Japan's Beach Erosion: Reality and Future Measures (Advanced Series on Ocean Engineering)

JAPAN’S BEACH EROSION Reality and Future Measures

7332 tp.indd 1

5/24/10 5:29 PM

ADVANCED SERIES ON OCEAN ENGINEERING Series Editor-in-Chief Philip L- F Liu (Cornell University) Published Vol. 20 The Theory and Practice of Hydrodynamics and Vibration by Subrata K. Chakrabarti (Offshore Structure Analysis, Inc., Illinois, USA) Vol. 21 Waves and Wave Forces on Coastal and Ocean Structures by Robert T. Hudspeth (Oregon State Univ., USA) Vol. 22 The Dynamics of Marine Craft: Maneuvering and Seakeeping by Edward M. Lewandowski (Computer Sciences Corporation, USA) Vol. 23 Theory and Applications of Ocean Surface Waves Part 1: Linear Aspects Part 2: Nonlinear Aspects by Chiang C. Mei (Massachusetts Inst. of Technology, USA), Michael Stiassnie (Technion–Israel Inst. of Technology, Israel) and Dick K. P. Yue (Massachusetts Inst. of Technology, USA) Vol. 24 Introduction to Nearshore Hydrodynamics by Ib A. Svendsen (Univ. of Delaware, USA) Vol. 25 Dynamics of Coastal Systems by Job Dronkers (Rijkswaterstaat, The Netherlands) Vol. 26 Hydrodynamics Around Cylindrical Structures (Revised Edition) by B. Mutlu Sumer and Jørgen Fredsøe (Technical Univ. of Denmark, Denmark) Vol. 27 Nonlinear Waves and Offshore Structures by Cheung Hun Kim (Texas A&M Univ., USA) Vol. 28 Coastal Processes: Concepts in Coastal Engineering and Their Applications to Multifarious Environments by Tomoya Shibayama (Yokohama National Univ., Japan) Vol. 29 Coastal and Estuarine Processes by Peter Nielsen (The Univ. of Queensland, Australia) Vol. 30 Introduction to Coastal Engineering and Management (2nd Edition) by J. William Kamphuis (Queen’s Univ., Canada) Vol. 31 Japan’s Beach Erosion: Reality and Future Measures by Takaaki Uda (Public Works Research Center, Japan) Vol. 32 Tsunami: To Survive from Tsunami by Susumu Murata (Coastal Development Inst. of Technology, Japan), Fumihiko Imamura (Tohoku Univ., Japan), Kazumasa Katoh (Musashi Inst. of Technology, Japan), Yoshiaki Kawata (Kyoto Univ., Japan), Shigeo Takahashi (Port and Airport Research Inst., Japan) and Tomotsuka Takayama (Kyoto Univ., Japan) Vol. 33 Random Seas and Design of Maritime Structures, 3rd Edition by Yoshimi Goda (Yokohama National University, Japan)

*For the complete list of titles in this series, please write to the Publisher.

YHwa - Japan's Beach Erosion.pmd

2

8/2/2010, 6:16 PM

Advanced Series on Ocean Engineering — Volume 31

JAPAN’S BEACH EROSION Reality and Future Measures

Takaaki Uda

Public Works Research Center, Japan

World Scientific NEW JERSEY

7332 tp.indd 2

•

LONDON

•

SINGAPORE

•

BEIJING

•

SHANGHAI

•

HONG KONG

•

TA I P E I

•

CHENNAI

5/24/10 5:29 PM

Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

JAPAN’S BEACH EROSION: REALITY AND FUTURE MEASURES Advanced Series on Ocean Engineering — Vol. 31 Copyright © 2010 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher.

ISBN-13 978-981-4277-12-9 ISBN-10 981-4277-12-6

Typeset by Stallion Press Email: [email protected]

Printed in Singapore.

YHwa - Japan's Beach Erosion.pmd

1

8/2/2010, 6:16 PM

May 26, 2010

13:35

9in x 6in

b807-fm

PREFACE

In recent years, beach erosion has become severe, and beaches with white sand and pine trees, characteristic of the natural coast of Japan, are rapidly disappearing. When visiting a coast, of which we have many fond memories, after a long interval, we find that much of it has been covered with concrete blocks, and the coast we remember is gone. However, even if an ordinary person thinks about why this happened, he does not know who to ask for the answer, and even if he asks the administrator of the coast for the reason, he cannot always receive a satisfactory reply. The beach erosion problem, to be sure, can be classified as a sophisticated and difficult issue from the scientific point of view. The coast is the common property of the people, and therefore, everyone has a right to know why the coast has changed the way it has. The coastal areas suffering severe beach erosion are not limited to only some regions, and there are a number of eroding coasts in Japan. Taking this condition seriously into account, beach erosion is not a problem only for engineers, but a problem to be tackled by all people. First, from among the number of examples of worsening beach erosion nationwide, some typical examples are selected and classified into several patterns. Then, the details of these classified examples are described. Second, concerning the possibility of predicting topographic changes, several predictive models, which have mainly been developed by this author and co-workers and are useful for applications in practical engineering, are introduced along with their results. These discussions may not differ from ordinary discussions and lack originality, except for the inclusion of many actual examples. When we investigate the issue of beach erosion in depth, we soon arrive at a very important conclusion. Apart from the superficial technological discussion, almost all beach erosion originates from the v

May 26, 2010

vi

13:35

9in x 6in

b807-fm

Japan’s Beach Erosion: Reality and Future Measures

many anthropogenic activities that we Japanese have carried out extensively throughout our coastal zone during the last 30 to 40 years.Accordingly, even if measures are taken to solve the superficial problems, the basic problems will not be solved at all, and actually many become worse. Under the strong sector-by-sector administration system in Japan, the administration tends to think that even if some scientific problem proves to be an obstacle to a once-determined plan, they should stick obstinately to the previous decision, instead of considering that fundamental issues must be learned from events that have occurred, and the points to be reflected upon must be recognized to prevent future recurrences of problems. This behavior has been a sturdy barrier in solving fundamental issues. Here, we consider these fundamental issues to really improve the condition of the coast in Japan. Many administrators involved with coastal work must be directed toward the common goal, although they will each take charge of their own work. For this purpose, information must be made available to the public to change the sector-by-sector system and to accelerate cooperative work among several sectors. In the text, a number of figures and photographs have been used and detailed discussion was avoided, while keeping clarity in mind. Takaaki Uda 3 October 2004, in Japanese edition 3 March 2010, in English edition Executive Director Public Works Research Center and Visiting Professor Department of Oceanic Architecture and Engineering College of Science and Technology Nihon University

May 26, 2010

13:35

9in x 6in

b807-fm

CONTENTS

Preface

v

1. What is Beach Erosion?

1

2.

Beach Erosion — Current Reality

7

2.1. 2.2.

7

Classification of Causes of Beach Erosion . . . . . . . Beach Erosion Due to Obstruction of Longshore Sand Transport . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Shibetsu Coast in Eastern Hokkaido . . . . . . . 2.2.2. Monbetsu Coast in Hidaka in Hokkaido . . . . . 2.2.3. Misawa Fishing Port in Aomori Prefecture . . . 2.2.4. Momosaki-hama Coast in Niigata Prefecture . . 2.2.5. Shinkawa Fishing Port in Niigata Prefecture . . 2.2.6. Fuji Coast in Shizuoka Prefecture . . . . . . . . 2.2.7. Fukude Fishing Port in Shizuoka Prefecture . . 2.2.8. Imazu-sakano Coast in Tokushima Prefecture . . 2.2.9. Method of Addressing Issues . . . . . . . . . . 2.3. Beach Erosion Triggered by Construction of Wave-Sheltering Structures . . . . . . . . . . . . . . . 2.3.1. Teradomari and Nozumi Coasts in Niigata Prefecture . . . . . . . . . . . . . . . . . . . . 2.3.2. Kashiwazaki Port and Arahama Coast in Niigata Prefecture . . . . . . . . . . . . . . . . . . . . 2.3.3. Ohtsu Fishing Port in Ibaraki Prefecture . . . . 2.3.4. Ajigaura Beach and Naka Coast in Ibaraki Prefecture . . . . . . . . . . . . . . . . . . . . 2.3.5. Kemigawa Beach in Chiba Prefecture . . . . . . vii

13 14 19 23 27 39 54 61 64 68 73 73 81 93 100 109

May 26, 2010

viii

13:35

9in x 6in

b807-fm

Japan’s Beach Erosion: Reality and Future Measures

2.3.6. Tojo–Maebara Coast in Kamogawa City in Chiba Prefecture . . . . . . . . . . . . . . . . . . . . 2.3.7. Shimobara Fishing Port in Tateyama City in Chiba Prefecture . . . . . . . . . . . . . . . . . 2.3.8. Asamogawa Coast in Kyoto Prefecture . . . . . 2.3.9. Tsutsuki Beach on Iki Island in Nagasaki Prefecture . . . . . . . . . . . . . . . . . . . . 2.3.10. Shiratsuru Beach in Amakusa District in Kumamoto Prefecture . . . . . . . . . . . . . . 2.3.11. Kashiwabara Coast in Kagoshima Prefecture . . 2.3.12. Methods of Addressing Issues . . . . . . . . . . 2.4. Beach Erosion Due to Decrease in Fluvial Sediment Supply . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Shizuoka and Shimizu Coasts in Shizuoka Prefecture . . . . . . . . . . . . . . . . . . . . 2.4.2. Suruga Coast in Shizuoka Prefecture . . . . . . 2.4.3. Tenryu River and Enshu-nada Coast in Shizuoka Prefecture . . . . . . . . . . . . . . . . . . . . 2.4.4. Methods of Addressing Issues . . . . . . . . . . 2.5. Beach Erosion Triggered by Offshore Sand Mining/Dredging . . . . . . . . . . . . . . . . . . . . 2.5.1. Mouth of Sagami River in Kanagawa Prefecture . . . . . . . . . . . . . . . . . . . . 2.5.2. Off Mouth of Niyodo River in Kochi Prefecture . . . . . . . . . . . . . . . . . . . . 2.5.3. Off Sumiyoshi-hama Sand Spit in Oita Prefecture . . . . . . . . . . . . . . . . . . . . 2.5.4. Methods of Addressing Issues . . . . . . . . . . 2.6. Beach Erosion Triggered by Construction of Detached Breakwater as Countermeasure . . . . . . . . . . . . . 2.6.1. Ariake–Takahama Coast in Ibaraki Prefecture . 2.6.2. Ghotsu Coast in Shimane Prefecture . . . . . . 2.6.3. Methods of Addressing Issues . . . . . . . . . .

117 121 130 133 140 151 155 162 162 184 191 195 198 198 200 208 213 215 216 230 234

May 26, 2010

13:35

9in x 6in

b807-fm

Contents

Disappearance of Natural Sand Dunes Due to Excess Planting of Coastal Forest . . . . . . . . . . . . . . . 2.7.1. Nakamura-hama Coast in Niigata Prefecture . 2.7.2. Node Coast in Chiba Prefecture . . . . . . . . 2.7.3. Southern Kujukuri Coast in Chiba Prefecture . 2.7.4. Heisa-ura Coast in Chiba Prefecture . . . . . . 2.7.5. Methods of Addressing Issues . . . . . . . . . 2.8. Disappearance of Sandy Beach Triggered by Construction of Gently Sloping Revetment . . . . . . 2.8.1. Isewan-seinan Coast in Mie Prefeture . . . . . 2.8.2. Kitanowaki Coast in Tokushima Prefecture . . 2.8.3. Uchihama Coast on Miyako Island in Okinawa 2.8.4. Methods of Addressing Issues . . . . . . . . .

ix

2.7.

3.

. . . . . .

236 236 249 262 269 278

. . . . .

283 284 291 296 301

Practical Models for Predicting Beach Changes

309

3.1. 3.2.

309 311 311 312

Characteristics of Practical Models . . . . . . . . . . . Prediction of Stable Shoreline on Pocket Beach . . . . 3.2.1. Predictive Model . . . . . . . . . . . . . . . . . 3.2.2. Example . . . . . . . . . . . . . . . . . . . . . 3.3. Three-Dimensional Model for Predicting Beach Changes Using Hsu and Evans’ Model . . . . . . . . . . . . . . 3.3.1. Predictive Model . . . . . . . . . . . . . . . . . 3.3.2. Numerical Calculation Procedure . . . . . . . . 3.3.3. Example . . . . . . . . . . . . . . . . . . . . . 3.4. Predictive Model of Three-Dimensional Beach Changes on Coast With a Seawall by Expanding Hsu and Evans’ Model . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1. Predictive Model . . . . . . . . . . . . . . . . . 3.4.2. Example . . . . . . . . . . . . . . . . . . . . . 3.5. Simple Model for Predicting Three-Dimensional Beach Changes on Statically Stable Beach . . . . . . . . . . . 3.5.1. Predictive Model . . . . . . . . . . . . . . . . . 3.5.2. Example . . . . . . . . . . . . . . . . . . . . .

315 315 317 318

327 327 329 331 331 336

May 26, 2010

13:35

x

9in x 6in

b807-fm

Japan’s Beach Erosion: Reality and Future Measures

3.6.

4.

Shoreline Change Model on Coasts Composed of Sand of Mixed Grain Size . . . . . . . . . . . . . . . . . . . 3.6.1. Predictive Model . . . . . . . . . . . . . . . . . 3.6.2. Example . . . . . . . . . . . . . . . . . . . . . 3.7. Predictive Model of Shoreline and Grain Size Around River Mouth . . . . . . . . . . . . . . . . . . . . . . . 3.7.1. Predictive Model . . . . . . . . . . . . . . . . . 3.7.2. Example . . . . . . . . . . . . . . . . . . . . . 3.8. Contour-Line Change Model Considering Stabilization Mechanism of Longitudinal Profile . . . . . . . . . . . 3.8.1. Calculation Method . . . . . . . . . . . . . . . 3.8.2. Comparison of Results of Experiments and Numerical Simulations . . . . . . . . . . . . . 3.9. Contour-Line Change Model Solved on x−y Meshes . 3.9.1. Predictive Model . . . . . . . . . . . . . . . . . 3.9.2. Examples . . . . . . . . . . . . . . . . . . . . .

375 386 386 393

Beach Erosion as Structural Problem

405

4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4.2. Institutional (legal) Issues Related to Beach Erosion . . 4.2.1. Occurrence of Issues at Boundaries Between Shore Protection, Port, and Fishing Port Areas . 4.2.2. Relationship Between Dredging Operations and Beach Erosion . . . . . . . . . . . . . . . . . . 4.2.3. Issues Arising from Conceptual Differences in Land Management by Coastal Act and Forest Law 4.2.4. Issues Related to Method by Which Public Sectors Expend Their Budgets . . . . . . . . . . 4.2.5. Work of Restoring Damaged Coast . . . . . . . 4.2.6. System of Administration and Difficulty of Training of Specialists . . . . . . . . . . . . . . 4.3. Technical Issues Related to Beach Erosion . . . . . . . 4.4. Concrete Measures . . . . . . . . . . . . . . . . . . . .

405 406

Index

342 342 347 352 352 354 363 363

406 408 409 410 411 412 412 413 417

May 26, 2010

13:35

9in x 6in

b807-ch01

Chapter 1 WHAT IS BEACH EROSION?

Beach erosion is a phenomenon in which land is worn away and disappears due to wave action. Generally, coasts exposed to open ocean are separated into sandy beaches and rocky coasts. The eroding velocity of the rocky coast with its large compressive strength is extremely small compared with that of the sandy beach. From an engineering point of view, erosion of the rocky coast may be neglected, except when unconsolidated strata of the sea cliffs are eroded. On the other hand, one of the characteristics of the sandy beach is that rapid response can take place in reaction to changes in wave field and sand supply, because the sandy beach is composed of a collection of grains of sand, the positions of which can be freely changed depending on wave action. Regarding the transport of sand, various studies have been carried out over many years, and still the study on the movement of sand is one of the main themes in coastal engineering currently. In this document, beach changes with these characteristics and resultant beach erosion are described. When visiting sandy beaches, which once had a wide foreshore, at various places in Japan after a long absence, we often find the situation that the beach has been rapidly eroded and the beach we remember has been totally lost without a trace. In addition, when visiting some coasts for which we have fond memories, we often hear that the coastline had been covered with a high seawall and concrete armor units. In this case, as for the reason beach was severely eroded, it is impassively explained that it is only due to the attack of high storm waves, but this reasoning is often difficult for us to understand. For example, as in the reason for the formation of a high scarp, as shown in Fig. 1.1, the explanation that “it is due to high storm waves and the eroded sand was transported offshore” has been offered. Many specialists and researchers provide the same explanation. Undoubtedly, high waves could have attacked the coast when the scarp was formed, but in many 1

May 26, 2010

2

13:35

9in x 6in

b807-ch01

Japan’s Beach Erosion: Reality and Future Measures

Fig. 1.1. High scarp formed north of Misawa fishing port (12 June 1998).

cases, the disappearance of a sandy beach also depends heavily on other factors. If this line of reasoning was correct, then the sandy beaches throughout Japan and the world would be destined to rapidly disappear, and there would be no way that we could build up impregnable defenses against waves so that waves could no longer reach the land. However, there are no beaches that have disappeared for this reason in all the examples of eroded coasts that were studied. The sea reached its present level around 8,000 years ago in Japan. Since then, high waves would have repeatedly occurred, whereas severe beach erosion took place in the last 20–30 years at most in Japan. Comparing the wave actions during this period and the geologically long period of several thousand years, the disappearance of the sandy beach is too abrupt. One researcher suggests that this is due to the rise in sea level. However, if beach erosion is triggered by the rise in sea level, such an event must

May 26, 2010

13:35

9in x 6in

What is Beach Erosion?

b807-ch01

3

happen uniformly all over the country and the world, but there are a number of beaches with the same wide foreshore as in the past. Taking these points into consideration, the man-made effects must inevitably be studied in considering beach erosion in Japan. A number of construction works have been carried out in the last several decades nationwide during the period when beach erosion has become severe. In rivers carrying sediment to the coast, riverbed excavation for sand mining or dam construction has been carried out, resulting in the obstruction of the continuous movement of sand in the river course. At present, the number of large dams with a height over 15 m is 2,532 in Japan (http://wwwsoc. nii.ac.jp/jdf/Dambinran/binran/Top). Sediment flowing into the sea at the mouth of a river is transported alongshore, but such continuous movement may also be obstructed by the breakwaters of ports and fishing ports, or river-mouth jetties. The construction and maintenance of these structures have been carried out independently by the Fishery Agency of the Ministry of Agriculture and Forestry, the Port Bureau of the Ministry of Land, Infrastructure and Transport, and the River Bureau of the same Ministry, on the basis of the Fishing Port Act, Port Act and River Law, respectively. The number of fishing and other ports are 2,931 (http://www.pref.hokkaido.jp/ srinmu/sr-ggson/contents/gyoko/sub2.htm) and 1,079 (http://www.mlit.go. jp/kowan/), respectively, at present. This means that they are distributed with one port per 8.5 km nationwide, since the total length of the coastline in Japan is 34,000 km. In each case, various problems have resulted. Similarly, countermeasures against cliff erosion have been taken along the sea cliffs, which are one of the supply sources of sediment to coasts, resulting in a decrease in sand supply. Furthermore, a long offshore breakwater was extended to form the wave-shelter zone. A large amount of sand accumulated on the lee side of the wave-sheltering structure associated with the elongation of the breakwater, and the beach was eroded in the neighboring area. Of course, these public works have been carried out to produce good results on the basis of various laws, and it is a fact that such works supported the rapid economic growth of Japan in the past. Every construction work was carried out most efficiently, and it is obvious that they were not carried

May 26, 2010

4

13:35

9in x 6in

b807-ch01

Japan’s Beach Erosion: Reality and Future Measures

out to cause beach erosion. However, there must be no other nation that conducted such rapid and large-scale land alteration of its national land with both limited space and many steep rivers. All these works have been carried out as public works. In that case, the work itself was implemented by the sector-by-sector system under the thinking that it was vitally important to enhance the efficiency of the work, even if the influence on the surrounding coastline was overlooked. Their results were not deliberate, but consideration of the environment of the surrounding coasts was significantly lacking at that time, compared with the present era, when the importance of the coastal environment must be adequately considered. Since the information was not available to the public at that time, discussion of the negative points was taboo among the people concerned, and significant time passed before many people realized the facts. When such influences accumulated and appeared at many coasts, the coasts of Japan came to a situation in which fundamental improvements were no longer possible in every respect because of the astronomical budget required. The most feared situation is that since the number of extensively artificial coastal areas have increased, young people, who carry the future of Japan on their shoulders, seem to pay no attention and have no interest in the nature of the coast or the coastal environment. The author still has memories of natural coasts in the decades before development. However, with the decrease in the number of people in this generation with such memories of natural coasts, it becomes difficult to convey the facts to young people. Remembering the original, past scenery of the coast will become totally impossible, even if people want to recover it. In considering the future perspective on the coasts of Japan, first we must clearly realize why Japan’s coasts arrived in their current condition. In this document, the real situation of the coasts in Japan is described from this point of view. Next, to prevent the recurrence of beach erosion, predicting the topographic changes of the coast, where many kinds of artificial alterations are carried out, is required. Therefore, some practical

May 26, 2010

13:35

9in x 6in

What is Beach Erosion?

b807-ch01

5

models for predicting beach changes are introduced, which have been developed to simulate real topographic changes occurring on coasts. In the process of investigating the reality of beach erosion deeply, it has been noted that the barrier for solving the erosion problem does not exist primarily as a scientific problem that cannot be solved, but rather in the present land management system that includes the coastal zone. Although this is a considerably difficult problem to tackle, it is unavoidable, if a true solution is to be achieved. Therefore, we discuss these fundamental problems related to the land management system. In Chap. 2, beach erosion is classified into several categories depending on its characteristics, and the current reality of erosion is discussed. In this case, we avoid a detailed analysis of the bathymetric survey data and show instead the effectiveness of the simple method by combining the comparison of aerial photographs and field observations at the eroded coasts, collected while walking along the coastline with a measuring stick. The past aerial photographs of all coasts of Japan taken between 1947 and 2000 are available from the Geographical Survey Institute (http://www.gsi.go.jp) for free, or at a cost of about $10 each for the recent aerial photographs. The long-term and large-scale shoreline changes can be investigated quantitatively using these aerial photographs, and in addition to this, if field observations are carried out, the causes of beach erosion on almost all coasts can be clearly identified. Anyone can use this method, and comparatively reasonable results can be obtained. In what follows, examples of many coasts are described from this point of view. In Chap. 3, some practical models for predicting beach changes, which are needed to suggest countermeasures against beach erosion, are introduced for several types of beach erosion as mentioned in Chap. 2. In Chap. 4, we show for the beach erosion problems, many examples of which were given in Chap. 2, that structural problems related to the social system in Japan must be considered in order to really address the situation, apart from which a perspective on the future cannot be obtained. Finally, a way to avoid the occurrence of the same kind of beach erosion problems in other countries is discussed on the basis of the Japanese experience as an

May 26, 2010

6

13:35

9in x 6in

b807-ch01

Japan’s Beach Erosion: Reality and Future Measures

example of behavior that should be avoided in conducting shore protection projects. Finally, for the convenience of the readers, English and Japanese references are shown separately at the end of each section, and Japanese articles are denoted by an asterisk next to the reference in the text.

May 26, 2010

13:35

9in x 6in

b807-ch02

Chapter 2 BEACH EROSION — CURRENT REALITY

2.1.

CLASSIFICATION OF CAUSES OF BEACH EROSION

Beach erosion is caused by an imbalance in the sediment budget of a coast, except in the case of the ground subsidence associated with the excess pumping of ground water or a crustal change of the earth. On a coast, seasonal variation in the beaches depending on the occurrence of storms and calm waves can usually be seen, but the long-term stability of the beach is governed by longshore sand transport; longshore sand movement is produced by waves obliquely incident to the shoreline. If sand supply from rivers and sea cliffs decreases compared with the longshore sand transport of a coast, beach erosion inevitably takes place. Similarly, when sediment is artificially removed from a coast by dredging or mining, beach erosion occurs in the neighboring coast. Thus, there are many causes of beach erosion, and they relate to each other in a complicated manner. Here, seven types of beach erosion, which can be found at many places along Japan’s coast, are selected and discussed in detail with real examples (Uda, 1997∗ ). (1) (2) (3) (4) (5)

Beach erosion due to obstruction of longshore sand transport. Beach erosion triggered by construction of wave-sheltering structures. Beach erosion due to decreased fluvial sediment supply. Beach erosion triggered by offshore sand mining/dredging. Beach erosion triggered by construction of detached breakwater as countermeasure. (6) Disappearance of natural sand dunes due to excess planting of coastal forest. (7) Disappearance of sandy beach triggered by construction of gently sloping revetments. 7

May 26, 2010

8

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Of the seven types of beach erosion, types (1)–(4) have been widely discussed in many places, and there are many actual examples of types (1) and (2). Since beach erosion associated with these types is severe in Japan and there are many examples, we discuss these themes selectively. The topics of types (5)–(7) have never been discussed before in Japan and are peculiar cases especially for Japan’s coasts; they are closely related to the system of coastal conservation in Japan. In the following, their features are outlined. (1) Beach erosion due to obstruction of longshore sand transport When a breakwater, jetty, or groin is extended offshore on a coast with predominant longshore sand transport, part or all of the longshore sand transport is obstructed, as shown in Fig. 2.1.1, and erosion occurs downcoast, whereas accretion takes place upcoast. When the offshore distance of the structure protruding seaward is short, longshore sand transport is obstructed soon after the construction of the structure, but it gradually discharges downdriftward, turning around the tip of the structure after a sufficient volume of sand deposits on the updrift side of the breakwater. Simultaneously, sedimentation into the navigation channel occurs in the port. In this case, it should be noted that the eroded area gradually expands with time. (2) Beach erosion triggered by construction of wave-sheltering structures On a coast where waves are incident from the direction normal to the shoreline, longshore sand transport is induced from outside the waveshelter zone to inside in the vicinity of a large-scale port breakwater or an artificial island, as shown in Fig. 2.1.2(a), resulting in erosion outside the wave-shelter zone and accretion inside.

Fig. 2.1.1. Accretion and erosion due to obstruction of longshore sand transport.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

9

Fig. 2.1.2. Accretion and erosion associated with formation of wave-shelter zone by constructing port breakwater.

May 26, 2010

10

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

When waves are incident counterclockwise or clockwise with respect to the direction normal to the shoreline, as shown in Figs. 2.1.2(b) and 2.1.2(c), sand accumulates inside the wave-shelter zone of the oblique breakwater every time when the wave direction seasonally varies, as shown in Fig. 2.1.2(d). Deposited sand is trapped inside the wave-shelter zone, and even if the wave direction changes, sand movement from inside the wave-shelter zone to outside becomes impossible. The shoreline behind the breakwater continuously deforms responding to the change in wave direction due to the construction of the port breakwater in such a way that the shoreline at each point is normal to the wave direction. Therefore, in Fig. 2.1.2 for example, if sand deposited behind the oblique breakwater is dredged, soon after the dredging longshore sand transport from outside to inside the wave-shelter zone is induced, resulting in erosion in the adjacent area. (3) Beach erosion due to decrease in fluvial sediment supply Since many Japanese rivers originate from mountains with much sediment yield, a large amount of sediment is transported to coasts, resulting in the formation of fluvial lowlands. In this case, the shoreline in the vicinity of the mouth of a river advances due to deposition of sediment, as shown in Fig. 2.1.3. At the same time, wave action at the shoreline of the river-mouth delta generates longshore sand transport in the direction away from the river’s mouth. When the angle between the incident wave direction and the normal to the shoreline increases due to the advance of the shoreline around the river’s mouth, the longshore sand transport rate grows, resulting in

Fig. 2.1.3. Development of river-mouth delta due to deposition of sand supplied from river.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

11

shoreline advance ceasing, and attainment of the equilibrium state of the sediment budget. Inversely, when the sediment supply from a river decreases, the shoreline around the river’s mouth retreats, resulting in a decrease in the angle between the incident wave direction and the normal to the shoreline, which in turn means a decrease in longshore sand transport; a new equilibrium condition is established. In reality, beach erosion that started in the vicinity of a river’s mouth was not allowed to continue in Japan, and various countermeasures against it have been taken in the vicinity of the mouths of rivers to locations far from them. If sediment supply from the river is significantly reduced, the shoreline of the protruding river-mouth delta as shown in Fig. 2.1.3 does not keep its form, and the shoreline around the river’s mouth retreats to be normal to the wave direction. A sea cliff has the same function as a river with respect to being a source of sediment for coasts. The collapse of a sea cliff mainly due to wave action, which is composed of unconsolidated layers, produces much sediment, and sediment is supplied to the coasts. A typical example is the Byobugaura sea cliff extending north of the Kujukuri beach in Chiba Prefecture. As a result, the application of countermeasures against sea-cliff erosion has the same influence on the surrounding coasts as that in the case of a decrease in the fluvial sediment supply from rivers. (4) Beach erosion associated with offshore sand mining/dredging Sand mining in the river channel was fundamentally prohibited in 1967, since excess riverbed degradation occurred because of it. However, sand mining of the seabed has been extensively carried out, and at present it continues at many places along the coast, especially in western Japan. Sand mining can be reasonably conducted if its influence is sufficiently evaluated. Sand mining, the influence of which has been sufficiently evaluated scientifically, may be allowed, but in reality, there are many examples for which finding a solution becomes very difficult. This problem is often regarded as taboo, and beach erosion due to sand mining is often explained by other reasons. In fact, despite the fact that the sand volume of a coast decreases as a result of sand mining from the shoreline area or an offshore bed, the cause of the erosion is officially considered to be the offshore

May 26, 2010

12

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

sand transport by waves; thus inappropriate corrective measures are taken, thereby accelerating the transition of a natural coastline into an artificial one. The influence of offshore mining may reach an extensive area depending on the wave conditions, even though sand is not removed directly from the shoreline. Sand movement due to longshore sand transport occurs in a depth zone shallower than the depth of closure, hc , which is approximately equal to 10 m on well-exposed beaches. Since sand is always exchanged in this depth zone, the removal of sand from a depth zone shallower than hc leads to the same results as sand mining in the vicinity of the shoreline. (5) Beach erosion triggered by construction of detached breakwaters as countermeasure In this type of erosion, the zone to be protected is unevenly distributed along part of a long coastline, so that if shore-protection facilities, such as a detached breakwater or an artificial reef, are built to protect a location, the construction of the facilities leads to erosion along the rest of the coastline. Although the necessity of protection at a densely built-up area is acknowledged, when a wave-dissipating structure with a very high efficiency is built at a location, longshore sand transport is newly induced in the direction of the wave-shelter zone from the surrounding area, resulting in the reduction of the protection level. In other words, a trade-off issue arises in this case. The mechanism of sand movement is equivalent to that associated with the construction of wave-sheltering structures, such as an offshore breakwater, but it is distinguished from that is that the shore-protection facilities themselves cause the erosion. (6) Disappearance of natural sand dune due to excess planting of coastal forest This is a fundamental problem tracing back to the land-utilization policies in the vicinity of a shoreline. When beach erosion takes place, measures can be taken only in the shore-protection zone designated by the Coastal Act in Japan. On the other hand, in the sand dune area extending along the coastline, the planting of coastal forest to prevent wind-blown sand has been unremittingly carried out. As a result, wind-blown sand was undoubtedly

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

13

prevented by these activities, and the sand dune area was covered by pine trees, but the excess advance of the coastal forest up to the vicinity of the shoreline led to the loss of the buffer zone between the coastal forest and the shoreline, and the habitat of many kinds of vegetation and animals living there in this delicate environment disappeared. At the same time, since this buffer zone functioned as a temporal sand reservoir against seasonal variations of the shoreline, its disappearance enhanced the potential for disaster on the coast. Furthermore, the construction of earth dikes and seawalls built to guard the coastal forest excessively advanced from wind-blown sand or salinity accelerated the generation of the artificial coastline. This is a fundamental issue originating from land use in the coastal zone. (7) Disappearance of sandy beach triggered by construction of gently sloping revetment This is a relatively new problem occurring frequently in the last decade. Gently sloping revetments are built on coasts with a narrow foreshore as a protective measure, and as a result, the foreshore is buried by the long slope of the gently sloping revetment. Originally, these structures had to be built to “create access to the shoreline and enhance recreational usage of the beach,” but the unnatural construction of the gently sloping revetment on a coast with a narrow foreshore produced a highly artificial coastline in place of a natural coastline.

REFERENCE (in Japanese) Uda, T. (1997∗ ). Beach Erosion in Japan (Sankaido Press, Tokyo) p. 442.

2.2.

BEACH EROSION DUE TO OBSTRUCTION OF LONGSHORE SAND TRANSPORT

When predominant longshore sand transport is obstructed by a port breakwater or a jetty extending normal to the shoreline, sand accumulates upcoast of the structure, whereas downcoast the beach is eroded, resulting in shoreline recession. There are many examples of beach changes of this

May 26, 2010

14

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.1. Locations of eight typical coasts, where erosion due to obstruction of predominant longshore sand transport has taken place.

type in Japan, eight of which are shown here to illustrate the phenomena. For convenience, the coasts are shown in order from north to south. Figure 2.2.1 shows the locations of the eight coasts.

2.2.1.

Shibetsu Coast in Eastern Hokkaido

The Shibetsu coast faces the Nemuro Strait in eastern Hokkaido, as shown in Fig. 2.2.2, and is located north of the Notsukezaki sand spit, being of the largest scale and classified as a compound spit. In the vicinity of this coast,

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

15

Fig. 2.2.2. Location of Shibetsu coast in eastern Hokkaido.

southward longshore sand transport prevails, since incident waves from the northeast via the Nemuro Strait, which works as a channel because of its great depth, are predominant. The Notsukezaki sand spit was formed by southward longshore sand transport, which carries a large amount of sand supplied from the northern sea cliffs and rivers. Since the breakwaters of a fishing port were extended along the coast, where predominant longshore sand transport prevails, the upcoast shoreline advanced, whereas the shoreline downcoast of the fishing port retreated. The eroded area has now extended over the entire zone of the Notsukezaki sand spit (Uda et al., 1991a∗ ).

May 26, 2010

16

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.3. Alignment of survey lines on Shibetsu coast and shoreline changes.

The shoreline positions were read at 150 m intervals from aerial photographs taken between 1947 and 1985. Figure 2.2.3 shows the shoreline changes during this period with reference to the shoreline position in 1947. On the basis of this comparison, in a region between No. 0 and No. 3 where the mouth of Churui River is located, the shoreline protrudes on average

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

17

in spite of the large variation of the shoreline position. This implies that a river mouth delta was formed due to the fluvial sand supply of this river during floods, whereas a storage space was formed at the river’s mouth, but occasionally sand was carried away to a significant extent due to longshore sand transport. In the area between No. 45 and No. 49 including the Shibetsu fishing port, the shoreline advanced 50–100 m compared with that in 1947, and in particular, the shoreline advance between 1965 and 1978 was large. This occurred because southward longshore sand transport was interrupted by the north breakwater of the Shibetsu fishing port during the period between 1962 and 1972. In contrast, in the adjacent area south of this fishing port, large shoreline recession can be seen, and in particular, the maximum shoreline recession reached 97 m in 1947 in the area between No. 50 and No. 59. Similarly, the shoreline south of No. 60 started to retreat in 1970, and the area of shoreline recession was gradually extended. However, the shoreline reached a stable form in 1978 in the zone between No. 53 and No. 78, and in 1983 in the zone between No. 70 and No. 80. These stable forms arise because the shoreline recession was completely stopped artificially by the construction of the seawall, resulting in the complete disappearance of the sandy beach. Figure 2.2.4 shows an aerial photograph of the vicinity of the Shibetsu fishing port, taken in July 1985. A wide accretion zone can be seen north of the fishing port, whereas on the south side of the fishing port, there was no foreshore and the coastline was protected by a seawall and many T-shaped groins. The contrast of the coastline with respect to the fishing port is very clear. Figure 2.2.5 shows the bathymetry in December 1987 of the area between No. 47 and No. 61, where the largest shoreline changes were observed around the Shibetsu fishing port. An oblique detached breakwater was built off the north breakwater to prevent sand deposition inside the port, and sand accumulated in an area between the north breakwater and this detached breakwater, resulting in the formation of a tombolo. In the

May 26, 2010

18

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.4. Aerial photograph showing Shibetsu fishing port taken in July 1985.

Fig. 2.2.5. Bathymetry around Shibetsu fishing port measured in December 1987.

vicinity of this detached breakwater, the contour lines shallower than −4 m depth extended approximately parallel to the shoreline and along the north breakwater, and finally these contours turned around the tip of the breakwater, implying sand deposition inside the port.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

19

At the harbor entrance, the water depth is kept at −4.5 m by dredging the navigation channel. In the area between No. 53 and No. 61, the contour lines shallower that −3 m extend approximately parallel to each other, except in the vicinity of the fishing port, but the intervals between the contour lines deeper than −4 m are very wide, implying that a very gentle seabed slope was formed downcoast by erosion. These features of the contour lines observed upcoast and downcoast of the Shibetsu fishing port are the most typical beach changes when longshore sand transport is obstructed by construction of a groin or breakwater.

2.2.2.

Monbetsu Coast in Hidaka in Hokkaido

The Monbetsu coast is located at Hidaka in Hokkaido, as shown in Fig. 2.2.6, and stretches 19.4 km between the mouths of the Atsubetsu and Saru Rivers. Along the coastline, Atsuga, Monbetsu, and Tomihama fishing ports are located. On this coast, since the direction of predominant waves is obliquely incident to the shoreline, northwestward longshore sand transport prevails on the whole.

Fig. 2.2.6. Location of Monbetsu coast in Hidaka in Hokkaido.

May 26, 2010

20

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.7. Shoreline changes of Monbetsu coast based on comparison of aerial photographs between 1947 and 1983.

Figure 2.2.7 shows the shoreline changes with reference to the shoreline configuration in 1947 and 1953, based on aerial photographs (Uda et al., 1991b∗ ; Uda andYamamoto, 1993∗ , 1994). The reference year was selected as 1947 and 1953 in the regions between No. 0 and No. 151 and between No. 160 and No. 194, respectively. In this figure, No. 0 is located at the Saru River’s mouth and No. 194, at a location 1.2 km southeast of Atsuga fishing port.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

21

At No. 181, the eastern end of the study area, Atsuga fishing port had already been constructed before 1953. Here, a marked obstruction effect due to the fishing port breakwater was observed because of the long lapse of time after the construction of the breakwater. Since 1953, the shoreline recession area gradually expanded, reaching No. 110 in 1983. The shoreline recession was largest at No. 160 in the vicinity of the Gahari River’s mouth, and it reached 150 m in 1975. In the region between No. 163 and No. 181, the receded shoreline reached the seawall line, resulting in no further shoreline recession after 1971 because of the attachment of the shoreline to the seawall. In contrast, the shoreline significantly advanced in the region between No. 181 and No. 194 southeast of the fishing port, reaching a maximum of 220 m. Figure 2.2.8 shows an aerial photograph of the Atsuga fishing port taken in 1989. On the right-hand (southeast) side of the fishing port, the triangular foreshore extends toward the breakwater, whereas the coastline northwest

Fig. 2.2.8. Aerial photograph of Atsuga fishing port taken in November 1989.

May 26, 2010

22

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

of the fishing port is totally covered with a seawall, showing a distinct contrast in the coastal conditions. In Fig. 2.2.7, the shoreline has gradually retreated in the area between No. 40 and No. 50, but after the initiation of the construction of the fishing port in 1977 at No. 58, severe shoreline recession began, resulting in the extension of the shoreline recession zone between No. 40 and No. 57. In contrast, in the area between No. 59 and No. 74 on the southeast side of the Monbetsu fishing port, sand accumulated and the shoreline advanced by a maximum of 140 m. Figure 2.2.9 shows an aerial photograph of the Monbetsu fishing port in 1987, and it clearly shows that northwestward (left in the photograph) longshore sand transport was blocked by the port breakwater similar to the case in the Atsuga fishing port shown in Fig. 2.2.8. As mentioned, fishing port breakwaters were extended on a coast with dominant longshore sand transport, and the transport was obstructed. As a result, sand accumulated upcoast, whereas beach erosion occurred downcoast.

Fig. 2.2.9. Aerial photograph of Monbetsu fishing port taken in November 1989.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

2.2.3.

23

Misawa Fishing Port in Aomori Prefecture

(1) Shoreline changes The Misawa fishing port is located in Aomori Prefecture and faces the Pacific Ocean, as shown in Fig. 2.2.10. Although northward longshore sand transport prevails at the coast, this longshore sand transport was obstructed by the breakwaters of the fishing port, resulting in accretion and erosion on the south and north sides of the fishing port, respectively (Uda et al., 1999∗ ; Watanabe et al., 2000∗ ). Figure 2.2.11 shows an aerial photographs around the Misawa fishing port. In 1966, no artificial structures protruded from the shoreline, and a natural sandy beach with a foreshore ranging in width between 150 and 200 m extended from it. In 1977, an L-shaped breakwater was completed, and simultaneously sand accumulated on the south side of the breakwater, forming a triangular foreshore. The shoreline advance achieved a maximum value next to the breakwater, and the advance decreased with distance southward. On the other hand, since a wave-shelter zone was formed on the north side of the breakwater, southward longshore sand transport was locally induced, resulting in shoreline advance in the wave-shelter zone of the port.

Fig. 2.2.10. Location of Misawa fishing port in Aomori Prefecture.

May 26, 2010

24

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.11. Aerial photographs around Misawa fishing port taken from 1966 to 1997.

In 1982, a straight breakwater was constructed along the first breakwater, and the construction of north and offshore breakwaters began. Until this stage, the area where the shoreline advanced further extended south of the port, whereas the shoreline receded on the north side. In 1997, a groin was built south of the first breakwater of the fishing port, and the construction of a new groin and breakwater started on the north side of the port. The construction of a new breakwater started on the beach south of the fishing port, where the shoreline advanced extensively due to the obstruction of northward longshore sand transport by the old breakwater. The sandy beach bounded by this new breakwater and the south groin was later altered into the land for the utilization of the port. The same situation can be seen in the wave-shelter zone north of the port. Longshore sand transport toward the wave-shelter zone was generated locally, resulting in sand accumulation on the north side of the port, and the sandy beach formed in such a way was also utilized as a zone of the fishing port.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

25

Fig. 2.2.12. Shoreline changes around Misawa fishing port.

Figure 2.2.12 shows the shoreline configurations measured from three sets of the aerial photographs taken in 1966, 1982, and 1997. It is found from these photographs that the shoreline advanced, while forming a triangular foreshore south of the Misawa fishing port with a maximum shoreline advance of 235 m. The shoreline advance zone extended to a location 3.5 km south of the fishing port, whereas the shoreline tended to retreat south of the location. In contrast, north of the fishing port, a maximum shoreline recession of 130 m was observed at a location 2.7 km north of the port, but it was small compared with the shoreline advance on the coast south of the port. Finally, the zone where a triangular stable foreshore had been formed by blocking northward longshore sand transport by the fishing port breakwater only extended 3.3 km south of the port, where sand was sufficiently supplied from the southern coast; but at present the beach is also being eroded, because of the exhaustion of the sand supply from the southern coast. (2) Field observations Field observation of the Misawa coast was carried out on 12 June 1998. First, Fig. 2.2.13 shows berm formation in the vicinity of a small river flowing into the Pacific Ocean at a location 2.5 km south of the fishing port. When a small river flows into the ocean, the river stream meanders, eroding the beach to form a scarp, and thereafter a berm is formed by the accretive action of waves in front of the scarp. The formation of a berm implies that a sufficient amount of sand exists in this area. In contrast, north of the Misawa fishing port, the entire shoreline was covered by concrete armor units or gently sloping revetments, as shown in Figs. 2.2.14 and 2.2.15, and a typical example of the totally artificial coast

May 26, 2010

26

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.13. Berm formation upcoast of Misawa fishing port observed on 12 June 1998.

Fig. 2.2.14. Concrete armor units laid down along the shoreline.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

27

Fig. 2.2.15. Gently sloping revetment built in eroded area.

was observed. The beach erosion triggered by the exhaustion of longshore sand transport was further extended, and a high scarp of over 2 or 3 m height was formed north of the artificial headland under construction 2.1 km north of the fishing port, as shown in Figs. 2.2.16 and 2.2.17. Thus, on the Misawa coast, beach erosion north of the Misawa fishing port has intensified with time, since northward longshore sand transport has been obstructed by the breakwaters of the fishing port.

2.2.4.

Momosaki-hama Coast in Niigata Prefecture

(1) Shoreline changes The Momosaki-hama coast is located south of the Arakawa River in the northern part of Niigata Prefecture, as shown in Fig. 2.2.18, and faces the Sea of Japan. In this region, southward longshore sand transport prevails, but in recent years sand supply from upcoast has decreased significantly, resulting in severe beach erosion. Here, focusing on the stretch 1.8 km

May 26, 2010

28

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.16. Scarp formed on north side of artificial headland.

Fig. 2.2.17. Scarp 3 m high formed north of artificial headland.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

29

Fig. 2.2.18. Location of Momosaki-hama coast in northern Niigata Prefecture.

long along the coastline, as shown by a rectangular area in Fig. 2.2.18, the long-term shoreline changes are investigated by comparing the aerial photographs taken in 1971 before beach erosion with those taken in 2000 after the erosion (Uda et al., 2003∗ ). In 1971, a natural sandy beach without any artificial structures extended alongshore, as shown in Fig. 2.2.19(a). At that time, there was a 70 mwide foreshore on average and a straight coastline. Although southward longshore sand transport prevails in this area, sand supply to this coast was greatly reduced for various reasons, such as the obstruction of longshore sand transport at Iwafune Port located north of the study area, decrease in sand supply from the Arakawa River, construction of the training jetties at

May 26, 2010

30

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.19. Aerial photographs of Momosaki-hama coast taken in (a) 1971, (b) 1987, (c) 1996, and (d) 2000.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

31

the Kinoto-dainichi River next to the Arakawa River, and dredging at this river’s mouth. In 1987, three detached breakwaters were built in the northern part of the study area to protect a small village located north of the study area against waves, and tombolos were formed on the lee side of these detached breakwaters as shown in Fig. 2.2.19(b). Then, sand supply significantly decreased south of these detached breakwaters, and the seawall was exposed to waves over a distance of 200 m, resulting in the disappearance of the sandy beach. Another detached breakwater was built at x = 1.24 km. The shoreline south of this detached breakwater retreated greatly, and a 400 m stretch of the coastline was totally covered by a seawall, resulting in the disappearance of the foreshore in front of the seawall, although a narrow foreshore was left north of the breakwater. This contrast in shoreline configuration on both sides of the detached breakwater explains well the predominance of southward longshore sand transport. Until 1996, seven detached breakwaters had been built, as shown in Fig. 2.2.19(c). These detached breakwaters were aligned parallel to the shoreline before the erosion, but the offshore distance between the shoreline and the detached breakwaters increased southward because of further shoreline recession after the installation, implying that the wave-dissipating effect of detached breakwaters was reduced southward. In 2000, beach erosion has further intensified in the entire zone, as shown in Fig. 2.2.19(d), and the length of the coastline covered with coastal structures exposed to waves increased. Although the salients had been left behind the three detached breakwaters in the northern part of the coast, the foreshore totally disappeared along the straight seawall south of these breakwaters. Similarly, in the vicinity of the detached breakwater located at x = 1.32 km, there was a foreshore north of the detached breakwater until 1996, but the sandy beach in front of the seawall totally disappeared by 2000, exposing the seawall to waves. Finally, the shoreline behind the detached breakwater crossed the seawall line discontinuously. It is clear that the sandy beach in this area has been eroded monotonically with time.

May 26, 2010

32

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.20. Shoreline change of Momosaki-hama coast.

Based on the aerial photographs shown in Fig. 2.2.19, the shoreline changes in the study area were read with reference to the shoreline in 1971, as shown in Fig. 2.2.20, where the base line was selected approximately parallel to the shoreline in 1971. It is clear that the shoreline receded south of the structures, where the shoreline position is fixed by the structures. As mentioned, on the Momosaki-hama coast, where a natural sandy beach with a 70 m-wide foreshore had been located until 1971, beach erosion has intensified since 1987. The detached breakwaters and seawall were constructed as countermeasures against beach erosion, but the seawall only resulted in fixing the shoreline at the seawall, causing downcoast erosion. In addition, despite the fact that the detached breakwaters at first exhibited sand accumulation behind the structure, the effect of sand accumulation was lost with time, resulting in the disappearance of the sandy

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

33

beach north of the detached breakwaters. Thus, detached breakwaters were ineffective in controlling the longshore sand transport in the long term and in stabilizing the shoreline. On the Momosaki-hama coast, sand supply from the upcoast has significantly decreased, and therefore the construction of detached breakwaters was useful only for the reduction of longshore sand transport. It was difficult to form a stable shoreline. This shows that the beach will be further eroded at this coast, unless a fundamental measure such as sand bypassing is taken. Although the coastline is protected by many facilities at present, the protective effect and stability of these shore-protection facilities will be lost with time, and wave overtopping at the gently sloping revetments will become severe. (2) Field observations On 30 April 2002, field observations were carried out. The results of these field observations are shown in comparison with an aerial photograph in Fig. 2.2.19(d) taken in May 2000. Since the field observations were carried out two years after the aerial photograph, beach erosion had become more serious in the southern part of the coast. Figure 2.2.21 was taken at A in Fig. 2.2.19(d), looking south, and shows the condition of the gently sloping revetment built at this coast. At this location, the gently sloping revetment was exposed to waves because of erosion, and concrete armor units had been placed straight along the revetment, leaving a narrow band of water at the foot of the revetment. Gently sloping revetments have been built in Japan to enhance the accessibility to the waterfront from the land, as its gentle slope permits walking. However, in this case, the wave run-up height was raised, because the toe of the gently sloping revetment protrudes into the seabed with no foreshore. Concrete armor units have been placed as a measure to reduce wave run-up height. The coastal condition in which the shoreline area in front of the long concrete slope is enclosed by many concrete armor units significantly spoils the scenic beauty and utilization of the coast. Figure 2.2.22 was taken at a location, looking north, south of the corner, where the concrete armor units gradually approach the shoreline and then

May 26, 2010

34

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.21. Gently sloping revetment built at Momosaki-hama coast (30 April 2002).

Fig. 2.2.22. Damaged gently sloping revetment.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

35

connect to the gently sloping revetment, as shown in Fig. 2.2.21. The same location is also shown by B in Fig. 2.2.19(d). At this site, the gravel mound of the gently sloping revetment with a slope of 1/4 was washed away due to waves, the slope was severely damaged, and the concrete blocks were scattered as shown in the photograph. The extreme scale of the damage can be clearly realized by comparison with the height of a man standing in the central part of the gently sloping revetment. Figure 2.2.23 shows the condition of the same damaged gently sloping revetment. There is no foreshore in front of the collapsed revetment, and the toe of the revetment is fully exposed to waves. The incident waves break just at the toe of the revetment without a large dissipation in the offshore zone, exerting a strong wave force on the slope. A scarp formed by the run-up waves over the revetment can be seen at the back of the revetment, illustrating that high storm waves in winter can overtop the revetment and reach the hinterland. After extending a stretch of the gently sloping revetment, as shown in Fig. 2.2.23, this protective facility ends at a distance. At this location

Fig. 2.2.23. Damaged gently sloping revetment, looking south.

May 26, 2010

36

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.24. Formation of scarp at south end of gently sloping revetment.

(C in Fig. 2.2.19(d)), a spatial imbalance of longshore sand transport was caused, and a scarp was formed at the south end of the revetment, as shown in Fig. 2.2.24, because the seawall had been built continuously upcoast, reducing southward longshore sand transport. The scarp was highest next to the south end of the revetment, and its height gradually decreased with the southward distance from the end. As shown in Fig. 2.2.24, another gently sloping revetment at the downdrift side of the hooked shoreline was formed downcoast of the gently sloping revetment, and this exhibited a solid boundary for shoreline change. Due to this effect, further extension of scarp erosion as shown in Fig. 2.2.24 was prevented. However, in the south part of this gently sloping revetment, large-scale subsidence of concrete blocks was found as shown in Fig. 2.2.25. In the central part of the gently sloping revetment, there was no foreshore, and the toe of the revetment was fully covered by many abraded concrete armor units placed along the coastline. They were placed to prevent the toe from scouring and to reinforce the foundation of the revetment. Despite these measures, the concrete blocks on the slope of the

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

37

Fig. 2.2.25. Partly subsided gently sloping revetment.

gently sloping revetment had subsided significantly. These damages were caused by the gravel mound beneath the slope being washed away through the gap between the toe of the revetment and the seabed lowered by the scouring associated with the beach erosion, and it became difficult for the concrete blocks to withstand their weight. Since the coast erodes because of the exhaustion of sand supply from upcoast, the seabed elevation in front of the revetment may decrease in the long term. Taking this condition into account, the possibility of subsidence damage of the revetment occurring as shown in Fig. 2.2.25 increases. The gently sloping revetment shown in Fig. 2.2.25 has been built only for a longshore distance of 200 m. Figure 2.2.26 was taken from the end (D in Fig. 2.2.19(d)) of this gently sloping revetment, looking south. In this photograph, another gently sloping revetment can be seen at the far side. Between two gently sloping revetments, the coastline greatly retreats downcoast, forming a high scarp. The scarp height just downcoast of the south end of the gently sloping revetment reached around 3 m, as shown

May 26, 2010

38

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.26. Scarp formed downcoast of south end of gently sloping revetment.

Fig. 2.2.27. Coastal forest fallen due to beach erosion forming scarp.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

39

in Fig. 2.2.27, and pine trees of the coastal forest fell down, permitting the invasion of the scarp deep into the coastal forest. As mentioned, on the Momosaki-hama coast, a gently sloping revetment was constructed intermittently under the condition that the longshore sand supply from the northern coast was exhausted, and severe beach erosion occurred at the end of the gently sloping revetment, as well as the collapse of concrete armor units at the south end. Unless the sand supply is increased in the future, the erosive zone will further expand southward, damaging the coastal forest area.

2.2.5.

Shinkawa Fishing Port in Niigata Prefecture

(1) Shoreline changes The coastline west of the Shinkawa fishing port located in the western part of the Niigata coast has a convex form, and a sand dune 2 km wide developed along the shoreline, as shown in Fig. 2.2.28. On the Niigata coast, westward longshore sand transport prevails due to storm waves in winter,

Fig. 2.2.28. Location of Niigata coast facing Sea of Japan.

May 26, 2010

40

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

and a long stretch of sandy beach had been formed by the sand supply from the Shinano River. However, the floodway was built in 1923 upstream of the Shinano River as shown in Fig. 2.2.28, which bypassed sand to the other coast facing the Sea of Japan. A long breakwater at Niigata Port was also constructed, resulting in the disruption of westward longshore sand transport. Since then, the sand source in the eastern area of the Shinkawa fishing port was exhausted, and an erosion wave propagated from east to west. Following this, severe erosion has occurred west of the Shinkawa fishing port. In 1966, beach erosion took place next to the fishing port, but thereafter the erosion zone steadily expanded westward. Since the coastal forest area controlled by the Forest Law extends along the coastline on this coast, the restoration work against the damage aimed at the protection of the coastal forest was carried out as the erosion expanded. In this restoration work, a vertical seawall with concrete armor units as foot protection was first constructed, and then the construction of a gently sloping revetment was begun. Since the protection facilities have been constructed from upcoast to downcoast of the longshore sand transport, a vicious circle was initiated, such that soon after the completion of the protective facilities, the downcoast erosion began (Uda and Kanda, 1998∗ ; Uda et al., 2000∗ , 2002∗ ). Figure 2.2.29 shows the location of the study area around the Shinkawa fishing port, as well as the bathymetry along the entire Niigata coast. Beach changes were investigated in detail in the rectangular region shown by the

Fig. 2.2.29. Bathymetry between Sekiya floodway and Shinkawa River on Niigata coast measured in October 1995.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

41

Fig. 2.2.30. Shoreline changes around Shinkawa fishing port with reference to shoreline in September 1985.

dotted line in this figure. Figure 2.2.30 shows the shoreline changes in the study area up to 1995 with reference to the shoreline position in 1985. The shoreline advanced on the northeast side of the fishing port as a whole, whereas it retreated on the southwest side, except in the vicinity of the port. Figure 2.2.31 shows the bathymetry in September 1985 around the fishing port. At this time, parallel jetties had already been built at the Shinkawa River’s mouth, and the shoreline northeast of the mouth protruded

May 26, 2010

42

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.31. Bathymetry around Shinkawa fishing port measured in September 1985.

seaward compared with that on the southwest side of the river’s mouth, forming a discontinuity in the shoreline. Although the sea-bottom contours at depths between 1 and 4 m had an irregular shape because of the formation of bars and troughs, the contours at depths between 4 and 6 m gradually protruded north of the river’s mouth in the same manner as along the shoreline, turning around the tip of the jetties. South of the river’s mouth, they extended again parallel to the shoreline. In 1985, a seawall 1 km long was built south of the river’s mouth, but a foreshore 40 m wide was at least left in front of the seawall. By October 1991, three detached breakwaters had been constructed at a location around 2.5 m deep on the southwest side of the jetty, as shown in Fig. 2.2.32, and a groin was also constructed, which later became part of the port breakwater, at a location 400 m northeast of the Shinkawa River’s mouth. Due to the construction of these detached breakwaters, the shoreline advanced in the area next to the jetty, resulting in deepening further downcoast. In addition, the fact that the scouring hole formed off the tip of the jetty extended southwestard, while keeping a narrow band as it gradually approached the shore, also illustrates the predominance of southwestward longshore currents. Figure 2.2.33 shows the bathymetry in October 1995. The bar and trough topography 3 m deep obliquely extended to the tip of the jetties on the northeast side, whereas this topography significantly shifted landward

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

43

Fig. 2.2.32. Bathymetry around Shinkawa fishing port measured in October 1991.

Fig. 2.2.33. Bathymetry around Shinkawa fishing port measured in October 1995.

on the southwest side of the jetty. This again implies that southwestward longshore sand transport was significantly obstructed at the river’s mouth. By comparing the bathymetries in 1995 with those in 1985 shown in Fig. 2.2.31, it is clear that the sea-bottom contours advanced on the northeast side of the river’s mouth, whereas they retreated on the southwest side. (2) Field observations Figure 2.2.34 shows the aerial photographs of the study area, taken in May and October 1993. A wide sandy beach extended east of the Shinkawa fishing port as shown in the photograph, whereas in contrast the foreshore

May 26, 2010

44

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.34. Aerial photograph of west part of Niigata coast taken in 1993.

was very narrow west of the fishing port. However, in the area between Yotsugoya beach and the Maki fishing port, a foreshore 80 m wide extended continuously. In the zone shown in Fig. 2.2.34, several field observations were conducted in the vicinity of and south of the Shinkawa fishing port. Figure 2.2.35 shows the sand dune behind the fishing port, taken on 4 July 1999, from the top of the sand dune with a mild slope covered with various dune vegetation, leaving the original scene of the past dune area of the Niigata coast. Figure 2.2.36 was taken on the way back to the shoreline from the top of the dune, and shows the shoreline east of the Shinkawa fishing port. It is clear that a wide sandy beach extended in front of the sand dune. As mentioned, a triangular foreshore was left on the east side of the Shinkawa fishing port because of the effect of obstructing westward longshore sand transport, as schematically shown in Fig. 2.2.37. In contrast, west of the fishing port, the shoreline significantly retreated and beach erosion progressed, because of a decrease in the longshore sand supply.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

45

Fig. 2.2.35. Natural sand dune covered with various types of dune vegetation (4 July 1999).

Fig. 2.2.36. Wide sandy beach left on east side of breakwater of Shinkawa fishing port.

May 26, 2010

46

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.37. Schematic shoreline change around breakwater obstructing predominant longshore sand transport.

Fig. 2.2.38. Seawall extending straight and wave-dissipating structures.

Figure 2.2.38 shows the seawall and concrete armor units immediately west of the Shinkawa fishing port. The same armored coastline continuously extended to 2 km west of this location. The earth dike extending straight behind the seawall was the protective measure against beach erosion for the coastal forest planted behind the earth dike. Figure 2.2.39 was taken further west of the location where Fig. 2.2.38 was taken. A large subsidence of the concrete armor units can be seen.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

47

Fig. 2.2.39. Subsided concrete armor units.

The measures attached over the subsided concrete armor units show the original elevation of the wave-dissipating structures, and repair work to recover the original elevation was planned at the time of the photo. According to the observation, the zone where the concrete armor units subsided coincided with the area with a concave shoreline west of three detached breakwaters built in the adjacent area of the fishing port. Thus, beach erosion had been progressing west of the Shinkawa fishing port, and therefore, on 20 December 2001, a second field observation was carried out. Figure 2.2.40 shows the gently sloping revetment built at the west end of the seawall. A very long, stepwise concrete slope was constructed. This large-scale, gently sloping revetment was built to protect the slope of the earth dike extending alongshore in front of the coastal forest. There was no foreshore in front, and the transition zone from land to sea, which is vital for the purification of seawater passing through the shore face, was completely divided into two parts. Under these conditions, it became impossible for organisms to live on the shore face. The steps near

May 26, 2010

48

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.40. Gently sloping revetment extending west of seawall area (20 December 2001).

the shoreline are also always slippery because of attached sea organisms, prohibiting the recreational use of the coast. This is totally contradictory to the concept of the Coastal Act, requiring a balance between the protection, environment, and use of a coast. Figure 2.2.41 shows the downcoast erosion of the gently sloping revetment shown in Fig. 2.2.40. Here, the scarp height reached 3 m and the upper part of the scarp fell down because of the formation of a notch at the foot of the scarp. Sand in the upper part slid down, forming a steep slope with an angle of repose and intersecting the backshore surface. The administration responsible for conservation of the coastal forest could not permit this severe erosion, and protective measures against beach erosion would have also been taken in this area, resulting in the further extension of the coastline covered with seawall and concrete armor units. In the field observation on 26 June 1996, a scarp 3 m high was formed at a location 1.7 km west of the Shinkawa fishing port, but this area was totally covered by a seawall on 14 July 1999, and a new scarp downcoast of the seawall was formed as well. This downcoast area had also been protected

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

49

Fig. 2.2.41. Land slide on scarp surface caused by toe erosion.

by the gently sloping revetment until 20 December 2001. Thus, the scarp formation zone was extended westward with the extension of the seawall. Sand transported alongshore accumulated on the east side of the Maki fishing port located in the western part of the coast, as shown in Fig. 2.2.34, and part of the sand accumulated inside the fishing port, turning around the tip of the breakwater. This excess deposition of sand became an obstacle for maintaining the fishing port, and sand mining on the beach was carried out as a measure to prevent excess sand deposition on the east side of the fishing port. Figure 2.2.42 shows a scene of sand mining using bulldozers and trucks. Although significantly rough waves were incident during the field observation, the sand mining was carried out at the shoreline behind the detached breakwaters, and therefore the mining work was done without any obstruction. Beach materials taken from this site were used to nourish the beach on the Niigata coast or to provide the construction materials. In the adjacent area east of the Maki fishing port, a wide, triangular foreshore had been formed due to the obstruction of westward longshore sand transport by the fishing port breakwater, resulting in the occurrence

May 26, 2010

50

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.42. Loading of mined foreshore sand.

of severe wind-blown sand. As a measure against this wind-blown sand, a duralumin fence was built along the dune to protect against the invasion of wind-blown sand into the access road of the fishing port. Since the fence line gradually curved parallel to the access road and the ground elevation rose in the east, the fence line obliquely intersected the predominant wind direction. At the corner of this fence, wind-blown sand deposited to a thickness of 2 m, because the exposed part was only about 0.6 m out of the fence’s total height of 2.6 m, as shown in Fig. 2.2.43. At the end of this fence, a large amount of sand deposited, while forming a slope with an angle of repose, as shown in Fig. 2.2.44. Field observations in 1999 showed that the coast west of the Shinkawa fishing port was severely eroded due to the exhaustion of the longshore sand supply from upcoast and the obstruction of longshore sand transport by the breakwaters of the port, and that a gently sloping revetment had been built as a measure against severe beach erosion. However, scarp erosion occurred again at the west end of the gently sloping revetment

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.2.43. Accumulation of wind-blown sand in front of fence.

Fig. 2.2.44. Wind-blown sand deposited in access road through end of fence.

51

May 26, 2010

52

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

(Uda et al., 2000∗ ). A comprehensive measure considering the entire coastline between the Shinkawa and Maki fishing ports was required to prevent the coastline from rapidly becoming artificial and to preserve the natural sandy beach. However, in reality, no concrete measures were taken, permitting the westward extension of the gently sloping revetment, because the eroded area belonged to the coastal forest area designated by the Forest Law, where protection using only a seawall and concrete armor units was permitted. In other words, fundamental issues were not solved at all, and the problem was simply postponed, resulting in the change of the natural sandy beach to an artificial coast covered with a gently sloping revetment. Much investment was simply useful for extending the gently sloping revetment in the longshore direction. The observation-based results are schematically summarized in Fig. 2.2.45. The real coastline has a smoothly curved, convex form, but the longshore coordinate fixed on this curved coastline expanded into a straight line. The longshore sand transport is assumed to be 0 at the Shinkawa fishing port as one of the boundary conditions. Since the water depth at the tip of the breakwater in this port is as shallow as −3.5 m, westward longshore sand transport is not fully obstructed, but part of the sand transport may supply the coasts west of the fishing port. Taking into consideration that many structures including seawall, detached breakwaters and gently sloping

Fig. 2.2.45. Main causes of beach changes in littoral system between Shinkawa and Maki fishing ports.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

53

revetments have been built on coasts east of the fishing port, and considering as well the reduction of natural sand supply, westward longshore sand transport at the fishing port can be assumed to have been significantly reduced. Therefore, the above-mentioned boundary conditions may be assumed. On the Niigata coast, since the balance in the westward longshore sand supply has totally been lost, the erosion wave, which was generated from the adjacent area of Niigata Port, has expanded westward and passed the Shinkawa fishing port. In the adjacent area of this fishing port, scarp erosion had occurred until 1996, but the seawall was built to restore damages. The beach erosion became severe in 1999 immediately west of the area where the seawall was built, and a gently sloping revetment was built there. Furthermore, scarp erosion is currently underway west of this gently sloping revetment in the same manner. In the erosion zone, serious problems are occurring, whereas a large amount of sand accumulated in the adjacent area east of the Maki fishing port, thereby widening the foreshore. Sand deposition inside the Maki fishing port turning around the tip of the breakwater is feared, so that sand mining has been carried out as a restorative. The widening of the foreshore accelerated the passage of wind-blown sand to the access road to the fishing port, and the manager of the road was required to remove the accumulated sand. Sand mining in the adjacent area east of the Maki fishing port and the removal of sand deposited on the road are equivalent to the action of removing sand from the system by littoral drift. Severe scarp erosion upcoast and sand accumulation in the adjacent area east of the Maki fishing port are independent events. Accordingly, even though beach erosion upcoast occurs, no matter how severe it is, sand accumulation near the fishing port takes place. Since the sediment supply to the littoral system has already been almost exhausted, sand mining at the shoreline and the removal of wind-blown sand deposited on the access road decreases the total sand volume of the system and alters the coast to a level from which recovery is impossible. Since the shoreline west of the Shinkawa fishing port of the Niigata coast has a convex shape as a whole, the effect of the shoreline advance

May 26, 2010

54

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

upcoast due to the blockage of longshore sand transport by the breakwater of the Maki fishing port is very limited. Thus, it is difficult to reduce scarp erosion by gradual recovery of the foreshore from downcoast, in spite of sufficient sand accumulation in the adjacent area east of the Maki fishing port. However, the decrease in total sand volume in a littoral system leads to the alteration of the coast from natural to artificial, unless the protection line is set back. From this point, taking sand out outside the littoral system should be re-examined, and some measures including appropriate distribution of sand by nourishment may be required, taking the whole littoral system into account.

2.2.6.

Fuji Coast in Shizuoka Prefecture

The Fuji coast is located at the base of Suruga Bay and has a longshore stretch 25 km long, as shown in Fig. 2.2.46. This coast was formed by eastward longshore sand transport, carrying sand supplied from the Fuji River. On this coast, Tagono-ura Port was built 4.8 km east of the mouth of the Fuji River, and the Showa floodway is located a further 3.5 km east of it. The beach around this floodway has been stable for a long time since the completion of the floodway, but dominant beach changes were triggered due to beach erosion, finally resulting in the collapse of the floodway itself (Uda et al., 1994∗ ; Uda, 1997∗ ). Figure 2.2.47 shows the recent shoreline change of the Fuji coast. The vertical axis is the shoreline change in each year with reference to the shoreline in 1968, and the abscissa is the survey line number at 250 m intervals from the Fuji River’s mouth, as shown in Fig. 2.2.46. West of No. 12 next to the river’s mouth, there were large variations of the shoreline position until 1982, but in the vicinity of No. 12 a large amount of sand was deposited around the river’s mouth due to a large flood that occurred in June 1982. The fact that the shoreline significantly advanced in the vicinity of the river’s mouth without discharging a large amount of sand by eastward longshore sand transport is due to the effect of the detached breakwaters built east of No. 12 that reduces the longshore sand transport rate.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

55

Fig. 2.2.46. Location of Fuji coast in Suruga Bay.

In the area between No. 12 and No. 22, the shoreline recession zone gradually expanded eastward between 1968 and 1977, but the shoreline had been stabilized since the construction of the detached breakwater whose building was begun in 1977. The construction of Tagono-ura Port began in 1959 and had almost been completed by 1967. In the area between No. 22 and No. 27 west of Tagonoura Port, the shoreline gradually advanced during the period between 1968 and 1977, because eastward longshore sand transport was blocked by the port breakwater, but after 1977 the shoreline tended to retreat. The period

May 26, 2010

56

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.47. Shoreline changes of Fuji coast.

during which the shoreline had the tendency to recede corresponds with the period of construction of the detached breakwaters. Accordingly, one of the reasons for the shoreline recession is a decrease in sand supply from upcoast by the construction of the detached breakwaters under conditions in which sand turning around the tip of the breakwater and deposited in the navigation channel was dredged out, and part of the sand discharged offshore because of the steep slope off the tip of the breakwater, while the overall balance was lost. East of Tagono-ura Port, the shoreline recession area expanded east monotonically in the period between 1968 and 1979. Simultaneously, concrete armor units had been placed along the shoreline in 1974 as a measure against beach erosion. Because these wave-dissipating structures prevented foreshore sand landward of these structures from discharging, shoreline recession east of these structures was accelerated. After the

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

57

Fig. 2.2.48. Shoreline east of Tagono-ura Port of Fuji coast (February 1992).

wave-dissipating structures were constructed up to the location of the Showa floodway in 1979, severe shoreline recession began east of this floodway. On the other hand, east of No. 50, shoreline changes were not observed during 1968 and 1992, and only the vast foreshore area disappeared between Tagono-ura Port and No. 50, implying the offshore discharge of sand from the shoreline area. In the vicinity of No. 30, no shoreline changes are observed in Fig. 2.2.47, because the coastline in this area was totally covered with concrete armor units, resulting in an artificially fixed shoreline. Figure 2.2.48 shows the condition of such a coast. Nine groins were built in total, and many concrete armor units weighing 50 tons were uninterruptedly installed between the groins. These measures against beach erosion apparently succeeded in preventing the shoreline from receding, but shoreline access was completely lost and the coastal scenery was very much spoiled. Figure 2.2.49 shows the coastal condition of No. 42 east of the Showa floodway, where the shoreline has greatly retreated since 1981. Since sand

May 26, 2010

58

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.49. Shoreline in vicinity of Showa floodway (February 1992).

discharge due to eastward longshore sand transport continued to develop, the shoreline east of this floodway (upper part of the photograph) retreated significantly. On the adjacent area of the floodway, where intensive erosion took place, the foreshore was extremely narrowed, and concrete armor units were installed to protect the foot of the front of the coastal dike, which can be distinguished by the white color in the photograph. Originally a wide foreshore existed in the vicinity of the Showa floodway, but it gradually narrowed due to erosion. Figure 2.2.50 shows the shoreline changes associated with the gradual beach erosion from 1974 to March 1991. In 1974, the impact of the obstruction of longshore sand transport due to the breakwaters of Tagono-ura Port, being under construction at that time at a location 3.5 km west of the floodway, did not reach this area, and a uniform foreshore 140 m wide extended in front of the sea dike. The outlet of the floodway was located exactly at the shoreline, and the littoral drift was able to freely pass through the tip of the floodway without any obstruction. Then, since the eroded zone extended eastward

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

59

Fig. 2.2.50. Shoreline changes in vicinity of Showa floodway (December 1974 versus March 1991).

because of a marked decrease in sand supply from upcoast, wave-dissipating structures were installed along the shoreline west of the Showa floodway, as shown in Fig. 2.2.50(b), as a measure against beach erosion to fix the shoreline position. The tip of the floodway was located just at the shoreline in 1974, and no east–west discontinuity along the shoreline was observed with respect to the location of the floodway. However, this channel protruded seaward across the shoreline with the beach erosion, resulting in significant shoreline recession east of the channel, although it was effective in preventing sand discharging from the coast west of the floodway.

May 26, 2010

60

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.51. Spatial change of shoreline position east of Showa floodway between December 1978 and February 1994.

Figure 2.2.51 shows the changes in shoreline positions over a typical four-year period with reference to the shoreline in 1975, based on the results shown in Fig. 2.2.47. In 1978, the shoreline did not change because of the construction of the floodway, although there was a shoreline variation of 10 m. Then, the shoreline recession increased from 1984 to 1994. The maximum shoreline recession was observed at No. 48 adjacent to the floodway, and it gradually decreased with distance from the channel. The location with dominant shoreline recession (for example, the location of the 10 m shoreline recession) extended eastward with time. As an example of the change in longitudinal profile associated with shoreline recession, the beach profile at No. 48 is shown in Fig. 2.2.52. In 1975, the beach profile had a convex shape, but with the shoreline recession, the depth range between −5 and −13 m was significantly eroded, resulting in a concave profile. The elevation at the shoreline in 1975 was deepened

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

61

Fig. 2.2.52. Longitudinal profile along survey line of No. 48 adjacent to Showa floodway.

Fig. 2.2.53. Location of Fukude fishing port facing Pacific Ocean.

to −9 m in 1993. Due to this beach erosion, the Showa floodway had to be reconstructed.

2.2.7.

Fukude Fishing Port in Shizuoka Prefecture

The Fukude fishing port faces the Pacific Ocean and is located 10 km east of the mouth of the Tenryu River, as shown in Fig. 2.2.53. At this fishing port, eastward longshore sand transport was obstructed by the construction

May 26, 2010

62

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.54. Bathymetry around Fukude fishing port measured in February 1975.

of breakwaters, resulting in accretion on the west side and erosion on the east side of the breakwaters. Beach changes around the fishing port can be investigated by comparing the bathymetries (Tomiya et al., 1988∗ ). Figure 2.2.54 shows the bathymetry in February 1975. The Fukude fishing port was built at the mouth of the Ohta River. In February 1975, there were no structures projecting seaward from the shoreline at the river’s mouth, and contours deeper than −6 m extended approximately parallel to the shoreline, whereas contours between −6 and −8 m protruded slightly seaward from the river’s mouth, and the configuration of the contours was asymmetrical as a whole with respect to the centerline crossing the river’s mouth, with an eastward shift of the location where the contours protruded most. Furthermore, the contours between −2 and −4 m were parallel with respect to each other and significantly protruded from the river’s mouth. The asymmetry of these contours in the east–west direction was similar to that in the offshore zone. The asymmetry of the contours with respect to the location of the river’s mouth clearly shows the predominance of eastward longshore sand transport at this location. In short, longshore sand movement was partially obstructed at the river’s mouth flow, and beach changes similar to those occurring around a groin were observed at this stage.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

63

Fig. 2.2.55. Bathymetry around Fukude fishing port measured in February 1986.

Figure 2.2.55 shows the bathymetry in February 1986. By this time, the facilities of the fishing port, including a long, oblique breakwater 600 m in length, had been completed. Compared with the bathymetries in 1975 and 1986, the contours between −1 and −4 m and the shoreline (D.L. 0 m) significantly advanced in the vicinity of survey lines No. 9 and No. 10 west of the oblique breakwater. In contrast, all contour lines between 0 and −7 m became concave east of the fishing port, and these differences clearly explain the beach changes due to the obstruction of predominant eastward longshore sand transport by the fishing port breakwater. In this section, the bathymetries before and after the construction of the breakwaters of the Fukude fishing port were compared. In the analysis of the bathymetric survey data, seabed changes are often numerically processed using discrete mesh data on the basis of the fundamental concept that quantitative analysis is vital and numerical information is always required. However, as a result of such processing, sufficient consideration of the characteristics of bathymetry is often forgotten, producing a superficial analysis of the discrete numerical data and insufficient consideration of its physical meaning. Taking into consideration that not only waves but also beach changes associated with longshore sand transport are governed by water depth, the analysis of temporal and spatial changes in the configuration of the contour lines, being one of the characteristic curves, are also significant. In fact,

May 26, 2010

64

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

the contour-line change model described in Chap. 3 was developed on the basis of this concept.

2.2.8.

Imazu-sakano Coast in Tokushima Prefecture

The Imazu-sakano coast is located in the eastern part of Tokushima Prefecture, as shown in Fig. 2.2.56, and is a sandy beach 7 km long bounded by the mouth of the Naka River and Komatsujima Port at the south and north ends, respectively. This beach was formed by the successive movement of sand originally supplied from the Naka River and transported by northward longshore sand transport due to waves obliquely incident from the southeast via the Kii Channel. Figure 2.2.57 shows the long-term shoreline changes of this coast during the 65 years between 1907 and 1972 (Uda et al., 1993∗ ). It is shown from Fig. 2.2.57 that beach erosion occurred along the entire coastline of the Imazu-sakano coast, and the maximum shoreline recession reached 150 m in the vicinity of No. 15 and between No. 20 and No. 23. In contrast, the shoreline advanced 130 m at the north end of the Wadano-hana sand spit,

Fig. 2.2.56. Bathymetry off Imazu-sakano coast in Tokushima Prefecture.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

65

Fig. 2.2.57. Long-term shoreline changes of Imaza-sakano coast between 1907 and 1975.

corresponding to the general tendency of sand deposition at the tip of a sand spit. Furthermore, another sand spit surrounding a lagoon developed near the Naka River’s mouth, and the shoreline advanced 400 m there. The shoreline changes mentioned show a clear contrast in the sense that the shoreline advanced at the tip of the Wadano-hana sand spit on the north side of the Naka River’s mouth, whereas it retreated in the middle of the coast. The main causes of the discontinuity in longshore sand transport are the development of a sand spit and the construction of the breakwaters of Nakajima Port in the adjacent area of the Naka River’s mouth. Here in particular, Fig. 2.2.58 contains a series of aerial photographs showing temporal changes of the sand spit extending from the Naka River’s mouth. In 1953, the bar at the mouth of the Naka River extended 300 m offshore, and it turned there to the northwest. This narrow bar continued to

May 26, 2010

66

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.58. Aerial photographs showing deformation of sand spits north of Naka River’s mouth between 1953 and 1990.

develop to the northwest parallel to the coastline, while enclosing a long lagoon behind it. A compound spit was formed at the tip of this bar along with a very shallow sea bottom off the bar extending in the alongshore direction.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

67

In 1961, the slender bar at the river’s mouth widened as a whole, and further extended to the northwest. Another small sand spit also began to develop to cover the existing bar. The development of a sand spit with these complicated shapes means that the fluvial sediment supply from the Naka River was abundant at that time, and a very shallow area was extended around the river’s mouth. In 1974, seawall and groins were built along the outer shoreline of the elongated sand bar. By examining the shoreline configuration around 13 groins, in particular the groins located near the river’s mouth, the shoreline on the south side of each groin protruded compared with that on the north side. This shows that northward longshore sand transport prevailed at least in the period when the aerial photograph was taken. Before 1961, a natural sandy beach extended without any artificial structures, but after the construction of the breakwater of Nakajima Port on the north side of the Naka River in 1969, longshore sand transport was totally interrupted, resulting in severe beach erosion. Finally, a seawall was built along the coastline as a countermeasure. In the central part of Fig. 2.2.58, the shoreline advance can be seen in the vicinity of the location where the tip of the sand spit connects to the opposite shore, implying that littoral drift was able to reach the opposite shore, because a continuous shoreline was formed between the tip of the sand spit and the opposite shore. Otherwise, continuous longshore sand movement was interrupted due to the existence of the deep channel at the tip of the sand spit, as a general feature of the sand spit. Finally, during the period when a sand spit extends alongshore, sand moving along the shoreline is used only for the formation of the sand spit itself, and continuous movement of longshore sand transport is interrupted at the tip of the sand spit, resulting in downcoast erosion of the sand spit. In 1990, a seawall was built along the entire coastline and the shoreline was fixed by the seawall, as shown in Fig. 2.2.58. In the offshore zone, 11 detached breakwaters had been built, covering the entire shoreline of the sand spit, while accelerating the alteration of the artificial coast. At the base of the north breakwater, sand deposition can be seen due to wave diffraction. The lagoon area landward of the sand spit observed in the photograph in 1974 had disappeared by 1990 due to land reclamation. This means that, by

May 26, 2010

68

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

that time, a very important wetland had been lost in this area, on the basis of the current point of view that the preservation of the coastal environment is vitally important. In the example of the Imazu-sakano coast, the historical beach erosion along the shoreline around a sand spit was discussed. In addition to this, the phenomenon was noted that a sand spit in a very shallow sea around a river’s mouth rapidly elongated due to the effect of longshore sand transport, as shown in Fig. 2.2.58, in that period when abundant sediment was supplied from the river’s mouth. In this case, since the sand spit was extended while forming a slender barrier island, a lagoon with a wide salt-marsh area was formed landward of the barrier island. This type of wetland existed nationwide in Japan in the past, but such land was reclaimed during the era of rapid economic growth. Simultaneously, the wetlands were lost by shoreline recession due to a decrease in the sand supply from rivers. This brackish-water zone was a part of the habitat of fish and shellfish. Thus, the loss of sandy beach brought about the degradation of the ecosystem in the coastal zone.

2.2.9.

Method of Addressing Issues

(1) Classification of beach erosion In Sec. 2.2, eight examples of eroded coasts were described, in terms of the beach erosion that occurred when predominant longshore sand transport was obstructed by the construction of breakwaters. Future measures for these coasts are mainly classified into two groups, depending on the current condition of the erosion, although strict definition is difficult. A coast in Category 1 is one where a seawall and a large number of concrete armor units have already been built in the eroded area, and the study of improvements is unrealistic, because too much cost is incurred to improve the existing condition: the Shibetsu coast in Sec. 2.2.1, Monbetsu coast in Sec. 2.2.2, and Imazu-sakano coast in Sec. 2.2.8 are three such coasts. A coast in Category 2 is one for which, although beach erosion occurred in the past, the possibility still exists that some improvements can be

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

69

instituted to recover the sandy beach, or the possibility exists that beach erosion may become more severe, so that urgent measures are required to prevent the coast from further deteriorating: the Misawa fishing port in Sec. 2.2.3, Momosaki-hama coast in Sec. 2.2.4, Shinkawa fishing port in Sec. 2.2.5, Fuji coast in Sec. 2.2.6, and Fukude fishing port in Sec. 2.2.7 are examples of such coasts. In the management of the coasts belonging to Category 1, large-scale improvement of coasts to recover sandy beaches is very difficult, but constant measures against the degradation of the durability of the seawall and the subsidence of concrete armor units will be necessary forever. (2) Future measures against erosion in Category 2 Here, future measures against beach erosion of Category 2 are discussed, taking the Misawa coast as a typical example. The breakwater of the Misawa fishing port at first blocked northward longshore sand transport, resulting in the erosion in the northern area of the fishing port. However, since a sandy beach also disappeared on the coast south of this fishing port due to the exhaustion of longshore sand supply from further south, the shoreline has been rapidly retreating. This means that the coast south of the Misawa fishing port will be eroded due to the exhaustion of longshore sand supply whether or not the port exists, and that the triangular sandy beach is left now as a stable beach, because the breakwater of the Misawa fishing port blocks northward longshore sand transport like a long groin. Another view may be considered; the facilities obstructing the longshore sand transport and causing downcoast erosion are reciprocally useful to form a stable shoreline upcoast of these facilities. This point infers that measures must be taken from the broad point of view, as schematically shown in Fig. 2.2.59, in the case of the Misawa fishing port, for example. First, the manager of the fishing port must recognize that the breakwater stabilizes the coastline 3 km long south of the fishing port, and that, since the sand deposited in the fishing port was originally transported from the southern coast, sand should be used as the recycled material for beach nourishment on the southern coast.

May 26, 2010

70

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.2.59. Schematic diagram of method of dynamically stabilizing beach upcoast of fishing port breakwater.

Next, the ordinary sand bypassing method of preventing erosion and accretion as shown in Fig. 2.2.59(a) is ineffective. One must be very careful in simply assuming that sand bypassing is the best way to solve beach erosion. Sand bypassing from upcoast to downcoast of the fishing port simply accelerates the erosion of the coasts located further on the updrift side of the fishing port, where longshore sand supply to the coast is exhausted, and the sand volume of the upcoast is limited. Furthermore, when the foreshore that widened most in the area adjacent to the fishing port is to be used, it is required that part of the sand be exchanged with other earth materials, as shown in Fig. 2.2.59(b), and excavated sand must be returned to the beaches by beach nourishment in

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

71

order not to reduce the volume of natural coastal sand. At present, there is no more sand south of the Misawa fishing port so that downcoast erosion can be prevented by beach nourishment and the foreshore is recovered north of the fishing port. Shore protection must be set forward on the basis of the concept that sand left on a beach now is a very limited natural resource. By looking only at the erosion of the Misawa coast, one can say that the cause of severe beach erosion lies in the existence of the Misawa fishing port, but this thinking is too simplistic. One may find that another facility is blocking longshore sand transport on the coast, or that the reduction in sediment supply from rivers is caused by the construction of large dams and riverbed excavation for sand mining, when the sediment source for a coast is investigated in the upstream direction. Under these circumstances, various discussions are repeated, such as “What breaks the continuous sand movement?,” “Each organization related to erosion should carry out its responsibility,” or “The cause of beach erosion must be fully investigated first.” What is important is that beach erosion will steadily continue, regardless of human expediency, and sandy beaches will disappear. This is the reason why concrete measures have to be taken urgently to prevent coastal conditions from deteriorating further.

REFERENCE Uda, T. and K. Yamamoto (1994). Beach changes caused by obstruction of longshore sand transport: An example of the Hidaka coast in Hokkaido, Coastal Eng. Japan 37(1), 87–106.

REFERENCES (in Japanese) Tomiya, Y., T. Uda and T. Yamamoto (1988). Beach changes around Fukude fishing port on Enshu-nada coast, Annual J. Coastal Eng. JSCE 35, 382–386. Uda, T., K. Yamamoto and S. Kawano (1991a). Beach erosion of Shibetsu coast in Hokkaido, Annual J. Coastal Eng. JSCE 38, 286–290.

May 26, 2010

72

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Uda, T., K. Kitagami, K.Yamamoto and S. Kawano (1991b). Beach changes of Hidaka coast in Hokkaido, Annual J. Coastal Eng. JSCE 38, 276–280. Uda, T. and K. Yamamoto (1993). Beach changes caused by obstruction of longshore sand transport — The example of Hidaka coast, Japan Geomorph. Union 14, 407–421. Uda, T., T. Fujisaki, K. Yamamoto and K. Odagiri (1993). Beach erosion of Imazu-sakano coast in Tokushima Prefecture, Annual J. Coastal Eng. JSCE 40, 446–450. Uda, T., K. Yamamoto and T. Cho (1994). Beach erosion of Fuji coast and measures, Annual J. Coastal Eng. JSCE 41, 526–530. Uda, T. (1997). Note on design of a culvert-type divergent channel on a coast with predominant longshore sand transport, Proc. Civil Eng. in the Ocean, JSCE 13, 627–632. Uda, T. and Y. Kanda (1998). Beach erosion associated with construction of breakwaters of Shinkawa fishing port located in the southwest part of Niigata coast — Issues and future measures, Proc. Civil Eng. in the Ocean, JSCE 14, 269–274. Uda, T., S. Seino, S. Watanabe, M. Serizawa and T. San-nami (1999). Field observation of beach changes around Misawa fishing port in Aomori Prefecture and future measures, Proc. Civil Eng. in the Ocean, JSCE 15, 529–534. Uda, T., S. Seino, M. Serizawa, T. San-nami, K. Furuike and H. Gomi (2000). Rapid disappearance of natural sandy beach caused by the difference in principle between the protection of coastal forest and shore protection — An example of Niigata coast, Proc. Civil Eng. in the Ocean, JSCE 16, 613–618. Uda, T., M. Serizawa, T. San-nami and K. Furuike (2002). Beach erosion expanding on the coast between Shinkawa and Maki fishing ports on Niigata coast and future perspective, Proc. Civil Eng. in the Ocean, JSCE 18, 713–718.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

73

Uda, T., T. Kuroki, T. Nakamura and K. Kaki-ichi (2003). Beach erosion of Momosaki-hama beach in northern Niigata Prefecture and measures, Proc. Civil Eng. in the Ocean, JSCE 19, 327–332. Watanabe, S., S. Seino, T. Uda, M. Serizawa, T. San-nami and K. Furuike (2000). Beach changes around Misawa fishing port in Aomori Prefecture and future shore Protection, Proc. Civil Eng. in the Ocean, JSCE 16 607–612.

2.3.

BEACH EROSION TRIGGERED BY CONSTRUCTION OF WAVE-SHELTERING STRUCTURES

When a wave-sheltering structure such as an offshore breakwater or artificial island is built off the shoreline, a calm-wave zone is produced behind the structure. In this case, wave direction changes from outside the wave-shelter zone to inside due to wave diffraction and wave height is reduced as well. As a result, even if the beach topography was in equilibrium before the construction of the breakwater, longshore sand transport from outside the wave-shelter zone to inside will be induced, resulting in erosion in the area adjacent to the wave-shelter zone and sand deposition inside the zone. There are many examples of coasts where erosion and accretion have occurred by this mechanism. Here, 11 examples as shown in Fig. 2.3.1 and are described in detail.

2.3.1. Teradomari and Nozumi Coasts in Niigata Prefecture (1) Shoreline changes The Teradomari and Nozumi coasts are located in the southern part of Niigata Prefecture, and facing the Sea of Japan. In this area, the Shin-Shinano River was excavated in 1923 as the floodway of the Shinano River, the largest river in Japan, and almost all sand was supplied to the Teradomari and Nozumi coasts through this floodway instead of flowing down the lower Shinano River, resulting in the formation of a large-scale

May 26, 2010

74

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.1. Location of 11 coasts taken as typical examples of beach erosion associated with formation of wave-shelter zone due to extension of port breakwater.

river-mouth delta (Tsuchiya et al., 1994∗ ). However, in recent years, beach changes associated with the construction of the breakwater for Teradomari Port were triggered, and the natural coastline has since been disappearing (Uda et al., 2003b∗ ). Figure 2.3.2(a) shows an aerial photograph taken in 1947. In the center of the photograph, the Shin-Shinano River flows into the Sea of Japan, and sand supplied from this river’s mouth was deposited along the coastline,

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.3.2. Aerial photograph of Teradomari and Nozumi coasts.

75

May 26, 2010

76

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

resulting in a large shoreline advance, while forming a large-scale rivermouth delta. From the historical records, the past coastline before the formation of the river-mouth delta ran along the line smoothly connecting the seaward boundary of villages located at the foot of the mountains. At that time, the river meandered significantly northward at its mouth and then flowed out to the sea at a location 700 m north of the center of the river channel. Figure 2.3.2(b) shows an aerial photograph taken in 1967. In comparison with the one from 1947, the foreshore area greatly increased, and in particular the shoreline advance north of the river’s mouth was much larger than that south of the mouth. The asymmetry of the shoreline configuration on both sides of the river’s mouth indicates that predominant waves were incident counterclockwise relative to the direction of the centerline of the river’s mouth. These shoreline advances around the river’s mouth become negligibly small at Ogama, located at the north end of the coastline. Furthermore, although a breakwater had been extended at Teradomari Port, the impact of this breakwater on the surrounding coastline associated with the formation of a wave-shelter zone was negligibly small, and the shoreline advance north of the port breakwater was small, since the breakwater was short at that time. Consequently, almost all sand supplied from the Shin-Shinano River is considered to have been deposited on the coast between Teradomari Port and Ogama. On the other hand, north of the river’s mouth, a coastal dune area was developed, and a paddy field 250 m wide with a longshore stretch 2.5 km long was created seaward of the villages. However, almost all the sand dune area seaward of this paddy field was left intact. In the area between the river’s mouth and the location 1.5 km to the south, pine trees were also planted to create a coastal forest parallel to the shoreline. Figure 2.3.2(c) shows an aerial photograph taken in 2001. In the area north of the river’s mouth, the natural sand dune area seaward of the paddy field, shown in Fig. 2.3.2(b), was densely covered with pine trees, resulting in a maximum advance seaward of the coastal forest of 160 m since 1967. Similarly, the coastal forest advanced seaward south of the river’s mouth,

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

77

but at the same time, in particular, an embayed shoreline was formed north of the offshore breakwater, resulting in a maximum shoreline advance of 480 m, since the offshore breakwater was constructed at a location 900 m off the previous shoreline at the south end of the coastline, while a large wave-shelter zone north of the breakwater was formed. Since the breakwater of Teradomari Port protruded offshore for a long distance, it became a solid boundary against shoreline changes due to longshore sand transport. Sand required to form an embayed shoreline was transported from the northern area of the coast, and the shoreline retreated significantly in an extensive region between the drainage channel, which partly obstructed longshore sand transport as a groin, located 1 km north of the river’s mouth and Teradomari Port. When comparing the shoreline in 2001 with that in 1967, not only the coastal forest advanced while the width of the shore decreased, but also the shoreline receded due to the formation of the wave-shelter zone by the construction of the breakwater of Teradomari Port, both of which resulted in the narrowing of the natural sandy beach. In order to investigate this situation in detail, the change in the seaward marginal boundary of the coastal forest and shoreline changes with reference to that in 1947 were investigated on the basis of the aerial photographs in Fig. 2.3.2. Figures 2.3.3 and 2.3.4 show the shoreline configuration, the shoreline change with reference to that in 1947, and the change in the seaward marginal boundary of the coastal forest, respectively.

Fig. 2.3.3. Shoreline changes of Teradomari and Nozumi coasts.

May 26, 2010

78

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.4. Change in shoreline position and seaward marginal boundary of Teradomari and Nozumi coasts.

Regarding the changes until 1967, it is clear that the shoreline advanced due to the deposition of sand supplied from the river’s mouth to the coasts, and resultantly the coastal forest was expanded seaward to prevent windblown sand. Within this period, the scale of the breakwater of Teradomari Port was not large enough to give rise to the overall shoreline changes of the river-mouth delta. In contrast, regarding the shoreline changes until 2001, the seaward expansion of the coastal forest took place, as well as the sand accumulation in the wave-shelter zone of Teradomari Port associated with the extension of the port breakwaters, and the shoreline recession in the surrounding area became striking. The shoreline recession extended not only to the coast southwest of the mouth of the Shin-Shinano River, but also reached the coast northeast of the river’s mouth, resulting in a maximum shoreline recession of 180 m, 500 m north of the river’s mouth. In addition, on the right-hand (northeast) side of the river’s mouth, the beach width in front of the coastal forest, which had been expanded until 1967, assuming the further development of a river-mouth delta, was greatly narrowed. (2) Field observations Field observations of the shoreline between Teradomari Port and Ogama were carried out on 25 and 26April 2002. First, Fig. 2.3.5 shows the northern view from the top of the earth dike 10 m high on the west Teradomari coast.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

79

Fig. 2.3.5. Northern view from top of earth dike 10 m high on west Teradomari coast.

In front of the continuous wooden fence were a wide bank with a gentle slope and a sandy beach. In Fig. 2.3.6, taken at the shoreline seaward of the slope of the earth dike looking north, the shoreline extending north and the earth dike line seem to intersect each other. When moving 500 m alongshore, a scarp over 2 m high was discovered, as shown in Fig. 2.3.7. In the observation of the scarp face, lamination patterns usually observed at a natural sand dune area were found, implying that this scarp was formed by the erosion of natural sand dunes. When approaching the mouth of the Shin-Shinano River, the earth dike protecting the coastal forest was severely eroded, and an extremely high scarp 10 m high was found, as shown in Fig. 2.3.8. The scarp was continuously formed along the shoreline, and a wooden fence placed on top of the earth dike was about to collapse. Since the coastal forest area designated by the Forest Law remained intact, even if the shoreline retreated by erosion, the earth dike, which provided the seaward boundary of the coastal forest, was severely eroded.

May 26, 2010

80

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.6. Shoreline view of Teradomari coast.

Fig. 2.3.7. High scarp formed on northern Teradomari coast.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

81

Fig. 2.3.8. High scarp deeply cutting earth dike on left bank of Shin-Shinano River’s mouth.

Figure 2.3.9 shows the condition of the location where the shoreline was eroded most severely on the right-hand (northeast) side of the river’s mouth, as shown in Figs. 2.3.3 and 2.3.4. To protect the coastal forest, a gently sloping revetment was built in 2001. Behind this revetment were an earth dike and a coastal forest. If longshore sand transport toward the waveshelter zone of Teradomari Port was not generated due to the extension of a large port breakwater, the construction of this revetment would have been unnecessary. In fact, a long breakwater was built, narrowing the sandy beach, which was extremely wide in the past.

2.3.2.

Kashiwazaki Port and Arahama Coast in Niigata Prefecture

(1) General condition The study area includes the Kashiwazaki Port and the Arahama coast, located in the middle of Niigata Prefecture facing the Sea of Japan, as

May 26, 2010

82

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.9. Gently sloping revetment built to guard earth dike of coastal forest.

shown in Fig. 2.3.10. The study area is a sandy beach of about 15 km long from Kannon Point in Kashiwazaki City at the north to Banjin Point at the south. In this study, a 2.4 km-long stretch separated by the Kashiwazaki Port and the Sabaishi River was selected for detailed investigation of beach changes (Uda et al., 1993∗ ; Uda and Noguchi, 1993). The mean direction of the shoreline of the Arahama coast is N37◦ E. The Kashiwazaki Port has been developed at the south end of the sandy beach as shown in Fig. 2.3.10, and a harbor breakwater 2,400 m long was constructed. Kashiwazaki-Kariha Nuclear Power Station and the related exclusive port are located about 6 km northeast of the Kashiwazaki Port. Since this port is located at the middle of the coastline between Kannon and Banjin Points, the littoral transport system is basically divided into two independent systems at this location. Pouring into this coast are the Sabaishi and Ukawa Rivers, with catchment areas of 312 and 112 km2 , respectively. In addition, the old sand dune roughly 2 km wide extends north of the Sabaishi River along the coastline, as shown in the classification of

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

83

Fig. 2.3.10. Location of Kashiwazaki Port and Arahama coast and geomorphological classification.

geomorphology in Fig. 2.3.10. Shoreward from this point extends a new sand dune along the current coastline. This sand dune is relatively wide to the south of the Sabaishi River. As is commonly seen on coasts facing the Sea of Japan, a wide alluvial lowland extends behind the sand dune. Due to the directional probability of significant wave height, measured at a location 15 m deep (1.5 km off Kashiwazaki-Kariha Nuclear Power Station) between 21 August 1979 and 31 March 1992, the wave incidences

May 26, 2010

84

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

from N-NNW are sheltered by Sado Island, and therefore the predominant waves are incident from WNW and NW. Although the breakwater at Kashiwazaki Port is extended to shelter the waves incident from WNW, waves are incident from both counterclockwise (WNW) and clockwise (NW) directions with respect to the direction normal to the shoreline in the vicinity of this port. (2) Topographic changes In the study area, bottom sounding has been conducted twice a year since September 1979. Here the long-term beach changes have been mainly investigated using four sets of bathymetries obtained in the period from 1980 to 1992. Figure 2.3.11 shows the bathymery of April 1980. This reveals the condition in which the west breakwater extended 750 m from the turning point, changing its direction from NE to N15◦W. At this stage, a sandy beach existed on the Arahama coast. Contours deeper than −10 m smoothly extended from SW to NE, except in the navigation channel behind the breakwater. In contrast, contours between −9 and −6 m protruded into the lee of the harbor breakwater. These contours approached the shoreline along the northeastward sector from the breakwater. In the region between 0.3 and

Fig. 2.3.11. Bathymetry of Arahama coast in 1980.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

85

Fig. 2.3.12. Bathymetry of Arahama coast in 1985.

1.2 km, the sea was very deep with concave contours. These characteristics imply that the sediment that eroded in the region between 0.3 and 1.2 km outside the wave-shelter zone was transported to the lee of the breakwater. In addition, many bars and troughs developed at a depth of −5 or −6 m. Figure 2.3.12 shows the bathymetry of April 1985, when the west breakwater was extended to 1,100 m in length. The bottom contours shallower than −5 m protruded more apparently in the region between the Ukawa River’s mouth and x = 0.5 km to form a fan-shaped shoal in the vicinity of the river’s mouth. The comparison of this figure with Fig. 2.3.11 showing the bathymetry in 1980 reveals the protrusion of −5 through −9 m contours behind the breakwater as well as the northeastward movement of the point where the contours are closest to the shoreline. The movement of this point corresponds well to the artificial extension of the zone sheltered by the breakwater. In 1990, the west breakwater was further extended to 1,230 m in the N15◦ E direction, with the addition of 95 m in the N20◦ E direction as well as the extension of the south jetty, as shown in Fig. 2.3.13. It is found from the comparison of this figure with Fig. 2.3.12 that the protrusion of the −5 through −10 m contours further evolved, and at the same time the location of their closest point to the shoreline largely moved to the northeast.

May 26, 2010

86

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.13. Bathymetry of Arahama coast in 1990.

Fig. 2.3.14. Bathymetry of Arahama coast in 1992.

In 1990, the foreshore of the Arahama coast almost disappeared, and the contour line −5 m deep continued to approach the shore. The effect of the south jetty is considered to be negligible compared with that of the west breakwater. Finally, Fig. 2.3.14 shows the bottom contours in 1992, in addition to the construction of a 410 m jetty in the lee of the breakwater. Since the tip of the breakwater extended parallel to the shoreline, the sheltering

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

87

effect of the breakwater was greatly enhanced, and therefore the indentation of the contour lines increased. In particular, the indentations of the −5 through −7 m contours are very large; they protrude seaward in the lee of the breakwater, whereas they are very close to the shoreline at about x = 1 km. It is clear from a comparison of this figure with Fig. 2.3.11 that the sea bottom off the Arahama coast has deepened. The Arahama coast has thus been altered from a natural sandy beach to an artificial coast totally protected by seawalls and wave-dissipating structures, as shown in Fig. 2.3.15, for the prevention of beach erosion and wave overtopping. The condition for sand accumulation inside the port area was studied by comparing oblique aerial photographs. Figure 2.3.16 shows a photograph of this port taken in April 1986. The contours measured nearest in time are shown in Fig. 2.3.12. In the photograph, the Ukawa River flows into the harbor. At this stage, the shorelines on both sides of the Ukawa River are straight and the development of the bar at the river’s mouth is not very rapid. Regarding the shoreline formation around the detached breakwaters to the north of the harbor (towards the bottom of the picture), the shoreline extends

Fig. 2.3.15. Seawall and wave-dissipating works at Arahama coast (16 December 1992).

May 26, 2010

88

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.16. Overview of Kashiwazaki Port in April 1986.

obliquely from the seawall to the detached breakwater, implying that the dominant incident waves enter from the opening between the shoreline and the harbor breakwater. The coastal landform in June 1990 is shown in Fig. 2.3.17, and the corresponding contours are shown in Fig. 2.3.13. A large sand spit extended from the right-hand shore of the river’s mouth, accumulating in front of the mouth. In contrast, sand deposits cannot be observed in the river channel upstream of the sand spit. At the location of the jetty northeast of this sand spit, the shoreline became discontinuous and the shoreline on the northwest side of the jetty became concave. These facts suggest that the sand spit was formed, not by sand supply from the river itself, but from the deposits of the sand transported alongshore from the northeast to the wave-shelter zone. (3) Distribution of accretion and erosion zones In order to quantitatively analyze the beach changes around Kashiwazaki port, the changes in the sea bottom and the accumulated volume of sand

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

89

Fig. 2.3.17. Overview of Kashiwazaki Port in June 1990.

Fig. 2.3.18. Arrangement of calculation zone for sand volume.

in the lee of the breakwater were calculated over the zone composed of 58 and 33 meshes in N–S and E–W directions, respectively, as shown in Fig. 2.3.18. The mesh size is 50 m and 1980 was selected as the reference year for the calculation.

May 26, 2010

90

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.19. Sea-bottom change due to erosion and accretion on Arahama coast until 1985.

Changes in the sea bottom between 1980 and 1985 are shown in Fig. 2.3.19, where the bottom contours of 0, −5, −10, and −15 m for the reference year are shown by the broken line and those for 1985 by the solid line. Sand accumulated in the area up to around −10 m depth in the lee of the breakwater, while erosion continued in the vicinity of the shoreline south of the Sabaishi River and in the zone deeper than −5 m of the river’s mouth. In 1990, the accumulation zone behind the breakwater contrasted with the erosive zone northeast of the breakwater, as shown in Fig. 2.3.20. Regarding the sea bottom changes from 1980 to 1992, the characteristics seen in Fig. 2.3.20 appear more apparent. A large amount of sand accumulated in almost all the areas behind the breakwater, whereas beach erosion continued in a wide zone −5 to −10 m deep off the mouth of the Sabaishi River, as shown in Fig. 2.3.21. It is clear from Figs. 2.3.18–2.3.20 that the accretion zone behind the breakwater has monotonically extended northeastward along with the extensions of the breakwater. Furthermore, in Fig. 2.3.20, the jetty of the port is totally included inside the accretion zone. This implies

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

91

Fig. 2.3.20. Sea-bottom change due to erosion and accretion on Arahama coast until 1990.

Fig. 2.3.21. Sea-bottom change due to erosion and accretion on Arahama coast until 1992.

May 26, 2010

92

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

that the location and length of this jetty were inadequate for preventing large-scale beach erosion and accretion of this coast. In preventive applications employing the same length of jetty, it is believed that the jetty blocks the longshore sand transport much more effectively if it is located about 0.7 km northeast of the present location, that is, at the critical point between erosion and accretion zones. The change in sand volume in the accretion zone behind the breakwater was estimated from the change in sea-bottom level in each subregion multiplied by the area of the subregion (2,500 m2 ) with reference to the original condition in 1980, as shown in Fig. 2.3.22. This figure shows that the volume of sand accumulating inside the wave-shelter zone has monotonically increased with time. The average rate of accumulation between 1980 and 1992 is 1.5 × 105 m3 /yr. Since the sand discharge from the Ukawa River can be ignored for the reason mentioned earlier, almost all sand must have been transported alongshore from the northeast side of the port. Finally about 1.5 × 105 m3 /yr of sand on an average has been carried into the wave-shelter zone behind the breakwater by longshore sand

Fig. 2.3.22. Change in sand volume accumulated on lee side of breakwater of Kashiwazaki Port.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

93

transport because of the breakwater extension at the Kashiwazaki port, and the total volume of sand that has accumulated inside the wave-shelter zone since 1980 has reached about 1.6 × 106 m3 .

2.3.3.

Ohtsu Fishing Port in Ibaraki Prefecture

(1) Comparison of aerial photographs Typical beach changes associated with the formation of a wave-shelter zone were triggered by constructing a port breakwater in the vicinity of the Ohtsu fishing port in Ibaraki Prefecture, facing the Pacific Ocean, as shown in Fig. 2.3.23. The detailed changes can be made clear by comparing past aerial photographs (Uda et al., 1997a,b∗ ; Uda et al., 2005).

Fig. 2.3.23. Location of Ohtsu fishing port.

May 26, 2010

94

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.24. Aerial photograph between Ohtsu fishing port and Tenpisan Point in Ibaraki Prefecture taken in October 1975.

Figure 2.3.24 shows an aerial photograph of the area taken in October 1975. The large promontory at the northeastern end is Ohtsu Point, and the rocky coastline called the Izura coast extends alongshore. A small rock protruding 100 m from the shoreline in the southern part of the figure is called “Tenpisan Point,” and the Okita River meandering northward flows into the sea south of this point. In 1975, the Ohtsu fishing port was located at the foot of Ohtsu Point, and a port breakwater 500 m long extended southwestward from the point, along with a jetty 200 m long at the tip of the breakwater. Since the port breakwater extended into the wave-shelter zone of Ohtsu Point, the influence of this port breakwater on the coastline south of the fishing port was negligible, and a sandy beach 50 m wide extended continuously from the Ohtsu fishing port to Tenpisan Point. Figure 2.3.25 shows an aerial photograph taken in April 1992. Until that time, the Ohtsu fishing port was extended. An offshore breakwater 750 m long was built parallel to the old offshore breakwater at a location 250 m from the shoreline. A jetty 500 m long was also constructed at the mouth of the Satone River along with another jetty 500 m long at the mouth of the Edogami River, and three detached breakwaters were installed. Since the new offshore breakwaters protruded out of the wave-shelter zone which was formed naturally by Ohtsu Point, a large wave-shelter

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

95

Fig. 2.3.25. Aerial photograph between Ohtsu fishing port and Tenpisan Point in Ibaraki Prefecture taken in April 1992.

zone was formed west of the offshore breakwater, significantly accelerating shoreline advance. The maximum shoreline advance reached 250 m at the mouth of the Edogami River. The shoreline advance gradually decreased southward from the river’s mouth. In contrast, the shoreline retreated 20 m in the area between 200 m south of the detached breakwater and Futatsujima Island. In Fig. 2.3.24, Futatsujima Island is located near the shoreline, but as a result of beach erosion, it has become isolated in the sea, as shown in Fig. 2.3.25. South of Futatsujima Island, four detached breakwaters were built, and sand accumulated behind the structures, forming cuspate forelands. These detached breakwaters, built between Futatsujima Island and Tenpisan Point, induced southward longshore sand transport that supplied the sand for forming the cuspate forelands behind the structures; this accelerated beach erosion north of Futatsujima Island. Fishing port facilities were built on the new sandy beach that was formed by successive sand deposition due to longshore sand transport toward the wave-shelter zone of the Ohtsu fishing port. This resulted in a decrease in the volume of sand available to be moved freely by wave action in the zone south of the fishing port. On the Kamioka-kami coast south of the Ohtsu fishing port, the shoreline greatly retreated, and it was protected heavily by concrete armor units.

May 26, 2010

96

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.26. Narrow foreshore in front of seawall in vicinity of Futatsujima Island.

(2) Field observations On 28 December 1996, field observations of the Kamioka-kami coast were carried out south of the Ohtsu fishing port. Figures 2.3.26–2.3.28 were taken on this coast. At that time, a narrow foreshore was left in front of the seawall in the vicinity of Futatsujima Island as shown in Fig. 2.3.26. However, north of this location, the foreshore in front of the seawall disappeared, resulting in the exposure of the seawall to waves, as shown in Fig. 2.3.27. In Fig. 2.3.28 taken at a position further north, many concrete armor units were placed along the seawall. The concrete armor units in front of the seawall significantly subsided as shown in these photographs, and further maintenance work was required. Thus, a long stretch of the sandy shoreline of this coast disappeared. (3) Beach erosion triggered at boundary between different coastal management areas The beach changes around the Ohtsu fishing port and the resultant formation of an artificial coastline arises not only as a scientific phenomenon of sand

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.3.27. Seawall and subsided concrete armor units in eroded zone.

Fig. 2.3.28. Seawall, subsided concrete armor units, and coastal forest.

97

May 26, 2010

98

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.29. Coastal management areas and related laws.

accumulating at part of a long coastline while erosion takes place in other regions, but also from the system of management of coastal land in Japan (Uda et al., 2005). Considering the case of the Ohtsu fishing port, the coastal management areas around the fishing port are schematically drawn in Fig. 2.3.29. One breakwater of the fishing port is assumed to be extended at the corner of the coastline. In this case, the management area of the coast is subdivided into the fishing port (port) area and the coastal protection zone outside the fishing port on the basis of the Coastal Act. Landward of the coastal protection zone is the coastal forest zone for preventing wind-blown sand under the jurisdiction of the Forestry Agency of the Ministry of Agriculture, Forestry and Fisheries on the basis of the Forest Law. When the port breakwater is extended, longshore sand transport, which is otherwise stable, is induced from outside to inside the wave-shelter zone as a result of the wave-sheltering effect of the breakwater at the sandy beach. Physically, sand is transported toward the wave-calm zone and is deposited there. The fundamental problem lies not in this physical process, but in the management of the coastal land. The construction of fishing ports, the

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

99

management of the coastal forest, and coastal protection work are carried out by the Fishery Bureau, the Agricultural and Forestry Bureau, and the Civil Engineering Bureau, respectively, of the prefectural government. Each work is planned so as to present the maximum economical rationality in the short term without sufficiently considering the degradation of the environment in the surrounding area; this is local optimization. For instance, sand deposited in the navigation channel of the fishing port was regarded as an obstacle and dumped in the offshore zone or used as material for land reclamation by one management office, as schematically shown in Fig. 2.3.29. On the other hand, at the southern coast, where sand was removed by longshore sand transport, the beach slope became very steep. A seawall, concrete armor units, and detached breakwaters were installed as countermeasures against beach erosion by a different coastal management office. True adjustment of the flow of sand, which is an obvious physical phenomenon, through cooperative work among various management offices, was troublesome, and it has been postponed. Regarding the issues common to several agencies, there was no common principle for soundly maintaining national land. Countermeasures against beach erosion are required when a beach is eroded by sand movement due to longshore sand transport. In this case, even if the cause of beach erosion is scientifically proven to be the result of the construction of port breakwaters, it has been difficult for the manager responsible for coastal protection in the surrounding areas to clearly criticize this situation, because all management offices belong to the same prefectural government. Furthermore, it is difficult for the manager responsible for the eroded coast to report the real cause, because the state budget is allotted only for natural disasters on the basis of the disaster restoration system in Japan. If managers report the true cause, they would not receive any state budget for restoration work, a consequence which must be avoided, taking the reality of coastal erosion into account. Abandonment of countermeasures is also difficult because local residents always request urgent measures. Thus, fundamental measures are difficult to adopt and, instead, the stopgap measure of installing a seawall with concrete armor units is selected, which rapidly produces an artificial coastline.

May 26, 2010

100

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

2.3.4. Ajigaura Beach and Naka Coast in Ibaraki Prefecture Ajigaura beach and the Naka coast are located at the south and north parts of the coastline of a 10 km stretch between the mouth of the Kuji River and Isozaki Point. In recent years, a very long offshore breakwater at Hitachinaka Port has been constructed in the middle of the coastline. Although Ajigaura beach was very famous as a bathing beach and a surf spot, severe beach erosion was triggered by the extension of the offshore breakwater, resulting in the disappearance of a natural sandy beach. Figure 2.3.30 shows an aerial photograph of Ajigaura beach and the Naka coast taken in 1998. Originally, a continuous sandy beach extended 10 km from Isozaki Point to the north of the Kuji River, but the Tokai Port was constructed first in the northern part, and now the construction of Hitachi-naka Port is underway in the middle of the coastline, as shown in Fig. 2.3.30. In particular, the scale of Hitachi-naka Port is large, and a long offshore breakwater 3.6 km long was extended 2.4 km off the shoreline in 1998, and was further extended 4.3 km to P in Fig. 2.3.30 by November 2002, resulting in great changes in the wave field. Here, the changes at Ajigaura Beach and the Naka coast located south and north of the port area, respectively, are described in detail (Uda et al., 2003a∗ ). (1) Ajigaura beach The south end of Ajigaura beach is bounded by the Isozaki fishing port next to Isozaki Point, and a hooked shoreline was formed in the wave-shelter

Fig. 2.3.30. Aerial photograph of Ajigaura beach and Naka coast taken in 1998.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

101

zone north of this fishing port. The foreshore width in front of the seawall was as narrow as 38 m in the vicinity of the fishing port in 1988. On the other hand, sand accumulated in the southern part of the breakwater of the construction base of Hitachi-naka Port, but the shoreline changes were not as large because the wave-sheltering effect of the offshore breakwater did not reach Ajigaura beach. On 4 November 2002, field observations were carried out at Ajigaura beach. Figure 2.3.31 shows the sand bags urgently placed on top of the seawall to prevent wave overtopping over the seawall at the south of Ajigaura beach. It is seen from Fig. 2.3.31 that the seawall line breaks at a location further north, and there the shoreline gradually approaches the seawall line, resulting in a narrowing of the foreshore north of the Isozaki fishing port. Figure 2.3.32 shows the condition in the vicinity of the location where the shoreline coincided with the toe of the seawall. The color of the lower half of the seawall is white compared with that of the upper half, implying that the toe of the seawall has been buried underneath the sandy

Fig. 2.3.31. Sand bags placed on top of seawall (4 November 2002).

May 26, 2010

102

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.32. Very narrow foreshore, where seawall line almost coincides with shoreline.

beach until recently. Furthermore, it was observed from comparison with a measuring stick with 20 cm intervals that the ground elevation declined by 1.2 m. Beach erosion was further confirmed by the appearance of iron sand with a large specific gravity on the foreshore in front of the toe of the seawall. In front of the location where the seawall line breaks, shown in Fig. 2.3.32, the seawall collapsed at the end of October 2002, and urgent measures were taken. In the central part of Fig. 2.3.33, the seawall destroyed by local scouring can be seen. Sand in front of the sloping revetment was carried away by northward sand transport, resulting in an increase in the toe depth of the revetment and the discharge of foundation material under the revetment. Then, the slope of the revetment fell down. Several gabions with concrete blocks and stones inside were placed in front of the damaged revetment as an urgent measure, but the iron wires of the gabions had been broken, permitting the outflow of stones. This area coincides with the location of a very narrow foreshore, as shown in Fig. 2.3.30.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

103

Fig. 2.3.33. Seawall destroyed due to local scouring.

North of the damaged revetment, a sandy beach appeared once again, even though the shoreline retreated. However, a beach that had been composed of fine sand was now totally covered with gravel, as shown in Fig. 2.3.34. Figure 2.3.35 shows the scale of the gravel, in comparison with a measuring stick with 20 cm intervals; the grain size of the gravel ranges from several centimeters to several tens of centimeters. Thus, a large amount of fine sand disappeared at Ajigaura beach due to severe erosion, and the foreshore was covered with gravel, thereby steepening the longitudinal slope. Many sea bathers without knowledge of the erosion were injured by the gravel bed. In contrast to this, a large amount of fine sand accumulated in the wave-shelter zone of the offshore breakwater of Hitachi-naka Port while forming a gentle slope, as shown in Fig. 2.3.36. The accumulation of fine sand contrasts well with the formation of a scarp, the exposure of iron sand with a large specific gravity on the foreshore, and the covering of the shore face by gravel observed on the southern Ajigaura beach.

May 26, 2010

104

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.34. Foreshore covered with gravel.

Fig. 2.3.35. Size of gravel.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

105

Fig. 2.3.36. Accretion of fine sand in wave-shelter zone of offshore breakwater of Hitachinaka Port.

(2) Naka coast The Naka coast is located north of Hitachi-naka Port. A natural coastline with sand dunes once extended straight from Ajigaura beach to the Naka coast before the construction of Hitachi-naka Port. At the south end of the Naka coast, the Shinkawa River flows into the sea, and there is a training jetty at the mouth of this river, as shown in Fig. 2.3.30. At the Naka coast, the dominant wave-sheltering effect due to the offshore breakwater of the port had been observed already in 1998, and a triangular foreshore was formed north of the river’s mouth. This triangular foreshore had a 1 km stretch alongshore. When extrapolating the shoreline of this foreshore, the line makes an angle of 12◦ counterclockwise to the seawall line south of Tokai Port. From this, it can be seen that the sandy beach in front of the seawall south of Tokai Port was eroded due to the wave-sheltering effect of the offshore port breakwater, and eroded sand was transported southward, resulting in sand deposition in the vicinity of the training jetty of the Shinkawa River.

May 26, 2010

106

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.37. Shoreline north of training jetty.

Figure 2.3.37 shows the shoreline north of this training jetty. The foreshore is wide in the southern part of the beach; it gradually narrows northward, and finally the seawall line coincides with the shoreline. Figure 2.3.38 shows the vicinity of the intersection between the seawall and the shoreline. The shoreline became attached to the seawall, resulting in the disappearance of the foreshore and the wave run-up over the seawall slope. Almost all the concrete armor units installed along the seawall to protect the toe subsided and thereby lost the wave-dissipation effect. The foreshore totally disappeared north of this location. Further north of the location shown in Fig. 2.3.38, a large cave-in 2 m deep was found as shown in Fig. 2.3.39. An extremely deep hole through the foundation of the seawall was observed, and it was very dangerous to walk along the seawall. North of the cave-in area, the seawall was completely destroyed, resulting in the formation of a very high scarp in the coastal forest, as shown in Fig. 2.3.40. Similarly, the picture in Fig. 2.3.41 was taken from the top of the seawall looking north, showing erosion and the collapsed

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.3.38. Intersection between seawall and shoreline.

Fig. 2.3.39. Large cave-in 2 m deep behind seawall.

107

May 26, 2010

108

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.40. Severe erosion with formation of high scarp.

Fig. 2.3.41. Erosion and collapse of seawall, looking northward from top of seawall.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

109

seawall. Debris from the collapsed seawall can be seen along the extension of the existing seawall. Thus, on the Naka coast as well, a strong longshore sand transport was triggered by the extension of an offshore breakwater of the port, flowing from outside the wave-shelter zone to inside, and severe beach erosion took place in the surrounding area of the port.

2.3.5.

Kemigawa Beach in Chiba Prefecture

(1) Shoreline change The Kemigawa beach is located at the bottom of Tokyo Bay, and its shoreline faces southwest in the bay as shown in Fig. 2.3.42. The most frequent wind direction through the year is NNW (frequency 11.9%), the second is SSW (10.9%), and the third is NNE (9.7%) at the wave observation tower of Chiba Port, as shown in Fig. 2.3.42. These wind directions are affected by

Fig. 2.3.42. Location of Kemigawa beach in Tokyo Bay.

May 26, 2010

110

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.43. (a) Plane view and (b) cross-sectional view of Kemigawa beach.

the geographical features of Tokyo Bay. Most of the strong winds blow from SSW. The construction of the artificial beach was started in 1977 and completed in 1991. As shown in Figs. 2.3.43(a) and 2.3.43(b), an artificial reef to prevent sand from transporting offshore and two curved jetties were built at both ends of the beach, as well as a straight groin at the center of the beach. The beach was 1,300 m long and 50 m wide at the high-water level, and 130 m wide at the low-water level. As for the beach materials for nourishment, the median diameter (d50 ) was 0.162 mm, the sand volume of the nourishment was 1.23 × 106 m3 , and the initial slope was 1/20. Since the completion of the beach, erosion started, resulting in the formation of a dominant scarp in 1993, and aY-shaped groin was completed at a location 150 m north of the center of the beach in 1995 as one of the countermeasures against beach erosion. The past shoreline changes of Kemigawa beach can be studied using aerial photographs (Kumada et al., 2001a,b∗ ; Kumada et al., 2002).

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

111

Fig. 2.3.44. Aerial photographs of Kemigawa beach.

Figure 2.3.44(a) shows an aerial photograph taken in 1987 after beach nourishment. The tip of the curved groins had not been built at this stage, and the shoreline was straight alongshore. In 1993, the curved groins were completed as shown in Fig. 2.3.44(b), and the shoreline in the vicinity of the foot of the curved groins advanced, whereas the shoreline at the center of the pocket beach retreated, because calm-wave zones were formed by the construction of curved groins.

May 26, 2010

112

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Figure 2.3.44(c) shows an aerial photograph taken in 1999. Further dominant shoreline changes were observed. The short groin at the center of the beach had been removed, and a Y-shaped groin had been built at a location 150 m north of the center of the beach, although it extended straight from the shoreline in 1999 because it was under construction. With the construction of the Y-shaped groin, the shoreline advanced at the foot of the groin, whereas the shoreline retreated between these structures. Figure 2.3.45 shows the bathymetry in 1999. The contours between the elevations of 2 and −3 m changed their forms to concave, and a scarp was formed near the shoreline. The initial contours were parallel to the seawall, but those in the zone shallower than −3 m changed their form to concave. The depth of closure, hc , is assumed to be approximately −3 m in the erosion zone. On the other hand, a flat surface with an elevation of 2 m was formed due to accretion, while a berm near the north and south curved groins and a very steep seabed slope were formed toward the tip of the groin due to the deposition of sand in the deeper zone. It is concluded from the shoreline and contour line changes that beach changes were triggered by the construction of curved groins under the action of longshore sand transport on Kemigawa beach. The shoreline positions were read from the aerial photographs taken from 1987 to 1999, and the shoreline change (
Y ) was calculated with

Fig. 2.3.45. Bathymetry of Kemigawa beach measured in 1999.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

113

Fig. 2.3.46. Shoreline changes with reference to shoreline in 1987.

reference to the shoreline form in 1987. The measured shoreline positions were corrected to determine the location of the shoreline at the mean sea level (MSL) by harmonic analysis using a tide table at an interval of
X = 10 m in the longshore direction. Figure 2.3.46 shows the value of
Y with reference to 1987. The maximum shoreline recession in 1999 was
Y = −34.6 m at X = 710 m. This shoreline recession pertains to 43% of the entire initial beach width of 80 m. In contrast, shoreline advance reached
Y = 90.6 m at X = 1,290 m. The eroded area, A1 , and accreted area, A2 , are determined as A1 = −1.6 × 104 m2 and A2 = 2.2 × 104 m2 , respectively, by multiplying the change in shoreline position
Y and
X. (2) Field observations Field observations of Kemigawa beach were carried out on 9 September 2000. Figure 2.3.47 shows the shoreline in the vicinity of the north groin, looking south from the top of the groin. A protruding shoreline was formed at this site, along with a berm with several high-water marks and a flat plain behind it. This clearly indicates that this flat plain was formed by successive accretion due to waves in the wave-shelter zone. Figure 2.3.48 shows the shoreline at a site further approaching the central part. The shoreline becomes concave from this site in contrast to the shoreline in the vicinity of the groin, as shown in Fig. 2.3.47, and a scarp can be seen at the far side. A berm is also formed along the shore

May 26, 2010

114

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.47. Protruding shoreline in vicinity of north groin widened by successive sand accumulation (9 October 2000).

Fig. 2.3.48. Shoreline in vicinity of neutral profile without erosion and accretion.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

115

Fig. 2.3.49. Overall view of eroded zone.

at this area, but the presence of a berm becomes obscure. In comparing Figs. 2.3.47 and 2.3.48, it is found that a transition point exists, where the shoreline configuration changes from convex to concave. The longitudinal profile through this transition point gives an approximately neutral profile without the advance and recession of the shoreline position. Figure 2.3.49 shows the shoreline zone in the central part of the northern half of the pocket beach. A scarp was formed on the face of the nourished beach, which was created parallel to the seawall line, and the buried concrete armor units were exposed. Since wave overtopping was severe in this area because of the shoreline recession and the appearance of a vertical wall, these concrete armor units were installed, but they destroyed the scenic beauty, diminishing the ability to use the coast. The scarp height reached 1.8 m, as shown in Fig. 2.3.50, and it is clear that a large amount of nourished sand was carried away and moved toward the lee side of the north groin, as shown in Fig. 2.3.47, due to northward longshore sand transport.

May 26, 2010

116

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.50. Scarp formation in eroded zone.

Fig. 2.3.51. Line of color on surface of seawall.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

117

Fig. 2.3.52. Overview of eroded zone.

Figure 2.3.51 shows the south end of exposed blocks. Judging from the change in color on the surface of the seawall, the seawall was buried up to the horizontal line as shown by the measuring stick, and then it was exposed by erosion. Figure 2.3.52 shows an overview of the central part of the beach, revealing severe erosion. As mentioned, the topographic changes occurring at Kemigawa beach are another typical example of beach erosion and accretion associated with the formation of a wave-shelter zone.

2.3.6. Tojo–Maebara Coast in Kamogawa City in Chiba Prefecture (1) Shoreline changes The Tojo–Maebara coast is a pocket beach 3.9 km long, located in the southern part of the Boso Peninsula facing the Pacific Ocean, as shown in Fig. 2.3.53. It is bounded by Kuzugasaki Point and Benten Island at the

May 26, 2010

118

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.53. Location of Tojo–Maebara coast.

northeast and southwest ends, respectively. In recent years, the breakwaters of Kamogawa fishing port have been extended at the southwest end, resulting in shoreline changes (Uda et al., 2000∗ , 2001c∗ ). Figure 2.3.54(a) shows an aerial photograph in 1967. At this time, a natural sandy beach continuously extended along the coastline without any large-scale wave-sheltering structures, but the shore of the Maebara coast in front of the streets of Kamogawa City was extremely narrow. In 1996, significant shoreline changes were induced in the vicinity of the south end of the pocket beach, as shown in Fig. 2.3.54(b), compared with the condition in 1967. Between the mouths of the Kamogawa and Matsuzaki Rivers, two detached breakwaters 200 m long were built and a short detached breakwater 60 m long was constructed in the opening of these detached breakwaters. Due to the construction of these detached breakwaters, wave calmness was enhanced behind the breakwaters,

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

119

Fig. 2.3.54. Aerial photograph of Tojo–Maebara coast: (a) 1967, (b) 1996, and (c) 1998.

May 26, 2010

120

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

resulting in the formation of the salient behind them. Simultaneously, the shoreline in the central part of the pocket beach significantly retreated and the foreshore was narrowed. Before 1967, the shore width was small south of the mouth of the Matsuzaki River, but in 1996, the shore south of the mouth was greatly widened; the relative shore width was totally reversed with respect to the mouth. In addition, the initiation of the construction of the new breakwaters of the Kamogawa fishing port next to the mouth of the Kamogawa River can be also seen in Fig. 2.3.54(b). Figure 2.3.54(c) shows an aerial photograph taken in 1998. Although the overall coastal condition is similar to that in 1996, dominant shoreline changes were observed south of the detached breakwater located next to the Matsuzaki River. Since the new port breakwater, constructed until 1998, created a new wave-shelter zone, the shoreline significantly retreated in the opening between the detached breakwaters, which were 200 m long, where the central part of the Maebara coast in front of the city is. Sand that eroded from the shoreline at the opening of the detached breakwaters was carried to the lee side of the southernmost detached breakwater and the new breakwater of the Kamogawa fishing port, and part of the foreshore that formed on the lee side of the new breakwater was buried and used as reclaimed land. Figure 2.3.55 shows the shoreline changes between 1974 and 1998. It is clear that the shoreline retreated in the central part of the pocket beach as a whole, and sand deposited in the wave-shelter zone that was formed on the lee side of the southernmost detached breakwater and the breakwater of the fishing port. The maximum shoreline advance at the south end of the shoreline was 100 m, whereas the shoreline recession in front of the Sea World Aquarium was 21 m. The entire eroded and accreted foreshore areas were 4.88×104 m2 and 4.48×104 m2 , respectively, and they approximately agreed with each other. As mentioned, on the Tojo–Maebara coast as well, dominant shoreline recession and advance were caused on a pocket beach, because the wavesheltering structures were built at the corner of the pocket beach with a closed system of littoral drift.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

121

Fig. 2.3.55. Shoreline changes of Tojo–Maebara coast (1974 versus 1998).

2.3.7.

Shimobara Fishing Port in Tateyama City in Chiba Prefecture

(1) Shoreline changes The area studied is the coast surrounding the Shimobara fishing port in Tateyama Bay in Chiba Prefecture, as shown in Fig. 2.3.56. Since the coastline in this area is located deep in Tateyama Bay, it is sheltered from high waves in the Pacific Ocean, and wind waves from the north are mainly incident to the coast. In this area, the breakwater of the fishing port has been rapidly extended since 1997, and beach erosion occurred due to the wave-sheltering effect of this breakwater (Hoshigami et al., 2003∗ ). Figure 2.3.57 shows aerial photographs of the area between 1997 and 2002. Before 1997, there was a natural hooked shoreline 600 m long west of the Shiomi fishing port. The Shimobara fishing port was constructed along a coastline 500 m long, bounded by a small rock at the western end and the breakwater of the Shiomi fishing port at the eastern end. By 1999, the port breakwater was obliquely extended, and then shoreline recession started from the eastern part of the coastline. In 2002, a large shoreline recession could be seen between the central part of the coast and the rock located in the eastern part, as well as a large shoreline advance in the vicinity of the fishing port. As a result, the

May 26, 2010

122

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.56. Location of Shimobara fishing port in Tateyama Bay.

shoreline in the adjacent area of the fishing port advanced greatly, whereas the shoreline receded in a region between the central and eastern parts of the coastline. Figure 2.3.58 shows the shoreline configurations and their changes with reference to the situation in 1996. Rocks located in the eastern part behaved as a solid boundary for longshore sand movement, and the shoreline retreated west of the rocks and vice versa. The transition point of the shoreline advance and recession is located at x = 300 m, and the shoreline greatly advanced toward the lee side of the port breakwater west of this point. The maximum shoreline advance and recession in the five-year period between 1997 and 2002 reached 47 m and 15 m, respectively. The shoreline

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.3.57. Aerial photographs of Shimobara fishing port.

123

May 26, 2010

124

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.58. Shoreline configurations and their changes with reference to the shoreline in 1997.

changes occurred monotonically with time due to the extension of the breakwater. (2) Field observations Field observations were carried out on 24 November. Figure 2.3.59 shows the overview of the area studied, looking west from the top of the sand dune next to the Shiomi fishing port. The breakwater of the Shimobara fishing port extends in the far distance in the photograph. There were no coastal facilities, such as a jetty, blocking longshore sand transport toward the wave-shelter zone of the breakwater. Therefore, a large amount of sand was transported westward. Two exposed rocks can be seen in Fig. 2.3.59. These rocks had been buried under the surface before erosion, but they were then exposed because of erosion.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

125

Fig. 2.3.59. Overview of area studied, looking west from east end of coast (24 November 2002).

The area covered with coastal vegetation in Fig. 2.3.59 is a sand dune formed by wind-blown sand, but at that time, a scarp 1.6 m high was observed at the front surface, as shown in Fig. 2.3.60. There was a steep slope with an angle of repose of sand on the foot of the scarp, but many plants overhung the upper part. In contrast, the beach slope in front of the scarp was very mild. A mudstone rock with a flat surface near the center of the photograph is a bench raised from the water during the Great Kanto Earthquake of 1923, which produced a rise in ground level of 1.5 m. Figure 2.3.61 shows another view of the scarp formed at a location closer to the fishing port than the location shown in Fig. 2.3.60. At this location, the scarp height was lowered to 1.1 m. Severe beach erosion, caused when the scarp was formed, occurred at this site until recently, judging from the fact that many tree roots, which were deep in the ground, were exposed. At this location, several high trees can be seen in front, and a small stream flows into the sea in the area adjacent to this forest. At a location close to

May 26, 2010

126

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.60. Scarp 1.6 m high formed on natural sand dune.

Fig. 2.3.61. Scarp formation in middle of locations between sand dune and small stream.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

127

Fig. 2.3.62. Scarp formation west of small stream.

the right-hand-side bank of this stream, a new scarp was formed, as shown in Fig. 2.3.62, but the scarp height was reduced to 0.7 m. Figure 2.3.63 shows a rock stabilizing the mouth of the small stream as a training jetty. This rock has functioned as a solid boundary for the shoreline changes before the beach changes associated with the extension of the breakwater of the Shimobara fishing port occurred, forming a hooked shoreline downcoast under the predominant condition of eastward longshore sand transport. This rock also fixed the downcoast boundary of the hooked shoreline extending from the rock at its current location in the fishing port. Furthermore, the shoreline was discontinuous at the rock as shown in Fig. 2.3.63, and this caused a difference in the position of the shoreline extending from the western part of the coast.After the construction of the fishing port breakwater, westward longshore sand transport flowed into the wave-shelter zone from the outside, resulting in the reversal of the direction of longshore sand transport and the disappearance of the discontinuity in the shoreline at the rock.

May 26, 2010

128

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.63. Rock functioning as training jetty.

West of this stream, scarp formation was not observed, implying that this location was the transition point from erosion to accretion zones. Figure 2.3.64 shows the shoreline extending from the river’s mouth to the fishing port. A curved shoreline can be seen, which gradually advanced in the vicinity of the fishing port. In contrast, in Fig. 2.3.65, showing the shoreline east of the fishing port, a berm formed by sand accumulation in the wave-shelter zone behind the offshore breakwater of the fishing port can be clearly seen. It is well known that when an offshore breakwater is constructed on a straight coast, forming a wave-shelter zone behind the breakwater, longshore sand transport from outside the wave-shelter zone to inside is induced, resulting in erosion outside the wave-shelter zone and accretion inside. There are many examples of this in Japan. The stable shoreline configuration in such a case can be sufficiently predicted by Hsu and Evans’ method as described in Sec. 3.2 or by the model in Sec. 3.4, which is applicable to the prediction of beach changes on a coast with a seawall (rock).

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

129

Fig. 2.3.64. Curved shoreline in vicinity of fishing port.

Fig. 2.3.65. Berm formation due to sand accumulation in wave-shelter zone of fishing port.

May 26, 2010

130

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Nevertheless, absolutely the same event was repeated in only four years between 1998 and 2002 at the Shimobara fishing port, and erosion intensified in the surrounding area of the fishing port, brought about by sand movement from outside the wave-shelter zone to inside. Thus, the condition of the natural coast, which had long been used as a bathing spot, was altered to that of an artificial coastline, losing the coast’s recreational value. This area belongs to a seminational park with a high environmental value, and many resort houses are located in the hinterland, implying that the coast has a great potential for utilization. Beach erosion triggered by the construction of the fishing port endangered the use of the coast by narrowing the beach width and steepening the foreshore slope.

2.3.8. Asamogawa Coast in Kyoto Prefecture The Asamogawa coast is located on the Tango Peninsula of Kyoto Prefecture, as shown in Fig. 2.3.66, facing the Sea of Japan. On this coast, a new seawall was built at a location seaward of the original seawall line to widen the public parking space next to the beaches, a groin was constructed and the beach nourished. Originally, the Asamogawa coast was located in part of a pocket beach with a dynamically varying shoreline, but due to the construction of a groin at the west end of the coastline, a wave-shelter zone was formed, resulting in erosion and accretion along the entire pocket beach (Uda et al., 1996∗ ). The shoreline changes between August 1987, before the construction of the groin, and March 1991 are shown in Fig. 2.3.67. The predominant direction of the incident waves on this coast is N24◦W. When a straight line is drawn in this direction through the tip of the groin, and the point of the intersection with the shoreline in August 1987 is set to be point Q, the shoreline can be described as retreating in a wide area east of point Q, whereas it is advancing west of this point. The maximum shoreline advance and recession reached 70 m at the foot of the groin and 25 m at the central part of the pocket beach, respectively. As mentioned, the beach changes along the Asamogawa coast were a typical example of the phenomenon in which significant shoreline advance

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

131

Fig. 2.3.66. Location of Asamogawa coast in Kyoto Prefecture facing Sea of Japan.

Fig. 2.3.67. Shoreline changes of Asamogawa coast between August 1987 and March 1991.

May 26, 2010

132

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

and recession occur on an entire pocket beach, when an impermeable structure is built at the corner of the pocket beach. To clearly understand the temporal changes in the shoreline configuration, a curvilinear coordinate system perpendicular to the seawall line in March 1991, as shown in Fig. 2.3.67, is selected first, the shoreline changes along the axis are read, and the results are shown in terms of cumulative longshore distance. The measurement interval along the shoreline is 25 m, and the reference year is taken as August 1987. The results are shown in Fig. 2.3.68. It is clear that sand movement from outside the wave-shelter zone to inside was triggered by the construction of the groin, which accelerated sand deposition inside the wave-shelter zone.

Fig. 2.3.68. Shoreline change with reference to shoreline in August 1987.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

133

2.3.9. Tsutsuki Beach on Iki Island in Nagasaki Prefecture (1) Shoreline changes Tsutsuki beach is located at the eastern side of Iki Island in the Sea of Genkai, as shown in Fig. 2.3.69. Tsutsuki Point, located on the east end of the beach, is a headland forming a barrier against incident waves. There is a rocky coast at the west end. This is a typical pocket beach 0.8 km long, exposed to waves coming from the SSE. Furthermore, small-scale rocky coasts and pocket beaches developed along the 1.5 km long stretch south of this beach to Otsujima Point. On this beach, significant beach changes were likewise triggered by the elongation of the breakwater of the fishing port in a corner of the pocket beach (Uda et al., 2001a∗ , 2002). For Tsutsuki beach, seven sets of aerial photographs are available, which were taken during 33 years from 1966, before the occurrence of major beach

Fig. 2.3.69. Location of Tsutsuki beach on Iki Island.

May 26, 2010

134

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

changes, to 1999, when the shoreline had essentially stabilized. The change in the shoreline position due to tide level fluctuation was corrected using the foreshore slope obtained in the beach survey and the tide level at the time the aerial photographs were taken. In December 1966, a sandy beach with a uniform width of 50 m existed, as shown in Fig. 2.3.70(a). At the east end of the beach, the shore-parallel and shore-normal breakwaters of the Nana-minato fishing port were located. The length of these breakwaters was short at this stage, so that they did not affect the sheltering zone that Tsutsuki Point forms. For this reason, the shoreline of the pocket beach was stable. In April 1971, the west breakwater was extended to the tip of the shoreparallel breakwater, as shown in Fig. 2.3.70(b), and the rectangular fishing port was completed. Since the extended breakwater protruded slightly into the wave-shelter zone of Tsutsuki Point at this stage, a new wave-shelter zone was formed north of this breakwater, increasing sand accumulation. In October 1977, two oblique breakwaters were built from one end of the old breakwaters that had formed a rectangular basin, as shown in Fig. 2.3.70(c). The length of the east breakwater measured from its base is 180 m, whereas that of the west breakwater is 250 m, and the direction of the breakwater altered inward near the tip to increase the level of wave calmness inside the port. Since the west breakwater is located in the waveshelter zone that was formed by the east breakwater against the dominant incident waves to the pocket beach, the influence of the construction of the west breakwater on beach change is minimal. On the other hand, the east breakwater was extended approximately parallel to the shoreline of the pocket beach, resulting in the formation of a large wave-shelter zone behind the breakwater to accelerate sand deposition. Since the sand volume of the pocket beach is considered to be approximately constant on the engineering time scale, the shoreline retreated due to erosion from the central part to the western part of the pocket beach, whereas sand was deposited in the eastern portion along the shoreline in the vicinity of the breakwater. The cause of sand drift is the occurrence of eastward longshore currents induced by the formation of the wave-shelter zone.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

135

Fig. 2.3.70. Aerial photographs of Tsutsuki beach: (a) December 1966, (b) April 1971, (c) October 1977, (d) May 1985, (e) May 1993, and (f) February 1999.

In May 1985, the shoreline behind the breakwater advanced further, whereas the shoreline along the western part of the beach retreated further, as shown in Fig. 2.3.70(d). In 1966, the sandy beach had a constant alongshore width of 50 m, but in 1985 an asymmetrical shoreline was

May 26, 2010

136

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

formed: its maximum widths were about 100 m and 20 m at the east and west ends, respectively. A close observation of the distribution of plants on the beach, as shown in Fig. 2.3.70(d), leads to the recognition that afforestation had begun to create a coastal forest in the natural sand dune area where the sandy beach was largely widened west of the breakwater. The photograph taken in May 1993, shown in Fig. 2.3.70(e), shows that a considerably large area was covered with pine trees due to the planting begun in 1985. At the same time, the shoreline was still advancing in the area next to the breakwater. In 1985, the shoreline was located 40 m landward from the western corner of the old fishery facility, but in 1993 the shoreline became attached to this corner, giving distinct evidence of the shoreline advance. In February 1999, the zone covered with coastal vegetation extended 30 m at a maximum in front of the coastal forest in the sand deposition zone next to the breakwater, as shown in Fig. 2.3.70(f). Off the corner of the west breakwater, the seabed with exposed rocks can be seen as black in color. Figure 2.3.71 shows that the areas covered with the coastal forest and vegetation have extended with time on the eastern half of the beach; the evolution of shoreline changes in this area is also shown in Fig. 2.3.71. The area of the coastal forest advanced by 100 m at maximum, as the beach was widened by sand accretion along the west side of the breakwater. Thus, the original natural sandy beach was artificially altered to the coastal forest by the shoreline advance, and the sandy beach was stabilized against windblown sand by the coverage of pine trees. In the erosion area of the western portion of the beach, the sandy beach was narrowed by 20 m because of the shoreline recession of 37 m at maximum, despite the fact that the location of the seaward borderline of the coastal forest did not change. This became a large factor for diminishing the use of the beach in summer. Comparison of aerial photographs showed that the shoreline advance and recession were triggered by the construction of breakwaters, both occurring at a pocket beach with a closed system of littoral drift, but the former in the wave-shelter zone and the latter far from the wave-shelter zone. This point is further analyzed quantitatively. Figure 2.3.72 shows the shoreline changes over 33 years between 1966 and 1999.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

137

Fig. 2.3.71. Evolution of change in areas covered with coastal forest and coastal vegetation and shoreline changes.

Fig. 2.3.72. Shoreline changes determined from aerial photographs.

May 26, 2010

138

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Until 1966, a sandy beach with a uniform width of 50 m existed, but the shoreline gradually advanced to the wave-shelter zone at the east side, and retreated at the west side of the pocket beach after the construction of port breakwaters. The maximum shoreline advance and recession since 1966 were 51 m and 37 m, respectively. The erosion and accretion zones are divided clearly at P (X = −390 m), and the shoreline recedes west of this point, whereas it advances east of it. This pattern of shoreline change indicates that the original, stable shoreline, before the construction of breakwaters, changes into another stable form in response to altered wave field: this is due to wave diffraction by the breakwaters. Figure 2.3.72 shows the result of the theoretical solution of the stable shoreline of a pocket beach given by the modified Hsu and Evans’ method as mentioned in Sec. 3.2, taking the sand budget of the pocket beach into account. The shoreline measured in 1999 agrees well with the theoretical result. Figure 2.3.73 shows the change in shoreline position at survey lines A and H located in the erosion and accretion zones, respectively, as shown in Fig. 2.3.72, with reference to the shoreline position in 1966. The shoreline

Fig. 2.3.73. Temporal change in shoreline position at survey lines A and H.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

139

change is measured perpendicular to the shoreline of 1966. The breakwater had already been constructed by 1971, producing a wave-shelter zone, and its influence on the shoreline gradually increased. The advance and recession of the shoreline is expressed as follows:
= 56.2 (1.0 − exp (−0.0095 t))
= −40.8 (1.0 − exp (−0.0060 t))

at survey line H, at survey line A,

(2.3.1) (2.3.2)

where t is the time. (2) Discussion The beach changes, which occurred on Tsutsuki beach, were triggered by the construction of the breakwaters of the fishing port along the pocket beach, which was stable before the construction. Elongation of the breakwater at one end of the pocket beach induced the formation of a wavecalm zone on the lee side of the breakwater, which in turn caused longshore sand transport toward the lee side from the outside. The shoreline on the pocket beach changed its form so as to be stable in response to the condition of incident waves formed due to the existence of the breakwaters. The easternmost breakwater, which formed a large wave-shelter zone, protruded 180 m alongshore in contrast to the alongshore length of around 0.5 km of the pocket beach. The ratio (0.36) of the breakwater length to the pocket beach length was not negligible and caused the overall shoreline changes of the pocket beach. On the Tsutsuki beach, the breakwaters had been constructed by 1971, but the beach changes associated with the formation of the wave-shelter zone triggered by the construction of the breakwaters continued for another 28 years until 1999, when the beach reached an almost stable form. The fact that the beach erosion stopped means that countermeasures against beach erosion are not needed, and rather, it is important to keep this beach intact without installing any kind of artificial coastal structures, such as seawalls and groins, to satisfactorily maintain a natural sandy beach. In fact, no measures for armoring the coast have been taken for this beach, and no problems have occurred up to the present. Thus, a natural sandy beach was prevented from becoming an artificial coast as a result of

May 26, 2010

13:35

140

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

excessive defense by the construction of a seawall and the installation of concrete armor units against beach erosion. Generally, measures are quickly taken in restoration work for coastal damages in Japan to respond to beach erosion and related damage to structures. It is often that such measures have made the coastal environment conditions worse or have induced a new coastal damage. In order to break the vicious cycle, considering leaving the beach deformation as it is important if the coastal conditions permit this step.

2.3.10.

Shiratsuru Beach in Amakusa District in Kumamoto Prefecture

(1) Shoreline changes Shiratsuru beach is located in the central part of Amakusa District in Kumamoto Prefecture in Kushu, Japan, as shown in Fig. 2.3.74. It is a sandy beach along a stretch 900 m long, facing the Sea of Amakusa. The Takahama River flows into Shiratsuru beach. This river meanders largely counterclockwise near the river’s mouth, finally pouring into the sea at the southern end of the pocket beach. Shiratsuru beach is a typical pocket beach bounded by Kami-ose and Shimo-ose Points at the northern and southern ends, respectively. Landward of the shoreline, towns are concentrated in the northern half of the Shiratsuru beach, whereas the natural sand dunes remain a coastal forest area along the right bank of the Takahama River in the southern half. The Takahama fishing port is an important port for the emergency shelter of small fishing boats in this district, and it is located on the rocky southern coastline. It is reasonable that this port was built along the southern coastline of the pocket beach to create a calm-wave zone inside the port, where relatively calm-wave conditions are expected due to the wave-sheltering effect of the headland of Shimo-ose Point against rough waves incident from the southwest during typhoon seasons. However, in recent years, the wave conditions of the pocket beach have changed greatly due to the extension of the breakwater of the Takahama fishing port at the southern end of the

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

141

Fig. 2.3.74. Location of Shiratsuru beach in Amakusa District in Kumamoto Prefecture, Kyushu, Japan.

pocket beach, resulting in shoreline recession and advance in the northern and southern parts of the beach, respectively (Uda et al., 2001b∗ , 2004). Eight sets of aerial photographs of the Shiratsuru beach have been taken in the 52 years between 1947 and 1999. Here, the long-term shoreline changes are investigated by comparing aerial photographs taken in 1947, 1965, 1994 and 1999. Figure 2.3.75(a) shows an aerial photograph of Shiratsuru beach taken in May 1947 before the large landform change. At that time, a natural sandy beach extended without any facilities such as breakwaters. The pocket beach had a longshore stretch of 900 m and was bounded by Shimo-ose and Kamiose Points at the southern and northern ends of the beach, respectively. These points protruded over 700 m from the base of the headland. The Takahama River meandered southward in the vicinity of its mouth and

May 26, 2010

142

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.75. Aerial photographs of Shiratsuru beach: (a) May 1947, (b) October 1965, (c) September 1994, and (d) October 1999.

poured into the sea along the headland located at the south end. A slender sandy beach extended as a sand spit on the right-hand side of the mouth of the Takahama River, and a coastal forest with many pine trees existed, the seaward marginal line of which was parallel to the shoreline. Since this beach opened to the sea between the west and the south, rough waves were mainly incident from the southwest. For this reason, the Takahama River flowed into the sea from the wave-shelter zone, where the wave-sheltering effect of the headland was most dominant. This characteristic agrees well with the results of the study on closure of the

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

143

river’s mouth by Uda et al. (1997a,b∗ ). At this time, large-scale structures affecting wave conditions at this pocket beach did not exist except for a straight jetty built at the Takahama River’s mouth. Figure 2.3.75(b) shows an aerial photograph taken in 1965. At the south end of the pocket beach, the breakwater of the Takahama fishing port had already been constructed. The breakwater was separated into two portions: the west and east breakwaters. The west breakwater was L-shaped and extended straight for 200 m from the foot of the breakwater. Since the wave-shelter zone was formed with the construction of the breakwater, sand accumulated in the region between the right bank of the Takahama River’s mouth and the entrance of the port, as shown by the white-colored beach. The outflow of the Takahama River channeled along the east breakwater of the port. Since the breakwater of the fishing port was located inside the wave-shelter zone of Shimo-ose Point against waves incident from the southwest, the impact on the surrounding shoreline was small. Figure 2.3.75(c) shows an aerial photograph taken in September 1994. The construction of the Takahama fishing port was complete, and the new Lshaped breakwater extended from west of the old breakwater and enclosed the breakwater of the old fishing port. The shortest distance from the base of the new breakwater to the other shore of the pocket beach was 900 m, along which the breakwater extended for 300 m. Hence, one-third of the total distance was covered by an artificial structure. This greatly affected the wave field inside the pocket beach, causing longshore sand transport toward the wave-shelter zone of the breakwater from the north to the south. The shoreline advanced considerably in the southern part of the pocket beach, resulting in an advance of 90 m at the mouth of the Takahama River. Two T-shaped groins had been built at the central part of the beach by this time, but these groins were inadequate to control the longshore sand transport, judging from the fact that there was no difference in shoreline positions on either side of the groin. This is because the crown height of the groins was low in the vicinity of the shoreline, permitting waves to overtop the foot of the groins. Sand transported southward along the shoreline was deposited on the lee side of the breakwater of the Takahama fishing port, but this location coincided with the mouth of the Takahama

May 26, 2010

144

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

River. An open mouth is ordinarily required to prevent the water level of the river from rising during floods due to the backwater mechanism. However, sand was carried inside the river’s mouth in spite of the installation of the jetty, causing the river’s mouth to close. Some sand was transported into the fishing port by currents flowing from the river’s mouth to the fishing port, causing deposition of sand into the channel. Thus, excessive sand deposition at the southern end of the beach gave rise to two new problems: closing of the river’s mouth and excessive sand deposition in the channel. Figure 2.3.75(d) shows an aerial photograph taken in October 1999. The shape of the breakwaters of the new Takahama fishing port were the same as that in 1994, as shown in Fig. 2.3.75(c); but the surrounding shoreline greatly changed after the construction of the breakwater of the fishing port. The shoreline retreated in the northern part of the pocket beach and advanced in the southern part. The configuration of the natural pocket beach was artificially altered due to the construction of the breakwater of the new Takahama fishing port, and the shoreline configuration responded to the newly formed wave field. Considerable shoreline changes occurring on the pocket beach can be clearly identified by comparing the aerial photographs in Fig. 2.3.75(a) before the construction and Fig. 2.3.75(d) after the construction. In addition, coastal utilization facilities were built along the outer margin of the coastal forest extending near the Takahama River’s mouth next to the embankment on the newly expanded beach, obstructing the mobility of natural sand. (2) Field observations Field observations of Shiratsuru beach were conducted on 16 December 2000. Coastal characteristics were recorded along the shoreline from the north to the south. Figure 2.3.76 shows Kami-ose Point and northern Shiratsuru beach. This headland is composed of hard rock, and Shiratsuru beach extends at the bottom of the pocket beach 600 m landward from the tip of the headland. As mentioned, the breakwater of the Takahama fishing port was extended at the south end of Shiratsuru beach, inducing longshore sand transport from outside to inside the wave-shelter zone. Therefore, the foreshore

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

145

Fig. 2.3.76. Kami-ose Point located at northern end of pocket beach and northern Shiratsuru beach.

totally disappeared at the north end of the beach, accelerating wave overtopping over the seawall. A gently sloping revetment was built there as a measure against wave overtopping, dubbed an “improvement of the coastal environment.” This gently sloping revetment is shown by the white structure in the picture; a continuous rubble mound was placed along the foot of the revetment. In front of this gently sloping revetment, beach nourishment was carried out, but at that time no sandy beach existed. Figure 2.3.77 shows the overall picture of the gently sloping revetment. At the end of the revetment, large stones were placed as pavement, above which a concrete revetment with a slope of 1/4 can be seen. The white color of the gently sloping revetment is striking in the surrounding coastal environment. The length of the revetment was only 100 m, because it was still under construction, and the end of the revetment was protected by concrete armor units. The toe of the gently sloping revetment protruded directly into the sea, not only because the revetment had a gentle slope, but

May 26, 2010

146

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.77. Overall picture of gently sloping revetment built at most severely eroded location.

also because its location advanced seaward compared with the location of the old seawall, leaving reclaimed land behind the revetment. Figure 2.3.78 shows an overall picture of the end of the gently sloping revetment taken from a location on the extension of the old seawall. It is clear that the new revetment protruded greatly from the old seawall. Furthermore, it should be noted that the foreshore width is only several meters in front of the old seawall at the corner, because the gently sloping revetment buried the natural sandy beach. The shoreline change of Shiratsuru beach was triggered by the construction of the breakwaters of the Takahama fishing port located at the southern end of this pocket beach. In this case, the shoreline retreated most at the northern end according to the fundamental theory of beach changes. The location shown in Fig. 2.3.78 is 300 m closer to the center of the pocket beach from the northern end. Nevertheless, the fact that the foreshore is very narrow in front of the seawall at this site implies that the foreshore totally disappeared near the northern end of the pocket beach, causing the exposure of the foot of the old seawall to the sea. Such a condition resulted

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

147

Fig. 2.3.78. Overall picture of extension of gently sloping revetment.

in severe wave overtopping of the seawall, causing the local citizens to request preventative measures. (3) Comparison of measures and predicted shorelines Figure 2.3.79 shows the shoreline positions in 1947 and 1999 taken from aerial photographs. The shoreline greatly advanced mainly in the waveshelter zone of the Takahama fishing port, and the maximum shoreline advance was 95 m, whereas shoreline recession in the northern part was small compared with the shoreline advance in the southern part. Prior to 1999, the amount of shoreline advance was greater than that of shoreline recession, and no balance between the planar areas of eroded and accreted beaches existed, implying an apparent increase in sand volume. The primary cause of this increase is considered to be sediment inflow from the Takahama River, but accurate calculation was difficult because of a lack of sounding maps of this coast. Therefore, the increase in the plane area of the beaches was calculated first from the shoreline positions in 1947 and 1999. Then, taking the increased plane area into account, the stable shoreline was predicted using the modified Hsu and Evans’ model, as mentioned in

May 26, 2010

148

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.79. Measured and predicted shoreline positions.

Sec. 3.2. The predicted shoreline in 1947, as shown in Fig. 2.3.79, agrees very well with the measured one, whereas the predicted shoreline change in 1999 is greater than the measured one, implying that further shoreline changes should be expected in the future. (4) Discussion The sequence of phenomena that occurred on the Shiratsuru beach can be summarized as follows: (a) Since the Shiratsuru beach is located on southern Amakusa Island, as shown in Fig. 2.3.74, strong waves are incident from the southwest. Under these conditions, it is reasonable for the fishing port to be on the southern site of the pocket beach near Shimo-ose Point in order to create a calm-wave zone to enhance the safety of the fishing boats. Simultaneously, it is natural for the Takahama River to pour into the sea from the wave-shelter zone on the lee side of the headland, on the basis of the theory by Uda et al. (1997a,b∗ ).

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

149

(b) Since the Takahama River meanders extensively and pours into the sea in the southern region, the plane area is wider in the north, where the town was founded. (c) The Takahama fishing port has been very important as a fishing port for the emergency shelter of fishing boats during the occurrence of rough waves in this district, which accompanied the continuous development of the sea cliff, and the fishing port facilities were expanded for this reason. (d) As a result, the wave field changed greatly, causing longshore sand transport from the north to the south of the pocket beach. The transported sand accumulated at the mouth of the Takahama River, causing both closure of the river’s mouth and sand deposition in the channel of the Takahama fishing port. Under the new boundary conditions created by the construction of the breakwaters of the fishing port, the shoreline changed and attained a stable form. This is a very natural phenomenon. (e) Accordingly, the removal of the deposited sand to prevent closure of the river’s mouth and sand deposition in the channel has caused further southward longshore sand transport. As a result, erosion was accelerated in front of the town located in the northern region, and this further accelerated wave overtopping of the seawall. (f) Before the erosion in the northern area, the beach in front of the town was a famous nesting spot for the loggerhead turtle, Caretta caretta. However, the sandy beach suitable for the nesting of loggerhead turtles totally disappeared. (g) For this reason, more loggerhead turtles started to nest on the accreted southern beach; but there, friction arose between people using the beach during the summer season as one of the few bathing spots in the district and the loggerhead turtles using the beach for nesting. (h) The shoreline approached a new equilibrium state due to erosion and accretion, but the town was located so close to the shoreline that the setback of the shoreline was not possible. Instead, the seawall line was advanced by land reclamation as a measure against wave overtopping, and a gently sloping revetment was built to protect this reclaimed land.

May 26, 2010

150

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Since the foot depth of the coastal facilities increased due to the seaward advance of the facilities, the instability of sand nourished in front of the facilities was increased. (i) Only from the aspect of the prevention of wave overtopping were land reclamation and the construction of the gently sloping revetment effective. However, the natural sandy beach, which had been poetically described as “the expanded wing of a crane” and which had been the only beach in this district, began to change from a natural to an artificially induced coastline in the northern area. This has been criticized by many tourists visiting this area. Under these circumstances, various measures have been considered, but the recovery of the shoreline to its condition prior to the construction of the Takahama fishing port is theoretically impossible, unless the new breakwater of the fishing port is removed. Important issues in considering future practical measures without large changes to existing coastal facilities or the town area located behind the shoreline are summarized as follows: (a) In considering the prevention of sand deposition at the river’s mouth and the channel of the fishing port, beach nourishment is impossible under the current conditions of the facilities, because the supplied sand is rapidly transported and diffused southward. (b) In order to recover the sandy beach in front of the gently sloping revetment built in the northern part of the pocket beach by beach nourishment, the construction of a long groin that separates the pocket beach into two parts is required. A large environmental change will be caused by this action and, therefore, discussions in view of the coastal environment and coastal utilization will be required. (c) Since the current shoreline configuration is approaching a stable form, it would be reasonable, with respect to fundamental theory, not to artificially recover the sandy beach in the northern region of the beach, and to leave it as it is. It is unreasonable to expect the complete recovery of the natural sandy beach in the northern area to its previous condition, considering the theory of beach changes.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

151

Changes in Shiratsuru beach were caused by extending the long breakwater of the Takahama fishing port at the southern end of the pocket beach to the shoreline that was previously rather stable. A large wave-shelter zone was formed on the lee side of the breakwater, resulting in an increase in wave height outside the breakwater and a change in the wave direction. The same phenomenon can be observed at many coasts in Japan (Uda, 1997∗ ). In those cases, much sand is deposited in the calm-wave zone, whereas sand is discharged from the rest of the pocket beach, accelerating shoreline recession. Land reclamation and the construction of the gently sloping revetment were carried out, despite the beach approaching a stable form via shoreline recession. Beach nourishment in front of these facilities protruding into the sea increased the instability of sand. If the hinterland is a natural sandy beach, a new stable shoreline will be formed, even if the scarp formed by shoreline recession is left as it is. However, on Shiratsuru beach, houses were concentrated in the hinterland of the shoreline recession zone, and various measures had to be taken against the natural changes. The direct cause of the narrowing of the foreshore in front of the seawall protecting the houses was the construction of the breakwater of the Takahama fishing port. Construction sites of good harbors are very limited in the Amakusa District facing the Sea of Amakusa, since a large part of the coastline is composed of sea cliffs. It is difficult to say that the construction of the fishing port itself is a problem, only for the reason that shoreline recession was triggered by the construction of the fishing port. The fishing port is valuable as a base that supports the major industry in this area. In addition, it should be noted that the Takahama fishing port was built at the southern end of the pocket beach, where the hinterland of the affected zone was coincidentally the same as the housing zone.

2.3.11.

Kashiwabara Coast in Kagoshima Prefecture

(1) General conditions The Kashiwabara coast is located on southern Shibushi Bay which opens to the Pacific Ocean, as shown in Fig. 2.3.80, in Kagoshima Prefecture and has

May 26, 2010

152

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.80. Location of Kashiwabara coast.

a 16.8 km-long coastline. The sandy beach extends nearly 14.2 km along the coast. An artificial island for storage of crude oil with a rectangular shape 1.5 km long and 1.5 km wide was constructed off the Kashiwabara coast after 1985, as shown in Fig. 2.3.81. The construction of the outer facility of the island was completed in 1987, and then the island was reclaimed using the material dredged from the sea bottom. After the construction of this island, a large cuspate foreland was formed in the wave-shelter zone by wave refraction and diffraction. Since the major sediment source for the deposition was the neighboring northern sandy beach, large shoreline recession up to 80 m occurred on the adjacent beaches, resulting in the formation of dune and beach scarps (Nishi et al., 1998∗ , 1998). The highest dune scarp reached nearly 7 m at that time. The refraction and diffraction patterns were also modified by the shape of the borrowed site for reclamation to cause an uneven wave-height distribution along the shoreline.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

153

Fig. 2.3.81. Orientation of coastal structures and transects.

(2) Shoreline changes The bathymetry and shoreline positions have been measured twice a year since 1984, and wave data has been collected since 1980. The shoreline configurations of this coast after the construction of the artificial island are shown in Fig. 2.3.82. The reference year is taken as 1985. As seen in the shoreline in June 1986, when an outlying facility of the artificial island was being constructed, the effect of wave sheltering on the sandy beach had begun particularly in the area further south than x = 4.3 km. It is clearly seen that sediment had been deposited in the wave-shelter zone, and the shoreline configuration showed quasi-double tombolo features. The peaks in the shoreline were located at x = 4.9 km and x = 6.3 km in June 1987. The migration of southern quasi-tombolo along the shore was blocked by the outer facility of the Hami fishing port, but the size of the depositional area had continued to grow. The north tombolo had shifted south along the shore, and the size of the tombolo had enlarged year by year.

May 26, 2010

154

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.3.82. Shoreline changes after construction of island.

Regarding the growth of the northern tombolo, where a significant amount of sand had been deposited, the offshore distance of the peak of the tombolo had advanced from 40 m in June 1986 to 170 m in June 1992. The maximum shoreline advance in this period was 130 m. In addition, the peak of the tombolo migrated southward from x = 4.75 km in 1986 to x = 5.25 km in 1992; hence the tombolo had been shifted 500 m in this period. Once tombolo generation was initiated by the diffracted waves in the wave-shelter zone, the water depth at the tip of the tombolo got deeper according to its offshore growth. As a result, more diffracted waves with higher wave energy flux propagated southward in the wave-shelter zone. Therefore, the peak of the tombolo migrated southwards, whereas the size of the tombolo increased in the wave-shelter zone. As seen in the figure, the boundary of the northern tombolo and southern quasi-tombolo

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

155

also shifted 270 m to the south at an annual mean speed of 45 m/yr. In accordance with the migration of tombolos behind the island, the boundary of depositional and erosional areas of this coast shifted 132 m toward the wave-shelter zone. The sandy beach in the wave-shelter zone had widened, but the neighboring sandy beach outside the wave-shelter zone had suffered severe coastal erosion after the construction of the artificial island. The shoreline changes mentioned give a typical example of the beach changes generated when sand is transported alongshore from outside to inside the wave-shelter zone, an effect associated with the construction of an offshore artificial island.

2.3.12.

Methods of Addressing Issues

(1) Classification of beach erosion In this chapter, 11 examples of beach erosion generated by longshore sand transport from outside to inside a wave-shelter zone were described in detail. To consider future measures for these coasts, the erosion pattern can be classified into three main groups depending on the state of the erosion, although strict definition is difficult. The classification of the coasts is as follows. The coasts of Category 1 have such characteristics that although the beach was eroded, a considerably wide foreshore is still left, permitting the coast to be left as it is; Kemigawa beach in Sec. 2.3.5 and Tsutsuki beach in Sec. 2.3.9 are classified under this category. Category 2 has a seawall and a large number of concrete armor units already constructed in the eroded zone, and the improvement of these structures requires large investment, so that the study of improvement methods is unrealistic; classified in this category are Kashiwazaki Port and the Arahama coast in Sec. 2.3.2, the Ohtsu fishing port in Sec. 2.3.3 and the Asamogawa coast in Sec. 2.3.8. Category 3 has characteristics indicating that the beach was eroded in the past, but some improvement of the coast may still be possible; the Teradomari and Nozumi coasts in Sec. 2.3.1, Ajigaura beach and Naka

May 26, 2010

156

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

coast in Sec. 2.3.4, Tojo–Maebara coast in Sec. 2.3.6, Shimobara fishing port in Sec. 2.3.7, Shiratsuru beach in Sec. 2.3.10, and Kashiwabara coast in Sec. 2.3.11 are classified in this category. (2) Future measures for Category 3 — Shiratsuru beach as example Out of the three categories, the concrete measures of Category 3, to which some adjustment may be still possible, are considered here, taking Shiratsuru beach as the typical example. On the other coasts belonging to the same category, one of these measures or their combination may be adopted. On Shiratsuru beach, concrete measures enabling the preservation of a natural beach as much as possible are as follows: Case 1: Seawall raising to prevent wave overtopping Shoreline recession has occurred inevitably due to the extension of the port breakwater. Accordingly, even though inconvenience may arise in terms of coastal scenery and utilization, the local people must be told the truth and lasting measures must be proposed. This is the cheapest approach. Case 2: Setting-back of seawall in accordance with shoreline recession The seawall line is set back landward in the area where the shoreline receded. This method is effective for preserving a natural sandy beach and for reducing cost as well, but changes of residence are required. For those who can change their residences, the change must be encouraged. The disadvantage of this method is the necessity of moving a residential area. Public consensus on the movement of the residential area is considered to be difficult to achieve for those who are requested to move their residences, because the necessity to move a residential area was not the result of the residents’ own activities, but rather was triggered by the construction of the fishing port breakwaters far from their residential area. Case 3: Separation of beach by extending groin with sufficient length in central part Figure 2.3.83 shows the stable shoreline predicted using Hsu and Evans’ model, which will be described in Sec. 3.2, as well as the shoreline configurations in 1947 and 1999. Both the shoreline separated by a groin

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

157

Fig. 2.3.83. Prediction of shoreline changes in case of construction of groin at center of pocket beach.

100 m long and the shoreline with 10,000 m3 of sand nourishment are shown in the figure. Since the long stretch of the natural sandy beach is completely separated into two parts, users of the beach such as sea bathers and surfers may vigorously complain about this situation, but if a sandy beach must be recovered in the northern part of the coast, this is the only way. If the foreshore width north of the groin is narrow and a wider beach is required, beach nourishment should be carried out, as shown in Fig. 2.3.83. The fundamental issue with this method lies in the disappearance of the natural, original scenery of Shiratsuru beach, which has been maintained for a long time, because of the construction of a new structure. Case 4: Recycling sand A groin of such a length that minimizes influence on beach utilization is built to control longshore sand movement, and sand deposited inside the fishing port crossing the tip of the groin is continuously dredged and transported to

May 26, 2010

158

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

the northern beach. The difficulty with this method is that it requires certain maintenance costs per year over the long term. On the other coasts, such as the Teradomari and Nozumi coasts in Sec. 2.3.1, Ajigaura beach and Naka coast in Sec. 2.3.4, Tojo–Maebara coast in Sec. 2.3.6, and Shimobara fishing port in Sec. 2.3.7, the same measures as those used on Shiratsuru beach can be considered. It should be noted that, referring to the schematic diagram of Fig. 2.3.84, since longshore sand transport induced by the formation of a wave-shelter zone occurs in a depth zone shallower than the depth of closure, which is approximately equal to 10 m at exposed beaches, erosion will continue if detached breakwaters or artificial reefs have been built as a measure against erosion in the depth zone shallower than the depth of closure, because some longshore sand transport can pass through these wave-dissipating structures. With this in mind, a specific example is described in Sec. 3.8. As explained in the example of Ajigaura beach in Sec. 2.3.4, fine sediment is selectively transported to the wave-shelter zone, and resultantly, the grain size of the seabed materials becomes coarser in the eroded zone due to erosion, and the slope of the beach increases as well. Therefore, when accumulated materials mainly composed of fine materials are transported to the eroded beach and artificially supplied to the beach, the discharging velocity of the nourished sediment becomes extremely large compared with the original conditions. The beach changes associated with beach

Fig. 2.3.84. Schematic view of beach changes in vicinity of wave-sheltering structure.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

159

nourishment should be fully investigated using some models for predicting not only topographic changes but also the sorting of sand of mixed grain size, as described in Sec. 3.6. In order to ultimately form a stable beach, the longshore sand transport over the entire depth range between the berm height, hR , and the depth of closure, hc , must be stopped. This automatically means that the size of the countermeasure structure increases, and in turn the structure itself creates another wave-shelter zone behind it; the same situation is repeated as when the wave-sheltering structure was first built. To maintain the natural sandy beach as much as possible, reuse of sand is inevitably important. It should be realized that countermeasures such as the detached breakwaters with ordinary scale are only useful for reducing the eroding velocity; it is difficult to form a stable beach in an extensive area with them, except for coasts facing the inland sea with calm waves.

REFERENCES Kumada, T., A. Kobayashi, T. Uda and T. San-nami (2002). Field observation of three-dimensional changes of artificial beach and application of expanded Hsu model — The example of Kemigawa beach in Chiba Prefecture, Japan, Proc. 28th ICCE, pp. 3711–3723. Nishi, R., T. Uda, M. Sato, M. Wakita, Y. Ohtani and T. Horiguchi (1998). Coastal erosion caused by construction of an artificial island and performance of beach nourishment, Proc. 26th ICCE, pp. 1679–1692. Uda, T. and K. Noguchi (1993). Beach changes caused, by elongation of breakwater of Kashiwazaki Port, Coastal Eng. Japan 36(2), 229–244. Uda, T., M. Serizawa, T. San-nami and K. Furuike (2002). Shoreline changes of a pocket beach caused by elongation of harbor breakwater and their prediction, Trans., Japan. Geomorph. Union 23(3), 395–413. Uda, T., R. Nishi, A. Kikuchi, T. San-nami and T. Kumada (2004). Shoreline changes of a pocket beach triggered by construction of port breakwaters and future measures — The example of Shiratsuru beach, Asian and Pacific Coasts 2003, Proc. 2nd Int. Conf., paper 64, pp. 1–11.

May 26, 2010

160

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Uda, T., T. San-nami, M. Serizawa and K. Furuike (2005). Beach erosion in Japan as a structural problem, Proc. 14th Biennial Coastal Zone Conf., New Orleans, Louisiana, pp. 1–5.

REFERENCES (in Japanese) Hoshigami,Y.,A. Kobayashi, T. Uda, M. Miura, T. Kumada and T. San-nami (2003). Method for preventing beach erosion and accretion associated with construction of wave-sheltering structure — The example of Shimobara fishing port in Tateyama City, Proc. Civil Eng. in the Ocean, JSCE 19, 481–496. Kumada, T.,A. Kobayashi, T. San-nami, T. Uda, M. Serizawa and K. Furuike (2001a). Mechanism of topographic changes of Kemigawa artificial beach in Chiba Prefecture, Proc. Civil Eng. in the Ocean, JSCE 17, 559–564. Kumada, T., A. Kobayashi, T. Uda, M. Serizawa, T. San-nami and Y. Hoshigami (2001b). Beach changes of Kemigawa beach in Chiba Prefecture and prediction by extended three dimensional Hsu model, Annual J. Coastal Eng. JSCE 48, 536–540. Nishi, R., T. Uda, M. Sato, S. Wakita, Y. Ohtani and K. Horiguchi (1998). Shore processes associated with construction of an offshore island and measures against beach erosion, Annual J. Coastal Eng. JSCE 45, 561–565. Tsuchiya, Y., T. Yamashita and M. Saito (1994). Beach changes due to expansion and reduction of a river mouth delta (1) — Formation of a river mouth delta at Teradomari and Nozumi coasts, Annual Report of Kyoto Disaster Prevention Res. Institute, Vol. 37, B-2, pp. 539–568. Uda, T., K. Noguchi and T. Osawa (1993). Beach changes of the surrounding coasts associated with construction of breakwater of Kashiwazaki Port, Annual J. Coastal Eng. JSCE 40, 436–440. Uda, T., K. Sekinishi and Y. Nishioka (1996). Analysis of beach changes on a pocket beach — The example of Asamogawa coast in Kyoto Prefecture, Proc. Civil Eng. in the Ocean, JSCE 12, 391–396. Uda, T. (1997). Beach Erosion in Japan (Sankaido Press, Tokyo), p. 442.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

161

Uda, T., T. Sako and K. Nomura (1997a). Determining mechanism of a river mouth flowing into a pocket beach and river mouth improvement, Annual J. Hydraulic Eng. JSCE 41, 863–870. Uda, T., M. Serizawa, T. San-nami, K. Furuike and Y. Kanda (1997b). Causes of formation of artificial coast associated with construction of wavesheltering structure and preventive measures, Proc. Civil Eng. in the Ocean, JSCE 13, 651–657. Uda, T., T. Ishikawa, S. Seino, S. Watanabe, T. San-nami and K. Furuike (2000). Beach changes due to construction of detached breakwaters at the corner of a pocket beach — The example of Tojo coast in Kamogawa City in Chiba Prefecture, Proc. Civil Eng. in the Ocean, JSCE 16, 595–600. Uda, T., M. Serizawa, T. San-nami and K. Furuike (2001a). Beach changes of a pocket beach triggered by constructing wave-sheltering structure — The example of Tsuzuki beach on Iki Island, Annual J. Coastal Eng. JSCE 48, 671–675. Uda, T., R. Nishi, A. Kikuchi, T. San-nami and T. Kumada (2001b). Deformation of a pocket beach by elongation of harbor breakwater and countermeasures — The example of Shiratsuru beach in Amakusa, Annual J. Coastal Eng. 48, 686–690. Uda, T., T. Ishikawa, S. Seino, S. Watanabe, M. Serizawa and T. San-nami (2001c). Beach changes due to construction of detached breakwaters at the corner of a pocket beach: The example of Tojo–Maebara coast in Kamogawa City in Chiba Prefecture, Japanese Geomorphological Union 22, 217–226. Uda, T., S. Seino, T. Kumada, Y. Hoshigami, M. Serizawa and T. Sannami (2003a). Large-scale beach erosion of Ajigaura beach and Naka coast triggered by construction of wave-sheltering structure, Proc. Civil Eng. in the Ocean, JSCE 19, 363–368. Uda, T., K. Sakai, T. Kumada, Y. Hoshigami, M. Serizawa and T. San-nami (2003b). Narrowing of natural sand dune due to excess advance of coastal forest and change in longshore sand transport induced by the formation of wave-shelter zone — The example of Teradomari and Nozumi coasts, Proc. Civil Eng. in the Ocean, JSCE 19, 475–480.

May 26, 2010

162

2.4.

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

BEACH EROSION DUE TO DECREASE IN FLUVIAL SEDIMENT SUPPLY

On a coast where sediment supply from rivers is the main source for its formation, a change in sediment supply significantly affects beach topography. When the predominant waves on a coast are incident from the direction normal to the coastline, sediment supplied from a river’s mouth is transported to both sides of the mouth, resulting in the formation of a symmetric river-mouth delta. In this case, a decrease in sediment discharge triggers beach erosion along the coastline on both sides of the river’s mouth. One typical example is the case of the Tenryu River flowing into the Enshunada coast facing the Pacific Ocean. On the other hand, on a coast where waves are obliquely incident to the shoreline and make a large angle, almost all sediment supplied from a river’s mouth is transported downdriftward, resulting in the formation of an asymmetrical river-mouth delta. Typical examples are given by the Abe and Ohi Rivers with steeply sloping beds that flow down into Suruga Bay. In these rivers, a large amount of sediment discharged from the river’s mouth at the coast in the past, and an extremely protruding river-mouth delta was developed, but in recent years, erosion became dominant, because of a rapid decrease in sand supply from the river due to sand mining from the river bed or the construction of large dams. Here, taking the Shizuoka and Shimizu coasts extending north of the Abe River, the Suruga coast extending on both sides of the Ohi River’s mouth and the Enshu-nada coast extending on both sides of the Tenryu River’s mouth as examples, we consider how the decrease in sediment supply from the rivers affects the surrounding coastlines.

2.4.1.

Shizuoka and Shimizu Coasts in Shizuoka Prefecture

(1) General conditions The Shizuoka and Shimizu coasts are located along the outer margin of the Mihono-matsubara sand spit in Suruga Bay, as shown in Fig. 2.4.1. This bay opens south to the Pacific Ocean, and rough waves are incident from

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

163

Fig. 2.4.1. Location of Shizuoka and Shimizu coasts in Suruga Bay facing the Pacific Ocean.

the south. Since the coastline runs southwest to northeast, northeastward longshore sand transport prevails along these coasts. The Shizuoka coast extends 7.8 km northeastward from the mouth of the Abe River to the mouth of the Takigahara River, and the 9.8 km coastline from the latter river’s mouth to the tip of the sand spit is called the Shimizu coast. Figure 2.4.2 shows the bathymetry off the Shizuoka and Shimizu coasts. Here, the beach slope near the shoreline is as steep as 1/10. A continental shelf with a mild slope of 1/150 spreads along the offshore zone ranging from 10 to 30 m in depth, but the sea-bottom slope at the tip of the Mihonomatsubara sand spit is as steep as about 1/5 (Uda et al., 1991∗ ; Uda and Yamamoto, 1994). During the Jomon Transgression of the sea level about 6,000 years ago, erosion of the side of Mt Kuno, as shown in Fig. 2.4.2, supplied sediment

May 26, 2010

164

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.2. Bathymetry off Shizuoka and Shimizu coasts.

to the Mihono-matsubara sand spit; but at the current sea level, the only source of sediment to the sand spit is the Abe River. Before 1968, riverbed excavation was extensively carried out in the Abe River, causing a sharp decrease in fluvial sand supply from the river, and the dynamic balance of longshore sand transport was lost, giving rise to the northeastward extension of the eroded area from the river’s mouth. Uda (1997∗ ) revealed that the erosion wave propagated 0.8 km/yr in the period between 1975 and 1983 and 0.5 km/yr between 1983 and 1988 along the Shizuoka coast. Currently, the most severely eroded portion of the beach is near the tip of the Mihono-matsubra sand spit at the northeastern end of the Shimizu coast. Most of the sediment that is moved by northeastward

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

165

Fig. 2.4.3. Detailed bathymetry off Shimizu coast and alignment of measuring lines.

longshore sand transport discharges into the submarine canyon located at the northeastern end of the Shimizu coast. Figure 2.4.3 shows the bathymetry around the tip of the Mihonomatsubara sand spit, as well as the alignment of measuring lines on the Shimizu coast. In this area, sea-bottom surveys have been conducted once a year in March since 1988. The interval of measuring lines is 100 m. The origin of the measuring lines is located at No. 0 at the tip of the sand spit, and measuring lines are set alongshore in the southwestward direction. In Fig. 2.4.3, the turning point of the shoreline is located at No. 12, and a submarine canyon exists between No. 12 and No. 30. Hagoromo-no-matsu, as shown in Fig. 2.4.4, is located at No. 35. According to the sampling test of bottom materials conducted at nine points at 1 km intervals alongshore from the mouth of theAbe River on 20 February 1989, the median diameter of beach materials near the shoreline of the Shizuoka coast is around 7.5 mm.

May 26, 2010

166

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.4. Sandy beach, Mt Fuji, and pine trees at Hagoromo-no-matsu taken at No. 35 on 10 February 1995.

(2) Changes in shoreline position and sand volume changes Figure 2.4.5 shows the shoreline change with reference to the shoreline position in 1983 obtained from the bathymetric surveys. The mouth of the Abe River is located at the right end of the figure, and the left end is the tip of the Mihono-matsubara sand spit called Masaki Point. Along the Shizuoka coast, the sand body formed by sand accumulation moves northward (left in the figure) with time and its propagation velocity is around 250 m/yr, as shown in Fig. 2.4.5. Uda and Itabashi (1997∗ ), and Itabashi and Uda (1998) showed that the propagation velocity of the leading edge of this sand body between 1984 and 1993 was 233 m/yr. The value obtained in the present study is slightly larger due to the selection of a longer comparison period. The shoreline of the Shizuoka coast was totally covered by concrete armor units before the propagation of the sand body, and no sandy beaches existed, which means there is no littoral sand to be transported alongshore. The sand body was considered to be moving northeastward, whereas sand

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

167

Fig. 2.4.5. Shoreline changes along Shizuoka and Shimizu coasts showing propagation of erosion and accretion waves.

also covered the concrete armor units set along the shoreline, since a large amount of sediment was supplied from the mouth of the Abe River located at the southwestern boundary of the coast. Furthermore, Uda et al. (1996) and Uda et al. (1997∗ ) revealed, by numerical simulation using the contourline change model, that the movement of the sand body was triggered by the blocking of longshore sand transport due to the presence of a number of detached breakwaters constructed off the coastline. Along the coast between the location of the leading edge of the sand body at a longshore distance of around 12.5 km, which it reached in 1996, and the location around 0.5 km north of the mouth of the Takigahara River, no shoreline change was observed, because this area is totally covered by concrete armor units, as shown in Figs. 2.4.6 and 2.4.7. Along the Shimizu coast, beach erosion has clearly occurred since 1983. Shoreline recession first began from a location of around 9 km and extended northeastward with time, approaching Hagoromo-no-matsu in

May 26, 2010

168

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.6. Coastline totally covered by seawall and concrete armor units on Shimizu coast (July 1995).

Fig. 2.4.7. Coastline protected by continuous seawall, concrete armor units and several detached breakwaters further north of location shown in Fig. 2.4.6 on Shimizu coast.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

169

1996. Beach erosion along the Shizuoka coast was originally triggered by a sharp decrease in fluvial sand supply from the Abe River due to largescale sand mining from the riverbed to obtain construction materials in the period around 1970, and the erosion propagated from the Shizuoka coast to the Shimizu coast (Uda, 1997∗ ). Currently, it is approaching the tip of the Mihono-matsubara sand spit. Shoreline changes of a zigzag type have occurred after the construction of artificial headlands using two sets of detached breakwaters built as a countermeasure against beach erosion, as shown in Figs. 2.4.8–2.4.10. The eroded zone monotonically extends northward, and its propagation velocity is around 270 m/yr. This propagation velocity is comparable to the propagation velocity (250 m/yr) of the sand body on the Shizuoka coast, but exceeds it by 8%. Since the distance between the tip of the eroded zone (No. 40) and Hagoromo-nomatsu (No. 35) was only 500 m in 1996, only two years remain before severe beach erosion reaches Hagoromo-no-matsu, if the propagation velocity of the erosion zone remains unchanged. Uda (1997∗ ) determined that the rate

Fig. 2.4.8. Shoreline around detached breakwaters I. Several detached breakwaters were built alongshore.

May 26, 2010

170

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.9. Shoreline downcoast of detached breakwaters II . Downcoast erosion was so severe that beach nourishment is being conducted, as shown in the photograph of the most severely eroded location.

Fig. 2.4.10. Shoreline around detached breakwaters III. In this area, the foreshore is still fairly wide.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

171

of extension of the erosion zone was about 271 m/yr on average in the period between March 1990 and March 1993; this means that the same propagation velocity of the erosive zone has been maintained since then. In addition, the north end of the erosive zone was located at No. 46 in 1993. The change in foreshore area in the accretion zone of the Shizuoka coast and the erosion zone of the Shimizu coast relative to that in 1983 was calculated and is shown in Fig. 2.4.11. Along the Shizuoka coast, the rates of increase of the foreshore area in the accretion zone during each of the three periods shown in Fig. 2.4.11 differ; the rate of increase was 1.42×103 m2 /yr in the first period between 1983 and 1987, which decreased by one order of magnitude to 0.44 × 103 m2 /yr in the second period between 1987 and 1992, and in the last period between 1992 and 1995, the rate increased to 1.98 × 103 m2 /yr, which is larger than that in the first period. The only sand source to the Shizuoka coast is from the Abe River. In addition, the shoreline north of the tip of the sand deposition zone is covered by a large number of concrete armor units, and there are no sandy beaches. This means that this artificially protected coast suffered from a shortage of sand before the arrival of the tip of the sandy body. Accordingly, the only reason for

Fig. 2.4.11. Changes with time in eroded and accreted areas along Shizuoka and Shimizu coasts.

May 26, 2010

13:35

172

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures Table 2.4.1. Records of large floods since 1980. Year

Month

1982 1982 1982 1983 1985 1990 1991

August August September August July August September

Day

Average discharge (m3 /s)

Maximum discharge (m3 /s)

2 3 12 2 2 10 2

1,129 1,466 1,723 1,731 1,523 1,005 1,396

3,857

2,981 — — 2,511

increase in foreshore area in the accretion zone as shown in Fig. 2.4.11 must be the sand supply from the mouth of the Abe River. Table 2.4.1 shows the record of large floods since 1980 at a location 4.7 km upstream from the river’s mouth. Daily average flood discharges of 1,000 m3 /s or larger occurred seven times between 1980 and 1993, as shown in Table 2.4.1, and on 12 September 1982, the flood with the largest discharge of 3,857 m3 /s occurred; thereafter, large-scale floods have rarely occurred. This indicates that the variation in the rate of increase of the foreshore area in the accretion zone of the Shizuoka coast mainly corresponds to the occurrence of large floods that supply large amounts of sediment to the coast. Along the Shimizu coast, the eroded area monotonically increased from 1983 to 1996 at the rate of 1.42 × 103 m2 /yr, as shown in Fig. 2.4.11. This rate is approximately equal to that in the first period along the Shizuoka coast. The long-term rate of increase of the foreshore area of the Shizuoka coast between 1983 and 1996 reached 1.21 × 103 m2 /yr, although shortterm variations in this rate occur, as shown in Fig. 2.4.11. This value is only 14% smaller than that obtained on the Shimizu coast. This indicates that the foreshore area exhibits a large variation along the Shizuoka coast in the area next to the river’s mouth depending on the variation in the amount of river sediment discharge, but along the Shimizu coast located far from the river mouth, such a variation is minimal and monotonous change is dominant, resulting in a smooth shoreline change. Along the Shizuoka and Shimizu coasts, the shoreline change correlates well with the change in the cross-sectional area of the beaches, and the regression coefficient between these parameters gives characteristic beach

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

173

change heights of 7.7 and 7.2 m on the Shizuoka and Shimizu coasts, respectively (Uda, 1997∗ ). The changes in foreshore area were multiplied by the characteristic height of beach changes to calculate the change in sand volume due to erosion and accretion. Since the coast near the boundary between the Shizuoka and Shimizu coasts is totally covered by concrete armor units and no sandy foreshore exists as shown in Figs. 2.4.6 and 2.4.7, longshore sand transport passing this location is assumed to be 0. Accordingly, the calculated change in sand volume is approximately equal to the inflow or outflow rate of longshore sand transport into the zone under study from the continuity of sand mass. Table 2.4.2 shows the results of the calculation in each period. The inflow rate of longshore sand transport to the Shizuoka coast varies considerably from 3.4 × 104 to 15.2 × 104 m3 /yr. The average rate of longshore sand transport is 10.1 × 104 m3 /yr. In contrast, the rate of longshore sand transport on the Shimizu coast has an approximately constant value of 10.2 × 104 m3 /yr. Figure 2.4.12 shows the temporal change in sand volume calculated by multiplying the decrease in the foreshore area in the region between No. 55 and No. 85 by the characteristic height of beach changes of 7.2 m. The reference year is taken in 1983, the same as that in Fig. 2.4.11. In Fig. 2.4.12, it was clear that the total volume of sand in the erosion zone on the Shimizu coast had increased monotonically since 1983. However, precise examination of the change in sand volume in each zone in Fig. 2.4.12 reveals another feature. Eroded sand volume in the region between No. 55 and No. 85 attained to an equilibrium state after it increased up to 1991, whereas the eroded sand volume in the zone between No. 40 and No. 55 increased after 1991 at Table 2.4.2. Longshore sand transport rates on Shizuoka and Shimizu coasts.

Coast Shimizu Shizuoka

Duration

Change in foreshore area (×104 m2 /yr)

Characteristic height of beach changes (m)

Longshore sand transport rate (×104 m3 /yr)

1983–1996 1983–1987 1987–1992 1992–1995 1983–1986

1.42 1.53 0.44 1.98 1.21

7.2 7.7

10.2 11.8 3.4 15.2 9.2

May 26, 2010

174

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.12. Changes in sand volume with time in regions between No. 55 and No. 85, and between No. 44 and No. 55 along Shimizu coast.

a rate equal to the increase rate of the eroded area along the entire Shimizu coast. This means that the construction of an artificial headland using a couple of detached breakwaters prevented further beach erosion at the site, but the erosion zone extends further downcoast. In short, accretion and erosion waves were triggered by the imbalance in the longshore sand transport rates. In Fig. 2.4.5, Hagoromo-no-matsu was about 7.9 km from the tip of the sand body (x = 12.5 km) in 1996, and therefore it should take 30 years for the tip of the sand body to reach the most severely eroded location of Hagoromo-no-matsu, if the movement of the sand body takes place in an unchanging manner. During this period, natural sand supply by longshore sand transport cannot be expected along the Shimizu coast, and therefore it is necessary to effectively utilize the sand existing along the coast. This is a long period compared with that required for erosion waves to reach the north tip of the Shimizu coast. During this period, measures to stabilize the shoreline must be taken without relying on sand supply from upcoast. Beach nourishment is one of the possible measures, but the longshore sand transport along this coast is around 10 ×104 m3 /yr, with ultimate discharge into the deep ocean through

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

175

the submarine canyon located at the tip of the sand spit, implying that beach nourishment only is not a sufficient measure. (3) Beach-profile changes along Shimizu coast Beach-profile changes were investigated in detail along several measuring lines where large shoreline recession was observed in Fig. 2.4.5. As typical measuring lines, four lines are selected: No. 44, located north of the group of detached breakwaters III very close to Hagoromo-no-matsu; No. 50, located between detached breakwaters II and II ; No. 55, located north of detached breakwaters II; and No. 62, to the north of detached breakwaters I, which were built the earliest.

Fig. 2.4.13. Profile changes along No. 44.

May 26, 2010

176

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Figure 2.4.13 shows the beach-profile changes along No. 44. The beach profile in 1985 is selected as a reference, and beach profiles after this year are shifted along the vertical axis. At this location, accretion prevailed until 1995, corresponding to the shoreline advance relative to the beach profile in 1985, but beach erosion became dominant in 1996 in the zone shallower than −4 m. At No. 50, 0.6 km south of survey line No. 44, intense beach erosion has taken place since 1992, as shown in Fig. 2.4.14. In this case, it should be noted that the beach-profile change around 1994 is very similar to that in 1996 in Fig. 2.4.13, implying that shoreline recession extended gradually alongshore while maintaining almost the same profile.

Fig. 2.4.14. Profile changes along No. 50.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

177

Fig. 2.4.15. Profile changes along No. 55.

Figure 2.4.15 shows the beach-profile changes along No. 55. As shown in Fig. 2.4.5, the shoreline retreated considerably until 1994 in the vicinity of this measuring line (No. 55) because detached breakwaters II were constructed between detached breakwaters II and III. As indicated by this shoreline change, the beach was eroded until 1994, but thereafter sand accumulated again until 1996 and almost the same beach profile as in 1993 was recovered. Reaccretion occurred at No. 55 due to the sand accumulation effect of the detached breakwaters constructed immediately downcoast. Figure 2.4.16 shows the beach-profile changes at No. 62. Severe beach erosion can be observed in the profile after 1989, corresponding to the

May 26, 2010

178

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.16. Profile changes along No. 62.

extensive shoreline recession shown in Fig. 2.4.5. It is noted that beach erosion became dominant at this location eight years before the initiation of beach erosion at No. 44 located 1.8 km north of No. 62. In addition, an upward convex profile near the shoreline reduced to a concave profile, because the seabed in the zone shallower than −6 m was eroded as a result of beach erosion. The beach-profile changes mentioned above are similar to each other except in terms of the initiation time of the erosion. As mentioned previously, only two years remain before the shoreline recession zone reaches Hagoromo-no-matsu. After that, the shoreline profile is predicted to become steep, as shown in Figs. 2.4.14–2.4.16.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

179

(4) Topographic changes around detached breakwaters Since dominant beach changes along the Shimizu coast can be observed in the area between No. 40 and No. 60, as shown in Fig. 2.4.5, beach changes in this area from March 1992 to March 1996 are investigated in detail by comparing bathymetric survey maps. Figure 2.4.17(a) shows the bathymetry in March 1992. At this time, northeastward longshore sand transport was partly blocked by detached breakwaters II located at around No. 57 and No. 58; therefore, sea-bottom contours shallower than −3 m became concave at the northeast end of the detached breakwaters, whereas the contours far from this area were straight. In contrast, on the southwest side of the detached breakwaters, the shoreline connects smoothly to the detached breakwaters and offshore contours between −4 m and −6 m protrude off the detached breakwaters, implying that some longshore sand transport is discharged downdriftward while passing around the detached breakwaters, since longshore sand transport is blocked by the detached breakwaters. In March 1993, as shown in Fig. 2.4.17(b), sea-bottom contours in the vicinity of No. 55 became more concave, and the foot depth in front of the seawall increased considerably. In March 1994, as shown in Fig. 2.4.17(c), according to the dense contours near the shoreline, the seawall was directly exposed to waves in the vicinity of No. 55, and a steep scarp was also formed between No. 55 and No. 51. Beach erosion near No. 55 was so severe in causing the failure of the seawall that not only detached breakwaters III were set in the vicinity of No. 46, but also detached breakwaters II were constructed at No. 53 to prevent further beach erosion. As a result, the foreshore was widened near No. 53, as shown in Fig. 2.4.17(d), and the seawall was protected against scouring. The shoreline north of detached breakwaters II is considerably stable because the shoreline is fixed at detached breakwaters III. In contrast, shoreline recession has begun northeast of detached breakwaters III. Figure 2.4.17(e) shows the bathymetry in March 1996. A stepped shoreline was formed due to the construction of detached breakwaters. However, the contours northeast of detached breakwaters III are still unstable because of the open boundary condition; therefore, urgent

May 26, 2010

180

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.17. Topographic changes around detached breakwaters between March 1992 and March 1996.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

181

measures to stabilize the shoreline in front of this area should be adopted by constructing a facility to stabilize the shoreline, such as a groin or detached breakwaters downcoast of this point. As shown in Fig. 2.4.4 taken at point No. 35, a snow-capped Mt Fuji in the distance beyond the sandy beach and pine trees produces a beautiful scene; the disappearance of the sandy beach is considered to constitute definite damage to this coast. (5) Comparison of bathymetry around gently sloping revetment In the vicinity of No. 72, where the shoreline significantly retreated as shown in Fig. 2.4.5, a gently sloping revetment was built as a measure against beach erosion in the late 1980s. This construction of the gently sloping revetment triggered heavy downcoast erosion because of the exhaustion of longshore sand supply. Figure 2.4.18 shows the beach changes in this area. In 1988, a gently sloping revetment was located at the northeast end of the protection area, the protection of which had been carried out from upcoast (southwest side of the coast). Although erosion occurred near the northeast end due to northward longshore sand transport, a narrow foreshore on the northeast side of the gently sloping revetment was still left in 1988. Contours shallower than −4 m receded most at the location of the gently sloping revetment, and they gradually advanced northeast with distance. In contrast, offshore contours deeper than −5 m were smoothly extended parallel to the coastline. By 1992, a detached breakwater had been built downcoast of the gently sloping revetment to stabilize the shoreline between the revetment and the detached breakwater; however, during the construction work, sand discharged northward by longshore sand transport, forming a scarp in the adjacent area of the gently sloping revetment. As a result, this revetment was exposed to waves, and concrete armor units were installed to prevent wave overtopping. It was known that the gently sloping revetment was not useful as a measure against beach erosion associated with forming a scarp, when it was built on a coast where the erosion occurred due to the spatial imbalance of longshore sand transport, and it only postponed the erosion site to downcoast. A typical example occurred in the early 1990s at this site. Between No. 54 and No. 68, artificial headlands using a couple of detached breakwaters had been constructed to stabilize the shoreline. The

May 26, 2010

182

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.18. Beach changes around the groups I and II of detached breakwaters between No. 54 and No. 69.

beach changes associated with the construction of the artificial headlands can be investigated by comparing the bathymetries. Figure 2.4.19(a) shows the bathymetry in 1988 before the construction of the detached breakwaters. At this stage, all contour lines smoothly extended alongshore, with a steep slope of 1/7 between the shoreline and points of 5 m depth. In 1992, as shown in Fig. 2.4.19(b), the shorelines extending parallel in the past changed form in a stepwise manner due to the construction of groups I and II of the detached breakwaters. For instance, the shoreline once extended straight toward the detached breakwater on the west side of group I, whereas an embayed shoreline was formed on the east side with a maximum shoreline recession next to the detached breakwater. Although the water depth at the construction site of the detached breakwater was 3.5 m, the contour lines 5 m and 6 m deep off the detached

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

183

Fig. 2.4.19. Bathymetries in 1988 and 1992 between No. 54 and No. 68.

breakwater protruded offshore in the vicinity of group I, as well as east of it, where they gradually approached the shoreline. Taking into consideration that these contour lines smoothly extended alongshore in 1988 before the construction of the detached breakwaters, it is clear that the advance of the offshore contours was triggered by the construction of the detached breakwaters, explaining that part of longshore sand transport passed through off the detached breakwaters. As mentioned, the Shizuoka and Shimizu coasts were formed by continuous sand supply from the Abe River, but severe erosion took place in recent years due to decreasing sand supply from the river. Beach erosion started from the area adjacent to the river’s mouth, and it gradually propagated downcoast from the river’s mouth as an erosion wave. Fortunately, no large-scale dams obstructed sand movement in the upper reaches of the Abe River, and sand supply from the river’s mouth started once again after riverbed excavation to obtain construction materials was

May 26, 2010

184

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

prohibited in 1967. The sandy beach has been recovering at locations once eroded and covered with seawalls and detached breakwaters. At present, the area with the recovered sandy beach extends northward at a speed of 250 m/yr.A long time is still needed for this accretion wave to reach the tip of the Mihono-matsubara sand spit, but if we continue beach nourishment until that time, the restoration of the past wide sandy beach becomes possible at least in the long term. Taking this fact into account, the Shizuoka and Shimizu coasts are in favorable condition compared with the other coasts, where the natural sand supply is interrupted by large-scale dams and can no longer be expected in the future.

2.4.2.

Suruga Coast in Shizuoka Prefecture

The Ohi River originates from the central high mountains and flows down Suruga Bay, facing the Pacific Ocean, as shown in Fig. 2.4.20. Because of a steep bed slope and a large amount of sediment in this river, a significantly protruding river-mouth delta has been formed. The coastline along the river-mouth delta is called the Suruga coast. This coast has a steep foreshore slope of 1/10 and is a shingle beach. Despite the fact

Fig. 2.4.20. Location of Ohi River and Suruga coast.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

185

Fig. 2.4.21. Aerial photographs of Suruga coast in 1991.

that the Ohi River’s mouth is located on the west coast of Suruga Bay, as shown in Fig. 2.4.20, and predominant waves are incident from the south in Suruga Bay, southwestward longshore sand transport prevails on the south Suruga coast because of the significantly protruding coastline, which induces longshore sand transport with such characteristics locally (Uda et al., 2000∗ ). Figure 2.4.21 shows an aerial photograph of this coast taken in 1991. At the northeast end, the Ohi River flows into the sea and the Ohigawa port has been built at a location very close to the left bank of the river’s mouth. Another breakwater of Yoshida Port can be seen 2.5 km southwest of the right bank of the river’s mouth, as well as the Sakayaguchi and Katsumata Rivers flowing into the sea at locations 2 km and 4.5 km southwest of Yoshida Port, respectively. When investigating the overall coastline configuration between the mouth of the Ohi River and Sagara-hirata Port, the shoreline has advanced east of Yoshida Port, and a 170 m difference in the shoreline position can be observed at maximum, compared with that west of the port. In this area, southwestward longshore sand transport prevailed before the construction of Yoshida Port, and this transport was obstructed by the port breakwaters. At settings A, B, and C as shown in Fig. 2.4.21, the characteristics of the coastline configuration of region AB differ from those of region BC. In region AB, the shoreline tends to retreat with distance from the river’s mouth, but in region BC the shoreline advances to the breakwater ofYoshida Port. These characteristics elucidate that, in region AB, the protruding river mouth itself obstructs northward longshore sand transport as a groin, preventing the south Suruga coast from eroding.

May 26, 2010

186

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

In region BC where the shoreline advances to the breakwater of Yoshida Port, the direction of the predominant waves becomes approximately normal to the shoreline (S37◦ E), taking into consideration the fact that the shoreline of the region BC has already achieved a stable form. This direction makes an angle of 9◦ counterclockwise to the shoreline of region AB, generating southwestward longshore sand transport in region AB. Finally, it can be summarized that region AB works as a temporary storage reservoir in which sand accumulates when a sufficient volume of sand is supplied from the Ohi River, and sand discharges, while permitting the occurrence of northeastward longshore sand transport, when sand supply decreases. Along the coastline 1,620 m long, at B as shown in Fig. 2.4.21, detailed photographs show the evolution of seawall damage due to storm waves during Typhoon No. 26, which attacked on 30 and 31 October in 1972. Figure 2.4.22 shows the condition of the coast in May 1972, looking south from the mouth of the Ohi River to Yoshida Port. A seawall extended parallel to the coastline. In front of the seawall there was a narrow foreshore, and concrete armor units were placed along the shoreline. Judging from the photograph, the foreshore width before the storm was several meters at most.

Fig. 2.4.22. Suruga coast in May 1972.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

187

Fig. 2.4.23. Wave overtopping seawall at 14:00 on 30 October 1972.

Figure 2.4.23 shows the condition of the seawall at 14:00 on 30 October. Although wave overtopping was prevented by the parapet of the seawall, high run-up waves over the seawall can be seen. At this stage, the seawall kept its original form without deformation due to waves. Figure 2.4.24 shows the condition at 14:20 on 31 October. Waves jumped up very high in front of the seawall, and the subsidence of the promenade and a slight gap can be observed behind the parapet. At 15:00 on the same day, a straight crack was formed behind the parapet, as shown in Fig. 2.4.25. At 15:08 the seawall was completely destroyed, as shown in Fig. 2.4.26. Although the seawall barely stood at a location where an observer was standing, the full-scale collapse of the seawall started at the far side. At 15:45, the area where the seawall collapsed extended up to the location where an observer was standing (Fig. 2.4.27). Figure 2.4.28 shows the coastal conditions at 15:56 on 1 November 1972, after the storm. The seawall was completely destroyed and foundation gravel was fully carried away. A shingle beach, observed in the past, appeared from underneath the destroyed seawall.

May 26, 2010

188

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.24. Wave overtopping seawall at 14:20 on 31 October 1972.

Fig. 2.4.25. Gap behind parapet, taken at 15:00 on 31 October 1972.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.4.26. Collapsed seawall measured at 15:08 on 31 October 1972.

Fig. 2.4.27. Extension of damaged area measured at 15:45 on 31 October 1972.

189

May 26, 2010

190

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.28. Completely destroyed seawall taken at 15:56 on 1 November 1972.

As mentioned, the seawall in this area was destroyed by wave attack in 1972. The damaged area is closer to the mouth of the Ohi River than the area where a stable beach was formed due to the obstruction of longshore sand transport by port breakwaters, as shown by region BC in Fig. 2.4.21; therefore, the area is affected strongly by the sediment supply of the Ohi River during floods and the discharge of sand due to longshore sand transport with significant shoreline variations. From this, it is considered that the main cause of the collapse of the seawall was constructing the seawall in an area with such large shoreline variations. The damage of the seawall at the Suruga coast explains well that it is very dangerous to build a seawall on a sandy beach without a sufficiently wide foreshore. Even if a seawall is a solid concrete structure 0.5 m thick, it may be easily destroyed by wave action, if the seawall is built on a sandy beach without sufficient foundation. If the seabed elevation in front of the seawall degrades due to sand movement, first the foundation is destroyed, responding to this elevational change, and then the seawall itself is destroyed by the discharge of the foundation gravel.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

191

Taking these conditions of the seawall into account, the location of the construction of a seawall becomes very important. When a seawall is built at a landward position some distance apart from the shoreline where it is not subjected to strong wave action, it becomes stable. The construction of a seawall on a shoreline, where severe sand movement is occurring, may thus increase the vulnerability of the seawall. The shoreline around a river’s mouth repeatedly advances or retreats in response to the variation in sediment supply from the river, but if a seawall is built at a position with a sufficient foreshore width in front, the stability of the seawall could be maintained well, even though there is a variation in the shoreline position.

2.4.3. Tenryu River and Enshu-nada Coast in Shizuoka Prefecture (1) Sand accumulation in dam reservoirs in Tenryu River watershed and sand mining from riverbed The Tenryu River originates from the central mountains in Honshu and flows down with a steep riverbed slope of 1/871 upstream of the river’s mouth. The riverbed materials are composed of coarse gravel with a mean diameter of 14 mm. In this river, riverbed excavation for sand mining was extensively carried out before 1968, and many dams were constructed, resulting in a rapid decrease in sediment discharge and beach erosion on the coasts around the river’s mouth (Nagashima et al., 2005∗ ). The Tenryu River supplied a large amount of sediment to coasts in the past, because the riverbed has a steep slope and the watershed is close to the Median Techtonic Line of the Japanese Archepelago, which yields much sediment. After World War II, however, several dams for electric power plants, such as the Sakuma Dam, were built in various places, resulting in a decrease in sediment supply to the coasts and severe erosion. Therefore, the river-mouth terrace as a sand reservoir has been disappearing, and severe erosion is currently occurring near the shoreline. Figure 2.4.29 shows the sand accumulation in the reservoirs of the main dams and sand mining in the riverbed in the Tenryu River watershed. On the

May 26, 2010

192

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.29. Volume change of sand accumulated in main dam reservoirs in Tenryu River watershed and volume change of sand excavated as construction material.

Enshu-nada coast, which was formed by sediment supplied by the Tenryu River, beach erosion occurred in the vicinity of the river’s mouth with the decrease in fluvial sand supply. The causes of the beach erosion are sand accumulation upstream of many dams constructed in this watershed and sand mining from the riverbed to obtain construction materials. The annual volume of sand mining since 1970 and the volume of sand accumulated since 1956 are shown in the figure. Taking into consideration the fact that sand mining had been carried out extensively before 1967, at a time when sand mining was prohibited by law, and that the sand volume actually mined was twice as much as the permitted volume in general in the past, the volume shown in Fig. 2.4.29 differs from the total volume of mined sand in all reaches of the Tenryu River, but the total sum reaches 2.5 × 107 m3 at least since 1970. Sand deposition occurs at a level of 1.25 × 108 m3 in the upstream reaches of the dams. The sum of both is equal to 1.5 × 108 m3 . Kawata et al. (1997∗ ) showed that fine materials such as the wash load of the river are ineffective for the formation of a sandy beach, and only coarser materials are effective, the deposition rate of which accounts for about 8% of all the total sediment yield in the Tenryu River watershed. If we assume that the sand component related to beach erosion is 8% of the

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

193

total volume of sand deposited in the dam reservoir and 100% of the sand volume mined in the riverbed, then the cumulative volume of such sand reached 3.5 × 107 m3 in the period between 1956 and 2004, resulting in a sand discharge of 7.0 × 105 m3 /yr. Torii et al. (2004) also showed that the annual sand supply of this river in the long term is approximately 8.0 × 105 m3 /yr from calculations using the model for predicting one-dimensional riverbed change. Thus, around 90% of the annual sand supply of the river was artificially cut. (2) Reduction of river-mouth terrace and erosion Beach erosion around the river’s mouth due to the reduction of sediment discharge from the river can be studied using aerial photographs of the river’s mouth, as shown in Fig. 2.4.30. In the figure, the alignment of the five transects at 200 m intervals from the left bank of the river and the shoreline position in 1962 determined from the aerial photographs are shown. Figure 2.4.31 also shows the longitudinal profiles along the four transects between 1984 and 2001. At No. 218, the terrace at the river’s mouth developed in 1984, but its topography had almost disappeared by 2001. The seabed depth at the outer edge of the terrace increased by about 5 m at maximum. The depth of closure, where the longitudinal profile change diminishes, was 14 m at No. 216 and 15 m at No. 218. In this way, at the Tenryu River mouth, both large shoreline recession and the complete disappearance of the terrace

Fig. 2.4.30. Aerial photograph at mouth of Tenryu River and alignment of transects.

May 26, 2010

194

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.4.31. Change in longitudinal profiles along four transects around mouth of Tenryu River.

topography, which was a reservoir of sand for longshore sand transport, occurred simultaneously. A rectangular examination area 1.6 km long alongshore and 1 km long in the cross-shore direction as shown in Fig. 2.4.30 was selected, and the volume change of this area was calculated as shown in Fig. 2.4.32, using bathymetric survey data. The rate of decrease of the total sand volume was very large in the period between 1986 and 1993, and reached 4.0 × 105 m3 /yr. After 1993, the rate of decrease diminished, but still maintained a value of 1.5×105 m3 /yr. The total decrease in volume reached 4.6×106 m3 in the period between 1984 and 2004. As mentioned, a terrace 500 m wide developed in 1984 off the mouth of the Tenryu River, but the terrace topography had entirely disappeared

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

195

Fig. 2.4.32. Change in sand volume in area examined.

by 2004, resulting in an increase in the seabed depth of 5 m at the outer edge of the terrace. As a result, the shoreline zone is being severely eroded at present beyond the erosion in the offshore zone including the terrace at the river’s mouth. Although the measures against beach erosion have been taken locally, the erosion of the entire area where longshore sand supply was interrupted is feared at present along the long stretch of the shoreline protruding as a river-mouth delta.

2.4.4.

Methods of Addressing Issues

When beach erosion is triggered by decreasing the fluvial sediment supply on a coast surrounding a river’s mouth, the most effective and fundamental measure is the artificial increase of the sediment supply. Recently, sand bypassing at dams has been tentatively tried at many locations, but the trial level must be raised to the practical level to really solve the problems. It is well known that rivers with steeply sloping beds originating from the central mountain areas in Japan, such as the Kurobe, Fuji, Abe, Ohi, and Tenryu Rivers, supplied much sediment in the past, but the sand supply greatly decreased due to sand mining of the riverbeds and the blockage

May 26, 2010

196

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

of sand transport by the construction of dams, resulting in beach erosion around the river’s mouths at first and then the propagation of an erosion wave downcoast. Here, the case of the Abe River and the Shizuoka and Shimizu coasts extending north of the river’s mouth was studied in detail as an example. An example of the littoral transport system including the Abe River and the Shizuoka and Shimizu coasts is a very rare case in which after the prohibition of riverbed mining, a large amount of sediment started to be supplied again to the Shizuoka and Shimizu coasts, and the sandy beach has been recovering until now. However, in almost all rivers with large dams in the watershed, except the Abe River, decreased sediment supply remains as it was. At the same time, a number of concrete blocks have already been installed in the vicinity of the river’s mouth, resulting in the alteration of the coast from natural to artificial. On these coasts, shore-protection facilities must be maintained for a long time, but more serious problems may exist elsewhere; namely, the supply of fine sediment. Almost all sediment supplied from rivers is composed of fine sediment, as mentioned in Sec. 3.7, and this diffuses and deposits in an extensive area off the shoreline. The diffusion of fine sediment is assumed to closely relate to the ecosystem of fishes and shellfish in the coastal zone. The interruption of sediment supply may ultimately affect the preservation of a healthy ecosystem in a coastal zone. At present, people may think that the problem lies in the formation of a severely eroded coastline covered by a number of concrete blocks, spoiling the coastal scenery and usability, but the true problem lies in the supply of fine sediment. Sufficient caution must be paid to not only the apparent visible problems, but also phenomena occurring underwater.

REFERENCES Itabashi, N. and T. Uda (1998). Field observation of erosion and accretion waves on Shizuoka and Shimizu coasts in Suruga Bay in Japan, Proc. 26th ICCE, pp. 3178–3191.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

197

Torii, K., S. Sato, T. Uda and T. Okayasu (2004). Regional sediment management based on sediment budget for graded sediments — A case study of Tenryu watershed and Enshu-nada coast, Proc. 29th ICCE, pp. 3110–3122. Uda, T. and K. Yamamoto (1994). Beach erosion around a sand spit — An example of Mihono-matsubara sand spit, Proc. 24th ICCE, pp. 2726–2740. Uda, T., Y. Yamamoto, N. Itabashi and K. Yamaji (1996). Field observation of movement of sand body due to waves and verification of its mechanism by numerical model, Proc. 25th ICCE, pp. 137–150.

REFERENCES (in Japanese) Kawata, Y., M. Inoue, M. Uemoto, T. Maruya and M. Ishikawa (1997). Effect of dams to beach processes — The example of Tenryu River watershed, Annual J. Coastal Eng. JSCE 44, 606–610. Nagashima, I., N. Iwasaki, T. Uda and T. Arimura (2005). Beach erosion of Enshunada coast west of Tenryu River mouth, Annual J. Coastal Eng. JSCE 52, 596–600. Uda, T., K. Yamamoto and S. Kawano (1991). Beach changes around a sand spit — The example of Mihono-matsubara sand spit, Trans. Japanese Geomorphological Union 12, 117–134. Uda, T. and N. Itabashi (1997). Propagation at erosion and accretion waves on Shizuoka and Shimizu coasts, Annual J. Coastal Eng. JSCE 44, 631–635. Uda, T., Y. Yamamoto, N. Itabashi and K. Yamaji (1997). Movement of sand body observed on Shizuoka coast and its generation mechanism, Proc. JSCE, No. 558/II-38, pp. 113–128. Uda, T. (1997). Beach Erosion in Japan (Sankaido Shuppan, Tokyo), p. 442. Uda, T., S. Seino, T. Ishikawa and M. Serizawa (2000). Historical erosion damages of southwestern Suruga coast and issues related to coastal protection, Proc. Civil Eng. in the Ocean, JSCE 16, 601–606.

May 26, 2010

198

2.5. 2.5.1.

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

BEACH EROSION TRIGGERED BY OFFSHORE SAND MINING/DREDGING Mouth of Sagami River in Kanagawa Prefecture

The Sagami River flows into Sagami Bay. In the past, the fluvial sand supply from this river was very large, developing a river-mouth delta, and sand dunes were formed along the coastline due to wind-blown sand in winter. In recent years, the protruding river-mouth delta has changed greatly, inducing upstream movement of the sand bar at the river’s mouth. The cause is closely related to the dredging of the navigation channel to the Suga fishing port located upstream of the river’s mouth on the right bank, as shown by an arrow in Fig. 2.5.1: the photograph was taken obliquely from the east side of the mouth. The river’s mouth retreated significantly, forming an embayment enabling the tip of the sand bar to reach the Shonan Bridge. This bar was

Fig. 2.5.1. Retreated bar at mouth of Sagami River.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

199

located seaward of the seawall protecting the sewage treatment facilities, as seen at the bottom of the figure until 1985, but the development of the bar in the transverse direction across the river stream and the dredging of the deposited sand in the navigation channel had been repeated, resulting in a decrease in sand volume at the river mouth bar and causing its recession. Figure 2.5.2 also shows the river’s mouth and new fishing port of Hiratsuka. The sand bar at the river mouth extends in the traverse direction, leaving a narrow opening between the right bank and the bar, whereas a scarp is formed in the vicinity of the shoreline at the base of the slender bar. The opening between the tip of the river-mouth bar and the right bank was extremely narrow, which caused strong currents, and the river channel also meandered significantly. These features made the navigation channel dangerous. For this reason, a dredging operation was carried out at the tip of the bar. After the dredging, longshore sand movement was induced from the base of the bar to the tip and upcoast of the bar was eroded, because the bar

Fig. 2.5.2. Mouth of Sagami River and new fishing port at Hiratsuka.

May 26, 2010

200

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

was approximately at equilibrium. The scarp observed in the area between the central part of the bar and its base was formed during this erosion. On the other hand, a sand bar was produced at the tip of the right bank by temporarily reclaiming part of the dredged materials in the river channel. The artificial sand bar had wave diffraction effects like a breakwater, accelerating the extension of the bar toward the right bank. The shoreline of the bar of the mouth of the Sagami River retreated significantly compared with the surrounding coastline. The shoreline of the river’s mouth must protrude seaward in general, if a large amount of sand is supplied from the river. The above-mentioned condition at the Sagami River’s mouth clearly shows that the fluvial sand supply has greatly decreased; particularly, the supply of the sand useful for the formation of the beach has been reduced. The coast surrounding the mouth of the Sagami River is starving for sand, and the recovery of a sandy beach has become impossible except by artificial nourishment.

2.5.2.

Off Mouth of Niyodo River in Kochi Prefecture

(1) Comparison of aerial photographs The Niyodo River is a large river with a catchment area of 1,560 km2 . It empties into Tosa Bay alongside Tosa City in the western part of Kochi Prefecture, as shown in Fig. 2.5.3. The riverbed slope is around 1/2,000 from the river’s mouth to 2 km upstream. In the past, the opening of the river-mouth bar was consistently located near the left-hand-side training jetty. In recent years, the location of the opening has become destabilized and has moved westward because of the effect of offshore sand mining and floods (Uda et al., 1985∗ , 1986). The shore sand is composed of coarser materials, and relatively long period waves are incident from the Pacific Ocean at the mouth of the Niyodo River. These two factors combine to produce a steep foreshore slope of roughly 1/13. The wave characteristics, H1/3 and T1/3 , around the mouth of the Niyodo River ranged within 3–4 m and 10–12 s, respectively, at the time of Typhoon 9109, which caused a severe disaster on the Kochi coast in 1991. For convenience of analysis, the origin of the coordinate system

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

201

Fig. 2.5.3. Location of Niyodo River’s mouth and coordinate system.

is set at Bunkobana Point, and the x axis is taken along the shore with the y axis perpendicular to the shore, as shown in Fig. 2.5.3. Figure 2.5.4, photographed on 19 September 1975, shows the bar of the mouth of the Niyodo River and the Kochi coast extending eastward from the river mouth. At this time, the wide bar distinctly protruded seaward, and the piers of the bridge across the Niyodo River’s mouth can be seen. To the

May 26, 2010

202

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.5.4. Aerial photograph of Niyodo River’s mouth (19 September 1975).

left of the river’s mouth, a narrow lagoon extended between the seawall and the bar. The shoreline was discontinuous at Bunkobana Point, but extended beyond the point. This feature indicates that, before 1975, the sediment supply from the Niyodo River was sufficient to nourish the downcoast. A comparison of Figs. 2.5.4 and 2.5.5, photographed on 23 February 1991, reveals that the shoreline had retreated considerably, as indicated by the location of the bridge piers. It is inferred that the offshore sand mining conducted until 1988 may have been the main cause of this shoreline

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

203

Fig. 2.5.5. Aerial photograph of Niyodo River’s mouth (23 February 1991).

recession. The opening of the river’s mouth was located in the vicinity of the training jetty on the left bank, but in recent years it has moved to the right-hand side of the river’s mouth because of the shoreline recession. (2) Bathymetric changes Figure 2.5.6 shows the bathymetry in 1981 (broken lines) and in 1984 (solid lines). The contours off the river’s mouth in 1981 were convex along the centerline of the mouth. In 1984, a large-scale dredging hole with a maximum depth of 14 m was formed only 150 m off the shoreline. In addition, another dredging hole with a maximum depth of 11 m was formed on the east side of the mouth. Both holes were mined to obtain building sand, as already reported by Uda et al. (1986). Figure 2.5.7 shows the bathymetry in November 1988. Until 1984, dredging of the sea bottom was conducted off the eastern part of the river’s mouth, and in 1988 the area was further extended to almost the entire area off the river-mouth bar, when a large-scale dredged hole 14 m deep at maximum was formed. The deepest part was located around 200 m from

May 26, 2010

204

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.5.6. Bathymery off Niyodo River’s mouth (1981 versus 1984).

Fig. 2.5.7. Bathymetry off Niyodo River’s mouth in 1988.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

205

the shore. The longshore distance of the 10 m-deep isocontour adjoining the dredged hole, selected to represent the scale of the hole, expanded from 400 m in 1984 to 900 m in 1988. Figure 2.5.8 shows the bathymetry off the river’s mouth in 1993. The opening of the river-mouth bar extends straight toward the right bank. The berm height of the bar is, at its highest, 6.5 m in the vicinity of the left bank, and it gradually decreases westward. It should be noted that both the formation of the river-mouth bar with a concave shoreline and the development of a high berm on the bar are strongly related to the presence of the large-scale hole. Furthermore, it can be seen from a comparison of Figs. 2.5.7 and 2.5.8 that, since 1988, the hole was refilled by floods with its maximum depth decreasing from 14 m to around 11.5 m, although no significant changes appeared in the location of the deepest part and in the scale of the hole. Beach profiles along a typical survey line A–A , as shown in Fig. 2.5.3, were compared. The A–A cross-section is located 2.2 km west of the origin of the coordinate system shown in Fig. 2.5.3 and almost corresponds to the

Fig. 2.5.8. Bathymetry off Niyodo River’s mouth in 1993.

May 26, 2010

206

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.5.9. Comparison of beach profiles (A–A cross-section).

center of the hole. Figure 2.5.9 shows the change in the beach profile. It can be seen that, between 1983 and 1988, a large-scale hole had formed because of sand mining in a range of 100–200 m off the 1981 shoreline, and thereafter this dredging hole was gradually refilled until 1992, concurrently with shoreline recession. During this observation period, large beach-profile changes took place in the zone shallower than 10 m. Regarding the dredged hole in the vicinity of the east bank of the Niyodo River’s mouth, the topographic changes with time have been monitored (Uda et al., 1994∗ , 1995). Figure 2.5.10(a) shows the bathymetry in November 1981. In 1981, a dredged hole with the shape of a concentric circle and a maximum depth of 11 m was formed just off the shoreline. The

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

207

Fig. 2.5.10. Topographic changes of dredged hole off left bank of Niyodo River’s mouth.

location of the maximum depth was only 110 m away from the shore, and a steep slope was formed in the vicinity of the shoreline. In November 1982, bathymetric changes occurred as shown in Fig. 2.5.10(b). Since two large floods occurred with a maximum discharge of 8,900 m3 /s on 27 August and 8,000 m3 /s on 25 September in 1982, the contour lines around the river’s mouth, which extended straight in the past, advanced significantly and a river-mouth terrace with the shape of a tongue was formed off the mouth. This change was due to the deposition of part of the sand supplied from the Niyodo River. The tip of the river-mouth terrace reached to a depth of 8 m, and the contours between the depths of 1 m and 8 m protruded parallel to each other. Taking into consideration the fact that the 11 m contour disappeared and the area surrounded by the 10 m deep contour was significantly reduced, it is clear that the dredged hole was naturally refilled. The shoreline behind the dredged hole retreated with respect to the surrounding shoreline and assumed a concave form in the offshore direction. The formation of this concave shoreline is a general feature formed in the shoreward zone of an offshore dredging hole due to the effect of wave diffraction and refraction. Figure 2.5.10(c) shows the bathymetry in October 1983. By comparing this figure with Fig. 2.5.10(b), it is found that sand discharged from the river’s mouth fell down into the dredged hole due to eastward longshore sand transport. In Fig. 2.5.10(b), the terrace of the river’s mouth had the shape of tongue, but it filled the dredged hole from the western side by eastward movement of sand.

May 26, 2010

208

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

As mentioned, large-scale dredged holes were formed by offshore sand mining off the Niyodo River’s mouth, and sand originally supplied to the Kochi coast by longshore sand transport was trapped in these offshore holes; the reverse longshore sand transport took place from the surrounding coast to the river’s mouth. Thus, longshore sand supply from the river’s mouth to the Kochi coast was greatly reduced, resulting in downcoast erosion.

2.5.3.

Off Sumiyoshi-hama Sand Spit in Oita Prefecture

(1) General conditions Sand spits are formed by the deposition of sand from longshore sand transport at the mouth of a bay, where the shoreline direction suddenly changes. Usually the topographic change of a sand spit remains small after the lapse of a long period of time since its formation. However, in recent years, at the Sumiyoshi-hama sand spit, which is approximately 1.2 km long, at the mouth of Moriye Bay located in the northern part of Beppu Bay in Kyushu, as shown in Fig. 2.5.11, new phenomena of rapid sand accumulation at the central part of a sand spit and rapid deformation of

Fig. 2.5.11. Location of Moriye Bay and Sumiyoshi-hama sand spit in Oita Prefecture.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

209

a spit have been observed. These phenomena cannot be explained by the effect of longshore sand transport alone, which is the main driving force of sand spit formation. At the Sumiyoshi-hama sand spit, a sand body appeared in a zone of very shallow water, and this sand body moved as a unit toward the tip of the sand spit. Finally, the sand body joined the tip of the sand spit, resulting in large changes to the beach. This study aims at investigating the process of sand body deformation by aerial photography and field observations (Uda et al., 2001∗ ; Seino et al., 2001∗ ; Uda et al., 2002). It is generally considered that the formation of a dredging hole in the vicinity of a shoreline accelerates sand deposition into the dredging hole, but the results of this study indicate the possibility of the occurrence of a contradictory rapid shoreward sand movement. (2) Comparison of aerial photographs Long-term topographic changes of the Sumiyoshi-hama sand spit were investigated by comparing aerial photographs taken between 1948 and 1996. Figure 2.5.12 shows an aerial photograph taken during low tide on 18 July 1996. Since the photograph was taken during low tide, the seabed topography in the zone of shallow water can be read well. Although almost

Fig. 2.5.12. Aerial photograph of Sumiyoshi-hama sand spit taken on 18 July 1996.

May 26, 2010

210

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

all of the sand spit was surrounded by the seawall, a fishhook-shaped sand bar extended from the tip of the sand spit. The tip of the sand bar reached the lighthouse, and low land extended into the central part of this fishhook sand bar. By 1996, the dredging hole had approached the vicinity of the tip of the sand spit. Figure 2.5.13 summarizes the process of the longshore extension of an offshore dredging hole. The western end of the dredging hole advanced approximately 1.56 km westward along the shoreline, and it gradually approached the shoreline; the shortest distance to the shoreline was only 115 m after 1996. (3) Rapid shoreline change triggered by offshore mining Aerial photographs taken in July and October of 1996 and August of 1998 were compared in detail to investigate the rapid shoreline changes around the Sumiyoshi-hama sand spit. In July 1996, as shown in Fig. 2.5.12, the southern coastline of this spit can be divided into western and eastern halves with respect to the turning point of the shoreline, shown by point P. On the western side, no sandy beaches exist in front of the seawall, whereas a sandy beach extends along the eastern side. The dotted line off the shoreline shows the margin of the dredging hole. Five rows of longshore sand bars had formed around the offshore dredging hole off the southern coastline

Fig. 2.5.13. Process of longshore extension of offshore dredging hole.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

211

of the spit. The offshore dredging hole extends alongshore, and two rows of longshore sand bars are also formed off the eastern shore of the turning point P of the coastline. However, no longshore sand bar is formed in the eastern zone of shallow water far from the dredging hole. These longshore sand bars have the distinctive feature that the top of the sand bar is exposed during low tide. The development of longshore sand bars surrounding the shoreward margin of the dredging hole strongly suggests the possibility that these sand bars were formed by shoreward sand movement due to wave action at the steep shoreward slope of the dredging hole. Figure 2.5.14 shows an oblique aerial photograph taken in August 1998. A semicircular sand bar was newly formed in front of the playground near the tip of the sand spit. This semicircular sand bar did not exist in July 1996, as shown in Fig. 2.5.12, and therefore it is clear that this sand bar was formed in the period between July 1996 and August 1998. In contrast, the longshore sand bars that had existed around the margin of the offshore dredging hole, as shown in Fig. 2.5.12, have disappeared in Fig. 2.5.14. These results imply that the semicircular sand body observed in Fig. 2.5.14 was formed by the accumulation of sand transported westward from the longshore

Fig. 2.5.14. Oblique aerial photograph taken in August 1998.

May 26, 2010

212

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

sand bar off the turning point P of the coastline. Furthermore, the tip of the Sumiyoshi-hama sand spit extended westward considerably during the same period, as observed by comparing Figs. 2.5.12 and 2.5.14. This clearly shows the predominance of westward longshore sand transport, precluding the possibility that the semicircular sand body in front of the playground, shown in Fig. 2.5.14, was formed by temporal eastward longshore sand transport from the tip of the sand spit. The Sumiyoshi-hama sand spit had originally been elongated by the deposition of abundant sand carried by westward longshore sand transport. However, currently, the sand supply from the updrift side of the spit is totally exhausted, judging from the disappearance of the sandy beach in front of the seawall. In addition, at the west end of the eastern half of the coastline, point P in Fig. 2.5.12, the seawall protrudes, acting as a groin that obstructs westward longshore sand transport. (4) Comparison of bathymetries Bathymetries in 1972 and in 1996, before and after offshore dredging, respectively, were compared using Figs. 2.5.15 and 2.5.16. The formation

Fig. 2.5.15. Bathymetry around Moriye Bay in 1972 before offshore sand mining.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

213

Fig. 2.5.16. Bathymetry around Moriye Bay in 1996 after offshore sand mining.

of a large dredging hole off the southern coastline of the Sumiyoshi-hama sand spit is clear. The shoreward seabed slope of the hole is very steep, and the maximum water depth reaches 12 m below the MSL. The sand volume of the dredged hole in the trapezoidal zone shown in Fig. 2.5.16 was 4.4 × 106 m3 , which amounts to 44% of all the sand deposited to form the sand spit (Seino et al., 2001). As mentioned, the Sumiyoshi-hama sand spit has changed its form with time, largely due to artificial causes. The sand spit, formed by abundant sand supply due to longshore sand transport in the past, is now starving for sand, because the longshore sand supply from the eastern coast is exhausted. Furthermore, offshore sand mining has decreased the total amount of sand deposited to form the sand spit.

2.5.4.

Methods of Addressing Issues

When offshore mining to obtain construction material or the maintenance dredging of navigation channels are carried out in a littoral zone extending from the berm height to the depth of closure of a coast, the impact inevitably expands to the surrounding coastline. A restoring force is exerted to recover

May 26, 2010

214

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

the original state of the beach, when the water depth is artificially increased at a location on a beach with a stable form, because the longshore and cross-shore slopes of the seabed change. The dredged hole is refilled until the longshore inclination of the offshore contours and the local slope of the longitudinal profile return to equilibrium before offshore mining. These phenomena can be predicted by numerical simulation using the contour-line change model as described in Sec. 3.8. To prevent the influences of offshore sand mining from expanding, mining in a zone shallower than the depth of closure, hc , must be prohibited, and even plans are made to carry out mining in a zone deeper than hc , the shoreline locally retreats, so that sand mining in a zone close to the shoreline must be avoided. In the dredging of a navigation channel, if the dredged materials are removed and used, the total sand volume of a coast decreases in the same way as in the case of sand mining in the nearshore zone; therefore, the reuse of the dredged materials for sand bypassing or sand recycling should be considered. Similarly, dredging at a river’s mouth as a method of improving the mouth may cause beach erosion on the surrounding coastline. Taking this discussion into consideration, investigating the details of dredging operations at ports and river mouths is inevitably required in the area adjacent to an eroded coast, as is the evaluation of activities from a quantitative point of view, when the causes of beach erosion are not clearly grasped, even if severe beach erosion has already been underway. Excluding the influence of dredging without sufficient examination will prevent the erosion problem from being solved and erosion will further deteriorate the beach.

REFERENCES Uda, T., C.Agemori and N. Chujo (1986). Beach changes caused by offshore dredging, Coastal Eng. Japan 29, 215–226. Uda, T., A. Takahashi and M. Fujii (1995). Bar topography changes associated with a dredged hole off the Niyodo River mouth, Coastal Eng. Japan 38(1), 63–88.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

215

Uda, T., S. Seino, M. Serizawa, T. San-nami and K. Furuike (2002). Rapid formation of sand body at shoreward edge of offshore dredging hole and resulting large change in sand spit, Proc. 28th ICCE, pp. 3461–3473.

REFERENCES (in Japanese) Uda, T., S. Agemori and T. Chujo (1985). Beach changes associated with offshore mining, Annual J. Coastal Eng. JSCE 32, 410–414. Uda., T., M. Fujii, A. Takahashi and K. Ito (1994). Deformation of a rivermouth bar associated with offshore dredging, Annual J. Coastal Eng. JSCE 41, 496–500. Uda, T., S. Seino, H. Kugimiya, M. Serizawa, K. Furuike and T. San-nami (2001). Rapid formation of sand body at shoreward edge of offshore dredging hole and resulting large change in sand spit, Annual J. Coastal Eng. JSCE 48, 606–610. Seino, S., T. Uda, T. San-nami, M. Serizawa and K. Furuike (2001). Beach changes around Sumiyoshi-hama sand spit in Beppu Bay due to exhastion of littoral drift, offshore mining and construction of seawall, Trans. Japan. Geomorph. Union 22, 59–73.

2.6.

BEACH EROSION TRIGGERED BY CONSTRUCTION OF DETACHED BREAKWATER AS COUNTERMEASURE

Detached breakwaters have been extensively used in Japan since 1972 after their application to the Kaike coast in Tottori Prefecture, and they are still one of the main countermeasures against beach erosion.Almost all detached breakwaters have been built by installing concrete armor units off the shoreline, and their depth ranges from around 3–5 m.A detached breakwater significantly dissipates incident wave energy, resulting in the formation of a wave-shelter zone behind it. The scale of this wave-shelter zone is usually smaller than that of a port breakwater, as described in Sec. 2.3. There are many examples in which beach erosion on the surrounding coast

May 26, 2010

216

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

was accelerated by constructing detached breakwaters as a measure against beach erosion. In particular, such examples are often observed on pocket beaches bounded by headlands. Here, examples of the Ariake–Takahama coast in Ibaraki Prefecture and the Gotsu coast in Shimane Prefecture are shown.

2.6.1. Ariake–Takahama Coast in Ibaraki Prefecture (1) General features of area studied The Ariake–Takahama and Ishihama coasts are located on a pocket beach around 6 km long in the northern part of Ibaraki Prefecture, as shown in Fig. 2.6.1; they face the Pacific Ocean. The Takado and Uno Points are at the north and south ends of this pocket beach, respectively. These points prevent inflow and outflow of longshore drift beyond them, forming a closed system of littoral drift. In the hinterland of this coast, Takahagi City extends north of the Hananuki River’s mouth. In contrast, a coastal forest covering sand dunes spreads continuously south of this river’s mouth. Since northerly winds prevail in winter, sand dunes are well developed in the southern part of the coastline. Three rivers, the Sekine, Hananuki, and Koishikawa, with catchment areas of 38, 65, and 20 km2 , respectively, flow into this pocket beach. Figure 2.6.2 shows the bathymetry measured in August 1998, in the area studied extending from Takado to Uno Points. Offshore contours up to 10 m deep extend parallel to the shoreline, but further offshore contours protrude considerably off the Hananuki River’s mouth. This feature is due to the existence of a submerged reef in the offshore zone. In addition, a sea bottom with concave contours exists on both sides of this submerged reef. In past studies of the coast in the vicinity of this area, Mogi and Iwabuchi (1961∗ ) clarified that historically, several rivers extending offshoreward from the present river mouths flowed into the ocean, forming erosional valleys through the land during the period of rising sea level after the maximum fall of the sea level in the last glacial age. These valleys remain as submarine canyons at depths of 20–50 m. In the present study area, the

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

217

Fig. 2.6.1. Location ofAriake, Takahama, and Ishihama coasts in northern Ibaraki Prefecture.

same kind of offshore sea bottom can be observed. Shidai et al. (1997∗ ) found that rocky beds are exposed on the sea bottom off the Takado Point and off the area between Uno Point and the Hananuki River’s mouth, in field observations using a side-scan sonar in an area approximately 20 km long stretching from Uno Point to the Izura coast, which included the present study area. Furthermore, nine detached breakwaters have been built north of the Hananuki River’s mouth and four in the vicinity of Uno Point.

May 26, 2010

218

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.6.2. Map of area studied and alignment of survey lines of profile.

(2) Shoreline changes On the basis of the aerial photographs taken by the Geographical Survey Institute, shoreline changes between 1946 and 1996 were investigated (Uda et al., 2000∗ , 2003). Out of seven sets of aerial photographs taken during this period, three sets are shown. Figure 2.6.3(a) shows an aerial photograph taken on 12 April 1950. At this time, a wide natural sandy beach extended along the shoreline. The maximum width of this sandy beach was around 160 m between the Hananuki and Sekine Rivers flowing into the central and at the north end of the shoreline, respectively. In addition, the maximum shoreline width around the Koishikawa River was about 180 m. Since a training jetty had not yet been constructed at the Hananuki River’s mouth, the channel near the river’s mouth meandered approximately 600 m southward before flowing into the ocean. The projecting shoreline in the vicinity of the Koishikawa River’s mouth shows the formation of a cuspate foreland leeward of the offshore reefs, as shown in Fig. 2.6.2, due to the wave-sheltering effect of the reefs. Figure 2.6.3(b) shows an aerial photograph taken on 29 August 1976. Comparison of this aerial photograph with that in Fig. 2.6.3(a) reveals that the width of the sandy beach had decreased considerably. Two reasons are given.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

219

Fig. 2.6.3. Aerial photographs of Ariake, Takahama, and Ishihama coasts taken by Geographical Survey Institute: (a) 12 April 1950, (b) 29 August 1976, and (c) 2 December 1993.

The first is the seaward advance of the boundary of Takahagi City; Fig. 2.6.3(a) shows that natural vegetation grew sparsely in the hinterland of the wide sandy beach north of the Hananuki River, whereas in Fig. 2.6.3(b), newly built townhouses are seen in this area. The second is the seaward advance of the coastal forest, composed mainly of pine trees, in the area south of the Hananuki River. The maximum advance of the coastal forest was around 150 m. Thus, it is concluded that the decrease in beach width at this coast did not result from beach erosion, but from artificial disruptions such as the seaward advance of the town and coastal forest. Figure 2.6.3(c) shows an aerial photograph taken on 2 December 1993. The coastal forest concentrated in the area south of the Hananuki River’s mouth. Southward blown sand prevails in winter along the shoreline extending from south to north, because of the predominance of a northerly

May 26, 2010

220

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

wind. Furthermore, since the shoreline orientation changes in the vicinity of the Hananuki River’s mouth from N20◦ E north of the river’s mouth to N5◦ E south of it, sand can be easily transported landward by wind from the shoreline south of the river’s mouth. This is the cause of the development of a wide area of sand dunes in the southern part of the coast, whereas north of the Hananuki River’s mouth, land is densely covered by houses. Detached breakwaters have been constructed at the southern and northern halves of the coastline; two detached breakwaters next to Uno Point and seven north of the Hananuki River’s mouth, as well as one under construction. A cuspate foreland has been formed behind the detached breakwaters. The narrow width of the sandy beach between the Hananuki River’s mouth and the detached breakwater north of Uno Point contrasts sharply with the very wide sandy beach behind the detached breakwaters, indicating longshore sand transport from south to north on this pocket beach. In particular, the width of the beach in the zone between the Koishikawa River’s mouth and a point 700 m to the south of it became as narrow as approximately 25 m. On the other hand, a relatively wide foreshore was formed north of the Koishikawa River’s mouth due to the wave-attenuation effect of the offshore reef, as shown in Fig. 2.6.2. Shoreline position was determined from aerial photographs as well as from bathymetry obtained in August 1998, and the shoreline configurations obtained for the different years were compared. In the determination of shoreline position from aerial photographs, a correction was made to calculate the shoreline position corresponding to the MSL using the tide level at the measuring time and a mean foreshore slope of around 1/10. Figure 2.6.4(a) shows the shoreline change for 1984 and 1998 with reference to the shoreline position in 1976 as examples of long-term shoreline change. Similarly, Fig. 2.6.4(b) shows the shoreline change for 1990 and 1996 with reference to 1976 as examples of the shoreline change during the period of construction of detached breakwaters. Figure 2.6.4(a) shows that the shoreline retreated between 1976 and 1984 in the zone from the Sekine River’s mouth and approximately 1 km to

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

221

Fig. 2.6.4. Change in shoreline position from August 1976 based on aerial photographs.

the south, whereas the shoreline advanced in the vicinity of the Hananuki River’s mouth. It should be noted that the shoreline recession between 0.45 and 1.3 km south of the Koishikawa River’s mouth and the shoreline advance near the south end of the coast can be observed simultaneously, although the shoreline change between the mouths of the Hananuki and Koishikawa Rivers is not significant. The shoreline change up to August 1998, as shown in Fig. 2.6.4(a), indicates that the construction of detached breakwaters caused shoreline advance behind the detached breakwaters and shoreline recession in a zone without detached breakwaters. In 1990, soon after the construction of detached breakwaters began, as shown in Fig. 2.6.4(b), a large tombolo was formed behind the detached

May 26, 2010

222

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

breakwaters next to Takado Point, but the shoreline in the zone next to this sand-accumulation area retreated. In 1996, the size of the tombolos behind the already completed detached breakwaters decreased, and further shoreline recession occurred in the zone south of the newly built detached breakwaters with the southward extension of the construction zone of detached breakwaters on the Ariake–Takahama coast. The wave observation conducted by Shidai et al. (1997∗ ) off Tenpisan Point located around 9 km north of Takado Point revealed that the wave direction changes seasonally in a cyclic mode, causing southward or northward longshore sand transport depending on the seasonal wave conditions; this results in the seasonal shoreline change. Therefore, the shoreline variation observed between 1976 and 1984 is considered to be caused by the development of southward longshore sand transport under wave incidence from the northeast, while the shoreline retreats (advances) on the south (north) side of a structure or a point that obstructs sand movement. (3) Field observations Figure 2.6.5 shows the condition of the coastal dike, and is taken from the top of the coastal dike located approximately 1 km south of the Koishikawa River’s mouth on 7 June 1998, looking north. The seaward slope of this dike is reinforced by a concrete revetment. Foreshore width gradually decreases northward from this area, and at a point approximately 0.8 km south of the river’s mouth, the seaward slope was eroded, as shown in Fig. 2.6.6, owing to the wave action, exposing the core of the dike. At a location further north, the coastal dike was totally eroded, as shown in Fig. 2.6.7. Figure 2.6.8 shows the overall condition of the severely eroded location. The beach was eroded over a distance of several 100 m, leaving a high scarp. As mentioned above, the area of the sandy beach had decreased between 1946 and 1976, although the shoreline itself was stable. This is partly due to the seaward extension of the coastal forest, which transformed the backshore into a coastal forest in the southern part of the coast, and partly due to the seaward advance of the city of Takahagi in the northern part of the

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

223

Fig. 2.6.5. Coastal dike protecting forest at southern part of Ariake–Ishihama coast (7 June 1998).

Fig. 2.6.6. Scarp of coastal dike in front of coastal forest (7 June 1998).

May 26, 2010

224

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.6.7. Scarp formed on seaward slope of coastal dike (7 June 1998).

Fig. 2.6.8. Overall condition of the severely eroded location.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

225

Fig. 2.6.9. Beach-profile changes along survey line A–A as shown in Fig. 2.6.2.

coast. Then in the period between 1976 and 1996, tombolos were created behind the detached breakwaters built north of the Hananuki River’s mouth and near Uno Point, and the foreshore was widened at the expense of the natural sandy beach without detached breakwaters. (4) Beach-profile changes and depth of closure Figure 2.6.9 shows the beach-profile changes between August 1985 and December 1997 measured along the survey line A–A located at around 1.5 km south of the Sekine River’s mouth, as shown in Fig. 2.6.2. Along this survey line close to the detached breakwaters, the foreshore slope was as steep as 1/6 with bar formation before the construction of the detached breakwater. After the construction of the detached breakwater, sand accumulated shoreward of the breakwater. A dominant seabed change is observed only at depths less than −9 m and the bottom slope becomes as gentle as 1/100 beyond this depth, implying that the depth of closure of this coast is around 9 m. Uda (1997∗ ) and Uda et al. (1997a∗ ) measured depth changes in the median diameter of bed material, and showed that coarse grain size near the shoreline decreases with depth while grain size converges to a constant

May 26, 2010

226

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

value of around 0.15 mm at a certain depth (critical depth for grain-size change). This critical depth agreed well with the depth of closure of the coast. In this study, the critical depth for grain-size change was determined by the same method as described previously. Survey lines a–a and b–b were set at around 0.5–1.5 km south of the Sekine River’s mouth, as shown in Fig. 2.6.2, and sediment sampling of bed materials was carried out at ten points along each survey line using a Smith-Mackintire dredger. The change in d50 with depth shown in Fig. 2.6.10 indicates that d50 ranges between 0.37 mm and 0.44 mm, and relatively coarse materials can be found near the shoreline; d50 decreases with the water depth to 0.2 mm and 0.15 mm at depths of −8 m and −10 m, respectively. Then, d50 becomes constant regardless of the water depth. This means that the critical depth for grain-size change is around −9 m at

Fig. 2.6.10. Relationship between median diameter and elevation. Sampling was carried out along lines a–a and b–b in Fig. 2.6.2.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

227

this coast, which agrees well with the depth of closure determined from beach-profile changes. (5) Discussion Beach changes occurring on this coast are summarized in Fig. 2.6.11(a). There is a training jetty at the Hananuki River’s mouth, but the point depth is only 3 m, as shown in Fig. 2.6.2. Since this depth is much less than the

Fig. 2.6.11. Summary and schematic explanation of beach erosion process.

May 26, 2010

228

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

depth of closure of this coast of around 9 m, a large part of longshore sand transport can turn around the tip of the training jetty. The length of the jetty located at the Koishikawa River’s mouth is so short that longshore sand transport can freely pass through this location. For these reasons, when the sand accumulation effect was enhanced, due to the construction of detached breakwaters north of the Hananuki River’s mouth, sand was carried away from the coast south of the Hananuki River’s mouth by northward longshore sand transport. On the coast south of the Koishikawa River’s mouth, the coastal dike was destroyed by severe beach erosion. The failure of the coastal dike is schematically summarized in Fig. 2.6.11(b). As the shoreline retreated in front of the coastal dike, foreshore elevation was reduced, enhancing direct wave impact on the coastal dike and resulting in the erosion of the earthen dike. Figure 2.6.11(b) shows the schematic profile change at a cross section. Here, it should be noted that this cross-sectional change was not caused by cross-shore sand movement, but was triggered by northward longshore sand transport, resulting in a decrease in the total sand volume in this area. As a consequence, the beach-profile change shown in Fig. 2.6.11(b) resulted. Beach erosion was caused not by a local imbalance of sand volume on this pocket beach, but by the construction of detached breakwaters that exert a high wave-dissipation effect on part of the coastline, which caused a longshore imbalance of the sediment budget in a wave field with a seasonally changing wave direction. Large amounts of sand accumulated behind the detached breakwaters in contrast to shoreline recession in the southern part of the coastline. Further, sand accumulation does not take place if the sand volume behind the detached breakwaters saturates. A sandy beach will be stabilized only if a sufficient volume of sand is supplied from the south coast. If the alignment of the coastal dike is withdrawn to protect the retreating shoreline, which penetrated into the coastal forest located in the southern part of the coast, as shown in Fig. 2.6.11(c), the overall shoreline of this coast will be stabilized. However, the local manager of the coastal forest, which is an independent authority different from the authority for the coastal

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

229

protection, desired to protect the coastal forest using hard structures, without considering the overall stability of the coastline. In this case, it is clear that a reconstructed coastal dike of the same earthen structure as that before the erosion cannot withstand the wave force, because the total sand volume is decreased and the foreshore width is reduced. Consequently, the seaward slope of the coastal dike will be reinforced by a concrete wall, or concrete armor units will be installed along the foot of the coastal dike to protect the dike, as schematically shown in Fig. 2.6.11(d). The construction of detached breakwaters or artificial reefs in a closed system of littoral drift, such as a pocket beach, triggers beach erosion, and the potential for damage is increased on the coast adjacent to these structures. Furthermore, in the construction of structures to protect the shore of the pocket beach, lack of consideration from the geomorphological standpoint, in which the overall stability of the coastline is considered on a long-term basis, may in fact accelerate beach erosion and increase the artificiality of the coastline through the construction of hard structures. On the Ariake–Takahama coast, each activity regarding protection had been carried out to satisfy only one individual purpose, and various construction projects were carried out without discussing how the overall coastline of the pocket beach should be stabilized. This triggered new coastal damages, and recovery work in each case accelerated the artificiality of the coastline, which was altered with concrete armor units. This is a common situation along Japanese coastlines as described by Uda (1997∗ ). In addition, it was found that a change in land use in a zone next to the shoreline becomes an important factor in coastline changes and in the formation of an artificial coastline. Sandy beaches on a pocket beach should be considered as one entity; the impact of artificial landform change in one part is felt throughout the entire pocket beach. Research on coastal erosion has been focused mainly on understanding physical mechanisms. The value of these kinds of studies is still high, but the finding the solution to problems that have arisen or which are now arising on real coasts is deterred, not only by the lack of understanding of

May 26, 2010

230

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

the mechanisms of littoral drift, but also by the lack of consideration from regional planning and geomorphological standpoints, as well as the lack of comprehensive coordination between several related authorities. Finding a solution does not depend on a tactical methodology, but it strongly depends on a broad, strategic methodology. This example can be utilized for future planning of coastal stabilization on a pocket beach.

2.6.2.

Ghotsu Coast in Shimane Prefecture

(1) Shoreline changes The Ghotsu coast is located in the central part of Shimane Prefecture and is a sandy beach with a 4 km stretch of the coastline, extending west of the Ghono River. On this coast, the construction of the detached breakwaters began in 1978, as shown in Fig. 2.6.12, as a measure against the severe beach erosion of the 1970s, and seven detached breakwaters with a longshore length of 200 m and an opening width of 50 m were built by October in 1984. After the completion of these detached breakwaters, salients were formed behind the detached breakwaters, whereas severe beach erosion began south of the detached breakwaters (Uda et al., 1997b∗ ). The coastline extending between the Ghono River’s mouth and Mashima Island 4 km south of the mouth, as shown in Fig. 2.6.12, was selected as a study area. In the northeast part of the study area, seven detached breakwaters have been built.

Fig. 2.6.12. Location of Ghotsu coast and alignment of survey lines.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

231

Fig. 2.6.13. Changes in shoreline of Ghotsu coast with reference to shoreline in 1976.

Figure 2.6.13 shows the shoreline changes between 1980 and 1985 with reference to those in 1976 immediately before the construction of detached breakwaters. By 1980, three detached breakwaters had been completed from the north end, and thereafter salients started to form behind these detached breakwaters. Simultaneously, shoreline recession began in a zone between Mashima Island and the Shinkawa River’s mouth. With the construction of seven detached breakwaters by 1985, salients have been formed behind each of the detached breakwaters. On the other hand, maximum shoreline recession reached 70 m at a location 1 km east of Mashima Island. The eroded and accreted shoreline areas reached 4.6 × 104 m2 and 5.2 × 104 m2 , respectively, and they approximately agreed with each other. These detached breakwaters have a high efficiency for wave dissipation, because the ratio of the opening width to the length of the breakwater is as small as 0.25. This also means that these detached breakwaters have a strong sand accumulation effect. Due to the construction of these highly efficient detached breakwaters, eastward longshore sand transport was induced, and transported sand was trapped behind the detached breakwaters, leaving a clear contrast in the shoreline configuration of this coast. (2) Field observations Figure 2.6.14 shows the shoreline east of Mashima Island, taken from the top of the hill on this island on 10 July 1986, looking eastward. There is a

May 26, 2010

232

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.6.14. Shoreline of Ghotsu coast on 10 July 1986, looking northeast from hilltop on Mashima Island.

small rock just east of the island, and a tombolo has been formed behind the rock. This photograph was taken right after the time when the large shoreline changes shown in Fig. 2.6.13 were observed. A concave shoreline can be seen at the far side of the photograph, and this part corresponds well to the shoreline recession area south of the detached breakwaters. The sandy beach around the rock has not yet been eroded, and there is a wide foreshore landward of the rock. Figures 2.6.15 and 2.6.16 show photographs taken on 8 April and 23 December of 1987 from the same location. By comparing Fig. 2.6.15 with Fig. 2.6.14, it is clear that the sandy beach east of the rock was severely eroded after only 17 months. In addition, not only the wide foreshore behind the rock disappeared, but also a seawall was destroyed, while a high scarp was formed. In the center of Fig. 2.6.15, a collapsed seawall can be seen. Before the erosion, there was a wide foreshore in front of the sand dune, but the beach

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

233

Fig. 2.6.15. Shoreline of Ghotsu coast on 8 April 1987, looking northeast from hilltop on Mashima Island.

Fig. 2.6.16. Shoreline of Ghotsu coast on 23 December 1987, looking northeast from hilltop on Mashima Island.

May 26, 2010

234

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

was severely eroded including the sand dune area, since a large amount of sand was carried away by northeastward longshore sand transport induced by the construction of the detached breakwaters. As mentioned, on the Ghotsu coast, with a closed system of littoral drift being separated by headlands on both sides, beach erosion in the southwest part was triggered by the construction of the detached breakwater along the northeastern half of the coast. Thus, beach erosion on this coast was totally due to artificial causes.

2.6.3.

Methods of Addressing Issues

In Japan, the coastal zone has been highly used, but land utilization in the fishing villages is uneven and often concentrated on a location adjacent to headlands. In this case, even if beach erosion occurred temporarily due to the seasonal variation in the wave direction, the administration responsible for the protection of the local coast responds excessively to the demands made by local people, requesting urgent protection against beach erosion, instead of telling them that this is a temporal phenomenon, and therefore it is better to leave the system intact. Taking measures against beach erosion itself is acceptable in terms of the Coastal Act, but a problem is encountered when the measures are taken shortsightedly. This example clearly shows that when detached breakwaters or artificial reefs are built in part of the coastline of a pocket beach, thereby enhancing wave calmness behind these structures, sand accumulates, resulting in shoreline recession in the adjacent area and an increase in the potential for disaster. This is a problem in exact trade-off. It is clear that there is no way to locally accumulate sand using wave-dissipating structures without associated shoreline recession in the surrounding coastal area. For shore protection, the entire pocket beach must always be taken into account, and local optimization of the coast will cause a domino effect, rapidly accelerating the artificialness of the entire coastline. If the protection of a fishing village against beach erosion is required, the influence of the wave-dissipating structures to be built on the surrounding coastline must be predicted first, and the wave-dissipating effect of the

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

235

structures must be adjusted beforehand using some models for predicting beach changes, as mentioned in Chap. 3. In such cases, it is better to plan the structures with the expectation of a reasonable effect, rather than planning very effective facilities and causing a strongly adverse effect on the adjacent beaches.

REFERENCE Uda, T., T. Takano, M. Serizawa, T. San-nami and K. Furuike (2003). Beach changes triggered by seaward development of towns, expansion of coastal forest and construction of detached breakwaters at a pocket beach, Coastal Sediments ’03 (World Scientific Publishing Company), pp. 1–15.

REFERENCES (in Japanese) Mogi, A. and Y. Iwabuchi (1961). Submarine topography and sediments on the continental shelves along the coasts of Joban and Kashimanada, Geographical Rev. 34(3), 159–177. Shidai, A., T. Kawamura, M. Tanaka, Y. Ohkuma and T. Uda (1997). Field observation of large-scale beach changes in the region between Ohtsu fishing port and Takado coast in northern Ibaraki Prefecture, Annual J. Coastal Eng. JSCE 44, 656–660. Uda, T. (1997). Beach Erosion in Japan (Sankaido Press, Tokyo), p. 442. Uda, T., S. Kosuge, M. Serizawa, T. San-nami and K. Furuike (1997a). A method predicting closure depth from depth distribution of d50 , Annual J. Coastal Eng. JSCE 44, 521–525. Uda, T., K. Fukuda, G. Ueda, H. Kihara, Y. Fujikawa and K. Togawa (1997b). Beach erosion triggered by construction of detached breakwaters in a closed system of littoral transport and measures, Annual J. Coastal Eng. JSCE 44, 551–555. Uda, T., M. Sumiya, K. Shimoyamada, S. Namasuya, Y. Takano, Y. Kanda, Y. Oki, M. Serizawa, T. San-nami, K. Furuike and T. Igarashi (2000). Beach

May 26, 2010

236

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

changes triggered by seaward development of towns, expansion of coastal forest and construction of detached breakwaters on Ariake and Ishihama coasts, Trans. Japan. Geomorph. Union 21, 17–30.

2.7.

DISAPPEARANCE OF NATURAL SAND DUNES DUE TO EXCESS PLANTING OF COASTAL FOREST

After World War II, the land, including coastal zones, in Japan was highly devastated, and wind-blown sand was severe, developing sand dunes along the coastline, causing damage by burying houses and farm lands. As a measure against wind-blown sand, planting pine trees started after World War II, resulting in the coastal forest. This activity has been carried out on the basis of the Forest Law, resulting in a substantial decrease in windblown sand or damage from salinity, but the excess seaward advance of the coastal forest up to a location very close to the shoreline caused a loss of a buffer zone against shoreline variations. In this section, examples of the Nakamura-hama coast in Niigata Prefecture, the Node and Ichinomiya coasts in Chiba Prefecture, and the Heisa-ura coast in Boso Peninsula are described.

2.7.1.

Nakamura-hama Coast in Niigata Prefecture

(1) Shoreline changes In the northern part of Niigata Prefecture, a coastline 18 km long extends between the mouths of the Arakawa and Kaji Rivers, as shown in Fig. 2.7.1. Between these two, the Tainai and Ochibori Rivers also flow into the Sea of Japan. The Nakamura-hama coast is located 2.5 km southsouthwest of the Tainai River’s mouth. Past studies revealed that southward longshore sand transport prevailed in the vicinity of this coast (Uda, 1997∗ ). Izumiya and Sunako (1994∗ ) analyzed the shoreline changes between 1975 and 1990 with reference to the shoreline position in 1965, and they noted that the shoreline retreated with time in the adjacent area south of the Tainai River’s mouth, and the eroded zone expanded southward.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

237

Fig. 2.7.1. Location of Nakamura-hama coast in Niigata Prefecture.

On this coast, a longshore stretch of 2.6 km was selected, as shown in Fig. 2.7.1, and the coastal changes were investigated by comparing past aerial photographs (Uda et al., 2003∗ ). Figures 2.7.2(a) and 2.7.2(b) show the coastal conditions in the same area in 1971 and 1998, respectively. It is clear that the coast experienced great changes during the 27 years since 1971. In 1971, a white sandy beach 60 m wide extended continuously alongshore. The coastal vegetation zone irregularly distributed seaward the black coastal forest, and a foreshore extended seaward of this zone, as is typically seen north of the mouth of a small river located at x = 1.1 km. In 1998, almost the entire shore, 56 m wide on average, disappeared as compared with the conditions in 1971. This fact does not necessarily mean that the foreshore disappeared as a result of severe erosion. The foreshore can be narrowed as a consequence of the advance of the coastal forest zone behind the shore, even if the shoreline does not recede. Selecting an area between x = 1.1 and 2.6 km as a typical example with uniform land utilization in the coastal zone, survey lines were set at 100 m intervals, and the distances from the reference line to the seaward

May 26, 2010

238

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.2. Aerial photographs of Nakamura-hama coast taken in 1971 and 1998.

marginal line of the coastal forest and coastal vegetation zone as well as the offshore distance to the shoreline were recorded. The results are shown in Table 2.7.1. These distances were averaged over 16 points alongshore, and Fig. 2.7.3 shows the results. The average width of the coastal forest was 129 m in 1971, but it widened to 189 m in 1998, whereas the width of the sandy beach including the natural sand dunes covered with coastal vegetation was greatly narrowed from 90 m in 1971 to 22 m in 1998. Finally, the natural sand dunes completely disappeared. The seaward advance of the coastal forest was 60 m, resulting in the disappearance of the natural sand dunes by the same distance. The decrease in the total distance to the shoreline reached 68 m, and the contribution of the shoreline recession between 1971 and 1998 was only 8 m; 88% of the decrease in width of the sandy beach was due to the advance of the coastal forest, and the rest (12%) was due to the shoreline recession associated with beach erosion.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

239

(2) Field observations Field observations were carried out on 29 April 2002 in the coastal forest, as shown in Fig. 2.7.2. Figure 2.7.4 shows an explanation of the coastal forest works exhibited in the coastal forest zone, indicating that: “The Japanese black pine tree forest of a coast is a forest produced on coastal land with extremely severe natural conditions such as strong wind, wind-blown sand Table 2.7.1. Seaward distances from reference line to marginal line of coastal forest and shoreline.

1971

No.

Coastal forest (m)

Coastal vegetation (m)

Sandy beach (m)

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Mean

126 102 110 113 112 116 123 123 124 140 131 132 135 140 163 166 129

12 36 30 31 38 46 35 25 25 36 32 38 48 44 39 32 34

62 62 60 56 50 48 52 62 61 44 67 60 57 56 48 52 56 (Continued )

May 26, 2010

13:35

9in x 6in

240

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures Table 2.7.1. (Continued )

1998

No.

Coastal forest (m)

Coastal vegetation (m)

Sandy beach (m)

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Mean

177 168 175 180 183 185 187 185 188 191 191 191 198 202 203 212 189

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

23 32 25 20 17 25 23 15 22 9 29 29 22 18 17 18 22

Fig. 2.7.3. Schematic diagram of advance of coastal forest.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

241

Fig. 2.7.4. Explanation of coastal forest works in coastal forest zone (29 April 2002).

and salinity. The creation of the pine tree forest aims at protecting lives from strong winds and wind-blown sand.” Carefully studying this photograph, one notes that the Japanese red pine trees were planted in the most landward area, whereas the black pine trees extend seaward of the red pine trees. The height of the black pine trees is gradually reduced as the shoreline is approached, because of the strong wind action in the vicinity of the shoreline. This is very reasonable. In Fig. 2.7.4, an earth dike is built seaward of the black pine trees that have a gradually decreasing height to protect the trees against wind and wave actions. A gently sloping revetment is also built on the foot of the earth dike to protect the dike against erosion, since the earth dike is very weak in the face of wave action, and there is no foreshore in front of the gently sloping revetment, thereby exposing the foot to waves. In other words, the administrator responsible for the protection of the coastal forest may consider the existence of the foreshore as an obstacle, because it causes wind-blown sand, instead of viewing it as a resource to be preserved or as an important foot protector for the gently sloping revetment.

May 26, 2010

242

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

In fact, even if the gently sloping revetment is built in such a way as shown in Fig. 2.7.4, the revetment may be easily destroyed due to a decrease in longshore sand supply. In that case, the destruction may reach not only the revetment but also the earth dike itself or the coastal forest behind the dike. Accordingly, it is clear that coastal disasters will recur. In addition, taking the action due to longshore sand transport into account, the collapse of the revetment is by no means confined to one location, and the same damage will be repeated downcoast. Nevertheless, if such conservation methods continue to be used for the coastal forest, an entire stretch of the natural coastline will ultimately be covered with concrete structures. Here, in particular, the coastal conditions in the region north of X = 1.2 km were investigated by field observations in detail out of the entire area shown in Fig. 2.7.2. First, Fig. 2.7.5 shows the coastal forest. Seaward of the coastal forest, a paved road 3 m wide as well as the earth dike extends straight along the coastal forest as shown in Fig. 2.7.5. This exactly satisfies the conditions schematically shown in Fig. 2.7.4. However, in the vicinity

Fig. 2.7.5. Condition of coastal forest of Nakamura-hama coast.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

243

of the location shown in Fig. 2.7.5, a sign indicating that a seawall was under construction was found, and the earth dike was cut to make a gap along its length. This gap in the earth dike was temporarily made for the construction of the gently sloping revetment near the shoreline by removing part of the dike. The sign indicated that the extension of this revetment under construction was 219.5 m, adding, “Create beautiful coast and waterfront space.” On top of the earth dike stood a wooden fence for preventing windblown sand, continuously extended southward. On the right-hand side of the earth dike, as shown in Fig. 2.7.6, the Sea of Japan can be seen, but at the shoreline of this site, an unnatural scene was observed; the installation of straight sheet piles. Figure 2.7.7 was taken from a point (location A in Fig. 2.7.2(b)) closer to the shoreline, compared with the site where Fig. 2.7.6 was taken. On the shore seaward of the earth dike, sheet piles were installed in a straight line. In Fig. 2.7.7, it seems that a wide sandy beach extended on the righthand side of the construction site, and the construction of the seawall was

Fig. 2.7.6. Sign showing construction work of seawall.

May 26, 2010

244

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.7. Construction of sheet piles on foreshore.

initiated at part of the sandy beach, but in fact the gently sloping revetment had been built along the coastline north of this site, as shown in Fig. 2.7.8 (location B in Fig. 2.7.2(b)). Figure 2.7.9 shows the gently sloping revetment taken from the same position, looking north. No foreshore was left at the toe of the gently sloping revetment, and its front was exposed to the waves. Exactly the same conditions as schematically shown in Fig. 2.7.4 were achieved, accomplishing the goal. However, this scenery is absolutely at odds with the concept on the sign, “Create beautiful coast and waterfront space.” It is dangerous to approach the shoreline over the slippery surface of the revetment because of attached sea organisms, and the extremely artificial concrete slope is not at all what we would expect from a waterfront space suitable for human use. South of the temporally built sheet piles shown in Fig. 2.7.7, severe beach erosion took place, because of the decrease in sand supply by southward longshore sand transport. Figure 2.7.10 shows the downcoast erosion of the sheet piles. Scarp formation began from the southern end

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

245

Fig. 2.7.8. Gently sloping revetment built on Nakamura-hama coast, looking south, taken on 29 April 2002.

Fig. 2.7.9. Gently sloping revetment built on Nakamura-hama coast, looking north, taken on 29 April 2002.

May 26, 2010

246

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.10. Scarp erosion downcoast of sheet piles under construction.

of the sheet piles, and its height gradually increased southward. The scarp height at the location of the measuring stick reached 5 m. On the backshore, part of the fence, placed to prevent wind-blown sand, fell down from the top of the earth dike because of the scarp erosion. Figure 2.7.11 shows the scarp erosion looking south from the same point from which Fig. 2.7.10 was taken. An extremely high scarp 10 m in height continuously extended southward, and the fence on top of the earth dike had fallen down. It should be noted here that the dike was not composed of sand, but of adhesive materials, and it was built by making a high embankment on a rather flat backshore. The formation of an extremely high scarp at this location is at least not due to large shoreline recession, but is instead due to the high embankment in the vicinity of the shoreline. Figure 2.7.12 shows the collapsing fences, looking north, at a location further south from the site where Fig. 2.7.11 was taken. The vertical exposed face of the earth dike was observed. It is clear that the earth dike was composed of silt and mud, and that it was embanked on the flat backshore.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

Fig. 2.7.11. Scarp erosion, looking south.

Fig. 2.7.12. Collapsed fence, looking north.

b807-ch02

247

May 26, 2010

248

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.13. Collapsed fence, looking north.

Further south of this site, the scarp height gradually decreased to 2 m at a site shown in Fig. 2.7.13. Here, the slope covered with grass was left intact on the upper part of the earth dike, since the scarp height was reduced. In Fig. 2.7.14, taken at a site further south, the scarp height was lowered to 0.8 m, and driftwood buried underneath the backshore was exposed due to beach erosion. Scarp formation ended at this site, and the natural sandy beaches continued. As mentioned, an earth dike was built along the coastline as an embankment on the Nakamura-hama coast. Southward longshore sand transport prevails at this coast, but longshore sand transport is exhausting now on the whole due to the decrease in sand supply from the northern part of the coast. Dominant scarp erosion can be seen at many locations, and a gently sloping revetment has been built on these eroded coasts as a measure against beach erosion. These structures have suffered damage in terms of the suction of the foundation gravel associated with local scouring. On the gradually eroding coast, the earth dike was built seaward of the coastal

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

249

Fig. 2.7.14. Scarp 0.8 m high and exposure of driftwood buried underneath backshore.

forest to guard the forest, resulting in severe erosion because of the weak resistance of an earth dike against wave action. In response to this erosion, further construction of gently sloping revetments has been carried out for the ostensible reason of protecting the eroded coast, but simultaneously, downcoast erosion started south of the revetment. The area with scarp formation is extending south at the same rate as the southward extension of the gently sloping revetment. The same situation was observed at the Shinkawa fishing port in Niigata Prefecture, as described in Sec. 2.2.5. The endless beach erosion can be summarized as schematically shown in Fig. 2.7.15. The extension of the erosion area relates entirely to the construction of the gently sloping revetment.

2.7.2.

Node Coast in Chiba Prefecture

(1) Characteristics of Node coast The Node coast is located northeast of the Kujukuri coastal plain shown in Fig. 2.7.16. On the eastern side of this coast, a sea cliff named Byobugaura

May 26, 2010

250

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.15. Schematic diagram showing extension of eroded area, that relates to extension of gently sloping revetment.

Fig. 2.7.16. Location of Node coast on Kujukuri coastal plain and Byobugaura cliffy coast.

extends over a length of approximately 10 km and has a height of 30–50 m. This sea cliff has retreated approximately 0.75 m/yr, and sand from this cliff fed the Kujukuri coastal plain, which is 60 km long and 10 km wide. Figure 2.7.17 shows an aerial photograph of the east end of the Byobugaura sea cliff and the adjacent coast. The long breakwater of the Iioka fishing port extends seaward now, and a group of detached breakwaters has been installed downcoast of the Iioka fishing port, accumulating sand on the lee side of the breakwaters. On the Node coast, southwestward longshore sand transport dominates, carrying sand from the sea cliff to the central

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

251

Fig. 2.7.17. Aerial photograph of Iioka fishing port and Byobugaura cliffy coast taken in January 2000.

Kujukuri coast through the Iioka coast located on the downdrift side of the fishing port, as shown in Fig. 2.7.17. In order to investigate the overall changes of the Kujukuri coast, including the Node coast, the shoreline changes were investigated by comparing aerial photographs. Figure 2.7.18 shows the shoreline changes before 1984, when a large shoreline recession was observed. On the Node coast located southwest of the mouth of the Shinkawa River, the shoreline retreated around 40 m since 1970, and thereafter the erosion zone extended southwestward. In contrast, on the Iioka coast located on the updrift side of this coast, the shoreline advanced largely due to the sand deposition effect of the detached breakwaters built here to prevent downcoast erosion of the Iioka fishing port. It can also be seen from this figure that the shoreline recession of the Node coast was severe in the past, but in recent years the shoreline seems to have been approaching a stable condition, showing the absence of environmental and shore-protection issues. In reality, however, severe changes in the coastal environment still

May 26, 2010

252

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.18. Shoreline changes along entire Kujukuri coast before 1984.

continue, which cannot be detected only from shoreline changes (Uda et al., 2002∗ , 2004). On the Node coast, sand from the Byobugaura sea cliff is transported to the central part of the Kujukuri coast. Although longshore sand transport is generally seen from northeast to southwest, wave-dissipating breakwaters have been built for several decades along the foot of the sea cliff at Byobugaura to prevent the sea cliff from eroding. As a result, around 90% of the entire stretch of the sea cliff has been protected against wave erosion in recent years. The Iioka fishing port is a famous fishing base on the northern Kujukuri coast featuring fish such as sardines. The breakwaters of the fishing port shown in Fig. 2.7.17 were built by 1964 on the basis of the wishes of many fishermen and have been there for 38 years. When the Iioka fishing port had no breakwaters, which was the case for around 6,000 years since the Jomon transgression of the sea, sand was transported through the current location of the Iioka fishing port toward the central part of the Kujukuri coast, forming the Kujukuri coastal plain. However, in order to enhance the safety and efficiency of fishing, long breakwaters were extended, as shown

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

253

in Fig. 2.7.17, and as a result the movement of sand from the sea cliffs toward the Kujukuri coast became difficult. Sand supply from the sea cliff was originally reduced by the protective works along the sea cliffs, and in addition, it was blocked by the long breakwaters, resulting in severe erosion on the northern Kujukuri coast. (2) Comparison of aerial photographs Long-term shoreline changes along the Node coast between 1947 and 2000 were analyzed by comparing aerial photographs. Figure 2.7.19(a) shows the condition of the coast in 1947. In the central part of the photograph flows the Shinkawa River, which was excavated in the past to enhance drainage from the lowland. The riverbed meandered near the river’s mouth and opened to the south. This photograph was taken on 21 November 1947, and, in winter, easterly waves dominate on this coast. Therefore, waves are incident from the counterclockwise direction relative to the normal to the shoreline, as shown in Fig. 2.7.19(a), inducing a southwestward extension of the bar at the river’s mouth that resulted in the opening of the mouth to the southwest. In 1947, wide natural sand dunes around 300 m in width could be seen around the river’s mouth, and two rows of lagoons existed behind the sand dunes. In 1963, on the northeastern side of the river’s mouth, the coastal forest preventing wind-blown sand was widened to the natural sand dunes, as shown in Fig. 2.7.19(b), and as a result, the sandy beach was narrowed to around 92 m. A highway was also built parallel to the coastline behind the sand dunes. However, at this stage, there was a sufficiently wide sandy beach in front of the coastal forest. On the southwestern side of the river’s mouth, the marginal line of the coastal forest was located 31 m seaward relative to that on the northeastern side of the river’s mouth, implying that the vulnerability of the coast against shoreline recession associated with beach erosion was large at the southwestern side of the river’s mouth. In addition, the narrow lagoon area that extended alongshore behind the sand dunes was converted to paddy fields by land reclamation. In 1970, the sandy beach around the river’s mouth was considerably wider, as shown in Fig. 2.7.19(c), and the natural sand dunes were still

May 26, 2010

254

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.19. Aerial photographs of Node coast.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

Fig. 2.7.19. (Continued )

b807-ch02

255

May 26, 2010

256

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.19. (Continued )

intact, although almost all parts of the sand dunes were covered by the coastal forest. Also in this period, a straight drainage channel was built at the lagoon area behind the sand dunes, and further land use was in progress. The construction of a straight highway was completed. In 1975, on the northeastern side of the river’s mouth, a sandy beach 92 m wide was intact in front of the coastal forest, as shown in Fig. 2.7.19(d), whereas on the southwestern side of the river’s mouth, the narrowing of the sandy beach became apparent, because the marginal line of the coastal forest was located 31 m seaward on the southwestern side relative to that on the northeastern side. Construction of the earth dike to protect the coastal forest against wind-blown sand was also begun on the adjacent area southwest of the river’s mouth. At this stage, two rows of marks of the lagoon area located behind the sand dunes can be traced back as a smooth curve separating the coastal forest, housing area, and paddy field. In 1979, the coastal forest grew as shown in Fig. 2.7.19(e). On the southwestern side of the river’s mouth, a straight seawall built to protect the coastal forest was exposed to waves, whereas the sandy beach, around 62 m wide on average, was intact on the northeastern side of the river’s mouth. In 1990, as shown in Fig. 2.7.19(f), the 62 m-wide sandy beach was intact on the northeastern side of the river’s mouth, although a gently sloping

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

257

revetment was built in the northern part as shown by a white line. Inside the river’s mouth, a sand bar extended from the left bank. These points clearly show that a considerable amount of sand, which could freely move, had been present at least until 1990 at the mouth of the Shinkawa River. Furthermore, waves incident from the counterclockwise direction relative to the normal to the shoreline on the northeastern side of the river’s mouth clearly show the dominance of southwestward longshore sand transport. On the southwestern side of the river’s mouth, since the marginal line of the coastal forest was originally located 31 m seaward relative to that on the northeastern side, and since the seawall was built along the marginal line of this coastal forest, the seawall line was on the extended line of the shoreline on the northeastern side of the river’s mouth, showing the disappearance of the sandy beach at the coast adjacent to the river’s mouth. However, the sandy beach extended again from the vicinity of the playground located around 920 m southwest of the river’s mouth. Until 2000, a seawall was built along the overall shoreline as shown in Fig. 2.7.19(g), and the natural sandy beach disappeared. Artificial headlands were built 1,140 m northeast and 950 m southwest of the river’s mouth as a measure against beach erosion, with another artificial headland under construction at a location 280 m northeast of the mouth. Although a triangular sandy beach was formed at the corner of the artificial headland, a sufficiently wide sandy beach was not formed because of the decrease in the total amount of sand in this area. (3) Causes of narrowing of foreshore On the Node coast in 1947 a wide sandy beach (including the sand dune area) extended, but it was changed to an artificial coast protected by a seawall and concrete armor units installed along the shoreline in the past 50 years. In general, this situation is believed to be caused only by beach erosion. However, the narrowing of the foreshore is strongly related to other factors such as changes in land utilization in Japan. On the Node coast, it depends primarily on the excessive expansion of the coastal forest. Figure 2.7.20 shows the change in the cross-shore distance over time from the reference point to the shoreline, to the seaward edge of the coastal

May 26, 2010

258

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.20. Temporal changes in cross-shore distance from reference point to edge of coastal forest and shoreline position at Node coast.

forest, and to the seawall along the survey line A–A set 300 m southwest (right-hand side) of the Shinkawa River’s mouth as shown in Fig. 2.7.19(g). At this location, the foreshore was around 300 m wide in 1947, but twothirds of the foreshore had disappeared by 1963 due to the expansion of coastal forest. Simultaneously, shoreline recession had started by this time, retreating 26 m by 1970 and continuing thereafter. Shoreline recession from 1947 to 2000 reached 45 m. However, shoreline recession contributes only 15% of the narrowing of the foreshore. It should be noted that an earth dike for the protection of the coastal forest against wind-blown sand and salinity was built in front of the coastal forest, despite the widening of the coastal forest up to 1963 in contrast to shoreline recession. Figure 2.7.21 shows this earth dike built in front of the coastal forest. On the other hand, the possibility of erosion of the earth dike increased because of further shoreline recession, and the seawall was built in front of the earth dike. However, the shoreline retreated further because of the decrease in longshore sand supply due to a large-scale imbalance in sediment supply to this area, and the sandy beach totally disappeared in front of the seawall. As a result, the intensity of wave overtopping increased, and

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

259

Fig. 2.7.21. Coastal forest and earth dike on Node coast (1 September 2001).

the filling materials under the seawall were washed away by local scouring. As a measure against wave overtopping, concrete armor units were installed in front of the seawall, as shown in Fig. 2.7.22. It is clearly understood that an artificial coastline has been created due to the excessive expansion of a coastal forest as well as the construction of protection facilities for the coastal forest against erosion due to the exhaustion of sand supply from upcoast. Figure 2.7.23 summarizes these processes. (4) Discussion Beach erosion of the Node coast was not caused by a local phenomenon such as scouring around the seawall, but it was caused by a significant decrease in sand volume due to exhaustion of longshore sand supply caused by a structural problem. Minor changes, such as the improvement of the seawall shape, were not useful for shore protection. Despite beach erosion and shoreline recession being caused by a structural problem, fundamental measures have not been taken, and instead, coastal forests and related

May 26, 2010

260

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.22. Seawall and concrete armor units built to prevent wave overtopping (1 September 2001).

facilities have advanced seaward, increasing the potential for disaster of these facilities against waves. In Japan, coastal forests and related facilities are controlled by the Forest Law, whereas the coastal zone is controlled by the Coastal Act. Under the Coastal Act, the shore-protection zone is basically determined to be the range between a location 50 m landward of the shoreline position at hightide level and 50 m seaward of the shoreline position at low-tide level in March. Shore-protection work may be carried out only within this narrow band. On the Node coast, a sandy beach in the shore-protection zone totally disappeared due to erosion, and seawall and wave-dissipating breakwaters for the protection of the coastal forest were directly exposed to waves. Although there are rational reasons for the work conducted in each area, measures for overall adjustment and for attaining harmonic solutions have not been carried out; thus, the change from a natural coast to an artificial one was inevitable along the Node coast.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

261

Fig. 2.7.23. Change from natural sand dune to artificial coast.

Having a coast covered with concrete armor units, as shown in Fig. 2.7.22, is common not only on the Node coast, but also on many coasts in Japan. Many people desire the recovery of natural sandy beaches, but important aspects must be considered. To recover a wide sandy beach, a large amount of sand is required as well as a large budget. Who shoulders the cost? Where can such a large amount of sand be obtained? In Japan, there are few coasts left which can supply sand to other coasts because of starvation of sand supply. Administrative measures cannot be taken without clear

May 26, 2010

262

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

answers to these questions, and thus this issue is not a problem for scientific research. An overall comprehensive plan for the protection and preservation of the coastal environment must be considered in the wide coastal zone including the coastal forest in order to improve heavily armored coasts under conditions where the coastline coincides with the front marginal line of the coastal forest.

2.7.3.

Southern Kujukuri Coast in Chiba Prefecture

(1) Comparison of aerial photographs The Kujukuri coastal plain, which is 10 km wide and 60 km long was formed mainly by sand supplied by longshore sand transport from the coastal cliffs of Byobugaura and Taito Point located in the northern and southern ends of the coastline, respectively. In the southern part of the coastal plain, the Nabaki and Ichinomiya Rivers with a riverbed slope of 1/3,000 at their mouths flow into the Pacific Ocean, as shown in Fig. 2.7.24. In the area surrounding these river mouths, natural sand dunes have disappeared due to the excess seaward advance of the coastal forest. Such conditions in these areas have been investigated by comparing aerial photographs taken during 1994 and 2000 (Ichikawa et al., 2001∗ ).

Fig. 2.7.24. Location of Nabaki and Ichinomiya Rivers in Kujukuri coast plain.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

263

Fig. 2.7.25. Comparison of aerial photographs at Nabaki River’s mouth between 1947 and 1999.

May 26, 2010

264

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

(a) Mouth of Nabaki River Figure 2.7.25(a) shows an aerial photograph of the Nabaki River’s mouth taken in February 1947. The photograph was taken right after World War II, and there was a sandy beach 400 m wide at its maximum around the mouth, with a meandering river stream. There was also a lagoon, which was formed by past floods, at the back of the sand dune located at a distance 200 m landward from the shoreline. In July 1961, the river generally meandered northward and flowed out into the sea at a location 450 m north of the left bank of the river’s mouth, as shown in Fig. 2.7.26(b). Regarding the lagoons located north of the river’s mouth in 1947, the northern part was enclosed by the coastal forest which extended seaward at that time. In contrast, the southern part was connected to the meandering main stream. Comparing this photograph with Fig. 2.7.25(a), it is clear that inland development was in progress along with the expansion of the coastal forest to block wind-blown sand from the sand dunes. In April 1970, as shown in Fig. 2.7.25(c), the northern lagoon and the main stream were separated by a training jetty extended by July 1965 to the left bank, leaving a narrow channel. After the construction of this jetty, the river started to flow down along the jetty and then entered the sea while meandering clockwise near the shoreline. In the area south of the river’s mouth, pine trees planted in the coastal forest, as shown by the black color in Fig. 2.7.25(c), grew well, and the forest area expanded seaward from the road extending along the coastline, so that the sandy beach was narrowed to a width of 150 m. Furthermore, 11 small access ways at 70 m intervals crossing the coastal forest and reaching the shoreline can be seen in the coastal forest south of the river’s mouth. From this, it is clear that there was good access from the hinterland to the shoreline in those days. In February 1980, as shown in Fig. 2.7.25(d), a new right-bank jetty was built, in contrast to the conditions in 1970. Since this jetty blocked northward longshore sand transport, the shoreline extended straight toward the tip of the jetty from the south, although a semicircular bar at the river’s

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

265

mouth was formed from the right bank in 1970. After this, the upstream migration of the river-mouth bar took place. In the coastal zone, a sea dike surrounding the outer boundary of the coastal forest was built. Due to the construction of this sea dike, the shore was narrowed to as much as 1/3 its former width from 400 m in 1947 to 150 m in 1980 in the area south of the river’s mouth. Furthermore, a highway had been constructed by 1979, separating the coastal forest into two parts. In January 1988, as shown in Fig. 2.7.25(e), the right-bank jetty was extended by 70 m relative to its length in 1980 and, simultaneously, the shoreline south of the jetty advanced, reaching the tip of the extended jetty. In January 1999, as shown in Fig. 2.7.25(f), the bar at the river’s mouth was fixed approximately at the same location as the one in 1988 due to the stabilization effect of the river stream by the training jetty. To access the shoreline from the hinterland, underground access ways were built across the highway at 300 m intervals, but this required a significantly long detour, taking the increase in the intervals from 70 m before the construction of the highway to 300 m after its construction into account. (b) Mouth of Ichinomiya River Figure 2.7.26(a) shows an aerial photograph taken in July 1961. In this year, the river meandered near its mouth in the same manner as the Nabaki River in the absence of any structures such as a training jetty, while keeping the characteristics of the natural river’s mouth. The river meandered northward, and a long bar at the river’s mouth of 1 km length developed. In addition, there was a wide sand dune without a coastal forest, permitting the predominance of wind-blown sand, similar to the condition at the Nabaki River’s mouth. Figure 2.7.26(b) shows the condition at the river’s mouth in March 1970. By this year, a left-bank jetty had been built, which had a narrow opening to the northern lagoon left behind by the sand dune. The lagoon water connected to the sea through this opening. In the wide bar at the river’s mouth of the right bank, extending northward in 1961, pine trees were planted, and this area was altered to the coastal forest as a measure to block wind-blown sand.

May 26, 2010

266

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.26. Comparison of aerial photographs at Ichinomiya River’s mouth between 1961 and 1999.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

267

Figure 2.7.26(c) shows the river’s mouth in February 1980. Although only a left-bank jetty was built in 1970, a right-bank jetty 20 m longer than the left-bank jetty was built by 1974. As a result, the bar of the river’s mouth began to develop from the left bank. The coastal forest increased relative to its state in 1970, and a large part of the natural sand dune in 1961 became coastal forest. In January 1988, as shown in Fig. 2.7.26(d), the bar at the river’s mouth migrated upstream, since the continuity between the shoreline south of the river’s mouth and the bar was lost due to the extension of the left-bank jetty. On the Ichinomiya coast located south of the river’s mouth, the entire shoreline retreated due to a decrease in sand supply from upcoast. In 1995, as shown in Fig. 2.7.26(e), the shoreline of the Ichinomiya coast retreated further. Until January 1999, as shown in Fig. 2.7.26(f), the shoreline recession became more serious south of the river’s mouth, resulting in an increase in the protruding length of the right-bank jetty with reference to the shoreline. (2) Change in beach width and location of outer marginal boundary of coastal forest To investigate the change in land use with time in the vicinity of the Nabaki River’s mouth, the offshore distances from the reference point to the shoreline and the outer marginal boundary of the coastal forest in each year were measured, as shown in Fig. 2.7.27. The measurement was carried out along the survey line A–A south of the Nabaki River’s mouth, as shown in Fig. 2.7.25(f). Except for the shoreline position advancing about 30 m on average both before and after the construction of the right-bank jetty, the shoreline has had a stable form. In contrast, the beach was as wide as 400 m in 1947, but it was narrowed by the rapid advance of the outer marginal boundary of the coastal forest to 140 m (one-third of the beach width in 1947) in 1995. From this fact, it is clear that the cause of narrowing of the sandy beach at the Nabaki River’s mouth is not shoreline recession associated with beach erosion, but the seaward advance of the coastal forest into the natural sand dune area.

May 26, 2010

268

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.27. Change in shoreline position and location of outer marginal boundary of coastal forest measured along survey line A–A south of Nabaki River’s mouth.

Fig. 2.7.28. Change in shoreline position and location of outer marginal boundary of coastal forest measured along survey line B–B in vicinity of Ichinomiya River’s mouth.

Similarly, Fig. 2.7.28 shows the change in the shoreline position over time and the location of the outer marginal boundary of the coastal forest along the survey line B–B south of the Ichinomiya River’s mouth, as shown in Fig. 2.7.26(f). In this area, there existed a 200 m-wide sandy beach in

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

269

1961, which totally disappeared by 2000. The cause is the seaward extension of the coastal forest and shoreline recession. In reducing the beach width, the extension of the coastal forest accounted for 65%, and the rest was due to shoreline recession. In particular, shoreline recession had become serious by 1985, and the recession rate was as fast as 5 m/yr. Taking this case as well as the case of the Node coast mentioned in Sec. 2.7.2 into consideration, it is clear that the main cause of the disappearance of natural sand dunes on the Kujukuri coastal plain is the excess seaward advance of the coastal forest.

2.7.4.

Heisa-ura Coast in Chiba Prefecture

(1) History of Heisa-ura coast The Heisa-ura coast is a pocket beach 5.5 km long located between Mera and Suzaki at the tip of the Boso Peninsula south of Tokyo, as shown in Fig. 2.7.29. This beach faces the Pacific Ocean to the SSW. On this

Fig. 2.7.29. Location of Heisa-ura coast at tip of Boso Peninsula.

May 26, 2010

270

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

coast, a large-scale sand dune developed in the past, but as a result of the development work on the coastal forest to block wind-blown sand, the coastal forest extends very close to the shoreline now, as well as close to the seawall constructed to protect the coastal forest. One-fourth of the entire coastline was changed to artificial coast protected by a seawall, and the foreshore in front of the seawall completely disappeared by November 2002 (Hoshigami et al., 2003∗ ). Historically, the raising of land associated with earthquakes has occurred several times in this area. For example, the Farmland Division of the Chiba Prefectural Government (1957∗ ) describes such changes as follows: “The geology of this area is comparatively new and belongs to the Holocene alluvium. The basement is composed of slate and sandstone, and sand dunes develop over the basement. In November 1703, a large earthquake accompanied the land uplift and a large tsunami occurred, resulting in the sudden appearance of a large-scale sand dune. The ground elevation was raised again during the Ansei Great Earthquake in 1855, by which the sand dune was further expanded. During the Kanto Great Earthquake, a large tsunami occurred on September 1 in 1923, and the ground elevation was raised extensively over 2 m, and the coastal zone was altered into a large sand dune.” In the same historical records, another description says that “sand discharge via a stream using human strength has been continuously carried out to remove wind-blown sand deposited on the fields.” In the case of this large sand dune, efforts in blocking wind-blown sand have been carried out for the past several 100 years, but the sand dune area was bought by the Japanese Imperial Army in 1930, and the coastal forest was completely cut down to produce a training area for the army. Since October 1949, the creation of a coastal forest on an area of 75 ha has been carried out. (2) Shoreline changes along Heisa-ura coast Figure 2.7.30 shows aerial photographs taken in four typical years between 1947 and 2002. According to Fig. 2.7.30(a), a sand dune 800 m wide at its maximum developed landward from the coastline in 1947, and there was

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.7.30. Comparison of aerial photographs along Heisa-ura coast.

271

May 26, 2010

272

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

no road passing from Sunosaki to Mera along the shoreline. Almost the entire area was used as a dry field. In 1967, as shown in Fig. 2.7.30(b), the utilization of the coastal zone was similar to the present condition, and the coastal forest grew in the entire area. At that time, a sandy beach more than 100 m wide remained east of the central part of the coast, but the beach was narrowed between 20 and 40 m in the western part, because the seawall was built very close to the beach cusps formed on the shore face. In 1985, as shown in Fig. 2.7.30(c), pine trees were planted landward of the seawall in the eastern part of the coast, while in the western part a new seawall was built, reducing the beach width to 20 m. In 2002, as shown in Fig. 2.7.30(d), the pine trees planted in the hinterland of the seawall grew well, and the land between the seawall and the highway was densely covered with trees. The seawall was expanded several meters seaward, accelerating the disappearance of the foreshore. In particular, the foreshore width was reduced in the eastern part of the coast. Figure 2.7.31 shows the shoreline changes along the Heisa-ura coast determined from the aerial photographs. Figure 2.7.31(a) shows the superposition of the shoreline configurations in 1947 and 2002, and Fig. 2.7.31(b) shows the shoreline change in each year with respect to the shoreline position in 1947. In Fig. 2.7.31(a), it should be noted that no shoreline changes occurred between 1947 and 2002 in the vicinity of the locations x = 1.4 km and x = 6.5 km at both ends of the coastline, because the Heisa-ura coast has a closed system of littoral drift, being bounded by exposed rocks. Comparing the shoreline configurations in 1947 and 2002, shoreline recession occurred over almost the entire zone, implying that the foreshore sand was lost for some reason. To investigate the reason, the shoreline changes, as shown in Fig. 2.7.31(b), were examined in detail. (1) Before 1967, the shoreline receded in an area between the east end of the coastline and the location x = 3.6 km, but it had a stable form west of the location x = 3.6 km. (2) In the period between 1967 and 1985, the shoreline recession ceased east of the location x = 3.6 km, where the shoreline retreated

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

273

Fig. 2.7.31. Shoreline changes of Heisa-ura coast.

significantly before this period. Instead, large shoreline recession occurred west of the location x = 3.6 km, where the shoreline was stable until 1967. (3) In the period between 1985 and 2002, the shoreline west of the location x = 4 km was stabilized, but the eastern shoreline which approached a stable form up to 1985 started retreating again. For the shoreline changes as described, an area with shoreline advance compensating for the shoreline recession does not exist, and it seems that a large amount of sand disappeared. The decrease in beach areas from 1947 was 1.2 ×105 m2 , 7.0 ×104 m2 , and 9.0 ×104 m2 by 1967, 1985, and 2002, respectively. (3) Field observations On 17 September 2002, field observations of the expanded coastal forest were carried out in the vicinity of point P shown in Fig. 2.7.30(d). Figure 2.7.32 was taken from the top of the earth dike looking west to

May 26, 2010

274

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.32. Coastal forest extending inland of sea dike (17 September 2002).

show the coastal forest. The earth dike as well as the coastal forest extends straight alongshore. Figure 2.7.33 was taken at a point little seaward from the location shown in Fig. 2.7.32. An earth dike with a gradual slope was artificially built by banking. In the overview of the earth dike taken from a seaward point along the same transect, shown in Fig. 2.7.34, there is almost no foreshore in front of the revetment made of concrete mats, and foot protection using many gabions had been placed along the shoreline for a long distance. Two months later, during the field observation of the same area, many concrete blocks were placed on the gabions in front of the revetment, as shown in Fig. 2.7.35, resulting in the complete disappearance of the foreshore. (4) Cause of beach erosion at Heisa-ura coast The Heisa-ura coast is a pocket beach, and if the direction of incident waves varies seasonally, the shoreline may change in a seesaw mode, in which the shoreline recedes at one side and simultaneously advances on the other side, in response to variations in wave direction. On the Heisa-ura coast, since the shoreline monotonically receded over the entire zone without a

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.7.33. Earth dike with gradual slope artificially built by banking.

Fig. 2.7.34. Concrete revetment of earth dike.

275

May 26, 2010

276

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.35. Totally armored coastal revetment (23 November 2002).

shoreline advance corresponding to the shoreline recession, as shown in Fig. 2.7.31, it is clear that beach changes due to eastward and westward longshore sand transport associated with the seasonal variations in wave direction have not occurred. In addition, wind-blown sand prevails due to westerly winds in winter at this coast, but the creation of the coastal forest has induced a decrease in the amount of wind-blown sand and hence maintains the sand volume of the beach. There are also no large-scale rivers carrying much sand to this beach, implying that the influence of fluvial sediment supply from rivers can be neglected. Similarly, the effect of the construction of the artificial structures, such as port breakwaters, can be neglected, because the largescale coastal structures used to create a large wave-shelter zone behind a structure have not been used in this area. Taking these conditions into consideration, it is suggested that the shoreline of the Heisa-ura coast has reached a statically stable form due to wave action over the long term. The fact that beach erosion occurred in the entire zone of the Heisa-ura coast despite these conditions strongly implies that the shoreline recession was triggered not naturally but artificially.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

277

It should be noted that the shoreline retreated at first in the eastern part with respect to the centerline of the coast (x = 4 km); then, in 1985, it retreated in the western part, and in 2002 once again the shoreline recession in the eastern part became severe similar to the conditions in 1967. The time and location of the occurrence of the shoreline recession agree well with those of the construction of the sea dike, implying that beach sand was used for constructing the artificial sand dunes and sea dike, and this triggered beach erosion. As a consequence of the field observations, the artificial sand dunes and seawall armored with concrete blocks have been constructed close to the shoreline in the western and eastern parts of the coast, respectively. The cross-sectional area required for the construction of the artificial sand dunes and sea dikes per unit length of coast was calculated from the design, and multiplied by the entire length read from the aerial photographs. The sand volume necessary for the construction was 2.0 × 105 m3 (90 m3 /m × 2.2 × 103 m) by 1967 for the eastern artificial sand dune, 1.8 × 105 m3 (90 m3 /m × 2.0 × 103 m) by 1985 for the western artificial sand dune, and 9 × 104 m3 (40 m3 /m × 2.2 × 103 m) by 2002 to widen the sea dike in the eastern part of the coast. In contrast, the eroded volume calculated from the decreased foreshore area in each year was 1.2 × 105 m3 in 1967, 7.0 × 104 m3 in 1985, and 9.0 × 104 m3 in 2002, assuming that a sand layer about 1m thick on average was removed from the shore by one bulldozer. Thus, the results are internally consistent. As mentioned, on the Heisa-ura coast, the artificial sand dunes and sea dike have been constructed for a long time using beach material, resulting in the disappearance of a wide sandy beach in the eastern part of the coast. The revetment was exposed to waves in the central part of the coast. These situations were not caused by the threat of erosion to the coastal forest. In actuality, the coastal forest zone was excessively expanded very close to the past coastline, and the earth dikes and seawalls were built using beach sand to protect the coastal forest against wind-blown sand and salinity from the sea. As a result of this, the sandy beach is considered to be rapidly disappearing. It is concluded that on the Heisa-ura coast, the present condition was not produced by beach erosion as a natural phenomenon;

May 26, 2010

278

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

rather the excess seaward advance of the coastal forest and protection facilities led to the disappearance or reduction of the foreshore.

2.7.5.

Methods of Addressing Issues

(1) Excess advance of coastal forest and shore protection The characteristic commonly observed at the Nakamura-hama coast in Niigata Prefecture, and at the Node, Kujukuri, and Heisa-ura coasts in Chiba Prefecture is the planting of the coastal forest based on the Forest Law as a measure against wind-blown sand in the devastated coastal land after World War II. The plantation of the coastal forest was carried out excessively very close to the shoreline, the earth dike was built to guard against wind-blown sand and salinity, and finally the seawall was built to protect the seaward slope of the earth dike. All these actions resulted in the loss of the buffer zone against waves, while altering the coast’s vulnerability to erosion and wave overtopping. A series of constructions led not only to the loss of the buffer zone against waves in the vicinity of the shoreline, but also to the loss of valuable space with a delicate environment extending from the sandy beach to the forest with high trees and low coastal vegetation, and therefore, was vital for the growth of many kinds of plants and animals. Although many of the coastal forests are located in the less developed areas rich with natural features, a paradoxical result is obtained in that the more natural the coast is, the more artificial the coast becomes, while being covered by concrete. Management of the coastal forest and shore-protection works are based on the Forest Law and the Coastal Act. The Coastal Act is based on the concept that a sandy beach is a very important resource, and both the coastal environment and its utilization must be well preserved. In contrast, in the case of land management, based on the Forest Law, only the appropriate protection of the coastal forest is emphasized and concern with the adjacent beaches seems to be insufficient. The coastal management appears to have been guided by the opposite concept. Under these circumstances, many issues, several examples of which were discussed here, cannot be solved, and the situation will continue to get worse.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

279

(2) Coastal forest and measures to deal with natural disasters On the Nakamura-hama coast in northern Niigata Prefecture, seawalls had been built one after another to protect the coastal forest, and the area with no foreshore in front of the seawall was expanding. A series of constructions of the seawall ultimately lead to the coverage of the entire coastline with a seawall made of concrete. Concurrent with the new Coastal Act’s request for harmony between protection, environment, and use of coasts, the continuation of this type of work was very questionable. When the earth dike was destroyed and a very high scarp was formed, it was reasonable to consider that this was an extraordinary event, and urgent restoration works were required. However, the earth dike itself did not exist at this site from the beginning, but it was built by raising the ground level. Taking this into account, it is difficult to justify the restoration work. The manager of the coastal forest may bring forward a counterargument, saying that “we have been simply carrying out restoration work based on the Forest Law, and the protection of the coastal forest is totally reasonable.” “The original reason for the erosion is not the construction of the coastal forest, and the administration of the coastal forest is rather a victim.” “The collapse of the earth dike is a fact, and it is difficult for the damaged dike to remain intact.” “The littoral transport phenomenon has not yet been adequately solved, and we cannot carry out the restoration work comprehensively along the entire coastline under the current sector-bysector system.” The thinking of the administrations managing the land near the coastline is not consistent as a whole, and this causes confusions. What is important is that since the tendency of longshore sand supply to be exhausted will be further accelerated in the future, and the sand volume supplied will also be reduced, coastal damage and disaster potential will also increase. In addition, the amount of repeated damage to the gently sloping revetment may increase, and finally a long stretch of the natural sandy beach will be converted to an artificial coastline. (3) Method of addressing issues: Technological approach Consider the Nakamura-hama coast, as described in Sec. 2.7.1, as an example. In this case, the organizations governing the erosion problem

May 26, 2010

280

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.7.36. Several countermeasures against beach erosion.

include the managers responsible for coastal protection, the coastal forest, and the rivers. Finding a solution among these organizations is not easy. Nevertheless, some adjustments must be made to carry out cooperative measures considering the entire system of littoral transport, such as the stabilization of the shoreline using artificial headlands, sand bypassing, or sand back passing (sand recycling). Otherwise, the possibility greatly increases that measures to save the disappearing natural coasts will come too late. Concrete measures for the solution are summarized in Fig. 2.7.36. The first method is to leave the sandy beach in its current condition and to extend the seawall protecting the coastal forest at the eroded sites every year. As a result, the construction of the seawall will be continued, and the artificial coastline will rapidly expand alongshore. Even if the earth dike is protected by a seawall, the subsidence of the concrete slope due to the suction of the foundation gravel will take place at many locations, requiring repetition of the restoration of the seawall. Finally, a long stretch of the coastline will be covered with concrete structures. The second approach is to select methods, such as sand bypassing or sand recycling, to maintain the sandy beach as far as possible, while leaving

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

281

the coastal forest and the earth dike as they are. In the rivers flowing into the sea around the Nakamura-hama coast, sand accumulates inside the river’s mouth due to storm waves in winter, and every year dredging has been carried out. Taking this condition into account, it is required that the materials dredged from the river’s mouth be transported downcoast as in sand bypassing or upcoast as in sand recycling, and that the materials be reused for nourishment. Even if this method is used, since a large amount of sand has already been lost, and the total volume of sand is insufficient, beach nourishment is merely useful for the maintenance of the status quo, or the coast will gradually be devastated. In order to improve conditions, it is necessary for a volume of sand to be transported from another place to nourish the beach. But in this case, two new problems arise. First, the problem of the principle that beneficiaries should pay for a project may arise between organizations, and agreement among them may become difficult, since the beneficiary of beach nourishment differs from the organization that carries out the dredging operation. Second, since such maintenance work must be continued forever, and the Finance Ministry, which has given priority to the construction of hard structures, dislikes such action, it is difficult to budget. However, it is still a reasonable approach, taking the protection of the national land from the comprehensive standpoint into account. The third method is to force the formation of a stable shoreline by obstructing longshore sand transport by the construction of artificial headlands at some places along the coastline. The application of this method on the eroded coast is possible at least in principle, but on an already eroded coast, the artificial headlands must be built under conditions without sufficient foreshore width, and erosion always takes place downcoast of the artificial headlands. The downcoast shoreline recession often exceeds the allowable level. In this case, the drastic setback of the shoreline into the coastal forest area is required to avoid further damage to the coast. An open discussion of how to manage these conditions is necessary to reach public consensus, because people’s values differ.

May 26, 2010

282

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

(4) Method of addressing issues: Legal and institutional approaches Another method of solving the problem is the legal and institutional approach. Although the fundamental issues originate mostly from the sector-by-sector system of work, ultimately the problem arrives at the issue of how to balance the protection, environment, and use of coasts in the zone including both the coastal forest and the shore. In this case, we do not discuss how to maintain a balance among the protection, environment, and use of coasts within the limits of the law, where each work is grounded, but we must discuss the issue extensively from the general public’s point of view and determine a direction. Since it is difficult for only the people within the organizations concerned to adjust the direction, information must be totally open to the public and agreement among the people is required. As with the issue before measurements were taken, recognition of the facts of beach erosion by many people is currently still lacking, while the artificial coastline is rapidly extending. Taking these points into account, it is important to make many people realize the present situation concerning the devastated coasts of Japan.

REFERENCE Uda, T., S. Seino and T. San-nami (2004). Mechanism of rapid change from natural to artificial coast in Japan — The example of Node coast in Kujyukuri coastal plain, Asian and Pacific Coasts 2003, Proc. 2nd Int. Conf. paper 126, pp. 1–10.

REFERENCES (in Japanese) Farmland Div., Farmland and Agricultural Department, Chiba Prefectural Government (1957). History of Sabo in Heisa-ura, p. 6. Hoshigami,Y., A. Kobayashi, T. Uda, T. Kumada, K. Sakai and T. San-nami (2003). Development of coastal forest and beach erosion on Heisa-ura coast located at most southern area of Boso Peninsula, Proc. Civil Eng. in the Ocean, JSCE 19, 487–492.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

283

Ichikawa, S., T. Uda, T. San-nami, K. Furuike, H. Kido and Y. Hoshigami (2001). River mouth improvement on exposed beaches and response of river mouth topography — The examples of Nabaki and Ichinomiya Rivers in southern Kujukuri, Annual J. Coastal Eng. JSCE 48, 621–625. Izumiya, S. and H. Sunako (1994). Comprehensive investigation of extensive beach erosion in northeast part of Niigata Prefecture, Annual J. Coastal Eng. JSCE 41, 531–535. Uda, T. (1997). Beach erosion in Japan (Sankaido Press, Tokyo), p. 442. Uda, T., S. Seino, T. Yoshida, E. Sakai and T. San-nami (2002). Beach changes of Node coast in Kujukuri and consideration on formation of artificial coastline, Annual J. Coastal Eng. JSCE 49, 541–545. Uda, T., T. Kuroki, T. Nakamura and K. Kaki-ichi (2003). Protection of coastal forest and beach erosion — The example of Nakamura-hama coast in northern Niigata Prefecture, Proc. Civil Eng. in the Sea, JSCE 19, 327–332.

2.8.

DISAPPEARANCE OF SANDY BEACH TRIGGERED BY CONSTRUCTION OF GENTLY SLOPING REVETMENT

During the period between the late 1980s and early 1990s, the boom in waterfront utilization was remarkably enhanced throughout Japan. This nationwide boom affected the coastal works in Japan, and gently sloping revetments were built on many coasts to create access to the shoreline. These targets themselves were reasonable, but the extensive construction of gently sloping revetments without sufficient consideration was questionable. Despite the discussion that when a gently sloping revetment is built on a coast with a narrow foreshore, the foreshore will be buried under the concrete slope, resulting in the disappearance of a natural sandy beach (Uda, 1994∗ ), the construction of gently sloping revetments was carried out nationwide. A gently sloping revetment has the disadvantage that it is easily destroyed by the outflow of foundation gravel through the toe of the revetment, unless the structure has sufficient toe protection. Another

May 26, 2010

284

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

disadvantage is that the wave-overtopping rate of a gently sloping revetment becomes greater than that of a vertical seawall, lowering the effective protection against wave overtopping (Serizawa et al., 2003∗ ; Uda et al., 2005). Nevertheless, gently sloping revetments have been built in recent years. In the Coastal Act, revised in 1999, balance among protection, the environment, and use of the coast is required, but a gently sloping revetment is inferior in each of these functions. Taking these points into account, we discuss here the use of the gently sloping revetment from the point of view that its use must be avoided, except under conditions where the shore is sufficiently wide. First, a general relationship between the foreshore width and the location of the seawall is investigated through the example of the Isewan-Seinan coast, and two other examples, the Kitanowaki coast in Tokushima and the Uchihama coast on Miyako Island in Okinawa Prefecure, are discussed in detail.

2.8.1.

Isewan-seinan Coast in Mie Prefeture

The Isewan-seinan coast is located in the south part of Ise Bay, as shown in Fig. 2.8.1, and has an 11 km-long stretch. Since this coast is located in Ise Bay, it usually has calm-wave conditions, but storm surges caused by past typhoons caused heavy damage. In particular, during a typhoon in 1959, the coast was inundated and heavily damaged. After this typhoon, the construction of a continuous seawall was carried out. In order to investigate the relationship between the construction of the seawall and its impact on the environment and coastal scenery, field observations were carried out on 6 November 1999 (Uda et al., 2000∗ ). Figure 2.8.2 shows the coastal conditions on the east side of the Oyodo fishing port on this coast. There is a vertical seawall, as well as a coastal vegetation zone several tens of metres wide in front. Seaward of this vegetation zone, a foreshore with a uniform width extends alongshore, as shown by the white region in Fig. 2.8.2. This photograph was taken during low tide, and lots of shell debris was thrown up forming a berm during high tide. The foreshore became white because of the rich content of the shell

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

285

Fig. 2.8.1. Location of Isewan-seinan coast in south part of Ise Bay.

debris. A wide vegetation zone exists landward of the backshore because the seawall was built far from the shoreline, leaving a sufficiently wide buffer zone free from wave action under a usual wave conditions between the seawall and the shoreline. Coastal vegetation which can bear dryness and salinity grew in this buffer zone. Figure 2.8.3 was photographed east of the location where Fig. 2.8.2 was taken. The width of the buffer zone between the seawall and the beach was significantly narrowed, but the zone was still there at this site. According to the beach survey, the distance from the top of the berm to the marginal line separating the coastal vegetation zone and the foreshore was 10 m, and the distance from the top of the berm to the shoreline at the time of the observation was 15 m. Accordingly, it is assumed that the width of the vegetation zone in the vicinity of this location as shown by an arrow in Fig. 2.8.3 was only 5–6 m, which was small compared with the total distance of 25 m from the shoreline to the vegetation zone.

May 26, 2010

286

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.2. Coastal conditions at site with sufficiently wide buffer zone between seawall and beach (6 November 1999).

Fig. 2.8.3. Coastal conditions with narrowed vegetation zone in front of seawall.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

287

Fig. 2.8.4. Coastal conditions in which only sandy beach extends in front of seawall.

Immediately east of the point where Fig. 2.8.3 was photographed, the coastal condition as shown in Fig. 2.8.4 was observed. By comparing the distance to the location shown by the arrow in Figs. 2.8.3 and 2.8.4 in front of the seawall, the relative distance of the locations where the two photographs were taken seems to be several tens of meters. At the site where the picture in Fig. 2.8.4 was taken, the vegetation zone completely disappeared in front of the seawall, because the seawall line and the shoreline obliquely intersected and the vegetation zone was narrowed in the eastward direction. At a site a little east from the point where Fig. 2.8.4 was photographed, the top of the berm and the toe of the seawall coincided as shown in Fig. 2.8.5. Under these conditions, the wave action directly reached to the slope of the seawall during high tide, and growth of the vegetation zone became impossible. Figure 2.8.6 shows the coastal conditions photographed from the same position as that in Fig. 2.8.5, looking east. A gradually curving seawall extended along the shore as well as many kinds of structures that were

May 26, 2010

288

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.5. Coastal conditions of site where seawall line and shoreline intersect during high tide.

Fig. 2.8.6. Coastal conditions of coastline protected by groins and concrete armor units.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

289

installed, such as small groins and concrete armor units. The toe of the seawall was located seaward of the berm, because in this area the seawall was built seaward of the foreshore that existed previously. The observations at the Isewan-seinan coast were obtained from the comparison of photographs taken on a single day (at a fixed time) at various locations (changing space), while moving along the coast. The same results can be obtained from the process of observing beach changes when the beach erosion proceeds with time at a fixed position. It is well known that both results agree well (Paine, 1985; Uda, 1997∗ ). Using this concept of the ergodic rationale for beach changes, the recovery of past conditions of the coast may be possible. With reference to Fig. 2.8.7(a), Figs. 2.8.2–2.8.5 as described above were taken while moving alongshore at several locations indicated by numbers 1–5 in Fig. 2.8.7(a). Using the ergodic rationale for beach changes, a sequence of photographs can be used to explain temporal changes in coastal conditions, by substituting time for space. Since beach topography in the vicinity of the shoreline has uniformity alongshore, the foreshore extends alongshore with an approximately uniform width as shown by the broken line in the figure; the seawall was built, while intersecting with this foreshore. For this reason, the width of the vegetation zone decreased with distance eastward in the order of 1, 2, and 3. The beach profiles can be superimposed along each measuring line as shown in Fig. 2.8.7(b). Here, we consider substituting time for space; the longshore changes of the beach profiles are transformed into temporal changes. Consider the condition under which time changes of the beach profiles occur along survey line 1ias a two-dimensional phenomenon. There are two modes of possible change along survey line 1i. The first is shoreline recession, and the latter is an advance in the location of the sea dike as a result of the artificial alteration. The change in the environmental conditions is not governed by the absolute location of the seawall, but it depends on the relative distance between the shoreline and the seawall, so that both result in the same consequence. Here, we consider the case in which the shoreline recedes due to beach erosion.

May 26, 2010

290

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.7. (a) Schematic relation between width of coastal vegetation in front of seawall and shoreline and (b) superimposed beach profiles and temporal changes in profile and width of coastal vegetation.

When the shoreline retreats as a result of beach erosion, the profile changes measured along survey lines 1i through 4i are observed just as if time changes. In this case, Fig. 2.8.2 can be considered to show the coastal condition before beach erosion takes place. Since the location of the sea dike is fixed, the width of the vegetation zone behind the backshore decreases with shoreline recession, and finally it completely disappears. In

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

291

this situation, only foreshore is left in front of the seawall. With further shoreline recession, the berm disappears, and wave-dissipating structures or detached breakwaters are built, as shown in Fig. 2.8.6, resulting in an acceleration of the artificialness of the coast, because of severe wave overtopping during storms. Thus, the improvement of the coastal environment by building structures is very limited under conditions without space in the coastal zone. Constructing a gently sloping revetment in front of an existing vertical seawall at survey line 5i, as shown in Fig. 2.8.7(a), further narrows the spare room for a buffer zone. The seaward movement of the seawall due to artificial structures results in the disappearance of the potential coastal vegetation zone with a fixed shoreline position. It should be noted that the growth of the vegetation cannot be expected, unless the space is sufficiently wide, as shown in Fig. 2.8.7(a), between the seawall line and the shoreline.

2.8.2.

Kitanowaki Coast in Tokushima Prefecture

The Kitanowaki coast is located 4.5 km south of the Naka River in Tokushima Prefecture and is a pocket beach 1.2 km long bounded by headlands at both ends, as shown in Fig. 2.8.8. The coastline runs in the SSW–NNE direction, and it is considerably sheltered by Hiuchizaki and Kamouta Points and many islands off the coast, resulting in the incidence of relatively calm waves instead of swells from the Pacific Ocean. On 10 November 2000, field observations were carried out to investigate an example in which the construction of a gently sloping revetment in front of an existing seawall caused the disappearance of the foreshore (Uda et al., 2001∗ ). Figure 2.8.9 shows the northern half of the beach, looking northward from the central part of the beach. A seawall extends along the coastline with a 20 m-wide sandy beach in front. Since storm waves can run up to the highest point of the sandy beach in front of the seawall, there were no coastal vegetation zones in front of the seawall. In addition to this, since the seawall has a high parapet, it was inconvenient to directly approach the shoreline from land, and several small stairs have been attached to the seawall to improve accessibility to the shoreline.

May 26, 2010

292

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.8. Location of Kitanowaki coast in Tokushima Prefecture.

South of the location in Fig. 2.8.9, a gently sloping revetment as well as a terrace with the same crown height as the seawall was built in front of the existing seawall, as shown in Fig. 2.8.10. The slope of the gently sloping revetment extended from the top of this terrace to the shoreline, as shown in Fig. 2.8.11. Figure 2.8.12 shows the corner of the terrace attached to this gently sloping revetment as well as the headland located at the south end of the pocket beach. It is clear that this terrace protrudes significantly from the location of the parapet of the existing seawall on the south side of the structure. Figure 2.8.13 shows the slope of the gently sloping revetment. The steps of the gently sloping revetment protrude into the sea, crossing the shoreline,

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

293

Fig. 2.8.9. Coastal conditions of northern half of beach, looking northward from central part of beach (10 November 2000).

Fig. 2.8.10. Terrace with the same crown height as that of seawall built in upper part of gently sloping revetment.

May 26, 2010

294

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.11. Toe of gently sloping revetment.

Fig. 2.8.12. Corner of terrace attached to gently sloping revetment.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

295

Fig. 2.8.13. Slope of gently sloping revetment with steps protruding into sea.

resulting in the complete burial of the natural foreshore under the concrete slope. The construction of a terrace with the same crown height as that of the existing seawall accelerated the seaward protrusion of the toe of the gently sloping revetment. The example of the Kitanowaki coast shows that the disappearance of a natural sandy beach was triggered by the construction of a gently sloping revetment which aimed simply at enhancing access to the shoreline and by the construction of the terrace on top of the revetment because of the severe limitation in construction space, because the seawall line could not be set back toward the hinterland. The conversion of the natural sandy beach into an artificial coast is entirely contradictory to the concept of the preservation of the valuable coastal environment; it is extremely difficult for environment-oriented people to accept this. In a sense, it can be said that the production of an artificial coastline covered with concrete blocks is accelerated for the nominal reason of conserving the environment.

May 26, 2010

296

2.8.3.

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Uchihama Coast on Miyako Island in Okinawa

The Uchihama coast is a small-scale pocket beach located in Yonaha Bay on Miyako Island, as shown in Fig. 2.8.14. On 8 September 2001, field observations of this coast with a tidal range of around 2 m were carried out. Figure 2.8.15 shows the gently sloping revetment of the Uchihama coast built along the beach park in the hinterland. This photograph was taken 3 h after low tide, but the toe of the gently sloping revetment was exposed to waves with no foreshore in front of the structure. This coast has calm waves because of the location in the bay, so that people could freely approach the shoreline on bare feet from the park. However, the natural access way to the beach was completely interrupted due to the construction of hard structures just over the shoreline (Uda et al., 2002∗ ). Walking along the gently sloping revetment gives an impression of walking along a hot asphalt road in the city: it destroys the pleasure of beach combing and of watching sea organisms along the natural shoreline. Figure 2.8.16 shows the protruding part of the gently sloping revetment. Arrow A is shown in Fig. 2.8.16 at the same location of that in Fig. 2.8.15 for comparison. In the vicinity of this area, the gently sloping revetment has a curvilinear shape along the shoreline, and the back slope of the gently sloping revetment separates the low-vegetation zone at the backshore from

Fig. 2.8.14. Location of Uchihama coast of Miyako Island.

May 26, 2010

13:35

9in x 6in

b807-ch02

Beach Erosion — Current Reality

Fig. 2.8.15. Gently sloping revetment on Uchihama coast (8 September 2001).

Fig. 2.8.16. Gently sloping revetment built over sandy foreshore and vegetation.

297

May 26, 2010

298

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.17. Picture of crab drawn on crown of concrete sea dike.

the shoreline, resulting in the coverage of the fragile zone with by a concrete structure. A picture is illustrated on the crown of the gently sloping revetment at a location shown by arrow A in Fig. 2.8.16. Figure 2.8.17 shows the details. A picture of a crab was drawn in a circle with a diameter of 5.6 m on the crown. A large expenditure was probably required to have the picture drawn, but the idea that a picture of a crab on the crown of the concrete dike improves the coastal environment is undoubtedly shortsighted. Figure 2.8.18 shows a sandy beach left intact north of the northern end of the gently sloping revetment. A foreshore several meters wide and a low-vegetation habitat extended in front of the forest of Casuarina equisetifolia. This low-vegetation habitat extends below the surface for a considerable length, and there are many crab holes in the habitat and the foreshore, implying that many kinds of marine organisms live there. Figure 2.8.19 shows a large crab Cardisoma hirtipes around 20 cm in size observed at the shoreline.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

299

Fig. 2.8.18. Natural sandy beach left intact north of northern end of gently sloping revetment.

Fig. 2.8.19. Crab Cardisoma hirtipes living in habitat of Sporobolus virginicus near shoreline.

May 26, 2010

300

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.20. Habitat of Sporobolus virginicus expanding below surface.

Comparing Fig. 2.8.16 with Fig. 2.8.18, it is clear that the gently sloping revetment was built at the cost of the low-vegetation habitat and the foreshore. Figure 2.8.20 shows a scene of low-vegetation habitat protruding extensively toward the sea. Here, the coastal zone, covered with many subtropical trees, connected very gradually to the sea, resulting in extremely attractive scenery, where people could walk along the shoreline without obstruction. On the Uchihama coast, there is no rough wave action from the Pacific Ocean, because of its location in the deep bay. Storm surges may be expected, but the construction of a small-scale seawall at the boundary separating the backshore and the land is sufficient to prevent this, since the simultaneous action of high waves is not a concern. Instead, the shoreline in front of the seaside park, produced for use of many people, was covered with the concrete blocks of a gently sloping revetment, resulting in the disappearance of the most important fragile front with a rich ecosystem existing between land and sea. This is totally contradictory to the idea of

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

301

the 1999 Coastal Act; it is a typical example of the destruction of the coastal environment. In addition, since small-scale steps attached to the seawall function well compared with the large-scale concrete structures as an access way to the shore, this facility is inferior in terms of coastal use and protection of the natural environment. The construction of a gently sloping revetment on Miyako Island south of Okinawa Main Island is feared, because it causes a rapid production of artificial coast, while degrading the coastal environment; the same phenomena occurred on the main island of Japan in the past two decades. Such artificial coasts differ greatly from the expectations of visitors, reducing the potential for tourism on Miyako Island, because there are a number of such artificial coasts on the mainland of Japan. Once a structure is constructed on a coast, it is difficult to demolish it in the near future, because of the legal restrictions designated by “the Law related to appropriate managing of financial help by the central government”; such structures will exist for several decades. To prevent the repetition of these events in the future, the examination of why the construction of such structures is inferior is strongly required. The example of the Uchihama coast on Miyako Island in Okinawa as described here is not a special case, but the same situation can be observed on many coasts in Japan. To prevent the same mistake from occurring throughout Japan’s coasts, while faithfully abiding with the concept of the Coastal Act, the use of gently sloping revetments in this manner must be avoided.

2.8.4.

Methods of Addressing Issues

(1) Disappearance of foreshore due to construction of gently sloping revetments Issues regarding the disappearance or narrowing of a natural sandy beach due to the construction of gently sloping revetments are not unique problems on the Kitano-waki coast in Tokushima Prefecture, but can be commonly found along many coasts in Japan. Therefore, the fundamental issues are reconsidered in general (Uda et al., 1999∗ , 2005).

May 26, 2010

302

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.21. Schematic view of shore protection zone of coast.

The shore-protection zone in Japan is schematically shown in Fig. 2.8.21. A reference line is drawn parallel to the coastline. The distances from this reference line to the shoreline, the seawall, and the boundary between public and private lands are represented by XA , XB , and XC , respectively. The beach width is given by XA − XB . In shore-protection work in Japan, stringent restrictions exist in terms of the selection of the boundary line, C, between public and private lands under the Coastal Act, and it is very difficult for the boundary to be set inland by purchasing private land. Consequently, construction is carried out only seaward of this boundary. Consider the case of coastal protection work conducted on the coast schematically shown in Fig. 2.8.21. In the new Coastal Act revised in 1999, sufficient consideration must be given not only to coastal protection, but also to the coastal environment and use. However, in Japan, it is widely mistakenly believed that coastal use is achieved only by improving the accessibility to the shoreline through the construction of gently sloping revetments, even if the construction space, XA − XC , is not necessarily wide. In fact, when construction space is limited, such activity causes the destruction of the environment of a natural sandy beach. Consider a case where the seawall is replaced by a gently sloping revetment on such a coast. Since the revetment has a gentle slope, its toe is inevitably located seaward of the existing seawall. When the offshore distance to the toe of the gently sloping revetment is set as XF , the improvement from the existing seawall to the revetment results in narrowing

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

303

the foreshore from XA − XB to XA − XF . In other words, construction of the gently sloping revetment means the burial of a valuable natural sandy beach instead of the improvement of the coastal environment. Furthermore, since the shoreline position generally varies seasonally depending on the wave conditions, the situation that XA − XF
0 may arise. In this case, the toe of the gently sloping revetment protrudes directly into the sea and, therefore, the subsidence of the concrete blocks of the gently sloping revetment is apt to take place via discharge of the gravel layer in the foundation under the concrete slope. Another problem also arises in that the surface of the gently sloping revetment becomes slippery due to the attachment of sea organisms, devaluating the conditions of use of the gently sloping revetment as well as promoting the disappearance of a valuable sandy beach. People simply assume that it is sufficient for the shoreline to be advanced by the seaward protrustion of the gently sloping revetment, XF − XB , since this condition is determined as a relative problem between the values XA and XF ; this is the idea of artificial beach nourishment. In other words, the shoreline in front of the gently sloping revetment is forced to advance, as shown in Fig. 2.8.21, by beach nourishment. In this case, two new problems may arise. First, it is very difficult to stabilize the foreshore within an expected width, because the original beach slope is inevitably steepened due to the nourishment of the shoreline zone, and particularly in the case of fine sand, the offshore movement of sand may arise to restore the original slope, while causing shoreline recession. Second, longshore sand transport flowing in both directions is generated because of the local protrusion of the shoreline, as shown in Fig. 2.8.21. This sand does not remain at the nourishment position, permitting discharge of sand alongshore. To advance the shoreline in front of the gently sloping revetment, groins or jetties must be built to prevent nourished sand from discharging in the longshore direction at both ends, D and D , of the nourishment area. However, it is questionable to try to widen the foreshore by building the gently sloping revetment in such a fashion.

May 26, 2010

304

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

On an eroding coast, another issue may arise in addition. The erosive coast is one where XA decreases with time. When XA − XB decreases with time, the seaward protrusion of the toe of the gently sloping revetment leads to a rapid decrease in the foreshore width XA − XF not only by natural causes, but also by artificial causes. (2) Burial of natural sandy beach by gently sloping revetment In Japan, a gently sloping revetment has often been built in front of an existing seawall, as schematically shown in Fig. 2.8.22. The construction of a gently sloping revetment in such a manner causes serious problems (Uda et al., 1999∗ , 2005). In the construction of a gently sloping revetment, ensuring easy access to the shoreline has become the main goal, where the meaning of access to the shoreline is considered only superficially. Thus, the slope of the revetment is selected to be as gentle as 1/4. If the crown height of the revetment is assumed to be 5 m above MSL, considering the prevention of wave overtopping during rough conditions, the gently sloping revetment would have a 20 m-long concrete slope, which would bury the sandy beach

Fig. 2.8.22. Schematic view of constructing gently sloping revetment.

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

305

remaining in front of the existing seawall, as shown in Fig. 2.8.22. Since the average beach width in Japan is around 30 m, this means that two-thirds of the sandy beach is buried. This is equivalent to artificial beach erosion. If a promenade is built on top of the gently sloping revetment or stones are placed at the toe to prevent scouring, the buried area of the foreshore is further increased. On the other hand, if stairs are built at appropriate positions along the existing seawall, access to the shoreline can be easily established, while retaining the natural sandy beach in front. It is clear that fundamental reform of the shore-protection method is now required in Japan, taking these issues into account. In the 1999 Coastal Act, it is required that sufficient consideration be paid not only to coastal protection, but also to preservation of the coastal environment and use. Construction of a gently sloping revetment on the Kitanowaki coast apparently enables the safe enjoyment of the coastal scenery and improves access to the shoreline, taking the concept of the new law partly into account. Therefore, it may be argued that the narrowing of the foreshore is an unavoidable result, but the construction of a coastal facility that is ineffective in preventing wave overtopping compared with the seawall and in accelerating the narrowing of a sandy beach is unreasonable from all angles (Uda et al., 2005). In conclusion, since the change from a vertical wall to a gentle slope upon the construction of a revetment accelerates the loss of the foreshore, which is important as a buffer zone, and spoils the ecosystem of the sandy beach, using gently sloping revetments along a coast must be avoided. (3) New type of seawall suitable to Okinawa’s subtropical environment In Sec. 2.8.3, the loss of a natural environment due to the construction of a seawall was described in detail, taking the Uchihama coast on Miyako Island as an example. Taking the future of the coasts on Miyako Island, which feature excellent subtropical scenery, into account, another method of building a seawall suitable for Okinawa’s subtropical environment may be recommended (Uda et al., 2002∗ ). A inspiration for this exists in the form of the historical old-fashioned masonry seawall.

May 26, 2010

306

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

Fig. 2.8.23. New type of seawall suitable for Okinawa’s subtropical environment.

Figure 2.8.23(a) shows the current condition of the seawall on the Uchihama coast, but we have devised a plan shown in Fig. 2.8.23(b). Namely, the seawall is built using Ryukyu limestone instead of the concrete slope of a gently sloping revetment, and the growth of subtropical vegetation

May 26, 2010

13:35

9in x 6in

Beach Erosion — Current Reality

b807-ch02

307

is permitted between the gaps of this limestone seawall, which is also useful in creating a habitat for many organisms living in the vicinity of the coastline. By this method, the seawall may melt into the scenery at the site. There may be another alternative of setting the coastline back as shown in Fig. 2.8.23(c) and burying the seawall underneath the ground surface, leaving the coastal zone intact as a buffer zone against waves. Building a masonry structure using Ryukyu limestone was a traditional part of the culture in Okinawa. Creating technologies most suitable for this region is strongly required.

REFERENCES Paine, A. D. M. (1985). Ergodic reasoning in geomorphology: Time for review of the term?, Prog. Phys. Geog. 9(1), 1–15. Uda, T., M. Serizawa, S. Seino, Y. Hoshigami, T. San-nami and K. Furuike (2005). Summary of gently sloping revetment in Japan, Proc. Inter. Conf. on Coastlines, Structures and Breakwaters, ICE, pp. 1–10.

REFERENCES (in Japanese) Serizawa, M., T. Uda, A. Kobayashi, Y. Hoshigami, T. San-nami and K. Furuike (2003). Evaluation of wave overtopping discharge of gently sloping revetment in comparison with seawall and problems, Proc. Civil Eng. in the Ocean, JSCE 19, 237–242. Uda, T. (1994). Questions and Answers on Coastal Engineering (National Coastal Association), p. 236. Uda, T. (1997). Beach Erosion in Japan (Sankaido Press, Tokyo), p. 442. Uda, T., M. Serizawa, T. San-nami, K. Furuike and S. Seino (1999). Various issues regarding usage of gently sloping revetment, Proc. Civil Eng. in the Ocean, JSCE 15, 523–528. Uda, T., H. Shimada, H. Ohta, M. Ishikawa, M. Serizawa and T. San-nami (2000). A note on protection of natural environment of coasts and shore

May 26, 2010

308

13:35

9in x 6in

b807-ch02

Japan’s Beach Erosion: Reality and Future Measures

protection based on field observation at Isewan-seinan coast, Proc. Civil Eng. in the Ocean, JSCE 16, 399–404. Uda, T., M. Serizawa, T. San-nami, K. Furuike and S. Seino (2001). Undesired usage of gently-sloping revetment and its improvement method, Proc. Civil Eng. in the Ocean, JSCE 17, 631–636. Uda, T., A. Kikuchi, R. Nishi, M. Serizawa, T. San-nami and K. Furuike (2002). Formation of artificial coast and disappearance of coastal ecosystem due to construction of seawall on Miyako Island, Proc. Civil Eng. in the Ocean, JSCE 18, 695–700.

May 26, 2010

13:36

9in x 6in

b807-ch03

Chapter 3 PRACTICAL MODELS FOR PREDICTING BEACH CHANGES

3.1.

CHARACTERISTICS OF PRACTICAL MODELS

As many examples mentioned in Chap. 2 illustrate, several patterns of beach erosion have been occurring. When artificial actions causing beach erosion, such as the extension of a breakwater and offshore sand mining/dredging, are planned or when artificial headlands or detached breakwaters are planned as a measure against beach erosion, the effect and influences of each construction project must be quantitatively predicted beforehand. When a detached breakwater is built as a measure against beach erosion without sufficient study of the influences on the adjacent area, and beach erosion takes place as a result, the administrator of the coast is confronted with difficulties in accounting for the erosion. From this point of view, a model for predicting beach changes practically and with sufficient accuracy, and which is understandable for practical engineers is required, even though scientific rigour may be somewhat sacrificed and sufficient field data are lacking. Few practical models satisfy these conditions. Furthermore, the models, the results of which were only compared with the results of small-scale movable bed experiments and for which the verification with field data was insufficient, may face many difficulties in practical applications. Some models that the present author has developed in cooperative studies with other researchers have the advantage that they have practical use and are applicable to real coasts. Here, the outline of these models is described. As a matter of course, other models may predict beach changes with a high accuracy, but the inclusion of all models is not the aim of this book. Here, the outlines of the models which were developed by the author and others are introduced and examples of their application are described. The models in the following are classified into three groups. 309

May 26, 2010

310

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Group 1 consists of four models derived from Hsu and Evans’ model, in which the final stable beach topography is directly predicted without calculating the process by which the beach changes occur. (1) Prediction of stable shoreline on pocket beach (Sec. 3.2). (2) Three-dimensional model for predicting beach changes using Hsu and Evans’ model (Sec. 3.3). (3) Three-dimensional model for predicting beach changes on coasts with seawall (Sec. 3.4). (4) Simple model for predicting three-dimensional beach changes on statically stable beach (Sec. 3.5). Group 2 consists of two models for predicting not only the shoreline position but also the temporal and spatial changes in grain size by expanding the conventional one-line model. (1) Shoreline change model on coasts composed of sand with mixed grain size (Sec. 3.6). (2) Predictive model of shoreline and grain size around river-mouth delta (Sec. 3.7). Group 3 consists of two models for predicting three-dimensional beach changes, taking both longshore and cross-shore sand transports based on the concept of the equilibrium profile into account by expanding the contourline change model developed by Uda and Kawano (1996∗ ). (1) Contour-line change model including stabilization mechanism of longitudinal profile (Sec. 3.8). (2) Contour-line change model solving for x–y meshes (Sec. 3.9). Out of these models, the contour-line change model including the stabilization mechanism of the longitudinal profile to be described in Sec. 3.8 can be applied to the prediction of beach changes with any complicated boundary conditions and is of high usability. Here, the outline of the model and some examples are described.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

3.2. 3.2.1.

311

PREDICTION OF STABLE SHORELINE ON POCKET BEACH Predictive Model

This method was developed by Hsu and Evans (1989) and revised by Serizawa et al. (1996∗ ) and Kumada et al. (2002). The stable shoreline formed behind an impermeable offshore structure, such as an offshore breakwater or a headland, as shown in Fig. 3.2.1, can be easily predicted. This has the advantage that the configuration of the stable shoreline can be directly calculated in place of tracing changes in shoreline position. First, the shoreline position is expressed using the polar coordinate system as shown in Fig. 3.2.1. A point P is set at the tip of a breakwater and another point Q at a location sufficiently distant from the breakwater, where the direction of the shoreline becomes parallel to the wave crest line. Denoting the angle SPQ as β, the distance PQ as R0 , and the coordinate of a point on the shoreline as (R, θ), Hsu and Evans (1989) derived the equation R/R0 = C0 + C1 (β/θ) + C2 (β/θ)2 ,

(3.2.1)

where the coefficients C0 , C1 , and C2 are the functions of β. Since the relations R = R0 and θ = β are satisfied at point Q in Fig. 3.2.1, we obtain C0 + C1 + C2 = 1.

Fig. 3.2.1. Definition of variables and polar coordinate system.

(3.2.2)

May 26, 2010

312

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

In addition, the following relation is obtained from the condition that the length SQ = R sin θ is invariant at point Q by slightly changing θ in order for the direction of the shoreline at point Q to be parallel to the wave crest line. C1 + 2C2 = β/ tan β.

(3.2.3)

If one of the coefficients C0 , C1 , or C2 is obtained, the other values are determined from Eqs. (3.2.2) and (3.2.3). C0 is expressed as a function of β, considering convenience in practical use. The relation is given by the polynomial of β as follows in the range of 10
β(deg)
80: C 0 = A0 + A 1 β + A 2 β 2 + A 3 β 3 + A 4 β 4 ,

(3.2.4)

where A0 = −0.0116, A1 = 0.376, A2 = −0.451, A3 = 0.276, A4 = −0.331, and the correlation coefficient becomes 0.999. The variable β in Eq. (3.2.4) has units of radians. Regarding C1 and C2 , the following relations are derived from Eqs. (3.2.2) and (3.2.3). C1 = −2C0 − β/ tan β + 2,

(3.2.5)

C2 = C0 + β/ tan β − 1.

(3.2.6)

Figures 3.2.2 and 3.2.3 show the relationship among C0 , C1 , C2 , and β, and the comparison between Hsu and Evans’ model and the present one in the domain of R/R0 and β, respectively. The value of coefficient An was redetermined by a second-order regression analysis so as to exactly satisfy the orthogonal condition of the shoreline in the wave direction at point Q, and it differs slightly from the value in the original paper.

3.2.2.

Example

When the construction of an artificial headland is planned as a measure against beach erosion on an erosive coast where the foreshore was narrowed by successive beach erosion, an issue arises that the construction of a headland will trigger beach erosion in the adjacent area, resulting in an increase in wave overtopping or damage to the seawall. These shoreline

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

313

Fig. 3.2.2. Relationship among C0 , C1 , C2 , and angle β in Hsu and Evans’ model and this model.

Fig. 3.2.3. Relationship between R/R0 and β in Hsu and Evans’ model and this model.

May 26, 2010

314

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.2.4. Stable shoreline configuration between headland and groin.

changes associated with installing a headland can be predicted by this method. Figure 3.2.4 shows the predicted stable shoreline on a pocket beach separated by a groin located at X = 0 and a headland installed at X = 500 m as an example. Here, the longshore distance between these structures is defined as c. Regarding the conditions of the headland, we assume that the half-length of the head, a, is 100 m, and the offshore distance from the shoreline to the head, b, is 200 m. The broken line is a solution for a stable shoreline smoothly connecting to the initial shoreline at x = 0, without considering the sand budget in the calculation. However, since this solution is unrealistic because it neglects the sand budget, the calculation was carried out again. The solid line shows the solution for a stable shoreline determined in such a way that the sand budget between the eroded and accreted areas is satisfied. In this case, the sand required to form a salient behind the headland is gathered from the surrounding area, and therefore the shoreline retreats by Y min at the location of the groin. Y min depends on the lengths a, b, and c. As mentioned, when an artificial headland is built on a straight coastline, the shoreline in the surrounding area must retreat. It can be seen from the

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

315

calculation results that the construction of an artificial headland on a coast without a sufficiently wide foreshore which becomes a buffer zone against shoreline recession should be carried out very carefully. The primary advantage of this method lies in that the final, stable shoreline configuration can be simply predicted without a complicated calculation of the wave field. In particular, one of the greatest features is that the final, stable shoreline can be directly calculated instead of approaching the answer by a stepwise method.

3.3. THREE-DIMENSIONAL MODEL FOR PREDICTING BEACH CHANGES USING HSU AND EVANS’ MODEL 3.3.1.

Predictive Model

Hsu and Evans’ model, as described in Sec. 3.2, can predict the stable shoreline on a pocket beach very quickly, but the application is restricted to predicting only the shoreline position because of the model’s limitation. Here, a model for predicting not only shoreline changes but also changes in all contour lines after beach nourishment is given (Serizawa et al., 2000∗ ; Kumada et al., 2002; Uda et al., 2010), considering the longshore sand transport rate as a function of depth using the same equation as used by Uda and Kawano (1996∗ ): ε(z) = 2/ h3c · (hc /2 − z)(z + hc )2 , = 0,

−hc
z
hR , z < −hc , hR < z,

(3.3.1)

where z is the elevation with reference to the mean sea level; hc , the depth of closure; and hR , the berm height. It is assumed that longshore sand transport distributes between −hc and hR as a cubic function of the water depth. At the shoreline (z = 0), the following equation holds: ε(0) = 1.

(3.3.2)

The longshore transport rate decreases both landward and seaward from the shoreline. Equation (3.3.1) was originally derived from the ratio of the horizontal displacement of each contour line relative to the shoreline

May 26, 2010

316

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

change, for beach changes occurring due to longshore sand transport. In this case, if a successive change in each contour line is induced under a constant wave height at one place, the ratio of the total displacement due to the horizontal movement of each contour line relative to the total shoreline change can be approximated using Eq. (3.3.1). When the shoreline change, Y(0), is given, the horizontal displacement of each contour, Y(zk ), at water depth zk is given by Y(zk ) = ε(zk ) · Y(0).

(3.3.3)

On the other hand, the integrated value of Eq. (3.3.1) in the range of −hc
z
hR is equal to the ratio of the cross-sectional area of the beach, A, to the shoreline change, Y(0), and becomes the characteristic height of beach changes, Ds :
Ds = A/Y(0) = ε(z)dz = hc /2(1 − hR / hc )(1 + hR / hc )3 . (3.3.4) Thus, Ds can be calculated from hc and hR , and it is used to calculate the shoreline change, which satisfies the sand budget. The quantities hc and hR are determined from the breaker height by the same method as in the contour-line change model (Uda and Kawano, 1996∗ ; Uda, 1997∗ ): hc = 2.5Hb ,

hR = 0.32hc = 0.8Hb .

(3.3.5)

In the calculation, the total sand volume in the calculation zone should always be constant. This condition is satisfied as follows. The product of shoreline change, Y (0)(i) , at a point (i) and the characteristic height of beach changes, Ds(i) , is equal to the change in crosssectional area, A(i) . The suffix i corresponds to the position along the x-axis. Total sand volume change in the calculation zone, V , is given by the integration of A(i) in the x direction; V must always be zero in order for the sand budget to be satisfied:   V = A(i) dx = Ds(i) Y (0)(i) dx = 0, (3.3.6)

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

317 (i)

where dx is the interval along the x-axis. In particular, if Ds is constant,  Y(0)(i) dx = 0. (3.3.7) In other words, the erosion area of the beach becomes equal to the accretion area. In this case, it is sufficient to satisfy the condition that the eroded and accreted areas of the beach are equivalent, regardless of the Ds value.

3.3.2.

Numerical Calculation Procedure

Figure 3.3.1 shows the numerical calculation procedure. The input data are the initial topography, parameters of structures, incident wave direction, breaker height, and critical slopes, iR and ic , on land and seabed, respectively. First, the calculation zone is divided in the x direction, and hc and hR are calculated from Eq. (3.3.5), Ds from Eq. (3.3.4), and ε(zk ) from Eq. (3.3.1). The shoreline change, Y(0), can be predicted using Hsu

Fig. 3.3.1. Numerical calculation procedure.

May 26, 2010

318

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.3.2. Correction of location of each contour line due to offshore sand movement.

and Evans’ method, which satisfies the sand budget. Since the change in contour line, Y(zk ), at each depth is obtained from Eq. (3.3.3), the new position of each contour line can be determined. The location of each contour line can be determined by this method. At this stage, however, there may be cases in which the local slope exceeds the critical slope of the seabed determined by the angle of repose in the sea or on land. If the local bed slope exceeds the critical slope, correction of the location of each contour line due to offshore sand movement must be carried out, maintaining the sand budget in the cross section restricted by the seawall, as schematically shown in Fig. 3.3.2.

3.3.3.

Example

A model calculation was carried out on the domain in which the initial beach slope is assumed to be uniform at 1/10 and the beach is separated by a groin and L-shaped breakwater at both ends as shown in Fig. 3.3.3. Critical slopes on land and seabed are set to be 1/2. Six cases of the calculations were carried out. Table 3.3.1 summarizes the calculation conditions. Case 1 is a base for comparison. In Case 2, the critical slope on the seabed is milder. In Case 3, the incident wave height is decreased. In Case 4, the wave height distribution in the wave-shelter zone is predicted using the

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

319

Fig. 3.3.3. Calculation domain and coordinate system. Table 3.3.1. Calculation conditions. Case 1 2 3 4

Initial beach topography

Uniform beach with 1/10 slope

Hb (m)

hc (m)

hR (m)

ic

iR

2.0 2.0 0.4 Kd∗ 2.0

5.0 5.0 1.0 2.5 Hb

1.6 1.6 0.32 0.8 Hb

1/2 1/4 1/2 1/2

1/2 1/2 1/2 1/2

5

Stable beach of case 1

2.0

5.0

1.6

1/2

1/2

6

Uniform beach with 1/10 slope

2.0

5.0

1.6

1/2

1/2

angular spreading method. In Case 5, the effect of dredging in the accretion zone behind the port breakwater on the surrounding coast is evaluated. Figures 3.3.4 and 3.3.5 show the results of the calculation of Cases 1 through 4. Figure 3.3.4 shows the bathymetry of the stable beach, while Fig. 3.3.5 shows the superimposed longitudinal profiles. In Case 1, the shoreline protrudes significantly behind the breakwater due to its wavesheltering effect and the advance of the offshore contours as well. In this case, sand deposits in a deeper zone while keeping an angle of repose of sand on the seabed, as shown in Fig. 3.3.4(a), in the salient behind the breakwater.

May 26, 2010

320

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.3.4. Comparison of stable beach contours in Cases 1–4.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

Fig. 3.3.4. (Continued)

b807-ch03

321

May 26, 2010

322

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.3.5. Comparison of superimposed longitudinal profiles.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

Fig. 3.3.5. (Continued)

b807-ch03

323

May 26, 2010

324

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

The seabed slope off the shoreline becomes very steep at the salient, whereas a flat foreshore is formed corresponding to the wave run-up height on land. Outside the wave-shelter zone of the port breakwater, the contours near the shoreline including land contours retreat, resulting in the formation of a scarp. In conclusion, both erosion with a scarp forming outside the wave-shelter zone and the formation of a salient behind the offshore breakwater due to the deposition of sand transported alongshore are well predicted in Case 1. Figures 3.3.4(b) and 3.3.5(b) show the results of the calculation of Case 2. Sand in Case 2 deposits in a deeper zone with a decrease in the critical slope, which corresponds to a smaller grain size, than in Case 1, as shown in the changes in the longitudinal profiles, but the beach changes in the erosion zone are the same as those in Case 1. Figures 3.3.4(c) and 3.3.5(c) show the results of the calculation of Case 3, in which the breaker height is reduced from 2.0–0.4 m. Since the longitudinal profiles in the accretion zone are mainly determined by the critical slope on the seabed and do not depend on the wave height, the same changes as in Case 1 are predicted. In contrast, significant changes are observed in the erosion zone. Depending on the reduction in wave height, the depth of closure is reduced, and therefore beach erosion occurs in a zone landward from the vicinity of the shoreline. The eroded sand is transported to the right by longshore sand transport. The formation of the scarp is the same as in Case 1, but erosion on land with scarp formation is accelerated, instead of a decrease in eroded sand volume in the subaqueous zone. It is noteworthy that scarp erosion becomes significant when beach erosion takes place over a long time with a low wave height, compared with rapid erosion with a high wave height, since the sand volume required for the formation of a salient behind the offshore breakwater is approximately constant, if the critical slope is constant. Figures 3.3.4(d) and 3.3.5(d) show the results of Case 4. The difference between Case 4 and other cases is the consideration of longshore changes in the breaker height in this case. Figure 3.3.6 shows the longshore distribution of breaker height. Since the wave height decreases in a zone where the longshore distance, x, is greater than 300 m, the beach topography in

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

325

Fig. 3.3.6. Longshore distribution of breaker height (Case 4).

the zone x
300 m is the same as that in Case 1. In the salient, sand deposition associated with the wave run-up on land remains only in a lowelevation zone, because of a significant decrease in wave height behind the breakwater. For example, the movement of the contour line with an elevation of 1 m gradually becomes difficult toward the corner, since waves cannot reach there, as shown in Fig. 3.3.4(d). When a wave-sheltering structure such as a port breakwater is extended off the shoreline, a salient is formed due to sand accumulation behind the structure, as shown by many examples in Sec. 2.3. In this case, when the tip of the salient formed on the lee side of the offshore breakwater buries the navigation channel, deposited sand is usually removed by dredging. The influence of dredging is considered not to remain in the vicinity of the wave-shelter zone, but to reach the entire littoral cell. However, such recognition is still insufficient. From this point, a quantitative prediction of the impact of dredging on the surrounding coastline was carried out. By assuming that the seabed topography before dredging is given by Fig. 3.3.4(a), the tip of the salient formed behind the offshore breakwater is scraped away in a triangular shape, as shown in Fig. 3.3.7(a). Figure 3.3.7(b) shows the results of the prediction. On the lee side of the offshore breakwater, a salient with approximately the same shape as shown in Fig. 3.3.4(a) is reformed. Sand, which is required to reshape the salient, is supplied from the zone to the right of the groin located at x = 0 m, and erosion takes place in the entire area of this zone. Comparison of

May 26, 2010

326

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.3.7. Impact of dredging in salient behind offshore breakwater.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

327

Fig. 3.3.7(a) with Fig. 3.3.7(b) clearly explains landward and longshore expansion of scarp formation. In conclusion, a salient develops to reach a statically stable form behind the offshore breakwater, and therefore, if the tip of the salient is removed, a salient is reformed at the same location because of the restoring mechanism against the perturbation from the stable form. Since sand required for the reformation of the salient is supplied from the entire zone in the same littoral cell, the impact involves the entire zone, implying that a dredging taking insufficient measures for sand redeposition accelerates beach erosion on the surrounding coastline. This is an appropriate method for the prediction of beach changes in the surrounding coastline when a wave-sheltering structure is built off the shoreline.

3.4.

3.4.1.

PREDICTIVE MODEL OF THREE-DIMENSIONAL BEACH CHANGES ON COAST WITH A SEAWALL BY EXPANDING HSU AND EVANS’ MODEL Predictive Model

Consider an initial shoreline formed parallel to the straight seawall by beach nourishment on a pocket beach bounded by L-shaped groins, as shown in Fig. 3.4.1. The shoreline advances in the wave-shelter zone and retreats in the central part of the pocket beach due to the wave-sheltering effect of the L-shaped groins. In this case, the stable shoreline can be predicted by Hsu and Evans’ model, as described in Sec. 3.2. However, the shoreline may retreat landward beyond the seawall in the erosion zone, even if the seawall exists in the calculation zone. This leads to an overestimation of the shoreline change. Here, this weak point is addressed to predict threedimensional stable beach topography, taking the method in Sec. 3.3 into account (Sakai et al., 2002∗ ; Uda et al., 2010). Assuming that the shoreline area that retreated landward beyond the seawall is A1 , as shown in Fig. 3.4.1, since this area is not eroded in reality because of the existence of the seawall, the sand volume to be supplied to

May 26, 2010

328

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.4.1. Method of calculating shoreline configuration on coast with seawall.

the accretion area near both ends of the beach should be reduced to yield a smaller shoreline change. Therefore, it is necessary to reduce A2 by the amount A1 to adjust the shoreline area, given by Eq. (3.4.1), and then the change in shoreline position is reduced by Y , given by Eq. (3.4.2), in order to satisfy the continuity of sand volume. The shoreline located landward from the seawall is corrected to the seawall line. In the accretion zone, YQ(i) , located on the new shoreline, is calculated using Eq. (3.4.3), and the shoreline configuration can be calculated by Hsu and Evans’ model given point YQ(i) . A2  2Y · b, Y = A2 /2b = A1 /2b, YQ(i) = YQ(0) − Y.

(3.4.1) (3.4.2) (3.4.3)

In expanding the three-dimensional model, the change in each contour line is calculated from the change in the longshore sand transport rate with depth, by the same method as in Sec. 3.3, and the location of the new contour line is calculated by the addition of this change to the initial values. Furthermore, the seawall condition is taken into consideration by the same technique as described previously for all contour lines. The change in sand volume in the region characterized by each contour that retreated landward beyond the seawall is reduced to satisfy the total sand budget in the area between contours.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

3.4.2.

329

Example

The model is applied to Kemigawa beach described in Sec. 2.3.5 as the example. On Kemigawa beach, the straight seawall has been built parallel to the initial shoreline, and the condition described previously is satisfied. Figure 3.4.2 shows the bathymetry of this beach measured in 1999. Figure 3.4.3 shows the results of the calculation assuming appropriate

Fig. 3.4.2. Bathymetry of Kemigawa beach in 1999.

Fig. 3.4.3. Predicted bathymetry of Kemigawa beach.

May 26, 2010

330

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.4.4. Predicted longitudinal profile of stable beach at Kemigawa.

coefficients. Although the predicted value is slightly overestimated compared with the measured one, they agree well. The contour lines advanced at the corner of the pocket beach, where the wave-sheltering effect is dominant due to the curved groin, whereas, in the central part of the south beach, contours shallower than −1 m coincide with the location of the seawall, and similarly in the north part of the beach, contours between 0 and 2 m coincide with the seawall as well. Figure 3.4.4 shows the comparison of the longitudinal profiles at X = 40 and 1,200 m in the accretion zone, and at X = 500 and 970 m in the erosion zone. A berm 1 m high was formed by successive accretion of sand at X = 40 m on the south beach, whereas at X = 500 m the water depth at the toe of the seawall increased up to 1 m at maximum, although erosion of the foreshore is prevented. This is due to longshore discharge of sand from the seawall. On the north beach, a berm 1 m high was formed at X = 1,200 m, whereas the exposure of the seawall at the shoreline was predicted at X = 970 m. It is realized that sand further deposits in the wave-shelter zone, and that outside the wave-shelter zone the exposure of the seawall is intensified as erosion proceeds. One characteristic of this method is that the stable beach topography associated with the formation of a wave-shelter zone can be predicted in a three-dimensional manner without calculating the wave field.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

3.5.

3.5.1.

331

SIMPLE MODEL FOR PREDICTING THREE-DIMENSIONAL BEACH CHANGES ON STATICALLY STABLE BEACH Predictive Model

(1) Prediction of wave diffraction coefficient and wave direction by angular spreading method In Sec. 3.2, a method of predicting a statically stable shoreline on a pocket beach, developed by Hsu and Evans (1989), was presented. This model uses a curve fitting of the second-order regression analysis to predict the stable shoreline on a pocket beach, and it can be applied to problems with simple boundary conditions, but not to a coast with complicated boundary conditions, because the method assumes that various structures do not exist on a pocket beach. Here, a new model for predicting threedimensional topography under any type of structural conditions based on the fundamental characteristics of a statically stable beach is presented (Sakai et al., 2003∗ ). The diffraction coefficient and wave direction around the wave-sheltering structures are calculated first by the angular spreading method. Then, the statically stable beach topography is very quickly predicted on x–y meshes under the conditions including wave-sheltering structures. The diffraction coefficient and wave direction around the wavesheltering structures are calculated using the angular spreading method of irregular waves developed by Serizawa et al. (1993∗ ). First, the cumulative function, M0 (θ), of the directional distribution of wave energy, D(θ), is expressed by Eq. (3.5.1) after some revision of the equation given by Kraus (1984) by introducing an approximation function instead of numerical integration:  
θ 1 tanh(A0 θ) D(θ)dθ = 1+ , (3.5.1) M0 (θ) ≡ 2 tanh(A0 (π/2)) −π/2 0.489 (S A0 = 0.425 × Smax max = 10∼100),

(3.5.2)

where Smax is the directional spreading parameter. Equation (3.5.2) was empirically obtained by curve fitting based on the least-squares method

May 26, 2010

13:36

9in x 6in

332

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

against the cumulative function determined by numerical integration, and the correlation coefficient was 0.999. The maximum error is 2% in Kraus’s (1984) method, whereas it is less than 0.1% in this method, and the approximation may be carried out with a high accuracy. The directional distribution function, G(θ), is given by Eq. (3.5.3) by differentiating Eq. (3.5.1) with respect to the variable θ.
∞ dM0 G(θ) ≡ S(f, θ)df = dθ 0 A0 1 = , (3.5.3) × 2 tanh(A0 (π/2)) cosh2 (A0 θ) where S(f, θ) is the directional spectrum combining frequency spectra of both the Bretschneider and the Mitsuyasu types, and a directional function of the Mitsuyasu type. G(θ) is given by the integration of S(f, θ) over the entire frequency domain (Goda, 1985). Furthermore, by defining the moment of the wave direction, M1 , as Eq. (3.5.4), its integration reduces to
θ M1 (θ) ≡ D(θ) · θdθ −π/2

  π  θ tanh(A0 θ) − tanh(A0 (π/2))   1  2  = . cosh(A0 θ) 1  2 tanh(A0 (π/2))   −  ln e A0 cosh(A0 (π/2))

(3.5.4)

Using Eqs. (3.5.1) and (3.5.4), the wave diffraction coefficient, Kd , and the mean wave direction, θm , in the range of wave directions, θ = θ1 ∼θ2 , are given by
θ2

Kd = D(θ)dθ = M0 (θ2 ) − M0 (θ1 ), (3.5.5) θ1

 θ2 θ

θm =  1θ 2 θ1

D(θ)θdθ D(θ)dθ

=

M1 (θ2 ) − M1 (θ1 ) . M0 (θ2 ) − M0 (θ1 )

(3.5.6)

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

333

When there are a number of domains of wave incidence such as in the case of detached breakwaters, Kd and θm are calculated by  N  2 [M (θ Kt(j) (3.5.7) Kd =  0 2(j) ) − M0 (θ1(j) )], j=1

N

2 j=1 Kt(j) [M1 (θ2(j) ) − M1 (θ1(j) )] , 2 j=1 Kt(j) [M0 (θ2(j) ) − M0 (θ1(j) )]

θm = N

(3.5.8)

where N is the number of domains of wave incidence, and Kt is the wave transmission coefficient. Using Kd and θm obtained from Eqs. (3.5.7) and (3.5.8), stable beach topography is calculated as follows. (2) Calculation of statically stable beach topography A statically stable beach is defined as a beach with no cross-shore and longshore sand transport at any time on the long-term basis. On such a beach, the following conditions must be satisfied on the basis of the concept of the equilibrium profile (Dean, 1991) and that of the fundamental mechanism of longshore sand transport: (1) the seabed slope at any point in the longitudinal profile becomes an equilibrium slope and (2) the contour lines are perpendicular to the wave direction at any point in a zone where longshore sand transport develops. Figure 3.5.1 schematically shows this concept. The shoreline and contour lines as shown in Fig. 3.5.1(a) are stabilized, only if they become normal to the wave direction, as shown in Fig. 3.5.1(b). Given the location of point 1, the location of point 2 is determined as the point at a distance m away from point 1 along the direction normal to the wave direction. When the same procedure is repeated, the location of all points can be determined in the order of points 1, 2, . . . , 11. The same calculation can be carried out in the longitudinal direction. Referring to Fig. 3.5.1(c), the longitudinal slope at each point coincides with the equilibrium slope, tan βc , in the stable longitudinal profile. Accordingly, given the location of point 1, the location of point 2 is determined by extending a line with a length l and the slope tan βc . The procedure is

May 26, 2010

334

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.5.1. Schematic explanation of statically stable beach topography.

repeated and the location of all points is determined in the order of points 1, 2, . . . , 7. By combining these calculations, a statically stable beach topography can be calculated, when a seabed elevation at any point on the x–y plane is given. This is a fundamental method of solving this problem, but the seabed elevation Z(x, y) is actually obtained as follows.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

335

Fig. 3.5.2. Explanation of calculation method.

By setting the wave direction θw and contour lines Si and Si+1 at point P at depth Zi,j , as shown in Fig. 3.5.2, wave direction θw becomes normal to contour line Si at point P. The depth difference, Z, between the depths corresponding to contour lines Si and Si+1 is expressed by Eq. (3.5.9) from the geometry of the change in the contour line. Z = X · sin θw · tan βC = Y · cos θw · tan βC .

(3.5.9)

Here, βC is the angle of the equilibrium slope. The following differential equations are obtained by differentiating both sides of Eq. (3.5.9) with respect to x and y. ∂Z/∂X = sin θw · tan βC ,

(3.5.10)

∂Z/∂Y = cos θw · tan βC .

(3.5.11)

Equations (3.5.10) and (3.5.11) are the fundamental equations for the calculation. These equations can be solved by the finite difference method using discrete meshes in the x–y plane. Given the wave direction, the equilibrium slope, and the sea depth at a point as an initial value, sea depths at adjacent points in the x and y directions are calculated. This procedure is repeated until the sea bottom depth at all grid points is determined. To satisfy the continuity equation of sand, a cumulative value of the difference in the sea bottom depth from the initial value is obtained first. Depending on this cumulative value, the

May 26, 2010

336

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

depth at a selected initial point is modified, so that the total volume change converges to zero. The domain where beach changes take place ranges between the depth of closure, hc , and berm height, hR . As for the equilibrium slope beyond this domain, the angle of repose on land and under the sea surface is used to express the sinking of sand due to gravity. In the wave-shelter zone formed by detached breakwaters, hc and hR are reduced in proportion to the wave-diffraction coefficient: hc = Kd · hc ,

(3.5.12)

hR = Kd · hR ,

(3.5.13)

where hc and hR are the values in the case without a wave-sheltering structure. The wave direction varies due to wave refraction resulting from depth change, but here the wave refraction effect was neglected and the wave diffraction angle was used, since the calculation of stable beach topographies on real coasts is governed mainly by wave diffraction rather than wave refraction.

3.5.2.

Example

The model was applied to the prediction of a stable beach topography on a pocket beach separated by T-shaped groins at both ends. The initial beach topography was assumed to have a uniform slope of 1/10, and a model beach with a longshore length of 400 m and a cross-shore width of 200 m was assumed. The initial shoreline was set at the location of Y = 100 m. As incident wave conditions, the deepwater wave height, Ho , and the wave period, T , were assumed to be 2 m and 8 s, respectively. The directional spreading parameter, Smax , was set to 25, and waves were incident normal to the shoreline. The equilibrium slope, tan βc , and the angle of slopes (iR and ic ) on land and below the sea surface were set to 1/10, 1/2, and 1/3, respectively. Furthermore, we assumed that hR = 2 m and hc = −5 m. The model was also compared with the results of the movable bed experiment. The initial beach topography of the experiment had a 400 cm width and a uniform slope of 1/10. The initial shoreline was located at

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

337

Y = 80 cm. The median diameter of the bed materials was 0.22 mm. An impermeable detached breakwater 100 cm long was placed at a depth of 8 cm in the central part of the wave basin. The regular waves with Ho = 4 cm and T = 0.8 s were incident normal to the initial shoreline. The wave generation period was set to 10 h to obtain a stable beach topography due to sufficiently long wave action. In the verifying calculation, we assumed that Smax was 75, considering a swell wave with a high directional spreading parameter of irregular waves, rather than regular waves, although the experiment was carried out using regular waves. The equilibrium slope, tan βc , and the angle of the slope on land and under the sea, iR and ic , were set to 1/10, 1/2, and 1/3, respectively. Table 3.5.1 summarizes the calculation conditions. Figure 3.5.3 shows the distribution of the mean wave direction, θm , and the wave diffraction coefficient, Kd , at each mesh point. Since the phenomenon is symmetric with respect to the centerline crossing the opening of the T-shaped groins, the distributions of θm and Kd are shown in the left and right halves of the figure, respectively. A wave-shelter zone is formed due to the T-shaped groin at both ends, and the incident wave angle near the shoreline becomes large as the inside of the wave-shelter zone is approached. The wave height gradually decreases from the central part of the opening to inside the wave-shelter zone corresponding to this distribution of wave directions. Figure 3.5.4 shows the predicted stable beach topography. The contour lines advanced significantly to be normal to the diffracted wave direction in the wave-shelter zone of the T-shaped groins, resulting in the formation of a tombolo. Around the tip of the tombolo, a very steep seabed slope Table 3.5.1. Calculation conditions. Initial slope

Structures

Ho (m)

Smax

hR (m)

hC (m)

βc

ic

iR

2.0

25

1.5

−5.0

1/10

1/3

1/2

4.0

75

2.0

−5.0

1/10

1/3

1/2

Prediction of stable beach topography 1/10

T-shaped groin

Comparison with the results of experiment 1/10

Detatched breakwater

May 26, 2010

338

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.5.3. Distribution of mean wave direction, θm , and wave diffraction coefficient, Kd .

Fig. 3.5.4. Bathymetry of stable pocket beach.

was formed due to sinking sand transported by longshore sand transport, whereas a flat foreshore corresponding to the wave run-up height was formed in the wave-shelter zone. In the central part of the pocket beach, contour lines shallower than the depth of closure receded, because sand was carried away by longshore sand transport toward the wave-shelter zone, resulting in the formation of a scarp 5 m high on land. Off the shoreline, a

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

339

Fig. 3.5.5. Bathymetry of stable beach behind detached breakwater (experiment).

wave-cut terrace with a depth of −5 m was formed by successive erosion. These bathymetric changes of a pocket beach explain the results obtained on the Kemigawa beach well as described in Sec. 2.8.3. Figure 3.5.5 shows the bathymetry around the detached breakwaters after 10 h of wave generation in the model experiment. The initial, parallel contours of −6 through −2 cm in the opening of the detached breakwater largely retreated, forming a concave hollow. In contrast, the contour lines advanced significantly behind the detached breakwater, resulting in the formation of a salient. Figure 3.5.6 shows the calculated distributions of θm and Kd at each mesh point. Behind the impermeable detached breakwater, diffracted waves were transmitted from the opening, and the Kd value decreased due to the wave-sheltering effect of the detached breakwater. In addition, the Kd value decreased with the offshore distance along the Y -axis behind the detached breakwater. Figure 3.5.7 shows the predicted stable beach topography behind a detached breakwater. Although the scale of the salient behind the detached breakwater was a little underestimated, the overall features were well reproduced. Figures 3.5.8(a)–3.5.8(c) show a comparison of the measured and predicted longitudinal profiles at X = 0 cm through the opening of the

May 26, 2010

340

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.5.6. Distribution of mean wave direction, θm and diffraction coefficient, Kd .

Fig. 3.5.7. Predicted stable beach topography behind impermeable detached breakwater.

detached breakwaters, X = 200 cm in the center of the wave-shelter zone of the detached breakwater, and X = 100 cm between the two. Along the longitudinal profiles at X = 0 and 100 cm, a concave profile was formed by erosion, resulting in the formation of a scarp on land and a gentle slope below the sea surface. Although erosion on land is slightly overestimated in the calculation compared with that in the experiment, the overall profiles agree well. Along the longitudinal profile at X = 200 cm, a large amount of sand was transported from outside the wave-shelter zone and deposited behind the breakwater in the experiment.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

341

Fig. 3.5.8. (a) Comparison of longitudinal profiles at X = 0 cm. (b) Comparison of longitudinal profiles at X = 100 cm. (c) Comparison of longitudinal profiles at X = 200 cm.

Maximum deposition was observed in the vicinity of the shoreline. In the calculation, the overall pattern of accretion was slightly underestimated, but the formation of the flat foreshore in the accretion zone and the advance of offshore contours due to sand sinking into the deep seabed are well reproduced.

May 26, 2010

342

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

This method is also a model for directly predicting a stable beach configuration, but the wave field was predicted by the angular spreading method for irregular waves, and the stable beach topography was predicted, taking the equilibrium state in the longshore and cross-shore directions at each local point into account. Thus, this model has greater generality than those described before Sec. 3.4.

3.6. 3.6.1.

SHORELINE CHANGE MODEL ON COASTS COMPOSED OF SAND OF MIXED GRAIN SIZE Predictive Model

(1) Fundamental equations The ordinary one-line model is a most practical model for predicting beach changes. It can predict changes with a considerably high accuracy, although it assumes that the beach profiles move parallel to each other in the crossshore direction. However, it was not able to predict changes in grain size. Here, the change in the content of a specific grain size due to mixing in a sand-exchange layer is taken into account in the longshore sand transport formula, and a model for predicting the shoreline changes as well as the grain size change is developed (Kumada et al., 2002a∗ ). In order to take the sorting effect of grain size into account, the ordinary one-line model with uniform grain size is expanded so that the calculation of the shoreline change on a beach composed of grains of mixed sizes is possible. In considering the sorting effect of sand composed of grains of mixed sizes, the longshore sand transport formula corresponding to each grain size is needed. For this purpose, in the CERC formula, the relationship K1 ∝ d −1/2 between the coefficient K1 of the CERC formula and grain size d is introduced, as was given by Kamphius et al. (1986). The coefficient K1 expresses the mobility of sand. If the grain size is large, K1 and the longshore sediment transport rate Q become small, because sand with a large grain size is difficult to move. Here, a simple linear assumption is made: the exposure ratio of the shoreline corresponding to each grain size is equal to the content of each

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

343

Fig. 3.6.1. Definition of longshore component of wave energy flux per unit length of shoreline.

Fig. 3.6.2. Exposed ratio of shoreline corresponding to each grain size.

grain size (Nth) in the exchange layer of sand, µ(K) ; K = 1, 2, . . . , N, and the longshore component of wave energy flux acting on each grain size of d (1) , d (2) , . . . , d (N) is proportional to Fx µ(1) , Fx µ(2) , . . . , Fx µ(K) , as schematically shown in Figs. 3.6.1 and 3.6.2. Fx is the longshore component of wave energy flux per unit length of the shoreline and αbs is the angle between the wave crest line and the shoreline. The longshore sand transport rate of each grain size in mixed sand is as follows: (K)

Q(K) = µ(K) K1 Fx ,

(3.6.1)

Fx = (ECg )B sin αbs cos αbs ,

(3.6.2)

(K) K1

= A(d (K) )−1/2 ,

(3.6.3)

May 26, 2010

344

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

where the coefficient A in Eq. (3.6.3) is determined on the basis of the physical condition of the beach. Tanaka and Suzuki (1998∗ ) tried to consider this effect in the formulation, but they did not include the content of each grain size (Nth) at each time step in the exchange layer of sand in the longshore sand transport formula. Therefore, even if the component of sand of a specific grain size disappears at a time step in the exchange layer as a result of erosion, further sand discharge continues until the shoreline attains a stable form, whereas Eq. (3.6.1) in this study takes into account the decrease in longshore sand transport corresponding to the decrease in the content of a grain size. (2) Conservation of sand volume corresponding to each grain size The shoreline change corresponding to each grain size is calculated separately from continuity equations as follows:   (K) ∂yS 1 ∂Q(K) =− , K = 1, 2, . . . , N, (3.6.4) ∂t Ds ∂x where x is the longshore coordinate and Ds is the characteristic height of beach changes. The change of the shoreline composed of sand grains of mixed sizes is the sum of the shoreline changes for each grain size as follows: N (K)  ∂yS ∂YS = . ∂t ∂t

(3.6.5)

K=1

(3) Calculation of content of each grain size in exchange layer The content of each grain size in the exchange layer of sand is calculated on the basis of the concept of the exchange layer proposed by Hirano (1971∗ ). The exchange layer of sand is defined as the sand layer that is mixed by wave action throughout its thickness; in this layer, grain size change occurs. Figure 3.6.3 illustrates the definition of the exchange layer. It is assumed that beach profiles of uniform slope move parallel to each other, maintaining a constant characteristic height of beach change, Ds , and beach slope, tan β, as assumed in the one-line model. The area of width B and depth Ds shown

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

345

Fig. 3.6.3. Definition of exchange layer mixed by wave action.

in Fig. 3.6.3 is the exchange layer of sand. The width B of the exchange layer is geometrically related to the mixing depth Ds as follows: B = Ds

1 = Ds cot β. tan β

(3.6.6)

B is determined from Ds on the basis of the mixing depth given by Kraus (1985). In the one-line model, the beach profiles move parallel to each other depending on shoreline recession or advance. After a shoreline change, sand in the exchange layer is assumed to be mixed immediately. The state of mixing in the accretion area is different from that in the erosion area. Mixing occurs within the shoreward width B from the shoreline position after the change. In the accretion zone, layer  1 is newly formed, and layers  2 and  3 are mixed, leaving layer  3 in the initial exchange layer without mixing, as shown in Fig. 3.6.4. The mean diameter becomes small in the accretion zone, because much sand of small grain size is deposited in the new exchange layer. In contrast, in the erosion zone, layer  1 is newly eroded, and the new exchange layer is formed shoreward of layer , 2 which invades the further landward layer . 3 Layer  3 is a newly mixed layer, as shown in Fig. 3.6.5. In this case, the mean diameter becomes large in the erosion zone because sand of small grain size is liable to be carried away, leaving grains of a large size in the new exchange layer.

May 26, 2010

346

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.6.4. Sand mixing range after shoreline advance.

Fig. 3.6.5. Sand mixing range after shoreline recession.

Next, the sand volume for each grain size being carried into and out of the exchange layer at each time is formulated, and the content of each grain size in the exchange layer is calculated. The infinitesimal change in time is denoted as t. The sand volume of each grain size and the total sand volume after the shoreline change in the accretion area shown in Fig. 3.6.4 are expressed as follows:   (K) (K) (K) Ds , K = 1, 2, . . . , N, = (B − YS )µ + yS V (3.6.7) N  V (K) = BDs . (3.6.8) V = K=1

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

347

The content of each grain size in the new exchange layer is given as  V (K) 1  (K)  µ(K) = (3.6.9) = µ(K) + yS − YS · µ(K) , V B where the prime indicates the value at t + t. Equation (3.6.9) can be transformed into the analytical form via the expression of the finite difference equation.  (K)   1 yS YS µ(K) − µ(K) = − · µ(K) , (3.6.10) t B t t   (K) ∂µ(K) 1 ∂yS ∂YS (3.6.11) ∴ = − · µ(K) . ∂t B ∂t ∂t Similarly, the sand volumes of each grain size and total sand volume after the shoreline change in the erosion area, shown in Fig. 3.6.5, are given by   (K) (K) V (K) = B · µ(K) + yS − YS · µB Ds , (3.6.12) V =

N 

V (K) = BDs ,

(3.6.13)

K=1 (K)

where µB is the content of each grain size in the sandy beach landward of the initial exchange layer. The content of each grain size in the new exchange layer is  1  (K)  (K) (3.6.14) µ(K) = µ(K) + yS − YS · µB . B Equation (3.6.14) is transformed into an analytical form as follows:   (K) ∂µ(K) 1 ∂yS ∂YS (K) . (3.6.15) = − · µB ∂t B ∂t ∂t

3.6.2.

Example

The model is applied first to changes for a beach bounded by groins at both ends to investigate fundamental characteristics of the model. Waves

May 26, 2010

348

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.6.6. Comparison of shoreline changes on beaches with uniform grain size and mixture of two grain sizes.

are obliquely incident from 10◦ normal to the shoreline on a beach with a slope of 1/100, and the breaker height, Hb , is assumed to be 2 m. The sandy beach is composed of well-mixed sand with mean diameters of 0.1 and 1.0 mm and a content of each grain size of 50%. In order to investigate the difference between this model and the ordinary one-line model with uniform grain size, a calculation using the average of the two grain sizes (0.55 mm) is also carried out. Figure 3.6.6 shows the shoreline changes. The shoreline change of the beach composed of sand of mixed grain sizes does not show large variance from the result for uniform sand during 0.01 and 0.2 years, but the shoreline change predicted by this model is more rapid than that with uniform sand after 1.1 years, because of the faster movement of fine sand in the sand layer composed of a mixture of two grain sizes. This difference strongly depends on the composition of the grain sizes. Since the CERC formula was used in this model, the stable shoreline became normal to the wave direction. Figure 3.6.7 shows the longshore distribution of the mean diameters ¯ (d). The mean diameter approaches a constant value of 0.78 mm from the initial mean diameter from the updrift (left) end of the beach with time, while the foreshore is gradually covered by coarser material. On the other hand, a rapid change to finer material initially occurred in the vicinity of

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

349

Fig. 3.6.7. Longshore distribution of change in mean diameter.

the downdrift end, and this characteristic spread to the left with time after 1.1 years. Figure 3.6.8 shows the longshore distribution of the content of each grain size in the sand-exchange layer. The gradual change to coarser material propagates from upcoast to downcoast with time. It is noted that in the erosion zone, the ratio between contents of coarse and fine material approaches a constant, similarly to the change in mean diameter. This is due to the characteristics of the fundamental equation. We consider this point through the example of one mesh next to the groin in the erosion zone, as shown in Fig. 3.6.9, because longshore sand transport passing through the groin located at the updrift end is 0 and the boundary condition can be easily handled. The shoreline change corresponding to each grain size at this location is given as follows:  t t  (K) (K) QB − Q(K) = − Q(K) , K = 1, 2. (3.6.16) yS = Ds x Ds x Therefore, the shoreline change ratio corresponding to each grain size equals the ratio of the longshore sediment transport rate as follows: (1)

yS

(2)

yS

(1)

=

µ(1) K1 Q(1) = = const. (2) Q(2) µ(2) K1

(3.6.17)

May 26, 2010

350

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.6.8. Longshore distribution of content of each grain size in the exchange layer.

The content of each grain size in the erosion area is given by Eq. (3.6.14), and in order for the content of each grain size to be constant, the relation  µ(K) = µ(K) must stand. Therefore, we obtain (K)

yS

(K)

= YS · µB ,

(1)

∴

yS

(2)

yS

(3.6.18)

(1)

=

µB

(2)

µB

.

(3.6.19)

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

351

Fig. 3.6.9. Shoreline change in calculation mesh next to left boundary.

Equations (3.6.17) and (3.6.19) lead to the following relation: µ

(1)

:µ

(2)

=

µ(1) B (1)

K1

(1)

:

(2)

µ(2) B (2)

K1

(1)

= µB

√ √ (2) d (1) : µB d (2) .

(3.6.20)

Since the relation µB /µB = 1 is assumed in this calculation, the content of each grain size √ in the erosion area becomes equal to the reciprocal ratio K1 or the ratio d. It is concluded that the longshore distribution of the mean diameter at the final stage strongly depends on both the initial grain sizes and the initial content of grains, the characteristics of which strongly depend on the form of the equation used to calculate the change in the content. The ordinary one-line model can only predict beach changes, and no practical model for predicting grain size change was available. Here, the change in the content of a specific grain size due to mixing in a sandexchange layer was taken into account in the longshore sand transport formula, and a model for predicting the shoreline changes as well as the grain-size change was developed. This model may be applicable to the prediction of beach changes in beach nourishment using sand composed of grains of mixed sizes and changes in the ecosystem.

May 26, 2010

13:36

352

3.7. 3.7.1.

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

PREDICTIVE MODEL OF SHORELINE AND GRAIN SIZE AROUND RIVER MOUTH Predictive Model

In recent years, instead of separately investigating sediment transport in regions, such as sediment yield areas, rivers in the mountains and plains, river mouths, and the coast, a comprehensive management of sediment transport in a watershed including coasts has been required to develop a rational, effective policy. Fujita et al. (1995∗ ) defined the population of the grain size governing a river’s morphological changes in a specific area of the river as the “effective grain-size population,” and they showed that the grain-size populations mainly governing the morphological changes in the upstream, middle, and lower reaches of a river differed from place to place. This clearly shows that not only the volume but also the quality (grain size) of sediment must be adequately considered. One of the final goals of sediment management is to control the morphological changes of the rivers, river mouths, and coasts, but in past studies of river mouths and coasts, these concepts have not been sufficiently taken into account. Here, the concept of the effective grain-size population is applied to the phenomena on a coast, and a model for predicting the sorting of grain sizes and corresponding shoreline changes around a river’s mouth is developed (Kumada et al., 2002b∗ ). The fundamental equations of the model are the same as those in 3.6 with the inclusion of several additional terms as follows. In Sec. 3.6, the mass conservation equation for each grain size was given by Eq. (3.6.4), but when sediment is supplied from rivers, the shoreline change for each grain size is calculated from (K)

∂yS ∂t

=−

1 Ds



 ∂Q(K) − q(K) , K = 1, 2, . . . , N, ∂x

(3.7.1)

where x is the longshore distance, q(K) is the sediment inflow (here, sediment discharge from a river) for each grain size per unit length, and Ds is the characteristic height of beach changes. The shoreline change for

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

353

sand of mixed grain sizes is obtained as the summation N (K)  ∂yS ∂YS = . ∂t ∂t

(3.7.2)

K=1

In this model, when the composition according to grain size of the sediment discharged from a river is known, its value is used as the initial condition. If it is not known, the grain-size composition in the vicinity of the river’s mouth is used as the initial condition. Taking into consideration that accurate information on grain-size composition of sediment discharged from a river is often difficult to acquire in the application of the model to the real coast, (K) µIN is predicted from the composition of grain size at the river’s mouth, that is, the content for each grain size, µ(K) 0 (K = 1, . . . , N). The total sum of sediment discharge from a river and the longshore sand transport at the river’s mouth are set to 2QIN and 2Q0 , respectively, as shown in Fig. 3.7.1, and the right half of the river’s mouth is considered. Assuming that all the river sediment discharge contributes to longshore sand transport, the sand budget for each grain size is satisfied, and sediment

Fig. 3.7.1. Relationship between sediment discharge of river and composition of grain size around river’s mouth.

May 26, 2010

354

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures (K)

discharge from a river corresponding to a grain size of K, µIN QIN , becomes equal to Q(K) 0 . (K)

(K)

µIN QIN = Q0 . (K)

On the other hand, Q0

(3.7.3)

is derived from Eq. (3.6.1) as follows: (K)

Q0

(K)

(K)

= µ0 K1 Fx .

(3.7.4)

When the summation of Eq. (3.7.4) is taken in the range of K = 1, . . . , N and Eq. (3.7.4) is divided by this summation, we obtain (K)

(K) Q0

(K)

µ0 K1 = Q0 . N  (K) (K) µ0 K1

(3.7.5)

K=1

By substituting Eq. (3.7.5) into Eq. (3.7.4) and using the relation Q0 = QIN , the composition of the grain size of sediment discharged from a river’s (K) mouth, µIN , becomes (K)

µ(K) IN =

3.7.2.

(K)

µ0 K 1 (K)

(K)

.

(3.7.6)

µ0 K1

Example

(1) Shoreline change of river-mouth delta and grain-size changes In order to reproduce the sorting effect of grain sizes along the coastline of a river-mouth delta, a model beach with a uniform slope of 1/100 separated by groins at both ends is considered. The beach materials are assumed to be a mixture of three grain sizes of 0.074, 0.425, and 2.0 mm with the same composition. The numerical simulation of the shoreline changes was carried out under the condition that sand with the same composition was supplied into the sea through the river’s mouth. Table 3.7.1 shows a summary of the calculation conditions. For wave conditions, the wave angle and Hb were assumed to be 0◦ and 2 m, respectively. Figure 3.7.2 shows the shoreline changes. The shoreline reaches a stable form after 57.1 years from the beginning, and thereafter the river-mouth

May 26, 2010 13:36

Table 3.7.1. Calculation conditions.

Wave conditions

Characteristic height of beach changes (Ds )

Number of grain size (N)

1/100

Hb (m) 2.0

— —

ab ( ◦ ) 0

5.5

7.0

3

Teradomari and Nozumi coasts

1/100

H1/3 (m) 1.0

T1/3 (s) 6.0

a(◦ ) NNW

3.4

7.6

6

Model coast (dam)

1/100

Hb (m) 2.0

— —

ab ( ◦ ) 0

5.5

7.0

3

Sediment discharge from river (×103 m3 /yr) 0.106 mm

0.250 mm

0.425 mm

0.850 mm

2.00 mm

Coefficient A

100 6.4

— 5.5

— 56.4

100 312.2

— 37.3

100 1.1

0.10 0.01

150

—

—

90

—

60

0.10

b807-ch03

Model coast Teradomari and Nozumi coasts Model coast (dam)

0.074 mm

9in x 6in

Model coast

Practical Models for Predicting Beach Changes

Beach slope

Width of exchange layer (B)

355

May 26, 2010

356

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.7.2. Shoreline of river-mouth delta.

¯ Fig. 3.7.3. Longshore distribution of mean diameter, D.

delta advances offshore while keeping a similar shape. Figure 3.7.3 shows the longshore distribution of the mean diameter. Fine sediments move fast around the mouth, and the sorting effect gradually propagates in the longshore direction. The movement of coarser sand is slow compared with that of fine material, and the amount of sand transported becomes small around the mouth. As a result, the grain size around the river’s mouth becomes large, and it gradually decreases with distance from the mouth. In this model, this longshore sorting of sand can be reproduced well.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

357

Fig. 3.7.4. Longshore distribution of content in exchange layer, S0 .

Figure 3.7.4 shows the change in the content of sand in the exchange layer. Coarse sand is left in the vicinity of the river’s mouth, and fine sand accumulates far from the mouth. The change in the content of sand gradually propagates alongshore with time. It should be noted that the content of sand of each grain size approaches a stable condition simultaneously with the change in shoreline position approaching a stable form. In other words, the change in the content of sand closely relates to the change in shoreline position.

May 26, 2010

358

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.7.5. Weight cumulative curves of sand at two points and their changes with time.

Figure 3.7.5 shows the comparison of the weight cumulative percentage curves of the shoreline material at the end of the river-mouth delta (x = 10,100 m) and the mouth (x = 100 m). In the figure, the initial and predicted values after 57.1 years at both points are shown, where the minimum grain size is assumed to be 0.01 mm for convenience in drawing the curves. At the initial stage, the weight cumulative curves are the same alongshore, but the curves clearly show that, with time, the content of fine sand decreases around the mouth whereas the content of coarser sand increases. Far from the river’s mouth, the content of fine sand significantly increases. Using these weight cumulative curves, the median diameter, d50 , and the sorting coefficient, S0 , can be calculated. Figure 3.7.6 shows the distribution of d50 and S0 at the final stage. The value of d50 has approximately the same distribution as the mean grain size, and the sorting effect can be clearly confirmed. S0 assumes a smaller value around the river’s mouth and in the vicinity of both boundaries, because the content of coarse and fine sand increases there, respectively, whereas S0 assumes a large value near X = ±5,000 m because it is composed of sand that is a mixture of various grain sizes. Fujita et al. (1995∗ ) showed Fig. 3.7.7 as a schematic diagram explaining the relationship between the patterns of topographic changes of fluvial rivers in Japan and the transport of sand composed of grains of various

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

359

Fig. 3.7.6. Longshore distribution of d50 and S0 .

Fig. 3.7.7. Schematic concept of effective grain-size population.

sizes. The grain size of sediment supplied to an alluvial plain has a wide distribution, with grain type ranging from silt and clay to sand and gravel, and the sediment yield decreases in this order. In particular, the supply of the gravel component on the alluvial plain is generally much smaller than that of other materials. Out of these materials, almost all silt and mud is directly transported to the river’s mouth without depositing in the river channel. The change in the discharge of sand appears in the reach of sand beds (S-segment with the river bed slope Ib
1/2,000 and d60
1 mm).

May 26, 2010

360

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

The change in discharge of gravel appears in the reach of gravel beds (Gsegment with the riverbed slope Ib  1/500 and d60  15 mm). Thus, the transport pattern significantly differs and the effect on the riverbed is different, depending on the grain sizes of the supplied sediment. According to the calculation results, a similar phenomenon occurs around the delta at a river’s mouth. Figures 3.7.2 and 3.7.4 show that there is a relationship between the longshore inclination of the shoreline and the composition of sand in the exchange layer; the greater part of the material around a river’s mouth is composed of gravel and coarse sand when the longshore slope of the shoreline is large, whereas the content of fine sand increases far from the river’s mouth if the slope of the shoreline is smaller. These characteristics can also be derived from the equation of sediment transport for each grain size. Q(K) αBS ∼ = (K) . K1 Fx

(3.7.7)

Accordingly, the following relationship is obtained from Eqs. (3.6.3) and (3.7.7). √ 1 (3.7.8) αBS ∝ (K) ∝ D(K) . K1 Figure 3.7.7 shows the conceptual diagram. Along the coastline of a rivermouth delta, the transport mechanism of sand of each grain size differs, and the grain size of sand depositing on the shore strongly depends on the longshore inclination of the shoreline. Using these characteristics, the same kind of segment classification as in the case of rivers may be possible in terms of the grain size of the foreshore materials and the longshore inclination of the shoreline. (2) Effect of obstructing sediment discharge by dam on coast at river-mouth delta According to this model, the sand budget along the coastline of a rivermouth delta can be analyzed in terms of not only volumetric changes but also grain-size changes. Here, this model is applied to the evaluation of the effect of obstructing sediment discharge by the construction of a dam in a watershed on the grain-size population of the river-mouth delta coast.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

361

In general, when a large-scale dam is built, sedimentation in the dam reservoir, riverbed degradation in the downstream reaches of the dam, and a decrease in suspended sediment take place. Here, the shoreline and grainsize changes around the river-mouth delta coast are reproduced when the discharge of fine sediment decreases due to the construction of a dam. Consider a model beach with a slope of 1/100, separated by groins at both ends of the coast, where beach material is composed of grains of three sizes: 0.074, 0.425, and 2.0 mm. First, we assume that the fluvial sediment is supplied from the river’s mouth to the coast at a sediment discharge rate of 3.0 × 105 m3 /yr with their contents of 5:3:2, respectively. Then, the shoreline configuration and content of sediment are calculated until they reach a dynamically stable condition, and the supply of sand with a grain size of 0.074 mm is suddenly stopped. Here, the discharge rate of each component of sediment was determined with reference to the sand volume for each component grain size based on the results of the alluvium boring test (Fujita et al., 1995∗ ). The calculation conditions are shown in Table 3.7.1. As for the wave characteristics, the wave-incidence angle and the breaker height are 0◦ and 2 m, respectively. Figure 3.7.8 shows the comparison of shoreline changes before and after cutting the discharge of fine sediment (see broken and solid lines in Fig. 3.7.8). Although no striking difference occurs in the shoreline configuration itself, the longshore inclination of the dynamically stable

Fig. 3.7.8. Comparison of shoreline configurations before and after cutting discharge of fine sediment.

May 26, 2010

362

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

shoreline somewhat decreases. The longshore inclination is significantly affected by the reduction by half of the sediment supply from the river, but the influence on the composition change in terms of grain size is also included. In the ordinal calculation considering only a single grain size, when the sediment discharge rate is reduced by half, the longshore inclination of the shoreline is considered to become two times as mild as that before the reduction. However, since the composition of the sediment discharge becomes coarser due to the obstruction of fine material in this model, longshore sand transport is reduced as a whole, and the longshore inclination of the shoreline becomes steeper than indicated in the results with the ordinary model. By comparing this with the calculation results of the case in which sand composed of grains of three sizes discharges at the same content, the stable shoreline form differs (see broken and dotted lines in Fig. 3.7.8). From this, it can be seen that the composition in terms of grain sizes becomes coarser due to the effect of the construction of dams, and this change affects the shoreline inclination. Similarly, Fig. 3.7.9 shows the longshore distribution of the mean grain size of beach material before and after cutting the discharge of fine sand. While maintaining the fundamental feature of the sorting of grains by size,

Fig. 3.7.9. Longshore distribution of mean grain size before and after cutting discharge of fine sediment.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

363

Fig. 3.7.10. Content in exchange layer after cutting discharge of fine sediment.

grains become coarser as a whole after cutting the discharge of fine sand. Figure 3.7.10 shows the content in the exchange layer before and after cutting the discharge of fine sand. The content in the exchange layer changed alongshore and that of coarser material increased. As described, this model is useful for predicting the impact of each grain size on coasts surrounding a river’s mouth, when continuous sand movement is obstructed by dams, or when a population of sand composed of grains of a specific size is discharged downstream of a dam as one of the measures against beach erosion.

3.8.

3.8.1.

CONTOUR-LINE CHANGE MODEL CONSIDERING STABILIZATION MECHANISM OF LONGITUDINAL PROFILE Calculation Method

The contour-line change model has wide practical applicability. It has several outstanding features: it can predict the formation of a scarp in an

May 26, 2010

364

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

erosion zone and the sinking of sand into a zone deeper than the depth of closure due to the existence of a steep slope, and it is suitable for the prediction of long-term beach changes. However, in the previous contourline model, the meandering of the contours off groins built along a coast with dominant longshore sand transport was difficult to predict, and general procedures to deal with various kinds of coastal structures, such as detached breakwaters, were not developed. Here, a practical model is developed to solve these problems by taking a stabilization mechanism of the longitudinal profile due to cross-shore transport into account. (1) Cross-shore and longshore sand transport formulae The x- and y-axes are the longshore and cross-shore distances, as shown in Fig. 3.8.1, respectively. The contour lines corresponding to the elevations z = z1 , . . . , zk are set in the x–y plane. The elevation z is upward positive. In the contour-line change model, the seabed topography is expressed by the offshore distance Y(x, z, t) from the x-axis to the location of each contour line and the time t. First, the cross-shore sand transport rate qz per unit length of the coastline is considered. In this study, longitudinal beach change due to cross-shore sand transport is not a short-period beach change associated

Fig. 3.8.1. Definition of variables.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

365

with storm waves, but a long-term beach change with a time scale comparable with that of beach changes due to longshore sand transport. Dean (1991) showed that an equilibrium profile exists and that a restoring force is caused depending on the extent of the difference from this equilibrium profile. This gives rise to cross-shore sand transport. In this study, this stabilization mechanism of the longitudinal profile was combined with the contour-line change model developed by Uda and Kawano (1996∗ ). The stabilization mechanism of the beach profile based on the equilibrium between the effect of gravity and wave action is schematically shown in Fig. 3.8.2, with reference to the concept presented by Bakker (1968). If βc is the angle of the equilibrium beach slope at which the shoreward motion of sand particles due to wave action is balanced by the seaward motion due to gravity, it is assumed that seaward sand transport is caused if the angle of the seabed slope β exceeds βc and that shoreward sand transport

Fig. 3.8.2. Stabilization mechanism of beach profile based on equilibrium between effect of gravity and wave action.

May 26, 2010

366

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

is caused if the angle β becomes smaller than βc . This transport is expressed by Eq. (3.8.1) as assumed by Bakker (1968):   cot β −1 , (3.8.1) qz = A · cot βc where cot β is given by cot β = −

∂Y . ∂z

(3.8.2)

Coefficient A in Eq. (3.8.1) corresponds to the restoring velocity of the beach profile and is assumed to be proportional to the shoreward energy flux at the breaking point. Consider the relationship between the range of the cross-shore zone where sand is moved by cross-shore sand transport and the dissipation rate of wave energy as schematically shown in Fig. 3.8.3. We assume that sand movement occurs in the zone where the depth ranges between the depth of closure, hc , and the wave run-up height (berm height), hR . Under these conditions, the wave energy flux transported shoreward from offshore, (EC g )b , is dissipated by wave breaking and wave run-up on the foreshore. Work proportional to this wave dissipation

Fig. 3.8.3. Zone with cross-shore sand movement and dissipation of wave energy.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

367

should be applied to the beach surface, causing sand movement on the seabed ranging between hc and hR . Accordingly, coefficient A must be proportional to the energy flux at the breaking point divided by the seabed length extending between hc and hR . By considering the seabed length, S, as shown in Fig. 3.8.2, the wave dissipation per unit seabed length becomes   (ECg )b (ECg )b 1 sin β¯ ≈ (3.8.3) = · (ECg )b sin βc , S hc + hR hc + hR where β¯ is the mean slope in the range between hc and hR , and the relation β¯ ≈ βc is assumed. Furthermore, in general, it should be noted that large beach changes occur in the surf zone as well as in the vicinity of the shoreline with a gradual decrease in the changes in the offshore direction, implying that the intensity of cross-shore sand transport has a distribution that depends upon depth. Here, we introduce εz (z) as the intensity function and a coefficient of cross-shore sand transport Kz . A is proportional to Eq. (3.8.4) for the reason mentioned. In addition, in order to consider the depth distribution for A, εz (z) is assumed by analogy with the coefficient 1/(hc + hR ), which is the coefficient for uniform distribution. Then, A and the cross-shore sand transport rate are given by Eqs. (3.8.4) and (3.8.5), respectively. The integral of εz (z) in the range between hc and hR must also be unity, as given by Eq. (3.8.6). A = εz (z) · Kz · (ECg )b sin βc , qz = εz (z) · Kz · (ECg )b sin βc ·


hR

−hc

εz (z)dz = 1.





cot β −1 , cot βc

(3.8.4) (3.8.5) (3.8.6)

Kz in Eq. (3.8.5) is determined as the coefficient of longshore sand transport Kx , later described in Eq. (3.8.10), multiplied by an empirical proportional coefficient of 0.2.

May 26, 2010

368

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

The derivations described are obtained under the condition that waves are incident from normal to the shoreline. Under the condition of oblique wave incidence, the wave energy flux per unit length of the contour line may be used instead of (EC g )b in Eq. (3.8.5). If αb is the breaker angle, then, multiplying the shoreward component of wave energy flux (EC g )b cos αb by the crest width, cos αb , of the waves acting on the unit length of the contour line, we obtain   cot β 2 qz = εz (z) · Kz · (ECg )b cos αb sin βc · − 1 , (3.8.7) cot βc where cos2 αb 1 holds if αb is sufficiently small. Using this approximation, Eq. (3.8.7) results in Eq. (3.8.5) as a fundamental equation. For βc , the bed slope of the natural sandy beach in the equilibrium state before the construction of coastal structures is given. In the zones higher than the wave run-up height and deeper than the depth of closure, the angle of repose is assumed to be the critical angle for the sinking of sediment over the steep slope. Assuming that εz (z) is equivalent to the depth distribution of longshore sand transport εx (z), it is given by a cubic equation of the depth, using the same method as that employed by Uda and Kawano (1996∗ ), as follows: 
hR εz (z)dz, (3.8.8) εz (z) = εx (z) = ε(z) −hc

    = 2 hc − z (z + hc )2 h3c 2 ε(z) =  = 0

(−hc ≤ z ≤ hR ),

(3.8.9)

(z < −hc , hR < z).

Regarding the fundamental equation of longshore sand transport rate qx , we employ the same relations as Eqs. (3.8.8)–(3.8.10) of Uda and Kawano (1996∗ ). qx = εx (z) · Kx · (ECg )b cos αb sin αb .

(3.8.10)

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

369

The wave energy flux at the breaking point, (EC g )b , can be calculated using Eq. (3.8.11) and the breaker angle is obtained from Eq. (3.8.12).  ρg g 5/2 (ECg )b = H (γb = 0.8), (3.8.11) 8 γb b   −1 ∂Y , (3.8.12) αb = θb − tan ∂x Hb is the breaker height, γb the ratio of breaker height relative to breaker depth, ρ the water density, g the acceleration due to gravity, and θb the angle between the crest line of a breaking wave and the positive x-axis. Contour-line changes are calculated using a two-dimensional continuity equation expressing spatial balance between longshore and cross-shore sand transport. ∂qx ∂qz ∂Y =− − . ∂t ∂x ∂z

(3.8.13)

(2) Physical interpretation of fundamental equations Assuming that the depth distributions of longshore and cross-shore sand transport are unique for simplicity, the following holds. εx (z) = εz (z) = 1/(hc + hR ).

(3.8.14)

Furthermore, assuming that the wave height is constant alongshore, αb is sufficiently small and the equilibrium beach slope, βc , is constant, the following equations are derived from Eqs. (3.8.7), (3.8.10), (3.8.12), and (3.8.13). ∂2 Y ∂2 Y ∂Y = Bx 2 + Bz 2 , ∂t ∂x ∂z (ECg )b , Bx = Kx hc + hR (ECg )b Bz = Kz sin βc tan βc . hc + hR

(3.8.15) (3.8.16) (3.8.17)

May 26, 2010

370

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Equation (3.8.15) is the two-dimensional diffusion equation, and the contour-line changes are expressed by the contributions of longshore (first term) and cross-shore sand transport (second term). The first term shows the same smoothing effect of the irregular contours by longshore sand transport as in the past contour-line change model. The second term shows that the change in longitudinal profile, ∂Y/∂t, is proportional to the curvature of the profile, ∂2 Y/∂z2 . On the convex sea bottom the contour-line retreats (∂Y/∂t < 0) since ∂2 Y/∂z2 < 0. If the seabed slope at each point becomes equal to the equilibrium slope tan βc , then the beach profile is stabilized. As mentioned previously, two stabilization processes are taken into account in this model so that each contour line is stabilized to be normal to the wave direction at each location without cross-shore sand transport, and that the local slope of the longitudinal profile is stabilized to be equal to the equilibrium slope without longshore sand transport. (3) Procedure for calculation In the calculation, discretization of the coastal domain is conducted in 2D elements with widths x and z. The calculation points of the contourline position and sand transport rate are set in staggered meshes with a difference of 1/2 mesh, as shown in Figs. 3.8.4 and 3.8.5. In the vertical

Fig. 3.8.4. Discretization in vertical direction.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

371

Fig. 3.8.5. Discretization in longshore direction.

direction, the location of the contour line corresponding to each depth is set, and the calculation point of cross-shore sand transport is set between the contour lines, as shown in Fig. 3.8.4. Given the initial seabed topography, the distributions of the breaker height Hb and αb , hc , and hR , the cross-shore and longshore sand transport rates are calculated using Eqs. (3.8.5) and (3.8.10), respectively, and the contour-line change is calculated using Eq. (3.8.13). The explicit finite difference method is used with Eq. (3.8.13), as expressed by Eq. (3.8.18), and the new location, Y  , of the contour line after t is calculated. (k)

(k)

Y(i) = Y(i) −

 t  (k)  t  (k) − . qx(i+1) − qx(k) qz(i) − qz(k+1) (i) (i) x z

(3.8.18)

The subscripts i and k indicate the cell numbers in the longshore and crossshore directions, respectively. For the boundary condition, qx and qz are set to 0 at the outer boundary of the domain. (4) Restoration of beach profile due to gravity The sinking of sand in regions above the berm height and deeper than the depth of closure can be calculated by giving the angle of repose, βg , as βc if the seabed slope exceeds the repose slope. The sinking of sand is due to gravity, and it is considered to be caused much more quickly compared with beach changes due to waves. For this

May 26, 2010

372

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

reason, the largest value of Ag is selected as A, as long as the stability of the numerical simulation is confirmed. By representing the cross-shore sand transport by Eq. (3.8.19) and neglecting longshore sand transport, the continuity equation of Eq. (3.8.13) reduces to the one-dimensional diffusion equation given by Eq. (3.8.20). The stability condition of this equation in the calculation using a finite difference method is that Rs in Eq. (3.8.21) must be smaller than 0.5. Conversely, Ag can be expressed as Eq. (3.8.22) from Eq. (3.8.21). The upper limit of Ag can be obtained from Eq. (3.8.22), assuming 0.5 for Rs , but in this calculation, we assume 0.2 for Rs by considering a safety factor of 2.5. Finally, Ag is calculated from Eq. (3.8.22) with 0.2 for Rs , when β exceeds βg , and qz is calculated from Eq. (3.8.19).   cot β qz = Ag · −1 , (3.8.19) cot βg Ag ∂2 Y ∂Y = . ∂t cot βg ∂z2

(3.8.20)

As the stability condition, RS =

Ag t ≤ 0.5, cot βg (z)2

(3.8.21)

(z)2 (β > βg ). (3.8.22) t In the calculation, tan βg was assumed to be 1/2 on land and 1/3 on the seabed. Ag = RS cot βg

(5) Condition of structures Regarding the arrangement of the structures shown in Fig. 3.8.6(a), for example, a new method for generally handling conditions of various coastal structures such as a seawall, groins and detached breakwaters was developed. In this method, the longshore and cross-shore sand transport rates are decreased by a reduction coefficient µ, as shown by Eqs. (3.8.23)– (3.8.26), when a contour line approaches very close to the seawall, for example, by applying the method in the three-dimensional model proposed

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

373

Fig. 3.8.6. Arrangement of cells expressing location of contour-line positions with various structures.

by Ikeno et al. (2001∗ ) regarding sand transport on the seabed with exposed rocks. qx(k) = µ(k) · qx(k) ,

(3.8.23)

qz(k) = µ(k) · qz(k) ,

(3.8.24)

µ(k) = Yc(k)

Y (k)

(0 ≤ µ(k) ≤ 1), (k) Yc 1 = cot βc(k) · z. 2

(3.8.25) (3.8.26)

May 26, 2010

374

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.8.7. Reduction of sand transport rate in vicinity of seawall.

As shown in Fig. 3.8.7, µ becomes 0 when a contour line approaches very close to the seawall, and it is proportional to the distance Y between the contour line and the seawall. The distance, Yc , at which a decrease in sand transport begins, is given by Eqs. (3.8.25) and (3.8.26). Consider a cell of rectangular shape in front of the seawall and the seabed of slope βc . The effect of the seawall begins at µ = 1 via the exposure of the seawall. Since the calculation point of sediment transport differs from that of the contour line by 1/2 mesh, the value at the updrift cell is used for µ. This method is applicable to the calculation at the tips of the groins, as shown in Fig. 3.8.6(b). In a zone with a detached breakwater, the calculation domain is separated into offshore and shoreward parts, as shown by regions 2–1 and 2–2 in Fig. 3.8.6. The method dealing with the offshore boundary of a detached breakwater is the same as the one for the seawall as mentioned above. Regarding the shoreward boundary of a detached breakwater, the crossshore distance from each contour line to the detached breakwater is used instead of Y(k) in Eq. (3.8.25), and the value at the downdrift cell is used for µ, so that the sand transport rate becomes 0 when the contour line merges

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

375

Fig. 3.8.8. Reduction of sand transport rate shoreward of detached breakwater.

into the back of the detached breakwater because of tombolo formation on the lee side of the detached breakwater, as shown in Fig. 3.8.8. In addition, since there are several contour lines in the offshore and shoreward zones corresponding to each depth zk , as shown in Fig. 3.8.6(b) in the region with a detached breakwater, the continuity equation of Eq. (3.8.18) was solved, taking the mutual relation between longshore calculation cells into account.

3.8.2.

Comparison of Results of Experiments and Numerical Simulations

(1) Beach changes due to offshore dredging Figure 3.8.9 shows the prediction of beach changes due to offshore dredging on a coast with a closed littoral system bounded by headlands. The initial seabed is shown by the parallel contours. Under the condition of wave incidence perpendicular to the straight shoreline, offshore dredging is assumed to be conducted in the trapezoidal zone, the base length of which is 200 m, in the range of water depth between 3 and 10 m. The breaker height, Hb , wave direction at the breaking point at initial condition, hc , and hR are shown in the figure.

May 26, 2010

376

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.8.9. Simulation of refilling of offshore dredging hole.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

377

Configurations of the seabed after 140, 550, and 8,760 steps are shown in Figs. 3.8.9(a)–3.8.9(c), respectively. Seabed contours shallower than hc retreat in the vicinity of the dredging hole, and the hole is refilled over time. After 8,760 steps, refilling of sand into the dredging hole is complete and all contour lines become parallel to each other, as shown in Fig. 3.8.9(c). At the initial stage, the shoreward slope of the dredging hole is very steep, but this steep slope disappears due to refilling. Considering that the initial beach contours before dredging were parallel and their interval was constant, the effect of offshore dredging remains at the concave part of the offshore contour deeper than hc , as well as in the parallel recession of all the contours shallower than hc and the narrowing of foreshore contours due to the formation of a scarp throughout the entire zone, as shown in Fig. 3.8.9(c). This result is due to the effect of the stabilization mechanism described in Sec. 2.2, and the original equilibrium profile and the contour lines restore their shape under the condition of static equilibrium. Offshore dredging at depths shallower than hc affects the entire system through sand movement due to waves, causing shoreward movement of the profile as well as the formation of a scarp at the landward end. (2) Formation of new beaches in front of seawalls Consider the condition in which a seawall is built along the shoreline of a coast where longshore sand transport dominates, and where the location of the seawall coincides with the shoreline. Under this condition, we assume that continuous longshore sand transport is obstructed by the construction of a groin or artificial headland at the downdrift end of the beach. It is often observed on coasts that there is initially no foreshore in front of the seawall, but that sand accumulates, forming a foreshore in front of the seawall over time because of the obstruction of longshore sand transport by the groin or artificial headland. This phenomenon was difficult to simulate using the previous contour-line change model given by the authors, in which sand can only move in the longshore direction without cross-shore movement, because contours describing a dry beach appearing a new in front of a seawall were not included.

May 26, 2010

378

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

A calculation was carried out under the condition that waves were obliquely incident at a breaker angle of 13◦ on a beach with a uniform slope of 1/30. As a boundary condition, a solid boundary was assumed at both ends and the seawall extended along the shoreline. Because of oblique wave incidence, rightward longshore sand transport dominated. Figures 3.8.10(a)–3.8.10(c) show bathymetries at 14, 110, and 438 steps. Beach contours retreat in the left half of the center of the calculation zone and they advance in the right half over time. The formation of a planar beach with an elevation of 2 m is particularly noted, as shown by the shaded area in the vicinity of the right corner. This planar beach did not exist initially, but a berm 2 m high was formed for two reasons: sand accumulation triggered by the obstruction of longshore sand transport by the impermeable wall at the right end and shoreward sand transport associated with the formation of an excessively mild slope in front of the seawall. This sand accumulation zone extended with time. In addition, the sinking of sand into a zone deeper than hc is predicted off the sand accumulation zone on the right-hand side. (3) Beach changes around gently sloping revetment Figure 3.8.11 shows the predicted beach changes around a gently sloping revetment with a slope of 1/3 built on a coast with a foreshore slope of 1/10 and a foot coinciding with the shoreline position. Figures 3.8.11(a) and 3.8.11(b) show the experimental results obtained by Uda and Sakano (1991∗ ) and the simulated results, respectively. As for the conditions of the model experiment, the incident wave height, H0 , was set to 5 cm and the breaker angle was 10◦ . The calculation was performed scaled-up 100 times relative to the experimental scale. Calculated results reproduce well the recession of the contours with scarp formation on the downdrift side of the gently sloping revetment. Since the revetment is located downcoast of the sidewall that obstructs longshore sand transport, the sea bottom in front of the revetment was severely eroded and the contour 5 m deep merged into the foot of the revetment. The gradual exposure of the slope of the gently sloping revetment to waves is well reproduced, as indicated by the deepening of the seabed in front of the revetment.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

Fig. 3.8.10. Formation of new beach in front of seawall.

379

May 26, 2010

380

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.8.11. Beach changes around gently sloping revetment.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

381

(4) Meandering of contours off groins Figures 3.8.12(a) and 3.8.12(b) show the results of the experiment and the numerical simulations, where the experiment was conducted under conditions in which three groins were installed on a straight coast with an initial slope of 1/10, the equivalent deep-water wave height, H0 , was 3 cm, and the breaker angle was 10◦ to the normal of the initial shoreline.

Fig. 3.8.12. Beach changes off group of groins.

May 26, 2010

382

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

In the experiment, groin length was insufficient to totally obstruct longshore sand transport and some littoral transport occurred off the tips of the groins. Contours in the zone shallower than the depth of the tips of the groins were stabilized between groins when they were normal to the wave direction. In addition, a scarp was formed due to erosion immediately downcoast of the groins. It is noteworthy that meandering of contours off the tips of the groins was also reproduced. This prediction of meandering of offshore contours becomes possible only if a stabilization mechanism of the longitudinal profile is taken into account. The fundamental mechanism of this phenomenon was discussed first by Bakker (1968) using a two-line model. Before the construction of groins, the beach had an equilibrium profile. After the construction, the shoreline advanced (retreated) upcoast (downcoast) because of the obstruction of longshore sand transport, which caused the steeper (milder) slope. This resulted in offshore (shoreward) sand transport. Due to the effects of both these cross-shore and longshore sand transports off the tips of the groins, sand in the vicinity of the shoreline upcoast was transported offshore, around the tips of the groins, and it finally reached the vicinity of the shoreline downcoast. The meandering of the contour lines off the groins corresponded to this motion of sand. This phenomenon was well simulated by including the stability mechanism of the longitudinal profile in the contour-line change model. (5) Meandering of contours off detached breakwaters Figures 3.8.13(a) and 3.8.13(b) show the results of the experiment performed by Uda et al. (1987∗ ) and the numerical simulation of the beach changes around three detached breakwaters. Conditions at the sea bottom and incident waves were the same as those in the case of the groin. In the calculation of contour-line change behind the detached breakwaters, the distribution of the diffraction coefficient, Kd , and diffracted wave angle, θd , was obtained first by the angular spreading method developed by Serizawa et al. (1993∗ ), and wave height was reduced by multiplying the coefficient Kd with the breaker height without detached breakwaters. For the wave angle, the diffracted wave angle θd was used.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

383

Fig. 3.8.13. Beach changes around detached breakwaters.

Using these data sets, the longshore component of wave energy flux was calculated at each point, and the longshore sand transport rate was calculated using Eq. (3.8.10), adding the term given by Ozasa and Brampton (1980). The formation of cuspate spits behind the detached breakwaters and meandering of contour lines off the detached breakwaters under oblique wave incidence were well reproduced. (6) Change in sand budget caused by construction of detached breakwater in closed system Beach changes triggered by the construction of a detached breakwater on a coast with a closed littoral system, in which both ends are bounded by headlands, were studied on a model beach. Consider the case in which a 200 m long detached breakwater is built 100 m off a coastline 500 m in length. It is assumed that waves are incident perpendicular to the initial shoreline and hc is 9 m.

May 26, 2010

384

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Generally, construction of a detached breakwater induces sand deposition on the lee side of the detached breakwater due to its wavesheltering effect, resulting in erosion on the adjacent coast. To prevent this erosion, a groin is often built at the boundary between the detached breakwater and the adjacent beach to control the amount of longshore sand transport. Considering this condition, a groin with a point depth of 5 m was introduced at the center of the beach in the numerical simulation. Figure 3.8.14(a) shows the result after a sufficiently long time: 87,600 steps from the condition of the initial parallel contours. A cuspate foreland was formed due to the sand accumulation effect of the detached breakwater, and sand was supplied from the surrounding coast. Judging from only Fig. 3.8.14(a), it appears that beach changes were confined to the right-hand side of the groin. However, the distribution of topographic changes from the initial beach, as shown in Fig. 3.8.14(b), indicates that sand deposited on the lee side of the detached breakwater was supplied not only from off the detached breakwater, but also from the zone left of the groin. This deposition is due to the same mechanism as for the recovery of the offshore dredging hole. With sand accumulation behind the detached breakwater, beach erosion occurs in the zone shallower than hc in the zone right of the groin. This brings about a step in contours in front of the tip of the groin, causing longshore sand transport to turn to the right around the tip of the groin, which resulted in beach erosion on the left beach. Finally, the entire area shallower than hc was eroded. In other words, unless a long groin with a point depth equivalent to hc is built, sand can be transported away from the entire zone shallower than hc , and it is difficult to prevent sand discharge from the zone left of the groin. In this study, only one detached breakwater was built, but the same results should be obtained in the case that several detached breakwaters are built on the basis of the basic principle. This means that the construction of detached breakwaters in a closed system as a countermeasure against beach erosion induces sand loss in the surrounding zone, including the offshore zone, to compensate for sand accumulation on the lee side of the detached breakwaters as mentioned in Sec. 2.3. The decrease in seabed elevation in the erosion zone including

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

385

Fig. 3.8.14. Large-scale beach erosion triggered by construction of detached breakwater in closed littoral system.

the offshore zone induces an increase in the disaster potential relative to the condition before the construction of the detached breakwaters. As a result, if another detached breakwater is built in the newly formed erosion zone, beach erosion is accelerated further in the adjacent area. This result

May 26, 2010

386

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

shows that appropriate care must be taken in the construction of detached breakwaters in a closed littoral system (Figs. 3.8.15 and 3.8.16).

3.9. 3.9.1.

CONTOUR-LINE CHANGE MODEL SOLVED ON x–y MESHES Predictive Model

(1) Fundamental concept The application of the so-called three-dimensional model solving for depth changes on x–y meshes has been widely adopted, and it has the advantage that it is easy to handle the boundary conditions for various structures, and numerical calculation can be performed systematically. However, the application of a model of this type was restricted to a phenomenon occurring over a rather short term, such as beach changes during one storm or the seasonal beach changes associated with alteration in waves. The shoreline changes in which the eroded zone extends downcoast also cannot be predicted by this model. Here, the contour-line change model as described in Sec. 3.8 is further expanded. This model adopts the direct calculation method for depth changes on x–y meshes, as in the so-called conventional three-dimensional model, and it is applicable to the prediction of long-term beach changes (Serizawa et al., 2003∗ ). This model has two kinds of stabilization mechanisms in which each contour line becomes stable only if it is normal to the wave direction under the condition that cross-shore sand transport is neglected, and that the longitudinal slope becomes stable only if the local longitudinal slope approaches the equilibrium slope under the condition that longshore sand transport is neglected. Here, to confirm the ease with which the method can be handled in practical applications, the depth changes on x–y meshes are calculated in the same manner as with the conventional three-dimensional model. (2) Sediment transport equations The longitudinal profile changes due to cross-shore sand transport to be considered here are not the seasonal, short-period topographic changes due

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

387

Fig. 3.9.1. Coordinate system and definition of variables.

to the repetition of storm and calm waves, but changes with the time scale of long-term topographic changes due to longshore sand transport. First, the same concept as the contour-line change model described in Sec. 3.8 is applied on the x–y meshes. Figure 3.9.1 shows the coordinate system and the definition of the variables. Serizawa et al. (2002∗ ) took the stability mechanism of the longitudinal profile into account as cross-shore sand transport based on the equilibrium slope following Bakker (1968) in the contour-line change model. This study follows their model, and the model is expanded to be able to predict depth changes on x–y meshes. First, Cartesian coordinates (x, y) are selected. The sea-bottom elevation Z is a variable to be determined. For each point, a coordinate system (n, s) is defined: n- and s-axes are perpendicular and parallel to the each contour line, respectively. The cross-shore and longshore sand transport rates are expressed by qn and qs , respectively. The n-axis and qn assume positive values in the shoreward direction, qn is the cross-shore sand transport rate per unit length of the contour line, and qs is the longshore sand transport rate per unit length along the n-axis. The angles between the direction normal to the contour line and the x-axis and that between the wave direction at the breaking point and the x-axis are defined as θn and θw , respectively.

May 26, 2010

388

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Regarding the cross-shore sand transport rate, qn , we assume a onedimensional problem, in which waves are incident from the direction normal to the shoreline. According to Dean (1991), the longitudinal profile has an equilibrium shape, and if the longitudinal profile changes, a restoration force is exerted by cross-shore sand transport. By applying the same concept as that of Bakker as mentioned in Sec. 3.8, the stabilization mechanism restoring the equilibrium slope is included in the cross-shore sand transport rate, qn . When the seabed slope is equal to the equilibrium slope, tan βc , the downslope action due to gravity balances the upslope action due to waves, and the cross-shore sand transport rate is 0. If the seabed slope, tan β, is smaller than tan βc , qn becomes shoreward flux, and vice versa. Then, the variables qn and tan β are given by   tan β qn = A · 1 − , (3.9.1) tan βc tan β =

∂Z . ∂n

(3.9.2)

The coefficient A in Eq. (3.9.1) expresses the intensity of the restoration of the seabed slope to the equilibrium slope. Restoration of the seabed slope is considered to take place between the depth of closure, hc , and the berm height, hR , and restoration intensity A is proportional to the energy dissipation rate of the waves, φ, per unit seabed length, S, in the depth range between hc and hR . These are given by Eqs. (3.9.3) and (3.9.4), respectively,   (ECg )b (ECg )b 1 ¯ = = · (ECg )b sin βc , sin β ≈ (3.9.3) S hc + hR hc + hR S=

hc + hR h c + hR , ≈ ¯ sin βc sin β

(3.9.4)

where tan β¯ is the mean slope between the depth range of hc and hR , and the approximation of tan β¯ ≈ tan βc was applied.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

389

Since the beach changes dominate in the surf zone and in the vicinity of the shoreline and decrease in the offshore zone, the intensity of the crossshore sand transport has a distribution with depth. To express this effect, an intensity distribution function of cross-shore sand transport, εz (Z), is introduced. Considering that the term 1/(hc + hR ) in Eq. (3.9.3) expresses the intensity distribution function in the case with uniform distribution, this part is replaced by εz (Z). After further introducing a coefficient of crossshore sand transport rate, Kn , A and the cross-shore sand transport rate are given by Eqs. (3.9.5) and (3.9.6), respectively. In these equations, εz (Z), must be defined to satisfy Eq. (3.9.7). A = εn (Z) · Kn · (ECg )b sin βc ,

  tan β , qz = εn (Z) · Kn · (ECg )b sin βc · 1 − tan βc
hR εn (Z)dZ = 1. −hc

(3.9.5) (3.9.6) (3.9.7)

Regarding the condition of oblique wave incidence, the vector of the seabed slope can be expressed by Eq. (3.9.8), as shown in Fig. 3.9.1, and the absolute value and direction of the vector are tan β and the angle of the direction normal to the contour lines, respectively, where the shoreward direction is defined as the direction of gradually decreasing depth. In this case, the angle of the direction normal to the contour lines, θn , becomes Eq. (3.9.9), and the seabed slope, tan β, is given by Eq. (3.9.10). 

∂Z ∂Z , ∂x ∂y

 = (tan β cos θn , tan β sin θn ) , 

  ∂Z ∂Z θn = tan , ∂y ∂x ! ∂Z tan β = = (∂Z/∂x)2 + (∂Z/∂y)2 . ∂n −1

(3.9.8) (3.9.9) (3.9.10)

May 26, 2010

390

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

The value of tan β in Eq. (3.9.1) is given by Eq. (3.9.10). In the case that the water depth locally increases in the direction of wave propagation, tan θw , measured in the propagation direction of the breaking waves, assumes a negative value, and in this case we set tan β = 0. tan βw = cos θw

∂Z ∂Z + sin θw . ∂x ∂y

(3.9.11)

Regarding the determination of the coefficient A, (EC g )b in Eq. (3.9.5) must be changed to the shoreward component of the wave energy flux per unit length of the contour line using the breaker angle, αb , under oblique wave incidence. By multiplying the shoreward component of the wave energy flux, (EC g )b cos αb , by the length of the wave crest, | cos αb |, acting as the unit length of the contour line, A and the cross-shore sand transport rate are given by Eqs. (3.9.12) and (3.9.13), respectively, A = εn (Z) · Kn · (ECg )b · |cos αb | cos αb · sin βc ,

(3.9.12)   tan β , qn = εn (Z) · Kn · (ECg )b · |cos αb | cos αb · sin βc · 1 − tan βc (3.9.13) where αb is the angle between the direction normal to each contour line and the wave direction at the breaking point. αb = θw − θn .

(3.9.14)

In the numerical calculation, the seabed slope in the natural condition before the construction of the structures is given as tan βc . In the depth domains higher than the berm height and deeper than the depth of closure, the angle of repose of sand is also assumed. εn (Z) is assumed to be equal to the depth distribution, εs (Z), of the longshore sand transport rate proposed by Uda and Kawano (1996∗ ). As for the fundamental equation of longshore sand transport rate, qs , Ozasa and Brampton’s term, is added to Uda and Kawano’s equation (1996∗ ). Here, the definition of qs is slightly changed, compared with the previous contour-line change model. In the previous model, qs was defined as the longshore sand transport rate crossing the unit depth (dZ = 1),

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

391

whereas in this model expanding in the x–y plane, qs is defined as the longshore sand transport rate crossing the unit length in the cross-shore direction. In introducing the intensity function of longshore sand transport, some modifications are required. The increment of the depth, dZ, corresponding to the length dn in the cross-shore direction is geometrically connected by Eq. (3.9.15). Since εs (Z) expresses the ratio of the transport rate per unit depth relative to the total longshore sand transport rate in the depth range between −hc and hR , the ratio per unit length in the cross-shore direction becomes εs (Z) tan βc , and finally the equation of longshore sand transport rate, qs , reduces to dZ = dn · tan β ≈ dn · tan βc ,

(3.9.15) # K2 1 ∂Hb , qs = εs (Z) tan βc · Ks · (ECg )b | cos αb | sin αb − Ks tan βc ∂s (3.9.16) ∂ ∂ ∂ = − sin θn + cos θn , (3.9.17) ∂s ∂x ∂y "

where Ks and K2 are the coefficients of longshore sand transport and the additional term of longshore sand transport introduced by Ozasa and Brampton (1980), respectively. For the distribution function, εs (Z), the depth distribution of longshore sand transport rate proposed by Uda and Kawano (1996∗ ) is used, and the intensity function of cross-shore sand transport is also assumed to be equal to this distribution. Then we obtain


εn (Z) = εs (Z), hR

−hc

(3.9.18)

εs (Z)dZ = 1, εs (Z) = ε(Z)

(3.9.19) 


hR −hc

ε(Z)dZ,

(3.9.20)

    = 2 hc − Z (Z + hc )2 (−hc ≤ Z ≤ hR ) , h3c 2 ε(Z) =  = 0 (Z < −hc , hR < Z) . (3.9.21)

May 26, 2010

392

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Here, (EC g )b is given by the wave energy flux at the breaking point,  ρ g 5/2 (ECg )b = H (γb = 0.7), (3.9.22) 8 γb b where ρ is the seawater density, Hb is the breaker height, and g is the acceleration due to gravity. By taking the absolute value of the term cos αb in Eqs. (3.9.13) and (3.9.16), cross-shore sand transport is always directed along the direction of wave propagation without moving backward, even if the water becomes deeper in the direction of wave propagation. Regarding longshore sand transport, similar conditions are satisfied as for cross-shore sand transport. In addition, to avoid the occurrence of instability in the numerical simulation, the upper limit of |αb | is set to 60◦ . The sand transport fluxes in the x and y directions are calculated by transforming the longshore and cross-shore sand transport rates, qn and qs , using Eqs. (3.9.23) and (3.9.24). Finally, the depth can be obtained from the continuity equation, Eq. (3.9.25), in the x–y plane. qx = cos θn · qn − sin θn · qs ,

(3.9.23)

qy = sin θn · qn + cos θn · qs ,

(3.9.24)

∂Z ∂qx ∂qy =− − . ∂t ∂x ∂y

(3.9.25)

After some simplification and deformation, the fundamental equations reduce to two-dimensional diffusion equations expressed in the local coordinate system, (n, s). Therefore, this model has not only the stability feature that each contour line is stabilized only if it becomes normal to the wave direction at the breaking point when cross-shore sand transport is neglected, but also the stability feature that the longitudinal profile is stabilized only if the local slope becomes equal to the equilibrium slope when longshore sand transport is neglected as is the case in 3.8. In the calculation of beach changes in the wave-shelter zone behind the offshore breakwater, the wave diffraction coefficient, Kd , and the direction of diffracted waves, θd , are predicted by the angular spreading method, and the breaker height in the case with no offshore structure is reduced by multiplying by the coefficient Kd . On the other hand, the direction of

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

393

diffracted waves, θd , is used as the wave direction to evaluate the wave energy flux at any point. The calculation procedure, the predictive method for slope failure due to gravity, and the handling of the model in the vicinity of the wave run-up height, hR , and depth of closure, hc , are omitted here.

3.9.2.

Examples

(1) Prediction of beach changes around groin The calculation was carried out given the conditions in Table 3.9.1. Beach changes are triggered by oblique wave incidence on a coast with a groin under the condition that the initial incident angle is 10◦ and the breaker height is 3 m. Figures 3.9.2(a) and 3.9.2(b) show the bathymetry, sediment transport flux, and plane distribution of topographic changes for 100 steps. At the initial stage, strong sediment transport fluxes were generated. In particular, sediment transport turning around the tip of the groin became strong, whereas sediment transport flux was reduced on both sides of the groin. At the same time, sand accumulated on the left of the groin, and the beach was eroded on the right. Figures 3.9.3(a) and 3.9.3(b) show the results for 5,000 steps. Since the contour lines approached normal to the wave direction in the depth zone shallower than the tip of the groin, sediment transport flux was reduced, but still a concentrated sediment transport flux occurred off the tip of the groin. The beach changes occurred over almost the entire zone compared with the initial beach topography. Table 3.9.1. Calculation conditions.

Groin Kemigawa beach

Groin Kemigawa beach

hc (m)

hR (m)

1/ tan βc

1/ tan βg

Ks

9 4

2 1.5

20 40

2 Seabed : 5 Land : 2

0.2 0.2

K2 /Ks

Kn /Ks

X (m)

Y (m)

t (h)

0.0 1.5

0.2 0.2

20 10

40 10

1 0.5

May 26, 2010

394

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.9.2. Sediment transport flux and topographic changes around groin for 100 steps.

Figures 3.9.4(a) and 3.9.4(b) show the calculation results for 20,000 steps. The beach topography approaches a stable form, and sediment transport flux is gradually reduced with the increasing number of time steps. At this stage, the contour lines shallower than the depth of closure, within

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

395

Fig. 3.9.3. Sediment transport flux and topographic changes around groin for 5,000 steps.

which topographic changes can occur due to longshore sand transport, were stabilized to be normal to the incident wave direction, and a scarp was formed on the land upcoast, and a steep slope was formed due to sand sinking in the deep area off the accretion zone. Figure 3.9.5 shows the

May 26, 2010

396

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.9.4. Sediment transport flux and topographic changes around groin for 20,000 steps.

shoreline changes; the same kind of prediction as the ordinal one-line model is possible. The prediction of the process of topographic changes until a fully stabilized condition is reached was impossible in the so-called threedimensional model of beach changes, but this model makes it possible.

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

397

Fig. 3.9.5. Prediction of shoreline changes around groin.

Fig. 3.9.6. Sediment transport flux and topographic changes of Kemigawa beach for 100 steps.

May 26, 2010

398

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.9.7. Sediment transport flux and topographic changes of Kemigawa beach for 3,000 steps.

(2) Application to beach changes at Kemigawa beach Kemigawa beach is an artificial pocket beach 1,300 m long separated by curved groins at both ends. Given the straight contour lines before the construction of the curved groins as the initial topography, as mentioned in 2.3.5, waves of Hb = 1 m were assumed to be incident normal to the initial shoreline, and sediment transport flux and topographic changes after the construction of curved groins were calculated. Figures 3.9.6(a) and 3.9.6(b) show the bathymetry and sediment transport flux for 100 steps, respectively, as well as the changes from the

May 26, 2010

13:36

9in x 6in

b807-ch03

Practical Models for Predicting Beach Changes

399

Fig. 3.9.8. Seabed topography and sediment transport flux for 30,000 steps.

initial topography. At the initial stage, strong longshore flux was induced in the vicinity of the shoreline due to the wave-sheltering effect of the curved groins. Sand was transported behind the curved groins by this sediment transport flux, resulting in the advance of the contour lines and the formation of a scarp in the central part of the pocket beach, as shown in Figs. 3.9.7(a) and 3.9.7(b) for 3,000 steps. Finally, for 30,000 steps, sand that eroded from the central part of the pocket beach was transported toward the wave-shelter zone and deposited there, resulting in the formation of a very flat foreshore on the lee side of the curved groins, as shown in Figs. 3.9.8(a) and 3.9.8(b). Behind the curved

May 26, 2010

400

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

Fig. 3.9.9. Comparison of shoreline changes.

Fig. 3.9.10. Bathymetry of Kemigawa beach in 1999.

groin, a very steep slope was formed due to sand sinking into the deep zone. At this stage, the beach topography approached a final equilibrium form, so that sediment transport flux was almost stopped. Figure 3.9.9 shows a comparison of the shoreline configuration interpolated from the water depth at mesh points and the measured shoreline configuration. The predicted shoreline configuration for 3,000 steps corresponds very well with the measured value. Since good agreement was obtained between the measured and predicted values, the bathymetry

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

401

for 3,000 steps was selected and compared with the measured bathymetry. Figure 3.9.10 shows the measured bathymetry of Kemigawa beach in 1999. Except in the vicinity of the Y-shaped groin in the central part of the beach, which was completed in recent years, the measured values correspond well with the predicted value for 3,000 steps.

REFERENCES Bakker, W. T. (1968). The dynamics of a coast with groyne system, Proc. 11th Int. Conf. on Coastal Eng., pp. 492–517. Dean, R. G. (1991). Equilibrium beach profiles, characteristics and applications, J. Coastal Res. 7(1), 53–84. Goda, Y. (1985). Random Seas and Design of Maritime Structures (University of Tokyo Press, Tokyo), p. 323. Hsu, J. R. C. and C. Evans (1989). Parabolic bay shapes and applications, Proc. Int. Civ. Eng., Part 2, 87, pp. 557–570. Kamphuis, J. W., M. H. Davies, R. B. Narim and O. J. Sayao (1986). Calculation of littoral sand transport rate, Coastal Eng. 10, 1–12. Kraus, N. C. (1984). Estimation of breaking wave height behind structures, J. Waterway, Port, Coastal Ocean Eng. 110(2), 276–282. Kraus, N. C. (1985). Field experiment on vertical mixing of sand in the surf zone, J. Sedimentary Petrology 55, 3–14. Kumada, T., A. Kobayashi, T. Uda and T. San-nami (2002). Field observation of three-dimensional changes of artificial beach and application of expanded Hsu model — The example of Kemigawa beach in Chiba Prefectutre, Japan, Proc. 28th ICCE, pp. 3711–3723. Ozasa, H. and A. H. Brampton (1980). Mathematical modelling of beaches backed by Seawalls, Coastal Eng. 4, 47–64. Uda, T., M. Serizawa, T. Kumada and K. Sakai (2010). A new model for predicting three-dimensional beach changes by expanding Hsu and Evans’ equation, Coastal Eng. 57(2), 194–202.

May 26, 2010

402

13:36

9in x 6in

b807-ch03

Japan’s Beach Erosion: Reality and Future Measures

REFERENCES (in Japanese) Fujita, K., T. Uda and A. Hattori (1995). Effective grain size population required for analysing sediment budget of a watershed, Civil Eng. J. 37(12), 34–39. Hirano, M. (1971). Riverbed degradation with armoring, Proc. JSCE 195, 55–56. Ikeno, M., T. Shimizu, E. Kobayashi, T. Ishii and T. Saito (2001). Application of three-dimensional beach change model in constructing port on beaches with exposed bedrock, Ann. J. Coastal Eng. JSCE 48, 561–565. Kumada, T., A. Kobayashi, T. Uda, M. Serizawa, Y. Hoshigami and K. Masuda (2002a). Development of a model for predicting beach changes considering sorting of sand of mixed grain size, Proc. Ann. J. Coastal Eng., JSCE 49, 476–480. Kumada, T., A. Kobayashi, T. Uda, M. Serizawa, T. San-nami and K. Masuda (2002b). Model for predicting shoreline configuration and change in grain size of a river-mouth delta — Reproduction of sorting effect by longshore sand transport, Ann. J. Coastal Eng. JSCE 49, 481–485. Sakai, K., A. Kobayashi, T. Uda, M. Serizawa and T. Kumada (2003). Model for predicting topography of three-dimensional, statically stable beaches on a coast with wave-sheltering structures, Ann. J. Coastal Eng. JSCE 50, 496–500. Sakai, K., A. Kobayashi, T. Kumada, M. Serizawa, T. Uda and T. San-nami (2002). Prediction of topography of statically stable beaches with seawall on a pocket beach by three-dimensional Hsu model, Ann. J. Coastal Eng. JSCE 49, 631–635. Serizawa, M., A. Rabil, T. San-nami and H. Gomi (1993). Simple calculation method of irregular wave field in wave-diffraction zone, Ann. J. Coastal Eng. JSCE 40, 76–80. Serizawa, M., T. Uda, T. San-nami, K. Furuike and Y. Kanda (1996). A method for predicting optimum stable shoreline around a headland by applying modified Hsu model, Ann. J. Coastal Eng. JSCE 43, 646–650.

May 26, 2010

13:36

9in x 6in

Practical Models for Predicting Beach Changes

b807-ch03

403

Serizawa, M., T. Uda, T. San-nami, K. Furuike and Y. Kanda (2000). Expansion of Hsu method to a model for predicting three-dimensional beach changes, Ann. J. Coastal Eng. JSCE 47, 601–605. Serizawa, M., T. Uda, T. San-nami, K. Furuike and T. Kumada (2002). Contour-line change model considering stabilization mechanism of longitudinal profile, Ann. J. Coastal Eng. JSCE 49, 496–500. Serizawa, M., T. Uda, T. San-nami and K. Furuike (2003). Prediction of depth changes on x–y meshes by expanding contour-line change model, Ann. J. Coastal Eng. JSCE 50, 476–480. Tanaka, H. and M. Suzuki (1998). Predictive model of shoreline change and grain-size sorting, Ann. J. Coastal Eng. JSCE 45, 511–515. Uda, T., A. Omata and K. Yamamoto (1987). Experimental report on controlling of longshore sand transport using groins and detached breakwaters, Technical Notes of PWRI, No. 2507, p. 39. Uda, T. and A. Sakano (1991). Beach erosion on downcoast of gentlysloping revetment on a coast with predominance of longshore sand transport and measures, Civil Eng. J. JSCE 33(2), 37–42. Uda, T. and S. Kawano (1996). Development of contour-line change model for predicting beach changes, Proc. JSCE, No. 539/II-35, pp. 121–139. Uda, T. (1997). Beach Erosion in Japan, Sankaido Shuppan, Tokyo, p. 442.

May 26, 2010

13:36

9in x 6in

This page intentionally left blank

b807-ch03

May 26, 2010

13:36

9in x 6in

b807-ch04

Chapter 4 BEACH EROSION AS STRUCTURAL PROBLEM

4.1.

INTRODUCTION

As mentioned in detail in Chap. 2, the sandy beaches in Japan are rapidly changing into artificial coasts, and man-made scenery covered with concrete structures is expanding very rapidly. Most long stretches of beach, characterized for a long time by white sandy beaches and pine tree forests, have disappeared. As a result, there is concern among the populace about the rapid deterioration of the coast. Issues related to beach erosion have been occurring predominantly since the 1960s, but they became severe in particular during the high economic growth era since the 1970s. Since then, momentum has continued following the bubble economy of the 1990s in Japan. Large budgets have been spent every year, and many kinds of countermeasures have been implemented. Similarly, in coastal engineering, much research has been conducted every year, and models for predicting beach changes have been developed. Nevertheless, it seems that beach erosion problems are becoming more serious throughout Japan’s coasts. Taking these points into account, we must assume that huge mistakes were made in the methods for coping with the problems, as well as in the research. When we consider that finding the solution to beach erosion problems appears to be very difficult, despite the scientific research carried out, coastal researchers also have other concerns. To begin with, taking part in the research, the purpose of which is questionable, is arduous and meaningless for researchers. Finally, the participation of young researchers in coastal engineering projects will not be expected. The cause of the recent large deformation of the coast relates closely to the inertia of the system in the past. Accordingly, the continuation of past methods simply leads to no change in the present situation. Here, regarding 405

May 26, 2010

406

13:36

9in x 6in

b807-ch04

Japan’s Beach Erosion: Reality and Future Measures

the issues that obstruct finding a solution to the problems, discussions from the viewpoint of the true nature of the problem are carried out as much as possible, although comprehensive discussion is difficult.

4.2. 4.2.1.

INSTITUTIONAL (LEGAL) ISSUES RELATED TO BEACH EROSION Occurrence of Issues at Boundaries Between Shore Protection, Port, and Fishing Port Areas

The construction of ports and fishing ports has been carried out on the basis of the Port Act and the Fishing Port Act, respectively, and each project was optimized locally; this is the so-called sector-by-sector system of administration. It is only natural that when a breakwater is extended on a coast with predominant longshore sand transport, sand accumulates on the updrift side of the structure and the downcoast shoreline recedes. Similarly, when a wave-shelter zone is formed by extending an offshore breakwater, sand accumulates inside the wave-shelter zone and the shoreline retreats in the area surrounding the port. Thus, the fundamental problem grows from the human aspect. Land in the vicinity of the coastline in Japan is legally controlled by various laws, but the fundamental concept of laws requires a system in which the divided national land is governed by only a single coastal manager. In other words, the value of the moving sand itself is not grasped dynamically, but the national land is constantly preserved only from its static perception, resulting in a preference for the construction of hard structures. The construction work carried out by public organizations exclusively controlling a coastal zone are all based on related laws, but the work has been conducted most effectively to accomplish the single target expressed in each law. The era of high economic growth in Japan was exactly the period in which actions were concentrated. For example, consider a case in which an offshore breakwater forming a large wave-shelter zone is extended in a port or a fishing port. Even if beach erosion occurs on the surrounding coast in response to the extension

May 26, 2010

13:36

9in x 6in

Beach Erosion as Structural Problem

b807-ch04

407

of the breakwater, as mentioned in Chap. 2, the construction project of the port itself may advance rapidly, as long as it does not encroach on other projects that another coastal management office controls. The influence of building such a structure on the surrounding coast can be quantitatively predicted using one of the models described in Chap. 3. However, even if the influence of a project reaches outside the project area, the manager may simply consider that the event occurred outside his own management area, so that the measures against beach erosion should be taken by the other organization that has jurisdiction. For these reasons, the problems have not yet been solved. Such situations have continued for long periods in the past, and the administrator of a port or a fishing port showed no interest in the phenomena occurring outside his own management area. Managers considered that the work in each coastal management area should be carried out by each manager of the coast, and that sand deposited in the wave-shelter zone of the breakwater was an obstacle and not a valuable resource for the surrounding coasts. Accordingly, the administrators of a port or a fishing port did not build the structures on the basis of the thinking that their work might exert a negative effect on the surrounding coastline, but were simply indifferent to the condition of the surrounding coasts. This gave rise to serious problems. Similarly, when predominant longshore sand transport was obstructed by port breakwaters and downcoast erosion takes place, the coastal manager of the eroded coast had to go on the defensive, and the work of restoring the damaged facilities has simply been repeatedly carried out. At the same time, the coastal manager of the eroded coast was accustomed to consider that the erosion occurred locally due to offshore sand movement, and therefore believed that taking measures to attenuate waves was sufficient, instead of considering the erosion from a comprehensive point of view. The accumulation of the effects of local optimization being carried out under these circumstances aggravated the problems, outweighing the benefits gained from improvements, and since fundamental measures were not on the basis of their essence, beach erosion problems repeatedly occurred at the coast.

May 26, 2010

408

13:36

9in x 6in

b807-ch04

Japan’s Beach Erosion: Reality and Future Measures

Furthermore, sand accumulated on the updrift side of breakwaters or sand deposited inside the navigation channels was extensively used as the material for land reclamation at a number of ports and fishing ports, resulting in a decrease in sand volume at the coasts surrounding the ports. Almost all coastal land, except for a few privately owned areas, is public land controlled by the offices of the Central or Prefectural Government in Japan. Therefore, the administrator of the coast where erosion has taken place found it difficult to formally complain to other administrators controlling an accretion zone regarding the true cause of the beach erosion, because both administrators belong to the same organizations of the Central or Prefectural Government. Theoretically, it is impossible for a government office to take an event to court against another government office, and it is also difficult for people to sue the administration managing public property that is coastal land.

4.2.2.

Relationship Between Dredging Operations and Beach Erosion

Consider a case where the wave-shelter zone due to an offshore breakwater is used as a navigation channel or anchorage, as shown in Fig. 4.2.1. In such a case, dredging is commonly carried out to maintain sufficient channel depth on the basis of the standards designated in the Port Act and Fishing Port Act. When the offshore breakwater or the jetty that prevents sand deposition inside the port is not deep compared with the depth of closure, hc , of sand movement by waves, sand is transported again by littoral transport inside the port due to the wave-sheltering effect of the offshore breakwater after dredging. This in turn causes a decrease in the sand volume of the beaches surrounding the ports. Similarly, to prevent closure of a river’s mouth or to maintain a navigation channel of a small fishing port at a river mouth, dredging has long been the standard procedure, particularly at small rivers, the number of which is large nationwide. Taking these characteristics into consideration, when dredging is carried out according to the general standards of ports, fishing ports, and rivers, and dredged sand is disposed independently by each management authority, the

May 26, 2010

13:36

9in x 6in

b807-ch04

Beach Erosion as Structural Problem

409

Fig. 4.2.1. Impact of dredging navigation channel on surrounding coasts.

ultimate result is beach erosion at the surrounding coasts (Serizawa et al., 2005). Arguing that countermeasures against beach erosion are required without changes in this situation is ineffective. The adoption of countermeasures without solving the fundamental causes of beach erosion is tantamount to declaring that “we will alter natural sandy beaches into artificial coastlines with many man-made structures.” Sand deposited in the wave-shelter zone is a nuisance, but the same sand is a valuable resource on the adjacent coasts. It is required not only in terms of its protective function, but also to preserve a healthy coastal ecosystem. Taking these facts into consideration, it is clear that dredged sediment must be returned to the surrounding coasts by appropriate methods with the mutual understanding among different management authorities concerned with beach changes. Hereafter, various standards related to the works at ports, fishing ports, and rivers should be revised to include concrete measures for solving this problem.

4.2.3.

Issues Arising from Conceptual Differences in Land Management by Coastal Act and Forest Law

In the shore protection work in Japan, the coastal protection zone is basically defined as the range between a location 50 m landward from the shoreline at high tide and 50 m seaward from the shoreline at low tide on the vernal

May 26, 2010

410

13:36

9in x 6in

b807-ch04

Japan’s Beach Erosion: Reality and Future Measures

equinox. In recent years, beach erosion has become severe along Japan’s coastline, resulting in shoreline recession, whereas the coastal forest area has advanced seaward excessively, as mentioned in Sec. 2.7. Previous natural sand dune areas have been rapidly lost from the landward side, resulting in the disappearance of the buffer zone against extraordinary variations in the shoreline position. Furthermore, earth dikes and seawalls to guard the coastal forest have been built. Based on the Forest Law, the protection of the coastal forest is absolutely right and meaningful, and the protective work has been carried out on the basis of the concept that the foreshore in front of the coastal forest is not needed because it causes wind-blown sand. On the other hand, in the shore protection zone next to the coastal forest, various measures have been taken on the basis of the concept of the revised Coastal Act, that the sandy beach is very important and must be preserved. A line invisible for people in the vicinity of the shoreline becomes a boundary at which the aims of the Coastal Act and Forest Law contradict each other. Discussion of the basic idea of preserving sandy beaches, as a common property of the people, and of methods of adjustment is required. The solution to these problems has been obstructed not only by the sector-by-sector administration system, but also by a sector-by-sector approach in the science evaluating the coastal zone. Engineers and scientists must exchange their views among the different scientific societies and modify them if necessary. As a result of the excess differentiation of the sciences, the comprehensive judgment necessary for the community has become difficult to achieve.

4.2.4.

Issues Related to Method by Which Public Sectors Expend Their Budgets

Out of the countermeasures against beach erosion on a coast where predominant longshore sand transport is blocked by a structure have arisen two well-known methods that do not involve constructing hard structures: sand bypassing and sand back passing (sand recycling). These methods have been used for a long time in other countries as the way to maintain the

May 26, 2010

13:36

9in x 6in

Beach Erosion as Structural Problem

b807-ch04

411

natural system of sand movement. However, in Japan, they were usually experimental and ended in a rather short period, because the Financial Ministry disliked their huge maintenance costs in the long term. The revised Coastal Act was put into operation in 2000, requiring a well-balanced policy between concerns for shore protection, environment, and use; at the same time, the Ministry of Land, Infrastructure, and Transport published a new policy of sediment management in watersheds that included the coast. In spite of these new policies, the real administration runs into the formidable barrier of the sector-by-sector system arising from the management system for public land. In the development of a sand bypassing system between management offices, cost allocation for the project is usually requested by the manager of the eroded coast on the basis of the principle that beneficiaries should pay for a project. However, this often becomes difficult because of the shortage of budget in the management office of the eroded coast. This point also becomes a barrier for adopting the sand bypassing method because the erosion area and its severity expand with time and further measures are needed.

4.2.5. Work of Restoring Damaged Coast With the rapid expansion of erosion, it becomes difficult to take measures with only the ordinary budget in each year distributed according to the plan; the restoration work is usually carried out on the basis of the damage restoration system, in which the budget can be spent only after the occurrence of damage, and preventive actions cannot be taken legally. Thus, even though we know that beach erosion expands alongshore, preventing the erosion from expanding becomes difficult. Countermeasures against beach erosion are delayed, and the coast is further devastated. In this case, the budget for damage restoration itself is not troublesome, but the problem is in the budgetary system for the restoration, in which the ranges of expenditure are very limited when it comes to achieving the real goal of preventing beach erosion. This mismatch among the aim of the construction, the measures, and the budget is the real problem, and this must be solved at the policy level.

May 26, 2010

412

4.2.6.

13:36

9in x 6in

b807-ch04

Japan’s Beach Erosion: Reality and Future Measures

System of Administration and Difficulty of Training of Specialists

Engineers in both central and prefectural government offices engaged in coastal work usually change their positions every two years in Japan. They deal with projects for an insufficient time to study the phenomena of the coast. An engineer may return to the same coastal project at a higher position, but sometimes the misunderstanding may arise that he has sufficient knowledge of coastal engineering from his past experience on coastal projects, even though he may have studied coastal engineering insufficiently during his past service. In reality, he may not be experienced enough. In the current system, it is difficult to produce an engineer who can truly bear the responsibility and comprehensively consider the future growth of the coasts. The education system of true engineers in public positions must be considered.

4.3. TECHNICAL ISSUES RELATED TO BEACH EROSION When referring to a textbook of coastal engineering published a decade ago, we usually find the statement that the mechanism of littoral drift is so complicated that future studies are expected. Every year, sufficient results of studies of coastal engineering have been published in Japan to satisfy this concern, but even now, many specialists in coastal engineering have made the same statement. However, the influence of artificial alterations on surrounding coasts can be easily understood through the comparison of the coasts concerned with many examples of eroded coasts in Japan, as mentioned in Chap. 2. From this, it can be said that out of the severely eroded coasts in Japan, there are no coasts where the cause of beach erosion is totally unidentified. At least, the quantitative evaluation of longshore sand transport required to identify various suitable measures against beach erosion is feasible, and we can obtain solutions with a significantly high accuracy using various practical numerical models described in Chap. 3. Nevertheless, the

May 26, 2010

13:36

9in x 6in

Beach Erosion as Structural Problem

b807-ch04

413

explanation that the problem is so complicated that further research works are required is irresponsible, and rather it can be said that it prevents the problems from being solved. Since the final goal of coastal engineering is to solve the problems of the coast, the main purposes of coastal engineering should be stated in that fashion. Currently, in coastal engineering in Japan, a large number of research themes have been essentially separated from the real problems occurring on coasts, and therefore the discussion regarding the identity of the problem is lacking and discussions of minor relevance, such as how to fix the coefficient in a numerical model, are undertaken instead. Even though such studies have been eagerly carried out, it is obvious that they are not useful for solving the real problems. When a researcher in coastal engineering is asked what the merit of the development of the numerical simulation model is, he will answer that the model can be used to predict changes of a beach, for example. However, if the beach erosion has already occurred because of an institutional, structural problem, this answer will not provide the real solution, and the problem remains far from being solved. Taking these points into account, serious discussions looking at the reality of the coasts and aimed at the real problem are required.

4.4.

CONCRETE MEASURES

As mentioned earlier, there is no doubt that the issues of beach erosion have not arisen only from the basics of coastal engineering, but they relate deeply to the social system. Ultimately, there is even the possibility of addressing the argument that coastal engineering is unnecessary. Even if many coastal researchers carry out studies on the fundamental, basic technology, when the real coasts are devastated and the situation has been worsened up to a condition for which any measures can no longer be taken, it is reasonable that the social significance of the research work will eventually be lost. On the other hand, that researchers in coastal engineering realize deeply the essence of the issue is another fact. The researchers and engineers working in coastal engineering must actively urge the public that measures effective to solve the problems must be taken, including not only the technical problems, but also the institutional problems.

May 26, 2010

414

13:36

9in x 6in

b807-ch04

Japan’s Beach Erosion: Reality and Future Measures

In either the technical or institutional discussion, we must be careful to acknowledge that really solving the issues is important, instead of arguing for argument’s sake. Eventually, the existence of the issues must be clearly discussed at every opportunity, and it is the obligation of coastal researchers and engineers to urge that, when we fail to cope with the problem, a large debt will be left for the next generation. With respect to the artificial coasts now, it is difficult to enhance people’s interest in the coasts; many people may forget the issues of beach erosion, thus losing the opportunity to solve the problem at an early stage. Nevertheless, in order to eliminate this vicious circle, it is very important to have as many people as possible realize that leaving the problem unaddressed is unwise. In addressing the complicated issues, including the structural or institutional problems, it is obvious that the continuation of the past way of thinking has limitations. After the efforts to make many people recognize these issues, including the reform of the institutional framework, the adoption of a policy to really preserve sandy beaches as a common property of the people is required. For this purpose, regarding the problems between the Forest Law and the Coastal Act, for example, the enactment of a basic land management law is required, which will demonstrate the future direction of the method of adjusting land management and provides a common goal. Further discussions on how land use in the vicinity of the coastline should be adjusted and how this coastal land should be protected as the common property of the people are needed without delay. Indeed, it is true that the duty of researchers is to study and to produce good results, and there is no formal connection with the structural problems related to the administrative system. However, in the meantime, when sandy beaches disappear and the healthy coastal zone is devastated as a fishing ground, researchers in coastal engineering will be said to have been socially irresponsible. To leave a sound coastal environment to the next generation, it is necessary for people to know the real situation on Japan’s coast, and a new movement must be initiated regardless of the past framework. It is very difficult

May 26, 2010

13:36

9in x 6in

Beach Erosion as Structural Problem

b807-ch04

415

for the present conditions to be changed only by the administration’s efforts. Through cooperation among specialists in coastal engineering, engineers in public offices, and civilians, a system which can be modified within society, if necessary, should be developed, instead of depending on the sector-by-sector system to break the closed system by exchanging useful information among the participants. Facts are not known by the public, and incorrect explanations have been accepted. However, social conditions are changing. New laws, such as the freedom of information law and the Coastal Act, require that information must be opened to the public and, based on that information, necessary policies must be set up. It is required that the problems be solved on the basis of the perseverance that is deeply rooted in this thinking. Instead of accusing each coastal manager of responsibility regarding past events, a method of setting-back of the coastline should be considered in some cases to create a sound coastal zone, in place of the idea that the present coastline should be obstinately protected by hard structures, on the basis of the solid, long-term perspective. For this purpose, the opening of information to many people becomes vitally important, because the future of the coast as a part of the national land must not be determined only by the judgment of part of the people, and measures satisfactory to many people should be adopted. Taking into consideration that sediment deposited in ports or fishing ports has been transported originally from the adjacent coastal zones, it is natural that such sediment be returned to the original coastal zone. Throwing the dredged materials away in deep water leads to the net loss of material forming the country, because it is never expected that dumped sand will return by wave action. This behavior must be stopped to prevent the net loss of sand. It is also useful to maintain healthy fishing grounds in coastal waters. In this case, another problem arises. As mentioned in Sec. 3.6, in the wave-shelter zone behind an offshore breakwater, fine sediment is selectively transported from the surrounding coasts deposits, and therefore when beach nourishment is carried out using this fine sediment in the eroded

May 26, 2010

416

13:36

9in x 6in

b807-ch04

Japan’s Beach Erosion: Reality and Future Measures

zone, the nourished fine sand is quickly washed away compared with that in the initial stage of beach erosion, because the stability of fine sand is considered to have been lost already due to the steepening of the seabed slope. Accordingly, in the reuse of dredged sediment, the sorting effect of sand composed of grains of various sizes must be sufficiently taken into account, instead of simply assuming that deposited sand may be returned to the eroded coast. This is one of the technical problems that remain to be solved.

REFERENCE Serizawa, M., T. Uda, T. Kumada, T. San-nami and K. Furuike (2005). Beach erosion caused by dredging of navigation channels at ports and river mouth, Proc. 14th Biennial Coastal Zone Conf., New Orleans, Louisiana, pp. 1–5.

May 26, 2010

13:36

9in x 6in

b807-index

INDEX

accretion waves, 167 Ajigaura beach, 100 angular spreading method, 331 Arahama coast, 81, 84 Ariake–Takahama coast, 216 artificial headland, 169, 312 artificial island, 153 Asamogawa coast, 130 Atsuga fishing port, 21

Fishing Port Act, 406 fluvial sediment supply, 10, 162 Forest Law, 279 Fuji coast, 55 Fukude fishing port, 62 gently sloping revetment, 13, 27, 34, 48, 82, 146, 181, 245, 284, 295, 297, 302, 304, 380 Ghotsu coast, 230 gravity effect, 365 groin, 381, 396

barrier island, 68 beach nourishment, 70 beach profile changes, 175

hard structure, 406 Heisa-ura coast, 269 Hsu and Evans’ model, 311, 315

Coastal Act, 98, 279 coastal forest, 12, 78, 136, 236, 239, 256, 270 coastal forest zone, 98 coastal management area, 98 concrete armor units, 168 contour-line change model, 370 cross-shore sand transport, 365

Ichinomiya River, 265 Imazu-sakano coast, 64 Institutional issues, 406 Isewan-seinan coast, 285 Kashiwabara coast, 151 Kashiwazaki Port, 81 Kemigawa beach, 109, 329, 398 Kitanowaki coast, 292

damage restoration system, 411 depth of closure, 158 detached breakwater, 32, 180, 215, 220, 230, 339, 383, 385 dredging, 11, 199, 326, 376, 408

land management, 409 longshore sand transport, 8, 19, 173, 367, 406

effective grain size population, 359 Enshu-nada coast, 191 equilibrium slope, 370 ergodic rationale, 289 erosion process, 227 erosion wave, 164, 167 exchange layer, 343

median diameter, 226 Misawa fishing port, 23 mixed grain size, 342 mixing depth, 345 Momosaki-hama coast, 29 417

May 26, 2010

418

13:36

9in x 6in

b807-index

Japan’s Beach Erosion: Reality and Future Measures

Monbetsu coast, 19 Monbetsu fishing port, 22 Nabaki River, 264 Naka coast, 105 Nakamura-hama coast, 237 Niyodo River, 200 Node coast, 249 offshore sand mining, 11 Ohtsu fishing port, 93 pocket beach, 111, 118, 135, 142, 273, 338 Port Act, 406 prediction of stable shoreline, 311 restoration work, 99 river mouth delta, 10, 78, 352 river mouth terrace, 193 riverbed excavation, 164 Sagami River, 198 Sakuma Dam, 191 sand bypassing, 70 sand dune, 270 sand mining, 50 sand spit, 66, 209 scarp, 28, 36, 38, 80, 108, 116, 126, 223, 247 sea bottom change, 91

seawall, 97, 107, 186, 260, 261 sector-by-sector system, 406 sediment transport flux, 394 Shibetsu coast, 16 Shimizu coast, 162 Shimobara fishing port, 121 Shinkawa fishing port, 39 Shiratsuru beach, 141 Shizuoka coast, 162 shoreline changes, 16, 20, 25, 41, 56, 77, 113, 121, 124, 131, 137, 154, 167, 221, 231, 252 statically stable beach, 331 structural problem, 405 Suruga coast, 186 Tenryu River, 191 Teradomari coast, 73 three-dimensional beach changes, 327 Tojo–Maebara coast, 117 Tsutsuki beach, 133 types of beach erosion, 7 Uchihama coast, 297 wave diffraction coefficient, 332, 340 wave shelter zone, 9, 92, 129, 319, 406 wave sheltering structures, 8, 73, 158 wave-dissipating structures, 46, 87 wind-blown sand, 51