sewer) may increase as much as 300%, when the bleed/pumpback is spread over a 24-h period (for treatment residuals concentrations greater than 1% solids). The impact of such discharge will be proportionately less when the bleed/pumpback is spread over periods greater than 24 h. Excessive grit deposition in the sewer can cause odor, septicity, and blockage problems, and if flushed to the plant, it can adversely affect normal operation. The overload effect of bleed/pumpback of CSO treatment residuals may produce shock loads (hydraulic, solids, toxic heavy metal levels, PCB and pesticides, low volatile solids, etc.), which

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may adversely affect DWF treatment operation and performance (primary, secondary, and sludge handling and disposal). Thus, the limiting factor to consider is the solids loading to the final clarifier; accordingly, bleed/pumpback periods of 8 to 22 days may be necessary depending upon the CSO treatment method involved. Disadvantages of bleed/pumpback also include the adverse effect on the operation and efficiency of the dry weather plant caused by constantly loading the plant at excessive levels and by the difficulty in storing CSO residuals without stabilization for any excessive length of time. Reduction in the treatment efficiency of the DWF facilities due to bleed/pumpback, although small in terms of concentration, can add significant pollutant load in terms of mass loading on the receiving water body. Furthermore, even assuming no reduction in treatment efficiency, at least some fraction of the pumped-back/bled-back residuals would be discharged to the receiving water as a carryover in the treated effluent. This is a disadvantage of the bleed/pumpback concept that must be considered in its evaluation. Central CSO Biosolids Treatment Facility This option involves construction of dewatering CSO treatment biosolids at parallel facilities at the DWF plant or at central facilities separate from the DWF plant. Limiting factors include the transportation of untreated biosolids and potential space problems at the WWTP areas for construction of parallel facilities or a central location. Satellite CSO Biosolids Treatment Train This alternative includes dewatering and stabilizing biosolids at on-site CSO treatment facilities. The following four different sludge treatment trains should be evaluated with respect to health and ecological safety: 1. 2. 3. 4.

Lime Lime Lime Lime

stabilization stabilization stabilization stabilization

→ → → →

gravity thickening → vacuum filtration → landfill gravity thickening → vacuum filtration → land application gravity thickening → land application land application

Flow scheme 3, which utilizes lime stabilization plus gravity thickening and then land application, is the most cost-effective for CSO biosolids handling on a generalized basis. Because of the higher toxic substance content in CSO compared to that in DWF WWTP, biosolids impacts on surface water and groundwater quality near the land application site should be throughly evaluated before adopting the land application option. The logistics of operating and maintaining multiple CSO biosolids handling facilities at different locations throughout a city are formidable but not insurmountable. Similarly, greater logistics would be required for multiple CSO treatment facilities from which the sludges to be handled are derived. There is increased global interest in producing environmentally safe, pathogen-free biosolid products. The focus of new and improved technologies is aimed at reducing odor and air emissions and increasing efficient energy use. Thus, pollutant containment, resource recovery, and operational efficiency will be the three-pronged strategy that will determine the future direction of the biosolids management.

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Montrejaud-Vignoles, M., Roger, S., and Herremans, L., 1996. Runoff water pollution of motorway pavement in Mediterranean area, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 247. Mull, R., 1996. Water exchange between leaky sewers and aquifers, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 695. Murphy, C.B., Jr., Mac Arthur, D.A., Carloe, D.J., Quinn, T.J., and Stewart, J.E., 1982. Best Management Practices Implementation: Rochester, New York, Report EPA- 905/9-81-002; NTIS PB 82-171067, U.S. Environmental Protection Agency, Cincinnati, OH. Nebolsine, R., Harvey, P.J., and Fan, C.-Y., 1972. High-Rate Filtration of Combined Sewer Overflows, Report 11023EYI04/72; NTIS PB 211 144, U.S. Environmental Protection Agency, Washington, D.C. Nowakowska-Blaszczyk, A. and Zakrzewski, J., 1996. The sources and phases of increase of pollution in runoff waters in route to receiving waters, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 49. Ogden, H.N., 1898. Flushing in sewer pipes, Trans. ASCE, 40, 1–29. Osantowski, R., Geinopolos, A., Wullschleger, R.E., and Clark, M.J., 1977. Handling and Disposal of Sludges from Combined Sewer Overflow Treatment, Phase III — Treatability, Report EPA-600/2-77-053c; NTIS PB 281 006, U.S. Environmental Protection Agency, Cincinnati, OH. Pisano, W.C., 1996. Summary: United States sewer solids settling characterization methods, results, uses, and perspective, Water Sci. Technol., 33(9), 109. Pisano, W.C. and Queiroz, C.S., 1977. Procedures for Estimating Dry Weather Pollutant Deposition in Sewerage Systems, EPA600/277120; NTIS PB 270 695, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH. Pisano, W.C. and Queiroz, C.S., 1984. Procedures for Estimating Dry Weather Sewage InLine Pollutant Deposition — Phase II, Report EPA600/284-020; NTIS PB 84141 480, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH. Pisano, W.C., Connick, D., Queiroz, C., and Aronson J., 1979. Dry Weather Deposition and Flushing for CSO Pollution Control, EPA/600/2-79-133; NTIS PB 80 118 524, U.S. Environmental Protection Agency, Cincinnati, OH. Pisano, W.C., Novac, G., and Grande, N., 1997. Automated sewer flushing large diameter sewers, in Collection Systems Rehabilitation and O&M: Solving Today’s Problems and Meeting Tomorrow’s Needs, Wade, M., Ed., Water Environment Federation, Alexandria, VA, 12-9–12-20. Pisano, W.C., Barsanti, J., Joyce, J., and Sorensen, H., 1998. Sewer and Tank Sediment Flushing: Case Studies, EPA/600/R-98/157; NTIS PB99-127839INZ, U.S. Environmental Protection Agency, Cincinnati, OH. Pisano, W.C., O’Riordan, O., Ayotte, F., Barsanti, J., and Carr, D., 2000. Automated sewer and drainage flushing systems in Cambridge, Mass, in Proceedings of 2000 ASCE Joint Conference on Water Resources Engineering and Water Resources Planning and Management, July 30–August 2, 2000, Minneapolis, MN. Pitt, R., Field, R., Lalor, M., and Brown, M., 1995. Urban stormwater toxic pollutants: assessment, sources, and treatability, Water Environ. Res., 67(3), 260. Prah, P.L. and Brunner, P.L., 1979. Combined Sewer Overflow Treatment by Screening and Terminal Ponding, Fort Wayne, Indiana. Report EPA-600/2-79-085; NTIS PB 80-119 399, U.S. Environmental Protection Agency, Cincinnati, OH. Process Research, Inc., 1976. A Study of Pollution Control Alternatives for Dorcester Bay, Metropolitan District Commission, Boston, MA. Saget, A., Chebbo, G., and Bertrand-Krajewski, J.-L., 1996. The first flush sewer systems, Water Sci. Technol., 33(9), 101–108. Sakakibara, T., 1996. Roof runoff storm water quality, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 157. Sansalone, J., 1996. Immobilization of metals and solids transported in urban pavement runoff, presented at North American Environ. Congress ’96, Anaheim, CA. ASCE. Sansalone, J.J. and Buchberger, S.G., 1996. Characterization of solid and metal element distributions in urban highway stormwater, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 253. Sanudo-Wilhelmy, S.A. and Gill, G.A., 1999. Impact of the Clean Water Act on the levels of toxic metals in urban estuaries: the Hudson River Estuary revisited, Environ. Sci. Technol., 33(20), 3477.

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Saul, A.J., Houldsworth, J.K., Meadowcroft, J., Balmforth, D.J., Digman, C., Butler, D., and Davies, J.W., 1999. Predicting aesthetic pollutant loadings from combined sewer overflows, in Proc. the Eighth International Conference on Urban Storm Drainage, I.B. Joliffe and J.E. Ball, Eds., August 30–September 3, 1999, Sydney, Australia, The Institution of Engineers Australia, The International Association for Hydraulic Research, and The International Association on Water Quality, 482. Sawyer, C.N., McCarty, P., and Parkin, G.F., 1994. Chemistry for Environmental Engineering, McGraw-Hill, New York. Shaheen, D.G., 1975. Contributions of Urban Roadway Usage to Water Pollution, EPA-600/2-75/004; NTIS PB 245 854, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH. Shelly, G.L., 1981. Field Evaluation of a Swirl Degritter at Tamworth, EPA-600/2-81-063; NTIS PB81-187247, U.S. Environmental Protection Agency, Cincinnati, OH. Shiragami, N., Kajiuchi, T., and Hatayama, M., 1988. Enhancement of settling in a tank by inclined plates, Intl. Chem. Eng. (Japan), 28(4), 669. Sonnen, M., 1977. Abatement of Deposition and Scour in Sewers, EPA-600/2-77/212; NTIS PB 276 585, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH. Speller, F.N., 1951. Corrosion: Causes and Prevention, McGraw-Hill, New York. Squillace, P.J., Zogorski, J.S., Wilber, W.G., and Price, C.V., 1996. Preliminary assessment of the occurrence and possible sources of MTBE in groundwater in the United States, 1993–94, Environ. Sci. Technol., 30(5), 1721. St. Michelbach, G.W. and Brombach, H., 1999. Nutrient impact from CSOs on Lake Constance, in Proc. the Eighth International Conference on Urban Storm Drainage, August 30–September 3, 1999, Sydney, Australia, I.B. Joliffe and J.E. Ball, Eds., The Institution of Engineers Australia, The International Association for Hydraulic Research, and The International Association on Water Quality, 474. Stansbury, J., Bogardi, I., and Stakhiy, E.Z., 1999. Risk cost optimization under uncertainty for dredged material disposal, J. Water Res. Plan. Manag., 125(6), 342. Thistlethwayte, D.K.B., 1972. Control of Sulphides in Sewerage Systems, Ann Arbor Science, Ann Arbor, MI. Thorolfsson, S.T. and Brandt, J., 1996. The influence of snowmelt on urban runoff in Norway, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 133. Tomlinson, R.D., Bebee, B.N., Heyward, A.A., Munger, S.G., and Swartz, R.G., 1980. Fate and Effects of Particulates Discharged by Combined Sewers and Storm Drains, EPA-600/2-80-111, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, Cincinnati, OH. Trauth, R. and Xanthopoulos, C., 1996. Non-point pollution of groundwater in urban areas, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 701. U.S. Environmental Protection Agency, 1970a. A Rotary Vibratory Fine Screening of Combined Sewer Overflows, Report 11023FDD03/70; NTIS PB 195 168, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1970b. Microstraining and Disinfection of Combined Sewer Overflows, Report 11023EVO06/70; NTIS PB 195 674, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1970c. Dissolved-Air Flotation Treatment of Combined Sewer Overflows, Report 11020FKI01/70; NTIS PB 189 775, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1970d. Retention Basin Control of Combined Sewer Overflows — Springfield Sanitary District, Springfield, IL, Report 11023-08/70; NTIS PB 200 828, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1972. Screening/Flotation Treatment of Combined Sewer Overflows, Report 11020FDC01/72; NTIS PB 215 695/8BE, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1985. Design Manual Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants, EPA/625/1-85/018, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1989a. EPA Combined Sewer Overflow (CSO) Control Strategy, Fed. Regis., 54, 37370.

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U.S. Environmental Protection Agency, 1989b. Results of the Evaluation Groundwater Impacts of Sewer Exfiltration, NTIS PB95-158358, Municipal Facilities Division, Office of Water, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1994. EPA Combined Sewer Overflow (CSO) Control Policy, Fed. Regis., 59, 18688. U.S. Environmental Protection Agency, 1995a. Combined Sewer Overflows — Guidance for Nine Minimum Controls, EPA 832-B-95-003, Office of Water, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1995b. Combined Sewer Overflows — Guidance for Long-Term Control Plan, EPA 832-B-95-002, Office of Water, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 1999. Biosolids Generation, Use, and Disposal in the United States, Report EPA530-R-99-009, Office of Solid Waste, U.S. Environmental Protection Agency, Washington, D.C. U.S. Environmental Protection Agency, 2000. Exfiltration in Sewer Systems, Urban Watershed Management Branch, Water Supply and Water Resources Division, National Risk Management Research Laboratory, Edison, NJ (draft report). U.S. Environmental Protection Agency, 2001. Report to Congress on Implementation and Enforcement of the CSO Control Policy, EPA 833-R-01-033, U.S. Environmental Protection Agency, Office of Water, Washington, D.C. U.S. Geological Survey, 1999. Stormwater Runoff for Selected Watersheds in the Edwards Aquifer Recharge Zone, Bexar County, Texas, 1996–1998. Fact sheet FS-172-98. U.S. Geological Survey http://water.usgs.gov/pubs. Venkatesan, M.I., de Leon, R.P., van Geen, A., and Luoma, S.N., 1999. Chlorinated hydrocabon pesticides and polychlorinated biphenyls in sediment cores from San Francisco Bay, Mar. Chem., 64(1-2), 85. Vignoles, M. and Herremans, L., 1995. Metal pollution of sediments contained in runoff water in the Toulouse City, presented at NOVATECH 95, Second Intermational Conference on Innovative Technologies in Urban Storm Drainage, May 30–June 1, 1995, Lyon, France, organized by Eurydice 92 and GRAIE, 611. Weis, P. and Weis, J.S., 1996. Leaching from chromated-copper-arsenic (CCA) treated wood and effects in Chesapeake Bay, Abstract Book: SETAC 17th Annu. Meeting, Washington, D.C., 88. Welsh, F.L. and Stucky, D.J., 1974. Combined Sewer Overflow Treatment by the Rotating Biological Contactor Process, Report EPA-670/2-74/050; NTIS PB 231892, U.S. Environmental Protection Agency, Cincinnati, OH. Wong, T.H.F., 1997. Continuous deflective separation: its mechanism and applications, in Proceedings WEFTEC ’97, Vol. 2, Water Environment Federation, Alexandria, VA, 703–714. Zogorski, J.S., Morduchowitz, A.B., Baehr, A.L., Bauman, B.J., Conrad, D.L., Drew, R.T., Korte, N.E., Lapham, W.W., Pankow, J.F., and Washington, E.R., 1996. Fuel oxygenates and water quality: current understanding of sources, occurrence in natural waters, in Environmental Behavior, Fate, and Significance, Executive Office of the President, Office of Science and Technology Policy, Washington, D.C.

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9

Beneficial Use of Urban Stormwater Chi-Yuan Fan and Richard Field

CONTENTS Introduction ....................................................................................................................................257 Urban Water Resources Management............................................................................................258 Storm Runoff Flow .................................................................................................................258 Continuous Runoff Hydrographs ..................................................................................260 Urban Stormwater Quality......................................................................................................260 Stormwater Storage Treatment Systems........................................................................................261 Treatment Systems and Water Quality ..........................................................................................264 Hypothetical Case Study................................................................................................................266 Introduction .............................................................................................................................266 Water Demands and Quality Needs .......................................................................................266 Stormwater Reclamation System ..................................................................................266 Cost Analysis.................................................................................................................267 Conclusions ....................................................................................................................................268 References ......................................................................................................................................269

INTRODUCTION As population and industry grow, water demand increases and freshwater supply becomes more of a problem. Declination of per-capita freshwater availability has been dramatically accelerated in recent years (Gleick, 1999). For example, in the Middle East and North Africa, available fresh water was 3430 m3/year/capita in 1960 and is projected to be less than 670 m3/year/capita by the year 2025 (Simon, 1998). In the United States, total gross water usage exceeds the total available freshwater supply, especially in the states of California, Florida, Nevada, Texas, and Arizona (Simon, 1998). The challenge is to do more with less by conserving and recycling water instead of continuously reaching out for more freshwater sources. One attractive possibility involves using reclaimed stormwater as a source in developed areas. Millions of cubic meters of stormwater runoff discharge daily into urban streams and lakes by passing over developed land. If urban stormwater is properly controlled and treated, this water could be partially or fully harvested as a supplemental source for water supply systems. While reclamation of municipal wastewater for industry, subpotable domestic usage, and groundwater recharge has been practiced in the United States over the past several decades, application of urban storm runoff reclamation has not been fully utilized. A 1971 U.S. Environmental Protection Agency (U.S.EPA)-supported nationwide survey estimated that current reuse of treated municipal wastewater for industrial water supply, irrigation, and groundwater recharge was 203 million, 291 million, and 45 million m3/year, respectively (Schmidt, 1975). It is reasonable to expect that the reuse of the treated wastewater for industrial cooling, subpotable domestic water supply, and park and golf 257

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course irrigation should and will be substantially increased in the future. Various publications are available on the reuse of municipal effluent for subpotable water supply and groundwater recharges. An early study in Columbia, Maryland (Mallory, 1973) evaluated the use of treated storm runoff for part of the municipal water supply. In a hypothetical case, Field and Fan (1981) compared the cost of treating stormwater vs. the cost of using the city water supply for subpotable water usage. They found that reclaiming stormwater at an industrial site for lawn irrigation and cooling waters was effective. Anderson (1996) described new initiatives for reusing stormwater for urban residential and industrial water supply systems in Australia. Mitchell et al. (1996) analyzed urban catchment drainage by using a water budget approach that uses the combined stormwater and treated wastewater for two sites in suburban Melbourne, Australia. Nelen et al. (1996) described the planning of a new development for about 10,000 people in Ede, Netherlands using a dual water supply system. Herrmann and Hase (1996) reported the benefits of adopting rainwater utilization systems in Bavaria, Germany. The system reduced the quantity of drinking water supply and lessened the storm runoff flow rate to the collection sewer system during wet weather periods. The Building Services Research and Information Association (1997) in the United Kingdom evaluated the economic, environmental, social, and health-related implications of greywater and rainwater reuse with proposed water quality standards for several levels of usage. It was concluded that such usage could be economically viable, especially when coupled with water charges that take environmental costs into account. Furthermore, the Texas Water Development Board (TWDB) in cooperation with the Center for Maximum Potential Building Systems published a guidance manual for harvesting rainwater for household supplemental water use (TWDB, 1997). The fact that storm runoff can be treated to meet water quality standards for different types of subpotable usage will enable reclamation of stormwater to become a more significant means of augmenting water supply. This chapter discusses current urban stormwater control and treatment technology leading to the feasibility of reclaiming urban stormwater for various purposes. Subjects discussed include residential subpotable indoor water usage, outdoor lawn and garden irrigation, and industrial cooling and process water uses. In addition, a cost comparison of a hypothetical case study illustrating the cost-effectiveness of reclaiming urban stormwater for complete industrial supply is presented.

URBAN WATER RESOURCES MANAGEMENT An integrated water management approach for developed or urban watersheds considers the interrelationship of city water supply, wastewater disposal, and stormwater drainage systems. Integration of wastewater and stormwater systems will achieve a reduction in the net fresh water imported from other sources. This integrated urban water resources planning approach was first developed by the Urban Water Resources Research Council of the American Society of Civil Engineers (McPherson et al., 1968). Figure 9.1 illustrates an urban water resources management flow diagram that connects the urban hydrological cycle and water distribution into a holistic watershed-based system. An urban water cycle schematic that indicates the components of the hydrologic system and water demands is shown in Figure 9.2 (Heaney et al., 1999). With integrated water management, storm runoff could be used several times, at different points for different purposes, before leaving the urban watershed (McPherson et al., 1968).

STORM RUNOFF FLOW An initial phase of the stormwater reclamation feasibility study is to select methods that adequately describe the storm runoff characteristics of the urban watershed. Based on the local area hydrologic conditions, two different types of flow data are required to design the reclamation systems:

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FIGURE 9.1 Interrelationship of urban water resources distribution. (From Heaney, J.P. et al., EPA/600/R99/029, 1999.)

FIGURE 9.2 The urban hydrologic water cycle and distribution. (From Heaney, J.P. et al., EPA/600/R-99/029, 1999.)

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1. A continuous daily runoff hydrograph from each of the drainage catchments in the watershed to estimate the total volume of water available for reservoir storage and use for any time interval 2. Peak flows in each catchment as a function of rainfall recurrence interval to determine treatment process efficiency and system operation during large storms Methods for calculating both types of data are discussed in the following sections. Continuous Runoff Hydrographs Many methods exist for calculating peak flows from drainage areas; however, there are few methods that can be applied to the development of a daily runoff hydrograph in an area where basic hydrologic data are unavailable. Furthermore, the method in the derivation of a runoff hydrograph for urban subwatersheds has to provide a continuous hydrograph of daily runoff volumes for each catchment and be applicable to the variety of impervious conditions of the subwatersheds. Therefore, several existing approaches to the solution should be examined with the purpose of selecting the best-suited approach and modifying it as necessary to simulate the specific urban hydrologic characteristics. Unit Hydrograph: The unit hydrograph is produced by one unit of effective precipitation (i.e., actual rainfall minus infiltration and other losses) occurring during a specified period of time (Linsley et al., 1958). It is then used to calculate the runoff hydrograph from any distribution of effective rainfall. To determine runoff flow rate for engineering designs, the time intervals required for rainfall input are measured and recorded in minutes on small drainage catchments in urban watersheds. In addition, data of local infiltration and evapotranspiration losses during each rainfall are required for the calculation of effective precipitation. However, precipitation records do not exist for unmonitored drainage areas. Linear Storage Reservoir Model: This computer method developed by the U.S. Department of Agriculture utilizes routings of effective precipitation through linear storage reservoirs with a built-in lag time (Holtan and Overton, 1964). This method generates a runoff hydrograph of individual storms; however, it also requires short-time interval rainfall data as well as the infiltration rate to calculate effective precipitation. Digital Simulation Models: Runoff models estimate runoff flow rate from a drainage area tributary to a drainage system and receiving water based on rainfall data, land use, and physical characteristics. Several models of this type exist with many variations, including the EPA Storm Water Management Model (SWMM), the Corps of Engineers’ Storage, Treatment, Overflow and Runoff Model (STORM), and the Source Loading and Management Model for Stormwater Control (SLAMM) (Pitt et al., 1999). All the models require a great deal of detailed input information to describe the watershed, such as the subcatchment physical characteristics, soil infiltration, and evapotranspiration information. Other commercial stormwater computer packages provide more advanced simulation and are interfaced with geographic information systems. Selection of an appropriate methodology for quantifying stormwater flow should use the best model to simulate a site-specific wet weather flow system.

URBAN STORMWATER QUALITY The origin of stormwater pollution varies widely. Pollutants carried by runoff from urban watershed surfaces mainly come from automobiles, animals, industrial activities, soil erosion, lawn chemical application, decaying vegetation, dry dust fall, roadway deicing, and general litter (Nix, 1994). In recent years, a considerable number of characterization studies have been performed. The reported quality parameters vary considerably in concentration and mass as well as storm runoff flow rate. These variations occur not only with time as the storm progresses, but also with location during

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TABLE 9.1 Heavy Metal Distribution vs. Particle Size Metal Distribution (%)

Suspended Solids, µm

Cd

Co

Cr

Cu

Mn

Ni

Pb

Zn

>100 10–100 <10

18 36 46

9 31 60

5 24 71

7 30 63

8 21 71

8 29 63

4 23 73

5 35 60

Source: Data based on Ellis and Revitt, 1982; Vignoles and Herremans, 1995.

the storm. Additional and significant influences on quality are attributable to land use, drainage system configuration, antecedent dry weather periods (allowing pollutant accumulation), and degree of imperviousness. Because of these multiple variations and the difficulties associated with representative sampling, relationships between cause and effect are largely obscured, even though a considerable amount of data is available. Furthermore, the quality of urban stormwater varies greatly from one metropolitan area to another. Pollutants carried off by urban drainage systems during wet weather originate from many sources: e.g., commercial, industrial, and residential parking areas; roadways; automobile-service stations; sewer infiltration from leaking underground storage tanks; accidents and spills; park and residential lawns; construction sites; and active and inactive industrial sites. Urban storm runoff contains a wide variety of toxic substances that include organic chemicals — e.g., benzene, polynuclear aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), etc. — and heavy metals — arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), and zinc (Zn)) For example, in some studies, the total petroleum hydrocarbon concentration in runoff ranged from <1 to 480 mg/l, and concentrations of individual PAHs ranged from <1 to 120 µg/l, depending on the land use. Storm runoff from highways and industrial areas contain higher toxic organic pollutants than that from residential and commercial areas (Bomboi and Hernandez, 1991; Baldys et al., 1998). Table 9.1 summarizes the distribution of heavy metals and ranges of particle size in urban stormwater. As indicated, most of the heavy metal constituents are associated with solids finer than 10 µm. Thus, selection of a treatment technology for reclaiming urban storm runoff for water supplies must be capable of removing these small particles. Rainwater collected from roof drainage contains less solids, oxygen-demanding organic pollutants, and coliform bacteria than runoff from urban streets and parking lots. However, it still requires proper treatment before indoor usage. Appan (1998) compared the water quality of roof rainwater and laboratory storage tank water with that of potable water; the selected quality parameters are indicated in Table 9.2.

STORMWATER STORAGE TREATMENT SYSTEMS Because of intermittent storm runoff, water reservoirs are necessary for providing a uniform constant inflow to the treatment system for stormwater reclamation. Without upstream reservoir storage, the capacity of stormwater treatment processes would need to be very large, which is more costly, to handle the high peak stormwater flow rate and volume. Thus, it is always significantly more costeffective to store untreated stormwater upstream and treat it at a continuous rate than to treat all stormwater and then store the treated water. Furthermore, construction of a reservoir for stormwater reclamation will have additional benefits for flood control. In some cases, existing flood control structures (e.g., reservoirs) could be retrofitted to provide storage capacity for stormwater reclamation.

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TABLE 9.2 Potable, Rain, and Roof Drain Storage Tank Water Quality Comparison Parameter pH Turbidity (NTU) TSS (mg/l) TDS (mg/l) Hardness (as CaCO3 mg/l) Fecal coliform (MPN/100 mL) Total coliform (MPN/100 mL)

Potable Water

Rainwater

Tank Water

7.0–7.5 <5 N.S. 240–400 20–40 N.S. N.S.

4 5 9 20 0.1–0.4 7 2000

6.3–7.4 8–40 2–9 350–430 27–33 1–9 200–900

Legend: N.S. = not specified; TSS = total suspended solids; TDS = total dissolved solids.

Rainfall

Building Catchment Area/Roof and Driveway

Storage

Treatment

Residential Out-door Water Supply

Residential In-door Water Supply

FIGURE 9.3 Rainwater harvesting system main components.

An optimization analysis will result in the most cost-effective storage treatment of stormwater reclamation. Furthermore, the systems analysis technique can be employed to take into full account interactions between the various hydrologic and hydraulic characteristics of local subwatersheds. For residential area rainwater harvesting, cisterns or storage tanks are the main system components. Others include catchment area (roof and driveway), gutters and downspouts, leaf screens and roofwashers, conveying piping and pump, and water treatment equipment (filter and disinfection). Figure 9.3 shows four major components of the rainwater harvesting system. Selection and design of rainwater harvesting equipment are contained in other published reports, such as the Building Services Research and Information Association (1997) and the Texas Guide to Rainwater Harvesting (TWDB, 1997). Figure 9.4 illustrates the complete water system within a specific urban watershed. Rainfall and water from a public supply are inputs. The system outputs include evapotranspiration, subsurface infiltration, surface drainage out of the watershed, consumptive uses, and sanitary sewer flow. The parts of this overall system subject to the analysis are (1) stormwater storage, (2) stormwater pretreatment, (3) final treatment, (4) treated water storage, and (5) treated water uses. The first four elements can be varied in size and performance within physical and technological constraints. The last component (treated water uses) consists of some fraction of the total water demand, which is a fixed flow for any given watershed. A stormwater reclamation study conceptual diagram is shown in Figure 9.5. Stormwater storage facilities may be constructed in-line or off-line. The in-sewer storage concept is based on using the excess capacity of main trunk sewers for temporary detention of flow. Subsequently, the flow is released to the stormwater treatment facility. Off-line storage is used

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Precipitation

263

Evpo-transpiration

Urban Watershed

Stormwater Collection System

Stormwater

Stormwater

Subsurface

Reclaimed Water Storage

Raw Water Intake

Water Treatment for Public Supply Uses

Stormwater Final Treatment

Advanced Treatment

Reclaimed Water Uses and Public Supply Uses

Sanitary Sewer Collection and Treatment

FIGURE 9.4 An urban basin-wide complete water system.

Return to Effective Storm

Stormwater Storage and Treatment

Losses to Losses to

Reclaimed Stormwater for Supplemental Water Supply

FIGURE 9.5 Stormwater reclamation conceptual diagram.

to attenuate storm-flow peaks, and it also provides some degree of treatment by enabling suspended solids to settle. A multitude of storage facility types and configurations have been used, including geosynthetic membrane-lined basins, concrete tanks, deep tunnels, and underwater bags. Costs of storage structures are dependent on location, land, type, structure, and construction materials. Systems for urban stormwater reclamation include storage and pretreatment, preliminary, secondary, tertiary, and advanced treatment processes. Selection of the treatment level is dependent on the usage of reclaimed stormwater. For example, water for lawn irrigation or car washing requires

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a minimum degree of treatment, whereas that for boiler feed requires a high level. Many unit processes designed for treatment of water and wastewater can be used for stormwater treatment. One such device, the swirl concentrator, achieves both quantity and quality control of storm flow (Field et al., 1997). Because of the high volume and variability associated with storm flow, highrate physical treatment units are considered to be advantageous over biological systems in many situations. Physical treatment alternatives have demonstrated the capability of handling high, variable flow rates and solids concentrations, whereas biological processes are more vulnerable to the variable flow conditions and the relatively high concentration of nonbiodegradable solids in storm flow. A new, high-rate ballasted coagulation clarification process was developed that is capable of coagulating colloidal particles in water at a range of overflow rates from 50 to 100 m3/h/m2 (20 to 40 gpm/ft2) (Delsalle et al., 1998). This high-rate process consists of the addition of a microcarrier (MC) and coagulant into the influent in a mixing chamber followed by flocculation and sedimentation tanks. The initiation of the reaction of coagulation–flocculation processes is improved by the presence of the MC and coagulant, which increases the bonding of the floc to the MC, resulting in higher settling velocities. Other unit process description and performance of typical physicochemical treatment (e.g., sedimentation, microscreening, filtration, and activated-carbon adsorption) can be found elsewhere and are not included in this discussion.

TREATMENT SYSTEMS AND WATER QUALITY The basic treatment and control methods indicated in the previous section are essentially singular processes; however, system optimization should be considered to enhance overall treatment effectiveness and stormwater reclamation potential. This section deals with an integrated treatment train to produce four different levels of water quality classification. The various unit processes form the following control or treatment trains: • Control/pretreatment — Swirl degritter followed by storage or retention basin • Preliminary treatment — Coarse screen and disinfection (Class C) • Secondary treatment — Preliminary treatment plus sedimentation, fine-mesh screening/microscreening, filtration, or dissolved-air flotation, and disinfection (Class B) • Tertiary treatment — Secondary treatment plus activated-carbon adsorption, and disinfection (Class A) • Advanced treatment — Tertiary treatment plus ion-exchange with disinfection (Class AA) A process flow diagram for several advanced physicochemical treatment trains is shown in Figure 9.6. Each train produces a different degree of effluent water quality. The water quality classifications associated with different water uses are as follows: Class AA is intended for highquality application, e.g., steam-generation boiler feed. Class A is intended for routine industrial process supply, which has lower requirements for dissolved mineral removal than Class AA. Class B can be used for industrial cooling and recreational water for fishing. Additional nutrient removal may be warranted. Finally, Class C is intended for lawn irrigation, fire protection, and aesthetic ponds. Table 9.3 indicates maximum concentrations of selected quality parameters. The water quality demands of each use must be evaluated with respect to the four water quality levels described above and defined in Table 9.3. The treatment trains previously described are intended to be only representations of general conditions. Specific unit process selection and system design should only be determined after suitable investigation of the quality and quantity characteristics of the stormwater at the site and after pilot plant evaluations. Four water quality classification levels (AA, A, B, and C) were used to represent a range of urban stormwater reclamation application.

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Water Quality Classification

Treatment Process Train

Storm

Disinfection

C

High-Rate Filtration

B

High-Rate Filtration, GAC, and

A

Swirl Unit Micro Stormwater

High-Rate MC Settling, UlterFiltration, RO, High-Rate MC Settling and Disinfection

AA

B

FIGURE 9.6 Physicochemical treatment trains process flow diagram.

TABLE 9.3 Maximum Concentration of Selected Pollutant by Reuse Category Water Quality Classification and Required Water Quality Maximum Concentration (mg/l unless indicated) Constituent Ammonia (NH3) Arsenic Calcium Chloride Chromium (hexavalent) Copper Cyanide Fluoride Iron Lead Magnesium Manganese Nitrate (as NO3) Oxygen, dissolved (minimum) Phosphates Sulfate Suspended solids Total solids Zinc Coliform (MPN/100 ml) pH (units) Color (units) Turbidity (NTU)

AA

A

B

C

0.50 0.01 0.50 50.00 0.05 1.00 0.01 1.50 0.05 0.05 0.50 0.05 45.00 5.00 1.00 50.00 — 150.00 5.00 1.00 7.00 15.00 0–3

0.50 0.05 75.00 250.00 0.05 1.00 0.20 3.00 0.10 0.10 150.00 0.10 50.00 5.00 1.00 100.00 — 500.00 15.00 70.00 6.00 20.00 3–8

0.50 0.05 75.00 250.00 0.05 1.50 0.20 3.00 0.10 0.10 150.00 0.50 50.00 4.00 1.00 400.00 10.0b 500.00 15.00 240.00 6.00 30.00 8–15

0.50 0.05 75.00 250.00 0.05 1.50 0.20 3.00 0.10 0.10 150.00 0.50 50.00 4.00 1.00a 400.00 30.0b 1500.00 15.00 240.00 6.00 30.00 15–20

a

Based upon maximum concentrations allowed by the U.S. Public Health Service, the World Health Organization, and the Water Quality Standards of the State of Maryland. b Higher suspended solids are permitted by various water quality standards. Limit based on sediment control and water contact recreation. Source: Mallory, C.W., EPA-R2-73-139, 1973.

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HYPOTHETICAL CASE STUDY INTRODUCTION A case study for a hypothetical industrial complex is presented to illustrate an engineering economic analysis of urban stormwater reclamation. Three water supply combinations, including reclaimed urban stormwater or city water, or both, were considered as potential alternatives for this case study: Alternative 1: Use of city water for lawn irrigation, potable, cooling makeup, and process water supply Alternative 2: Use of reclaimed stormwater for lawn irrigation, cooling makeup, process water supply, and city water for potable water supply Alternative 3: Use of reclaimed stormwater for lawn irrigation and cooling makeup, and city water for potable and process water supply

WATER DEMANDS

AND

QUALITY NEEDS

An industrial complex is being established on a 40.5-ha tract, 25% of which is used for open lawn and 75% for buildings, facilities, and roadways. Overall demand for water in the case study is expected to be about 30,000 m3/day, based on an assumed population (10,000 people), lawn irrigation (100 mm/week and 8 months/year), and in-plant cooling and processes. An additional intermittent demand of 34,500 m3/day for storage basin overflow disinfection was estimated based on long-term hydrologic analysis. Table 9.4 summarizes the water demands and water quality classifications. Table 9.5 presents typical water quality requirements for cooling-tower makeup and process supply, which are the assumed hypothetical case study requirements. Stormwater Reclamation System The stormwater reclamation system includes treatment facilities and a storage reservoir. The basic purpose of the system is industrial water supply, but the system also provides the multibenefits of enhanced drainage control, water pollution abatement, and improved local aesthetics. In other words, the storage basin functions as a water reservoir, flow attenuation pond, pollutant load separator, and urban aesthetic lake. For pollution control, the reservoir should be designed and operated to prevent degradation of downstream receiving water quality. For water supply, the significant design criteria are industrial water demand rate, storage capacity, and the reliability with which the demand can be satisfied. In studying storage requirements for the case study, the basin drainage locations (or catchment areas) were analyzed first to determine the industrial plant yield vs. volume/capacity relationships, assuming 100% reliability over a representative 5-year buildup period of stormwater inflow volume. A mass flow–accumulation diagram with the known industrial process demand rate was used. Next, effluent water quality was analyzed in terms of system design and operating requirements. Finally, treatment process design criteria and capacities were developed that met all three plant water use (water classes A, B, and C) requirements. Fine-mesh screening has had relatively wide use for storm flow treatment; it has been adopted for both the main process and for pretreatment in such processes as high-rate filtration and dissolved air flotation (Nebolsine et al., 1972; Lager and Smith, 1974; Lager et al., 1977). Dual-media highrate filters hold promise for stormwater treatment based on large-scale pilot demonstrations (Nebolsine et al., 1972; Lager and Smith, 1974; Lager et al., 1977, Innerfeld and Forndarn, 1979). Carbon adsorption is usually employed as a tertiary polishing step if a higher water quality is desired and has a demonstrated ability to treat stormwater. The design flows and criteria of the unit processes are contained in Table 9.6.

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TABLE 9.4 Water Demands and Quality Classifications (Hypothetical Case Study) Use Category

m3/day

106 m3/year

Classification

Lawn irrigation Cooling makeup Potable Process Overflow disinfection

1,480 19,000 3,370 6,100 34,500

0.356 5.69 1.01 1.83 0.076

C B City water A C

TABLE 9.5 Water Quality Requirements (Hypothetical Case Study) Parameter Total dissolved solids (mg/l) Suspended solids (mg/l) pH (units) Turbidity (NTU) Calcium (mg/l) Iron (mg/l) BOD5 (mg/l) COD (mg/l) Sulfate (mg/l)

Cooling Makeup

Process Supply

1500 25–100 6.9–7.5 20 50 0.5 25 75 200

500 <10 7 5 50 0.3 2.0 5 50

TABLE 9.6 Design Criteria for Facilities (Hypothetical Case Study) Process Unit Flow control/treatment Storage basin (m3) High-rate disinfection units Class C, B, and A Pumping station Fine-mesh screens Class B and A High-rate filters Class A Carbon columns

103 m3/day

m3/s

Design Criteria

758,000 38

— 0.44

30 days 15 min

27 27

0.32 0.32

— 0.034 m/s

25

0.30

0.011 m/s

0.07

0.0034 m/s and 30 min

6.1

Cost Analysis The aforementioned design criteria were used to size the various units needed for the proposed stormwater reclamation treatment train, and then the capital and operating and maintenance (O&M) costs were estimated for the various alternatives. The capital costs include (1) structures, equipment, pumps, and piping; (2) engineering services, legal and administrative (estimated 20% of equipment

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TABLE 9.7 Capital and Annual Costs ($) for Stormwater Treatment (Hypothetical Case Study)a Process/Equipment Flow control/treatment Storage basin High-rate disinfection units Class C, B, and A Pump station Fine screening units Class B and A High-rate filtration unitsc Chemical feed equipment and building Class A Carbon columns Disinfectant feed system a b c

Capital Cost

Amortizationb

Annual Cost O&M

Total

5,326,000 464,000

587,000 51,000

20,000 231,000

607,000 282,000

2,074,000 549,000

229,000 60,000

178,000 49,000

407,000 109,000

2,176,000 362,000

240,000 40,000

71,000 197,000

311,000 237,000

2,346,000 294,000

258,000 32,000

178,000 197,000

436,000 229,000

Unit cost (Benjes, 1976). n = 25 year; i = 10%. Unit cost (Nebolsine et al., 1972).

and installation); and (3) land costs at $320,000/ha. The capital costs were adjusted using the June 1997 Engineering News-Record (ENR) cost indices (construction 5860). Annual costs were based on a 25-year amortization period at 10% interest and 300 days/year of plant operation. The proposed storage reservoir is of lined earthen construction and is 5.5 m deep with a 2.5:1 interior slope, a 3:1 exterior slope, and a 4.9 m top width of levee. The reservoir is aesthetically designed to harmonize with the local environment. Table 9.7 summarizes the capital, O&M, and total annual costs for the proposed stormwater reclamation treatment train (Nebolsine et al., 1972; Benjes et al., 1976). Table 9.8 presents the summary of total annual costs of the hypothetical case study water supply for each alternative. Based on the aforementioned analysis, Alternative 2, which is mainly dependent on stormwater supply, costs more than $6,000,000/year less than the exclusive city water supply alternative. Even Alternative 3, utilizing a combination of storm and city water supply, offers a clear savings of over $4,000,000/year vs. Alternative 1, which utilizes city water only. Further savings are realized when the multipurposes of sewer charges (based on water consumption), pollution control, drainage, and aesthetics are considered.

CONCLUSIONS This hypothetical case study provides evidence that the beneficial use of urban stormwater for industrial subpotable water supply is technically feasible and economically attractive when compared with using the city water source. In addition, other important benefits (e.g., reduction of pollutant discharges, drainage control, creation of recreation and aesthetic ponds, groundwater recharge, and improvement and preservation of ecology in urban areas) will be achieved. Cost for municipal water will vary on a case-by-case basis as will the stormwater inflow volume, industrial process demand, and storage basin and treatment process sizing; therefore, site-specific studies will be necessary for every case before stormwater reclamation is chosen. However, at the very least, the capture and use of stormwater for aesthetic ponds and lawn irrigation will most likely be costeffective, due to low maintenance and low-level treatment requirements for subpotable water supply

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TABLE 9.8 Estimated Total Annual Costs of Water Supply (Hypothetical Case Study) Purpose Alternative 1

Alternative 2

Alternative 3

Irrigation Cooling Process Potable Total Irrigation Cooling Process Potable Total Irrigation Cooling Process Potable Total

Annual Flow 103 m3

Unit Cost ($/m3)

Annual Cost ($)

water water water water

356 5700 1830 1010

0.92a 1.13b 1.25b 0.92a

Class C c Class B c Class A c City water

356 5700 1830 1010

0.19 0.27 0.66 0.92a

Class C c Class B c City water City water

356 5700 1830 1010

0.25 0.35 1.25b 0.92a

328,000 6,398,000 2,286,000 931,000 9,943,000 68,000 1,530,000 1,213,000 931,000 3,750,000 89,000 2,013,000 2,286,000 931,000 5,319,000

Source City City City City

a

Typical water consumption charge rate in Middlesex County, NJ (June 1995). Additional treatment required for control of biological growth, corrosion, or scaling. cTreated stormwater. b

and usage. This approach should not be limited to stormwater reclamation only; it could also be set up to enhance or supplement other recycling efforts for industrial water supply.

REFERENCES Anderson, J.M., 1996. Current water recycling initiatives in Australia scenarios for the 21st century, Water Sci. Technol., 33(1), 37. Appan, A., 1998. Integrated dual-mode roofwater collection system for non-potable uses in the NTU complex, in Proceedings of the 25th Annual Conference on Water Resources Planning and Management, Chicago, IL, June 7–10, 1998, American Society of Civil Engineers, Reston, VA. Baldys, S., Raines, T.H., Mansfield, B.L. and Sandlin, J.T., 1998. Urban Stormwater Quality, Event-Mean Concentration, and Estimates of Stormwater Pollutant Loads, Dallas-Fort Worth Area. Texas, 1992–1993, U.S. Geological Survey, Water Reources Investigation Report 98-4158, 51. Benjes, H.H., Jr., 1976. Cost Estimating Manual: Combined Sewer Overflow Storage and Treatment, EPA600/276286; NTIS PB 266 359, U.S. Environmental Protection Agency, Cincinnati, OH. Bomboi, M.T. and Hernandez, A., 1991. Hydrocarbons in urban runoff — their contribution to the wastewater, Water Res., 25(5), 557–565. Building Services Research and Information Association (1997. Water Conservation: Implications of Using Recycled Greywater and Stored Rainwater in the UK, DWI/BSRIA Final Report 13034/1, The Building Services Research and Information Association, Bracknell, Berkshire, U.K. Delsalle, F., Lepoder, N., and Binot, P., 1998. Method and Installation for Treating an Untreated Flow by Simple Sedimentation after Ballasting with Fine Sand, U.S. patent 5,840,195. Ellis, J.B. and Revitt, D.M., 1982. Incidence of heavy metals in street surface sediments: solubility and grain size studies, Water Air Soil Pollut., 17(1), 87. Field, R. and Fan, C.-Y., 1981. Industrial reuse of urban stormwater, ASCE JEED, 107, EE1. Field, R., Averill, D., and O’Connor, T.P., 1997. Vortex separation technology, Water Qual. Res. J. Can., 32(1), 185–214.

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Gleick, P.H., 1999. The World’s Water: 1998–1999, Island Press, Washington, D.C. Heaney, J.P., Wright, L., and Sample, D., 1999. Sustainable urban water management, in Innovative Urban Wet-Weather Flow Management Systems, EPA/600/R-99/029, U.S. Environmental Protection Agency, Cincinnati, OH. Herrmann, T. and Hase, K., 1996. Way to get water rainwater utilization or long-distance water supply: a holistic assessment, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 1641. Holtan, H.N. and Overton, D.E., 1964. Storage-flow hysteresis in hydrograph synthesis, J. Hydrol., 2. Innerfeld, H. and Forndran, A., 1979. Dual Process High Rate Filtration of Raw Sanitary Sewage and Combined Sewer Overflows, EPA600/279015; NTIS PB 296 626; U.S. Environmental Protection Agency, Cincinnati, OH. Lager, J.A., and Smith, W.G., 1974. Urban Stormwater Management and Technology: An Assessment, EPA670/274040; NTIS PB 240 687, U.S. Environmental Protection Agency, Cincinnati, OH. Lager, J.A., Smith, W.G., Lynard, W.G., Finn, R.M., and Finnemore, E.J., 1977. Urban Stormwater Management and Technology: Update and User’s Guide, EPA 600/877014; NTIS PB 275 654, U.S. Environmental Protection Agency, Cincinnati, OH. Linsley, R.K., Kohler, M.A., and Paulhns, J.L.H., 1958. Hydrology for Engineers, McGraw-Hill, New York. Mallory, C.W., 1973. The Beneficial Use of Stormwater, EPA-R2-73-139, US Environmental Protection Agency, Cincinnati, OH. McPherson, M.B., 1973. Need for metropolitan water balance inventories, J. Hydr. Div. ASCE, 99 (HY10), 1837–1848. McPherson, M.B. et al., 1968. Systematic Study and Development of Long-Range Programs of Urban Water Resources Research, Report to Office of Water Resources Research, NTIS PB 184 318, Washington, D.C. Mitchell, V.G., Mein, R.G., and McMahon, T.A., 1996. Evaluating the resource potential of stormwater and wastewater: an Australian perspective, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/AWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 1293. Nebolsine, R., Harvey, P.J., and Fan, C.Y., 1972. High Rate Filtration of Combined Sewer Overflows, EPA11023EYI04/72; NTIS PB 211 144, U.S. Environmental Protection Agency, Cincinnati, OH. Nelen, A.J.M., deRidder, A.C., and Hartman, E.C., 1996. Planning of a new urban area in a municipality of Ede, using a new approach to environmental protection, in Proc. 7th Int. Conf. on Urban Storm Drainage, IAHR/IAWQ Joint Committee on Urban Storm Drainage, Hannover, Germany, 259. Nix, S.J., 1994. Urban Stormwater Modeling and Simulation, Lewis, Boca Raton, FL. Pitt, R., Melissa, L., Durrans, S.R., Burian, S., Nix, S., Voorhees, J., and Martinson, J., 1999. Integrated WWF Collection and Treatment Systems for Newly Urbanized Areas (Draft), U.S. Environmental Protection Agency, Washington, D.C. Schmidt, C.J., 1975. A Current Municipal Wastewater Reuse Practices Research Needs for the Potable Reuse of Municipal Wastewater, EPA600/975007, U.S. Environmental Protection Agency, Cincinnati, OH. Simon, P., 1998. Tapped Out: The Coming World Crisis in Water and What We Can Do about It, National Press Books, of Washington, D.C./Welcome Rain Publishers, New York. TWDB (Texas Water Development Board), 1997. Texas Guide Manual to Rainwater Harvesting, Texas Water Development Board, Austin. Vignoles, M. and Herremans, L., 1995. Metal pollution of sediments contained in runoff water in the Toulouse City, in Proceedings NOVATECH 95, Second International Conference on Innovative Technologies in Urban Storm Drainage, May 30–June 1, 1995, Lyon, France. Edrydice 92 and GRAIE.

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Low-Impact Development: An Ecologically Sensitive Alternative for Stormwater Management Larry S. Coffman and Michael L. Clar

CONTENTS Introduction ....................................................................................................................................272 Background ....................................................................................................................................273 Why Develop an Alternative Stormwater Technology?.........................................................273 Technical Issues ......................................................................................................................273 Early Efforts — Level I ................................................................................................273 Transition Period — Levels 2 and 3.............................................................................275 Current Period — Levels 4 and 5.................................................................................275 Economic Issues .....................................................................................................................276 Maintenance Burdens of a Growing, Aging Infrastructure..........................................276 Urban Retrofit................................................................................................................277 Environmental Concerns.........................................................................................................277 Pipe-and-Pond Technology and Environmental Concerns ...........................................278 LID and Environmental Concerns ................................................................................279 Political Problems ...................................................................................................................279 Practical Problems ..................................................................................................................279 Better Technology or More Restrictive Land Use Policies ...................................................280 The LID Approach .........................................................................................................................280 Overview .................................................................................................................................280 Benefits of LID .......................................................................................................................281 Five Basic Concepts for LID Designs....................................................................................283 Conservation..................................................................................................................284 Minimization .................................................................................................................284 Timing ...........................................................................................................................284 IMPs ..............................................................................................................................285 Pollution Prevention ......................................................................................................286 Other Important LID Considerations .....................................................................................286 Potential Requirement for Additional Detention Storage ............................................286 Determination of Design Storm Event .........................................................................287 Integrated Management Practices...........................................................................................287 Urban Retrofit ................................................................................................................................288 International Microscale Experience and Case Studies ................................................................288

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German Watershed Planning Using Microscale Approaches ................................................289 Japanese Experimental Sewer System....................................................................................290 Lyon Residential Infiltration Strategies ..................................................................................290 Cost.................................................................................................................................................291 Roadblocks to LID.........................................................................................................................291 Summary ........................................................................................................................................292 References ......................................................................................................................................292

INTRODUCTION This chapter provides a brief introduction and overview of low-impact development (LID) technology. LID is the general term used to describe an alternative innovative comprehensive suite of lot-level land development principles and practices designed to minimize the hydrologic regime alterations that typically result from land use activities, particularly land development This new approach combines a variety of conservation strategies, minimization measures, strategic timing techniques, microscale management practices that are integrated into the landscape features and distributed throughout the site, and pollution prevention measures. Through the combined cumulative beneficial impacts of all the possible integrated site design and LID management techniques, it is now technically feasible to develop a site with little impact on hydrology or water quality. This innovative approach to site planning and design is considered to represent an ecologically sensitive and sustainable strategy for land development. To assist local governments in their efforts to develop more effective economically and environmentally sustainable stormwater management programs, Prince George’s County, Maryland, Department of Environmental Resources (PGDER) with the support of the U.S. Environmental Protection Agency (U.S. EPA) developed a two-volume set of national guidance manuals on the LID approach (U.S. EPA, 2000a, b). EPA provided grant funding to assist PGDER in its efforts to develop national guidance manuals to make this technology available to other local governments. This new approach is a significant step toward advancing the state of the art of stormwater management and will provide valuable and useful tools for local governments in their efforts to control urban runoff. Prince George’s County received the first place U.S. EPA 1998 National Excellence Award for Municipal Stormwater Management Programs for its pioneering work on LID technology. There are now many other efforts currently under way across the nation to further advance LID technologies such as improving the sensitivity of current hydrology and hydraulic analytical models, development of new microscale control approaches and practices for highway design, urban retrofit applications, and numerous monitoring efforts. Some practitioners have found the LID site-oriented microscale control approach to be controversial, as it sometimes conflicts with building codes, challenges conventional stormwater management paradigms, and is perceived by some to accommodate urban sprawl. However, many have found the LID distributed source control technology to be an economical commonsense management approach that can be used to achieve superior environmental protection for new development and provide extremely useful new tools to retrofit existing development. The need for more effective economically sustainable stormwater management technology has never been greater. With the wide array of very complex and challenging goals to be addressed by stormwater programs, many practitioners are beginning to question the efficacy of conventional stormwater management technology to meet these challenges. Communities are struggling with the economic reality of funding stormwater infrastructure maintenance, inspection, enforcement, and public outreach necessary to support an ever-expanding and aging infrastructure. Still more challenging are the exceptionally high costs of retrofitting existing urban development using conventional stormwater management end-of-pipe practices to protect the integrity of receiving waters and living resources.

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Across the country and around the world there are an amazing number of new lot-level control techniques being developed, such as incorporating multifunctional landscapes features, restoring soil functions, using bioretention plant/soil filtration, providing lot-level storage, runoff capture, and use, modifying timing, and preventing pollution. When these various strategies are integrated into the site design, they create a distributed decentralized management approach. The basic goal of LID is to engineer a site with as many small-scale retention, detention, and treatment techniques as needed to achieve the functional equivalent to predevelopment conditions. This chapter only briefly outlines the LID approach and its basic philosophy, control principles, and practices. A more-detailed explanation on the planning, design, and application LID technologies is provided in the two-volume national LID manuals: “An Integrated Design Approach” (EPA, 2000a) and “Hydrologic Analysis” (EPA, 2000b). For more information on how to obtain copies of the national LID guidance manuals, call the Prince George’s County Department of Environmental Resources at (301) 883-5834. It is hoped that the LID national manuals will help to stimulate debate on the state of current stormwater management, watershed protection, and restoration technology and its future direction.

BACKGROUND WHY DEVELOP

AN

ALTERNATIVE STORMWATER TECHNOLOGY?

Numerous jurisdictions throughout the United States have been using conventional “pipe and pond” technology (centralized best management practices, or BMP, treatment) over the past 20 to 30 years and have gained a tremendous amount of experience and insight into the economic and environmental sustainability issues related to the implementation and administration of a massive stormwater infrastructure. Essentially, practitioners have learned that there are serious economic, environmental, public safety, political, and practical limitations associated with many of the conventional BMPs. LID was developed to address and reduce many of these limitations and burdens, some of which are listed and discussed below: 1. Technical Issues: Particularly the validity and viability of pipe-and-pond control strategies 2. Economic Issues: Particularly soundness of existing control strategies, including maintenance burdens of a growing, aging infrastructure 3. Environmental Concerns 4. Political Problems 5. Practical Problems

TECHNICAL ISSUES Stormwater management (SWM) technology has been in use for over 30 years. The initial response of the technical community, largely dominated by municipal public works agencies and the soil conservation districts, was to apply the National Resources Conservation Service (NRCS) flood control technology for small watersheds. This technology, described in the NRCS publication, “Urban Hydrology for Small Watersheds: TR-55 (NRCS, 1986), and referred to as the pipe-andpond or end-of-pipe technology, consists of controlling the peak discharge from the postdevelopment land use conditions to a release rate equal to the predevelopment peak discharge for one or more design storms. Early Efforts — Level I During the first decade (1970s) of SWM the primary focus was on flooding and stream channel erosion. Flood control was provided by controlling the release rates for the 10- and 100-year storms to predevelopment conditions. Control of the 2-year storm was used as a surrogate for channel

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TABLE 10.1 Description of Various Levels of BMP Performance Level Level 1

Level 2

Level 3

Level 4

Level 5

Description This is the level provided by the NPDES Storm Water Program regulations. It provides two performance criteria that are closely related, flood control and peak discharge control. This requirement is generally implemented by controlling the postdevelopment peak discharges for one or more design storms to the predevelopment levels. The two most frequently used storms are the 2- and 10-year storms. Some degree of pollutant removal may be obtained with this level, depending on the type and design of BMP used to meet the peak discharge criteria. This level is provided by the Guidance Specifying Management Measures of Nonpoint Pollution in Coastal Waters, issued in 1993 by the U.S. EPA pursuant to the Coastal Zone Act Reauthorization Amendments (CZARA) of 1990. The CZARA guidance includes requirements for municipalities located in coastal states. This level specifies the same criteria as Level 1 but in addition requires 80% removal of the total suspended solids (TSS) from construction sites. This level is frequently encountered in more environmentally active municipalities and states. It is also the performance level used in the ASCE National Storm Water BMP Database. It defines performance with respect to three traditional criteria: (1) pollutant removal effectiveness, (2) peak discharge control effectiveness, and (3) flood control. It differs from Level 2 in that there is generally some mandated volume of control, typically the first 1/2 in. or first inch of runoff for water quality and pollutant removal. Although no specific pollutant removal requirements are typically used, it is generally assumed that the pollutant removal levels reported in the literature can be achieved. This level, which has recently been developed by the Center for Watershed Protection for the State of Maryland, is referred to as the “Unified Sizing Criteria” (MDE, 2000). It takes a broader definition of receiving water impacts and includes two additional criteria for BMP performance to supplement the three criteria found in Level 3. These additional criteria are maintenance of groundwater recharge functions and receiving channel protection criteria using extended detention control concepts. This level, which represents an attempt to provide an ecologically sustainable approach to SWM, is currently under development by a number of groups throughout the United States. It includes the joint effort by Tetra Tech, Inc., and Prince George’s County (U.S. EPA, 2000a,b), Yoder (1995), and Snodgrass et al. (1998) among others. This level uses an integrated approach including biological, chemical, and physical criteria to define BMP performance. A combination of water quality, biohabitat, and geomorphic criteria is used to evaluate whether a receiving stream is at the targeted goal of “fishable and swimmable,” or the extent of departure from this goal. A number of additional parameters are added to the Level 2 performance criteria: (1) stream buffer retention and thermal impact considerations, (2) volume control considerations, such as considerations presented in the low-impact development concept approach, are added to the peak discharge and groundwater recharge criteria to achieve maintenance of hydrologic function at a site-specific level, and (3) geomorphic criteria as described by Dunne and Leopold (1978), Lane (1955), Leopold et al. (1964), Leopold (1994), Rosgen (1996), and others are incorporated to supplement or replace extended detention approaches to achieving channel stability.

protection. This level SWM has been described as Level 1, and is summarized in Table 10.1 (Clar et al., 2001). This is basically the SWM requirements specified in the National Pollutant Discharge Elimination System (NPDES) Phase II program, and is the most widely used approach to SWM for many jurisdictions throughout the United States (Clar et al., 2001). Many stormwater practitioners are under the impression that this approach represents an adequate level of protection for our nation’s receiving waters as specified under the Clean Water Act. Unfortunately there is very little field monitoring data that relates BMP performance with receiving water conditions, and certainly no data that support this belief. Quite the contrary, watershed, hydrologic, geomorphic, biologic, and chemical science, as well as limited field monitoring data, clearly demonstrate that this approach by itself is not sufficient to adequately mitigate the impacts from land use changes and to prevent the degradation and impairment of designated uses of the receiving waters as mandated by the Clean Water Act (Swietlik, 2001).

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Transition Period — Levels 2 and 3 During the second decade (1980s) of SWM, the focus was expanded to include water quality control. This awareness of water quality issues was the result in large part of the U.S. EPA National Urban Runoff Program (NURP, 1983). This program characterized the concentration of pollutants in urban runoff from representative land uses throughout the United States. As an outgrowth of this information a number of jurisdictions added requirements to provide water quality control. This approach is described as the Level 2 and/or Level 3 approach (Clar et al., 2001). Water quality management added some new perspectives to stormwater hydrology. Water quality control required that the annual volume of runoff be considered as a basis for control rather that a single flood type design storm such as the 2-, 10-, or 100-year storm. Many jurisdictions selected the first 0.5 to 1.0 in. of rainfall as water quality design storm. Also, the concept of small storm hydrology was developed (Pitt, 1994). Although the addition of water quality control to SWM technology improved the level of management, there is no evidence that it stopped the degradation and impairment of urbanizing streams. Current Period — Levels 4 and 5 During the third decade (1990s) the growing concern for the continuing degradation of the receiving waters led a number of jurisdictions to continue development of improved SWM technology. Two significant developments in SWM technology included the development of the Maryland 2000 SWM Design Manual (MDE, 2000), and also the development of the LID technology by Prince George’s County, Maryland. The MDE 2000 manual provides a multiparameter control approach referred to as a Level 4 approach (Clar et al., 2001). This multiparameter approach includes five design criteria or parameters, referred to as the “Unified Sizing Criteria”: 1. 2. 3. 4. 5.

Groundwater recharge criteria, Rev Water quality criteria, WQv Channel protection criteria, Cpv Overbank flooding criteria, Q10 Extreme flood volume criteria, Qf

Criterion 1, the groundwater recharge criterion, is a relatively new criterion for SWM and was developed to address the concern for the impacts on groundwater recharge, lowering of wells, lowering or loss of base flow to small streams, saltwater intrusion in coastal areas, and settlement of structures. Only a few jurisdictions including the states of Maryland and Massachusetts have adopted this criterion. Criterion 2, the water quality criterion, is not new, but Maryland increased the control requirement from the first 0.5 in. to the first inch in order to capture and treat 90% of the annual runoff volume. Criterion 3, the channel protection criterion, is also not new, but Maryland replaced the use of the 2-year predevelopment storm as a surrogate for channel protection, with the use of the 1-year storm with extended detention, which effectively reduces the allowable release rate to a 2-in. storm event, or approximately 25 to 50% of the predevelopment peak discharge rate. Criteria 4 and 5 are the traditional flood control requirements for the 10- and 100-year storms. The LID approach to SWM is the subject of this chapter and is addressed in detail. It represents a Level 5 approach to SWM. This level has been described as an attempt to provide an ecologically sustainable approach to SWM (Clar et al., 2001). This level uses an integrated approach including biological, chemical, and physical criteria to define BMP performance. A combination of water quality, biohabitat, and geomorphic criteria are used to evaluate whether a receiving stream is at the targeted goal of “fishable and swimmable,” or the extent of departure from this goal. A number of additional parameters are added to the Level 2 performance criteria:

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1. Stream buffer retention and thermal impact considerations 2. Volume control considerations, such as considerations presented in the LID concept approach, which are added to the peak discharge and groundwater recharge criteria to achieve maintenance of hydrologic function at a site-specific level 3. Geomorphic criteria, as described by Dunne and Leopold (1978), Lane (1955), Leopold et al. (1964), Leopold (1994), Rosgen (1996), and others, which are incorporated to supplement or replace extended detention approaches to achieving channel stability In summary, it can be observed that SWM technology has experienced a considerable degree of maturity and improvement over the past 30 years, which has reflected our increased understanding of the complicated cause-and-effect relationships between land disturbance activities and the corresponding responses of the natural systems, particularly the riparian zones and receiving waters. This condition forms the background that led to the need and development of LID technology.

ECONOMIC ISSUES There are a number of economic issues that must be addressed with respect to SWM technology. The financial burden of providing SWM technology is considerable. Currently, the United States is converting on an annual basis approximately 2 million acres of land from a rural to an urban land use. The costs of providing SWM vary considerably by jurisdiction, and by the level of control that is required. However, using a conservative range of $1000 to $5000/acre for SWM control produces an annual national estimate of approximately $2 billion to $10 billion per year, just for the construction of new facilities. The total annual cost also includes the annual inspection, maintenance, and replacement costs of this infrastructure, and the cost of retrofitting existing areas that were developed prior to the existence of the Clean Water Act, but which contribute large volumes of runoff and pollutants to our nation’s waters. The costs associated with restoring these areas, particularly areas with combined sewer infrastructure, easily push the annual expenditure for SWM technology well over a trillion dollars per year. Given the level of expenditures that are being committed to SWM technology, it is crucial to assess the effectiveness of these expenditures to ensure that the overriding goals of these efforts are achieved. Unfortunately, as the previous discussion on technology pointed out, the majority of jurisdictions in the United States are requiring the use of Levels 1, 2 or 3, which has been documented to be inadequate to prevent the degradation and impairment of receiving waters. Maintenance Burdens of a Growing, Aging Infrastructure As experience is gained with the current management technology, many highly urbanized jurisdictions are beginning to question the efficacy of traditional structural approaches to meet complex environmental objectives. They are also finding it more difficult to fund the inspection and enforcement and to maintain programs necessary for the massive SWM infrastructure created by conventional approaches. Some larger, highly urbanized jurisdictions now have the responsibility for the maintenance, inspection, and enforcement of thousands of BMPs (ponds, infiltration practices, and filters), thousands of miles pipes and gutters, and tens of thousands of structures (inlets, manholes, and catchbasins). This infrastructure, like all urban infrastructure, is growing and aging at the same time. Many of the oldest BMPs have reached their expected service life and are failing. Most jurisdictions have reached the point where they can no longer afford to adequately pay for the upkeep of their stormwater BMP infrastructure. For example, in Prince George’s County (population 800,000) the annual stormwater maintenance budget is approximately $6.5 million and is rising every year by about $250,000. Generally, most jurisdictions cannot afford to have proactive maintenance programs for the current suite of conventional BMPs. Maintenance occurs when there are complaints or a total

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system failure that results in “detectable” damage. Many infiltration or underground treatment systems are never inspected or maintained. Survey and studies of these devices show a failure rate of about 50% after 5 years of operation. In most cases, neither the property owner nor the local jurisdiction can afford to maintain these BMPs and therefore do not. For those jurisdictions that do not have a dedicated funding source for their stormwater programs, the problem of affordability is only compounded, as they cannot successfully compete for resources against police, education, and fire services. The question arises: Why would we continue to build treatment systems that we simply cannot afford to maintain? Also, just how effective can current BMPs be in meeting protection goals if they are rarely or never maintained? Urban Retrofit Many highly urbanized areas are almost entirely covered by impervious surfaces such as roadways, parking lots, sidewalks, and buildings. Control of runoff in urban areas using conventional SWM practices is difficult and severely limited due to the lack of open space, cost of land, high construction costs, and high operational and maintenance costs. Typical urban stormwater controls involve construction of expensive end-of-pipe detention facilities, infiltration systems, underground storage, systems to optimize in pipe storage, and the use of water quality BMPs (filters and hydraulic separators). Conventional SWM retrofit approaches (ultraurban BMPs and end-of-pipe storage) also have limitations due to costs and physical requirements. To address urban runoff problems adequately, cities must have effective, low-cost, and politically acceptable tools. If dramatic improvements in urban runoff management are to be achieved, it will be necessary to fundamentally rethink current approaches and radically redesign and reengineer urban SWM technology.

ENVIRONMENTAL CONCERNS Knowledge of the environmental issues and concerns associated with the impacts of land use change activities on receiving waters has been increasing steadily over the years and is reflected in the increasing levels of SWM protection described above. In general, the degradation process to receiving waters caused by urbanization has the following pattern. Land use and cover changes, compacted soils, and creation of efficient drainage/conveyance systems using connected impervious surfaces lead to very significant changes in the hydrologic cycle or regime of the watershed. These changes consist primarily of a reduction or loss in the initial rainfall abstractions, Ia. These processes include water intercepted by vegetation, water retained in surface depressions, evaporation, and infiltration. The reduction or loss of these initial abstractions, together with accompanying decreases in the time of concentration, Tc (both sheet flow and overland flow), leads to significant increases in both the runoff volume and peak discharge values. The receiving streams respond to these hydrologic regime changes by increasing their crosssectional areas, usually through a combination of channel down-cutting and bank erosion that produces staggering volumes of sediment and results in the destruction and loss of stream habitat and the related reduction and loss of biologic species. In addition, the surface runoff flowing over the impervious surfaces typically displays elevated pollutant levels and temperatures that can be very damaging to most fish species. The reduction or loss of the initial abstractions can reduce groundwater recharge, lower water tables, and reduce or cease base flow to small streams, particularly during dry periods. The U.S. EPA has recently developed a comprehensive identification and assessment of the environmental impacts due to runoff. Table 10.2 summarizes the four major categories of impacts identified: (1) physical, (2) habitat, (3) biological, and (4) chemical or water quality (Clar et al., 2001). In this assessment the habitat and biologic categories are combined into one category as habitat/biological, because these two categories are so closely related. As Table 10.2 indicates there

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TABLE 10.2 Categories of Impacts Attributable to Construction and Land Development Activities Category

Impact Type/Metric

Physical

Hydrologic regime Runoff volume Peak discharge Flow duration Flow frequency Groundwater recharge Water table elevation Base flows Channel geometry Channel lining Flooding Sediment transport Thermal Attachment sites Embeddedness Fish shelter Channel alteration Sediment deposition Stream velocity and depth Channel flow status Bank vegetation protection Bank condition score Riparian vegetation zone Total taxa Ephemeroptera Plecoptera, Tricoptera EPT (taxa) % taxa % EPT Family biotic index (FBI) Sediment Nutrients Metals Oil and grease Pathogens Organic carbon Herbicides/pesticides MTBE Deicer

Habitat

Biologic

Chemical (water quality)

Impairment or Change in Beneficial Use

Groundwater recharge, hydrologic balance Flooding, channel erosion Channel erosion, habitat impairment Channel erosion, habitat impairment Water table, base flows, habitat Local wells, springs, base flow, wetlands, habitat Habitat Bank stability, vegetative cover, habitat Downstream erosion, channel stability, habitat Loss of property, or damage Channel stability, habitat Habitat impairment Impairment or loss of habitat structure results in reduction or losses in biologic conditions and communities

Biologic conditions and communities can be reduced or eliminated as a result of impairment or loss of habitat structure caused by physical impacts resulting from construction and development activities

Water quality degradation or impairment can have many negative consequences; drinking water violations, increased water treatment costs, beach closures, shellfish bed closures, loss of boating use, fishery loss, reduction of reservoir and lake volumes due to sediment volume

were 12 individual metrics identified for the physical impact category; 10 for the habitat category, 5 for the biological category, and 9 for the chemical or water quality category. Thus, for all the categories combined, a total of 36 impact metrics were identified. Pipe-and-Pond Technology and Environmental Concerns There are now many studies that demonstrate or strongly suggest that, for example, SWM ponds can create their own unique set of environmental impacts. These include problems associated with fish blockages, thermal pollution, groundwater contamination, bioaccumulation of toxics, export of

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nutrients, sediments, and toxics during high flows, and increases in stream erosion. Perhaps the greatest failure of the centralized pond approach is that it does not and cannot replicate predevelopment hydrology. Ponds can only be designed to reproduce peak discharges. They do not reestablish the predevelopment rainfall/runoff/recharge volume relationships or maintain the natural frequency of surface discharge. The changes in hydrology resulting from the pipe-and-pond approach accelerate stream channel erosion changing stream morphology and adversely affecting aquatic habitat structure such as pools, riffles, and shading needed to protect the biological integrity of aquatic biota. Because ponds have their own set of impacts, it is questionable whether this control technology can be helpful in maintaining or restoring the ecological integrity of a receiving stream and its biota. Furthermore, because current management practices only mitigation or lessens the effects of urban development, there is concern about the cumulative impacts of the widespread use of conventional mitigation practices. With the use of conventional management, continued growth will allow increases in pollutant loads and fundamental alterations in the hydrologic regime of a watershed. At best, conventional approaches only slow the rate of change but allow an overall net increase in adverse environmental impacts including pollutant loads and hydrodynamic modifications. LID and Environmental Concerns A watershed hydrologic cycle is changed by the way sites and drainage systems are designed and constructed, which creates an efficient drainage system that completely alters the natural hydrologic regime. The key to addressing and controlling these impacts must focus on controlling or at least minimizing the changes to the hydrologic cycle or regime. Simply relying on TIA reduction and conventional detention is not feasible, practical, or sustainable. The LID technology provides a comprehensive toolbox of techniques that allows the recreation of the initial abstraction volumes and frequency of discharges to mimic natural hydrologic processes and thus preclude the traditional impairment associated with urbanization. The effective use of LID site design techniques can significantly reduce the cost of providing SWM. Savings are achieved by eliminating the use of SWM ponds and reducing pipes, inlet structures, curbs and gutters, roadway paving, grading, and clearing. Where LID techniques are applicable and depending on the type of development and site constraints, stormwater and site development costs can be reduced by 10 to 25% compared with conventional approaches. LID allows for the same or in some cases higher lot yields compared to conventional approaches. Because SWM is control on each lot using a multifunctional landscape, that portion of the buildable area that would have been used for stormwater ponds can now be recovered and used for building, parking lots, open space, or habitat enhancements.

POLITICAL PROBLEMS A tremendous number of complaints are generated by the current predominant use of SWM ponds. These complaints deal with issues such as public safety (drowning and mosquito-borne diseases), lack of maintenance (aesthetic), maintenance costs, and property owner legal liabilities, i.e., insurance costs. Justifying the continued use of technology that is viewed (real or perceived) by the public as a liability is becoming problematic as political pressure rises to find more acceptable, sustainable, and safer solutions.

PRACTICAL PROBLEMS A dilemma for local governments is that they are confronted with many protection and restoration goals. They must respond to a wide variety of state and federal regulations and address unique local needs associated with the adverse impacts of urban runoff. Local governments have the difficult task of developing complex multiobjective SWM programs. They require multidisciplinary and integrated approaches with the need for as many tools as necessary to meet the desired objective.

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New regulatory programs such as NPDES Phase II and/or TMDLs now focus on specific targeted issues of concern or compliance requirements. Can conventional technologies meet new goals in a cost-effective and sustainable manner? LID provides many new cost-effective principles and practices that can be added to existing technologies that to help “tailor” a program that meets the economic and environmental needs of each community.

BETTER TECHNOLOGY

OR

MORE RESTRICTIVE LAND USE POLICIES

Generally, land use decisions and development are not typically organized around a set of strong environmental or conservation principles. For the most part, development occurs and is organized around typical land use principles based on economic needs, individual property rights, public policy, and politics. There are exceptions where environmental laws (wetland and endangered species) can delay development until an acceptable mitigation option is worked out by the regulators or courts. In the end, there is a high degree of certainty that development (perhaps conditioned) will not be stopped based solely on environmental constraints. Furthermore, most local governments need continued development or redevelopment to maintain an adequate tax base and are almost always supportive of economic development projects. The problem with relying too heavily on land use controls (conservation measures alone) to protect natural resources and receiving waters is that they depend on firm and continued political support. The practical reality is that political support is ephemeral, especially in the face of economic development pressures. Given that development cannot be stopped and that current technology does not reduce new development impacts to predevelopment levels, the impacts of urbanization will continue to increase (perhaps at a slower rate). There is a need to develop appropriate technology that will ensure no net increase in pollutant loads or change in the ecosystem hydrology. The development and use of better technology is key to protecting receiving waters and ecosystems from continued and rapidly increasing urbanization. Better technology does not mean just better, more efficient BMPs. Technology is defined in the broadest sense as a comprehensive spectrum of planning and design techniques that balances the appropriate level of conservation, minimization, and control techniques to ensure land uses will not impact receiving waters and economic development can continue. The advantage of a better technology approach is that once established it is slow to change and its application is less susceptible to changes in political points of views or economic development pressures. Generally, technology is apolitical and can be supported by growth, no-growth, and conservation proponents, alike.

THE LID APPROACH OVERVIEW The LID approach combines a hydrologically functional site design with pollution prevention measures to compensate for land development impacts on hydrology and water quality. The primary goal of LID methods is to mimic the predevelopment site hydrology by using site design techniques that store, infiltrate, evaporate, and detain runoff. Use of these techniques helps to reduce off-site runoff and ensure adequate groundwater recharge. Because every aspect of site development affects the hydrologic response of the site, LID control techniques focus mainly on site hydrology. There are a wide array of impact reduction and site design techniques that allow the site planner/engineer to create stormwater control mechanisms that function in a manner similar to that of natural control mechanisms. If LID techniques can be used for a particular site, the net result will be to mimic more closely the natural hydrologic functions of the watersheds or the water balance among runoff, infiltration, storage, groundwater recharge, and evapotranspiration. With the LID approach, receiving waters may experience fewer negative impacts in the volume, frequency,

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The LID Approach

FIGURE 10.1 Major components of the LID approach.

and quality of runoff, so as to maintain base flows and more closely approximate predevelopment runoff conditions. Figure 10.1 summarizes the major components of the LID approach. These include (1) site planning, (2) LID hydrologic analysis, (3) selection of LID integrated management practices (IMPs), (4) LID erosion and sediment control, and (5) public outreach efforts. These components are described in detail in the U.S. EPA publication, “Low Impact Development Design Strategies: An Integrated Design Approach” (U.S. EPA, 2000a). This chapter highlights selected elements of these components, including the benefits of LID, five basic steps for LID designs, potential requirements for LID storage, determination of the design storm, and selection of IMPs.

BENEFITS

OF

LID

LID is a powerful technology that allows development to take place in a manner that can preserve water-related ecological functions/relationships and maintain development potential. LID achieves SWM and ecosystem protection goals through the cumulative effects of a wide array of techniques. LID uses new site design planning principles, microscale management practices, and pollution prevention to create environmentally sensitive landscapes that allow the developed area to remain a functioning part of the ecosystems instead being dysfunctional and apart from the ecosystem. LID maintains or restores the hydrologic regime and manages stormwater by fundamentally changing conventional site design to create a hydrologically functional landscape that mimics natural ecological hydrologic functions. LID provides numerous tools to maintain the predevelopment volume relationship between rainfall/runoff, recharge, interflow, and evaporation. This is accomplished in five basic steps for new development:

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1. Apply conventional conservation planning techniques to define the building envelope. These would include master zoning and environmental features such as streams, wetlands, forests, agricultural/historical preservation, trails, and open space, etc. 2. Apply impact minimization strategies to the extent practicable (or allowable) by reducing imperviousness, reducing use of pipes, and minimizing clearing and grading, etc. 3. Maintain predevelopment time of concentration by strategically routing flows to maintain travel time throughout the site. 4. Apply distributed IMPs to treat, detain, retain, and infiltrate runoff to restore predevelopment conditions. These practices would include use of multifunctional open swales, bioswales, infiltration practices, bioretention (rain gardens) water capture and use (rain barrels), and depression storage in conservation areas. 5. Provide effective public education and socioeconomic incentives to ensure property owners use effective pollution prevention measures and maintain on-site management practices. LID is a new and creative way of thinking about site design to make every site landscape, roadway, and building feature (green space, landscaping, grading, streetscapes, roads, parking lots, roofs, etc.) multifunctional, multibeneficial, and optimized to manage, treat, or use runoff to maintain/restore hydrologic functions. The effective use of LID site design techniques can significantly reduce the cost of providing SWM. Savings are achieved by eliminating the use of SWM ponds, and reducing pipes, inlet structures, curbs and gutters, less roadway paving, less grading, and clearing. Where LID techniques are applicable and depending on the type of development and site constraints, stormwater and site development design, construction, and maintenance costs can be reduced by 25 to 30% compared with conventional approaches. The creation of the LID lot-level management principles and practices have led to the development of new tools to retrofit existing urban development. Microscale decentralized management practices to recharge, filter, retain, and detain runoff can be easily integrated into the existing green space, parking lots, building design, landscaping, and streetscapes. These IMPs can be constructed as part of the routine maintenance and repair of urban infrastructure requiring less capital outlay for retrofit compared to conventional large-scale, highly capitalized centralized approaches. LID microscale techniques have been shown to reduce the cost of retrofitting existing urban development. Reducing urban retrofit costs will increase the ability of cities to implement effective retrofit programs to reduce the frequency, and improve the quality, of combined sewer overflows (CSOs) and improve the quality of urban runoff to protect receiving waters. Currently, every site is designed and constructed with one basic overriding goal — to achieve good drainage. Move runoff off the site as quickly as possible to the conveyance system and centralized BMP treatment device. As a site is developed, its hydrologic functions are first altered on a microscale to create a highly efficient drainage system. The cumulative impacts of these microscale changes result in drastically altered hydrologic regimes that end-of-pipe management practices typically try to mitigate. If sites can be designed to achieve good drainage, why not design sites with the opposite objective to maintain predevelopment hydrologic functions? Can sites be intelligently engineered to replace the microscale hydrologic functions, and would the cumulative beneficial effects result in the preservation of natural watershed hydrologic functions? Can a site be designed in a way to remain a functional part of the hydrological regime of an ecosystem or at least more closely mimic natural hydrologic functions? To create hydrologically functional sites there must be a radical change in thinking. It is necessary to restore hydrologic functions, not just mitigate development impacts.

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TABLE 10.3 Steps in LID Site Planning Process Step Step Step Step Step Step Step Step Step

1 2 3 4 5 6 7 8 9

FIVE BASIC CONCEPTS

Identify applicable zoning, land use, subdivision, and other local regulations Define development envelope Use drainage/hydrology as a design element Reduce/minimize total site impervious areas Integrate preliminary site layout plan Minimize directly connected impervious areas Modify/increase drainage flow paths Compare pre- and postdevelopment hydrology Complete LID site plan

FOR

LID DESIGNS

The LID approach comprises the following five basic concepts: 1. 2. 3. 4. 5.

Conservation Minimization Flow (Timing) manipulation IMPs (maintain curve number, or CN, and runoff volume) Pollution prevention

These concepts follow a systematic approach to site planning and design that are summarized in Table 10.3. The LID techniques are not necessarily new, but the principle of combining all of these techniques in a manner that produces a comprehensive approach of distributed management is new. What is also new is that the LID guidance manuals provide an analytical methodology based on TR-55, which allows one to determine the hydrologic impacts of the combined affects of all the LID practices. The objective of LID site design is to manage, recharge, detain, and retain runoff volumes uniformly throughout the site to mimic predevelopment hydrologic functions. Uniform distribution of small, on-lot retention and detention to control both runoff discharge volume and rate is the key to better replication of predevelopment hydrology. The relative change in the frequency and duration of runoff is also much closer to predevelopment conditions than can be achieved by typical application of conventional centralized BMPs such as ponds. Management of both runoff volume and peak runoff rate is included in the design of controls. This is in contrast to conventional endof-pipe treatment, which completely alters the watershed hydrology to create a new, modified hydrologic regime. The LID site analytical analysis and design approach focuses on four major hydrologic elements. These fundamental factors affect site hydrology and are introduced below: 1. Curve Number (CN): A factor that accounts for the effects of soils and land cover on the amount runoff generated. Minimizing the magnitude of change from the predevelopment to the postdevelopment CN by reducing impervious areas and preserving more natural vegetation will reduce runoff storage requirements and help to maintain predevelopment runoff volumes. 2. Time of Concentration (Tc): The time it takes runoff to travel through the watershed. Maintaining the predevelopment Tc reduces peak runoff rates and can be achieved by lengthening flow paths, reducing the use of pipes and paved channels, and conservation of natural drainage and depression storage.

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3. Permanent storage areas (Retention): Retention storage is needed for volume and peak control, as well as water quality control and to maintain the same CN as the predevelopment condition. 4. Temporary storage areas (Detention): Detention storage may be needed to maintain the peak runoff rate or to prevent flooding. Conservation The first concept of LID is to minimize or prevent avoidable land disturbances that increase the CN value and result in higher runoff volumes. This step is similar to traditional techniques of maximizing natural resource conservation, limiting disturbance, and restoring impacted natural resources. This includes considering conservation requirements and watershed plan components such as parks, open space, streams, steep slopes, and permeable soils. These are the typical conservation techniques that help to define the buildable area of the site. Conservation techniques also include maintaining natural drainage patterns, topography, and depressions, preserving as much existing vegetation as possible in pervious soils, hydrologic soil groups A and B, and revegetating cleared and graded areas. These measures help to minimize the change in the pre- and postdevelopment CN. Minimization Calculation of the LID CN is based on a detailed evaluation of the existing and proposed land cover so that an accurate representation of the potential for runoff can be obtained. This calculation requires the engineer/planner to investigate the following key parameters associated with LID: (1) land cover type, (2) percentage of and connectivity of impervious cover, (3) hydrologic soils group (HSG), (4) hydrologic conditions (average moisture or runoff conditions), and (5) existing drainage patterns and natural retention features. Reducing the change in CN alone will reduce both the postdevelopment peak discharge rate and volume. The following are examples of LID site planning practices that can be utilized to achieve a substantial reduction in the change of the calculated CN: narrower driveways and roads (minimizing impervious areas), site fingerprinting (minimal disturbance), open drainage swales, preservation of soils with high infiltration rates, location of BMPs on high-infiltration soils, disconnecting impervious surfaces to direct and disburse runoff to soil groups A and B, flattening slopes within cleared areas to facilitate on-lot storage and infiltration, and construction of impervious features on soils with low infiltration rates. Timing The LID hydrologic evaluation requires that the postdevelopment Tc be close to the predevelopment Tc. This is important because LID is based on a homogeneous land cover and distributed retention and detention of on-site BMPs. The following site planning techniques can be used to maintain the existing Tc: 1. Maintaining predevelopment flow path length by dispersing and redirecting flows using open swales and natural or vegetated drainage patterns 2. Increasing surface roughness (e.g., preserving woodlands, vegetated swales) 3. Detaining flows (e.g., open swales, bioretention) 4. Minimizing disturbances (minimizing compaction and changes to existing vegetation) 5. Flattening grades in impacted areas 6. Disconnecting impervious areas (e.g., eliminating curb/gutter and redirecting down spouts) 7. Connecting pervious areas to vegetated areas

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TABLE 10.4 Hydrologic Functions of LID IMPs IMP Hydrologic Function

Bioretention

Dry Well

Filter/Buffer

Grass Swale

Rain Barrel

Cistern

Infiltration Trench

Interception Dep. storage Infiltration Groundwater recharge Runoff volume Peak discharge Runoff frequency Water quality Base flow Stream quality

H H H H H M H H M H

N N H H H L M H H H

H H M M M L M H H H

M H M M M M M H M M

N N N N L M M L M N

N N N N M M M L N L

N M H H H M M H L H

Note: H = high; M = moderate; L = low; N = none.

TABLE 10.5 Reported Pollutant Removal Efficiency of IMPs IMP Bioretention Dry well Infiltration trench Filter/buffer Grass swale Infiltration swale Wet swale Rain barrel Cistern

TSS

Total P

Total N

Zinc

Lead

BOD

Bacteria

99 99 99 20–99 30–65 90 80 NA NA

81 40–60 40–60 0–60 10–25 65 20 NA NA

43 40–60 40–60 0–60 0–15 50 40 NA NA

99 80–100 80–100 20–100 20–50 80–90 40–70 NA NA

99 80–100 80–100 20–100 20–50 80–90 40–70 NA NA

NA 60–80 60–80 0–80 NA NA NA NA NA

NA 60–80 60–80 NA Negligible NA NA NA NA

Source: CRC, 1996; Davis et al., 1997; MWCG, 1987; Urbonas and Stahre, 1993; Yousef et al., 1985; Yu et al., 1992; 1993.

Combined use of these techniques, and those to reduce the change in the CN, can modify runoff characteristics to effectively shift the postdevelopment peak runoff time toward that of the predevelopment condition. IMPs Once the postdevelopment Tc is maintained at the predevelopment conditions and the change of predevelopment to postdevelopment CN is minimized, any additional reductions in runoff volume must be accomplished through distributed on-site stormwater IMPs. The goal is to select the appropriate combination of IMPs that simulates the hydrologic functions of the predevelopment condition to maintain existing CN and corresponding runoff volume. Table 10.4 summarizes the hydrologic functions that are addressed by various BMPs. Table 10.5 summarizes the reported pollutant removal efficiency of IMPs. LID design strives to maximize the runoff use and retention practices distributed throughout the site to provide the required volume controls at the source.

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Retention storage allows for a reduction in the postdevelopment volume and the peak runoff rate. The increased storage and infiltration capacity of retention BMPs allow the predevelopment volume to be maintained. The most appropriate on-lot retention BMPs include, (1) bioretention cells (rain gardens), (2) infiltration trenches, and (3) rain barrels. Other possible retention BMPs include retention ponds, rooftop storage, cisterns, and irrigation ponds. It may be more difficult to distribute these types of controls throughout a development site, but often they are part of the site drainage patterns and have only to be left untouched. As retention storage volume is increased, there is a corresponding decrease in the peak runoff rate in addition to runoff volume reduction. If a sufficient amount of runoff is stored, the peak runoff rate may be reduced to a level at or below the predevelopment runoff rate. This storage may be all that is necessary to control the peak runoff rate when there is a small change in CN. However, when there is a large change in CN, it may be less practical to achieve flow control using volume control only. Pollution Prevention Pollution prevention and maintenance of on-lot BMPs are two key elements in the overall LID comprehensive approach. Effective pollution prevention measures can reduce the introduction of pollutants to LID BMPs, thereby enhancing their ability to reduce pollutant levels and extend the life of the facilities. Public education is essential to successful pollution prevention and BMP maintenance. Not only will effective public education complement and enhance BMP effectiveness, it can also be used as a marketing tool to attract environmentally conscious buyers, to promote citizen stewardship, awareness, and participation in environmental protection programs, and to help to build a greater sense of community based on common environmental objectives and the unique environmental character of LID designs. Education is the key to effective public participation. With LID techniques all stakeholders (public officials, engineers, builders, realtors, and buyers) must be educated about the positive environmental impacts of LID and its maintenance savings and burdens. Once LID controls are integrated into the lot landscape, property owners will need to know how to maintain these features. This can be achieved through easements, covenants, brochures, and environmental committees. Although additional landscape means more landscape maintenance, there is no major stormwater infrastructure (ponds and pipes and structures) to maintain and the scale of the maintenance is reduce to what an individual property owner can afford for routine landscape maintenance costs. Property owners have responded to the LID landscape-level control in two very important ways. First, they feel good about their property helping to protect the environment. Second, they believe that the additional landscape material adds greater value to their property. Thus, LID controls provide a strong economic incentive to maintain LID practices because property values are also maintained!

OTHER IMPORTANT LID CONSIDERATIONS Potential Requirement for Additional Detention Storage In cases where very large changes in CN cannot be avoided, retention storage practices alone may either be insufficient to maintain the predevelopment runoff volume and peak discharge rates or require too much space to represent a viable solution. In these cases, additional detention storage will be needed to maintain the predevelopment peak runoff rates. A number of traditional detention storage techniques are available that can be integrated into the site planning and design process for an LID site. These techniques include (1) swales with check dams, restricted drainage pipes, and inlet/entrance controls; (2) wider lower gradient swales; (3) rain barrels; (4) rooftop storage; (5) shallow parking lot storage; and (6) constructed wetlands and ponds. These detention practices can easily be integrated into the site design features.

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Where downstream flooding is a problem, additional flow control may be necessary to protect property and ensure safe conveyance. Also, when there is a need to retrofit existing development, additional detention may be needed to control off-site flows using regional ponds. Determination of Design Storm Event The hydrologic approach of LID is to retain the same amount of rainfall within the development site as that retained prior to any development (e.g., woods or meadow in good condition) and then release excess runoff as the woods or meadow would have. By doing so, it is possible to mimic, to the greatest extent practical, the predevelopment hydrologic regime to maximize protection to aquatic ecosystems and groundwater recharge. This approach allows the determination of a design storm volume that is tailored to the unique soils, vegetation, and topographic characteristics of the developing watershed. This approach is particularly important in watersheds that are critical for groundwater recharge to protect stream/wetland base flow and groundwater or surface water supplies. For each watershed there is a unique amount of runoff that must be retained to mimic the natural conditions. With LID the volume of runoff to be controlled changes with each site in order to replicate the natural ecological conditions.

INTEGRATED MANAGEMENT PRACTICES Site design techniques and IMPs can be organized into three major categories as follows: 1. Runoff prevention measures designed to minimize impacts and changes in predevelopment CN and Tc 2. Retention facilities that store runoff for infiltration, exfiltration, or evaporation 3. Detention facilities that temporarily store runoff and release through a measured outlet Table 10.6 lists examples of only some of a wide array of LID IMPs and their primary functions. Placing these IMPs in series (treatment train) and uniformly dispersing them throughout the site provide the maximum benefits for hydrologic controls.

TABLE 10.6 Examples of LID IMPs and Primary Functions BMP Bioretention Infiltration trench Dry wells Rooftop storage Filter strip Rain barrels Vegetative swales and small culverts Swales Infiltration swale Reduce imperviousness Strategic clearing and grading Engineered landscape Eliminate curb and gutter Vegetative buffer

Prevent/Reduce Runoff x x x x x x x x x x x x x

Detention

Retention

x x

x x x

Conveyance

Water Quality

Channel Protection x x x

x

x x x x x

x

x

x

x x

x x

x x x x x x x

x x x x x

x

x

x

x x x x

x

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URBAN RETROFIT LID management strategies and practices can be modified to address the unique characteristics of each individual watershed. This makes the protection strategy even more effective as it requires the designer, regulator, residents, and stakeholders to have a better understanding of the complexities and water resource protection needs of each watershed. To apply LID to any land use is simply a matter of developing numerous ways to creatively manage, prevent, retain, detain, use, and treat runoff within multifunctional landscape features unique to a that land use. Factors that influence LID strategies and practices include specific land use, soils, climate, rainfall distribution, water/natural resource protection objectives, and the regulatory framework. Initially LID practices and principles were developed to accommodate typical suburban land uses. During the initial development of the technology it was realized that LID principles, such as creating a hydrologically functional landscape, uniform distribution of controls, strategic timing of flows, micromanagement, and use of plant soil filter technology, e.g., bioretention, are universally applicable regardless of the land use. LID can be applied to address SWM goals and water resource objectives for urban, suburban, and rural development. To apply LID to highly urbanized areas, a new set of specific LID urban management principles and management practices must be developed to address the unique landscape features and water resources protection objectives of cities. Each city may have different urban runoff management objectives that are based on compliance with required local, state, and federal regulations and the economic, environmental, and human health needs to protect receiving waters, sensitive environmental features, and living resources. The ability of cities to meet urban runoff objectives will depend greatly on the cost of control measures and the available economic resources. The cost of current state-of-the-art conventional CSO and BMP technologies can be staggering. The potential for cost savings using LID for volume and water quality control to reduce or eliminate these systems in urban areas is tremendous. This is because many of the techniques used in suburban applications are extremely cost-effective and can be modified for use in the existing urban infrastructure.

INTERNATIONAL MICROSCALE EXPERIENCE AND CASE STUDIES Conventional stormwater institutions in the United States have not encouraged the use of incorporating multiple objectives at small scales. To determine how effective sustainable microscale techniques and small-scale multiple objective programs can be, the experiences of other countries are instructive. The following section highlights and explores some of the experiences that other countries have had using microscale techniques that are similar to LID approaches for wet weather control. Although many communities in the United States are rapidly accepting and planning for the use of LID and other integrated microscale techniques, there are a limited number of sites that have been constructed using these approaches. Examples of these developments in the United States are Village Homes in California, Sumerset and Greenbelt Plaza in Prince George’s County, Maryland, Wonderland Creek in Colorado, and the NEMO project in Jordan Cove, Connecticut. The collection of long-term monitoring and modeling data at these sites are also limited. Because of the limited U.S. experience with LID and other microscale approaches, one must look to other regions to evaluate the effectiveness of these programs to meet receiving water objectives. Then, this existing experience can be built on to develop a knowledge base of planning, design, and program management tools to help communities evaluate and develop successful LID programs. European and Asian countries have a significant amount of experience with microscale approaches. Some of the critical environmental and economic factors that have been responsible for the development and acceptance of microscale techniques in other countries include the following: • In Europe, much of the infrastructure is based on combined sewer systems. For example, England has over 70% combined sewers. The original design capacity cannot account

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for the amount of infill and increased imperviousness and the systems are surcharging. It is difficult to construct new systems because of the expense, conflicts with other infrastructure, and disruption to the city due to construction. New ways to reduce runoff volumes to these systems must be developed. • The cost of land in many urbanized and core city areas in Europe and Asia makes the sacrifice of land for developable area for conventional BMPs economically and politically prohibitive. Techniques that provide environmental protection must be incorporated into infrastructure, and site designs that have minimal impact on the development are essential. • Land subsidence in many Asian countries and saltwater intrusion in European countries such as the Netherlands due to reduction of groundwater levels that result from decreased infiltration capacity of soils and increased impervious cover that has resulted from urbanization is a critical issue. Techniques for groundwater recharge to maintain a sustainable water balance must be developed. National, local government, and watershed compacts have adopted comprehensive strategies that incorporate microscale planning programs, demonstration projects, financing districts, and subsidies to improve water quantity and water quality control using microscale and sustainable techniques. Described below are some key studies that have been published in the literature that demonstrate the planning, design, and environmental effectiveness of these programs, as well as some of the institutional issues associated with microscale approaches.

GERMAN WATERSHED PLANNING USING MICROSCALE APPROACHES The Emscher catchment in Germany is approximately 330 square miles with 2.5 million inhabitants residing in over 17 cities. Although traditionally a coal mining and farming area, over the last 100 years the area has evolved into a more industrialized region. The beginning of the transition caused a decision between economic growth and good environmental conditions. Water resources were exploited and watercourses and soils were contaminated. The long-term records have indicated a frequency of flooding corresponding with increased urbanization during recent years. In addition, over 98% of the infrastructure utilized combined sewers and there were many open systems. In the early 1990s, a strategy was developed to reduce runoff volume and improve environmental quality. The first step was to find strategies to maintain a river ecology that met the aesthetic and recreational needs of the citizens, although the planners recognized the challenge of restoring the system to its predevelopment state due to the intense urbanization. Certain precepts such as not increasing peak flows and volumes and reconstructing straightened channels to a more natural morphology were employed. This approach helped improve the opportunity for ecological systems to reestablish themselves and led to a minor, 3 to 5% reduction in flood frequency. The planning for the drainage area was restructured so that subbasins were analyzed in more discrete units, which enabled the development of targeted strategies for volume and pollution reduction. The next step was the introduction of source control. Because of the long-standing tradition of efficient conveyance systems, this new concept was at first difficult for many communities within the drainage district to embrace. A competition was held and grants were awarded for communities to incorporate source controls as pilot projects. The emphasis of the project areas was disconnection of imperviousness and incorporation of small-scale simple source controls. In one project area of approximately 400 acres, of which 40% was impervious, 70% of the facilities was constructed by private landowners, 20% was constructed with neighborhood development grants, and 10% was constructed by contractors. Polls showed that environmental protection was the main reason, and reducing fees and receiving financial incentives for construction secondary. The level of disconnection in this area was 5%. In the seven areas studied, which included this area, the range of disconnectivity achieved varied widely with percentages from 5% to almost 100%. It is estimated that this program will achieve a 10% reduction in the peak of the 2-year interval storm event.

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JAPANESE EXPERIMENTAL SEWER SYSTEM In Japan, the Experimental Sewer System (ESS) has been a highly effective method to control runoff volume. This approach is based on the development of an integrated, highly efficient infiltration approach to reduce runoff volumes and increase groundwater recharge in urban areas. From 1983 to 1995, the Tokyo Metropolitan Government built within a 5.5 square mile area 33,300 infiltration pits, 122 acres of permeable pavement, and over 175 miles of infiltration trenches. This approach has also been adopted in many other Japanese cities including Sapporo, Shiogama, Chiba, Yokohama, Nagoya, and Amagaski. Representative areas within the ESS were analyzed to determine the cost-effectiveness and efficiency of the program. It was determined that the cost of using microscale infiltration techniques, such as permeable pavement, and infiltration inlets was approximately 33% less than conventional open pond detention systems and 10% of the cost of storage vaults. This alternative was so much more cost-effective because of the high cost of land and the complexity of the existing infrastructure. Perhaps equally impressive was the reduction in monitored storm drain flow volumes of up to 50%. Most important, was the reduction in CSO events from 36 to 7. The following are some of the keys to the success of the acceptance of these programs: • • • • • • • • • • •

Evaluation of positive effects Development of maintenance programs Obtaining cooperation of the public Evaluating disadvantages Evaluating effect on groundwater Subsidizing private construction and maintenance Providing administrative guidance Improvement of administrative model district Promotion of technology Inclusion of infiltration in planning Development of a political base

Demonstration projects have been one of the most effective ways to educate people and gain acceptance for this approach. In Yokohama, model areas were constructed to demonstrate the technology. Within a 15-acre section of the city, 1.8 miles infiltration pipes (typically smaller than 8 in. with a gravel base), 2.5 acres permeable pavement, and 10 acres conventional pavement were constructed as part of the infrastructure. The results, including volume reduction, maintenance, and public acceptance, are continuously monitored.

LYON RESIDENTIAL INFILTRATION STRATEGIES The area of Lyons in France has a history of over 50 years of developing infiltration strategies for stormwater runoff control. Lyons is approximately 230 square miles in area with 1.2 million inhabitants. The storm sewer and sewage system infrastructure includes 1600 miles of sewers, 56 pumping stations, and nine treatment plants in a combined system. Because of the cost and disruption required to build large-scale relief sewers to reduce CSO events, alternative strategies had to be developed. Although one large-scale interceptor project was planned and built in the 1970s, it was recognized that these systems could not continuously be built and that a reduction in flows to the interceptor was also required. In the 1980s, a strategy of large-scale centralized infiltration ponds and pits with pretreatment devices was incorporated as part of the control strategy. Concerns over large-scale loadings on groundwater, maintenance of the systems, construction of long networks of storm drain pipes to the end-of-pipe facilities showed the limitations of this approach. In the early 1990s, a new approach to infiltration was proposed as part of the water and sewer master plan. Specific approaches were recommended:

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In housing areas, stormwater from private surfaces must be infiltrated. In commercial and industrial areas, stormwater from roofs must be infiltrated. Infiltration areas must take place as close as possible to the source. SWM must preserve the natural water cycle and not affect the water quality. Stormwater facilities must fit into their urban surroundings as well as possible.

As an example of this approach for residential development, houses are required to have private infiltration pits, cisterns, or rain barrels to divert rainwater from roofs away from the public drainage system. Residents are responsible for constructing and maintaining these systems. Overflow from the private systems, as well as water from public streets and driveways, is collected in a similar system along the street before entering the collector system. Education and maintenance programs were included in this approach. Over 2000 private devices were inspected. The maintenance program also included the development of a standard system for the retrofit and rehabilitation of older systems.

COST LID case studies and pilot programs show at least a 25 to 30% reduction in site development, stormwater, and maintenance costs for residential development. This is achieved by reducing clearing, grading, pipes, ponds, inlets, curbs, and paving, which in turn lowers construction costs allowing builders to add greater value (features) to the property or to be more flexible and competitive in pricing their products. One of interesting results of the LID on-lot microscale approach is that the SWM controls become a part of each property owner’s landscape (natural areas, rain gardens, open space, open swales, etc.). This reduces the public burden to maintain large centralized management facilities and reduces the cost and scale of maintenance to a level the homeowner can easily afford — the cost of routine landscape/yard care and pollution prevention. Prince George’s County does not rely on enforcement to ensure maintenance of LID landscape practices. Instead, it believes the economic incentive of maintaining property values will ensure most property owners will adequate maintain their LID landscape.

ROADBLOCKS TO LID There are a number of roadblocks that must be overcome for the successful implementation of LID. Regulatory agencies, the development community, and the public may all have concerns about the use of new technology. In the development of the PGDER LID design manual a multiagency task force spent more than 2 years addressing all the concerns and issues. Some of the major concerns are the following: 1. Development of an hydrologic analytical methodology to demonstrate the equivalence of LID to conventional approaches 2. Development of new road standards that allow for narrow roads, open drainage, and use of bioretention 3. Streamlining the review process for innovative new LID designs, which allows for easy modification of site, subdivision, road, and stormwater requirements 4. Development of a public education process that informs property owners how to prevent pollution and maintain on-lot LID BMPs 5. Development of legal and educational mechanisms to ensure BMP maintenance 6. Demonstration of the marketability of green development 7. Demonstration of the cost benefits of the LID approach

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8. Provision of training for regulators, consultants, the public, and political leaders 9. Conduct of research to demonstrate the effectiveness of bioretention BMPs 10. Lack of field monitoring data to demonstrate the effectiveness of LID in controlling runoff quantity and quality

SUMMARY LID is a viable, cost-effective alternative approach to SWM and the protection of natural resources. LID is designed to provide tangible economic incentives to a developer to save more natural areas and reduce stormwater and roadway infrastructure costs. LID can achieve greater natural conservation by using conservation as a stormwater BMP. As more natural areas are saved, less runoff is generated and SWM costs are reduced. This allows multiple uses of landscape features to achieve environmental, economic, aesthetic, and natural resource benefits. Additionally, developers have economic incentives to provide better environmental protection by reducing short- and long-term infrastructure costs by reducing impervious areas and eliminating curbs/gutters and stormwater ponds to achieve LID stormwater controls. Reduction of the infrastructure also reduces infrastructure maintenance burdens making LID development more economically sustainable. LID allows for the same or in some cases higher lot yields compared with conventional approaches. Because SWM is controlled on each lot using multifunctional landscapes, that portion of the building area that would have been used for stormwater ponds can, in some cases, be used for additional flood control or recovered and used for building, parking lots, open space, or habitat enhancements. LID promotes public awareness, education, and participation in environmental protection. As every property owner’s landscape functions as part of the watershed, each must be educated on the benefits and the need for maintenance of the landscape and pollution prevention measures. LID developments can be designed in a very environmentally sensitive manner to protect streams, wetlands, forests, and habitat and to save energy. The unique environmental protection objectives of an LID development can create a greater sense of community pride based on environmental stewardship. In the development of the LID hydrologic analysis, TR-55 was used because this model is most widely used by site engineers in Prince George’s County and throughout most of the United States. During the development of LID it was learned that current analytical models such as TR-55 are not well suited for use with very small watersheds. There is a significant amount of work needed to upgrade current models to better quantify the effects of microscale site design control techniques. One extremely fascinating aspect of LID is that, when controlling runoff on a microscale, there exists a whole new world of possible control practices and strategies. So move out of the box of conventional pipe-and-pond technology, take up the LID challenge, and try thinking small!

REFERENCES Clar, M., Collins, J., Loftin, H. et al., 2001. Stormwater BMP technology assessment protocols — preliminary findings, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. Collins, J., Clar, M., Loftin, H. et al., 2001. Compilation of regulatory requirements for stormwater runoff, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. CRC (Chesapeake Research Consortium), 1996. Design of Stormwater Filtering Systems, prepared for the Chesapeake Research Consortium, Inc., Solomons, Maryland by The Center for Watershed Protection, Silver Spring, MD.

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Darr, 1990. A Technical Manual for Woodland Conservation with Development, Prince George’s County, Maryland, Maryland National Capital Planning Commission. Davis, A., Shokouhian, M., Sharma, H., and Minami, C., 1998. Optimization of Bioretention Design for Water Quality and Hydrologic Characteristics, Report 01-04-31032, Final report to Prince George’s County, MD. Dunne, T. and Leopold, L.B., 1978. Water in Environmental Planning, W.H. Freeman, San Francisco, 818 pp. Jones, R.C., Via-Norton, A., and Morgan, D.R., 1997. Bioassessment of BMP effectiveness in mitigating stormwater impacts on aquatic biota, in L.A. Roesner, Ed., Effects of Watershed Development and Management on Aquatic Ecosystems, American Society of Civil Engineers, New York. Lane, E.W., 1955. The importance of fluvial morphology in hydraulic engineering, in American Society of Civil Engineering Proceedings, 81, paper 745, 1–17. Leopold, L.B., 1968. Hydrology for Urban Land Planning — A Guidebook on the Hydrologic Effects of Urban Land Use, U.S. Geological Survey, Water Supply Paper, 1591-C. Leopold, L.B., 1994. A View of the River, Harvard University Press, Cambridge, MA, 298 pp. Leopold, L.B. and Maddock, T., Jr., 1954. The Flood Control Controversy, Ronald Press, New York. Leopold, L.B., Wolman, M.G., and Miller, J.P., 1964. Fluvial Processes in Geomorphology, Freeman & Sons, San Francisco, 522 pp. MacRae, C. 1996. Experience from morphological research on canadian streams: is control of the two-year frequency runoff event the best basis for stream channel protection? in Effects of Watershed Development and Management on Aquatic Ecosystems, L. Roesner, Ed., American Society of Civil Engineers, New York, 144–162. Maxted, J. and E.Shaver, 1997. The use of retention basins to mitigate stormwater impacts on aquatic life, in Effects of Watershed Development and Management on Aquatic Ecosystems, L.A. Roesner, Ed., American Society of Civil Engineers, New York. McCuen, R.H., Moglen, G., Kistler, E., and Simpson, P., Policy Guidelines for Controlling Stream Channel erosion with Detention Basins, prepared by the Department of Civil Engineering, University of Maryland, College Park, MD, for the Water Management Administration, Maryland Department of the Environment, Baltimore, MD. MDE (Maryland Department of the Environment), 2000. 2000 Maryland Stormwater Design Manual, Vol. I and II, prepared by the Center for Watershed Protection and the Maryland Department of the Environment, Water Management Administration, Baltimore, MD. MWCOG (Metropolitan Washington Council of Governments), 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs, Department of Environmental Programs, Washington, D.C. NRCS (Natural Resources Conservation Service), 1986. Urban Hydrology for Small Watersheds, Technical Release 55, U.S. Department of Agriculture, Conservation Engineering Division, Washington, D.C. PGC (Prince George’s County, Maryland), 1993. Design Manual for Use of Bioretention in Stormwater Management, prepared by Engineering Technologies Associates, Inc., Ellicott City, MD. PGC (Prince George’s County, Maryland), 1997a. Department of Environmental Resources, Low-Impact Development Design Manual, prepared by Tetra Tech, Inc., Fairfax, VA. PGC (Prince George’s County, Maryland), 1997b. Department of Environmental Resources, Low-Impact Development Guidance Manual, prepared by Tetra Tech, Inc., Fairfax, VA. Pitt, R., 1994. Small storm hydrology, paper presented at Design of Stormwater Quality Management Practices, Madison, WI, May 17–19, University of Alabama. Rosgen, D.L., 1996. Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO. Schueler, T., 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs, Metropolitan Washington Council of Governments, Washington, D.C. Skupien, J.J., 2000. Establishing effective development site outflow rates, paper presented at the Delaware Sediment and Stormwater Issues for a New Millennium, Conference 2000, University of Delaware, Newark. Snodgrass, W.J., Kilgour, B.W., Leon, L., Eyles, N., Parish, J., and Barton, D.R., 1998. Applying ecological criteria for stream biota and an impact flow model for evaluation sustainable urban water resources in southern Ontario, in Sustaining Urban Water Resources in the 21st Century. Proceedings for an Engineering Foundation Conference, A.C. Rowney, P. Stahre, and L.A. Roesner, Eds., Malmo, Sweden, September 7–12, 1997.

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Stribling, J.B., 2001. Relating instream biological condition to BMP activities in watersheds, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. Swietlik, W.F., 2001. Urban aquatic life uses — a regulatory perspective, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. Tetra Tech, Inc., 2000. Effluent Limitations Guidelines, Draft Report, prepared for Office of Science and Technology, U.S. Environmental Protection Agency, Washington, D.C. Urbonas, B.R., 1995. Recommended parameters to report with BMP monitoring data, J. Water Resour. Planning Manage. Div., 121(1). Urbonas, B. and Stahre, P., 1993. Best Management Practices and Detention for Water Quality, Drainage and CSO Management, Prentice-Hall, Englewood Cliffs, NJ. U.S. EPA, 1983. National Urban Runoff Program, NURP, study. U.S. EPA, 2000a. Low Impact Development Design Strategies: An Integrated Design Approach, prepared by Tetra Tech, Inc., Fairfax, VA prepared for Department of Environmental Resources, Prince George’s County, MD, funding provided by the U.S. Environmental Protection Agency, Washington, D.C. U.S. EPA, 2000b. Low Impact Development (LID) Hydrology, prepared by Tetra Tech, Inc., Fairfax, VA, for Department of Environmental Resources, Prince George’s County, MD, funding provided by the U.S. Environmental Protection Agency, Washington, D.C. Yoder, C.O., 1995. Incorporating ecological concepts and biological criteria in the assessment and management of urban nonpoint source pollution, in National Conference on Urban Runoff Management: Enhancing Urban Watershed Management at the Local, County and State Levels, EPA/625/R-95/003, U.S. Environmental Protection Agency, Washington, D.C., 183–197. Young, G.K., Stein, S., Cole, P., Kammer, T., Graziano, F., and Bank, F., 1996. Evaluation and Management of Highway Runoff Water Quality, FHWA-PD-96-032, Federal Highway Administration, Office of Environment and Planning. Yousef, Y., Wanielista, M., Harper, H., Pearce, D., and Tolbert, R., 1985. Best Management Practices for Removal of Highway Contaminants by Roadside Swales, Final report, University of Central Florida, Florida Department of Transportation, Orlando. Yu, S.L., Kasnick, M., and Byrne, M., 1992. A level spreader/vegetative buffer strip system for urban stormwater mangement, in Integrated Storm Water Management, R. Field, Ed., Lewis Publishers, Boca Raton, FL, 93–104. Yu, S.L., Barnes, S., and Gerde, V., 1993. Testing of Best Management Practices for Controlling Highway Runoff, FHWA/VA 93-R16, Virginia Transportation Research Council.

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Low-Impact Development Hydrologic Analysis Mow-Soung Cheng, Larry S. Coffman, and Michael L. Clar

CONTENTS Introduction ....................................................................................................................................296 Development Impacts and Mitigation ...........................................................................................296 LID Fundamental Concepts ...........................................................................................................297 The Distributed Control Approach .........................................................................................297 Microscale Integrated Management Practices........................................................................298 The Hydrologically Functional Landscape ............................................................................298 LID Hydrologic Analysis Components .........................................................................................298 Hydrologic Evaluation ............................................................................................................299 LID Runoff Potential ..............................................................................................................300 Maintaining the Predevelopment Time of Concentration ......................................................301 Maintaining the Predevelopment Runoff Volume ..................................................................303 Potential Requirement for Additional Detention Storage ......................................................305 Process and Computational Procedure ..........................................................................................305 Data Collection .......................................................................................................................305 Determining the LID Runoff Curve Number.........................................................................305 Step 1: Determine Percentage of Each Land Use/Cover .............................................305 Step 2: Calculate Composite Custom CN ....................................................................306 Step 3: Calculate LID CN Based on the Connectivity of Site Impervious Area ........306 Development of the Time of Concentration...........................................................................307 LID Stormwater Management Requirements.........................................................................307 Step 1: Determine Storage Volume Required to Maintain Predevelopment Volume or CN Using Retention Storage....................................................................................309 Step 2: Determine Storage Volume Required for Water Quality Control ...................309 Step 3: Determine Storage Volume Required to Maintain Peak Stormwater Runoff Rate Using 100% Retention..........................................................................................310 Step 4: Determine Whether Additional Detention Storage Is Required to Maintain the Predevelopment Peak Runoff Rate .........................................................................311 Step 5: Determine Storage Required to Maintain Predevelopment Peak Runoff Rate Using 100% Detention .........................................................................................311 Step 6: Use Hybrid Facility Design..............................................................................311 Step 7: Determine Hybrid Amount of IMP Site Area Required to Maintain Peak Runoff Rate with Partial Volume Attenuation Using Hybrid Design ..........................312 Determination of Design Storm Event...................................................................................312 Step 1: Determine the Predevelopment CN .................................................................313 Step 2: Determine the Amount of Rainfall Needed to Initiate Direct Runoff ............313 Step 3: Account for Variation in Land Cover...............................................................313 References ......................................................................................................................................313 0-56676-916-7/03/$0.00+$1.50 © 2003 by CRC Press LLC

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INTRODUCTION This chapter provides an overview of low-impact development (LID) hydrologic analysis and computational procedures used to meet stormwater management requirements for low-impact development. LID uses a combination of conservation site planning, pollution prevention, and stormwater management techniques to achieve the desired hydrologic objective. The stormwater management facilities used in LID designs are called integrated management practices (IMPs). IMPs are microscale facilities that are evenly distributed and integrated into the buildings, landscape, and infrastructure of a site or project area to control the timing or volume of runoff or filter pollutants. The techniques described here are used to determine the required volumes of IMPs that are required to maintain the predevelopment runoff volume, peak flow rate, and flow duration and frequency for a site. The method to determine the requirements and size of these facilities is based on the National Resources Conservation Service (NRCS) procedures developed for small urban watersheds (NRCS, 1986). A series of design charts has been developed using the pre- and postdevelopment runoff conditions to determine the required volume to be constructed as IMPs in order to achieve the desired LID site design objectives. LID achieves stormwater management controls by fundamentally changing conventional site design to create an environmentally functional landscape that mimics natural watershed hydrologic functions (discharge, frequency, recharge, and volume). This is accomplished in four ways: 1. By minimizing impacts to the extent practicable by reducing imperviousness, conserving natural resources/ecosystems, maintaining natural drainage courses, reducing use of pipes and minimizing clearing and grading. 2. By recreating detention and retention storage dispersed throughout a site with the use of open swales, flatter slopes, rain gardens (bioretention), and rain barrels. 3. By maintaining predevelopment time of concentration by strategically routing flows to maintain travel time. 4. By encouraging property owners to use effective pollution prevention measures and to maintain management measures.

DEVELOPMENT IMPACTS

AND

MITIGATION

Climate coupled with the topographic, geologic, and vegetative features of a watershed produce a unique hydrologic regime. Aquatic, wetland, and riparian biota have evolved by adapting to this unique regime (Cairns, 1993). Urban development changes this regime, resulting in a new annual and seasonal hydrologic balance, causing frequency distribution changes of peak flows, magnitude and duration of high flows, and magnitude and duration of low flows. Typical alterations to the hydrologic regime as a result of development and the related increase in impervious areas include, but are not limited to, the following: • • • • • •

Increased runoff volume Increased flow frequency, duration, and peak runoff rate Reduced infiltration (groundwater recharge) Modification of the flow pattern Faster time to peak Loss of storage

Conventional stormwater conveyance systems are designed to collect, convey, and discharge runoff as efficiently as possible. Conventional stormwater management controls (so-called best management practices, or BMPs) are typically sited at the most downstream point of the entire site

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Q

2 1 3 Time

FIGURE 11.1 Comparison of the hydrologic response of conventional and LID IMPs.

(end-of-pipe control). The stormwater management requirement is usually to maintain the peak runoff rates at predevelopment levels for a particular design storm event. Therefore, especially where a stormwater management pond is constructed, the peak flow will not be fully controlled for those storm events that are less severe than the design storm event. LID approaches, on the other hand, will fully control these storm events. This is a very important and significant difference between the two approaches. Figure 11.1 illustrates the hydrologic response of the runoff hydrograph to conventional BMPs. • Hydrograph 1 represents the response to a given storm of a site in a predevelopment condition (i.e., woods and meadow). The hydrograph is defined by a gradual rise and fall of the peak discharge and volume. • Hydrograph 2 represents a postdevelopment condition with conventional stormwater BMPs, such as a detention pond. Although the peak runoff rate is maintained at the predevelopment level, the hydrograph exhibits significant increases in the runoff volume and duration of runoff from the predevelopment condition. • Hydrograph 3 represents the response of postdevelopment conditions that incorporate LID stormwater management. LID uses undisturbed areas and on-lot and distributed retention storage to reduce runoff volume. The peak runoff rate and volume remain the same as the predevelopment condition through the use of on-lot retention and/or detention. The frequency and duration of the runoff rate are also much closer to the existing condition than those typical of conventional BMPs.

LID FUNDAMENTAL CONCEPTS The LID design procedure is based on the application of three powerful fundamental concepts: (1) the distributed control approach, (2) microscale integrated management practices, and (3) a hydrologically functional landscape. These concepts are described below.

THE DISTRIBUTED CONTROL APPROACH In comparison with conventional stormwater management, the objective of LID hydrologic design is to retain the postdevelopment excess runoff volume in discrete units throughout the site to emulate the predevelopment hydrologic regime. This is called a distributed control approach. Management of both runoff volume and peak runoff rate is included in the design. The approach is to manage runoff at the source rather than at the end of the pipes. Preserving the hydrologic regime of the predevelopment condition may require both structural and nonstructural techniques to compensate for the hydrologic alterations of development.

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MICROSCALE INTEGRATED MANAGEMENT PRACTICES To achieve the distributed control approach, LID technology employs microscale management techniques, called integrated management practices (IMPs), which are integrated throughout the site landscape to achieve desired postdevelopment hydrologic conditions. They can maintain the predevelopment runoff volume and peak and can be integrated into the site design. The design goal is to locate IMPs at the source or lot, ideally on level ground within individual lots of the development. Management practices that are suited to LID include the following: • • • • • • •

Bioretention facilities (rain gardens) Dry wells Filter/buffer strips and other multifunctional landscape areas Grassed swales, bioretention swales, and wet swales Rain barrels Cisterns Infiltration trenches

More information on IMPs can be obtained in the publication titled, “Low-Impact Development Design Strategies: An Integrated Design Approach,” prepared by Prince George’s County, Maryland, May 1999.

THE HYDROLOGICALLY FUNCTIONAL LANDSCAPE In LID, one of the design approaches is to leave as many undisturbed areas as practical to reduce runoff volume and runoff rates by maximizing infiltration capacity. IMPs are then evenly distributed throughout the site to compensate for the hydrologic alterations of development. The approach of maintaining areas of high infiltration and low runoff potential in combination with small, on-lot stormwater management facilities creates a “hydrologically functional landscape.” This functional landscape not only can help maintain the predevelopment hydrologic regime but also enhance the aesthetic and habitat value of the site.

LID HYDROLOGIC ANALYSIS COMPONENTS The LID approach emulates the predevelopment temporary storage (detention) and infiltration (retention) functions of the site. This approach is designed to mimic the predevelopment hydrologic conditions through runoff volume control, peak runoff rate control, flow frequency/duration control, and water quality control. Runoff Volume Control. The predevelopment volume is maintained by a combination of minimizing the site disturbance from the predevelopment condition and then providing distributed retention IMPs. Retention IMPs are structures that retain the runoff for the design storm event. Peak Runoff Rate Control. LID is also designed to maintain the predevelopment peak runoff discharge rate for the selected design storm events. This is done by maintaining the predevelopment time of concentration and then using retention and/or detention IMPs (e.g., rain gardens, open drainage systems, etc.) that are distributed throughout the site. The goal is to use retention practices to control runoff volume and, if these retention practices are not sufficient to control the peak runoff rate, to use additional detention practices to control the peak runoff rate. Detention is temporary storage that releases excess runoff at a controlled rate. The combined use of retention and detention to control the peak runoff rate is defined as the hybrid approach. Flow Frequency/Duration Control. Because LID is designed to emulate the predevelopment hydrologic regime through both volume and peak runoff rate controls, the flow frequency and

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TABLE 11.1 LID Techniques and Hydrologic Design and Analysis Components



  

Vegetation Preservation

  

Revegetation

 

Bioretention

 

Rooftop Storage



Rain Barrels

 

  

Reduce Curb and Gutter

 

Disconnected Impervious Areas



Constricted Pipes



Vegetative Filter Strips



Infiltration Swales



Flatten Slopes on Swales

Increase Roughness



Minimize Disturbance

Increase Sheet Flow

Lower postdevelopment CN Increase Tc Retention Detention

Increase Flow Path

Low-Impact Hydrologic Design and Analysis Components

Flatten Slope

LID Technique

  

  

  

duration for the postdevelopment conditions will be almost identical to those for the predevelopment conditions. Consequently, the impacts on the sediment and erosion and stream habitat potential at downstream reaches will be minimized. Water Quality Control. LID provides water quality treatment control for the first 1/2 or 1 in. of runoff from impervious areas using retention practices. LID also provides pollution prevention by modifying human activities to reduce the introduction of pollutants into the environment. The low-impact analysis and design approach focuses on the following hydrologic analysis and design components: • Runoff Curve Number (CN). Minimizing change in postdevelopment hydrology by reducing impervious areas and preserving more trees and meadows to reduce the storage requirements to maintain the predevelopment runoff volume. • Time of Concentration (Tc). Maintaining the predevelopment Tc to minimize the increase of the peak runoff rate after development by lengthening flow paths and reducing the length of the runoff conveyance systems. • Retention. Providing retention storage for volume and peak control, as well as water quality control, to maintain the same storage volume as the predevelopment condition. • Detention. Providing additional detention storage, if required, to maintain the same peak runoff rate and/or prevent flooding. Table 11.1 provides a summary of low-impact techniques that affect these components.

HYDROLOGIC EVALUATION The purpose of the hydrologic evaluation is to determine stormwater management requirements for LID sites. The evaluation method is used to determine the amount of retention and/or detention to control the runoff volume and peak discharge rate. Appropriate detention and/or retention techniques are then selected to meet these requirements.

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Ex. Woods Lawn Area

Detention area, if required

Driveway

Driveway

Lawn Area

Bioretention Drainage Swale

Curb & Cute

Reduced Road Width

Conventional CN for 1-Acre Lot [Table 2.2a TR 55] N.T.S.

Typical Low-Impact Development Lot N.T.S.

FIGURE 11.2 Comparison of land covers between conventional and LID CNs.

LID RUNOFF POTENTIAL Calculation of the LID runoff potential is based on a detailed evaluation of the existing and proposed land cover so that an accurate representation of the potential for runoff can be obtained. This calculation requires the engineer to investigate several key parameters associated with an LID: • • • •

Land cover type Percentage of and connectivity of impervious areas Soils type and texture Antecedent soil moisture conditions

A comparison of conventional and LID runoff potential using the Soil Conservation Service (SCS) curve number (CN) approach is presented. The CN for conventional development are based on the land cover assumptions and parameters shown in table 2.2a of TR-55 (SCS, 1986). The LID CN are based on a detailed evaluation of the land cover and parameters listed above. As illustrated in Figure 11.2, customizing the CN for an LID site allows the developer/engineer to take advantage of and receive credit for such LID site planning practices as the following: • • • • • •

Narrower driveways and roads (minimizing impervious areas) Maximizing tree preservation or forestation (tree planting) Site fingerprinting (minimal disturbance) Open drainage swales Preservation of soils with high infiltration rates to reduce CN Location of IMPs on high infiltration soils

Table 11.2 illustrates a comparison of LID CN land covers with those of a conventional development CN, as found in table 2.2a of TR-55 (SCS, 1986) for a typical 1-acre lot. Figure 11.2

TABLE 11.2 Comparison of Conventional and LID Land Covers Conventional Land Covers (TR-55 Assumptions) 20% impervious 80% grass

LID Land Covers 15% imperviousness 25% woods 60% grass

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illustrates a comparison of conventional land covers, based on the land covers in table 2.2a of TR-55, with an LID customized CN for a 1-acre lot. Table 11.3. provides a list of LID site planning practices and their relationship to the components of the LID CN. Key low-impact techniques that will reduce the postdevelopment CN, and corresponding runoff volumes, are as follows: Preservation of Infiltratable Soils: This approach includes site planning techniques such as minimizing disturbance of soils, particularly vegetated areas, with high infiltration rates (sandy and loamy soils), and placement of infrastructure and impervious areas such as houses, roads, and buildings on more impermeable soils (silty and clayey soils). Care must be taken when determining the suitability of soils for proposed construction practices. Adequate geotechnical information is required for planning practices. Preservation of Existing Natural Vegetation: Woods and other vegetated areas provide many opportunities for storage and infiltration of runoff. By maintaining the surface coverage to the greatest extent possible, the amount of compensatory storage for IMPs is minimized. Vegetated areas can also be used to provide surface roughness, thereby increasing the Tc. In addition, they function to filter out and uptake pollutants. Minimization of Site Imperviousness: Reducing the amount of imperviousness on the site will have a significant impact on the amount of compensatory IMP storage required since there is almost a one-to-one corresponding relationship between rainfall and runoff for impervious areas. Disconnection of Site Imperviousness: Impervious areas are considered disconnected if they do not connect to a storm drain system or other impervious areas through direct or shallow concentrated flow. Directing impervious areas to sheet flow onto vegetated or bioretention areas to allow infiltration results in a direct reduction in runoff and corresponding storage volume requirements. Creation of Transition Zones and Bioretention: Transition zones are vegetated areas that can be used to store and infiltrate runoff from impervious areas before they discharge from the site. These areas are located at the sheet or discharge points from graded and impervious areas. These areas affect the land cover type calculations of the LID CN. The use of these techniques will provide incentives in cost savings to the overall site development and infrastructure. It will also reduce costs for stormwater permit fees, inspection, and maintenance of the infrastructure as well as project-based costs. The hydrologic response using LID techniques to reduce the impervious areas and increase the storage volume is a reduction in both postdevelopment peak rate and volume.

MAINTAINING

THE

PREDEVELOPMENT TIME

OF

CONCENTRATION

The LID hydrologic evaluation requires that the postdevelopment time of concentration (Tc) be maintained close to the predevelopment Tc. The travel time (Tt) throughout individual lots and areas should be approximately the same so that the Tc is representative of the drainage. This is critical because LID is based on a homogeneous land cover and distributed IMPs. To maintain the Tc, LIDs use the following site planning techniques: • Maintaining predevelopment flow path length by dispersing and redirecting flows, generally, through open swales and natural drainage patterns • Increasing surface roughness (e.g., reserving woodlands, using vegetated swales) • Detaining flows (e.g., open swales, rain gardens) • Minimizing disturbance (minimizing compaction and changes to existing vegetation) • Flattening grades in impacted areas • Disconnecting impervious areas (e.g., eliminating curb/gutter and redirecting downspouts) • Connecting pervious and vegetated areas (e.g., reforestation, forestation, and tree planting) To maintain predevelopment Tc, an iterative process that analyzes different combinations of the above appropriate techniques may be required. These site planning techniques are incorporated

Land Cover type Percent of imperviousness Hydrologic soils group Hydrologic condition Disconnectivity of impervious area Storage and infiltration

Suggested Options Affecting CN







Reduce Road Length and Width



Limit Use of Sidewalks





Reduce Driveway Length and Width





  

Minimize Disturbance

Conserve Natural Resources Areas

 

Preserve Infiltratable Soils



Preserve Natural Depression Areas

 

Use Transition Zones



Use Vegetated Swales





Preserve Vegetation

302

TABLE 11.3 LID Planning Techniques to Reduce the Postdevelopment LID CN

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into the hydrologic analysis computations for postdevelopment Tc to demonstrate an increase in postdevelopment Tc above conventional techniques and a corresponding reduction in peak discharge rates. The hydrologic responses to maintaining equal predevelopment and postdevelopment Tc are to shift the post peak runoff time to that of the predevelopment condition and to lower the peak runoff rate. The greatest gains for increasing the Tc in a small watershed can be accomplished by increasing the Manning’s roughness n for the initial surface flow at the top of the watershed and increasing the flow path length for the most hydraulically distant point in the drainage area. After the transition to shallow concentrated flow, additional gains in Tc can be accomplished by decreasing the slope, increasing the flow length, and directing flow over pervious areas. In LID sites, the amount of flow in closed channels (pipes) should be minimized to the greatest extent possible. Swales and open channels should be designed with the following features: • • • • •

Increase surface roughness to retard velocity Maximize sheet flow conditions Use a network of wider and flatter channels to avoid fast-moving channel flow Increase channel flow path Reduce channel gradients to decrease velocity (minimum slope is 2%; 1% may be considered on a case-by-case basis) • Direct channel flow over pervious soils whenever possible to increase infiltration so that there is a reduction of runoff to maximize infiltration capacity

Table 11.4 identifies LID techniques and volume objectives to maintain the predevelopment Tc.

MAINTAINING

THE

PREDEVELOPMENT RUNOFF VOLUME

After all the available and feasible options to reduce the runoff potential of a site described have been deployed, and after all the available techniques to maintain the Tc as close as possible to predevelopment levels have been used, any additional reductions in runoff volume must be accomplished through distributed on-site stormwater management techniques. The goal is to select the appropriate combination of management techniques that emulate the hydrologic functions of the predevelopment condition to maintain the existing runoff CN and corresponding runoff volume. LID sites use retention to accomplish this goal. These facilities must be sited on individual lots throughout the site to provide volume controls at the source. Retention storage allows for a reduction in the postdevelopment volume and the peak runoff rate. The increased storage and infiltration capacity of IMPs allows the predevelopment volume to be maintained. IMPs that maintain the predevelopment storage volume include, but are not limited to the following: • • • •

Bioretention (rain garden) Infiltration trenches Vegetative filter/buffer Rain barrels

As the retention storage volume of the LID IMPs is increased, there is a corresponding decrease in the peak runoff rate in addition to runoff volume reduction. If a sufficient amount of runoff is stored, the peak runoff rate may be reduced to a level at or below the predevelopment runoff rate. This storage may be all that is necessary to control the peak runoff rate when there is a small change in runoff CN and storage volume. However, when there is a large change in CN, it may be less practical to achieve flow control using volume control only.

Minimize disturbance Flatten grades Reduce height of slopes Increase flow path (divert and redirect) Increase roughness n

LID Objective





On-lot Bioretention





Wider and Flatter Swales

 

 

Maintain Sheet Flow

 



Clusters of Trees and Shrubs in Flow Path





Provide Tree Conservation/ Transition Zones     

Minimize Storm Drain Pipes

LID Technique

 



Disconnect Impervious Areas

 



Save Trees

  

Preserve Existing Topography



 

LID Drainage and Infiltration Zones

304

TABLE 11.4 LID Techniques to Maintain the Predevelopment Time of Concentration

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POTENTIAL REQUIREMENT

FOR

305

ADDITIONAL DETENTION STORAGE

Even though the postdevelopment Tc and CN are maintained at the predevelopment level, in some cases additional detention storage is needed to maintain the predevelopment peak runoff rate due to the spatial distribution of the retention storage provided (i.e., storage areas are not uniformly distributed throughout the site). The amount of storage that maintains the predevelopment runoff volume might not be sufficient to maintain also the predevelopment peak runoff rate. Therefore, additional on-lot storage is required in the form of detention storage. LID stormwater management techniques for providing detention storage include, but are not limited to the following: • • • • •

Swales with check dams, restricted drainage pipe, and inlet entrances Wider swales Rain barrels Rooftop storage Diversion structures

The effect of this additional detention storage is to reduce the postdevelopment peak discharge rate to predevelopment conditions.

PROCESS AND COMPUTATIONAL PROCEDURE The hydrologic analysis of LID is a sequential decision-making process. Several iterations may occur within each step until the appropriate approach to reduce stormwater impacts is determined. The procedures for each step are given in this section. Design charts have been developed to determine the amount of storage required to maintain the existing volume and peak runoff rates to satisfy county stormwater management runoff storage requirements.

DATA COLLECTION The basic information used to develop the LID site plan and to determine the runoff CN and Tc is the same as the conventional site plan and stormwater management approaches.

DETERMINING

THE

LID RUNOFF CURVE NUMBER

The determination of the LID CN requires a detailed evaluation of each land cover within the development site. This will allow the designer to take full advantage of the storage and infiltration characteristics of LID site planning to maintain the CN. This approach encourages the conservation of more woodlands and the reduction of impervious area to minimize the needs of IMPs. The steps for determining the LID CN are as follows: Step 1: Determine Percentage of Each Land Use/Cover In conventional site development, the engineer would refer to figure 2.2.a of TR-55 (SCS, 1986) to select the CN that represents the proposed land use of the overall development (i.e., residential, commercial) without checking the actual percentages of impervious area, grass areas, etc. Because low-impact design emphasizes minimal site disturbance (tree preservation, site fingerprinting, etc.), it is possible to retain much of the predevelopment land cover and CN. Therefore, it is appropriate to analyze the site as discrete units to determine the CN. Table 11.5 lists representative land cover CN used to calculate the composite “custom” LID CN.

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TABLE 11.5 Representative LID CN CN for Hydrologic Soils Groups a Land Use/Cover

A

B

C

D

Impervious area Grass Woods (fair condition) Woods (good condition)

98 39 36 30

98 61 60 55

98 74 73 70

98 80 79 77

a

Figure 2.2a, TR-55 (SCS, 1986).

Step 2: Calculate Composite Custom CN The initial composite CN is calculated using a weighted approach based on individual land covers without considering disconnectivity of the site imperviousness. This is done using Equation 11.1. CN c =

CN1 A1 + CN 2 A2 … + CN j Aj A1 + A2 … + Aj

(11.1)

where CNc = composite curve number Aj = area of each land cover CNj = curve number for each land cover Overlays of SCS hydrologic soil group (HSG) boundaries onto homogeneous land cover areas are used to develop the LID CN. What is unique about the LID custom-made CN technique is the way this overlaid information is analyzed as small discrete units that represent the hydrologic condition, rather than a conventional TR-55 approach, which is based on a representative national average. This is appropriate because of the emphasis on minimal disturbance and retaining site areas that have potential for high storage and infiltration. This approach provides an incentive to save more trees and maximize the use of HSG A and B soils for recharge. Careful planning can result in significant reductions in postdevelopment runoff volume and corresponding stormwater management costs. Step 3: Calculate LID CN Based on the Connectivity of Site Impervious Area When the impervious areas are less than 30% of the site, the percentage of the unconnected impervious areas within the watershed influences the calculation of the CN (SCS, 1986). Disconnected impervious areas are impervious areas without any direct connection to a drainage system or other impervious surface. For example, roof drains from houses could be directed onto lawn areas where sheet flow occurs, instead of to a swale or driveway. By increasing the ratio of disconnected impervious areas to pervious areas on the site, the CN and resultant runoff volume can be reduced. Equation 11.2 is used to calculate the CN for sites with less than 30% impervious. P  CN c = CN p +  imp  × 98 − CN p × (1 − 0.5 R)  100 

(

)

(11.2)

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where R CNc CNp Pimp

= = = =

307

ratio of unconnected impervious area to total impervious area composite CN composite pervious CN percent of impervious site area

DEVELOPMENT

OF THE

TIME

OF

CONCENTRATION

The pre- and postdevelopment calculation of the Tc for LID is exactly the same as that described in the TR-55 (SCS, 1986) and NEH-4 (SCS, 1985) manuals.

LID STORMWATER MANAGEMENT REQUIREMENTS Once the CN and Tc are determined for the pre- and postdevelopment conditions, the stormwater management storage volume requirements can be calculated. The LID objective is to maintain all the predevelopment volume, predevelopment peak runoff rate, and frequency. The required storage volume is calculated using three series of design charts (A, B, and C) for different geographic regions in the nation. Examples of these chart series are shown in Attachment 11.A, 11.B, and 11.C. As stated previously, the required storage volume is heavily dependent on the intensity of rainfall (rainfall distribution). Because the intensity of rainfall varies considerably over geographic regions in the nation, the NRCS developed four synthetic 24-h rainfall distributions (I, IA, II, and III) from available National Weather Service (NWS) duration/frequency data and local storm data. Type IA is the least intense and Type II is the most intense short duration rainfall. The remaining LID hydrologic analysis techniques are based on the premise that the postdevelopment Tc is the same as the predevelopment condition. If the postdevelopment Tc does not

ATTACHMENT 11.A Representative Chart Series A.

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ATTACHMENT 11.B Representative Chart Series B.

ATTACHMENT 11.C Representative Chart Series C.

equal the predevelopment Tc, additional LID site design techniques must be implemented to maintain the Tc. Three series of design charts are needed to determine the storage volume required to control the increase in runoff volume and peak runoff rate using retention and detention practices. The required storages shown in these design charts are presented as a depth in hundredths of an inch (over the development site). Equation 11.3 is used to determine the volume required for IMPs.

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Volume = (depth obtained from the chart) × (development size) 100

309

(11.3)

It is recommended that 6-in. depth be the maximum for bioretention basins used in LID. The amount, or depth, of exfiltration of the runoff by infiltration or by the process of evapotranspiration is not included in the design charts. Reducing surface area requirements through the consideration of these factors can be determined by using Equation 11.4. Volume of site area for IMPs = (initial volume) × (100 − x ) 100

(11.4)

where x = % of the storage volume infiltrated and/or reduced by evaporation or transpiration, x% should be minimal (less than 10% is considered). Stormwater management is accomplished by selecting the appropriate IMP, or combination of IMPs, to satisfy the surface area and volume requirements calculated from using the design charts. The design charts to be used to evaluate these requirements are as follows: • Chart Series A: Storage Volume Required to Maintain the Predevelopment Runoff Volume Using Retention Storage • Chart Series B: Storage Volume Required to Maintain the Predevelopment Peak Runoff Rate Using 100% Retention • Chart Series C: Storage Volume Required to Maintain the Predevelopment Peak Runoff Rate Using 100% Detention These charts are based on the following general conditions: • The land uses for the development are relatively homogeneous throughout the site. • The stormwater management measures are to be distributed evenly across the development, to the greatest extent possible. • The design storm is based on 1-in. increments. Use linear interpolation for determining intermediate values. The procedure to determine the IMP requirements is described in the following sections. Step 1: Determine Storage Volume Required to Maintain Predevelopment Volume or CN Using Retention Storage The postdevelopment runoff volume generated as a result of the postdevelopment custom-made CN is compared to the predevelopment runoff volume to determine the surface area required for volume control. Use Chart Series A: Storage Volume Required to Maintain the Predevelopment Runoff Volume using Retention Storage (see Attachment 11.A for a representative Chart Series A). It should be noted that the practical and reasonable use of the site must be considered. The IMPs must not restrict the use of the site. The storage area is for runoff volume control only; additional storage may be required for water quality control. The procedure to account for the first 1/2-in. of runoff from impervious areas, which is the current water quality requirement, is found in Step 2. Step 2: Determine Storage Volume Required for Water Quality Control The surface area, expressed as a percentage of the site, is then compared to the percentage of site area required for water quality control. The volume requirement for stormwater management quality control is based on the requirement to treat the first 1/2-in. of runoff (approximately 1800 cubic feet per acre) from impervious areas. This volume is translated to a percent of the site area by

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FIGURE 11.3 Comparison of retention of storage volumes required to maintain peak runoff rate using retention and detention.

assuming a storage depth of 6 in. The greater number, or percent, is used as the required storage volume to maintain the CN. Step 3: Determine Storage Volume Required to Maintain Peak Stormwater Runoff Rate Using 100% Retention The percentage of site area or amount of storage required to maintain the predevelopment peak runoff rate is based on Chart Series B: Percentage of Site Area Required to Maintain Predevelopment Peak Runoff Rate Using 100% Retention. See Attachment 11.B for a representative Chart Series B. This chart is based on the relationship between storage volume, Vs /Vr , and discharge, Qo /Qi, to maintain the predevelopment peak runoff rate (where Vs = storage volume to maintain the predevelopment peak discharge; Vr = postdevelopment peak runoff volume; Qo = peak outflow discharge rate; and Qi = peak inflow discharge rate). The relationship for retention storage to control the peak runoff rate is similar to the relationship for detention storage. Figure 11.3 is an illustration of the comparison of the storage volume/discharge relationship for retention and detention. Curve A is the relationship of storage volume to discharge to maintain the predevelopment peak runoff rate using the detention relationship from figure 6.1 of the TR-55 manual (SCS, 1986) for a Type II 24-h storm event. Curve B is the ratio of storage volume to discharge to maintain the predevelopment peak runoff rate using 100% retention. Note that the volume required to maintain the peak runoff rate using detention is less than the requirement for retention. The following calculations apply to Design Chart Series B: • The Tc for the postdevelopment condition is equal to the Tc for the predevelopment condition. This equality can be achieved by techniques such as maintaining sheet flow lengths, increasing surface roughness, decreasing the amount and size of storm drain pipes, and decreasing open channel slopes. • IMPs are to be distributed evenly across the development site. If the Tc is equal for the predevelopment and postdevelopment conditions, the peak runoff rate is independent of Tc for retention and detention practices. The difference in volume required to maintain the predevelopment peak runoff rate is practically the same if the Tc for the predevelopment and postdevelopment conditions are the same.

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Step 4: Determine Whether Additional Detention Storage Is Required to Maintain the Predevelopment Peak Runoff Rate The storage volume required to maintain the predevelopment runoff volume using retention, as calculated in Step 1, might or might not be adequate to maintain both the predevelopment volume and peak runoff rate. As the CN diverge, the storage requirement to maintain the volume is much greater than the storage volume required to maintain the peak runoff rate. As the CN converge, however, the storage required to maintain the peak runoff rate is greater than that required to maintain the volume. Additional detention storage will be required if the storage volume required to maintain the runoff volume (determined in Step 1) is less than the storage volume required to maintain the predevelopment peak runoff rate using 100% retention (determined in Step 3). The combination of retention and detention practices is defined as a hybrid approach. The procedure for determining the storage volume required for the hybrid approach is described in Step 5. Step 5: Determine Storage Required to Maintain Predevelopment Peak Runoff Rate Using 100% Detention This step is required if additional detention storage is needed. Chart Series C: Storage Volume Required to Maintain the Predevelopment Peak Runoff Rate (see Attachment 11.C for a representative Chart Series C) is used to determine the amount of site area to maintain the peak runoff rate only. This information is needed to determine the amount of detention storage required for hybrid design, or where site limitations prevent the use of retention storage to maintain runoff volume. This includes sites that have severely limited soils for infiltration or retention practices. The procedure to determine the site area is the same as that of Step 3. Using Chart Series C, the following assumptions apply: • The Tc for the postdevelopment condition is equal to the Tc for the predevelopment condition. • The storage volume, expressed as a depth in hundredths of an inch (over the development site), is for peak flow control. These charts are based on the relationship and calculations from Figure 6.1 (Approximate Detention Basin Routing for Rainfall Types I, IA, II and III) in TR-55 (SCS, 1986). Step 6: Use Hybrid Facility Design (required for additional detention storage) When the percentage of site area for peak control exceeds that for volume control as determined in Step 3, a hybrid approach must be used. For example, a dry swale (infiltration and retention) may incorporate additional detention storage. Equation 11.5 is used to determine the ratio of retention to total storage. Equation 11.6 is then used to determine the additional amount of site area, above the site area required for volume control, needed to maintain the predevelopment peak runoff rate. x=

(

50 × −∀ D100 + ∀ D2 100 + 4 × (∀ R100 − ∀ D100 ) × ∀R (∀ R100 − ∀ D100 )

)

(11.5)

where ∀R = storage volume required to maintain predevelopment runoff volume (Chart Series A) ∀R100 = storage volume required to maintain predevelopment peak runoff rate using 100% retention (Chart Series B)

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∀D100 = storage volume required to maintain predevelopment peak runoff rate using 100% detention (Chart Series C) x = area ratio of retention storage to total storage and the hybrid storage can be determined as H = ∀R × (100 ÷ x )

(11.6)

Equations 11.5 and 11.6 are based on the following assumptions: • x% of the total storage volume is the retention storage required to maintain the predevelopment CN calculated from Chart Series A: Storage Volume Required to Maintain Predevelopment Volume Using Retention Storage. • There is a linear relationship between the storage volume required to maintain the peak predevelopment runoff rate using 100% retention and 100% detention (Chart Series B and C) Step 7: Determine Hybrid Amount of IMP Site Area Required to Maintain Peak Runoff Rate with Partial Volume Attenuation Using Hybrid Design (required when retention area is limited) Site conditions, such as high percentage of site needed for retention storage, poor soil infiltration rates, or physical constraints, can limit the amount of site area that can be used for retention practices. For poor soil infiltration rates, bioretention is still an acceptable alternative, but an underdrain system must be installed. In this case, the bioretention basin is considered detention storage. When this occurs, the site area available for retention IMPs is less than that required to maintain the runoff volume, or CN. A variation of the hybrid approach is used to maintain the peak runoff rate while attenuating as much of the increased runoff volume as possible. First, the appropriate storage volume that is available for runoff volume control (∀R′) is determined by the designer by analyzing the site constraints. Equation 11.7 is used to determine the ratio of retention to total storage. Equation 11.8 is then used to determine the total site IMP area in which the storage volume available for retention practices (∀R′) substitutes the storage volume required to maintain the runoff volume. X′ =

(

50 × −∀ D100 + ∀ D2 100 + 4 × (∀ R100 − ∀ D100 ) × ∀ R′ (∀ R100 − ∀ D100 )

)

(11.7)

here ∀R′ = storage volume acceptable for retention IMPs. The total storage with limited retention storage is H ′ = ∀R′ × (100 ÷ χ′)

(11.8)

where H′ is hybrid area with a limited storage volume available for retention IMPs.

DETERMINATION

OF

DESIGN STORM EVENT

Conventional stormwater management runoff quantity control is generally based on not exceeding the predevelopment peak runoff rate for the 2-year and 10-year 24-h Type II storm events. The amount of rainfall used to determine the runoff for the site is derived from Technical Paper 40 (Department of Commerce, 1963). For Prince George’s County, these amounts are 3.3 and 5.3 in.,

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respectively. The 2-year storm event was selected to protect receiving channels from sedimentation and erosion. The 10-year event was selected for adequate flow conveyance considerations. In situations where there is potential for flooding, the 100-year event is used. The criteria used to select the design storm for LID are based on the goal of maintaining the predevelopment hydrologic conditions for the site. The determination of the design storm begins with an evaluation of the predevelopment condition. The hydrologic approach of LID is to retain the same amount of rainfall within the development site as that retained by woods, in good condition, and then to release the excess runoff gradually as woodlands would release it. By doing so, one can emulate, to the greatest extent practical, the predevelopment hydrologic regime to protect watershed and natural habitats. Therefore, the predevelopment condition of the LID site is required to be woods in good condition. This requirement is identical to the State of Maryland definition of the predevelopment condition. The CN for the predevelopment condition is to be determined based on the land cover being woods in good condition and the existing HSG. The design storm is to be the greater of the rainfall at which direct runoff begins from a woods in good condition, with a modifying factor, or the 1-year 24-h storm event. The rainfall at which direct runoff begins is determined using Equation 11.9. The initial rainfall amount at which direct runoff begins from a woodland is modified by multiplying this amount by a factor of 1.5 account for the slower runoff release rate under the wooded predevelopment condition.  1000  P = 0.2 ×  − 10 CN   c

(11.9)

where P is rainfall at which direct runoff begins. A three-step process is used to determine the design storm event. Step 1: Determine the Predevelopment CN Use an existing land cover of woods in good condition overlaid on the HSG to determine the composite site CN. Step 2: Determine the Amount of Rainfall Needed to Initiate Direct Runoff Use Equation 11.9 to determine the amount of rainfall (P) needed to initiate direct runoff. Step 3: Account for Variation in Land Cover Multiply the amount of rainfall (P) determined in Step 2 by a factor of 1.5 to 1.8.

REFERENCES American Society of Civil Engineers (ASCE), 1994. Design and Construction of Urban Stormwater Management Systems, ASCE Manuals and Reports of Engineering Practice, 77, prepared by the Urban Water Resources Research Council of the American Society of Civil Engineers and the Water Environment Federation, Reston, VA. Cairns, J., 1993. Ecological restoration: replenishing our national global ecological capital, in Nature Conservation 3: Reconstruction of Fragmented Ecosystems, D.A. Saunders, R.J. Hobbs, and P.E. Eherlich, Eds., Beatty & Sons, Surrey. Chow, V.T., 1964. Handbook of Applied Hydrology, McGraw-Hill, New York. Department of Commerce, 1963. Rainfall Frequency Atlas of the United States for Durations from 30 minutes to 24 hours and Return Periods from 1 to 100 years, Technical Paper 40. U.S. Department of Commerce, Washington, D.C.

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Federal Interagency Stream Restoration Working Group (FISRWG), 1998. Stream Corridor Restoration: Principles, Processes, and Practices, PB98-158348LUW. Leopold, L.B., 1968. Hydrology for Urban Land Planning: A Guidebook on the Hydrologic Effects of Land Use, U.S. Geological Survey Circular 554. Leopold, L.B., Wolman, M.G., and Miller, J.P., 1964. Fluvial Processes in Geomorphology, Dover Publications, Mineola, NY. Maidment, D.R., 1993. Handbook of Hydrology, McGraw-Hill, New York. Maryland Department of the Environment (MDE), 1998. Maryland Stormwater Design Manual, vols. I and II, Baltimore, MD. NRCS, 1985. National Engineering Handbook, Section 4, Hydrology (NEH-4), Soil Conservation Service, U.S. Department of Agriculture, Washington, D.C. NRCS, 1986. Urban Hydrology for Small Watersheds, Technical Release 55, U.S. Department of Agriculture, Soil Conservation Service, Engineering Division, Washington, D.C. Prince George’s County, Maryland, 1997. Low-Impact Development Design Manual, Department of Environmental Resources, Prince George’s County, MD.

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12

Geomorphic Considerations in Stream Protection Michael L. Clar

CONTENTS Introduction ....................................................................................................................................316 Overview of Geomorphic Concepts ..............................................................................................317 Bankfull Discharge.........................................................................................................................318 Use of USGS Gauging Stations ....................................................................................................319 Flood Frequency Determination ....................................................................................................321 Hydraulic Geometry.......................................................................................................................323 Stream Channel Patterns ................................................................................................................325 Hydrologic Impacts of Urbanization .............................................................................................326 Physical Impacts .....................................................................................................................327 Hydrologic Regime.................................................................................................................327 Large Storm vs. Small Storm Hydrology.....................................................................328 Peak Discharge Control Strategies ......................................................................329 Design Storms......................................................................................................329 Small Storm Hydrology.......................................................................................329 Channel Stability and Degradation.........................................................................................331 Groundwater Recharge ...........................................................................................................332 Biologic Impacts ............................................................................................................................333 Receiving Waters Habitat .......................................................................................................333 Temperature.............................................................................................................................334 Traditional Control Approaches.....................................................................................................335 Level 1: Peak Discharge Control............................................................................................335 Level 2: Floodplain Zoning ....................................................................................................335 Level 3: Water Quality Control ..............................................................................................335 Peak Discharge Strategies and Control of Physical Impacts.................................................335 Control of Increased Flooding ......................................................................................335 Channel Instability, Bank Erosion, and Sediment Transport .......................................336 Reduction in Groundwater Recharge and Related Issues ............................................336 Thermal Impacts............................................................................................................337 Peak Discharge Strategies and Control of Habitat and Biologic Impacts.............................337 Summary of Peak Discharge Strategies .................................................................................337 Future Directions and Innovation in Control Technology ............................................................338 References ......................................................................................................................................339

0-56676-916-7/03/$0.00+$1.50 © 2003 by CRC Press LLC

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FIGURE 12.1 The hydrologic cycle and its components.

Urbanization

Moodflow

Changes in Watershed Hydrology

Changes in Stream Hydrology Hydraulic Geometry Relations Slope to Channel Length Relations

AGNPS Runoff Curve Number Universal Soil Loss Equation Changes in Stream Water Quality

Changes in Urban Stream Morphology

Changes in Riparian Zone Stream Habitat

Biotic Indices

Temperature Model

Habitat Suitability Changes In In-Stream Models Habitat and Ecology

FIGURE 12.2 The impact flow model. A conceptualization of how urbanization impacts aquatic resources. (From Snodgrass, W.J. et al., in Sustaining Urban Water Resources in the 21st Century, A.C. Rowney et al., Eds., Engineering Foundation, 1997. With permission.)

INTRODUCTION The many uses of the land resources including agriculture, silviculture, minerals extraction, and in particular urbanization can have very significant impacts on the elements of the hydrologic cycle illustrated in Figure 12.1, which in turn trigger undesirable physical, chemical, and biologic processes that lead to the degradation and impairment of receiving water resources. Figure 12.2 illustrates the general process by which the impacts occur (Snodgrass et al., 1998). Many land use changes create alterations in the hydrologic regime that ultimately can lead to the increased peak flow conditions and flood events as well as destabilization and degradation of receiving streams channels, their associated habitat metrics, and the related abundance and diversity of aquatic biologic species. Concurrently, land use changes can also alter the chemical balance usually introducing new or higher concentrations of chemical constituents that, although they do not affect the physical stability of channels, can also lead to biologic degradation and impairment of receiving waters. In response to understanding of these issues stormwater management technologies and land use management programs have been developed to mitigate these potential impacts. These technologies

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understandably focused first on the flooding issues and the associated loss of property and life. Later, in response to implementation of the Clean Water Act and the National Urban Runoff Program (NURP, EPA, 1983), water quality control measures were added to the flood control programs. However, understanding and application of appropriate technology to address the channel stability issues have lagged behind the other technical areas. Although some local programs are attempting to develop appropriate technical guidance for this problem, most local, state, and federal programs have yet to recognize the importance of this problem, and consequently to develop appropriate management tools. Currently in the United States, approximately 2 million acres of land annually is converted from a rural to urban condition. Without appropriate technical management programs to address the impacts of urbanization on the physical stability of receiving stream channels, there will be a steady and unnecessary continued degradation and impairment of receiving waters. Hundreds of miles of urbanizing streams will be degraded, thus undermining the intent of the Clean Water Act to preserve and restore the nation’s waters to a fishable and swimmable standard. This chapter addresses the issues associated with the physical impairment of stream channels associated with land use changes and the resulting modifications of the hydrologic regime. This chapter provides a review of the relevant geomorphic concepts that must be recognized and incorporated into suitable management programs. In addition, the physical and hydrologic impacts of land use changes are summarized. The current control strategies for stormwater management are reviewed and their effectiveness or lack thereof in addressing the physical degradation problem is discussed. The next chapter addresses the technology for the restoration of degraded urban streams. It includes an introduction to the elements of fluvial geomorphology and morphologic features that should be assessed, the role of sediment in determining channel stability, and the use of the Rosgen classification system as an integrating tool for the assessment of channel conditions. Chapter 13 also describes the application of the Rosgen classification system and provides a review of stream stabilization techniques using natural materials. Two case studies that demonstrate the use of this approach are provided.

OVERVIEW OF GEOMORPHIC CONCEPTS Fluvial geomorphology is the science that describes the physical characteristics and processes associated with river channels. A generalized relationship indicating the “stable channel balance” was proposed by Lane (1955) (Figure 12.3), and can be expressed as follows: Sediment Load (Qs ) × Sediment Size ( D50 ) ~ Stream Discharge (Q) × Stream Slope ( S)

FIGURE 12.3 Lane’s stable channel balance. (From Lane, E.W., Am. Soc. Civil Eng. Proc., 81, 1, 1955. With permission.)

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A change in any one of these variables sets up a series of mutual adjustments in the companion variables with a resulting direct change in the characteristics of the stream. These variables are closely related to both their spatial location within a watershed and the land use conditions within the watershed. Under natural watershed conditions, stable natural channels result with distinct ecological conditions that reflect the regional climate and geology, or ecoregion. Many human land use activities, including silviculture, agriculture, mining, road building, and urbanization, can create short- or long-term alterations to one or more of these variables, which can lead to rapid destabilization of the stream channel conditions and can have devastating impacts on both the stream ecology and its floodplain, or riparian zone. A brief description of the interaction among these variables and watershed conditions is provided. The description focuses on a description of important parameters.

BANKFULL DISCHARGE For the purpose of stream channel assessment and restoration design, the bankfull, dominant or effective discharge is the primary design storm. While some have attempted to draw clear distinctions among these three terms, in this chapter the definition provided by Dunne and Leopold (1978), which integrates these three terms, will be used: The bankfull stage corresponds to the discharge at which channel maintenance is the most effective, that is, the discharge at which moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the average morphologic characteristics of channels.

It is this discharge in concert with the range of flows that makes up an annual hydrograph, which governs the shape and size of the channel. Bankfull discharge is associated with a momentary maximum flow that on the average has a recurrence interval of 1.5 years as determined using a flood frequency analysis (Dunne and Leopold, 1978). Although great erosion and enlargement of steep, incised channels may occur during extreme flood events, it is the modest flow regimes that often transport the greatest quantities of sediment over time, due to the higher frequency of occurrence for such events (Wolman and Miller, 1960). An example of the relationship between flow magnitude and frequency of flow is shown in Figure 12.4. The dominant, effective, or bankfull

FIGURE 12.4 Relations between discharge, sediment transport, frequency, and the product of frequency and transport rate. (From Wolman, M.G. and Miller, J.P., J. Geol., 68, 54, 1960. With permission.)

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Discharge of 1.5 Year Recurrence Interval cfs

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Colorado Front Range Upper Green River, Wyo. S.E. Pennsylvania Salmon River, Idaho

1000

100

10 10 100 1000 Bankfull Discharge from Field Channel Geometry cfs

FIGURE 12.5 Comparison of field-determined bankfull discharge with discharge of 1.5-year recurrence interval for four regions of the United States. (From Leopold, L.B., A View of the River, Harvard University Press, Cambridge, MA, 1994. With permission.)

discharge is associated with the peak of cumulative sediment transport for a given stream flow magnitude and frequency of occurrence. The majority of work over time is accomplished at moderate flow rates, as shown in Figure 12.4. In the United States, Leopold and co-workers (1964; 1978; 1994) have reported that bankfull discharge occurs approximately every 1 to 2 years (Figure 12.5), although a wide range of values have been reported by others (Williams, 1978). Similar observation have been reported in New Zealand (Gordon et al., 1982), where Mosley (1981) reported that for a study of 72 rivers the average bankfull recurrence interval ranged from 1 to 10 years with a reported median value of 1.5 years.

USE OF USGS GAUGING STATIONS A common error in the field determination of bankfull discharge is the failure of observers to calibrate the elevations of appropriate field indicators of bankfull stage to known stream flows. The recommended procedure for calibrating field-identified bankfull stage with known stream flows has been summarized by Rosgen (1996) as follows: 1. Locate all current and discontinued stream gauging stations within the study basins and/or in nearby similar basins. 2. Make a field visit to each station to collect supplemental data, which will be needed to interpret existing hydrologic records at each station. Note that these field visits are not an unnecessary extravagance, nor are they likely to be a major time encumbrance. Investigators will be fortunate if they can find a half-dozen gauging stations within a selected study area, and often it may be necessary to travel outside the area of interest to obtain representative data for extrapolation. 3. The use of the information in Table 12.1 will serve as a checklist for procedures to be performed at the gauged site. A portion of the data to be collected is not entirely necessary

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TABLE 12.1 Checklist of Recommended Procedure for USGS Gauge, or Other Stream Flow Measurement Locations A. Describe Site 1. Geomorphic Setting — Valley Types I through XI (Rosgen, 1996, chap. 4) 2. Channel Materials (Pebble Count) (D16, 35, 50, 84, 95) a. Bed Material — Pebble Count b. Bank Material — Pebble Count and Core Sample c. Bar Material — Core Sample 3. Locate on Topographic Map 4. Photo Document — Up/Downstream 5. Compute Percentage of Watershed Hydraulically Impacted B. Longitudinal Profile 1. Measure Average Water Surface Slope a. Riffle Slope b. Pool Slope 2. Measure Valley Slope 3. Sequence of Riffle/Pool or Step/Pool as a Function of Bankfull Width 4. Locate Bankfull Stage along Longitudinal Profile C. Plan View 1. Measure Sinuosity (SL/VL) (VS/CS), where SL = stream length; VL = valley length; VS = valley slope; and CS = channel slope 2. Meander Geometry a. Meander Length (LM) b. Belt Width (BW) c. Radius of Curve (RC) d. Meander Arc Length (ML) e. Meander Width Ratio D. Cross Section (Dimension) 1. Cross Section of Channel + Valley Features — Terrace/Floodplain (to be identified on cross section plot) a. Bankfull Width (Wbkf) b. Bankfull Mean Depth (Dbkf) c. Bankfull Maximum Depth (Dmbkf) d. Flood Prone Area Width (Wfpa) e. Entrenchment Ratio (Wfpa/Wbkf) f. Bankfull Cross-Sectional Area (Abkf) g. Bankfull Velocity (Estimate from various sources) h. Estimated Bankfull Discharge (Qbkf) 2. Calibrate Bankfull Discharges a. Survey estimated bankfull stage. b. From gauge plate, extrapolate stage reading associated with estimated “bankfull.” c. Read discharge from rating Curve@gauge (Stage/Discharge Relation). d. Determine recurrence interval in years from flood frequency curves at station. e. Analyze hydraulic geometry data from 9-207 forms (discharge notes) for width, depth, velocity, and crosssectional area vs. stream discharge. Plot data on log/log paper and run a regression to obtain slope and intercept values for each variable. f. Develop dimensionless hydraulic geometry relations. This is to be applied for extrapolation purpose to rivers of the same stream type, but for various sizes. W/Wbkf vs. Q/Qbkf (Complete for depth, velocity, and crosssectional area). Source: Rosgen, D.L., Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO, 1996. With permission.

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for stream classification, but it is necessary to perform supporting sediment and hydraulic analyses. 4. Table 12.2 provides a form for recording gauged and field data. The primary uses of these data involve the calibration of field-estimated bankfull stage to a corresponding measured record. To this available published data, observers must add their own collected field data, describing the reference reach slope, particle size distribution, bankfull stage and channel characteristics, hydraulic geometry, and, of course, stream type. Examination and use of partial duration series is very useful for bankfull discharge analysis. The partial duration series provides a means of evaluating short duration records (i.e., <10 years). Also for urbanizing watersheds, they can provide a clear interpretation of the changing hydrologic regime. There is a relationship between recurrence intervals obtained from the annual-maximum series and the partial–duration series, as shown in Table 12.3 (Dunne and Leopold, 1978). The differences are negligible for return periods greater than 10 years. But there is a distinction between the meanings of recurrence interval of floods obtained from the two series. For the annual-maximum series the recurrence interval is the average interval within which a flood of a given size will occur as an annual maximum. The recurrence interval obtained from the partial-duration series is the average frequency of occurrence between floods of a given size irrespective of their relation to the year. It is the average time between flows equal to or greater than a given discharge. The usual method of obtaining return periods for the partial-duration series is to obtain them for the annualmaximum series and then to convert the frequencies by use of Table 12.3. All gauging stations normally will have a permanent benchmark installed for the purpose of maintaining an elevation control and referencing to a corresponding discharge measurement cross section. The established gauge station cross section should be resurveyed and expanded laterally to include the active floodplain, low terraces, and other valley features of interest. PROCEDURE FOR CALIBRATING BANKFULL DISCHARGE GAUGED STREAM

OF

UNGAUGED STREAM

TO

1. Identify a gauged stream within the same hydrophysiographic region that has a drainage area similar to the drainage of the stream of interest. 2. Visit the gauging station and survey a longitudinal profile through a minimum length equal to 20 to 24 channel widths. Record the elevation and position of bankfull stage indicators, water surface, and bed features along the thalweg. This profile should extend past the gauge plate. 3. Where the profile of bankfull stage intersects the gauge plate will give the gauge reading of field-calibrated bankfull stage. Then, from the gauge record of stage vs. discharge, determine the discharge at that stage. 4. Compare this discharge to the flow frequency distribution from the gauge record. If it is equal to a recurrence interval between 1 and 2 years it is acceptable. This is the fieldcalibrated bankfull discharge for the gauged stream. 5. Compare this discharge on a proportional area basis to the field-determined discharge for the stream of interest to calibrate the steam to a gauged stream in the same hydrophysiographic region.

FLOOD FREQUENCY DETERMINATION The USGS gauging station data available at the state office will usually include a flood frequency analysis computed using a Pearson Type III distribution expressed as an exceedence probability

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TABLE 12.2 Sample Form for Recording Gauge Station and Field Data SUMMARY of USGS GAGE DATA/RECORDS for STREAM CHANNEL CLASSIFICATION Station NAME: _______________________________ Station NUMBER: ____________________________ LOCATION: ____________________________________________________________________________________ Period of RECORD: ______________________ Yrs. Percentage of Watershed Hydraulically Impacted _________% Drainage AREA: ________________ Ac.___________ Sq. Mi. Drainage MEAN ELEVATION: ______________Ft. Reference REACH SLOPE:______________________ STREAM TYPE: ________________________________ BANKFULL CHARACTERISTICS Determined by FIELD MEASUREMENT Bankfull WIDTH ______________________Ft. (Wbkf) Bankfull MEAN DEPTH ________________Ft. (dbkf) Bankfull STAGE _____________________________Ft. Bankfull Xsec. AREA _______________Sq. Ft. (Abkf) Wetted PERIMETER ______________________Ft. WP Est. Mean VELOCITY______________Ft./Sec. (Vbkf) Est. Bankfull DISCHARGE _____________Cfs (Qbkf)

Determined by GAGE DATA Analyses Bankfull WIDTH ______________________Ft. (Wbkf) Bankfull MEAN DEPTH _________________Ft. (dbkf) Bankfull STAGE _____________________________Ft. Bankfull Xsec. AREA ________________Sq. Ft. (Abkf) Wetted PERIMETER ______________________Ft. WP Mean VELOCITY___________________Ft./Sec. (Vbkf) Bankfull DISCHARGE _________________Cfs (Qbkf)

Bankfull DISCHARGE associated with "field determined" Bankfull STAGE: __________Cfs. (Qbkf) (From Gauge Height reading at Staff Plate and tubular Stage-Discharge Curve Data) Recurrence Interval (Log-Pearson) associated with "field determined" Bankfull Discharge R.I. = _________Years From the Annual Peak Flow Frequency Analysis 1.0 Year R.I. Discharge = 1.5 Year R.I. Discharge = 2.0 Year R.I. Discharge =

data for the Gauge Station, determine: _________CFS _________CFS _________CFS

MEANDER GEOMETRY Determined by FIELD MEASUREMENT Meander Length (Lm) ___________________________ Ft. Radius of Curvature (Rc) _______________________ Ft. Belt Width (Wblt) _______________________________ Ft. Meander Width Ratio (Wblt/Wbkf) _________________

Based on USGS Discharge Summary Notes data (Form 9-207) and regression analyses of measured discharge (Q) with the hydraulic parameters of Width (W), Area (A), Mean Depth (d), Mean Velocity (V); determine the intercept coefficient (a) and the slope exponent (b) values for a power function of the form Y=aX^b; when Y is one of the selected hydraulic parameters, and X is a given discharge value (Q). Width (W)

Depth (d)

Area (A)

Velocity (V)

Coefficient (a) Slope Expn. (b) Hydraulic Radius (R=A/WP) ____________Ft. Manning’s "N" (Rough. Coeff.) at Bankfull Stage ___________. "N"=1.486 / Qbkf [(Area) (Hydraulic Radius ^2/3) (Slope ^1/2)] Source: Rosgen, D.L., Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO, 1996. With permission.

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TABLE 12.3 Relation between Recurrence Intervals (years) of the Annual-Maximum Series and the Partial-Duration Series Annual-Maximum Series

Partial-Duration Series

1.16 1.50 1.58 2.0 2.54 5.52 10.50 20.50 50.50 100.50

0.5 0.9 1.0 1.45 2.0 5.0 10.0 20.0 50.0 100.0

Source: Langbein, W.B., Water Supply Paper 1543-A, U.S. Geological Survey, 1960.

value. A return interval can be calculated from the probability value since the return interval (R.I.) is the inverse of exceedence probability (P), or R.I. = 1 P

(12.1)

If the annual peak series is downloaded through the Internet, the flood frequency graph is not provided, but a flood frequency analysis can be developed using either the annual or partial duration flood series using the equation: R.I.(years) = (n + 1) m

(12.2)

where n = the number of entries in the record m = the rank order This procedure is described in most hydrology textbooks (Linsley et al., 1958). The bankfull discharge previously described has a recurrence interval on the annual flood series of 1.5 years. Table 12.3 shows that a flow with a recurrence interval of 1.5 years in the annual series will have a recurrence interval of 0.9 years in the partial-duration series. Therefore, a discharge equal to or greater than bankfull may be expected to occur on the average once every 0.9 years. Thus, it can be stated that, on average, the bankfull stage will be equaled or exceeded about once a year.

HYDRAULIC GEOMETRY Hydraulic geometry describes the way in which channel properties change with stream flow. A stream’s cross-sectional area, for example, is generally determined by the amount of water it must carry; i.e., headwater streams are smaller than the rivers into which they flow. Leopold and Maddock (1953) demonstrated that some hydraulic characteristics of stream channels — mean depth (D), width (W), mean velocity (V), and suspended load (Qs) — vary with

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discharge as simple power functions at a given cross section. The interrelationships, which they termed the hydraulic geometry of streams, are described by the following functions. 1. 2. 3. 4.

W = aQb D = cQf V = kQm Qs = pDj

When plotted on log-log paper, the slopes of the curves, described by the exponents b, f, m, and j, indicate the average change of width, depth, velocity, and sediment load, respectively, with changes in discharge, and do not vary with the units used. The coefficients a, c, k, and p do have dimensions. They represent the intercepts of the lines and are, respectively, values of W, D, and V at a discharge of unity. Hydraulic geometry relationships can be applied to the description of how variables change with discharge (1) at a particular location (“at a station”) or (2) over a drainage basin (“downstream”). Leopold and Maddock (1953) cite average at-a-station coefficients of b = 0.26, f = 0.40, and 0.34. Thus, as discharge increases at a cross section, velocity rises and depth increases faster than width (the width–depth ratio drops) until the channel overtops its banks. It should be noted that these coefficients can be expected to differ considerably from reach to reach. Downstream changes in channel geometry can be investigated by linking information from a number of sites within a stream system. This method is valid only if the discharges used for comparison are of the same average recurrence interval. Values of mean annual flow or bankfull discharge are commonly used, under the assumption that these flows occur at approximately the same frequency on a large number of rivers. Using mean annual flows, Leopold and Maddock (1953) found that downstream increases in depth, width, and velocity relative to discharge were similar for rivers of varying drainage basin size and setting. Average values of hydraulic geometry exponents for the rivers studied were b = 0.5, f = 0.4, and m = 0.1. From these exponents it can be seen that large rivers tend to be wider and shallower than smaller streams, and that velocity increases slightly in a downstream direction. This latter conclusion may be somewhat surprising, because whitewater mountain streams give the impression of flowing faster than meandering valley streams. Although the headwater streams are steeper, the lower roughness in the valley streams due to reduced particle sizes can lead to increases in velocity. Discharge typically increases with distance downstream because of the increasing area of drainage. Therefore, drainage area at any point is closely related with many size and discharge characteristics. To provide a general picture of river channel dimensions then, the bankfull width, depth, and cross-sectional area as functions of drainage area are useful. Figure 12.6 presents these dimensions for four regions in the United States. It can be seen that there is a considerable consistency even among regions. For the same drainage area, the bankfull dimensions in the Eastern United States and the San Francisco Bay area are very nearly the same. Channels in Wyoming, in an area of lower annual precipitation and runoff, are correspondingly smaller. As usual, mean relations for rivers are drawn through points with considerable scatter, but the averages are useful tools in planning even though any individual place on a river may not agree closely with the means. Not only are average channel dimensions similar for streams of a given drainage area in a region, but all of the parameters applicable to a geographic location are similar as well. This means that having measured the basin area on a map, a quantitative estimate of channel dimensions and bankfull discharge can be made with the understanding that the variability is large. Figure 12.7 provides such a relationship of drainage area to bankfull discharge for five regions of the United States.

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Bankfull Dimensions 500 Cross-sectional 100 area (square feet) 50

Average values of bankfull channel dimensions as a function of drainage area for four regions

10 5 1 50

50 20

20 San Francisco Bay region Width (feet) 10 39" annual precipitation

Eastern United States Upper Green River, WY Upper Salmon River, ID (from Leopold, 1994)

Depth (feet)

5

10 5

1

1 .1 .2 .5 1 2 5 10 20 50 100 Drainage area (square miles)

500

FIGURE 12.6 Regional hydraulic geometry relationships. (Modified from NRCS, 1998.)

FIGURE 12.7 Regional bankfull discharge relationships. (From Leopold, L.B., A View of the River, Harvard University Press, Cambridge, MA, 1994. With permission.)

STREAM CHANNEL PATTERNS Channel pattern, also called planform, is the term used to describe the geometry of a stream channel along its floodplain. Stream channels are seldom straight, except for short distances (Leopold, 1994). This observation led to the development of a number of relationships among the meander wavelength, channel width, and the radius of curvature that are very useful in both the assessment of channel stability and the design of stable restoration planforms. Figure 12.8 identifies the primary meander geometry variables as follows (Leopold, 1994): • Bankfull width (w) • Meander wavelength (L), which typically averages about 11 times the bankfull width and nearly always is between 10 to 14 times bankfull width (Figure 12.9) • Meander arc length (ML), which is the distance of the meander wavelength measured along the thalweg

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L

Θ rc w

L ML W MA rc Θ

MA ML

meander wavelength meander arc length average width at bankfull discharge meander amplitude radius of curvature arc angle

FIGURE 12.8 Meander geometry variables. (From NRCS, 1998.) (Modified from Leopold et al., 1964.)

Meander Length (feet)

Relations between Meanderr Length and Channel Width and Mean Radius of Curvature 1,000,000 100,000

100,000 10,000

100,000

1.01

L = 10.9w

10,000 10,000

1,000 100 10

1,000 0.98 L = 4.7rm

10

100 1,0005 10

100

10 100 1,000 10,000 100,000

(from Leopold, 1994) Channel Width (feet) Mean Radius of Curvature (feet)

FIGURE 12.9 Relationship between meander length, channel width, and mean radius of curvature. (From NRCS, 1998). (Modified from Leopold, 1994.)

• Meander amplitude (MA), which is measured from the thalweg of the first curve or channel bend to the thalweg of the second curve • Radius of curvature (rc) of the central portion of a channel bend, which usually averages about one fifth of the wavelength or 2.3 times the bankfull width • Arc angle (θ) of the channel bend Rosgen (1996) has observed that stream flow regimes not only influence bankfull channels widths but can also change stream patterns depending on the magnitude and duration of flows. For example, as urban watersheds are developed, it should come as no surprise to observe widening of streams and changes in channel patterns. These systems of channel adjustment brought on by an acceleration of stream bank and bed erosion result in the noted increases in the width/depth ratio and corresponding bar development. These processes are discussed in more detail in Chapter 13.

HYDROLOGIC IMPACTS OF URBANIZATION Urban runoff, which includes runoff from impervious surfaces such as streets, parking lots, buildings, lawns, and other paved areas, is one of the leading causes of water quality impairment in the

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United States. Based on the 1996 State Water Quality Inventory reports, siltation (sediment discharged from urban runoff, as well as construction sites, agriculture, mining, and forests) is the leading cause of impaired water quality in rivers and streams. In the portion of the inventory identifying sources, urban runoff was listed as the leading source of pollutants causing water quality impairment related to human activities in ocean shoreline waters and the second leading cause in estuaries across the nation. Urban runoff was also a significant source of impairment in rivers and lakes. The hydrologic impacts of urbanization cover a broad spectrum of interrelated issues, sciences, and disciplines that makes it difficult to address in a comprehensive manner. The issues include (1) physical impacts such as flooding, channel erosion and degradation, and reduction in groundwater recharge; (2) chemical impacts such as increases in a wide range of water quality parameters; and (3) biologic and habitat impacts primarily related to the loss or impairment of habitat, both in the receiving waters and the adjacent riparian corridors, which is necessary for the sustainability of numerous biologic communities. These impact areas are summarized in Table 12.4. This discussion will focus on the physical impacts, although it should be noted that in general it is these changes in physical impacts, particularly the hydrologic regime and sediment supply alterations, which result in downstream channel degradation, related habitat degradation, and subsequent reduction or loss of biologic species. The chemical impacts or water quality issues and biologic issues are addressed in other chapters.

PHYSICAL IMPACTS As shown in Table 12.4, the physical impacts include a number of hydrologic regime alterations: increases in runoff volume and peak discharge, alterations to the flow duration and frequency, and reductions in groundwater recharge and base flows (Clar et al., 2001). In addition, the physical impacts typically include channel geometry alterations in the form of channel enlargement (widening and/or deepening) in response to the increased flow regime. The sediment supply is often increased, first during the construction phase and later from the channel enlargement processes. Other physical impacts can include increased flooding and thermal impacts.

HYDROLOGIC REGIME Hydrologic regime alterations identified in Table 12.4 include runoff volume, peak discharge, flow duration, flow frequency, groundwater recharge, water table elevation, and base flow. Figure 12.10 illustrates how these parameters are altered as a result of urbanization. The runoff volume, which spans the entire regime of flow, can be measured by number and by characteristics of rise in stream flow. The many rises in flow, along with related sediment loads, control the stability of the stream channel. The two principal factors governing flow regime are the percentage of area made impervious and the rate at which water is transmitted across the land to stream channels. The former is governed by the type of land use; the latter is governed by the density, size, and characteristics of tributary channels and thus by the provision of storm sewerage. Stream channels form in response to the regime of flow of the stream. Changes in the regime of flow, whether through land use or other changes, cause adjustments in the stream channels to accommodate the flows. The volume of runoff is governed primarily by infiltration characteristics and is related to land slope and soil type as well as to the type of vegetative cover. Thus, it is directly related to the percentage of the area covered by roofs, streets, and other impervious surfaces at times of hydrograph rise during storms. As runoff volume from a storm increases, the size of flood peak also increases. Runoff volume also affects low flows because in any series of storms the larger the percentage of direct runoff, the smaller the amount of water available for soil moisture replenishment and for groundwater storage. An increase in total runoff from a given series of storms, as a result of imperviousness,

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TABLE 12.4 Categories of Impacts Attributable to Construction and Land Development Activities Category

Impact Type/Metric

Physical

Hydrologic regime Runoff volume Peak discharge Flow duration Flow frequency Groundwater recharge Water table elevation Base flows Channel geometry Channel lining Flooding Sediment transport Thermal Attachment sites Embeddedness Fish shelter Channel alteration Sediment deposition Stream velocity and depth Channel flow status Bank vegetation protection Bank condition score Riparian vegetation zone Total taxa Ephemeroptera Plecoptera, Tricoptera EPT (taxa) % taxa % EPT Family biotic index (FBI) Sediment Nutrients Metals Oil and grease Pathogens Organic carbon Herbicides/pesticides MTBE Deicer

Habitat

Biological

Chemical (water quality)

Impairment or Change in Beneficial Use

Groundwater recharge, hydrologic balance Flooding, channel erosion Channel erosion, habitat impairment Channel erosion, habitat impairment Water table, base flows, habitat Local wells, springs, base flow, wetlands, habitat Habitat Bank stability, vegetative cover, habitat Downstream erosion, channel stability, habitat Loss of property, or damage Channel stability, habitat Habitat impairment Impairment or loss of habitat structure results in reduction or losses in biologic conditions and communities

Biologic conditions and communities can be reduced or eliminated as a result of impairment or loss of habitat structure caused by physical impacts resulting from construction and development activities

Water quality degradation or impairment can have many negative consequences; drinking water violations; increased water treatment costs, beach closures, shellfish bed closures, loss of boating use, fishery loss, reduction of reservoir and lake volumes due to sediment volume

results in decreased groundwater recharge and decreased low flows. Thus, increased imperviousness has the effect of increasing flood peaks during storm periods and decreasing low flows between storms. Large Storm vs. Small Storm Hydrology Historically, engineers and hydrologists have understandably focused their efforts on the protection of downstream areas from flooding conditions. This gave rise to a design approach to stormwater management based on peak discharge control of large design storm events.

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FIGURE 12.10 Hydrologic impacts of urbanization.

Peak Discharge Control Strategies Peak discharge control is the oldest and most widely used strategy for controlling the impacts of urban runoff. The strategy is relatively straightforward and consists of a general policy, or requirement, that postdevelopment discharge rates cannot substantially exceed existing or predevelopment discharge rates. Both postconstruction runoff conditions (total volume and the peak discharge values) are usually much greater than predevelopment conditions. Therefore, the peak discharge approach generally requires that storage facilities be provided to store temporarily the additional runoff volume, which is then discharged at the allowable release rate, based on the design storm. Design Storms The peak discharge control strategy is closely tied to the use of design storms. The selection of a specific design storm generally incorporates a number of implicit assumptions relating to the storm water runoff impacts being controlled, and thus provides a good starting point for a scientific assessment relating to actual benefits vs. perceived benefits of this strategy. As Table 12.5 documents, a number of the assumptions implicit in the selection of a design storm in conjunction with the peak discharge control strategies do not hold up under scientific scrutiny and have never been validated by field monitoring. As the table indicates, the implicit assumption that peak discharge control of the 2-year storm as a strategy for channel protection is not supported by geomorphic science or field-monitoring data. On the contrary, the geomorphic data predict that the strategy is flawed, and the prediction is confirmed by limited field monitoring data. Geomorphic science also indicates that use of the 10-year storm has no geomorphic significance within a stream valley and is simply a carryover of the cost–benefit basis for the design of storm drainage systems (ASCE, 1994). Watershed-based hydrologic analysis further reveals that the downstream flood control benefits from both the 10- and 100-year storms are very short-lived and that, in fact, due to the super-positioning of hydrograph peaks, flooding problems will tend to be transferred to downstream properties (Leopold and Maddock, 1954; Skupien, 2000). Small Storm Hydrology The nature and extent of hydrologic regime alterations can be estimated using a number of hydrologic models such as the NRCS TR-55 model described in the publication, “Urban Hydrology for Small Watersheds” (NRCS, 1986). The TR-55 model uses three principal parameters to define hydrologic regime. These include the drainage area (A), curve number (CN), and time of concentration (Tc).

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TABLE 12.5 Design Storm Frequencies and Assumed Benefits Design Storm ½-in. rainfall

1-in. rainfall 1-year

Assumed Benefits 70–80% control of annual runoff volume used for water quality volume control 90% control of annual runoff volume used for water quality volume control Water quality management and stream channel protection

2-year

Used by most municipalities to provide protection from accelerated channel erosion and for habitat protection

10-year

Used to provide flood protection from intermediate storm events

100-year

Used for flood control protection from major storms; also used to maintain 100-year floodplain limits

Comments

Ref.

Used by many municipalities

Replacing 1/2 in. as basis for water quality control Used by some municipalities for water quality management. Maryland is now using for channel protection. Geomorphic science does not support this assumption; very limited field monitoring indicates that the strategy is flawed Use of this storm frequency is mostly a carryover from storm drainage design practices; flood control benefits are very limited; in some cases increases potential for downstream flooding; there is no geomorphic basis for the use of this storm Flood control benefits are very limited; in some cases increases the potential for downstream flooding

MDE 2000 MDE 2000

Leopold, 1968; McCuen et al., 1987; McCrae et al., 1996; Jones et al., 1997; Maxted et al., 1997 Skupien, 2000

Skupien, 2000

TABLE 12.6 Comparison of Runoff Volumes for Selected Storms and CN Values Land Use

Storm/Rainfall Runoff Depth CN TIA %

Undeveloped 58 0

Res(1/2 acre) 70 25

% Increase

Commercial 90 85

% Increase

Paving 98 100

% Increase

1 in. 2 in. 1 yr/2.7 in. 1.5 yr/3.0 in. 2 yr/3.3 in. 5 in. 7 in.

0.00 0.044 0.18 0.27 0.36 1.17 2.41

0.00 0.24 0.50 0.71 0.89 2.04 3.62

0 545 278 259 247 174 150

0.32 1.09 1.63 1.98 2.26 3.88 5.82

NA 2477 906 733 628 332 241

0.79 1.77 2.47 2.77 3.07 4.76 6.76

NA 4022 1372 1026 853 407 280

In the process of converting from an undeveloped watershed to an urban land use a watershed can experience a significant change in hydrologic regime, as illustrated in Figure 12.10. An undeveloped watershed typically exhibits a negligible amount of total impervious surfaces (TIA), whereas urban land uses exhibit a wide range of impervious coverage as shown in Table 12.6.

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FIGURE 12.11 Impacts of urbanization on increase of overbank flows. (Note: Increase in number of flows per year equal to or exceeding original channel capacity (1-square mile) as a ratio to number of overbank flows before urbanization for different degrees of urbanization). (From Leopold, L.B., Water Supply Paper 1591-C, U.S. Geological Survey, 1968.)

The TR-55 model and hydrologic soil group type B was used in Table 12.6 to compare the change in runoff volume associated with a range of land use types and design storms or rainfall events. Thus, it can be observed that that the relative change or impacts of urbanization, as measured by a percent increase in runoff volume, are greater for the smaller more frequent storms, such as the 1- and 2-in. storms, and decrease as the storm size increases. It can also be noted that the runoff volume from a 1-in. storm for a commercial land use (0.32 in.) is greater than the runoff volume for the bankfull storm for an undeveloped condition (0.27 in.). This situation illustrates the problem with current design strategies that use the 2-year storm as a surrogate for channel protection. The strategies allow all release rates based on the predevelopment runoff volume. In this case a release equivalent to the predevelopment 2-year storm would occur with every rainfall event of 1 in. or more. In the Mid-Atlantic states the 1-in. storm can occur 8 to 12 times/year, thus producing the equivalent of 8 to 12 bankfull events per year, as shown in Figure 12.11. Figure 12.11 shows the increase in number of flows per year equal to or exceeding original channel capacity (1 square mile) as a ratio to number of overbank flows before urbanization for different degrees of urbanization. These types of altered hydrologic conditions would probably lead to considerable adjustment or enlargement of most stream types. This increase in runoff volume is further compounded by the accompanying change in the Tc that is typical in urban development. As areas urbanize, the time of concentration, that is, the time that it takes for the entire watershed to contribute runoff to its outlet point, generally decreases, which in turn increases the instantaneous peak discharge rate from the watershed.

CHANNEL STABILITY

AND

DEGRADATION

The volume and flow rate of stormwater discharges can have significant impacts on receiving streams. In many cases, the impacts on receiving streams due to high stormwater flow rates or volumes can be more significant than those attributable to the contaminants found in stormwater discharges. Although studies linking increased storm water flows due to urbanization to stream degradation are generally lacking in quantitative data, there are a number of studies that support this hypothesis. The U.S. EPA summarized studies that contain documented evidence of impacts

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TABLE 12.7 Impacts from Increases in Impervious Surfaces Increased Imperviousness Leads to:

Resulting Impacts Flooding

Habitat Loss

Erosion

Channel Widening

Streambed Alteration

X X X

X X X X X X

X X X

X X X

X X X

X

X

X

Increased volume Increased peak flow Increased peak duration Increased stream temperature Decreased base flow Changes in sediment loading

X

Source: U.S. EPA, 1999. Preliminary Data Summary of Urban Storm Water Best Management Practices.

on streams due to urbanization (U.S. EPA, 1997a). These physical impacts to streams are summarized in Table 12.7. Impacts of urbanization and increased stormwater discharges to receiving streams documented in this evaluation include the following: • • • • • • • • • •

Increase in the number of bankfull events and increased peak flow rates Sedimentation and increased sediment transport Frequent flooding Streambed scouring and habitat degradation Shoreline erosion and stream bank widening Decreased base flow Loss of fish populations and loss of sensitive aquatic species Aesthetic degradation Changes in stream morphology Increased temperatures

Similar results were observed by the Delaware Department of Natural Resources and Environmental Control, in a report, which identified a list of impacts on physical streams attributed to urban stormwater (DE DNREC, 1997). This list included the following impacts: • • • • • • • • •

Accelerated bank erosion Accelerated bank undercutting Increased siltation (burial of stable habitats) Elimination of meanders (canalization) Channel widening Reduced depth Reduced base flow Loss of shade Increased temperature

GROUNDWATER RECHARGE Urbanization can have a major impact on groundwater recharge. As shown in Figure 12.10, both shallow and deep infiltration decreases as watersheds undergo development and urbanization. Groundwater recharge is reduced along with a lowering of the water table. This change in watershed hydrology alters the base flow contribution to stream flow, and it of most pronounced during dry periods. Ferguson (1990) points out that “base flows are of critical environmental and economic

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concern for several reasons. Base flows must be capable of absorbing pollution from sewage treatment plants and non-point sources, supporting aquatic life dependent on stream flow, and replenishing water-supply reservoirs for municipal use in the seasons when water levels tend to be lowest and water demands highest.” Many urban areas are at least either partially or totally dependent on groundwater sources for their water supply needs. The alteration of the groundwater recharge relationship is a very slow process that is currently not given much attention, but has the potential to become a significant issue over a long period of time. Some coastal areas of the United States such as Florida are already experiencing water shortages as a result of lower water tables and are placing restrictions on summertime watering of lawns. Also, some coastal areas are experiencing saltwater intrusion problems associated with the lowering of water tables. Lowering of water tables can also lead to subsidence problems for buildings and other structures.

BIOLOGIC IMPACTS RECEIVING WATERS HABITAT Natural ecosystems are a complex arrangement of interactions among the land, water, plants, and animals. The relationship between stormwater discharge and the biologic integrity of urban streams is illustrated in Figure 12.12 (Masterson and Bannerman, 1994). As shown, habitat is influenced by changes in both water quality and quantity, and the volume and quality of sediment. The progressive degradation of urban stream ecosystems is directly related to the cumulative impacts of many interrelated individual factors, which include sedimentation, scouring, increased flooding, lower summer flows, higher water temperatures, and pollution. Schueler and Claytor (1995) have reported a direct relationship between watershed imperviousness and stream health (Figure 12.13), and found that stream health impacts tend to begin in watersheds when the imperviousness exceeds 10% (the 10% threshold). As shown, sensitive streams can exist relatively unaffected by urban stormwater with good levels of stream quality, where impervious cover is less than 10%, although some sensitive streams have been observed to experience water quality impacts at as low as 5% imperviousness. Impacted streams are threatened and

FIGURE 12.12 Relationship between urban stormwater and aquatic ecosystems.

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Watershed Impervious Cover

334

60

40

Non-supporting (>25%)

20

Impacted (11 to 25%) Sensitive (0 to 10%)

0 good

far

low

Source: Schueler and Clayton, 1995

FIGURE 12.13 Impervious cover vs. stream quality.

exhibit physical habitat changes (erosion and channel widening) and decreasing water quality where impervious cover is in the range of 10 to 25%. Streams in watersheds where the impervious cover exceeds 25% are typically degraded, have a low level of stream quality, and do not support a rich aquatic community.

TEMPERATURE Water temperature is an important measure of water quality. As described by Malina (1996), “the temperature of water affects some of the important physical properties and characteristics of water, such as, specific conductivity and conductance, salinity, and the solubility of dissolved gases (e.g., oxygen and carbon dioxide).” Specifically, water holds less oxygen as it becomes warmer, resulting in less oxygen available for respiration by aquatic organisms. Furthermore, elevated temperatures increase the metabolism, respiration, and oxygen demand of fish and other aquatic life, approximately doubling the respiration for a 10°C(18°F) temperature rise; hence the demand for oxygen is increased under conditions where supply is lowered (California, SWRCB, 1963). Certain species of fish, such as salmon and trout, are particularly sensitive and require relatively low water temperatures. Brown and rainbow trout die when water temperatures exceed 82°F. Brook trout die when water temperature exceeds 72°F. Even lower water temperatures are required for spawning and egg hatching (U.S. EPA, 1976). If the temperature of a stream reach is raised by 5 to 10°C (9 to 18°F), it is probable that such cold-water game fish will avoid this reach and that they will be replaced by “rougher,” more tolerant fish (California, SWRCB, 1963). Thus, even without direct mortality, the character of the fish life will change. Sudden changes in temperature directly stress the aquatic ecosystem. Some states have adopted varying criteria to protect fisheries from such stresses. Typically, states limit in-stream temperature rises above natural ambient temperatures to 2.8°C (5°F). Allowable temperature rises in streams that support cold-water fisheries may be lower; some states adopt values as low as 1°C (1.8°F) and 0.6°C (1°F) (U.S. EPA, 1988). The temperature of urban waters is often affected directly by urban runoff. Urban runoff can be heated as it flows over rooftops, parking lots, and roadways. When it reaches urban waterways it can cause a temporary fluctuation in the in-stream water temperature. Other factors that tend to increase summer water temperature in urban waters include the removal of vegetation from stream banks, reduced groundwater base flow, and discharge from stormwater facilities with elevated water temperature. Frequent fluctuations in steam temperatures stress the aquatic ecosystem, and make it difficult for temperature-sensitive species to survive. Severe thermal impacts associated with summer thunderstorms have been reported in the MidAtlantic states. Runoff from impervious surfaces such as roads and parking lots can reach temperatures of 85 to 90°F. This runoff is generally delivered to the nearest receiving stream through a series of storm drain inlets and pipes with little or no dilution with cooler water.

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TRADITIONAL CONTROL APPROACHES Technical efforts to control and manage these hydrologic impacts of urbanization have evolved through a number of phases or levels of control over the last 30 years in response to increasing understanding of the complexity of the issues that are involved. A brief review of past and current efforts to manage these impacts is provided using the concepts of “levels of control.”

LEVEL 1: PEAK DISCHARGE CONTROL The early efforts (1970s) at managing urban runoff, described as Level 1 focused on reducing the risk of downstream flooding. Often a secondary objective, Level 1a, was to reduce channel erosion. The major tool used in this effort was the use of a stormwater management pond, wet or dry, that temporarily stores and releases runoff from large storms to reduce peak stormwater discharges downstream of the pond. This approach referred to by some as the “end-of-pipe” or “pond” approach is very popular and remains the mainstay of most stormwater management programs throughout the country. Many areas of the United States are still currently using a Level 1 type of control as the only management tool for urban runoff.

LEVEL 2: FLOODPLAIN ZONING A second level of control, Level 2, implemented by some states and local governments at about the same time consists of restricting development along stream floodplains that are susceptible to frequent flooding. Typically, the 100-year floodplain limit is used for this purpose. Many local governments have also purchased floodplains for public use as stream valley parks. Because this is a nonstructural technique, it is often overlooked as a level of stormwater management.

LEVEL 3: WATER QUALITY CONTROL Although it was generally taken for granted that Levels 1 and 2 were reasonably effective in curtailing flooding problems, practitioners began to realize that these levels of control could not mitigate the adverse impacts of urbanization on stream habitat or increased pollutant export. This awareness gave rise to the development of Level 3 during the 1980s. In Level 3 a series of best management practices (BMPs) was developed for urbanizing areas that could remove urban pollutants and, it was hoped, provide some protection for downstream aquatic life. Most of these BMPs consisted of modifications to the old standby end-of-pipe pond, which involved the use of multiple outlet structures to provide extra detention or retention. The use of infiltration practices, both trenches and ponds, was added, as well as the use of wetland ponds and marshes. All these new BMPs were focused on enhancing pollutant removal and providing additional stormwater management. Efforts at improving and adding additional BMPs have continued throughout the 1990s.

PEAK DISCHARGE STRATEGIES

AND

CONTROL

OF

PHYSICAL IMPACTS

Some of the objectives and assumptions inherent in the peak discharge control strategy were described earlier in this chapter. Table 12.8 provides a brief qualitative assessment of the effectiveness of peak discharge strategies with respect to the physical impact category (Clar et al., 2001). Control of Increased Flooding The ability of land use changes, and in particular land development activities, to increase runoff quantity and cause downstream flooding and erosion has been recognized for several decades. This recognition has led many states, counties, municipalities, and other agencies to require on-site detention of increased project area runoff with peak site outflows set equal to the predeveloped conditions. This requirement has become popular, as it can be applied during development design

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TABLE 12.8 Qualitative Assessment of Peak Discharge Control Strategies with Respect to Physical Impact Category Physical Impact Category

Control Strategy

Increased flooding

Peak discharge control of 10- and 100-year storms

Channel instability and erosion

Peak discharge control of 2-year storm

Reduction in groundwater recharge and related issues Increased sediment transport

Not addressed by peak discharge control

Thermal impacts

Not addressed by peak discharge control

Peak discharge of 2-year storm

Assessment Peak discharge strategy provides limited downstream control; in some cases, it aggravates downstream flooding condition; requires coordinated permitting at watershed scale Both geomorphic science and limited fieldmonitoring indicate that this strategy does not work N/A Both geomorphic science and limited fieldmonitoring indicate that this strategy does not work N/A

and review process on a case-by-case basis without large-scale watershed analysis. This popularity has led to the frequent use of on-site detention and retention basins, which have become standard features on many land development projects. However, the limitations of peak discharge control strategies documented by Leopold and Maddock in 1954 have been largely ignored. Recent research conducted by the Somerset County, New Jersey and others (Skupien, 2000) indicates that this approach may not be adequate to prevent downstream peak flow increases and subsequent erosion and flooding problems. Studies conducted by the New Jersey Department of Environmental Protection (NJDEP) and the Natural Resources Conservation Service (NRCS) demonstrate that the sue of this standard peak outflow rate may in fact cause greater downstream peak flow increases than if no on-site detention had been used at all (Skupien, 2000). Additional research by Somerset County suggest that, if an effective on-site detention policy is to be pursued, peak allowable site outflow rates must be determined on a watershed basis and, in many instances, must be set at a rate 25 to 50% less than the predeveloped peak rates. Channel Instability, Bank Erosion, and Sediment Transport A related issue associated with the peak discharge control strategy is the well-documented problem of increases in the frequency and duration of stormwater discharges. As demonstrated by McCuen et al. (1987), the practice of detaining the extra volume of stormwater runoff and discharging it at preconstruction peak discharge rates until the extra volume is fully dissipated has the result of creating more in-stream erosion than if no stormwater control were present. This occurs when the selected design storm focuses predominant on downstream flood control and not on in-stream erosion (channel protection) and the protection of aquatic habitat and biology. Reduction in Groundwater Recharge and Related Issues Peak discharge control strategies are often referred to as end-of-pipe control strategies, because they typically make use of small BMP ponds placed at the low topographic point on development sites. This approach does not usually address groundwater recharge and related issues, such as

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TABLE 12.9 Qualitative Assessment of Peak Discharge Control Strategies with Respect to Habitat and Biological Impact Categories Habitat and Biologic Impact Category

Control Strategy

Assessment

Impairment or loss of habitat

Peak discharge of design storms (100-, 10-, 2-year)

Reduction or elimination of biologic species

Peak discharge of design storms (100-, 10-, 2-year)

BMP systems designed to control peak discharge are not protective of biologic habitats (Jones, 1997; Maxted, 1997; Stribling, 2001) BMP systems designed to control peak discharge are not protective of biologic habitats (Jones et al., 1997; Maxted and Shaver, 1997; Stribling, 2001)

lowering of groundwater levels and reduction or loss of base flows in small streams. One minor exception to this condition consists of recent initiatives in the State of Florida, where stormwater management ponds are used as sources of gray water for lawn watering. This initiative is in part a response to the alarming lowering of water tables in many areas of Florida. Thermal Impacts A negative consequence of the peak discharge control strategy and the associated use of pond BMPs is the associated increase in thermal warming of runoff waters. The problem is particularly acute in regions of the country that support cold-water habitat, particularly trout and salmon fisheries.

PEAK DISCHARGE STRATEGIES

AND

CONTROL

OF

HABITAT

AND

BIOLOGIC IMPACTS

With respect to the habitat and biologic impact categories, the major areas of impairment include impairment or loss of habitat, reduction or elimination of biologic species, and incursion of invasive species. Table 12.9 provides a brief qualitative assessment of the effectiveness of peak discharge strategies with respect to the habitat and biologic impact category (Clar et al., 2001).

SUMMARY

OF

PEAK DISCHARGE STRATEGIES

Peak discharge strategies represent a Level 1 approach to control or mitigation of impacts from urban runoff. As described in Chapter 10, this level of control is provided by the NPDES stormwater regulatory approach. It provides two performance criteria that are closely related: (1) flood control and (2) peak discharge control. The technology assessment for the major impact categories as presented in this Level 1 approach are based solely on peak discharge control and are not adequate to address the range of impacts associated with urban runoff issues. The following is a summary of findings: • Although this approach provides some limited degree of flood control from moderate and large storms, it can in some instances actually transfer or aggravate flooding conditions downstream of the control points. • This approach not only fails to provide protection for stream channel stability, but also may actually aggravate stream channel impacts. • The approach does not address groundwater recharge issues. • The approach does not address, but can actually aggravate, thermal impacts on receiving waters.

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• This approach does not address or guarantee water quality management and pollutant removal, although both can be achieved if the BMPs are properly designed. • This approach does not provide control for the degradation and loss of riparian habitat. • This approach does not provide control for the degradation and loss of biological communities.

FUTURE DIRECTIONS AND INNOVATION IN CONTROL TECHNOLOGY As knowledge and awareness of issues and problems associated with hydrologic impacts of urbanization continue to increase, many practitioners and local governments have come to realize that existing levels of stormwater management are falling short of the intended overall goal of maintaining receiving streams and rivers in urban areas “fishable and swimmable.” Consequently, they have begun to adopt a broader perspective of performance criteria for stormwater management programs, which addresses the full range of hydrologic impacts of urbanization including physical, chemical, and biologic issues. In adopting this parametric approach to performance definition, they have begun to identify gaps, or missing pieces, in the management strategies. These missing pieces include the following: • Criteria to maintain the natural groundwater recharge capacity, and the related base flow contributions during dry weather condition • Criteria to provide control of thermal impacts from urban areas • Criteria to provide effective geomorphically based channel degradation protection • Provision of stormwater credits for innovative site planning techniques Two significant developments in stormwater management technology include the development of the Maryland 2000 SWM Design Manual (MDE, 2000), and also the development of the lowimpact development (LID) technology by Prince George’s County, Maryland. The MDE 2000 manual provides a multiparameter control approach referred to as a Level 4 approach (Clar et al., 2001). This multiparameter approach includes five design criteria or parameters referred to as the “Unified Sizing Criteria,” as follows: 1. 2. 3. 4. 5.

Groundwater recharge criteria, Rev Water quality criteria, WQv Channel protection criteria, Cpv Overbank flooding criteria, Q10 Extreme flood volume criteria, Qf

Criterion 1, groundwater recharge, is a relatively new criterion for stormwater management and was developed to address the concern for the impacts on groundwater recharge, lowering of wells, lowering or loss of base flow to small streams, saltwater intrusion in coastal areas, and settlement of structures. Only a few jurisdictions including the States of Maryland and Massachusetts have adopted these criteria. Criterion 2, the water quality criterion, is not new, but Maryland increased the control requirement from the first 1/2 in. to the first inch to capture and treat 90% of the annual runoff volume. Criterion 3, the channel protection criterion, is also not new, but Maryland replaced the use of the 2-year predevelopment storm as a surrogate for channel protection, with the use of the 1-year storm with extended detention, which effectively reduces the allowable release rate to a 2-in. storm event, or approximately 25 to 50% of the predevelopment peak discharge rate. Criteria 4 and 5 are the traditional flood control requirements for the 10- and 100-year storms.

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The LID approach to stormwater management is the subject of a separate chapter and is addressed in detail in a number of publications (PGC, 1997a, b; U.S. EPA 2000a, b). It represents a Level 5 approach to stormwater management. This level has been described as an attempt to provide an ecologically sensitive approach to stormwater management (Clar et al., 2001). This level uses an integrated approach including biologic, chemical, and physical criteria to define BMP performance. A combination of water quality, biohabitat, and geomorphic criteria is used to evaluate whether a receiving stream is at the targeted goal of “fishable and swimmable,” or the extent of departure from this goal. A number of additional parameters are added to the Level 2 performance criteria: (1) stream buffer retention and thermal impact considerations; (2) volume control considerations, such as considerations presented in the LID concept approach, are added to the peak discharge and groundwater recharge criteria to achieve maintenance of hydrologic function at a site-specific level; and (3) geomorphic criteria as described by Dunne and Leopold (1978), Lane (1955), Leopold and co-workers (1964; 1994), Rosgen (1996), and others are incorporated to supplement or replace extended detention approaches to achieving channel stability. In summary, it can be observed that stormwater management technology has experienced a considerable degree of maturity and improvement over the past 30 years, which has reflected the increased understanding of the complicated cause-and-effect relationships between land disturbance activities and the corresponding responses of the natural systems, particularly the riparian zones and receiving waters. However, the massive land development activities that have occurred over the last 40 years have left a legacy of impaired receiving waters whose primary source of impairment is due to channel degradation resulting from hydrologic modifications. The application of the fluvial geomorphology concepts presented earlier, together with the Rosgen classification system, as tools to assess and restore these impaired streams are described in Chapter 13.

REFERENCES California State Water Resources Control Board (SWRCB). 1963. Water Quality Criteria. 2nd ed. Publication No. 3-A, pp. 284–285. Clar, M., Collins, J., Loftin, H. et al., 2001. Stormwater BMP technology assessment protocols — preliminary findings, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. Collins, J., Clar, M., Loftin, H. et al., 2001. Compilation of regulatory requirements for stormwater runoff, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. Delaware Department of Natural Resources and Environmental Control (DNREC). 1997. Conservation Design of Storm Water Management. A joint effort between DNREC and the Brandywine Conservancy, Dover, DE. Dunne, T. and Leopold, L.B., 1978. Water in Environmental Planning, W.H. Freeman, San Francisco, 818 pp. Ferguson, B.K., 1990. Urban stormwater infiltration: Purposes, implementation, results, J. Soil and Water Conservation, 45(6). Gordon, N., McMahon, T., Finlayson, B., 1982. Stream Hydrology: An Introduction for Ecologists, John Wiley & Sons, New York. Jones, R.C., Via-Norton, A., and Morgan, D.R., 1997. Bioassessment of BMP effectiveness in mitigating stormwater impacts on aquatic biota, in Effects of Watershed Development and Management on Aquatic Ecosystems, L.A. Roesner, Ed., American Society of Civil Engineers, New York. Lane, E.W., 1955. The importance of fluvial morphology in hydraulic engineering, Am. Soc. Civil Eng. Proc., 81, paper 745, 1–17. Langbein, W.B., 1960. Plotting positions in frequency analysis, U.S. Geological Survey Water Supply Paper 1543-A, A48–A51. Leopold, L.B., 1968. Hydrology for Urban Land Planning — A Guidebook on the Hydrologic Effects of Urban Land Use, U.S. Geological Survey, Water Supply Paper, 1591-C. Leopold, L.B., 1994. A View of the River, Harvard University Press, Cambridge, MA, 298 pp.

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Leopold, L.B. and Maddock, T., 1953. The hydraulic geometry of stream channels and some physiographic implications, U.S. Geological Survey Prof. Paper 252, U.S. Government Printing Office, Washington, D.C., 57 pp. Leopold, L.B. and Maddock, T., Jr., 1954. The Flood Control Controversy, Ronald Press Corp., New York. Leopold, L.B., Wolman, M.G., and Miller, J.P., 1964. Fluvial Processes in Geomorphology, Freeman & Sons, San Francisco, 522 pp. Linsley, R.K., Kohler, M., Paulhus, J., 1958. Hydrology for Engineers, McGraw-Hill, New York. MacRae, C., 1996. Experience from morphological research on Canadian streams: is control of the two-year frequency runoff event the best basis for stream channel protection? in Effects of Watershed Development and Management on Aquatic Ecosystems, L. Roesner, Ed., American Society of Civil Engineers, Snowbird, UT, 144–162. Malina, J.F., 1996. Water quality, in Water Resources Handbook, Mays, L.W., Ed., McGraw Hill, New York, chap. 8. Masterson, J.P. and Bannerman, R.T., 1994. Impacts of stormwater on urban streams in Milwaukee County, Wisconsin, National Symposium on Water Quality, Nov. 1994. Maxted, J. and Shaver, E., 1997. The use of retention basins to mitigate stormwater impacts on aquatic life, in Effects of Watershed Development and Management on Aquatic Ecosystems, L. A. Roesner, Ed., American Society of Civil Engineers, New York. McCuen, R.H., Moglen, G., Kistler, E., and Simpson, P., 1987. Policy Guidelines for Controlling Stream Channel Erosion with Detention Basins, prepared by the Department of Civil Engineering, University of Maryland, College Park, for the Water Management Administration, Maryland Department of the Environment, Baltimore. MDE (Maryland Department of the Environment), 2000. 2000 Maryland Stormwater Design Manual, Vol. I and II, prepared by the Center for Watershed Protection and the Maryland Department of the Environment, Water Management Administration, Baltimore. Mosley, M.P., 1981. Semi-determinate hydraulic geometry of river channels, South Island, New Zealand, Eart Suf. Prof. Landforms, 6, 127–137. MWCOG,1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs, Metropolitan Washington Council of Governments, Department of Environmental Programs, Washington, D.C. NRCS, 1986. Urban Hydrology for Small Watersheds, Technical Release 55, U.S. Department of Agriculture, Natural Resources Conservation Service, Conservation Engineering Division, Washington, D.C. NRCS, 1998. Stream Corridor Restoration: Principles, Processes and Practices, prepared by the Federal Interagency Stream Restoration Workgroup, published by the Natural Resources Conservation Service, U.S. Department of Agriculture, Washington, D.C. PGC (Prince George’s County, Maryland), 1993. Design Manual for Use of Bioretention in Stormwater Management, prepared by Engineering Technologies, Associates, Inc., Ellicott City, MD. PGC (Prince George’s County, Maryland), Department of Environmental Resources, 1997a. Low-Impact Development Design Manual, prepared by Tetra Tech, Inc., Fairfax, VA. PGC (Prince George’s County, Maryland), Department of Environmental Resources, 1997b. Low-Impact Development Guidance Manual, prepared by Tetra Tech, Inc., Fairfax, VA. Rosgen, D. L., 1996. Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO. Schueler, T., 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs, Metropolitan Washington Council of Governments, Washington, D.C. Skupien, J.J., 2000. Establishing effective development site outflow rates, paper presented at the Delaware Sediment and Stormwater Issues for a New Millennium, Conference 2000, University of Delaware, Newark, DE. Snodgrass, W.J., Kilgour, B.W., Leon, L., Eyles, N., Parish, J., and Barton, D.R., 1998. Applying ecological criteria for stream biota and an impact flow model for evaluation sustainable urban water resources in southern Ontario, in Sustaining Urban Water Resources in the 21st Century. Proceedings for an Engineering Foundation Conference, A.C. Rowney, P. Stahre, and L.A. Roesner, Eds., Malmo, Sweden, September 7–12, 1997. Stribling, J.B., 2001. Relating instream biological condition to BMP activities in watersheds, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York.

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Swietlik, W.F., 1997. Stormwater management in the United States — key challenges and possible solutions, in Proceedings of Conference on Sustaining Urban Water Resources in the 21st Century, September, Malmo, Sweden, United Engineering Foundation/American Society of Civil Engineers, Reston, VA. Swietlik, W.F., 2001. Urban aquatic life uses — a regulatory perspective, paper presented at the Conference on Linking Stormwater BMP Designs and Performance to Receiving Water Impacts Mitigation, Snowmass, CO, United Engineering Foundation, New York. Tetra Tech, Inc., 2000. Effluent Limitations Guidelines, Draft Report, prepared for Office of Science and Technology, U.S. Environmental Protection Agency, Washington, D.C. U.S. EPA, 1976. Water Quality Criteria for Water. Washington, D.C., pp. 218–231. U.S. EPA, 1983. Results of Nationwide Urban Runoff Program (NURP), Final Report, Water Planning Division, Washington, D.C. U.S. EPA, 1988. Water Quality Standards Criteria Summaries: A Compilation of State/Federal Criteria: Temperature. EPA 440/5-88/023. U.S. EPA, 1999. Preliminary Data Summary of Best Management Practices, Office of Water, U.S. Environmental Protection Agency, Washington, D.C. EPA-821-2-99-012. U.S. EPA, 2000a. Low Impact Development Design Strategies: An Integrated Design Approach, prepared by Tetra Tech, Inc., Fairfax, VA for Department of Environmental Resources, Prince George’s County, MD, funding provided by the U.S. Environmental Protection Agency, Washington, D.C. U.S. EPA, 2000b. Low Impact development (LID) Hydrology, prepared by Tetra Tech, Inc., Fairfax, VA for Department of Environmental Resources, Prince George’s County, MD, funding provided by the U.S. Environmental Protection Agency, Washington, D.C. Yoder, C.O., 1995. Incorporating ecological concepts and biological criteria in the assessment and management of urban nonpoint source pollution, National Conference on Urban Runoff Management: Enhancing Urban Watershed Management at the Local, County and State Levels, U.S. Environmental Protection Agency, Washington, D.C., EPA/625/R-95/003, 183–197. Williams, G.P., 1978. Bankfull discharge of rivers, Water Resour. Res., 14(6), 1141–1153. Wolman, M.G. and Miller, J.P., 1960. Magnitude and frequency of forces in geomorphic processes, J. Geol., 68, 54–74.

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13

Geomorphic Considerations in Stream Restoration James W. Gracie

CONTENTS Introduction ....................................................................................................................................344 Elements of Fluvial Geomorphology and Geomorphic Features .................................................344 Field Procedures.............................................................................................................................345 Bankfull Stage and Slope .......................................................................................................345 Bankfull Width and Depth......................................................................................................345 Sinuosity..................................................................................................................................346 Particle Size Distribution ........................................................................................................346 Sediment.........................................................................................................................................346 Channel Processes...................................................................................................................347 Stream Classification......................................................................................................................347 Applications of the Rosgen Classification System .......................................................................348 Stability/Instability..................................................................................................................350 Patterns of Adjustment and Disequilibrium ...........................................................................352 Factors That Lead to Instability .............................................................................................357 Erosion Rates ..........................................................................................................................358 Aggradation/Degradation or Vertical Instability ....................................................................358 State Assessment and Departure.............................................................................................358 Stream Restoration Techniques......................................................................................................359 Channel Geometry ..................................................................................................................359 Structures ................................................................................................................................360 Bank Stabilization Structures........................................................................................360 Root Wads............................................................................................................360 Vanes ....................................................................................................................360 Live Fascines .......................................................................................................360 Biologs .................................................................................................................361 Other Bioengineering Techniques .......................................................................361 Grade Control Structures ..............................................................................................361 Vortex Rock Weirs ...............................................................................................362 Step Pools ............................................................................................................362 Case Studies ...................................................................................................................................362 Quail Creek .............................................................................................................................362 Tributary 9 to Sawmill Creek.................................................................................................364 References ......................................................................................................................................368

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INTRODUCTION The Rosgen stream classification system has provided many engineers and environmental scientists with a much needed key to the understanding and application of the complex processes of fluvial geomorphology. The Rosgen stream classification system is a practical and universally applicable scheme for classifying stream channels, which involves the main parameters that operate in the processes of river mechanics and maintenance. The classification depends on knowledge of processes and is therefore useful not only to describe channels but also to evaluate how a stream will react to change through time.

ELEMENTS OF GEOMORPHOLOGY AND GEOMORPHIC FEATURES Bankfull stage is the elevation of the water surface during a bankfull discharge. Bankfull discharge is the flow that just overtops the floodplain. The floodplain is defined as the flat depositional surface adjacent to the stream and formed by the stream during the current hydrologic regime. In practical terms the bankfull stage is identified as a change in slope, the top of the point bar, or a change in vegetation. Most geomorphologists agree that the bankfull discharge is the discharge that forms and maintains the channel. It is the discharge that moves the most water and sediment over time. Streams that are stable are sized to move the sediment and water associated with the bankfull discharge. The bankfull discharge occurs on average about 2 out of every 3 years. Its return period is, therefore, about 1.5 years. Dimensions of the channel at bankfull discharge are critical properties of a stream channel. When attempting to identify bankfull stage in the field one looks for flat depositional surfaces adjacent to the stream. In meandering streams a good indicator is the tops of point bars that form on the inside of meander bends. There is usually a break in slope at the highest point on the bar, where the bar flattens out. This is the elevation of the bankfull stage. In many streams the floodplain is not fully developed and flat depositional surfaces are intermittent, not continuous, but the bankfull discharge is closely correlated with the drainage area. This fact is highly useful. However, even in those streams that do not have well-developed floodplains these surfaces exist and are at a consistent elevation. Field procedures for identifying the bankfull stage are discussed later. Identification of the bankfull stage is the first step in collecting the data that enable measurement of the critical parameters of a stream channel. The basic measurements that are taken are bankfull width, bankfull depth, sinuosity, entrenchment, slope, and particle size distribution of the materials in bed and bank. The bankfull width is the width at bankfull stage of the channel measured in a straight reach. Bankfull depth is the average depth of the bankfull stage in the same reach where width is measured. Sinuosity is simply the ratio of the channel length divided by the down-valley distance. In other words, it is a ratio expressing how much the stream meanders. Entrenchment (Figure 13.1) is the ratio of the width of the flood-prone area divided by the bankfull width. The flood-prone width is measured at a distance above the channel invert equal to twice the maximum bankfull depth. Slope is the change in elevation of bankfull stage divided by the channel length. It is usually measured by the slope of the water surface at base flow. It is measured over a distance equal to 20 to 28 times the bankfull width. Starting and ending points must be a similar geomorphic point in the channel so that the reach being measured has the same number of pools and riffles, for example, from top of riffle to top of riffle. Otherwise, the energy slope will be either overstated or understated. Particle size distribution is measured using the Wolman pebble count method. A cumulative distribution is plotted and the median size is calculated.

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FIGURE 13.1 Entrenchment. (From Rosgen, D.L., A classification of natural rivers, Catena, 22, 169–199, 1994. Elsevier Science. With permission.)

These parameters are a basic key for geomorphic classification, for discharge estimates, and characterization of pattern, energy, management implications, and many key elements of restoration design.

FIELD PROCEDURES BANKFULL STAGE

AND

SLOPE

The best way to identify and survey the bankfull stage is by use of what Emmett calls the “long profile method.” The investigator walks along the channel for a distance of at least two wavelengths and marks the apparent bankfull stage with pin flags or flagging tape. These points are surveyed in a long profile in which the invert of the channel, the water surface, and the bankfull indicators are all recorded. The profile is then plotted with all three features (bankfull, water surface, and invert) shown along the channel length. There will be some scatter in the data, but the best straight line fit to the bankfull indicators should represent the energy slope. A good check is that the high points along the bed should be parallel to the bankfull slope. A line through the tops of the riffles should also be parallel to the bankfull slope line. Divide the change in elevation between two convenient points by the distance along the channel between the points and the result is the slope in units of feet per foot. Slope is often represented as a percentage. To obtain the percent slope, divide the slope in ft/ft by 100.

BANKFULL WIDTH

AND

DEPTH

The procedure is as follows. Select a location for a cross section measurement in a straight reach free of obstructions such as large boulders, logs, midchannel bars, etc. Stretch a tape measure with zero on the left side of the channel while looking downstream. Stretch the tape from a point on the left and right banks that is at least twice the distance of the maximum depth above the channel invert. Install monuments with either cement and carriage bolts or reinforcing rods. Record the location and elevation of points at every break in slope along the tape measure. Make a note of the left and right bank bankfull stage indicators as well as the water surface on both sides of the channel. Draw a sketch map showing the location of the cross section monuments.

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SINUOSITY Sinuosity is most easily measured in the office from aerial photographs. Obtain recent aerial photographs that are taken in winter or that are color infrared photographs so the stream channel is visible. Select a typical reach from the aerials that includes the area to be measured. Mark off limits of the reach that is of interest and measure the distance between the upstream and downstream ends. Then measure the channel length between the same two points. Use a wheel scale so that the actual length along the channel can be measured. Divide the distance along the channel by the down-valley, straight line distance. The result is the sinuosity. Topographic maps should not be used for this purpose. They nearly always understate the channel length significantly, unless one is measuring a very large river. Sometimes it is not possible to obtain suitable aerial photographs for measuring sinuosity. If this is the case, sinuosity will have to be measured in the field. Select a reach that is at least 20 to 28 channel widths long and lay a tape along the centerline of the channel using chaining pins to approximate the channel meanders. Measure the channel length and measure the distance between the starting and ending points in a straight line. Divide the channel length by the down-valley distance to obtain the sinuosity.

PARTICLE SIZE DISTRIBUTION The pebble count procedure involves a stratified random sampling method of measuring the particle size distribution of the materials in the bed and bank of the active channel. The procedure involves an examination of the reach being characterized to estimate the percent of pool, riffle, and run in the reach. Once this is determined, select transects from pool, riffles, and runs in the same proportion that they exist in the reach. In other words if the reach is 25% pool, 25% riffle, and 50% run, the transects should be in the ratio of one pool, one riffle, and two runs. It will be necessary to measure at least 100 particles and also to measure the right proportions of complete transects of pool, riffle, and run. Once the transects have been selected, one walks along each one, taking a particle at every step by reaching under the big toe, without looking, and measuring the first particle touched. This is done until the minimum requirement of at least 100 particle and full transects in the correct proportions has been met. The particle dimension measured is the length of the intermediate axis. The reason for this is the intermediate axis will determine the size sieve that the particle will pass through. After measuring the required number of particles, group them into phi intervals (increasing powers of two; i.e., 20 = 1, 21 = 2, 22 = 4, etc.) and plot the cumulative distribution with the particle intermediate axis on the horizontal scale and the percent less than on the vertical scale. The resulting graph is a cumulative particle size distribution. Locate 50% on the vertical scale and draw a horizontal to intersect the curve. From that intersection draw a vertical line to the horizontal axis. Where it intersects the horizontal axis is the median size or D50.

SEDIMENT Streams move sediment and water. A stream may be thought of as the manifestation of a process converting the potential energy of elevation into the kinetic energy of movement. Streams shape their channels and form floodplains. The movement of sediment is not constant. It varies in both space and time. Sediment moving in streams is classified in two forms: suspended sediment and bed load sediment. Suspended sediment is fine enough to be suspended in the water and transported as “washload.” Bed load sediment is coarser and moves in a process called saltation. Saltating particles are momentarily lifted or entrained and move along the bed, bouncing and starting and stopping. Bed load does not move at normal flows but only when there is enough energy in the flowing water to move it. It has been estimated that bed load begins to move in most streams when flow exceeds about one third of bankfull depth.

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In general, sediment comes from two sources: overland and in channel. Usually, overland sources of sediment mainly comprise fine sediment because shallow flow cannot entrain and transport coarse sediment. Some mass failure such as landslides, hill slope failures, etc. can introduce coarse sediment.

CHANNEL PROCESSES Erosion of outside meander bends is a natural process in all meandering stream channels. In stable channels it occurs at low rates and and is matched by deposition rates on point bars. In this way the stream builds its floodplain behind the advancing point bars. When a channel is maintaining its pattern and dimensions but neither degrading nor aggrading it is said to be in equilibrium. Note that a stream in equilibrium may be changing its position with lateral migration, but as long as it meets the conditions of maintaining its pattern and dimension and neither degrading nor aggrading it is still in equilibrium. One of the conditions of this equilibrium is that sediment supply is in equilibrium with sediment transport. The farmer losing pasture and the engineer whose bridge is threatened may not like this definition of equilibrium, but geomorphologically it is correct. The river is following its natural equilibrium tendencies. As long as the climate and land use do not change, streams will stay in equilibrium. If something happens to change the flow regime or the sediment supply or the pattern or dimensions of the channel, streams respond in a process of adjustment and can enter a condition of disequilibrium. Disequilibrium results when a change in the flow regime, sediment supply, or energy distribution in the channel occurs. A change in climate can induce disequilibrium; a change in land use that alters the hydrologic regime can induce disequilibrium. A change in sediment supply, either an increase or decrease in sediment supply, can also induce this condition of disequilibrium. Degradation of channels is well documented downstream of dams because of the dramatic decrease in sediment supply, which is trapped in the pool of the dam. An increase in sediment supply from human activities such as construction mining is not uncommon. The distribution of energy in stream channels can change when channels are straightened, resized, or otherwise altered. A common process of destabilization is when changes in land use activities alter the hydrologic regime. Removal of vegetation and installation of impervious area and storm drain systems alter the hydrologic cycle by decreasing the amount of infiltration, increasing the amount of runoff, and reducing or eliminating evapotranspiration. Depression storage is also decreased. The peak flows from the same rainfall events increase, often severalfold. Since channels are in equilibrium with the peak annual flows from their watersheds, enlargement must follow these hydrologic changes. Enlargement occurs as erosion either by accelerated lateral erosion or by incision, or both. If erosion rates increase, enough sediment supply increases beyond the competence of the stream to transport it and disequilibrium occurs. Excess sediment from channel erosion exceeds transport capacity. This excess sediment forms depositional features, which affects channel capacity, thereby inducing more erosion in an effort to regain channel capacity. This sediment increases sediment supply even more, which leads to more erosion, which yields still more excess sediment, and on and on. Thus, a self-feeding, or positive feedback, mechanism is under way. Many streams in urbanizing areas are in this condition. Equilibrium may not reestablish itself for decades or longer. While these processes dominate, many stream are devoid of normal aquatic life. Either the fine sediment fills the interstitial gravel habitat or the frequent shifting of substrate grinds delicate creatures to death. One of the important goals of restoration is to reestablish the equilibrium between sediment supply and sediment transport and to reduce the rate of channel adjustment so that aquatic habitat can recover.

STREAM CLASSIFICATION Efforts to classify streams are not new. One of the early classification systems by Davis (1899) grouped streams into three stages of adjustment: youthful, mature, and old. The youthful streams

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were steep-gradient, low-sinuosity streams and had large channel materials. Mature streams were sinuous, lower gradient, and carried intermediate-sized sediment. Old streams were very sinuous, flowed in low-gradient valleys, and had still smaller sediment. Straight, meandering, and braided streams were identified by Leopold and Wolman (1957). Lane (1957) developed slope–discharge relationships for braided, intermediate, and meandering streams. Another classification system was developed by Schumm (1963) based upon descriptive and interpretive characteristics such as channel stability and mode of sediment transport. Descriptive classification systems were developed by Culbertson et al. (1967), Thornbury (1969), and Khan (1971). Most of these early classification systems relied on qualitative interpretation of geomorphic features, thus leading to inconsistency in classification and limited predictive abilities. There have been many attempts to classify streams most of which have limited usefulness. A current, widely useful classification system is the Rosgen classification system. Specific objectives of the system are as follows: 1. To predict the behavior of a river from measurable morphologic features 2. To develop specific hydraulic and sediment relationships for a given stream type and its state 3. To provide a mechanism to extrapolate site-specific data to stream reaches with similar characteristics 4. To provide a consistent frame of reference for communicating stream morphology and condition among a variety of disciplines and interested parties The Rosgen classification system is hierarchical. Combinations of morphologic variables useful for different scales of analysis from coarse to fine resolution are used in a hierarchy of river morphology. The coarse level analysis can be performed using aerial photographs and topographic maps. This level, Level 1, can distinguish the major stream types, A, B, C, D, E, F, and G. The next level, Level 2, is the reach specific classification; then there is a Level 3, which includes a state assessment and prediction, and, finally, Level 4, which is validation or monitoring. The discussion here concentrates on Level 2, the reach classification in this work. Table 13.1 is a key to classification of natural rivers (Rosgen, 1996). The parameters used have been discussed above along with the procedures for measuring them. The first step is to decide whether the stream is a single thread channel or a multiple thread channel. If it is a single thread, then entrenchment is the next determination and there are three categories: entrenched, moderately entrenched, and slightly entrenched. Examples of different entrenchment ratios are shown in Figure 12.6. Next, look at width/depth ratio. Bankfull width and depth are used in this simple calculation. Then sinuosity and, finally, slope. These parameters will enable one to determine the major stream type. Add the D50 from the cumulative particle size distribution and a number from 1 to 6 can be assigned based upon the median size from boulder to silt-clay. Figures 13.2 through 13.15 are examples of different stream types.

APPLICATIONS OF THE ROSGEN CLASSIFICATION SYSTEM The Rosgen classification system in its full version is a hierarchical system. It allows classification at different levels of specificity. At its most generic level, Level 1, it allows the classification of streams and rivers into the major stream types. This classification can be performed with available data such as topographic maps and aerial photographs. In its most familiar form, Level 2, it is a reach-specific classification, which can be performed only after the collection of data specific to the reach being classified. At an even finer level of detail, Level 3, it is a tool for state assessment and prediction of future adjustment direction and rates. Finally, at Level 4, the system suggests methods for monitoring and validating prediction from Level 3.

A4a+

A5a+

A6a+

Gravel

Sand

Silt/Clay

A6

A5

A4

A3

A2

A1

G6

G5

G4

G3

G2

G1

G6c

G5c

G4c

G3c

G2c

G1c

<0.02

F6b

F5b

F4b

F3b

F2b

F1b

0.02– 0.038

F6

F5

F4

F3

F2

F1

<0.02

Slope Range

F

High Sinuosity (>1.4)

Mod.-High W/O (>12)

B6a

B5a

B4a

B3a

B2a

B1a

0.04– 0.099

B6

B5

B4

B3

B2

B1

0.02– 0.039

Slope Range

B

B6c

B5c

B4c

B3c

B2c

B1c

<0.02

Moderate Sinuosity (>1.2)

Moderate (W/O (>12)

Mod. Entrenched (1.4–2.2)

E6b

E5b

E4b

E3b

0.02– 0.039

E6

E5

E4

E3

<0.02

Slope Range

E

Very High Sinuosity (>1.5)

C6b

C5b

C4b

C3b

C2b

C1b

0.02– 0.039

C6

C5

C4

C3

C2

C1

0.001– 0.02

Slope Range

C

C6c-

C5c-

C4c-

C3c-

C2c-

C1c-

<0.01

High Sinuosity (>1.4)

Mod.-High W/O (>12)

Slightly Entrenched (>2.2) Very Low W/O (<12)

Values can vary by ±0.2 units as a function of the continuum of physical variables within stream reaches. Values can vary by ±0.2 units as a function of the continuum of physical variables within stream reaches.

A3a+

A2a+

Boulders

Cobble

A1a+

Bedrock

0.02– 0.038

Slope Range

Slope Range

0.01– 0.999

G

A

>0.10

Moderate Sinuosity (>1.2)

Low Sinuosity (<1.2)

Low W/O (<12)

Entrenched (<1.4)

Single Thread Channels

Source: Rosgen, D.L., Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO, 1996. With permission.

b

a

Channel Material

*Sinuositya

**Width/Depthb

*Entrenchmenta

TABLE 13.1 Key to Classification of Natural Rivers

D6b

D5b

D4b

D3b

0.02– 0.039

D6

D5

D4

D3

0.001– 0.02

Slope Range

D

Low Sinuosity

Very High W/O (>40)

D6c-

D5c-

D4c-

<0.01

Multiple Channels

DA6

DA5

DA4

<0.01

Slope

DA

LowHigh

Low (<40)

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FIGURE 13.2 A1 stream type.

FIGURE 13.3 A2 stream type.

STABILITY/INSTABILITY A stable channel in geomorphic terms is said to be a channel that is maintaining its pattern and dimensions over time and is neither aggrading nor degrading. This definition allows for lateral adjustment, which over time causes the channel to change its position laterally over a floodplain and still be considered stable. When lateral adjustment is under way, there is still a question of stability based on the equilibrium between sediment supply and sediment transport. When sediment supply exceeds sediment transport, instability results. This is because excess sediment creates depositional features that alter pattern and dimension. The process exhibits a positive feedback mechanism in which excess sediment creates depositional features, which accelerate bank erosion, which in turn increases sediment supply even further, which creates more depositional features, which induce additional erosion, and on and on.

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FIGURE 13.4 B1 stream type.

FIGURE 13.5 B2 stream type.

351

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FIGURE 13.6 C1 stream type.

FIGURE 13.7 C2 stream type.

PATTERNS

OF

ADJUSTMENT

AND

DISEQUILIBRIUM

Many patterns of adjustment are recognizable and predictable once they are under way. For example, one of the classic adjustments is the oxbow lake formation. It involves lateral extension of a meander bend, followed by the formation of a cutoff channel resulting in the abandonment of the meander bend, often leaving a pond or lake isolated from the main flow of the stream. An understanding of sediment supply and transport helps explain the processes that drive this form of adjustment. The

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FIGURE 13.8 C3 stream type.

FIGURE 13.9 C4 stream type.

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FIGURE 13.10 D3 stream type.

FIGURE 13.11 D4 stream type.

lateral extension occurs as a result of erosion on the outside of a meander bend coupled with deposition of sediment on the inside of the bend. As the extension proceeds, the length of the meander bend increases while the elevation of the upstream and downstream ends of the bend do not change. This results in a reduction of the slope through the meander bend. Eventually, the slope is reduced enough so that sediment transport is reduced through the bend. Once this occurs, sediment deposition in the bend causes the channel to aggrade locally. As this proceeds, the bed of the channel rises and the capacity decreases. Eventually, discharges overflow the existing channel and form a new channel, cutting off the meander bend. That may not be the end of the process. The new channel connecting the reach upstream of the meander to the downstream end of the meander is much shorter that the meander it replaced. Again, the elevation of upstream and downstream

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355

FIGURE 13.12 E4 stream type.

FIGURE 13.13 F3 stream type.

reaches has not changed, so that the cutoff channel has a much steeper slope than the original channel. This area of increased local slope may headcut, causing channel incision. Figure 13.16 shows a sequence of adjustment once degradation has occurred. The initial incision may be caused by a chute cutoff, an increase in flow regime, or channelization. The incision results in floodplain abandonment and the formation of a fully entrenched G stream type. In this stage the

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FIGURE 13.14 F4 stream type.

FIGURE 13.15 G4 stream type.

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357

FIGURE 13.16 Evolutionary stages of channel adjustment. (From Rosgen, D.L., A classification of natural rivers, Catena, 20, 169–199, 1994. Elsevier Science. With permission.)

channel does not experience flood relief until flows significantly larger than bankfull occur. The result of this configuration is that the channel experiences much greater shear stress on the bed than it did before entrenchment. In this way, incision can accelerate after it starts. In the next stage the channel evolves to an F stream type as the oversteepened banks rejuvenate and the channel widens. Eventually, the channel can return to equilibrium at a lower base elevation. In its reestablished equilibrium condition it will form a C or E channel after the F channel has widened enough to provide an adequate flood-prone width commensurate with this final stable stream type.

FACTORS THAT LEAD

TO INSTABILITY

Stream channels can be destabilized in a number of ways. Some stream types are more susceptible to destabilizing factors than others. Understanding the stream classification system can help predict what stream types are most susceptible to various destabilizing factors. As an example take a C4 stream that is experiencing land development in its watershed and compare it with a B3 stream experiencing the same changes. The land development increases the magnitude and frequency of peak flows. As the peak annual flow increases in magnitude, the channel must enlarge to accommodate the change. This is true for both the C4 stream and the B3 stream. The C4 stream, with its well-developed floodplain, is heavily dependent on riparian vegetation for bank stability. As the erosive forces exerted on the bed and banks increase with the flows, it will probably incise. As its bed degrades, the erosive forces are exerted at lower elevations until they are below the depth of rooting of the riparian vegetation. The vegetation no longer stabilizes the banks. Lateral erosion is now unimpeded by vegetative stabilization. The C4 stream incises, then widens. As riparian trees and shrubs are undermined, they fall into the channel, directing erosion to new sites. The stream is following the classic process of evolution as outlined in Figure 13.16. The B3 stream, however, is not as dependent on vegetation for its stability. The coarser materials in the channel resist erosion because of their size and mass. The channel will enlarge, but it will do so at a lower rate of change. In most cases the B3 stream will enlarge slowly without increasing sediment supply enough to initiate a sediment disequilibrium. In general, streams with well-developed floodplains are dependent on vegetation for stability and streams that are moderately or fully entrenched obtain their stability from coarser material in the channel.

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While bearing in mind that some streams respond differently than others to an imposed change, the following can induce instability: • • • • • • •

Straightening Channel enlargement for flood conveyance Change in flow regime Removal of riparian vegetation Dams Introduction of excess sediment Gravel mining

EROSION RATES There is also a procedure for determining relative erosion rates for eroding banks: the bank erosion hazard index (BEHI). The BEHI involves a series of measurements on the condition of the factors that contribute to bank erosion: 1. 2. 3. 4. 5. 6.

Ratio of total bank height to bankfull depth Bank angle Ratio of rooting depth to total bank height Rooting density Size characteristics of materials in bank Presence or absence of stratification

These parameters coupled with the percentage of shear stress in the one third of bank width nearest the erosion site allow ratings to be developed that can be used as relative measures of rates of erosion or sediment supply. They can be quantified by monitoring actual rates and calibrating the model to predict actual rates. Comparison of these parameters to reference reaches will enable one to estimate departure from the ideal habitat and will give an understanding of the degree of instability in different specific locations on the stream of interest.

AGGRADATION/DEGRADATION

OR

VERTICAL INSTABILITY

Streams can be vertically stable, aggrading, or degrading. Although there are signs of such vertical instability that experienced observers can interpret, there is often no sure way to determine which condition of vertical stability is currently prevalent in a stream reach. The most obvious sign of instability is the “headcut,” an area of steep local slope, often appearing to be a small waterfall. These areas of local slope are a sure sign that degradation is under way. The steep local slope generates higher water velocities than areas upstream or downstream. These headcuts are so-called because they will move headward, leaving an incised or degraded channel in their wake. They also can generate enormous quantities of sediment in the process of moving headward. The excess sediment generated often creates an aggradation process downstream of the headcut. Furthermore, the incised or degraded channel causes the abandonment of the floodplain eliminating flood relief and creating oversteepened banks that will rejuvenate, delivering still more sediment over time.

STATE ASSESSMENT

AND

DEPARTURE

Once a stream has been classified, it is useful to know whether it is stable or unstable. This can often be determined by examining time series aerial photographs. To be able to say whether or not a stream is at its potential for aquatic habitat is also useful. To do this, a state assessment and comparison to a stable, healthy stream are necessary. To make these comparisons, it is necessary to have the critical data on what are considered healthy streams of the same stream type. The U.S.

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359

EPA Rapid Bioassessment Habitat Protocol is a useful tool in rating habitat. This procedure, too, requires a reference stream for comparison. How are these “reference” streams identified? First of all, it is helpful to select streams for which aquatic life data exist. Furthermore, the streams should appear in a healthy, stable condition. When such streams of the same stream type have been identified, the data compared are important. Select a healthy stream of a C4 stream type in the same hydrophysiographic province as the stream of interest. Obviously, the stream should be classified at Level 2 to confirm the stream type. In addition to the classification data, the parameters that should be measured are as follows: 1. Meander length ratio — The meander length divided by bankfull width 2. Meander belt width ratio — the belt width (distance between parallel lines drawn between outside of adjacent meander bends) divided by the bankfull width 3. Pool maximum depth/bankfull depth ratio 4. Pool width/bankfull width ratio 5. Pool slope 6. Riffle slope 7. Pool riffle ratio 8. Particle size distribution D15, D50, D85 In addition, there are other features that should be noted or quantified: 1. Embeddedness (visual estimate of the fraction of fine sediment filling the spaces between the coarse particles) 2. Presence or absence of midchannel bars 3. Evidence of frequent out-of-bank flows

STREAM RESTORATION TECHNIQUES CHANNEL GEOMETRY Different stream types have characteristic dimensions and patterns, but all stream types maintain a relationship between channel size and bankfull discharge. It is crucial that stream restoration projects be designed with a design discharge that is equal to bankfull discharge under the current hydrologic regime. Determining the correct design discharge is the most critical step in a restoration design. There are certain guidelines for recognizing the indicators of bankfull stage in the field, but these must be calibrated with indirect methods to confirm that the indicators are not misread. The recommended procedure is to perform a proportional area comparison with a gauged stream. The gauged stream must be calibrated for bankfull discharge as described in Chapter 12. The streams being compared must have the same degree of development and should be in the same hydrophysiographic region. If the bankfull discharge of the gauged stream (for the same drainage area) differs from the field-determined discharge on the restoration stream, then the field indicators have been misread. Another site visit to find indicators of bankfull stage that agree with the calibrated discharge must be made. The circumstances under which field indicators are not reliable are usually related to the state of the stream. The most difficult streams for which to determine bankfull stage are incised streams. In an incised channel, the stream has abandoned its historic floodplain, so the old floodplain is now a terrace. If one chooses this surface as the bankfull stage, the bankfull discharge will be overestimated. If there are other flat depositional surfaces below the terrace, they may or may not be indicators of the current bankfull stage. If the stream is still incising, the lowest flat depositional surface may overestimate bankfull discharge. If the channel is aggrading, the surface may underestimate bankfull discharge. If different degrees of development exist in the

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gauged and ungaged stream, then the relationship between rainfall and peak runoff is not the same for the two streams. Needless to say, the determination of design discharge is not always straightforward and can be difficult. Experience with streams in a developed region is very important. Once the design discharge has been determined, an appropriate stream type must be selected for the restoration design. Restoration does not always restore a stream to its historic stream type. Rather, the question is what stream type will be stable under the present conditions. Often the best indicator is the valley morphology. Such factors as valley slope, the presence or absence of a wide, well-developed floodplain, the size distribution of the materials in the valley are all useful to consider. However, in many situations existing constraints such as infrastructure locations limit the degrees of freedom in design. Sewer lines, roads, building structures, bridges, and culverts are examples of infrastructure constraints that must be considered in design. Once the decision of what stream type to design for has been made, the next step is to locate and characterize a reference reach to use as a template for design. Reference reaches are streams of the same stream type that are stable. Ideally, they are found in the same watershed upstream or downstream of the reach being restored. After a reference has been identified, it must be completely geomorphologically characterized. The parameters listed in the section on State Assessment and Departure are calculated and used to develop the design. The dimensions and pattern in plan view, profile, and cross section are drawn and structures are added to provide stability where needed.

STRUCTURES Structures are categorized in two groups: bank stabilization structures and grade control structures. The bank stabilization structures are used to roughen and toughen the banks to protect them from erosion and also to reduce near bank water velocities. They are often temporary stabilization devices to protect the bank from erosion until vegetation can be established to perform the permanent stabilization. Bank Stabilization Structures Root Wads Root wads are spaced along the banks on the outside of meander bends to protect these highvelocity areas from erosion during bankfull and greater discharges. Spacing varies with the size of the root wads and the depth of the stream as well as the radius of curvature on the meander bend. Details of plan and cross section of a root wad installation are given in Figure 13.17. The banks behind the root wad revetments are backfilled and planted with vegetation that will provide longterm stabilization. Usually hydrophytic woody shrubs such as willow, dogwood, or viburnum are used. Vanes Vanes are structures that direct the stress of high flows away from the bank and toward the center of the channel. They can be installed with logs or built with rock. The rock vanes are permanent bank stabilization structures. The areas at and above bankfull stage are planted with hydrophytic woody shrubs for overbank stabilization. Plan, profile, and cross section details are given in Figure 13.18. Live Fascines Live fascines are bundles of live cuttings buried in trenches and staked. The stems root and sprout and provide vegetative stabilization. The staked versions provide early stability in the event of high flows before the vegetation is established.

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Cutoff Log (Typ.)

361

Boulder (Typ.)

Root Wads (Typ.) Footer Logs (Typ.)

Fl o w

Bankfull Elevation Limit CL and PCL Proposed Stream Root Wad Typical Plan Not to Scale

Proposed Ground Boulders (As Directed by Engineer) Cut Off Log Bankfull Root Wad Elevation Invert Elevation Top of Footer Log is at Invert

Subgrade

1

From Bankfull Elevation Slope Existing Ground as Indicated By Proposed Contours on the Plans. Maximum 2:1 (50%) Slope

2

Boulder Footer Log

Fill

Root Wad Limit

Install Enkamat or and Erosion Control Matting With Equal Performance, as Approved By the Engineer. Matting Shall be Placed From the Edge of the Channel to 3' Beyond the Root wad Revetment Limit.

Root Wad Typical Section Not to Scale

FIGURE 13.17 Root wad revetment details.

Biologs Biologs are commercially available products made of coconut fiber rolled into tubes that can be staked along banks and planted with appropriate vegetation. They are a stable planting medium to help stabilizing vegetation become established. Other Bioengineering Techniques Other techniques are being developed almost daily. Soil fabric lifts can be used to build up a bank and allow for establishment of vegetation to provide permanent stabilization. Some suppliers provide sod packs with vegetation already growing in them. The vegetation can be custom-ordered to match the native materials growing along the steam under restoration. Grade Control Structures Cross Vanes Cross vanes are grade control structures that arrest downcutting. They are usually built of rock but can be constructed of logs. One approach to the log structures is a “K-Vane,” which is an adaptation of the K-Dam developed by the U.S. Forest Service. They protect the bed against scour by directing the stress into footers and taking the stress off the banks in the vicinity to prevent end cutting. Details are given in Figure 13.19.

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ull nkf Ba idth W

Channel Bed A

rom % Flweg 5 1 10 – to Tha pe Slo nkfull Ba

Bankfull

Flow

Invert Footer Rocks Length Varies A

A' Rock Vane Profile A–A' Not to Scale

A'

Thalweg to Bankfull

15°-30°

Bankfull Channel Bed Invert

Footer Rocks

Rock Vane Plan View

Rock Vane Cross Section Elevation View

Not to Scale

Not to Scale

FIGURE 13.18 Vane details.

Vortex Rock Weirs Vortex rock weirs are grade control structures that create velocity differential areas downstream of the vortex rocks to provide enhanced habitat variability. Details are shown in Figure 13.20. Step Pools Step pools are rock-lined structures that are useful for stabilizing channels on steep slopes (usually greater than 4%). They duplicate the natural function and shape of step pool channels in A3 or A2 streams. They are very useful when stabilizing headcuts and can be designed to maintain fish passage through steep reaches. Details are shown in Figure 13.21.

CASE STUDIES The following case studies are intended to illustrate the key features of restoration projects and to help highlight some of the pitfalls that can be avoided.

QUAIL CREEK This stream in Baltimore County, Maryland was severely destabilized by the failure of a regional stormwater management pond during a rainstorm of more than 5 in. of rain in a 24-hr period. When the pond failed 130 yd3 of sediment was discharged into the stream, which had a bankfull capacity of 50 cubic feet per second (cfs) and a drainage area of about 600 acres. The excess sediment initiated a process of disequilibrium, which resulted in more than 2500 yd3 of new sediment from

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1/3 w?

363

Horizontal Location Point Stationing Shown on Profile

1/3 w?

25'–30'

Vane Rock

2D

D = 1 Rock Diameter

Footer Rock 1/2 w?

1/2 w? 2D

1D

Excavation Area Cross Vane Plan View Not to Scale

Vertical Location Point for Rock Set at Channel Invert Rock Vane

Channel Bed Bankfull Elevation

Footer Rocks Cross Vane Cross-Section Elevation Not to Scale

Vane Rock

Bankfull Invert

e

15%

Slop

Footer Rocks Excavation Area

Cross Vane Profile Not to Scale

FIGURE 13.19 Cross vane details.

the resultant channel erosion in the following year. The restoration program included removal of excess sediment deposits with heavy equipment and a suction dredge. It included the stabilization of 15 banks totaling about 1200 ft of channel. The stream hosted self-sustaining populations of brook and brown trout before the pond failure. In the year after the failure, one trout and virtually no mayflies, caddis flies, or stone flies were found in the stream during studies conducted by Maryland Department of Natural Resources (DNR). The restoration project, finished in August 1990, resulted in dramatic improvements in the stream during the next year. In 1991, 13 species of macroinvertebrates were found. Particle size distribution of bed materials changed from a D50 of 2 mm to one of 32 mm in 1 year. In 1992, successful spawning of brook trout occurred. Monitoring of stability for 5 years with annual surveys of cross

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Bankfull Width 1/2 D' Typ.

30

Flow

30

Footer Rocks Vortex Rock Weir Typical Plan Outermost Four Rocks Touching One Another (Typical Both Sides)

Not to Scale

Ground Bankfull Width Elevation Bankfull Elevation Note: Tops of Vortex Rocks Set D Maximum 13% of Bankfull Depth 1/2 D

Max. Stream Invert Note: Vortex Rocks Shall be Place so That They Lean on Footer Rocks. Footer Rocks Shall be Place in Such Fashion so as to Dissipate Scour Forces by Vortex Rocks. The Tops of Vortex Rocks Shall be Placed at Maximum of 15% of Bankfull Depth.

Footer Rocks (Bottom Row of Footer Rocks Not Shown for Clarity)

Vortex Rock Weir Typical Cross-Section Not to Scale

FIGURE 13.20 Vortex rock weir details.

sections and pebble counts showed that the stream was stable and the particle size distribution coarsened as the excess fine sediment was transported from the stream. Today, there is a viable trout population in the stream. Figure 13.22 illustrates the stream prior to restoration and Figure 13.23 following restoration.

TRIBUTARY 9

TO

SAWMILL CREEK

This stream in the coastal plain of Maryland suffered from extensive development, which occurred shortly after World War II. The stream had its headwaters of nearly 1 square mile drainage area piped into a storm drain system in the early 1950s. Its bankfull discharge was 130 cfs. The regional relationships indicate that the bankfull discharge for this drainage area should be about 50 cfs. The channel had incised as much as 6 ft below its historic floodplain. Biologic surveys conducted by the Maryland DNR showed virtually no aquatic life except for an occasional American eel. The channel bed was covered with fine sand and silt, which shifted with every runoff event. The restoration plan restored 1100 ft of stream channel from G5 and F5 stream types to a B4c stream type. The entire length of the channel was reconstructed using root wads for bank stabilization and vortex rock weirs for grade control. The construction was completed in 1994. Monitoring shows that the channel is stable and the bed materials include a healthy component of gravel. The Maryland DNR introduced nine species of fish in 1996, and annual surveys show that seven species are surviving and reproducing. Figures 13.24 and 13.25 show the stream before and after restoration, respectively.

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FIGURE 13.21 Step pool details.

365

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FIGURE 13.22 Quail Creek before restoration.

FIGURE 13.23 Quail Creek after restoration.

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FIGURE 13.24 Tributary 9 before restoration.

FIGURE 13.25 Tributary 9 after restoration.

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REFERENCES Culbertson, D.M., Young, L.E., and Bryce, J.C., 1967. Scour and Fill in Alluvial Channels, U.S. Geological Survey, Open File Report, 58 pp. Davis, W.M., 1899. The geographical cycle, Geographical J., 14, 481–504. Dunne, T. and Leopold, L.B., 1978. Water in Environmental Planning, W.H. Freeman, San Francisco, 818 pp. Khan, H.R., 1971. Laboratory Studies in Alluvial River Channel Patterns, Ph.D. dissertation, Department of Civil Engineering, Colorado State University, Fort Collins. Lane, E.W., 1957. A study of the shape of channels formed by natural streams flowing in erodible material, Missouri River Division Sediment Series 9, U.S. Army Engineer Division, Missouri River, Corps of Engineers, Omaha, NB. Leopold, L.B., 1994. A View of the River, Harvard University Press, Cambridge, MA, 298 pp. Leopold, L.B., and Maddock, T., 1953. The hydraulic geometry of stream channels and some physiographic implications, U.S. Geological Survey Prof. Paper 252, U.S. Government Printing Office, Washington, D.C., 57 pp. Leopold, L.B. and Wolman, M.G., 1957. River channel patterns: braided, meandering and straight, U.S. Geological Survey Prof. Paper 282-B. Leopold, L.B., Wolman, M.G., and Miller, J.P., 1964. Fluvial Processes in Geomorphology, W.H. Freeman, San Francisco, 522 pp. Rosgen, D.L., 1996. Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO. Schumm, S.A., 1963. A tentative classification of alluvial river channels, U.S. Geological Survey Circ. 477, Washington, D.C. Thornbury, W.D., 1969. Principles of Geomorphology, 2nd ed., John Wiley & Sons, New York. Williams, G.P., 1978. Bankfull discharge of rivers, Water Resour. Res., 14(6), 1141–1153. Wolman, M.G. and Miller, J.P., 1960. Magnitude and frequency of forces in geomorphic processes, J. Geol., 68, 54–74

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Index A Activated-carbon adsorption, 264 Activated sludge systems, 243 Adjustable regulators, examples of, 22 ADT, see Average daily traffic Aeration equipment, maintenance costs of, 176 Aerobic sludge digestion, 243 Aerosols, removal of CFCs from, 148 Agriculture, impact of on hydrologic cycle, 316 Air bubbles, common process for forming, 30 Airfields, 166 Algal blooms, 131 Alkaline stabilization, 248 Alum pilot-scale treatment system, 131 American Petroleum Institute (API), 114, 115 Animal waste removal, 11 API, see American Petroleum Institute Aquatic ecosystems, relationship between urban stormwater and, 333 Aquatic habitat, disturbance of, 15 Asphalt pavement, construction of, 163 Attached film growth processes, 243, 244 Automobile maintenance and operation waste products, 120 Average daily traffic (ADT), 48

B Bacteria denitrifying, 126 fecal coliform, 124, 131 nitrifying, 126 soil matrix, 162 Balling, 214 Bank erosion, 336 accelerated, 350 hazard index (BEHI), 358 stabilization structures, 360 vegetation protection, 328 Bankfull discharge(s), 318 calibration of, 320 field-determined, 319 occurrence of, 319 Bankfull stage, identification of, 344, 345 Bankfull width, 325, 344 Bar screens, 26, 239 Basin(s) bioretention, 312 design, two-stage, 153 dry detention, 151, 152, 155

BAT, see Best available technology BCT, see Best conventional technology Bed load, 190, 191, 208 roughness, 207, 209 shear stress condition, 204 BEHI, see Bank erosion hazard index Belt filter, 248 Best available technology (BAT), 227 Best conventional technology (BCT), 227 Best management practices (BMPs), 3, 55, 71, 144, 234, 296, 335 application of vortex separators to stormwater, 166 constructable, 181 definition of, 147 design utility of downstream, 164 development of in Europe, Japan, and Australia, 150 in United States, 149 effectiveness of bioretention, 292 evaluation of as black boxes, 179 failure rate of, 8 goal of upstream, 17 heavy metal trap efficiency, 56 inappropriate, 180 infrastructure, upkeep of stormwater, 276 in situ treatment, 56 most-promising, 6 objective of, 170 on-lot, 286 overloaded, 165 partial exfiltration trench reactor, 167 performance of, 8, 182, 274 pollution prevention, 10 porous pavement, 164 pretreatment measure for downstream, 157 references, urban runoff and CSO, 7–8 role of, 149 as source control mechanisms, 148 stormwater mitigation, 159 structure, cleaning of, 181 ultraurban, 277 urban stormwater quantity and quality mitigation, 151 runoff mitigation, 178 vegetative, 14, 156 wet retention ponds as, 154 Best professional judgment (BPJ), 227 Biochemical oxygen demand, 74 Bioengineering techniques, 361 Biofilms, 193 Biological oxygen demand (BOD), 19, 126, 161, 171 Biological treatment, 25

369

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Biologs, 361 Biomound bio-oxidation systems, 249 Bioreactors, 249 Bioretention, 301, 303 basin, 312 BMPs, effectiveness of, 292 Biosolid(s), 246 characteristics, 247 products, pathogen-free, 251 removal of excess water from, 248 Black box(es) evaluation of BMPs as, 179 model, 81, 82 treatment system, 178 Bleed/pumpback, disadvantages of, 251 BMPs, see Best management practices BOD, see Biological oxygen demand BPJ, see Best professional judgment Brake pad, heavy metal content of typical, 49 Broom-operated equipment, 118 Bubble diffusers, 133 Building Services Research and Information Association, 262

C Calcined sludge, alum recovery from, 32 Captur, 19 Carcinogen formation, 33 Carcinogenic compounds, separation of, 165 Catalytic converters, 46 Catchbasin(s), 200 basic, 112 cleaning, 17, 18, 79, 80, 95, 97 design, basic, 113 idealized, 109 inlets, 136 monitoring of, 110 performance, 109 pollutants retained in, 19 sediments, mobility of, 109 sumps, 110, 111 CCTV, see Closed circuit TV CDFs, see Cumulative probability density functions CDM, see Technical Council of Camp Dresser & McKee Central CSO biosolids treatment facility, 251 Centrifuge, 248 Channel adjustment, evolutionary stages of, 357 erosion, 328, 330 geometry, 359 alterations, 327 downstream changes in, 324 instability, 336 pattern, 325 processes, 347 protection criteria, 275, 338 stability, 331 Chemical oxygen demand (COD), 29, 70, 85, 110 loadings, 47

removal, 165 Chlorophenol formation, 33 Cholesterol compounds, 122 CIRIA, see Construction Industry Research and Information Association Cisterns, 291, 298 Citrobacter, 32 City water, use of for lawn irrigation, 266 Clay soils, 88 Clean Water Act (CWA), 226–227, 274, 276 Climate modification, urban environment as heat island for, 46 Closed circuit TV (CCTV), 208 CN, see Curve number Coalescing plate interceptors (CPI), 114 Coarse filter unit, 111 Coarse screens, 26, 239 COD, see Chemical oxygen demand Cold-water habitat, 337 Colloids, 190 Combined sewer overflows (CSO), 3, 188, 236, 282 biosolids characteristics, 250 heavy metals in, 247 PCB in, 247 pesticide in, 247 treatment facility, 251 BMP references, urban runoff and, 7–8 control, 107 marine receiving water, 23 options used for, 18 disposing of biosolids from, 246 events, large-scale relief sewers built to reduce, 290 facilities, catenary screens used for, 26 HGMS treatment of, 31 pollution, control of, 241 prevention of, 176 production of undesirable solids in, 232 prohibition of, 227 regulator/separator, isometric view of swirl, 36 treatment, screening devices used in, 27 Combined sewer system (CSS), 150, 236 Compaction, effects of on clayey urban soils, 87 Compost(s), 249 -amended soils, 108 yard waste, 125 Computer models, simulation of sediment hydraulic effects in sewers, 201 Constructed wetland(s), 147, 172, 286 maintenance requirements for, 175 pollutant removal mechanisms, 174 systems, 15 Construction activities, impacts attributable to, 278, 328 site(s) runoff, suspended material transported by, 45 soil erosion from, 12 Construction Industry Research and Information Association (CIRIA), 199, 205, 206, 234 Continuous runoff hydrographs, 260 Control approaches, traditional, 335

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Index

371

Controlled street surface washoff test, 90 Controlled waste, 201 Corrosion, heavy metals generated from, 49 CPI, see Coalescing plate interceptors Cross vanes, 361, 363 CSOs, see Combined sewer overflows CSS, see Combined sewer system Cumulative probability density functions (CDFs), 106 Curve number (CN), 88, 283, 300 composite custom, 306 low-impact development, 306 CWA, see Clean Water Act

D DAF, see Dissolved air flotation Dambreak creation, 235 flushing wave, 211 Data quality objectives (DQOs), 180 Deicing agents, sand and salt applied to, 13 Denitrifying bacteria, 126 Deposition, approach to estimating, 209 Design charts, series of, 308 Design discharge, determination of, 360 Design storm(s), 105, 329 event, determination of, 312 frequencies, 330 Destabilization, 347 Detention, 299 basin, 56 pond effects, 82 storage, 284, 305, 312 Development characteristics, 91 Dewatering, 192, 248 Digital simulation models, 260 Discharge permit, 37 Disinfectant feed system, 268 Disinfection, 32 accomplishment of, 147 high-rate, 34 units, high-rate, 268 Disk screens, design parameters for, 28 Disposable diapers, 198 Dissolved air flotation (DAF), 26, 30, 237, 240 Dissolved heavy metals, 55 Dissolved phosphate reduction benefits, 98 Dissolved solids, 90 Distributed control, 297 Diversion structures, 305 DMHRF, see Dual-media high-rate filtration Domestic sewage, 197 Downstream channel degradation, 327 DQOs, see Data quality objectives Drainage area mapping, 121 candidate scenarios for urban, 106 design criteria, 105 efficiency, 91 grass swale, 96

inlet use, suggestions for optimal, 112 objectives, urban, 105 open, 291 patterns, natural, 284 problems, sanitary wastewater cross-contamination, 121 system, physical areas of, 4 Drinking water violations, 278 Drowning, 279 Drum screen, 27, 28 Dry detention basins, 151, 152, 155 Dry extended detention pond, capital costs of construction for, 153 Dry weather deposition, critical fluid shear stress theory for estimation of, 235 Dry weather flow (DWF), 11, 226 constant, 238 diluting, 200 plant, cost of, 244 sanitary wastewater input during, 229 sources, pH of, 122 treatment of, 242 Dry weather pollutant deposition loading, 229 Dual-media high-rate filtration (DMHRF), 27, 241 cost-competitivenesss of, 242 design parameters for, 29, 30 pilot study installations, 241 removal of heavy metals by, 242 DWF, see Dry weather flow

E Effluent quality, 36 Elaborate model, 230 EMC, see Event mean concentration Emerging stormwater controls for critical source areas, 103–139 design objectives and general approach in selection of stormwater controls, 104–108 candidate scenarios for urban drainage, 106–108 runoff and pollutant yields for different rain categories, 105–106 emerging critical source area controls, 123–135 chemical-assisted sedimentation, 130–132 combination practices, 132–135 filtration of stormwater, 124–130 public works activities historically used for stormwater control at critical areas, 108–123 catchbasins and other floatable and grit traps, 109–113 oil/water separators, 113–117 prevention of dry weather pollutant entries into sewerage systems, 120–123 street cleaning, 117–120 Emulsions stable, 114 unstable, 114 End-of-pipe controls, 80 End-of-pipe storage, 277 End-of-pipe treatments, 6, 25

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Engineering News-Record (ENR), 268 ENR, see Engineering News-Record Enterobacter, 32 Entrenchment, 344, 345 Environmental audit, 215 Enviro Whirl I, 119 EPS, see Extracellular polymeric substance Erosion bank, 336, 350 channel, 328, 330 /deposition model, 210 gully bed deposit, 200 meander bend, 347 rates, 358 resistant plant species, 160 Escherichia coli, 32, 33, 122 ESS, see Experimental Sewer System European Union landfill directive, 216 Evapotranspiration, 280 Event mean concentration (EMC), 50, 67 calculation of grand mean and standard deviation of, 74 as comparative index, 52 cumulative distribution for site-grouped TP, 73 estimated, 71 NURP, 76 pooled, 76 predicting, 92 statistical probability distribution of, 72 values, NURP, 71 variance of, 68 Exfiltration systems, 163 full, 163 partial, 163, 167 water quality, 163 Experimental Sewer System (ESS), 290 Extended detention dry ponds, 15 Extracellular polymeric substance (EPS), 193 Extractable zinc (EZN), 79 Extreme flood volume criteria, 275, 338 EZN, see Extractable zinc

F Family biotic index (FBI), 278, 328 FBI, see Family biotic index FBM, see Flow balance method FC, see Fecal coliform Fecal coliform (FC), 19, 124, 131 Fecal streptococci, 34 Federal Highway Administration (FHWA), 149 Fertilizer(s) application, 49 reduced use of, 11 washed off ground, 13 FHWA, see Federal Highway Administration Filter(s) belt, 248 bypassing, 128 design, 127, 128 fabric unit, 111, 112

peat–sand, 126, 129, 161 sand, 19, 160, 161 strip(s), 14, 158, 159 establishment costs, 160 performance, 160 vacuum, 248 Filtration effectiveness of for stormwater clarification, 146 media, selection of, 127 sand, 124 systems, 156 Fine screens, 26, 239 First flush, 52, 232 Fish kills, 131 populations, loss of, 332 sensitivity, 334 Fixed media device, 56 Fixed regulators, examples of, 22 Flash aerator, 133 Fleet vehicle maintenance washing facilities, 166 Flocculation treatment procedure, 130 Flood control, 2, 6, 181, 273 conveyance, channel enlargement for, 358 frequency determination, 321 Flooding control of increased, 335 damage, prevention of, 136 Floodplain(s) well-developed, 357 zoning, 335 Flow equalization, 176 frequency/duration control, 298 Flow balance method (FBM), 23 freshwater configuration, 24 quick construction potential of, 24 seawater configuration, 24 Fluidized-bed furnace, carbon regeneration in, 32 Fluidsep vortex separator, 35 Fluoride, 121 Flushing public attitudes to, 198 sewer, 214, 235 wave, dam-break, 211 WC, 216 Full exfiltration system, 163

G Gamma irradiation, 147 Garbage grinders, 199 Gauge station data, recording of, 322 Gauging stations, permanent benchmark of, 321 GIS models, 81 Gottingen Cleaning Balls System, 211 Grab sampling, 177, 178 Grade control structures, 361 Grass swales, 82, 95, 97, 108

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Index

373

Gravel mining, 358 Gravity separation, 35, 114 Grit deposition, 236 traps, 109, 113 Groundwater MTBE contamination, 233 recharge, 257, 280, 296, 332, 336 criteria, 275, 338 issues, 337 Gully bed deposits, erosion of, 200 Gully pot, 200

H Habitat cold-water, 337 degradation, 327 impairment, 278, 328 protection, 330 receiving waters, 333 Hazardous residuals management, 152 Hazardous waste disposal, 156, 181 emissions, 196 management, 11, 13 Hazen’s settling efficiency model, 145 HDG, see Hot-dipped galvanizing Headwater streams, 323, 324 Heavy metal(s) content of in typical brake pad, 49 in CSO biosolids, 247 diffusion of into water column, 56 distribution, particle size vs., 261 dry-deposited, 46 generation of, 49, 50 mass trends, cumulative, 64 partitioning, 53, 56 in pavement runoff, 53 removal of by DMHRF, 242 in sewer bed deposits, 194 treatment design implications for dissolved, 55 for particulate, 63 Helium, ultrahigh pure, 61 Herbicide(s), 328 application, 49 reduced use of, 11 washed off ground, 13 HGMS, see High-gradient magnetic separation High-gradient magnetic separation (HGMS), 26, 31 High-rate treatment processes, 237 Highway infrastructure, 46 Hot-dipped galvanizing (HDG), 49 Household toxic substances, 120 HRT, see Hydraulic residence time HSG, see Hydrologic soils group Human enteric viruses, 232 Human-entry sewers, 213 Human population growth, worldwide, 48

Hybrid facility design, 311 HYDRASS gate, 211, 212 Hydraulic controls, 148, 149 Hydraulic geometry, 323, 324, 325 Hydraulic loadings, 26 Hydraulic models, 81 Hydraulic residence time (HRT), 177 Hydrocarbons, distribution of in urban stormwater, 228 Hydroclones, solids cleaning using, 217 Hydrograph(s) continuous runoff, 260 modification to runoff, 44 unit, 260 Hydrologic cycle, 44, 316 Hydrologic soils group (HSG), 284, 306 Hydrology small storm, 84, 88, 89 urban environment, 51

I ICP, see Inductively coupled plasma spectroscopy IDF formulations, see Intensity-duration-frequency formulations I/I, see Inflow and infiltration Impervious surfaces, impacts from increases in, 332 IMPs, see Integrated management practices In-cloud processes, modified, 47 Inductively coupled plasma spectroscopy (ICP), 54 Infiltration basins, 16 device, clogging of, 124 practices, 16, 107 strategies, residential, 290 swale, 285 trenches, 16, 298, 303 Inflow and infiltration (I/I), 199 In-line storage, 22, 171 In-sewer conditions, 190 In-sewer processes, 187 In-stream habitat, protection of, 104 Integrated management practices (IMPs), 281 distributed, 282 examples of LID, 287 hydrologic functions of LID, 285 microscale, 298 reported pollutant removal efficiency of, 285 Intensity-duration-frequency (IDF) formulations, 143 Intermediate model, 230 Ion exchange, 134, 147 Irradiation gamma, 147 ultraviolet, 34, 147

J Jetting, 214 Juvenile sediments, 232

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K Kitchen grinders, importance of, 199 Klebsiella, 32 K-Vane, 361

L Lake formation, oxbow, 352 Lamella separation, cross-current, 116 Land cover, variation in, 313 development activities impacts attributable to, 278, 328 massive, 339 disturbance activities, 339 prices, 152 subsidence, 289 use changes, 316 decisions, 280 modifications, 44 regulations, 10 toxic pollutant concentrations vs., 228 Landfill disposal, alternatives to, 217 European Union directive for, 216 Laundry wastewater, 120 Lawn irrigation, 266, 267 LCA, see Life cycle assessment Lead reduction benefits, 98 LID, see Low-impact development Life cycle assessment (LCA), 215, 216 Limit of deposition (LOD), 204 Linear storage reservoir model, 260 Liquid pycnometer, 61 Litter control, 119 Litter monitoring, continuous, 119 Litter reductions, 112 Live fascines, 360 LOD, see Limit of deposition Long profile method, 345 Low-impact development (LID), 271–294 approach, 280–287 basic concepts for LID designs, 283–286 benefits of LID, 281–282 integrated management practices, 287 other important considerations, 286–287 overview, 280–281 background, 273–280 better technology or more restrictive land use policies, 280 development of alternative stormwater technology, 273 economic issues, 276–277 environmental concerns, 277–279 practical problems, 279–280 technical issues, 273–276 cost, 201 curve number

postdevelopment, 302 representative, 306 designs, concepts for, 283 integrated management practices comparison of hydrologic response of conventional and, 297 examples of, 287 hydrologic functions of, 285 international microscale experience and case studies, 288–291 German watershed planning using microscale approaches, 289 Japanese experimental sewer system, 290 Lyon residential infiltration strategies, 290–291 land cover, 300 major components of, 281 planning techniques, 302 roadblocks, 291–292 runoff potential, 300 site planning practices, 284 process, 283 stormwater management requirements, 307 technology, development of, 338 urban retrofit, 288 Low-impact development hydrologic analysis, 295–314 LID fundamental concepts, 297–298 distributed control approach, 297 hydrologically functional landscape, 298 microscale integrated management practices, 298 LID hydrologic analysis components, 298–305 hydrologic evaluation, 299 LID runoff potential, 300–301 maintaining predevelopment runoff volume, 303 maintaining predevelopment time of concentration, 301–303 potential requirement for additional detention storage, 305 process and computational procedure, 305–313 data collection, 305 determining design storm event, 312–313 determining LID runoff curve number, 305–307 development of time of concentration, 307 LID stormwater management requirements, 307–312

M Macrophyte algal growths, 131 Manholes deposits in, 194 monitoring of, 110 Marine ports, 166 Mass loading, 62 Mass transit park-and-ride lots, 166 Material flow accounting, 215 MC, see Microcarrier MCTT, see Multichambered treatment train Meander geometry variables, 326 Mechanical street cleaning, 118, 200

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Index

Metal flashing, 46 Methylene blue active substances (MBAS), 122 Microbial decomposition, 174 Microcarrier (MC), 264 Microclimate characteristics, 143 Microorganisms, engineered intracellular, 250 Microscale management practices, 281 Microscale planning programs, 289 Microscreens, 26, 239 Microstrainers, 27, 28 Microtox screening test, 134, 135 Minerals extraction, impact of on hydrologic cycle, 316 Mixing–coagulation–flocculation, 146 Model(s), see also U.S. Environmental Protection Agency Storm Water Management Model black box, 81, 82 categorized statistical, 82 commercial sewer flow quality, 209 computer, simulation of sediment hydraulic effects in sewers, 201 conceptual hydrologic, 82 digital simulation, 260 elaborate, 230 erosion/deposition, 210 GIS, 81 Hazen’s settling efficiency, 145 hydraulic, 81 intermediate, 230 linear storage reservoir, 260 MOUSETRAP, 201 rainfall–runoff, 82 risk, 208 sedimentation in sewers, 205 sewer flow quality, 202, 208 simplest, 230 stormwater, 87 problem with, 90 quality, 79 verification, 82 Monte Carlo sampling procedure, 92 Monte Carlo simulation techniques, 82 Monte Carlo statistical modeling, 123 Mosquito -borne diseases, 279 breeding, 9, 14 MOUSETRAP model, 201 MSAS, see Methylene blue active substances Multichambered enhanced treatment device, 21 Multichambered treatment train (MCTT), 117, 132 facility, Birmingham pilot-scale, 134 installed capital cost of, 134 pilot-scale, 132 sedimentation chamber in, 130 Multicriteria decision support systems, 215 Municipal sewage sludge, 249 Municipal wastewater, physical treatment units applied to, 176 Municipal water supply, 258

375

N NALMS, see North American Lakes Management Society National Coastal Pollution Discharge Inventory (NCPDI), 68 National Oceanographic and Atmospheric Administration (NOAA), 68, 105 National Pollution Discharge Elimination System (NPDES), 11, 68 data, incorporation of into Rouge River Program Oracle database, 71 permit application programs, 72 data, 70 program, 226 Phase I, 149 Phase II, 274, 280 regulations, 120 regulatory requirements, 151 stormwater regulatory approach, 337 National Stormwater Runoff Pollution Database, 67–78 CDM National Urban Stormwater Quality Research Grant, 69 major National Urban Runoff data collection efforts conducted since NURP, 68–69 methods, 69–75 analytical approach, 72–75 database compilation, 71–7 2 gathering of urban stormwater data, 69–71 Nationwide Urban Runoff Program, 67–68 results, 75–76 National Weather Service (NWS), 307 Natural drainage patterns, maintaining, 284 Natural resource protection, 10 Natural Resources Conservation Service (NRCS), 88, 273, 296, 336 NCPDI, see National Coastal Pollution Discharge Inventory New Jersey Department of Environmental Protection (NJDEP), 336 Nitrification, 174 Nitrifying bacteria, 126 NJDEP, see New Jersey Department of Environmental Protection NOAA, see National Oceanographic and Atmospheric Administration Nonequilibrium partitioning, 54 North American Lakes Management Society (NALMS), 131 Noxious gas buildup, 172 NPDES, see National Pollution Discharge Elimination System NRCS, see Natural Resources Conservation Service Nuisance control, 155 NURP, see U.S. Environmental Protection Agency Nationwide Urban Runoff Program Nutrient(s) loadings, reduction of, 154 removal, by wet retention basins, 156 soluble-phase, 161 NWS, see National Weather Service

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O Off-line storage, 23, 171 Oil -contaminated soil, 250 dissolved, 114 free-floating, 114 – grit separators, 8, 20 Oil/water separator(s), 112, 136, 165 American Petroleum Institute, 115 historical use of, 116 performance, 114 problems with, 115 processes occurring within, 116 O&M, see Operation and maintenance On-lot BMPs, 286 Open channel design, 303 Open drainage, 291 Operation and maintenance (O&M), 181 Outfall dry weather data/observations, analysis of, 123 Overbank flooding criteria, 275, 338 Overflow sieve, 166 Oxbow lake formation, 352 Oxidation, 174 Ozone, disinfecting rate of, 33

P PAC, see Powdered activated carbon PAH, see Polynuclear aromatic hydrocarbons Parameter definitions, relationships between granulometry parameters and, 58 Partial exfiltration systems, 163, 167 trench reactor (PER), 167 rehabilitation of clogged, 169 sizing of, 168 use of directly on bedrock, 169 Particle(s) behavior of on streets, 118 number, 57 size distribution (PSD), 57, 60, 62, 321, 344 heavy metal distribution vs., 261 metal concentration vs., 229 specific gravity, measurement of, 61 Particulate(s) accumulation rates, 82 matter, separation of, 58 treatment design implications for, 63 washoff, 89 Partitioning kinetics, 54 nonequilibrium, 54 reactions, 53 Pathogenic microorganisms, control of, 32 Pavement(s) asphalt, 163 porous, 16 BMP, 164

construction of, 163 cross-section of, 17 economics of installation, 164 performance of, 181 pollutant removal efficiencies for, 164 rainfall losses for, 86 runoff, heavy metals in, 53 slopes, 64 structures, aquaclude for, 87 surface, urban, 52 PCB, see Polychlorinated biphenyls PCC, see Portland cement concrete Peak discharge control, 329, 335 statistics, 335 Peak runoff rate control, 298 Peat moss, 126 Peat–sand filter (PSF), 126, 129, 161 Pebble counts, 344, 346, 364 PER, see Partial exfiltration trench reactor Pesticide(s), 250, 328 reduced use of, 11 washed off ground, 13 Petroleum-contaminated soils, 250 PGDER, see Prince George’s County, Maryland, Department of Environmental Resources pH degradation, 127 depression, rainfall, 46, 144 dry weather flow source, 122 Pharmaceuticals, 122 Phosphorus removal, 131 Photolysis, 174 Photo-oxidation, 147 Physicochemical treatment trains process flow diagram, 265 Phytoremediation/photo-oxidation, 147 Pipe full flow conditions, 205 Pipe and pond technology, 273, 292 Plant coverage, seasonal dormancy of, 157 species, erosion resistant, 160 Plate-and-frame press, 248 Pocket wetlands, 8 Pollutant(s) accumulative loadings of, 85 degradation, 127 deposition loading, dry weather, 229 discharges, predicting, 80 loading(s) accumulative, 84 equalizing of, 22 roadway area, 144 variability of, 144 loads methods for estimating, 177 reduction of downstream, 19 mixing, influence of topography on, 144 removal effectiveness, 274 efficiencies, 168, 177, 244, 285

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Index

majority of, 162 mechanisms, 157, 174 objectives, 170 rates, 159 soluble, 159 upstream, 9 reuse categories, 265 RTDs in presence of, 145 separation of in stormwater detention/settling tunnel, 172 source(s) control of, 11 investigation of, 123 use of SLAMM to identify, 92 Pollution control programs, 183 prevention, 10, 281, 286 reduction efficiencies, 178 sources, quantifying of, 5 water, abatement, 266 Poly pigs, 214 Polychlorinated biphenyls (PCB), 261 Polynuclear aromatic hydrocarbons (PAH), 261 Pond(s) detention, 82 drainage, costs incurred in, 156 dry extended detention, 153 extended detention dry, 15 vegetation, control of, 153 wet detention, 79, 95, 97 disinfection in, 130 outfall, 107 use of for controlling runoff, 107 wet retention cost-effectiveness of, 169 nutrient removal efficiencies of, 154 Pool riffle ratio, 359 Porous pavement, 16 BMP, 164 construction of, 163 cross-section of, 17 installation, economics of, 164 performance of, 181 pollutant removal efficiencies for, 164 Portland cement concrete (PCC), 54 POTWs, see Publicly owned treatment works Powdered activated carbon (PAC), 32, 146 Power bucket machine, 214 Power rodding, 214 PPCC test, see Probability plot correlation coefficient test Precipitation, 146, 174 Predevelopment condition, definition of, 313 Prince George’s County, Maryland, Department of Environmental Resources (PGDER), 272 Probability plot correlation coefficient (PPCC) test, 73 Property owner legal liabilities, 279 Protozoa, abundance of in composting operation, 249 PSD, see Particle size distribution PSF, see Peat–sand filter Publicly owned treatment works (POTWs), 227 capacity, 236

377

influent flow rate, 237 Public support, gaining of, 10 Public works practices, 107, 108, 136

Q QA/QC, see Quality assurance/quality control Quail Creek restoration, 366 Quality assurance/quality control (QA/QC), 70, 178–179 procedures, 180 program, U.S. EPA, 180

R Rain barrels, 286, 291, 296, 298, 303 count, accumulative, 84 depth, 94 gardens, 80, 296 Rainfall event, solids transported in, 51 pH depression, 46 Rainfall–runoff city street, 86 model, 82 Rainwater harvesting system main components, 262 natural acidity of, 45 RBC, see Rotating biological contactor Real-time control (RTC), system, 22 Receiving stream(s) fishable and swimmable, 275, 339 response of to hydrologic regime changes, 277 Receiving water(s) habitat, 333 quality, threat to, 226 Reclaimed stormwater, use of for lawn irrigation, 266 Redox conditions, 145 Residence time distribution (RTD), 145 Residential infiltration strategies, Lyon, 290 Residual(s) management, 158 treatment, decision-making process for, 64 Retention storage, 284, 303, 309 Retrofit costs, urban, 282 Risk modeling, 208 Rivers, classification of natural, 348, 349 Roadway areas, pollutant loadings in, 144 stormwater, 50 Rodents, 195 Roof disconnections, 95, 96, 97 runoff, 129 Roofing materials, 46 Rooftop storage, 305 Root wads, 360, 361 Rosgen classification system, 348 Rotary screen, 27, 28

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Rotating biological contactor (RBC), 25, 244, 245 Rotostrainer, 27 Rouge River Program Oracle database, 71 RTC system, see Real-time control system RTD, see Residence time distribution Runoff characteristics of storm, 34 construction site, suspended material transported by, 45 controls, 4 curve number, 299 flow rates, 104 hydrograph, modification to, 44 pavement, heavy metals in, 53 pollution characterization, 69 potential, low-impact development, 300 predictions, 88 quantity, magnitude of, 144 recession curves, 86 removal of contaminants from, 182 roof, 129 snowmelt, 229 solids from, 228 volume(s) comparison of for selected storms, 330 control, 298 increase in, 331 reduction benefits, 98 SLAMM results for, 96 strategy developed to reduce, 289 yields, 86

S SA, see Surface area SAGES Unit, 21 Saltation, 346 Saltwater intrusion, 289 Sand filter(s), 19, 60 beds, maintenance of, 20 objective of, 161 stormwater inlet, 20 system, conceptual design of, 19 Sand filtration, 124 Satellite CSO biosolids treatment train, 251 Screen(s) bar, 26, 239 categories of, 26 coarse, 26, 239 disk, design parameters for, 28 drums, design parameters for, 28 fine, 26, 239 rotary, 27, 28 static, 27 units, categories of, 239 Sediment(s) accumulated, 182 bed(s) dewatering of, 192 mature, 234 catchbasin, 109

control devices, active, 211 controlled depth of, 204 controlled design, 204 deposition, in sewers, 188 deposits, graded, 209 -free sewers, 195 in-sewer accumulation of, 200 juvenile, 232 loadings, excessive, 170 removal, 172 comparison of with sewer flushing, 210 cycles, 155 sewer leachate, 232 near-bed material, 190 size, 317 taxonomy, sewer, 188 transport, 203, 207, 318, 336 -trapping equations, 110 Sedimentation, 174 approaches to predict, 206 with chemicals addition, 240 high-rate, 242 Self-cleansing sewers, 188, 202, 203 Separator(s) American Petroleum Institute oil/water, 115 Fluidsep vortex, 35 oil–grit, 8, 20 oil/water, 112, 165 historical use of, 116 performance, 114 problems with, 115 processes occurring within, 116 Storm King hydrodynamic, 35 vortex, 166 vortex/swirl, 165 Settled sewerage systems, 197 Settling basins, 151 chemical-assisted, 130 velocities, soluble nutrient, 154 Sewage, domestic, 197 Sewer(s) accumulation of sediments in, 194 bed deposits, heavy metals in, 194 cleaning, 214 automatic, 212 techniques, 211 deposition in, 207 deposits organic material in, 191 physical characteristics, 192 design, 195, 201 excessive grit deposition in, 250 flow quality models, 202, 208 flushing, 210, 214, 235 gravity, slime thickness in, 193 human-entry, 213 interceptor, 213 large-scale relief, 290 organic particles in, 208

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Index

overdesigned, 202 pipe, structural deterioration of, 232 records, 199 sediment(s) -free, 195 leachate, 232 near-bed material, 190 origin, 227 taxonomy, 188, 189 traps in, 213 type A, 192, 193 self-cleansing, 188, 202, 203 small-diameter gravity, 197 solids effects caused by, 195 hydraulic characteristics of, 230 loading and transport, 229 management of, 209 systems, structural integrity of receiving, 196 underdesigned, 202 workers, dangers to, 194 Sewerage sediment accumulations, monitoring of, 110 solids by-products (SSBPs), 217 systems prevention of dry weather pollutant entries into, 120 settled, 197 Sewer sediments, management of, 187–223 characteristics and pollutant properties of sewer solids, 188–195 biofilm, 193–194 deposits and near-bed material, 190–193 effects on performance of sewer systems, 194–195 suspended solids and colloids, 190 transport, 188–190 design and operation of sewer systems to control problems of sewer solids, 201–205 management and disposal of sewer solids, 209–218 sediment management in large sewers, 210–213 sewer cleaning in smaller sewers, 214 sustainability and disposal of sediment removed from sewers, 214–218 nature of sewer sediments, 188 prediction of sedimentation and control of sediments in existing sewers, 205–209 sewer solids control at source and at inlets, 195–201 behavioral aspects, 197–199 gullies and other inlet controls, 200–201 SGDS, see Small-diameter gravity sewers Shapiro–Wilk test, 73 Silviculture, impact of on hydrologic cycle, 316 SIMAPRO LCA software, 216 Simplest model, 230 Sinuosity, 344, 346 Site imperviousness, minimization of, 301 SLAMM, see Source Loading and Management Model Slimes, 193 Slope, 344, 360 Sludge calcined, 32

379

municipal sewage, 249 Small-diameter gravity sewers (SGDS), 197 Small storm hydrology, 84, 88, 89, 329 Snowmelt particle data, fit of power law to, 58 degradation in, 61 granulometry, variation in urban, 63 size distribution for, 57 runoff, 229 Software, SIMAPRO LCA, 216 Soil(s) altered urban surficial, 43 compacted, 277 compost-amended, 108 effects of compaction on clayey urban, 87 erosion construction site, 12 control of, 6 fabric lifts, 361 incorporation, 174 infiltration BMP involving, 9 rates, low, 161 matrix, resident bacteria in, 162 moisture replenishment, 327 oil-contaminated, 250 petroleum-contaminated, 250 preservation of infiltratable, 301 saturated hydraulic conductivity of urban, 144 strata, pollutant removal in underlying, 162 Soil Conservation Service (SCS), 300 Solid(s) cleaning, 217 dissolved, 90 impact of on wastewater treatment plant, 195 – liquid separation activated sludge and, 243 enhanced, 240 loadings, street dirt total, 118 originating from sanitary wastewater sources, 229 recovery, poor, 75 removal efficiencies, 26 separation, 35 sewer effects caused by, 195 hydraulic characteristics of, 230 management of, 209 suspended, 9, 85, 90, 125, 226 control of, 127 loading capacities, 128 monitoring, 27 reduction benefits, 98 removal, 29, 239 SLAMM results for, 96 wastewater, 190 temporary systems for accumulating, 187 total dissolved, 267 total suspended, 45, 70, 74, 124, 145 transporting of in rainfall event, 51 volatile suspended, 135

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waste management, 11 water interfacial surface area, 62 WWF, 246 Soluble-phase nutrients, 161 Soluble pollutant removal, 159 Source control mechanisms, BMPs as, 148 Source Loading and Management Model (SLAMM), 3, 79–101, 260 computational processes, 91–92 future directions, 96–99 history, 80–81 major process descriptions in, 83 process descriptions, 81–82 unique attributes, 82–91 particulate washoff, 89–91 small storm hydrology, 84–89 use of SLAMM to identify pollutant sources and to evaluate different control programs, 92–96 Specific gravity, 62 Specific surface area (SSA), 57, 62 determination, 58 particulate matter, 59 SS, see Suspended solids SSA, see Specific surface area SSBPs, see Sewerage solids by-products Stabilization alkaline, 248 bank, 360 processes, 248 vegetative, 357 Stable channel balance, Lane’s, 317 Static screen, 27 Statistical properties, arithmetic and lognormal transform, 75 Step pools, 362, 365 Storage detention, 284, 305, 312 end-of-pipe, 277 in-line, 171 off-line, 23, 171 retention, 284, 303, 309 rooftop, 305 surface depression, 2 Storm design, 105, 309 drainage inlet use, suggestions for optimal, 112 drain inlet devices, evaluation of, 110 events, characteristics of, 2 overflow tanks, 166 runoff characteristics of, 34 flow, 258 infiltration of, 9 urban Cincinnati, 50 Storm King hydrodynamic separator, 35 Stormwater beneficial reuse of, 36 clarification, effectiveness of filtration for, 146 controls, see also Emerging stormwater controls for critical source areas problems in evaluating urban area for, 94 –

retrofitting, 107 disinfection, 130 filtration, 124 flood control, 2 gradation, particle number-based analysis for, 59 institutions, U.S., 288 management (SWM), 273 design guidelines, 104 LID approach to, 339 mitigation facility, 170 models, 87, 90 NPDES permit data, 70 particle(s) degradation in, 61 size distribution for, 57 SSA for, 59 particulate matter, physical and chemical characteristics of, 56 pollutant loads, removal of, 2 presettling, 127 problems, reduced, 104 public perception of, 6 quality control(s) structures, costs of, 162 water quantity reduction benefits of, 80 quality treatment facilities, designing of, 2 reclamation conceptual diagram, 263 roadway, 50 runoff controls, 4 storage treatment systems, 261 treatment alternatives, influence of hydrologic factors on, 143 capital and annual costs for, 268 volume discharges, 175 wetlands, 14 Stormwater, beneficial use of urban, 257–270 hypothetical case study, 266–268 stormwater storage treatment systems, 261–264 treatment systems and water quality, 264–265 urban water resources management, 258–261 storm runoff flow, 258–260 urban stormwater quality, 260–261 Stormwater runoff, treatment of from urban pavement and roadways, 141–185 assessment of treatment effectiveness, 177–180 analytical techniques, 178–179 evaluation of BMPs as black boxes, 179 event focus basis, 177–178 quality control and quality assurance, 179–180 seasonal or annual focus, 178 complicated nature of stormwater with respect to treatment potential, 143–145 environmental factors affecting treatment selection and effectiveness, 143–144 hydrologic factors affecting treatment selection and effectiveness, 143 physicochemical aspects of stormwater affecting treatment selection and effectiveness, 144–145 development and operational aspects of treatment, 180–183

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Index

cleaning, 181 construction, 180–181 cost and design life, 180 hazardous waste disposal, 181–182 operation and maintenance, 181 postconstruction considerations, 182–183 effectiveness of multiple unit operations, process controls, and BMPs, 167–177 below-grade treatment storage systems, 171–172 centralized treatment plants, 175–177 constructed wetlands, 172–175 modified partial exfiltration trench reactors, 167–169 partial exfiltration systems, 167 treatment trains, 170–171 effectiveness of single unit operation in situ control BMPs, 151 filtration systems, 156–164 filter strips, 158–160 porous pavement, 162–164 sand filters, 160–162 vegetated swales, 157–158 physical, chemical, and biological unit operations and processes for stormwater, 145–147 adsorption, 146 biological processes, 147 disinfection, 147 filtration, 146 ion exchange, 147 precipitation, 146–147 sedimentation and settling, 145–146 historical development of controls or best management practices, 147–151 best management practices, 147–149 development of BMPs in Europe, Japan, and Australia, 150–151 development of BMPs in United States, 149–150 infrastructure modification and appurtenances, 164–166 oil and water separators, 165–166 vortex/swirl separators, 165 settling basins, 151–156 dry detention basins, 151–154 wet retention ponds, 154–156 Stormwater runoff pollutants, physical and chemical nature of urban, 43–65 heavy metal partitioning, 53–56 influences on hydrologic cycle by built urban environment, 43–47 acidic deposition and urban environment, 45–46 impervious surfaces and hydrologic cycle, 44 modification to runoff hydrograph by urban environment, 44 transport of particulate matter due to urban anthropogenic activities, 45 urban environment as heat island for climate modification, 46–47 physical and chemical characteristics of stormwater particulate matter, 56–64 measurement of stormwater particulate matter, 57–63

381

treatment design implications for particulates and particulate heavy metals, 63–64 role of urban environmental hydrology, 51–53 sources and magnitude of anthropogenic urban constituent loadings, 47–50 Stream(s) A1, 350 A2, 350 B1, 351 B2, 351 banks, unstable, 106 buffer retention, 276 C1, 352 C2, 352 C3, 353 C4, 353, 357 channel(s) formation of, 327 patterns, 325 classification, 347, 348 D3, 354 D4, 354 destabilized, 362 discharge, 317 E4, 355 F3, 355 F4, 356 G, 355 G4, 356 habitat, impacts of WWF toxic pollutants on, 233 headwater, 323, 324 hydraulic geometry of, 324 quality, impervious cover vs., 334 restoration techniques, 359 slope, 317 Stream protection, geomorphic considerations in, 315–341 bankfull discharge, 318–319 biologic impacts, 333–334 receiving water habitat, 333–334 temperature, 334 flood frequency determination, 321–323 future directions and innovation in control technology, 338–339 hydraulic geometry, 323–325 hydrologic impacts of urbanization, 326–333 channel stability and degradation, 331–332 groundwater recharge, 332–333 hydrologic regime, 327–331 physical impacts, 327 overview, 317–318 stream channel patterns, 325–326 traditional control approaches, 335–338 floodplain zoning, 335 peak discharge control, 335 peak discharge strategies, 335–338 water quality control, 335 use of USGS gauging stations, 319–321 Stream restoration, geomorphic considerations in, 343–368 applications of Rosgen classification system, 348–359 aggradation/degradation or vertical instability, 358 erosion rates, 358

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factors that lead to instability, 357–358 patterns of adjustment and disequilibriun, 352–357 stability/instability, 350 state assessment and departure, 358–359 case studies, 362–367 elements of geomorphology and geomorphic features, 344–345 field procedures, 345–346 bankfull stage and slope, 345 bankfull width and depth, 345 particle size distribution, 346 sinuosity, 346 sediment, 346–347 stream classification, 347–348 stream restoration techniques, 359–362 channel geometry, 359–360 structures, 360–362 Street cleaning, 79, 80, 83, 95, 97, 136 effects, 82 mechanical, 200 methods, 200 operations, for traffic safety, 120 performance, factors affecting, 117 pollutant removal benefits of, 119 primary role of, 119 productivity, 118 Student’s t-test, 179 Surface area (SA), 57 depression storage, rainfall depth required to satisfy, 2 overflow rates, 152 roughness, 284 wetting, rainfall depth required to satisfy, 2 Suspended growth processes, 243 Suspended solids (SS), 9, 85, 90, 125, 226 control of, 127 loading capacities, 128 monitoring, 27 reduction benefits, 98 removal achievement of, 239 CSO-DMHRF average, 29 SLAMM results for, 96 wastewater, 190 Swale(s), 14 design, 303 infiltration, 285 operation rationale of vegetated grass, 156 performance, criteria for optimum vegetated, 158 pollutant removal mechanisms, 157 wet, 285 wider, 305 Swirl combined sewer overflow regulator/separator, isometric view of, 36 Swirl regulators/concentrators, 35 SWM, see Stormwater management SWMM, see U.S. Environmental Protection Agency Storm Water Management Model

T TCLP, see Toxicity characteristics leaching procedure Technical Council of Camp Dresser & McKee (CDM), 69 Test(s) Birmingham pilot-scale, 134 controlled street surface washoff, 90 laboratory bench-scale treatability, 133 Microtox screening, 134 probability plot correlation coefficient, 73 Shapiro–Wilk, 73 Student’s t-, 179 Texas Water Development Board (TWDB), 258 Time of concentration, 283, 299 development of, 307 predevelopment, 301, 304 TKN, see Total Kjeldahl nitrogen Toilet, use of in developed world, 198 Topography, influence of on pollutant mixing, 144 Total dissolved solids, 267 Total Kjeldahl nitrogen (TKN), 70, 74, 124 Total petroleum hydrocarbons (TPHs), 228 Total phosphorus (TP), 70, 244 Total suspended solids (TSS), 45, 70, 74 inverse trend in, 54 removal efficiency, 125, 245 Toxicant source areas, 20 Toxicity characteristics leaching procedure (TCLP), 153 Microtox, 135 reduction, 133 Toxic substances, household, 120 Toxic waste management, 11, 13 TP, see Total phosphorus TPHs, see Total petroleum hydrocarbons Traffic safety, 120, 218 Transition zones, creation of, 301 Transportation land uses, stormwater from, 50 Trap filling rates, estimation of, 213 Treatability tests, laboratory bench-scale, 133 Treatment -based controls, 148 holding times, median toxicity reductions for different, 133 lagoons, pollutant removal efficiencies by, 244 storage systems, below-grade, 171 Tree preservation, 305 Tributary 9 restoration, 367 Trickling filter unit, 245 Tri-halomethanes, 33 TSS, see Total suspended solids TWDB, see Texas Water Development Board Type A sewer sediment, 192, 193

U UHP helium, see Ultrahigh pure helium Ultrahigh pure (UHP) helium, 61 Ultraviolet (UV) light irradiation, 34, 147 Underground treatment systems, 277

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Index

383

Unit hydrograph, 260 Upstream surveys, 123 Urban areas, depressional storage in, 44 Urban drainage candidate scenarios for, 106 objectives, 105 Urbanization, hydrologic impacts of, 326, 327, 329 Urban pavement surface, 52 Urban retrofit costs, 282 Urban runoff management objectives, 288 Urban storm runoff pollution, stormwater management to control, 4 U.S. Environmental Protection Agency (U.S. EPA), 67, 272 National CSO Control Policy, 237 Nationwide Urban Runoff Program (NURP), 67, 80, 118 data, 69 EMC estimates, 76 EMC values, 71 indicator chemical constituents, 69 QA/QC program, 180 Rapid Bioassessment Habitat Protocol, 358–359 Storm Water Management Model (SWMM), 3, 231, 260 -supported nationwide survey, 257 swirl concentrator, 35 U.S. EPA, see U.S. Environmental Protection Agency U.S. Geological Survey (USGS), 68 gauging stations, 319, 320, 321 National Urban Storm Runoff Database, 70 USGS, see U.S. Geological Survey UV light irradiation, see Ultraviolet light irradiation

V Vacuum tankers, 201 Valley slope, 360 Vanes, 360 VDS, see Vehicles during storm Vegetated grass swales, operation rationale of, 156 Vegetated submerged bed (VSB), 173 Vegetation, preservation of existing natural, 301 Vegetative cover, preferred, 158 Vegetative stabilization, 357 Vehicle(s) during storm (VDS), 48 statistics for average, 48 Viruses, human enteric, 232 Volatile organic compounds, 195 Volatile suspended solids, 135, 168 Volatilization, 174 Vortex rock weirs, 362, 364 Vortex/swirl separators, 165 VSB, see Vegetated submerged bed

W Washload, 346 Waste controlled, 201

hazardous, 156 products, automobile maintenance and operation, 120 Wastewater collector of, 187 concentration of oil, 116 disposal, 258 laundry, 120 physical treatment units applied to municipal, 176 sources, sanitary, 229 stream, treatment scenario for, 143 total load in, 198 treatment facility, 175 treatment plant (WWTP), 23 design average flow rate, 238 impact of solids on, 195 improvements, 233 performance efficiency of, 196 pumpback of stormwater to, 23 sludge at, 215 threats to receiving water quality by discharges from, 226 treatment train mode of operation, 238 Water city, 266 closet (WC), 198, 216 consumption charge rate, 269 demands, 257, 266 pollution abatement, 266 quality classification levels, 264 criteria, 275, 338 exfiltration systems, 163 measure of, 334 quality control, 299, 335 storage volume required for, 309 volume (WQCV), 154 – solid interface, stresses at, 192 systems, urban basin-wide complete, 263 table conditions, 21 Watershed(s) challenge in protecting urban, 226 changes in hydrologic cycle of, 277 imperviousness, 333 instantaneous peak discharge rate from, 331 plan, 5, 289 variable contributing areas in urban, 85 WC, see Water closet Wet deposition aquasols, 45 Wet detention ponds, 79, 95, 97 disinfection in, 130 outfall, 107 use of for controlling runoff, 107 Wetland(s) channel, development of, 14 constructed, 147, 172, 286 maintenance requirements for, 175 pollutant removal mechanisms, 174 pocket, 8 stormwater, 14 systems, constructed, 15 Wet retention basins, nutrient removal by, 156

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Wet retention ponds cost-effectiveness of, 169 nutrient removal efficiencies of, 154 Wet swale, 285 Wet-weather flow (WWF), 25, 226 biosolids, ultimate disposal of, 247 control treatment, benefit–cost of, 237 intermittent, 238 solids, management of, 246 zeta potential of colloids in urban, 242 Wet weather flow solids, management of, 225–256 management of WWF solids, 246–251 characteristics and implication, 246–247 treatment processes, 247–251 regulatory background, 226–227 sewer sediment solids origin, impact, and control, 227–236 control methods, 234–236 impact of sewer sediment solids, 231–234 sewer sediment solids origin, 227–229 sewer solids loading and transport, 229–231 treatment of combined sewer overflow, 236–245 background, 236–237 high-rate treatment processes, 237–238 physical treatment, 239–245 Wet weather flow in urban watershed, management of, 1–41 beneficial reuse of stormwater, 36–37 end-of-pipe treatment, 25–35 biological treatment, 25 physical/chemical treatment, 25–35 use of existing treatment facilities, 25 general approach and strategy, 2–6 small storm hydrology, 2–3 strategy, 3–6 installed drainage system, 17–25

catchbasin cleaning, 18–19 critical source area treatment devices, 19–21 illicit or inappropriate cross-connections, 18 infiltration, 21 in-line storage, 22–23 maintenance, 25 off-line storage, 23–24 source treatment, flow attenuation, and storm runoff infiltration, 14–17 detention facilities, 15–16 infiltration practices, 16–17 vegetative BMPs, 14–15 storage and treatment optimization, 35–36 watershed area technologies and practices, 6–14 regulations, local ordinances, and public education, 10–11 source control of pollutants, 11–14 Wolman pebble count method, 344 Woodlands, 284 Wood preservatives, 46 Woods in good condition, 313 WQCV, see Water quality control volume WWF, see Wet-weather flow WWTP, see Wastewater treatment plant

Z Zinc coatings, 49 contamination, 132 extractable, 79