Environmental Geomorphology (Developments in Earth Surface Processes 4)

in Earth Surface Pro 4 ELSEVIER SCIENCE B. V. Sara Burgerhartstraat 25 P.O. Box 211, I 000 AE Amsterdam. The Netherla...

Author: Mario Panizza

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in Earth Surface Pro

4

ELSEVIER SCIENCE B. V. Sara Burgerhartstraat 25 P.O. Box 211, I 000 AE Amsterdam. The Netherlands

Library of Congress Cataloglng-ln-Publtcatton Data

Panizza, Mario, geomorphology 1 Mario Panizza. p. cra. -- and 1 ndex. ISBN 0-444-89830-7
ISBN 0-444-89830-1

© 1996 Elsevier Science B. V. All rights reserved. No part of this publication may be reproduced. stored in a retrieval system or transmitted in any form or by any means, electronic, mechanicaL photocopying. recording or otherwise. without the prior written permission of the publisher, Elsevier Science B. V .. Copyright & Permissions Department. P.O. Box 521. I 000 AM Amsterdam. The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc . (CCC). 222 Rosewood Drive. Danvers. MA 01923 . Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A .. should be referred to the publisher. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise. or from any use or operation of any methods. products. instructions or ideas contained in the material herein . This book is printed on acid-free paper. Printed in The Netherlands

Vll

Foreword Quippe ita formido mortalis continet omnis, quod multa in terris fieri caeloque tuentur quorum operum causas nulla ratione videre possunt ac fieri divino numine rentur. Quas ob res ubi viderimus nil posse creari de nihilo, turn quod sequimur iam rectius inde perspiciemus, et unde queat res quaeque creari et quo quaeque modo fiant opera sine divum. (Lucretius Titus, Caro: De rerum natura, liber I, 151-158)

This book was written in response to the growing interest of public opinion in environmental topics and by the increasing opportunities for applying Geomorphology in this field. In particular, the specific and sometimes distinctive role of Geomorphology in the study and the solution of some problems was considered as an important aspect to be emphasized. Indeed this discipline has now reached levels of methodology, scientific content and concrete results which make it worthy of being considered as a major aspect of environmental research. It is important for the most significant and decisive geomorphological components and indicators in the environment's integrated system to be assessed and applied from time to time. After all, it is necessary to have the right dialogue and relationship with the other disciplines making up this system so as to apply the most suitable methodologies and offer the most valid solutions. In preparing this text, priority has been given to both methodology and illustrative case histories. For the former the starting point was a conceptual framework, integrated by a methodological proposal and scientific approach, trying to avoid schemes and classifications that would be ill-suited for a naturalistic, empirical and nonsystematic discipline like Geomorphology. For the latter several examples derived from concrete or personal research and work experiences were quoted. The concepts outlined in the text are based on a subdivision of geomorphological resources and hazards, their links with Man and the consequent risk and impact problems. It is indeed obvious that each investigation, study, intervention concerning the environment cannot ignore either the human context in which it occurs or Man's history and prospects. Today's world is experiencing great scientific development, cultural challenges as well as ethical and social crises, hence the greater need to go back to some form of cultural unity. This can be achieved by means of integrated cooperation between humanistic, scientific and technological disciplines which will

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allow the comprehensive nature and significance of the environment system to be fully understood. Lastly, this will make us aware of the need to protect and properly manage the territory in which we live. Environmental reality is not simply made up of many elements added together but results rather from the interaction of various systems. This implies, today more than ever, that investigations should be carried out bearing in mind the planetary implications of problems, in order to properly direct the necessary energies to find solutions. The volume is addressed both to university students, who have to study topics of Geomorphology as part of their syllabus, and to researchers and consultants (geologists, geographers, engineers, naturalists, etc.) who work in the field. For some of the subjects discussed I made use of the collaboration of some colleagues who are solely responsible for these parts. To them I wish to express my warmest gratitude and appreciation. These collaborations are specified in a special Hst. I wish to thank Milena Bertacchini and Marco Coltellacci for their special assistance in computer elaboration management. My special, affectionate thanks to Sandra for her irreplaceable moral and scientific support. Mario Panizza

IX

LIST OF CONTRIBUTING AUTHORS

For 2.3 and 6.4 Mauro MARCHETTI

Dipartimento di Scienze della Terra, Universita degli Studi di Modena, Largo S. Eufemia, 19, 41100 Modena, Italy

For 3.2

Giuliano RODOLFI

Dipartimento di Scienza del Suolo e, Nutrizione della Pianta, Universita degli Studi di Firenze, Piazzale delle Cascine, 15, 50144 Firenze, Italy

Dino TORRI

Consiglio Nazionale delle Ricerche, Centro per la Genesi, Classificazione e Cartografia del Suolo, Piazzale delle Cascine, 15, 50144 Firenze, Italy

For 3.3 Alessandro PASUTO

Mauro SOLDATI

Consiglio Nazionale delle Ricerche, I.R.P.I., Bacini Italia nordorientale, Corso Stati Uniti, 4, 35020 Camin — Padova, Italy Dipartimento di Scienze della Terra, Universita degli Studi di Modena, Largo S. Eufemia, 19, 41100 Modena, Italy

For 3.4.2 Franca MARAGA

Consiglio Nazionale delle Ricerche, I.R.P.I., Bacino padano., Strada delle Cacce, 73, 10135 Torino, Italy

For 3.5.2 Paolo ORRU and Antonio ULGEZA

Dipartimento di Scienze della Terra, Universita degli Studi di Cagliari, Via Trentino, 51, 09127 Cagliari, Italy

For 3.6 Alberto CARTON

Dipartimento di Scienze della Terra, Universita degli Studi di Torino, Via Accademia delle Scienze, 5, 10123 Torino, Italy

For 3.7.4 Doriano CASTALDINI Dipartimento di Scienze della Terra, Universita degli Studi di Modena, Largo S. Eufemia, 19, 41100 Modena, Italy For 4 and 5.4 Sandra PIACENTE

Dipartimento di Scienze della Terra, Universita degli Studi di Modena, Largo S. Eufemia, 19, 41100 Modena, Italy

xi

Contents Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of contributing authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii IX

1 INTRODUCTION TO ENVIRONMENTAL GEOMORPHOLOGY ..... . 1.1 1.2 1.3 1.4 1.5 1.6

Definition of Geomorphology . Geomorphology and Geology . Geomorphology as a science . . Environmental Geomorphology Environmental impact . . . . . . . Geomorphological risk . . . . . .

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2 GEOMORPHOLOGICAL RESOURCES . . . . . . . . . . . . . . . . . . . . . . . . .

8

2.1 Geomorphological raw materials . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Earth materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Soil .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2.1.4 The contribution of Geomorphology in the search for other natural resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.1 Solar radiation . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.2 Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.3 Glaciers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.4 Geothermal energy . . . . . . . . . . . . . . . . . . . . . . . 2.2 Landforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 A method for surveying, mapping and assessing landforms as geomorphological assets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 GEOMORPHOLOGICAL HAZARDS

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3.1 Geomorphological instability . . . . . . . . . . . 3.2 Soil erosion . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Introduction . . . . . . . . . . . . . . . . . 3.2.1.1 Definition and terminology . 3.2.2 Physical bases of erosion . . . . . . . . 3.2.2.1 Forces produced by fluids in 3.2.2.2 Forces resisting detachment 3.2.3 Water erosion . . . . . . . . . . . . . . . . 3.2.3.1 Introduction . . . . . . . . . . .

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35 36 36 36 38 38 41 42 42

xii 3.2.3.2 Impact of raindrops . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.3 Overland flow erosion . . . . . . . . . . . . . . . . . . . . . . Flow detachment . . . . . . . . . . . . . . . . . . . . . . . . . . Transport capacity of rill flow . . . . . . . . . . . . . . . . Rill erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.4 Differential erosion . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.5 Subsuiface flow . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.6 Gully erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Aeolian erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5 Models for evaluating water erosion . . . . . . . . . . . . . . . . . . 3.2.5.1 General overview . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5.2 De Ploey 's erosion susceptibility model . . . . . . . . . . 3.2.5.3 PSIAC model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Landslide hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Types and causes of landslides . . . . . . . . . . . . . . . . . . . . . . 3.3.2.1 General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.2 Internal causes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2.3 External causes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Techniques for landslide investigation . . . . . . . . . . . . . . . . . 3.3.3.1 Retrospective research . . . . . . . . . . . . . . . . . . . . . . 3.3.3.2 Databases and GIS . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3.3 Remote sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3.4 Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3.5 Geotechnical investigations and monitoring . . . . . . . Methodological aspects . . . . . . . . . . . . . . . . . . . . . Sensors and instrumentation . . . . . . . . . . . . . . . . . . Monitoring of suiface deformations . . . . . . . . . . . . . Monitoring of subsuiface deformations . . . . . . . . . . Monitoring of groundwater pressure . . . . . . . . . . . . Monitoring of climate (weather) . . . . . . . . . . . . . . . 3.3.4 Landslide hazard assessment . . . . . . . . . . . . . . . . . . . . . . . . 3.4 River hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 River erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1.1 Erosion in the strict sense . . . . . . . . . . . . . . . . . . . 3.4.1.2 Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1.3 Abrasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1.4 Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 .4.1.5 The action of mountain streams . . . . . . . . . . . . . . . 3.4.2 River instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.1 General overview . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.2 Main channel shaping . . . . . . . . . . . . . . . . . . . . . . 3.4.2.3 Channel pattern mobility . . . . . . . . . . . . . . . . . . . . 3.4.2.4 Channel network adjustment . . . . . . . . . . . . . . . . . .

43 46 46 47 48 49 49 50 53 55 55 58 60 64 64 66 66 69 71 72 73 75 76 79 81 82 83 83 84 85 85 86 88 88 89 90 90 90 91 92 92 97 98 99

xiii 3.4.2.5 Headward erosion .. . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.6 Tributary junction change . . . . . . . . . . . . . . . . . . . 3.4.2.7 Flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.8 Inundation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Marine hazard ....................................... 3.5.1 Geomorphology ...... .. ...... ... ................ 3.5.1.1 Coastal environment . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.2 Coastal processes . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.3 Cliffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.4 Beaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 .1.5 The evolution of coastlines . . . . . . . . . . . . . . . . . . . 3.5.1.6 Continental shelf and slope . . . . . . . . . . . . . . . . . . 3.5.2 The Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2.1 Coastal hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2.2 Hazard affecting cliffs . . . . . . . . . . . . . . . . . . . . . . 3.5.2.3 Hazard affecting beaches . . . . . . . . . . . . . . . . . . . . 3.5.2.4 Hazard affecting lagoons .................... 3.5.2.5 Hazard affecting continental shelves . . . . . . . . . . . . 3.5.2.6 Hazard affecting continental slopes . . . . . . . . . . . . . 3.6 Glacial and periglacial hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 Ice-rock avalanches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.3 Supraglacial debris fall/slide outside the lateral moraine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.4 Ice fall from snout of glaciers (ice avalanches) . . . . . . . . . . . 3.6.5 Rapid advance of glacier snouts (surges of glaciers) ....... 3.6.6 Emptying of internal water-pockets, proglaciallakes and ice-dammed lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.7 Highwater events on the valley floor connected with the presence of glaciers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.8 Lahars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.9 Hazards derived from mountain climbing ............... 3.6.10 Mass falls owing to confluence glaciopressure . . . . . . . . . . . 3.6.11 Assessment of the hazard derived from the presence of ice masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.12 Avalanches ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.13 Rock glaciers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.14 Debris flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.15 Final remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Geomorphology and seismic risk .......................... 3.7.1 Seismic risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2 Morpho-neotectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3 Geomorphology and seismic susceptibility .............. 3.7.3.1 Slope angle . ... . . . . . . . . . . . . . . . . . . . . . . . . ..

101 102 103 106 110 110 110 112 113 115 118 119 120 120 121 122 124 125 128 135 135 137 139 141 143 144

148 149 149 150 150 152 159 162 164 165 165 166 172 174

xiv 3.7.3.2 Debris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3.3 Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3.4 Degradational slopes . . . . . . . . . . . . . . . . . . . . . .. 3.7.3.5 Paleolandslides . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3.6 Underground cavities . . . . . . . . . . . . . . . . . . . . . . . 3.7.4 Earthquake-triggered mass movements . . . . . . . . . . . . . . . . . 3.7.4.1 Worldwide examples .............. . ... . .... 3.7.4.2 Examples in Italy . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.4.3 Study methodologies in Italy . . . . . . . . . . . . . . . . . . 3.7.4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Geomorphology and volcanic hazard . . . . . . . . . . . . . . . . . . . . . . . 3.9 Geomorphological hazard maps . . . . . . . . . . . . . . . . . . . . . ...... 3.9.1 Causes of hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9.2 Effects of hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9.3 Synthesis of survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

174 175 176 179 179 180 180 181 183 185 187 189 191 193 194

4 MAN AS GEOMORPHOLOGICAL AGENT . . . . . . . . . . . . . . . . . . . . . 197 4.1 An approach to the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Man's activities and their geomorphological consequences ........ 4.2.1 General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Consequences of hunting . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Consequences of animal-farming . . . . . . . . . . . . . . . . . . . . . 4.2.4 Consequences of agriculture .... . . . . . . . . . . . . . . . . . . . . 4.2.5 Consequences of resource exploitation . . . . . . . . . . . . . . . . . 4.2.6 Consequences of engineering works . . . . . . . . . . . . . . . . . . . 4.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

197 200 200 20 I 201 203 205 207 209

5 VULNERABILITY AND GEOMORPHOLOGICAL RISK . . .......... 214 5.1 5.2 5.3 5.4

Vulnerability to hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Risk mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prediction and forecast ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental education . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

214 216 218 219

6 GEOMORPHOLOGY AND ENVIRONMENTAL IMPACT ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 6.2 Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 6.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

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6.3.1 Types of projects .. .... .... .. . . . .. . . . . . . . . . . . . . . . 6.3.2 Investigation phases ........ .... .......... .. ...... 6.3.3 Mapping .. . . . . . . . . . . . . . . . . . . . . . . . . ... . .. .. .... 6.3.4 Indicators .... . ........ . . . . . . . . . . . . . . . . . . . . . . . . 6.3.5 Evaluation of Hazards and Assets . . . . . . . . . . . . . . . . . . . . 6.3.6 Evaluation of Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.7 GIS Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . ....... 6.4 Quantification of impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Raw materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Landforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3 Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 CONCLUSIONS

227 228 228 229 230 231 232 233 233 234 236

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REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

BIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

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ENVIRONMENTAL GEOMORPHOLOGY

1 INTRODUCTION TO ENVIRONMENTAL GEOMORPHOLOGY

1.1 Definition of Geomorphology Geomorphology, from the Greek words yfj jiopcpfi X6joq, is a science which aims to study and interpret landforms and especially the causes that create and modify them. Geomorphology's main area of study is the contact surface between the lithosphere, on the one hand, and the atmosphere and/or the hydrosphere, on the other hand, that is, the interface between two different physical entities: a solid medium and a liquid and/or aeriform one. It is along this surface that geomorphological processes take place. Two types of forces are exerted along this interface: endogenetic and exogenetic. The former (internal) originate in the substance of which the Earth is made and produce changes in the lithosphere. They are diastrophic phenomena giving rise to crustal deformation, volcanic activity and the slow transfer of the inner heat of the earth to the surface. Exogenetic (external) forces originate in the solar system and modify the surface of the lithosphere. They are principally the force of gravity and solar energy, which determine vectorial movements, convection and tangential movements of portions of solid, liquid or aeriform masses on the above-mentioned interface. This scheme of subdivision leads to the distinction between landforms and processes caused by internal geodynamics, termed endogenetic, and those landforms and processes that are related to external geodynamics, termed exogenetic. The morphology of the Earth is a result of the interaction of both forces, with the prevalence of one or the other in terms of space and/or time. Sometimes and in some places, volcanic or tectonic phenomena may give the most significant features to the land surface, prevailing over the external processes of erosion or deposition. At other times or in other places, the external processes of landscape formation may hide or destroy reliefs or depressions of endogenetic origin. For example, in volcanic areas, such as Mt. Etna or Mt. Pelee, or in tectonically active areas, like California or in some Mediterranean regions, the internal geodynamic components are evident in the overall morphological configuration of landscape reliefs. On the other hand, erosion landforms such as the badlands in Dakota, or a landslide in the Alps, or on the coast of the English Channel more clearly reveal the external features related to the action of water and the force of gravity in the formation of that particular landscape.

1.2 Geomorphology and Geology A definition of the areas of investigation specific to Geomorphology compared with those specific to Geology might at this point be opportune. The criteria for a dividing line may be found in the concept that Geomorphology is concerned with the units on the Earth's surface that are genetically linked to the relief that can be observed at the present time, whereas Geology involves the investigation of other units linked to landscapes on the Earth, which in that particular area have undergone a radical change. Therefore, from the conceptual viewpoint, it is not a chronological criterion that enables the two fields of study to be distinguished since relief units of the same age may be features under investigation in both fields of study. For example, moraine deposits of the Gtinz in the Alps come under the heading of geomorphological investigations, whereas a deposit of the Calabrian, more or less the same age, in the Southern Apennines would belong to the field of Geology. In fact, the former is the result of deposition from a glacier that occupied a valley quite similar to the valley in which the deposit is now located. The latter, on the other hand, is the result of deposition in a marine environment, which differs considerably from a subaerial one, where the deposit now outcrops. At any rate, it is clear that the landscapes of the past which are still preserved on the earth's surface are prevalently the least ancient ones, as the probability of their undergoing (endogenetic or exogenetic) transformation processes has been reduced. As a result the least ancient units on the earth's surface often prove to be the objects of geomorphological studies. Therefore, the time frame in which geomorphological studies are concentrated is the Quaternary era (Table 1). It is from this conceptual boundary between Geomorphology and Geology that a difference in the method of scientific investigation is also derived. The geomorphologist must always seek a logical connection, an explanation, a cause and effect relationship between a particular outcrop and the surrounding landscape: between the moraine deposits and the valley, a landslide accumulation and the main scarp, a paleobeach and a marine coast, even if the latter has retreated, etc. On the other hand, the pure geologist relies on outcropping rocks alone, albeit in the context of surrounding formations, in order to obtain a paleogeographic framework by drawing on the stratigraphical, sedimentological, paleontological and petrographical characteristics together with other features of the rock. This framework may also be obtained through a palinspastic reconstruction of the original positions of the rocks in that specific moment in geological history to which the outcropping rock type belongs, before the rocks may have been subjected to any folding or thrusting.

1.3 Geomorphology as a science It must be said that Geomorphology is an ''empirical" science. That is, a science based on observation and experience, experimental research on the various processes

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and forms of landscape formation, that is, on individual cases chosen among the infinite variety of geomorphological phenomena, all of which are different. For this reason, it is impossible to apply a ''systematic" method following the procedures of other natural sciences such as Zoology or Botany. The adoption of methods of this type, that is, the conception of Geomorphology as a systematic science using schematic procedures to explain the genesis and evolution of a physical landscape, can lead, and in certain cases has unfortunately led, to misconceptions and a fossilisation of research, by weighing it down with rigid preconceived models and patterns. Should it become necessary to adopt models, they should either refer to specific examples or be obtained from cases that have been directly observed. In this way, they are given no genetic significance but may be used for descriptive and illustrative purposes. Other models may be of theoretical value alone and present an intentionally limited number of variables selected from the infinite number of variables that play a part in the evolution of geomorphological phenomena. For this reason, the sole purpose of such models is to define the role of these variables, considered individually or in connection with others. However, they should never be taken as rules for indicating specific aspects of morphogenesis. For the same reasons, it is not possible to "classify" either geomorphological processes or the landforms they create. In other words, they cannot be subdivided into specific "classes" like plants, animals, minerals, etc. They may instead be split into

groups which present certain genetic analogies, considering a Hmited number of parameters. A correct and satisfactory procedure for the genetic interpretation of geomorphological phenomena consists in an accurate scientific analysis of the landforms and the numerous causes that have generated them. Simplistic deductions based on exterior appearance alone should be avoided, genetic significance should not be attributed to models that are only descriptive and phenomena that are genetically different should not be grouped together. Geomorphological studies often deal with landforms that have the same appearance but a different genesis, which are examples of geomorphological convergence.

1.4 Environmental Geomorphology The term, "Environmental Geomorphology" was introduced by Coates in 1971. He defines this field as follows: Environmental Geomorphology is the practical use of geomorphology for the solution of problems where man wishes to transform or to use and change surficial processes. According to the same author (Coates, 1972), this discipline involves the following issues and themes: 1) The study of geomorphic processes and terrain that affect man, including hazard phenomena such as floods and landslides. 2) The analysis of problems where man plans to disturb or has already degraded the land-water ecosystem. 3) Man's utilisation of geomorphic agents or products as resources, such as water or sand and gravel. 4) How the science of geomorphology can be used in environmental planning and management. Many other researchers have dealt with Environmental Geomorphology, in the discussion of both specific topics and the various applications of Geomorphology in the forms of AppUed Geomorphology, Engineering Geomorphology and also Engineering Geology. It is not necessary to review the available literature here; it is sufficient to mention Tricart (1962, 1973, 1978), Verstappen (1968, 1983) and Cendrero, Luttig and Wolff (1992), among others. Nor is it within the scope of this book to discuss the evolution of the concepts and methods of the applications of Geomorphology. The reader should consult mainly Coates (1980) and Verstappen (1983) for a specific definition of these issues. A discussion on the dividing line between Geomorphology and Geology may, however, be found in part 2 of this chapter. In this book. Environmental Geomorphology is defined as that area of Earth Sciences which examines the relationships between man and environment, the latter being considered from the geomorphological point of view. It should be further specified that environment, in general, is defined (Panizza, 1988) as the "range of physical and biological components that have an effect on life

and on the development and activities of living organisms". It is therefore used in an ecological sense. The geomorphological components may be schematically subdivided into: — geomorphological resources; and — geomorphological hazards. Geomorphological resources include both raw materials (related to geomorphological processes) and landforms — both of which are useful to man or may become useful depending on economic, social and technological circumstances. For instance, littoral deposits can become important, economically valuable and considered as geomorphological resources when used for sand quarrying. Similarly a sea beach can acquire value and considered as a geomorphological resource when utilised as a seaside resort. Geomorphological hazards can be defined as the "probability that a certain phenomenon of geomorphological instability and of a given magnitude may occur in a certain territory in a given period of time". For example, in any one area, the possibility of a certain landslide occurring over a 50-year time span may be assessed. In the context of relationships with the environment, man represents: — Human activity; and — Area vulnerability. Human activity is the specific action of man which may be summarised under the headings of hunting, grazing, farming, deforestation, utilisation of natural resources and engineering works. Area vulnerability is the complex of all things that exist as a result of the intervention of man in a given area and which may be directly or indirectly sensitive to material damage. Included in this complex, we find the population, buildings and structures, infrastructures, economic activity, social organisation and any expansion and development programs planned for an area. Considering the relationships between Geomorphological Environment and Man, two main possibilities can be examined (Panizza, 1992) (Fig. 1): Geomorphological Environment

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a) Geomorphological Resources in relation to Human Activity, where the Geomorphological Environment is regarded as mainly passive in relation to Man (active). In other words, a resource may be altered or destroyed by human activity (e.g., a mountain landscape that has been modified by a bulldozer). b) Geomorphological Hazards in relation to Area Vulnerability, where the Geomorphological Environment is regarded as mainly active in relation to Man (passive). In other words, a hazard may alter or destroy some buildings or infrastructures (e.g., a landslide or river erosion that causes road collapse). To conclude this conceptual outline, we can define as IMPACT the consequences of Human Activity on a Geomorphological Resource and as RISK the consequences of Geomorphological Hazards on a situation of Area Vulnerability (Fig. 2).

1.5 Environmental impact "Environmental Impact", as it will be considered here, consists in the physical, biological and social changes that a human intervention brings about in the environment, the latter term being intended in its geomorphological elements. Therefore, impact equals the "product" of man's activity and geomorphological resources.

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Fig. 2. The "Diablo" Bridge (near Santander, Spain) is an example of risk or of impact. If we consider this natural bridge as a hazard and the man with the oxen as vulnerability; the eventual collapse of the bridge represents a risk. If, on the contrary, we consider the natural bridge as a geomorphological resource (owing to its singularity and rareness) and the man passing on it with his oxen as an activity' that in the long run can cause the collapse of the bridge, then we are in the case of impact.

Raw materials related to geomorphological processes are generally nonrenewable, at least not on a time scale comparable to the history of man. However plentiful the reserves may be, a rapid rate of consumption, as there has been in the recent years of our history, may lead to irreversible depletion. In addition, the uncontrolled utilisation of some materials, such as alluvial gravels from riverbeds, can lead to a negative impact not only in the area that is more or less directly involved, that is, the riverbed or the surrounding slopes, but also in locations that may be distant from the gravel pit, for example along the coast near the river mouth, with negative effects noticeable only after a certain length of time. Moreover, there are sites or landforms of great value in terms of scientific heritage. The assessment of this value is based on their rarity, critical position in the foundation of an ecosystem, basic scenic appeal, paleoheritage, etc. Some systems have a high educational value as they facilitate an approach to knowledge about the environment and encourage environmental awareness. Children and adults alike need to be informed about the environment if they are to manage, understand and support management activity in the future. To do this, the examples useful for educational purposes must be protected and safe; sensible access to them should also be provided. Impact may be assessed in terms of: a) magnitude, which consists of the quantifiable entity of the changes; b) the sign, that might be positive or negative; c) the extension, referring to the area involved; d) the duration, which refers to the persistence of the changes over time; e) the moment in which the impact occurred; f) the probability of its occurring; g) reversibility, that is, the possibility that the environmental situation may return to preimpact conditions; h) synergistic impact situations; and i) mitigation, that is, the possibility of mitigating effects, etc. It should be mentioned here that this type of impact is not entirely identical to the one found in the "Environmental Impact Assessment" standards/regulations, which will be discussed later.

1.6 Geomorphological risk "Geomorphological Risk" is a natural risk connected to a geomorphological hazard: the term refers to the probability that the economic and social consequences of a particular phenomenon reflecting geomorphological instability will exceed a certain threshold (Panizza, 1987). Therefore, "geomorphological" risk is equal to the "product" of geomorphological hazard and an area's social and economic vulnerability. Vulnerability refers particularly to an area's population, buildings and its economic and social infrastructures. Thus, the assessment of risk must necessarily include an analysis of all the man-made structures existing in a given area and take into consideration the extent to which the existing situation is vulnerable with regard

to a given geomorphological event (exposure and vulnerability). For example, landsliding is a hazard that may not involve any risk in a desert area, whereas in a densely populated or highly industrialised area, it may represent a high risk. However, the assessment of risk must also include the prospect of new buildings and infrastructures (planning and vulnerability). Therefore, an accurate investigation of a territory's geomorphological stability is indispensable before applying engineering solutions. Vulnerability is also related to the existing degree of preparedness and social organisation, i.e., environmental education, civil defence and prediction and monitoring techniques. In areas where the population is aware of the problem and safeguards its territory, where the boards responsible for intervention are efficient, where advanced warning systems exist for major instabilities (e.g., river high-waters, storms in coastal areas, etc.) and finally, where such a high level of social organisation exists, the levels of vulnerability are certain to be lower. As for "geomorphological" risk mitigation, intervention is possible against both the hazard and vulnerability. For example, in the case of a building that has been built or is scheduled for construction on a degraded slope affected by solifluction, it is possible to intervene with land upgrading works (against hazard) or with deep building foundations (against vulnerability). In the case of lateral river erosion, it is possible to implement hazard mitigation through diversions or rectifications of the flow (channelization) or through a reduction of its energy (dykes). Banks could also be reinforced and vulnerability thereby reduced. Nevertheless, in many of the most hazardous cases, such as large landslides, mitigation of the hazard is the only intervention possible (see 5.2). Therefore, mitigation of geomorphological risk must come to terms with aspect, planning and organisation problems, i.e., problems of a social and economic nature, as well as those associated with political administration. In fact, risk mitigation operations of a preventative nature, involve an evaluation of costs and benefits. Some guarantees and levels of protection must be determined for both man and the environment. In some cases a risk threshold will have to be established and the higher it is, the more onerous and expensive it will be. In other cases, a priority scale will have to be defined concerning the various environmental risks or a choice will have to be made between risk mitigation and other programmes.

2 GEOMORPHOLOGICAL RESOURCES

2.1 Geomorphological raw materials 2.1.1 General aspects Before discussing this subject in detail, it is useful to define three different terms that are commonly used as synonyms. They are raw materials, resources, and reserves.

In a literal sense, a raw material is intended as any element of a natural system which can be utilised for processing. In everyday language, only some of the basic substances used in industry are indicated with this word (e.g., coal, oil, rock, mineral and metal ore, chemical substance, etc.) and likewise those farming products used on an industrial scale (e.g., rubber, wool, cotton, silk, timber, etc.) since they are all raw materials for a large number of producers. On the other hand, the meaning of resource is derived from a cultural concept: a raw material is transformed into a resource when utilised by man under particular social, economic and technical conditions. Therefore, the transformation of a raw material into a resource is a reversible process. In fact, when a resource ceases to be necessary, it again becomes a raw material, with no particular value. This is what happened to the flintstone that was a resource of paramount importance throughout the Neolithic period, but became a raw material once more when the mines were no longer used. Resources as such are therefore determined according to the human concept of usefulness and, as a consequence, their value changes according to variations in social, economic and technological conditions. Reserves correspond to the resources that are actually available under present day socioeconomic and technological conditions: they are therefore the most specific but also the rarest of the three categories afore mentioned and assume a particular value only in a limited period of time: the present. If raw materials are valuable, they can be called assets: therefore if geomorphological raw materials are valuable, they can be called geomorphological assets which, in turn, may be considered, and really become, geomorphological resources when used by man (Fig. 3). In order to be consistent with the general aims of this book, which deals with geomorphological matters, the specific themes concerning the search and exploitation of natural resources will not be discussed here nor will the economic problems linked to them, but after a general review a detailed discussion of the relationships between some kinds of resources and Geomorphology will be presented. However, it is obvious that activities connected with the use of raw materials can interfere with other aspects of the territory resulting in environmental risk and impact. For example, in the geomorphological field, a rock quarry could: a) modify environmental characteristics such as a slope shape; b) destroy cultural assets such as a glacial cirque or a paleocliff; c) cause environmental risks such as a landslide on communication lines. At the same time, of course, everything concerning the use of natural resources has social and economic implications. Raw materials, in particular those that make up natural resources utilised by man, are generally subdivided into two categories: — Renewable. They are theoretically inexhaustible since they are found in very large GEOMORPHOLOGICAL ^GEOMORPHOLOGICAL ^ GEOMORPHOLOGICAL RAW MATERIALS if valuable ASSETS ~V^ RESOURCES Fig. 3. Relationships between geomorphological raw materials, assets and resources.

10 quantities or are periodically renewed or reproduced. In this category various kinds of energy sources are included, such as geothermic or solar energy, waves, wind and flowing waters, farming and forestry land and fishing products. Nevertheless, some of these could be destroyed or made nonrenewable as a consequence of man's actions: i.e., some farming territories in the Americas were transformed into irrecuperable wasteland by the speculative greed of the first European settlers. — Nonrenewable. They are on the contrary those subject to depletion since they are present in limited quantities and are not renewable on the same time scale as man's history. They are mineral ore, fossil coal, building materials, natural gas and oil. Although the reserves of these materials may be plentiful, a rapid rate of consumption, as has occurred in recent decades, could lead to their irreversible depletion. A progressive substitution of renewable resources by nonrenewable ones has been taking place during the past decades: farming activities once performed only with the aid of domestic animals now require the use of fossil fuels; natural fertilizers have been replaced by those derived from natural gas; containers once made of wood are nowadays made of plastic which, in turn, derives from crude oil. Moreover, the constant dwindling of available natural resources leads to the progressive exploitation of other resources containing smaller amounts of useful substances, whose utilisation leads to increased energy consumption and therefore to a further use of nonrenewable raw materials. Another problem to be taken into account is the degradation of the environment resulting from the excessive consumption of nonrenewable resources, for example, pollution problems caused by the use of chemical fertilizers and pesticides, or by the exhaust fumes from motor vehicles and emissions from heating plants using kerosene or natural gas. Other problems are created by the accumulation of refuse of industrial origin such as scrapped cars, electric appliances, plastic containers, mineral products discarded after extraction and refinery processing and radioactive waste. The treatment and disposal of urban and industrial waste is causing a hazardous decline in the quality of air, soil and water, as well as biological damage to plants and animals. Environmental alterations lead to changes or destruction of the ecosystems, while the possibilities of recreating renewable resources are progressively reduced: an emblematic example is given by the sea, where fishing resources are undergoing a constant decline owing to water pollution. The consequences of technological innovation, progressive industrialisation and urbanisation, rapid changes in the social systems and economic growth, are also accompanied by a strong population increase demanding an ever growing need of nonrenewable raw materials. This means there is a danger of a complete depletion and a change in ecological cycles, with the risk of hindering the reproduction of renewable resources. The situation is growing more and more serious and is leading to limits of survival close to a general catastrophe. The tendency towards an ever increasing shortage of these natural resources.

11 which are often used inadequately together with environmental degradation, can be fought only by a rational policy of territorial planning. There should be two fundamental parallel actions: conservation and innovation, which determine a more balanced management of natural resources and of the environment. This would aim to reduce consumption and artificially induced needs, privileging instead ways of life requiring lower levels of energy. At the same time, the technologies based on minimum waste production should be adopted, with the possibility of recycling refuse and reducing the built-in obsolescence of many products. The future availability of resources depends mainly on these two actions. Conservation must be part of a concerted effort to safeguard the landscape by means of a better knowledge of the environment around us. As for innovation, technological progress could delay the depletion of nonrenewable raw materials by shifting forward in time the limits imposed by Nature, for example, by improving the productivity of renewable resources. Finally, a more effective commitment to the research on alternative energy is necessary, in particular solar energy since it may be the only one capable of giving a safe and durable solution to the problem of energy resources. 2.1.2 Earth materials Metallic and nonmetallic minerals, fossil fuels and building materials are all included in the category of nonrenewable resources. Geomorphological processes and conditions can be more or less related to the origin, position, state of conservation and exploitation of these materials. Therefore the appUcations of Geomorphology also play a part in this picture. The relationships between the most widespread categories of these resources and the Earth's reliefs can be summarised as follows (see Demek, ed., 1972; Cooke and Doornkamp, 1974; with modifications): a) Resources directly connected with the relief: — building materials derived from competent and incompetent rocks alike (marble, granite, clay, sand, gravel, etc.); — clastic mineral ores (gold, diamonds, heavy minerals, etc.); — alteration mineral products (kaolin, limonite, bauxite, etc.); — basin mineral deposits (coal, peat, etc.). b) Resources indirectly connected with the relief; for example those associated with tectonic morpho-structures, such as gas and mineral oil, rock-salt, coal, etc. c) Resources not connected with the relief, such as deep metalliferous or nonconcentrated mineral ore deposits. Among mineral ore deposits, a further subdivision can be based on their genesis: eluvial, colluvial, alluvial, etc. The relationships between the Earth's reliefs and natural resources have been shown in order to emphasize the contribution of Geomorphology also in this field of Earth Sciences, in particular by means of geomorphological survey and mapping. These investigations might seem obvious considering their relations with clastic

12 deposits (e.g., glacio-fluvial sands or eolian silts) or with particular environmental implications (e.g., peat deposits or bauxite ore). Nevertheless, they are essential also in many other less immediately evident situations: morpho-structural and morphoneotectonic investigations are particularly useful for the search of minerals related to tectonic phenomena. Moreover, for those situations caused by successive weathering processes, rillwash, enrichment and deposition, a reconstruction of the geomorphological evolution of the area is indispensable. Earth materials more directly related to Geomorphology are sand and gravel. According to their origin, they can be subdivided as follows: — alluvial deposits; — marine-coastal deposits; — slope deposits; — glacial deposits. These kinds of Earth materials are sought after for the production of concrete, for the construction of road and railway beds, for macadam and ballast preparation, etc. But not all sands and gravels are suitable for these purposes: detritus derived from very brittle rocks should be excluded owing both to the intrinsic nature of the rock (e.g., gypsum) and to the weathering processes. Equally important are: the sort of material, the thickness of the deposit, the nature and distribution of available resources, the roundness of rock particles, etc. In order to know and properly assess all these characteristics it is necessary to carry out detailed geomorphological survey and mapping, as well as relying on in situ and laboratory measurements. For example, the composition of fluvial gravel will correspond to the lithological characteristics of the river's catchment basin upstream of the extraction area. From the examination of the geomorphological map it will be possible not only to deduce the general lithological character of the deposit but also the approximate percentage of the different kinds of rocks that constitute it, on the basis of the wideness of the outcrops and of their resistance to weathering. If the deposits are terraced, the material is often weathered and the thicker the alteration layer, the older the terrace and the higher its position compared with the present valley floor (Fig. 4). The search for gravel in these cases should be directed towards the lower terraces, considering in particular the foot of the terrace, where unweathered gravel deposits are more Ukely to be found. Moreover, from geomorphological maps and more detailed field surveys it will be possible to estimate the thickness and therefore the volume of exploitable gravels. In situ and laboratory analyses will also provide lithological and particle-size information. Similar considerations can be made for other kinds of deposits, such as morainic ones, although they are usually characterised by a certain amount of silt and clay which may reduce the materials' field of application and should therefore be removed before being used. The use of talus as a raw material depends on its lithological composition, its state of weathering and also its final purpose: it can generally be utilised for road beds, even when altered. Instead, for some constructions, such as retaining dams, the material must be very fresh.

13

Fig. 4. Alluvial terrace deposits near the Seche Torrent in Martinique. Notice the tyres incorporated within sands and gravels at the base of a deposit over 20 m thick: these singular "fossils" show an extremely recent age of the terrace deposits (photo M. Panizza, March 1994).

When a project of exploitation of a raw material is activated, it is indispensable to assess its consequences, especially in terms of environmental impact. For example, quarrying activities of slope deposits can cause mass movements in the adjacent slopes. Tips of waste materials create artificial scarps with negative consequences not only for the landscape but also for geomorphological stability. An example of irreversible environmental damage is given by the consequences of quarrying along and within riverbeds. The removal of gravel and sand from riverbeds can trigger a series of different consequences both in the short and long term. First of all, the lowering of the riverbed due to excavation causes an increase of the watercourse's profile acclivity upstream of the excavation area, thus augmenting the flow velocity and the erosive power of the water. The removal of alluvial deposits from riverbeds exposes the substratum to erosion, especially if it is made up of cohesionless or pelitic rocks, and causes a deepening of the runoff channels. Along erosion furrows the river's water attains a higher velocity that it would have in a wide riverbed and therefore it increases its erosive power. Such erosion phenomena can determine the drainage and depletion of the more superficial aquifers as well as the destruction of the underdrainage flow. Moreover, downstream of the quarrying area fluvial waters might carry a smaller load of coarse materials, compared with the period prior to the removal of riverbed

14 deposits, and a higher content of finer suspension elements deriving from the erosion of a peHtic substratum. As a consequence, in the lower plain the deposition of abnormal amounts of fine sediments could take place, leading to a raising of riverbeds already suspended in some stretches of their course and therefore to a greater flooding hazard. Less coarse material will also be deposited at the river's outlet and along the adjacent seashores whilst silty-clayey sediments not previously dispersed and deposited during floods will prevail in some coastal areas. A lack of coarse deposits causes a decrease of supply for the beaches, which are consequently more subject to regression and erosion. The above-described phenomena have occurred in several cases, following intense quarrying in the beds of many watercourses of northern and central Italy, the effects of which may be seen along the Romagna and Marche Adriatic coast. From the above-mentioned cases, it appears clear that the negative effects of quarrying do not only concern the territory directly affected by the exploitation activities of a natural resource, such as a riverbed or a mountain slope, but can have further repercussions on the environment, even in areas far from the quarry and after a considerable length of time has elapsed. 2.1.3 Soil Soil is a natural resource since it is the essential element in agriculture and is closely related to the geomorphological evolution of the site where it has been formed. A soil can be considered as a thin layer that coats the dry land at the interface between lithosphere and biosphere, surrounded by water, ice and rock outcrops. Soil is created by the transformation undergone by rocks from the moment they appear on the Earth's surface, in a very different physical and chemical environment from the one where they originated. These transformations occur in different ways in the various regions of the planet, according to processes governed by environmental factors which impart specific characteristics to the pedological covers. Soils vary according to geographical regions and geomorphological environments; they are therefore an essential component of the landscape and are subject to the same laws that control the Earth's modelling. For these reasons the study of soils cannot be separated from the study of the landforms on which they lie. One could say that the two processes of morphogenesis and pedogenesis are inversely proportioned: in fact erosional and depositional processes hinder or prevent the soil's formation. On the other hand, pedogenesis could develop where an adequate vegetation cover reduces or prevents soil mobilisation phenomena, i.e., the morphogenetic processes. The study of soils has important practical implications since food resources largely depend on their state of health. Notwithstanding this, every year large extensions of soil are destroyed by the advancing of deserts in tropical regions, by hydraulic and geological disruption and by uncontrolled development of urban and industrial areas. Soil must therefore be considered as a resource worthy of conservation, on which further advanced scientific knowledge must be acquired. Soil also keeps a historical record: the geological events of the Pleistocene and

15 the succession of man's cultures during the Holocene have given it recognizable characters that allow a proper reconstruction of the environmental changes and maninduced impact of the past. Soil behaves like an open physicochemical system, with constant matter exchanges with the surrounding systems and transformations within it (Fig. 5). The energy necessary for these processes to take place is derived from the Sun's radiant energy, from the Earth's endogenetic heat, from physicochemical reactions and from gravity. They all take place through processes that lead to progressive mineralisation of the coarse organic substance provided by the biosphere, the breakdown of mineral matter at an ionic level, the production of carbonates, silicates and neogenesis oxides and hydroxides, the transformation of already existing materials and the redistribution of all components. This complex sequence of processes imparts a series of proper, specific characters to the soil that differentiate it from the underlying bedrock. Pedogenetic processes and therefore the soil's characteristics are conditioned by several environmental factors which are usually expressed as a function of the ADDITIONS precipitation (with ions and solid particles), organic matter SURFACE

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LOSSES ions, H4Si04

Fig. 5. Scheme of the main processes acting in soil evolution (after Cremaschi, in Panizza, 1988).

16 geographical region, the substratum and the topographical features. The five main factors that can be related to the development of a soil are identified by Jenny's (1941) well-known equation: s = f(cl, o, r, /7, f, ...), where; — s corresponds to the soil's properties; — cl is the climate; — o is given by the organisms, including man's activities; — r is the relief and therefore the topographical factor; — p is the bedrock; and — t is time. As they are not included within the aims of this volume, the various factors regulating pedogenesis will not be discussed in detail nor will the soil's characters be classified. Similarly, the problems of paleosoils will not be considered: all these aspects are dealt with in specific Pedology textbooks. 2.1.4 The contribution of Geomorphology in the search for other natural resources Geomorphology can offer numerous contributions to the research and exploitation of natural resources: field survey and geomorphological mapping; remote sensing; elaboration of special morpho-neotectonic maps; investigations for recognizing characteristic relief landforms or features which help in localizing and exploiting particular resources; collaborations with interdisciplinary investigations; studies for the reclamation of exploited areas. Some examples are described below. 2.1.4.1 Solar radiation The Earth intercepts part of the energy radiated by the Sun and uses it as its main natural resource. The importance of this energy input is due not only to the fact that without it no vegetable or animal life would be possible on the Earth, but also because it is the engine of all the physical phenomena that take place in the atmosphere and of most of those occurring on the Earth's surface: the circulation of air, the whole hydrological cycle, the movement of ocean currents, the breaking up of rocks, as well as many others. Solar radiation is here considered in its role as a natural resource: one of the most important of its various effects is the generation and support of vegetable life: it is therefore an essential element since it conditions the formation of farming and forestry land. One of the main variables that influence the distribution of the Sun's incident rays on a slope at a certain latitude is slope aspect. Apart from plain areas, it is obvious that in the northern hemisphere southward slopes are those that most benefit from solar radiation: let's consider, for example, the vineyard and orchard-covered hills of the preAlpine area in Italy. Aspect has also conditioned the different distribution of

17 the forests: in many East-West stretching Alpine valleys the North-facing slopes are usually covered by woods while the South-facing ones are utilised for farming and grazing, since the aforementioned geographical conditions are more suitable for these activities. It is, therefore, important to collect adequate information on the morphological aspect of slopes. These data will be used not only for forestry-farming purposes but also for other applications connected with the selectivity of geomorphological processes such as the wind influence, evapo-transpiration, snow melt, etc., with relation to the different orientation of slopes. These characteristics can be represented by means of a thematic map of slope aspect which is one of the specific objects of research and elaboration of quantitative Geomorphology. For the drawing up of this map a certain number of classes (generally eight) having equal angular magnitude (45°) are chosen. Their central values correspond to the main cardinal points and to the intermediate ones (North, North-East, East, SouthEast, South, South-West, West, North-West). Figure 6 shows the eight classes of aspect and their relative angular values. Plain areas of course cannot have a defined orientation; for this reason a ninth class is represented for surfaces with an acchvity of less than 5%. Each of the nine classes is defined by using different symbols. Moreover, the convergence points of the contour lines, corresponding to

N

337^30

M

•^

^^

t^

VS^S^S^

Jr

,,30' N

Vxjc^

292° 30" /

^^^

111111\ ^^" *^^ 1

iTTTTv»^

247*' 30'

C•% \J

. .;

•'" v®

^ ^ ^

"Nj^

202° 30'

E

s

Surfaces with slope angle < 5%

Fig. 6. Legend for a map of slope aspect.

r°30'

18 the relief tops and depressions and areas where orientation is undefined since the tangent plane is horizontal, are usually emphasized by means of symbols such as little circles. An application of a cartographical elaboration of this kind for farming purposes is given by Tozsa, Molnar and Pelle (1985). The area considered is that of Mt. Somlo in Hungary, an isolated hill in a plain area, 432 m above sea level, made up of basalt rocks and cultivated with vineyards. The investigation methods here applied consist in the superimposition of the ''map of slope aspect" (Fig. 7) with a slope angle map (Fig. 8) of the same territory. In the case of Mt. Somlo, the area was subdivided into five slope angle categories, which make up the usual slope classes in agricultural applications in Hungary. The result is a map of solar energy distribution, subdivided into: 8 X 4 + 1 = 33 categories (Fig. 9). It is possible to calculate the theoretical quantity of energy that each category can receive during a year for a given region and therefore correctly assign farming crops on the basis of the minimum energy needed by each vegetable species. Therefore, the "map of slope aspect" is a fundamental document for the evaluation of the energy resource derived from solar energy, with particular applications in the field of farming land. It is also used in other sectors of applied Geomorphology, such as the representation of phenomena linked to wind, avalanches, etc.

|N

FFfflNE^E

^SE[TII]S

C^^SwflMlw

^NW[:;^<5»

Fig. 7. Map of slope aspect of Mt. Somlo, Hungary (modified after Tozsa et al., 1985).

19

<5%

5-12%

13-17%

18-25%

Fig. 8. Slope angle map of Mt. Somlo, Hungary (modified after Tozsa et al., 1985).

2.1.4.2 Water Water can be considered both as a raw material and a natural resource. It supports life on Earth and therefore directly and indirectly also man's life. It covers threequarters of the Earth's surface, is present in the atmosphere and makes up the main component of organisms. It also conditions man's socioeconomic life with waterways, fishing, drinking water-supplies, farming, industry, etc. This topic is undoubtedly very vast and complex and it has numerous geomorphological implications and applications. In this context only some aspects of water as a resource and as gravity-induced energy will be dealt with. Some problems regarding storage reservoirs created by river damming will now be considered. From artificial hydric basins the stored water is distributed and used in various ways and for different purposes: as a drinking supply, for irrigation, for shipping canals, for industrial applications but, above all, for the production of electric energy. The creation of storage reservoirs, whether they are large hydric basins for the production of electric energy or small impoundments in the hills for field irrigation,

20

6 Slope angle

1 < 5% 5-12% 13-17% 18-25% '

> 25%

1km t

EXPOSURE E SE

tlat

,

S

SW

W

1

-

-

-

-

-

—

—

—

— — —

2

3

4

8

11

15

19

26

27

28

21 29

16 24

9 17

18

12 20

6 14

7

10

5 13

30

NE

22

23 31

NW

32

25 33

Fig. 9. Solar energy distribution map of Mt. Somlo, Hungary (modified after Tozsa et al., 1985).

implies a certain variety of geomorphological applications, connotations and consequences. In this section the duties of the geomorphologist will be discussed

21 when a reservoir construction site has to be chosen giving particular attention to the identification of the most suitable place for the dam's foundation. When planning a dam, an essential instrument of data acquisition is offered by the "geomorphological map". Indeed, this descriptive document of the reliefs genesis and the dynamic processes affecting slopes and hydrological networks can show some of the points essential for a satisfactory outcome of the work and its economic viability. When a territory is going to be chosen for a storage reservoir, whether it is for hydroelectric or other purposes, two fundamental characteristics should be identified: a) it should be wide enough; and b) it should not show incompatible processes of degradation. Unfortunately in most cases these two conditions appear to be antithetical, since a valley is usually very wide and smooth when degradation processes are particularly intense. In any case one of the first evaluations should concern the amount and trend of the degradation processes, in order to assess their compatibility with the construction of the dam. A rapid silting of the reservoir would in fact shorten its existence since its economic viability depends on the amount of energy (or other kind of resource) that it can provide and also on its efficient durability in time. Such a disadvantage could indeed preclude the reservoir's construction unless adequate prevention works are carried out first. These works should be based on studies which identify the debris' provenance, its trend of transport, its particle size characteristics and its input rate. Geomorphological survey and mapping could perform most of these duties. First of all, along the mountain slopes the areas most subject to degradation should be recognised, since they provide most of the detrital materials. The lithological characteristics of these areas will be examined in detail, with particular attention given to superficial and weathering deposits, their thickness, rate of cohesion and particle size distribution. The degradation mechanisms should also be recognised, whether they are physical or chemical, for example due to frost cracking or hydrolythic alteration: the identification of the causes will lead to the choice of the most appropriate and effective remedial measures. The routes run by the debris to reach the hypothetical basin will be determined and by measuring the solid transport and the amount of rillwash processes, the rate of sheet-erosion will also be calculated. All this is part of the information that geomorphological maps and derived thematic maps can provide about the problems of active degradation. These geomorphological data will determine the choice of the remedies directed to a decrease of the erosion processes, in order to reduce the silting up of the reservoir. Particularly favourable situations for the choice of a reservoir site are given by glacial landforms such as overdeepened hollows, which can be found along valleys previously occupied by Pleistocene glaciers. Moreover, in some places these hollows are partially dammed downstream by rock-bars which can be the ideal site for the dam's foundation. Apart from being sufficiently wide, the valley which has to be chosen for the reservoir should also be characterised by low acclivity in order to permit the

22

construction of a retaining dam which is not too high. Also a lateral valley, suspended above the main one, in a geomorphological context derived from ancient glacial modelling, could be a very favourable site, considering also the high vertical interval at the confluence: the dam's construction in this point could rely also on the noticeable water gradient. Another favourable situation could be found in an area where there had been a lake in the past: this would indicate the imperviousness of the ancient basin. Nevertheless, it would be necessary to carry out a detailed geomorphological reconstruction of the area and discover the causes that had led first to the lake formation and afterwards to its extinction, considering particular events that could have given rise to cavities or joints, thus increasing the basin's permeability. It would also be essential to investigate slope stability assessment, paying particular attention to active, potential, quiescent and ancient mass movements. Reference should be made here to the huge landslide that affected the storage reservoir created by the Vaiont dam (district of Belluno, Italy): this vast rock block and debris slide occurred on 9 October 1963, claiming over 2,000 lives (Fig. 10). Another process to be taken into account concerns the degradation and gravity phenomena along the lake shores: this is a consequence of the artificial reservoir formation and is the effect of vertical variations of the waves' hitting lines and to moisturization and exsiccation alternations, as a result of water level fluctuations. These settlements are a function not only of the water dynamics but also of the material's cohesion and rate of permeability and of the acclivity of the surface in

Fig. 10. The Vaiont landslide: in the background Mt. Toe with its detachment scarp, in the centre the landslide heap fallen into the reservoir and in the foreground the dam which withstood the tremendous thrust (photo M. Panizza, June 1990).

23

contact with the reservoir's water. Once the reservoir's maximum level has been established, detailed investigations will have to be carried out on the characteristics of the materials that make up the shores of the future storage basin and, as far as Geomorphology is concerned, accurate morphometric measurements of the slopes will have to be made. Finally, another phenomenon should be mentioned: avalanches. They can have considerable consequences on the silting up of the reservoir, with hazards analogous to those induced by landslides. Even the choice of the site of the foundation requires the geomorphologist's expertise. A sufficiently narrow valley section will have to be recognised, confining a territory with the above mentioned requisites and capable of storing the greatest possible amount of water with the minimum economic commitments. It will have to assure indispensable stability and imperviousness requirements. Particular attention will also be given to karst landforms and processes. With reference to what has already been said about reservoirs, a favourable condition could also be given by a rock-bar along a valley once occupied by Pleistocene glaciers, or by a fluvial gorge in correspondence with particularly competent rocks. In any case, the choice should be directed towards those river stretches where a valley goes narrower, downstream of a wide and open sector. Investigations which require the contributions of Geomorphology are those concerning the nature of the dam's threshold, regarding its stability and imperviousness and more general observations on the territory's evolution, such as the identification of paleolandslides, that could be decisive for the choice of the damsite or the most suitable engineering works for the construction phase. One of the first investigations to be performed is a detailed geomorphological survey of the area where the dam is to be built: the research will aim at recognizing a) the rock-types, as well as their level, kind and grade of weathering; and b) the extent of the degradation processes along the slopes. Undoubtedly the ideal characteristics would be those corresponding to compact, impervious, non fractured, unweathered rocks, either unstratified, or with bedding and schistosity surfaces dipping upstream. Nevertheless, for certain kinds of dams, such as earth or stone-filler gravity dams or for small irrigation impoundments, a barrier threshold made up of superficial deposits could be suitable. Usually, deposits such as moraine arcs, landslide heaps and talus cones could assure appropriate geotechnical and hydrogeological characteristics or permit their optimisation by means of opportune engineering techniques. An important geomorphological problem is given by the presence of buried riverbeds, i.e., fossil fluvial forms hidden by superficial deposits. A well known and characteristic example is that of the epigenetic superimposition of the Avisio watercourse near the Camini gorge (Trento), illustrated by G.A. Venzo (1957) during the geological investigations on the site of the San Floriano d'Egna hydroelectric plant. The reconstruction of the valley's geomorphological evolution, starting from the Riss glaciation up to the present, led to the identification of a buried paleochannel

24

and to the construction of the reservoir in a particularly complex situation. Figure 11, taken from the above quoted article by Venzo, schematically shows the geomorphological features of the Avisio valley and the hydraulic plant built for the construction of the storage reservoir. At the end of the Riss glaciation the valley floor stood at an altitude of about 720 m. At the beginning of the Riss-Wurm Interglacial period, the watercourse further deepened its bed, subsequently depositing vast amounts of alluvial sediments up to a height of about 900 m by the end of that period. During the following advance of Wtirmian glaciers, some of these deposits were partially removed and substituted by moraine deposits. The Holocene 900 m Riss - WQrm

After the wormian glacier retreat

Actual

100

200 m

Fig. 11. Stages in the geomorphological evolution of the Avisio valley (Province of Trento, NE Italy) at Forra dei Camini and plan of the damming works carried out (after G.A. Venzo, 1957). Legend: al - RissWiirm alluvial deposits; mo = Wiirm moraine deposits; p = porphyries (bed rock); D = arch-dome dam; / = diaphragm grouted into the alluvial deposits; R = coating in bentonite-enriched earth; S = cement groutings in the rock.

25

watercourse redissected its own riverbed near the Camini gorge. But its course underwent a lateral shift, compared with the main axis of the old valley bottom, with a characteristic superimposition phenomenon, and deeply eroded the porphyries on the right hand side of the valley into ravines. In order to build the reservoir, it would not have been sufficient to dam the Camini ravine, since the water would have run off through the paleochannel filled with alluvial deposits. Therefore, before impounding the basin, it was necessary to make these sediments impermeable. Treacherous situations could be determined by the apparent presence of bedrock in correspondence with the barrier foundation: it is not uncommon in fact for the accumulations of rock-block slides to be misinterpreted as bedrock, since they show several analogies with primary rock outcrops on their outer surface. The same can happen with large rock bodies displaced from overhanging slopes and accumulated from falls or slides. Detailed geomorphological surveys can then detect these situations, which are not infrequent in the Alpine regions. Among the situations above described, a typical case would be a wide area that becomes narrower downstream, at the confluence of different catchment basins, forming gorges or ravines. Moreover, sites like these can offer a particularly suitable place for the choice both of the storage reservoir (confluence of several valleys) and of the dam foundation (valley narrows). However, particular kinds of landslide barriers, not rare in the areas affected by Pleistocene glacier advances, have been interpreted by the writer (Panizza, 1973) as due to confluence "glaciopressure" (see: 3.6.10). 2.1.4.3 Glaciers The use of glaciers as a natural resource has been a well known practice since remote times: both for agricultural purposes, with irrigation canals taking their water from a glacier, and as a source of cold for storing food and cooling drinks before the advent of refrigerators. Water from glaciers is used for the supply of hydroelectric storage reservoirs. The presence of glaciers provides a constant supply of waters and therefore an optimal performance of hydroelectric plants. This is true in particular in the Alpine regions, where summers are usually dry and warm, with hydrological regimes determined by rain and snow feeding in the winter and snow and ice melt-water in the summer. 2.1.4.4 Geothermal energy The inner heat of the Earth, generated and stored underground, makes up a very relevant energy resource, although it is difficult to exploit, at least at the present level of technological knowledge. Usually this kind of energy is found at too great a depth and in rather scattered areas for an adequate exploitation on an industrial scale. Moreover, in its natural state it is manifested through emissions of hot water and steam (fumaroles, geysers) with limited energy quantities or through volcanic eruptions, the latter being more dangerous than useful. Nevertheless, during the past few years, increasing attention has been given to the exploration and exploitation of geothermal resources and new extraction and

26 application technologies are eventually making feasible the use of geothermal basins which in the past had not been considered, both for their limited energy quantities and for the difficulties connected with their exploitation. The geothermal energy available has a low or very low temperature (between 30 and 90°C), consequently it excludes a transformation into mechanical or electric energy. Instead, it is useful for a direct use of heat, such as for urban heating plants, for greenhouses, exsiccation procedures, etc. In the near future, technological progress applied to the extraction and exploitation of this natural resource should make it more acceptable from the economic point of view, considering also that it does not pollute and its environmental risk is minimum. The contribution of Geomorphology in geothermal research is mainly related to the identification and location of open fractures or active faults, along which the circulation and eventual outlet to the surface of steam and hot water is possible. This field belongs to neotectonics, an area in which Geomorphology has many important applications, as will be shown in the following chapters.

2.2 Landforms As has already been stated, if landforms are valuable they can be called geomorphological assets and, therefore, may also be geomorphological resources in that they may be or become useful to man, depending upon the economic, social or technological circumstances existing at a given time (Fig. 12). Landforms are a part of our natural assets which also belong to our cultural assets. In order to provide a framework for this topic, it should be specified that the concept of culture is used here in its traditional meaning, which is derived from what the Romans called humanitas, that is, ''the whole body of knowledge and behaviour, which enables man to achieve his authentic human nature". Cultural assets may be subdivided into two categories: those that are more strictly ''natural" and those that are more directly related to the "work of man". This is, however, a somewhat contrived division, in that the two are closely linked, just as all cultural assets are and should be seen as interacting. "Man-made'' cultural assets include documentary, artistic, architectural, historical and archaeological assets. Natural assets may be subdivided into biological and nonbiological assets. Knowledge of these assets is an indispensable priority if an awareness of their worth and thus their correct collocation in Environmental Management is to be attained. In this way they may be protected and utilised without being destroyed or degraded, and may be used in the most opportune ways. Indeed, landforms are among the most widespread and spectacular natural nonbiological assets: a river gorge, a mountain peak, a natural bridge, a sea cliff and LANDFORMS if valuable

GEOMORPHOLOGICAL ASSETS

if used

• GEOMORPHOLOGICAL RESOURCES

Fig. 12. Relationship between landforms. geomorphological assets and resources.

27 the like. They have always aroused interest because of their scenic appeal. However, scenic aspects are not the only attributes that make such landscape elements important — at times, it is also their cultural significance that does so. Consider the scientific and educational aspects associated with the remains of Pleistocene glacialism for example, the spectacular nature of a Dolomite landscape or of a Mediterranean coast, the value in tourism and economic terms associated with a beach in the Caribbean or the cultural implications of a landscape in Greece. Landforms such as these ~ whose attributes of value lead to their definition as cultural assets — by analogy, can be called geomorphological assets, which in turn may be considered and really become geomorphological resources if used by man (Fig. 12). The attributes that may confer value to a geomorphological asset are listed below: — scientific; — cultural; — socioeconomic; — scenic. By a process of generalization, these attributes may also be seen in other natural assets. In the following paragraphs, they are described in more detail. From a scientific point of view and in the field of geomorphology, the importance of a natural asset may be assessed according to four characters (Panizza & Piacente, 1993): 1) As a model of geomorphological evolution', e.g., a Karst doline, or an earth pillar. 2) As an object used for educational purposes, e.g., a badlands area or a fluvial meander. 3) As a paleogeomorphological example such as a Pleistocene loop moraine or a river terrace. 4) A landform can be considered a geomorphological asset because of its scientific aspects also when it is an ecological support, maybe because it is the exclusive habitat of a particular animal or plant species, which are indispensable elements in an ecosystem: some wetlands or debris accumulations may provide examples. However, in this case, other disciplines, such as Botany or Zoology, rather than Geomorphology, will indicate the scientific attribute of the geomorphological asset. In other cases, the determination of the scientific value of a particular morphological feature, such as a cave or a ledge that were once the site of an ancient human settlement, may lie within the bounds of Archaeology. Each of these characters can assume a higher or lower value owing to its rareness, i.e., its interest in spatial terms; therefore a different level of interest can be ascribed to each of the four categories of the above defined characteristics: — local; — regional; — super-regional; — world-wide. From the cultural viewpoint, a geomorphological asset may belong to the world of art or to a cultural tradition, as, for example, the landscapes depicted by Venetian

28 painters in the sixteenth century or Mt. Olympus, the abode of the pagan gods. A geomorphological asset may also acquire socioeconomic value if it can be utilised for the purpose of tourism or sports, as for example an Alpine valley, a trail for Nature walks and excursions or a rock wall equipped for mountain climbing. The scenic element may also be a geomorphological asset both in an intrinsic spectacular sense and because of its appeal as an attraction, which can make it easier for people to approach environmental issues and increase their knowledge and awareness. From the above observations, one may deduce that a geomorphological asset may be either a landform or raw material, or even both. For example, a debris cone can be considered as a geomorphological asset, as a landform, if certain scientific characters (e.g., as a model of evolution or a feature of paleogeomorphological evidence) or if socioeconomic characters (e.g., as an area equipped for Nature excursions) are contained within it. It may instead be considered geomorphological asset as raw material if the debris it is made up of can be used as aggregate to be quarried. Legislative initiatives to safeguard and utilize natural resources in general, and geomorphological assets in particular, imply, in certain cases, an appraisal of their value in quantitative terms. This is especially the case in ''Environmental Impact Assessment''. Such problems are being posed with increasing urgency, owing to the rapid increase in the number of projects for urbanization, civil works, etc., in many areas. Growing human intervention on the earth's surface is progressively increasing environmental deterioration and the social demand for recreation areas, uncontaminated localities, parks and protected areas. Nevertheless, the appraisal of the above mentioned "value" and impact is a difficult problem to solve, linked as it is to the difficulties involved in providing an objective and quantifiable evaluation of the various geomorphological aspects of the landscape. The studies that must be conducted involve the concept of measure, that is, the necessity of assigning to each "environmental marker" (and therefore, also to geomorphological assets) a "weight", which in turn, permits an assessment in quantitative terms of their "environmental value" and a comparison with other markers, to permit the definition of a hierarchy of priorities. The criteria that can be used for the assignment of this "weight", that is, for a "measurement" of landscape features, may be one or more of the four types previously illustrated. The scenic/aesthetic criterion is to a great extent, of an intuitive nature. In this case, the approach to Nature depends upon the individual contemplating it and his/her state of mind at the time. It is derived from feelings which being personal perceptions are highly subjective, it is therefore difficult to value and compare with the feelings and perceptions of others. Several procedures for such evaluations exist in literature (i.e., Linton, 1968; Fines, 1968; Leopold, 1969). Some of these present morphometric methods of measuring several landscape components that have been judged as representative of the scenic quality of the landscape. Other procedures appear to be more subjective and they concern the perception of the landscape as a whole in qualitative terms.

29 Numerical rating scales are proposed. The limitations appear to be considerable in all cases, due to the subjectivity inherent in judgements conditioned by both the sensitivity and the experience of the observer, either because several important components are omitted and a vision of the landscape as a whole is lost, or because by breaking up the resource itself it is distorted and permanently stripped of its wholeness and essence. The merits of the cultural and socioeconomic criteria will not be discussed here because they concern disciplines other than geomorphology. The geomorphological approach, however, is based precisely on scientific knowledge of the natural resource, the perception of the laws regulating its evolution and an awareness of its significance for humanity. Therefore, this is a task that can only be performed by well-trained geomorphologists who can accurately identify and evaluate those attributes. On the other hand it is of fundamental importance to try to approach the asset in its entirety without preestablished tables and matrices because there are other intersecting or combining variables that will inevitably come into play and augment or detract from its value. We are referring here to human forces and particularly to socioeconomic, cultural and political interest groups, which may select a given asset as the most significant as a tourist attraction, for example, or as the best scientific expression of certain environmental processes, and so on. A landform becomes a geomorphological resource only if it has social implications, that is, only if other parameters, external parameters, come into play to invest it with value (Panizza & Piacente, 1993) (Fig. 13). As long as a particular river, or a particular landscape is studied by and known only to scientists and researchers, it remains ''private" knowledge and its potential as a resource does not materialise. However, if the scientist and the researcher publicise it, thus making its cultural and environmental significance known to the general public and thereby give it a social dimension, then the landform becomes a geomorphological resource in the eyes of society at large. One assessment, however, should certainly be carried out and that is the evaluation of the asset's continued existence over time. More specifically, this involves the

LANDFORM

1

ORGA^STION

GEOMORPHOLOGICAL RESOURCE Fig. 13. Relationship between landforms and society.

<

Economics Culture Politics Tourism

30 correct regulation of its utilization when conflicting interests develop. Such conflicts include scientific research versus its fruition, the restriction of access to the landform versus the demand for knowledge and educational purposes, direct use versus pure observation, etc. A preference for one alternative over the other in a sort of a priori discrimination is not possible. On the other hand, choices must be dependent upon guarantees for the asset's continuing existence. This can be achieved by defining the asset's "capacity load" in relation to various functions. In other words, when and where a geomorphological resource has a social implication (see human activity: e.g., tourism) an environmental impact is likely to occur (see Fig. 1). Thus, the hierarchy of evaluations may be summed up as follows: in the first place there is the existence of the landform; the evaluation as a geomorphological asset comes second, while its usage comes third. However, care must also be taken to create equal opportunities for use by all parts of society as the resource has also a collective, community value which is different from the value it has for the individual. These problems take on considerable complexity when considered also in the light of the present decline of two cultural concepts that originated in the first half of the seventeenth century with the Galilean and Cartesian dichotomy. The ascent of reason, on the one hand, had created the basis for the development of science and technology and led to the scientific empiricism of the Enlightenment, as well as to the methods of simplification and subdivisions of Positivism. On the other hand, the limits of science, conditioned by its subjection to free will, had led to conceptions based on abstraction and ideology. Environmental impact studies have appeared at a time in which the ideological evolution of postindustrial society is bringing about an increase in critical and negative attitudes towards technology and its effects on Nature and the environment (e.g., opposition to nuclear energy, genetic engineering, polluting industries, etc.). At the same time, however, the pragmatic application of these ideological trends to dayto-day social and political administration is a difficult operation without the support of science. One of the needs of this postindustrial age is perhaps a return to "neohumanism", a holistic concept of culture. Environmental assessment studies provide the means to combine scientific, economic, social and cultural aspects in a harmonious and integrated concept of the landscape, in order to achieve a balanced relationship between humanity and the environment. This kind of holistic approach to the environment should provide a comprehensive view of environmental problems, including the basic elements needed by policy- and decision-makers for correct choices.

2.3 A method for surveying, mapping and assessing landforms as geomorphological assets* *by Mauro MARCHETTI

The conceptual principles and scientific method here described were employed in an

31 interuniversity study and can be taken as an example for a research on evaluation of landforms as geomorphological assets (Carton et al., 1994, modified). In the first phase of the work a ''Map of geomorphological units" is compiled. A document of this type may be easily derived from a detailed geomorphological map, where the geomorphological units are grouped according to their morphogenetic nature (e.g., glacial, river, karst). These groups comprehend more detailed elements of the rehef (e.g., a glacial cirque, a river meander, a slope subject to a network of lapies, etc.). Other details may also be taken into consideration, depending on the scale and aims of the research. During the second phase the scientific characters of landforms, leading to the evaluation of each unit or part of it (depending on the detail required) as a geomorphological asset, are indicated (e.g., as a paleogeomorphological example, see Table 2). In some cases other disciplines may include other assets, such as wetlands or the caves mentioned in the previous chapter. The third phase is concerned with the assignment of a differentiated level of interest to each unit or element considered as a geomorphological asset, according to its degree of scientific importance in terms of space, that is, whether its importance is of a local or more general nature (Table 2). For example, a glacial landform on an African mountain may be of value on a superregional or even a world-wide level because of its rarity, even if it is poorly preserved, whilst the same type of landform in the Alps may not be of any particular geomorphological significance other than purely local, even when properly preserved, because it is part of a widespread and common morphology in that mountain range. The product of these operations is a Map of geomorphological assets. Its preparation can be carried out through two techniques: manual and automatic. The final data, although containing the same quality and quantity of information, differ on account of the type of graphic representation. For the representation of geomorphological assets, the ''manual" technique makes use of the same symbols as those applied in geomorphological maps: the value of each asset and its level of interest are deduced from a table/legend accompanying the map itself. From the map it is possible to recognise the level of interest and the TABLE 2 Scientific characters and spatial levels of interest of geomorphological assets

type of attribute

character model of geomorphological evolution

scientific

level of interest ^

/'"^

/

object used for educational purposes

paleogeomorphological example

ecological support

' y ^^

32 character of each landform that in itself or with others makes up a geomorphological asset. The "automatic" technique represents geomorphological assets by defining the assets' areal extension. Their character and level of interest are identified by means of a legend based on a scale of different colours or shades. In the first case (manual technique) the identification of the geomorphological asset is performed directly by the geomorphological map's surveyor who, thanks to his/her knowledge of the territory, will select those landforms to which one or more scientific characters could be attributed. Only the landforms selected will be represented on the map of geomorphological assets, with their specific geomorphological symbol (i.e., scarp, ridge, slide) and colour corresponding to their morphogenetic type (i.e., fluvial, glacial). Each geomorphological asset is progressively numbered and inserted in a matrix (Table 3), according to their characters and level of interest. Thanks to the use of the geomorphological symbology, this map gives a particularly plastic and direct representation of geomorphological assets. It is therefore more easily understood and particularly useful and suitable for general use (parks. Nature reserves, etc.). During the subsequent phases quantitative values will be assigned to each geomorphological asset, in order to give these components of the landscape their proper weight (see 6.4). This will permit not only a ranking of the assets, but also their comparison with other environmental components. In the second case, the ''map of geomorphological assets" can be prepared using automatic techniques (Carton et al., 1994; Cavallin et al., 1995). The use of computers and in particular of GIS supports is in fact necessary for processing megabytes of geomorphological data. Also in this case the basic map is the geomorphological map. This map must be subdivided into polygons which represent areas characterised by a morphogenetic type (i.e., fluvial, glacial, etc.). Also the geomorphological feature (i.e., scarp, ridge, slide, etc.) or the characteristics of the deposits are represented on it. TABLE 3 Matrix for landforms selected as geomorphological assets. The numbers (1 — 15) refer to each landform and to an explanatory list Level of interest

Worldwide Superregional Regional Local Nonsignificant

Character Model of geomorphological evolution

Objects used for educational purposes

Paleomorphological example

3 9 7, 13, 14 11

15 5 6, 10

1 2, 4

Ecological support

8 12

33

Each polygon is automatically associated to a progressive number inserted as a simple record in a geographical database system (Table 4). In it the first record is the number of the polygon (i.e., 80), then the morphogenetic type (i.e., periglacial = 3) and the landform (i.e., landslide degradational scarp = 7), then for each type of the four characters (i.e., paleogeomorphological example = c) (see Table 2) a number corresponding to the level of interest (1 = world-wide; 2 = super-regional; 3 = regional; 4 = local; and 5 = nonsignificant) is assigned. If a polygon does not show any kind of geomorphological interest, nothing will appear in the relative columns and in the next column (z = polygon not interesting as a geomorphological asset) a zero (0) will be inserted. Since one or more polygons can be grouped in a geomorphological asset in the next column, the progressive number (N) of the geomorphological asset to which the polygon belongs is expressed, and in the following columns the character and level of interest are expressed not only for the single polygon, as in the previous columns. TABLE 4 Geographical database for a digital map of geomorphological assets Polyg on n

M

Asset F

Character a

80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103

3 4 4 3 4 4 3 4 3 4 3 3 3 3 3 3 3 4 3 4 3 4 4 3

7 10 9 7 15 9 7 8 7 9 7 5 7 5 7 5 7 9 7 9 7 8 0 15

b

4 3

N c

d

4 3

4

Character a

z

b

c

d

4

7 7

4 3

4 3

9

4

4

10

4

9

11 11

4 4

0 4

4 0 0 0 0 0 0

4

9 0 0

4 4

4 4

4 4

0 0 0 0 0 0 0 12

4

4

0

Legend: n) progressive number of the polygons; M) morphogenetic typology; F) landform; a,b,c,d) characters of landforms; z) landform negligible as asset; N) progressive number of assets.

34

but for the whole geomoq^hological asset. If the latter is composed by only one polygon, the character and level of interest of the geomorphological asset are identical to those of the single polygon. On the contrary, if a geomorphological asset corresponds to more than one polygon, the relative column will show the same progressive number for the polygons that make up the asset, and in the next columns the characters and levels of interest of the geomorphological asset are the same for each polygon belonging to the asset. For each quality the level of interest is maximum and derives from the comparison of each polygon making up the asset. To this purpose, many kinds of software installed on suitable hardware can be used. In order to prepare a schematic digital asset map, a Central Processing Unit (CPU) is necessary, which is the heart of the system, connected to a graphic tablet which permits the digitalisation of geographical data in vectorial format. A scanner may be useful for a quick acquisition of geographical data. Also devices for the representation of the results are necessary, such as videos, plotters and printers. The product of digitalisation is a set of lines defining homogeneous areas (polygons). This further step is simply acquired by means of CAD-type software. The next step is given by the transformation of the vectorial data in raster data. In this way the map is divided into pixels whose dimensions are determined by the degree of final precision and by the density of the entry data. The passage from vectorial format to raster format is important and necessary since in this way an attribute can be given to each pixel of the map. Then each pixel of the map is linked to a large

0---rJ\

\ ^\ '-' C \

Fig. 14. Selection map of geomorphological assets in the Castelnuovo del Zappa — Annico area west of Cremona. Legend: 1) regional level of interest; 2) local level of interest (after Marchetti et al., 1994).

35

number of codes, corresponding in the case of the geomorphological assets to the codes previously described which, finally, are stored in a database system linked to the geographical data. In this manner each pixel holds all the attributes relative to morphogenetic typology, form, character and level of interest of the polygon, number of the relevant geographical asset, character and level of interest of the geomorphological asset. Therefore, the structure of the database allows the map representation to be carried out as a function of each single attribute. An example of "map of geomorphological assets" elaborated using automatic techniques is shown in Fig. 14 (Marchetti et al., 1994). By comparing it with the manual one, the character and level of interest of the asset are immediately emphasized but not the type of landform. Nevertheless, this system has the advantage of being constantly updated, and also provides precise information according to different requirements. It is therefore a useful tool for urban and territorial planning.

3 GEOMORPHOLOGICAL HAZARD

3.1 Geomorphological instability In chapter 1 the concept of geomorphological hazard was introduced, with reference to geomorphological instability phenomena. It is therefore opportune to define the meaning of the term unstable landform: it is either ''a form which is not in equilibrium with the natural environment and which tends to reach a balance by modifying itself or "a form which has already reached equilibrium although remaining a particularly dynamic one". They are all landforms evolving in a way which disturbs human activities: for example, a landslide or a meander cause more or less marked environmental modifications due to erosion and accumulation processes. Moreover, instability must be considered in relation to the phenomena that cause it; one cannot speak about instability in absolute terms since an area could prove to be unstable with respect to a certain process (e.g., slope movements) and, on the contrary, stable with respect to others (washing out, fluvial erosion). The indication of the type of instability can also offer the essential elements for the choice of the most opportune and effective remedies. Finally, there are more or less acceptable degrees of instability according to the various social uses: the type and degree of stability required for a nuclear plant is quite different from that considered sufficient for the construction of a road. The following paragraphs illustrate the main kinds of geomorphological hazard in the widest sense, where Geomorphology can give a research contribution with some examples of specific investigations. The last paragraph introduces a mapping methodology for geomorphological hazard representation.

36 3.2 Soil erosion* *by Giuliano RODOLFI and Dino TORRI

3.2.1 Introduction Soil can be considered as a natural consequence of a situation of dynamic equilibrium established in time in the midst of the following environmental components: climate, bedrock, relief morphology, flora and fauna, antropic activity. It can be ranked between the main natural resources because its peculiar position at the intersection among atmosphere, lithosphere and biosphere makes it one of the principal actors in myriads of interactions. Obviously, any given soil type can be modified in its potential capacities and characteristics by altering the dynamic equilibrium under which that soil was formed. Often a more or less sharp but progressive decline of soil production capacities accompanies quick and intense modifications. Therefore soil should be considered as a natural resource subject to deterioration, which may be renewed only with difficulty in the long term. This chapter will examine one of the main groups of processes trough which soil capacities are depleted: soil erosion. 3.2.1.1 Definitions and terminology Two main groups of processes may be distinguished: 1) Soil forming (i.e., pedogenetic) processes, that lead to a progressive increase of soil thickness at the expense of bedrock and to its vertical differentiation into horizons {soil profile). 2) Morphogenetic processes, that are dominated by factors external to the soil (mainly climate). Within the more general framework of a relief-demolishing physico-mechanical action, they determine soil disgregation with removal of soil material and consequent thinning of the profile. Two main situations, due to interactions between pedogenesis and morphogenesis, can be identified: — A biostasis regime, which is a state of biological equilibrium between soil, vegetation and climate. Here, pedogenesis prevails on morphogenesis, as in the intertropical belt with rain forests or at middle latitudes in flat morphology. Pedogenesis may produce very deep soils with profiles well differentiated into horizons. — A rhexistasis regime, when the biostasis regime is broken. This may happen in particular climatic situations, or following exceptional natural events that could alter the morphological situation of certain areas of the Earth's surface. Direct antropic activities also cause intense perturbations. All these cause morphogenesis to prevail on pedogenesis. Then the soil undergoes progressive thinning up to the point of being completely removed, with outcropping of the inert bedrock. This often leads to progressive desertification in arid and semiarid regions. Desertification, as well as in other cases of severe erosion, is often caused or accelerated by human activities: precipitation is rare and often determines intense and aggressive soil erosion,

37

preventing effective pedogenesis; the lack of vegetation, although discontinuous, favours aeolian erosion which becomes the fundamental morphogenetic agent. Morphogenesis and pedogenesis can also compensate each other. As the soil is developed in depth, erosion removes its superficial horizons at the same rate. Soil thickness remains constant and so does its characteristic profile; the only effect of this process is a slow but progressive relief levelling (geological erosion). In the last two situations, the prevailing morphogenetic process is called soil erosion or surface erosion or, more generally, erosion. A distinction can be made (Zachar, 1982) between natural erosion, essentially due to natural factors, and anthropogenic erosion, connected more or less directly with human activities. In each case the intensity of the process and its possible consequences (erosion hazard) are of different significance. In normal natural conditions without any noticeable interference due to anthropic activity, erosion is kept within acceptable limits (normal erosion) and the soil productivity remains constant. As for antropic activities -^ in particular farming if carried out with conservation techniques — a slow down of the erosional process (inhibited erosion) could take place achieving situations of low or nil hazard. An exceptional natural event (intense and concentrated rainfall, mass movement, seismic shock, prolonged drought) can rapidly alter the equilibrium conditions previously discussed and trigger soil abnormal erosion. Similar consequences are attained when disequilibrium conditions are directly or indirectly determined by human activities (deforestation, nonconservative farming systems, earth movements, etc.); in this case the degradation process rapidly increases and the term accelerated erosion is used. When abnormal erosion interacts with anthropic activities it is usually amplified. The interaction scheme is shown in Fig. 15.

Fig. 15. Relationships between soil erosion types and related hazard levels.

38 Any component of the natural environment (such as air or water in its different physical states) becomes an erosion factor when, assisted by gravity, it is capable of modelling in the soil (and/or bedrock) a particular form (erosional landform), usually characterised by a transversal or longitudinal upward concave profile. The same component can act as a vector, transporting the eroded material towards places with lower energy. Here it can build a corresponding accumulation or depositional landform which tends to fill pre-existing concavities or to assume and maintain a convex profile. These forms of the earth relief are mainly differentiated according to the nature of the modelling factor (Table 5) and since there are several factors, a noticeable variety is eventually produced. Therefore the use of a geomorphological criterion seems appropriate to illustrate the various aspects of the erosive process and attempt a classification of the derived forms. 3.2.2 Physical bases of erosion 3.2.2.1 Forces produced by fluids in motion Soil erosion is the result of detachment, transport and deposition processes. The developing of these processes, their relative importance and intensity are strongly influenced by the particular set of initial conditions that characterise the soilvegetation-management complex. Let us survey how processes act and interact and how they are linked to soil, vegetation and management characteristics, beginning with the forces generated by fluids in motion. The movement of a fluid may either be well organized, describable as the motion of parallel sheets gliding one over the other (laminar regime) or turbulent with development of whirls and eddies. The degree of turbulence of a fluid motion is measured by means of an adimensional ratio called Reynolds' number:

R^-aiL

(1)

V

TABLE 5 Types of erosion in relation to the determining factors Dominant factor

Erosion type

Water Snow Ice Wind Gravity Organisms Man

Water erosion (*) Nival erosion Glacial erosion Wind (aeolian) erosion (*) Mass erosion, mass movements Organic erosion Anthropogenic erosion

*Asterisks mark the types generally referred to with the general term "soil erosion"

39 where w, h and n are, respectively, the mean velocity, the height and the kinematic viscosity of the fluid in motion. If Rf: is less than 200/300 the flow is laminar and flow velocity profile is parabolic. When R^r is over 1,000 the flow turns completely turbulent and the velocity profile is logarithmic. In the latter case, velocity at a given depth is a temporal mean made in order to eliminate the main effect of turbulence, i.e., the continuous variation of local velocity values. In other terms, one could say that the velocity in a given point (x,y,z) within the fluid (moving along the x-axis) is characterised, at a given instant t, by a velocity u(x,y,xj) resulting from the vectorial sum of the following components: w(x,y,z,0 = w(x,y,z) - i7 (jc,v,z,r) - ii^{x,}\zJ) ' uix^\z.t)

(2)

where the components u- (/ = xs,z) have a nil temporal mean. Velocity i7(x,y,z,t) is the temporal mean value at point {x^\z) and time /. If the Oxy plane coincides with the soil surface, then u(x,y,z) (from now onward abbreviated as u(z)) is the temporal mean velocity of the fluid at height z from the soil surface. The relationship between u(z) and z is as follows:

uiz) = —u^Logi^^) k

(3) k s

where k is von Karman's universal constant {k = 0.4 for clear fluids), k^ is a measure of the bed roughness, u^ is the flow shear velocity in the neighbourhood of the bed. Equation 3 is valid for 30.2z » k^, otherwise the flow has laminar characteristics and the velocity distribution is of the following type:

u(z) - l^}I}^z(2h 2v

- z)

(4)

where g is gravitational acceleration, a the local slope angle and /i^, coincides with the fluid thickness when the latter is completely in the laminar state, otherwise it is a convenient value used for linking curves (4) and (3). The situation is depicted in Fig. 16. The fluid motion generates forces which act on the soil grain both for detaching and transporting it. Such forces can be ascribed both to the laminar and to the turbulent flow. The former are estimated by means of relations such as the following one, valid for spherical grains with diameter D:

F = 3K^uD(\-^R}!ll.) 16v

(5)

40

a«

turbulent layer

T3 T3

"5

transition zone

laminar layer fluid velocity Fig. 16. Flow velocity profile for turbulent regime.

where r| is the fluid dynamic viscosity and ii is the fluid velocity in the neighbourhood of the grain (u being the intensity of the velocity vector). If the second term in brackets is negligible, then Eqn. 5 becomes Stokes' equation, normally used in laboratory tests to determine the particle size distribution of fine sediments. When the motion of the fluid is turbulent, the term proportional to the 2nd power of velocity (Eqn. 5) becomes dominant. In general terms, the force expressed by the fluid becomes:

C^A

p\u\

(6)

where C^ is the drag coefficient ( Q is about 1.1 to 1.2 for a disk placed across the flow, 0.2 to 0.4 for a sphere, 0.004 for an ellipsoidal body very elongated in the flow direction); p is the fluid density and A is the surface of the grain cross-section. The fact that the forces in Eqns. 5 and 6 have the same direction as the velocity means that, in the case of laminar flow, the force can be considered as parallel to the ground surface whilst in turbulent flow the force always shows a component normal to the ground surface. When this component has a direction oriented upwards, lifting forces originate; they are extremely effective in causing the detachment of the soil grains and afterwards in keeping them in motion. In the case of water erosion, the flow shows transitional characteristics between laminar and turbulent. It is usually turbulent in the case of channeled flows (such as rill and gully flow), while it may be laminar in interrill, diffused overland flow. In the case of wind erosion, the flow is always turbulent. As previously stated, the local velocity variations of a fluid in motion are always remarkable and therefore also the force of Eqn. 6 will undergo violent fluctuations. When the fluid is water, an example of the variations can be given by Naden's (1987) results. For turbulence lasting 0.2 s Naden suggests that ujzj) be normally distributed

41 with zero average and standard deviation given by the following empirical relationship: a^iz) = 0A6u(z)(-f'' z

(7)

Similarly, the ujzj) component, which is always characterised by a zero average, has the following lower standard deviation: aXz.t) = 0.17a/z,0

(8)

3.2.2.2 Forces resisting detachment Soil primary particles are usually linked by interparticle forces of varying nature and intensity. The spatial variation of the bonding forces causes the creation of aggregates of primary particles, that are linked to each other by intra-aggregate forces more intense than those linking the aggregate to other surrounding aggregates. The direction of application and the intensity of the detaching and bonding forces determine the prevailing dimension of the removed aggregates. The spatial dispersion of the two types of forces — detachment and bonding — contributes to the size distribution of the eroded aggregates. Obviously this means that together with aggregates single primary particles will be eroded also, hence the use of the generical term "grains" throughout the text. This definition of aggregate implies the identification of the forces resisting erosion with low-intensity bonding forces which control grains of such dimensions that the efficiency of the entrainment process is maximized. Together with intense and time-stable bonds there are others, probably due to organic matter, which show seasonal and longer term dynamics (Nestroy, 1993; Rousseva, 1989), depending on microbiotic activities and therefore on the type of vegetation present, farming techniques, local climate (which include slope aspect with respect to sun and wind; see Imeson & Lavee, 1996; Torri, 1996). Still others are due to clay particles and are therefore conditioned by the presence of water and solute. In some cases, generally characterised by the presence of sodium in the exchange complex and low electrolyte concentration, bonding forces can even be transformed into deflocculant repulsive forces Also frost and thaw processes can modify the state of surface aggregates: an experience common to whoever walks in the countryside on ploughed fields after an overnight frost is to find little heaps of isodiametral aggregates (often around 1 to 3 mm in diameter). On the contrary, during warm and dry periods the ground surface is richer in microaggregates, which are often powdery. Naturally, any action favouring a greater or lesser packing density of the grains influences the bonding forces, their spatial distribution and the dimensions of the eroded grains. Among these actions, ploughing, grazing and the traffic of heavy

42

machinery have direct consequences on the distribution and intensity of the bonding forces. With regard to erosion, these resisting forces, concerning the grains exposed on the ground surface, can be considered as Coulomb-type forces, that is with components due respectively to internal friction and cohesion. Such a resistive force has a direction coincident and opposite to that of a detachment force. In the case that the detachment is mainly due to lift, the resisting frictional term may disappear and be substituted by the submerged weight of the grain. 3.2.3 Water erosion 3.2.3.1 Introduction Water erosion is responsible for the most outstanding and most widely represented landforms. Water is in fact the main modelling agent of the Earth's surface at present: it entrains, transports and deposits large volumes of material in a very short time. Its action can affect both the innermost continental environment and, to a much lesser extent, the thin belt of transition to the marine environment (Table 6). Indeed, the dynamic action of the sea (waves, tides, streams) dominates coast modelling: with high and rocky coasts, submerged shelves and cliffs are formed owing to abrasion, whilst low and sandy coasts can be affected by coastal erosion. In the continental environment water erosion produces effects which are proportionally related to the intensity of the process and the nature of the soils and bedrocks. In general terms it can be said that the intensity of erosion is strictly linked to the volume of rain that reaches the soil in a certain time unit (e.g., annual mean rainfall). One should nevertheless consider that with the increase of annual rainfall in natural conditions, the density of the vegetation cover also increases. The increase in vegetation cover acts as a limiting factor to erosion because it reduces rain-drop impact forces, favours infiltration, and limits overland flow volume and velocity. Therefore, the lowest values of water erosion are attained either in arid regions, owing to the scarcity of precipitation (desert or subdesert areas) or in regions with intense rainfall but characterised by a thick vegetation cover (rain forests). In general TABLE 6 The various types of water erosion subdivided according to the active agent and the environment affected (after Zachar, 1982, modified) Environment

Erosion factor

Erosion type

Coastal

Waves, tides, marine streams

Abrasion, littoral

Continental (slopes)

Rainfall Overland flow (runoff) Subsurface waters (internal flow) Surface and subsurface waters

Splash Interrill, rill, gully Eluviation, leaching, piping, mass movements Polymorphic (badlands)

Streams (torrents, rivers)

Vertical, lateral (undermining)

Continental (thalwegs)

43 terms, the erosion maxima are recorded in correspondence with mean rainfall values ranging between 700 and 1,000 mm/year, because of the competition between rainfall erosivity and vegetation protective effect. This is, for example, the situation taking place in circum-Mediterranean regions where erosion forms reach their maximum expression. This is due to the simultaneous effects of relatively high rainfall and a very discontinuous vegetation cover, with vast surfaces that have long been subjected to farming activities. The erosional process may assume various aspects that can remain isolated but can also be connected both in time and space and from higher altitudes to lower ones; each of these aspects produces its own landforms, which are often clearly identifiable and distinguishable from all others. In the next chapters the complexity of the erosional processes will be illustrated and the main relief forms derived from these processes will be defined, discussing in particular water erosion and, more generally, wind (aeolian) erosion. Finally, some characteristics of the most current models of erosion assessment will be described. 3.2.3.2 Impact of raindrops Let us now try to follow what happens when it rains. The soil is initially characterised by a distribution of resisting and bonding forces that is altered during the wetting process, especially when the soil is initially dry. Usually some clay dispersion occurs, which may sometimes be sufficiently intense as to dominate detachment In any case, clay dispersion, when present, influences the whole erosion mechanics (Shainberg et al., 1992). In particular it is an important cofactor in the formation of some kinds of gully that can evolve into proper badlands (Oostwood & Bryan, 1994). Still during the first stages of a rainstorm, when soil has an initial low moisture content, a rapid wetting can cause slaking of the aggregates with detachment of microaggregates and particles (Yoder, 1938, quoted in Bryan, 1969; Le Bissonnais, 1990). The process takes place when the water occludes the external orifices of the pores of an aggregate.Thus the air within must get out by passing through the water itself. If the penetration velocity of the water inside the aggregate is larger than to that of the air diffusion through the water, then air is compelled within lower and lower volumes and, as a consequence, there is a pressure build-up which eventually breaks the aggregates. If the material expands while water penetrates then other breaking forces develop due to differential volume expansion at the water penetration front (producing fissures transversal to water penetration direction). The processes so far described do not require any mechanical energy but simply water: they can be reproduced by utilising a foggy-like rain or by submerging a dry soil sample into water. On the contrary, during rainfalls a noticeable quantity of mechanical energy is transferred to the soil by the impact of the rain drops. The impulse generated during the impact is used to break the drop, deform a small volume of soil, detach particles and aggregates and eject them together with water droplets all around the impact areola. In particular, during the collision a nearly

44 circular crown of jets is formed, moving away at high velocity from the impact point. Thus very intense forces are produced. They detach and drag away particles and microaggregates (seldom exceeding 2 mm in diameter), each of them wrapped within a droplet. They are then ejected and, after having covered a nearly parabolic trajectory, fall to the ground. The summarised description given in the previous paragraph is derived from the work of several researchers among which worthy of quotation are Engels (1955), Harlow & Shannon (1967), Ghadiri & Payne (1988) and Mc Carthy (1980). The material ejected by an impact is redeposited around the microcrater, according to a law generally well approximated by an exponential (Poesen & Savat, 1981; Reizebos & Epema, 1985, Torri et al., 1987b). Let us now examine how the drop impact interacts with the soil surface morphology. Several studies (Ekern & Muckenhirn, 1947; De Ploey & Savat, 1968; Savat, 1981) showed that with respect to the drop impact point, more material is detached downslope rather then upslope and total detachment increases as soil surface slope angle increases. The first physical explanation of the phenomenon was given by Torri & Poesen (1992) and is based on the considerations hereafter illustrated. Let us consider a drop falling vertically. In this case (Fig. 17,a) the mass upslope of the vertical line, which passes where the drop first touches the soil, is compelled to move (in the form of lateral jets) with an upslope component of motion, whilst the mass on the left of the line will have a downslope component. If the force applied to the detachment of soil particles is proportional to the water mass passing over them, then the quantity of material removed downslope is necessarily greater than that removed upslope. In fact, the vertical line subdividing the two drop fractions passes by the drop centre, dividing it in two equal parts, only when the slope is nil. In all other cases the line is always located upslope. Moreover, as slope increases, also the total mass of detached soil increases. This is due to the fact that the component of the gravitational force (G in Fig. 17,b), parallel to the ground surface and acting on aggregates and particles, behaves as a detaching force when the direction of the erosive force is downslope. When F^ is directed upslope, then G acts as a resistance. Since a greater soil mass is ejected downslope, the gravitational force is mainly acting as a detaching agent. This fact corresponds to a net increase of the total detachment force, and therefore of the total mass removed, with increasing slope. The overall effect on detachment is described by an integral equation resolvable with numerical methods (Torri & Poesen, 1992). The overall result on detachment is shown in Fig. 17,c. This effect is important only on steep slopes such as those, for example, that can be measured on the surfaces of clods. In fact the gradient felt by the drop during impact is not that of the hillslope or of the field, but that of the impact areola (Torri & Poesen, 1992). The fact that the ground surface is locally inclined implies that the mean jump distance of the ejected material is itself asymmetrical. The downslope length is greater than the upslope one and this difference is even more evident with the increase of the slope. The global effect of a more intense downslope detachment and

45 impinging drop upstope splashing drop mass

downslope splashing . -^ drop mass

(a)

Fd Rd Relative Splash 1 Detachment 2 ]

(c) 0

10 slope {%)

30

Fig. 17. The effects of a vertically falling raindrop are schematically shown: a) the upslope splashing fraction of the impinging drop is smaller than the downslope part: b) the diagrams of forces depicting both upslope and downslope particle entrainment show that gravity may either help (downslope) or resist (upslope) detachment; c) the overall effect of slope on detachment is shown for three soils with differing angle of friction (25, 35 and 55°).

a longer jump length is a net downslope sediment flow. The flow direction, however, can be strongly modified by the wind (Poesen, 1986) which can even invert it. The effectiveness of the drop impact can be noticeably diminished by the presence of water on the soil surface. In fact, when crossing water, a drop reduces its velocity before reaching the soil. Hence the crown of lateral jets formed during impact will act over a smaller soil surface while their force will be partly used for removing the underlying water layer. Finally, the material is ejected with a more vertical trajectory. In other terms, with the increase of the thickness of water covering the soil there is a decrease of material detached and of the mean jump distance (Palmer, 1963; Torri et al., 1987b; Profitt et aL, 1991). In particular.

46 according to Torri et al. (1987b), the detachment decreases exponentially with water depth. Let us now consider how the above described processes interact. It is obvious that there is a net flow of material ejected downslope since both the mean distance run by the particles and the quantity of particles ejected downslope is greater. The downslope net flow increases as slope increases. The process is further intensified by the presence of water (in puddles among the clods or in interrill and rill flow). In fact the detachment in the hollows among the clods is greatly reduced by the protective effect of water, thus the flow towards the top of the clod is reduced. In other terms, marked sedimentation takes place among the clods, while erosion is observed at the top of the clods themselves. It can therefore be said that one of the main effects of the drop impact consists in the levelling of surface roughness. At the same time, the quantity of mobilised material is very high. Generally this is the main source of sediments that in part will be afterwards removed by surface runoff. The combined action of the mechanisms so far described contributes to the formation of a thin surface layer of soil with reduced porosity, therefore with low hydraulic conductivity. Where erosion is prevalent, such as on top of the clods or on small reliefs, this process -^ known as sealing is due to soil compaction caused by drop impacts (structural seal). On the contrary, where redeposition is dominant the formation of depositional seals takes place. The two types of seal differ also morphologically since the second type is characterised by relevant thickness and a particle organisation that partly depends on the local sedimentation environment. The first, on the contrary, depends basically on the depth (a few millimeters) to which the effect of drop impact is felt. Owing to erosion, a portion of the compacted material is eroded during subsequent drop impacts. This causes a maximum of possible reduction of porosity beyond which the effect of removal and remixing of the soil grains is no longer felt. 3.2.3.3 Overland flow erosion Flow detachment Detaching forces in concentrated as well as in diffuse overland flow are those previously discussed (Eqn. 1—8). Local flow velocity fluctuations (turbulences) cause the detaching forces to fluctuate, giving rise to a temporal distribution of forces in any given spot. Naden's Eqns. 8 and 9 show a temporal distribution of overland flow forces acting per unit of soil surface in a given position. In that particular position soil grains are found which are bonded to the soil by means of any strength belonging to a certain distribution of resisting forces (g(R)), generally more intense than detaching forces. If resistance falls within the intersection area between the distribution curves of the detachment forces and the resisting ones, then detachment takes place. Let us now suppose that the two distributions are representative of the general situation of a given surface: the probability that the detachment event takes place is given by the probability that during the observation time interval /,, the detachment force is greater than resistance. This can be written as follows:

47

Fig. 18. Erosional-depositional patterns resulting from "up and down" ploughing. Rills develop in correspondance of sow furrows as well as of tire tracks on a steep slope (Tuscany, Italy).

Df = pyJlgiR)

jf,(F)dF\lR

(9)

where D^ is the amount of material detached during r^, per unit of surface, p, and V^ are, respectively, the mean dry bulk density and volume of the eroded grains,/^ is the frequency distributions derived by Eqns. 6, 7 and 8. Similar conclusions had already been discussed by several authors as Raudkivi (1976), Yalin (1977), Naden (1987), Torri et al. (1990) and Hearing (1991). Transport capacity of rill flow Once particles and aggregates have been detached or, in any case, introduced in the

48 overland flow, transport begins. This lasts until particles and aggregates are redeposited. The length run by a given grain depends on various factors. Among them, the internal forces, the thickness of overland flow water and the particle weight and shape. Turbulences, hydraulic roughness, cross-section variations, and drag coefficients of grains, etc., make it difficult to find parameters for the system. In fact, the forces acting on the soil grain comprehend gravitational forces that tend to make the particle fall and hydraulic forces, of the kind described by Eqn. 5, where velocity should be considered as the difference between the grain and the fluid velocity at that point. The presence of forces transversally directed with respect to the general motion direction of the fluid make the grain zig-zag vertically and horizontally. Sometimes, with bigger grains, the motion takes place by means of rolling and sliding. Other times, the grain proceeds by a series of successive jumps. The overall picture is furthermore complicated by the difficulties in defining the values to attribute to drag coefficients. The above-stated difficulties led many modellists to the application of simplified concepts such as the transport capacity of a fluid in motion, considered as the concentration of sediments that that system can transport at a steady state. In other terms, the transport capacity is the maximum quantity of sediments that can be transported by an overland flow having motion characteristics that remain constant in space and time (steady-state flow). In order to give reality to this definition it should be added that the processes of sedimentation should be just as constant, as well as the processes of sediment supply. This implies also the constancy of some characteristics depending on the type of sediment transported, such as density, dimensions and shapes of particles and aggregates. In some cases also the chemical characteristics of the transported material and overland flow water are important, when the balance between flocculation and deflocculation processes is unstable. If changes in overland flow and erosion are slow in time and space, then it is possible to define a transport capacity of overland flow. Despite the fact that this is rarely the real situation during a natural rain, the use of transport capacity is extremely widespread in modelling and seems to produce reliable results (at least at the present level of precision). As previously said, the processes making up transport capacity are manifold and complex. Therefore, empirical relations are usually utilised. A raisonnee collection of formulae for transport is given by Covers (1992). Rill erosion The water of overland flow does not cover the soil surface in a uniform way. On the contrary, it usually flows in fairly winding microrills (or protorills), of low erosive capacity, which later join in rills. When the flow becomes more turbulent and therefore more erosive, irregular channels are formed having a V-shaped transversal section, usually 10 cm wide and 5 to 10 cm deep which tend to come together creating a miniaturised hydrographical network. In recently ploughed fields, which are the areas most subject to this process, the main collectors can deepen their "beds", cutting through the plough layer, in the

49 most intense cases the cut can affect the whole soil profile until it comes into contact with the parent material, which is usually more resistant. This phenomenon is called rill erosion. As previously said, the phenomenon assumes its greatest evidence with the least protected soils, i.e., those that have just undergone ploughing or, rather, when the seed bed has just being prepared. The most critical period then falls between ploughing and the moment when the crops begin to grow (in particular herbaceous vegetation), thus ensuring an adequate cover to the soil. Some ploughing systems {up and down, that is in the direction of the maximum inclination) and particular crops (row crops) noticeably facilitate the development of rill erosion which will exploit preconstituted courses (the sow furrows) and eventually produce the most conspicuous forms (Hradek, 1989). Very often these forms of erosion reach such dimensions that they can hinder the use of machines in farming operations; in any case, they are wiped out by the subsequent ploughing. Evidence of the phenomenon taking place is indirectly given by the thickening of colluvial deposit belts at the toe of the affected slopes owing to coalescence of colluvial cones or colluvial fans that are formed in correspondence with the "mouth" of each rill system (Fig. 18). 3.2.3.4 Differential erosion The morphological evidence of the action of a fluid differs according to the constitution of the bedrock (rock, sediment or soil) on which the action takes place. On the outcrops of competent massive rocks a uniform smoothing action is produced (exaration in the case of glaciers, erosion in the case of water, corrasion in the case of wind). On the contrary, on coherent alternating lithotypes the more articulated the relief the more diverse will be the resistance to grain entrainment {differential erosion, morphoselection (Panizza and Piacente, 1978)). In the particular case of rain-drop impact, the occurrence of differential erosion during the event is confirmed by a characteristic microrelief of earth pillars (splash pedestals, Fig. 19) due to the soil local protection by coarse elements of various nature and particle size (pebbles, gravels, vegetal residues, etc.). Soil grains exceeding 0.5 cm are removed only with difficulty. If the dimensions are more than a few centimetres, also interrill overland flow is incapable of removing them (Poesen et al., 1994). Therefore small earth pillars made up of material protected by the overlying rock fragment are formed. Differential erosion can take place even if the flow is channelled. Selective erosion lasts until the surface is completely armoured. 3.2.3.5 Subsurface flow When a certain amount of rain water percolates into the soil or regolith, water can act both as a solvent, therefore triggering a set of chemical transformations by destroying some compounds and/or generating new ones, and as a carrier of particles, transferring them from one position to another (leaching, eluviation). Moving under the influence of gravity, these particles will follow vertical paths, in conditions of horizontal surfaces, or more or less inclined along slopes (lateral

50

Fig. 19. "Splash pedestals", an evident consequence of rain-drop impact. The diameter of the coin in foreground is 25 mm.

movement). In the latter case and with particular soils and parent material, subsurface flow can become sufficiently erosive to trigger removal of particles that are afterwards evacuated by means of a proper network of underground passages with outlets at the surface, usually in the higher slope stretch. According to its different dynamics and intensity, various terms are attributed to this process: tunnel erosion, suffosion, seepage, tunnel-gullying, piping, pseudokarst. The diameter of the water paths can vary enormously, from those a few millimetres wide, due simply to soil porosity, to those properly developed into real underground channels (Fig. 20). Among the main factors generating and keeping the phenomenon active, besides the precipitation variability and the vegetational cover discontinuity, shrinkingswelling of soils and of some bedrocks have been recognised; they develop during the arid season and are connected with the presence of levels having particular texture characteristics (Selby, 1982). These erosion forms develop frequendy also on the sides and heads of ditches or at the crowns of landslides, thus contributing to their rapid withdrawal. 3.2.3.6 Gully erosion When cuts reach such dimensions that it becomes impossible to eliminate them

51

Fig. 20. Surface collapse due to tunnel erosion in a gypsum-rich colluvial deposit (near Zaragoza, Spain).

through normal farming operations, then the term gully erosion is used, sometimes with the definition of ephemeral gullies for forms which have already such dimensions as to be considered gully, but which are nevertheless eliminated with specific farming works. Actually, the term "gully" groups many different types of natural channels such as the Italian calanco, although with some reservations, the French ravine, the Spanish arroyo, barranco, carcava, the Brasilian-Portuguese vogoroca, the German Delle, the Arabic wadi, the Indian hulla. the Swahili donga, etc. This widespread use of terms referring to the same phenomenon is witness to its extension in particular morphoclimatic environments showing as a common characteristic a certain aridity or, in any case, a net separation between a rainy season and a more or less prolonged arid one (Fig. 21). A typical example is given by the circum-Mediterranean belt. Hudson (1986) defines gullies as ''erosional furrows characterised by very steep

52

Fig. 21. Gully erosion on silty-clayey alluvial deposits (Eritrean highlands).

slopes and subject to sudden and intermittent runoJf'\ Brice (in Selby, 1982) considers them as ''recently fanned drainage channels with occasional runoff, steep slopes, vertical or highly inclined head, width exceeding 30 cm and depth over 60 cm'\ Any survey must consider these forms of erosion and describe in detail their features, since their distribution, type and density within each portion of territory can be assumed as indicators of its state of degradation. Gullies develop in particular situations determined by the concomitance of several factors: poor resistance to erosion of both soil and bedrock, high intensity of the meteoric event, conditions of the vegetal cover, slope morphology, land use and management which favour the concentration of water along runoff paths that eventually become permanent. Gullies affect mostly cohesionless or semicohesive materials (loess-like deposits, tuffs, colluvial deposits, poorly cemented marine sediments, etc.). As for their origin.

53 several hypotheses have been brought forward. Some authors consider them as the natural evolution of a master rill in a rill system; this situation seems nevertheless to take place only when the affected slope is sufficiently long with morphological features favouring the concentration of flow lines. In more general terms, gullies retreat upslope following a given direction marked by a single headcut (axial gullies, in coarse sediment), or following several headcuts (digitated gullies, in clay loams). The latter type may explain landscapes where erosion has deepened numerous routes that, after zig-zagging upwards and intersecting each other, have left less eroded reliefs, remnant of a past landscape (Alexander, 1982; Hodges & Bryan, 1982). When a concentrated flow path intersects an earth bank (e.g., sunken lane or road bank, lynchet and terrace bank, etc.) bank gullies develop frequently (Poesen, 1993). A particular type of gully (frontal gull}\ where width largely prevails on length) is usually associated with piping (in loamy sand sediment with columnar structure) which destabihses river walls and banks causing their collapse. Ephemeral gullies form where overland flow concentrates in either natural drainage lines or linear landscape elements such as furrows, access roads, etc. (Poesen, 1993). Erosion rates have been observed to vary between 0.9 and 1.2 cm/a (Vandaele, 1993) in the Belgian loam belt, comparable to annual values of rill and interrill erosion. 3.2.4 Aeolian erosion Wind is the main factor of aeolian erosion (wind erosion). This kind of erosion is particularly active in climatic zones with scanty precipitation (with average values of 250 to 330 mm/a) distributed in rare rainy events isolated by long drought periods and, as a consequence, with a discontinuous or totally lacking vegetational cover. The most remarkable effects and forms are found in concomitance with high temperature, such as in the arid environments of the tropical and subtropical belt. The dynamic processes of aeolian erosion are controlled by the following factors: — soil structure and texture, given that the wind action is highly selective; — soil humidity, especially at its surface, which contributes to keeping particles linked; — climate, in particular the length of the arid periods with respect to the rainy ones; and — vegetation, that similarly to what happens with rains, reduces wind speed inhibiting its erosive power. Aeolian erosion can be defined as the capacity of wind to entrain and transport loose particles elsewhere. For a given type of soil, the quantity that can be removed is a function of wind velocity and surface roughness. Particles are carried with patterns that differ according to their diameter. Three main types of transport can be identified: — suspension, affecting the smallest particles (generally less than 0.1 mm) that can be carried even over long distances, as it happens during dust storms'.

54 — creep, a kind of movement that compels larger particles (0.5 to 2.0 mm) to "creep" or roll at the surface, moved by the power of wind or pushed by other particles; — jumping, that is by successive "jumps" affecting those particles having dimensions too large to be carried in suspension or too small for rolling (roughly between 0.05 and 0.5 mm); their movement takes place by means of a series of rebounds having curved trajectories, whose height is proportional to the particle mass. Let us see some of what has been just described in the light of the equations ruling wind shearing and lifting forces, i.e., of the equation already described for a fluid in motion. As already stated, laminar wind flow does not exist. This makes things simpler as only Eqns. 3 and 6 are to be examined. It comes out clearly that high roughness values reduce wind erosivity because high velocities are attained at more elevated height values. Differently from water erosion, wind erosion is mainly due to lift detaching forces. This depends on two factors: firstly, wind produces an intense lift. Secondly, soil is mainly eroded when it offers a minimal resistance to detachment. The latter sentence means that soil is more erodible when it is dry, when grains are loose (no cohesion) and when friction is nil. Consequently, resistance to grain detachment is given by grain weight. This situation makes fine sand and silt, the grain-size classes more prone to wind erosion. Grain transport by saltation causes a peculiar type of detachment. When a wind transported grain is impacting the soil surface a large impulse is released on a very small impact area and other grains are consequently displaced. As soon as the displaced grains are ejected some centimetres above the ground surface they are more easily entrained by wind because it blows more intensely (wind velocity increases with height, as stated in Eqn. 3). A detailed study of the physics involved in wind erosion in desert environments was made by Bagnold in the late thirties, early forties (see Raudkivi, 1976; chapter 4). Forces depend on wind velocity (Eqn. 6), which, in its turn, depends on roughness. Surface roughness is here made of soil grain roughness, clods or other man-made soil surface structures, vegetation (including trees), fences, some elements of the landscape, etc. It is evident that, controlling surface roughness, e.g.. via fences, wind speed can be controlled and consequently wind erosion, to a certain extent, modified. The effects of wind deflation are basically the following ones: — soil thinning at the expense of the overlying horizons; — indirect soil enrichment with coarser material since, as discussed in chapter 3.3, only the finer fractions are removed; the process comes to an end when a proper erosion pavement is formed, as it happens in several desert areas; and — general levelling of the soil surface since the material transported can deposit into depressions filling them and reducing roughness.

55 3.2.5 Models for evaluating water erosion 3.2.5.1 General overview In the previous chapters an overview was given of the erosive processes and the related forms. Before surveying models suitable for predicting water erosion let us firstly discuss how the various processes can be assembled and give some nomenclature. In the previous chapters the actions of overland flow and rain drops in the production and transport of sediments were considered. In order to assemble them it will be necessary to apply widely the law of conservation of matter (or continuity equation). Let us consider a rill segment of unitary length, width vv and cross-section A. The quantity of sediments coming out of the system minus that going into it must equal the quantity of removed or deposited sediments within the segment considered. Let us first consider what happens outside the rill segment. The variation of intensity in the sediment flow (in terms of mass) between the outlet downstream and the input upstream of the rill segment is given by dS / dx. From the interrill zones (one to the right and one to the left) 2^, material per unit of length arrives, which is detached by the drop impact and carried by overland flow. The dS/dx - le- quantity must equal the quantity of material which is temporarily produced, deposited or provisionally stored within the rill segment. If 3(p,^) / dt is the mass corresponding to variations in rill volume (where p, is the dry soil bulk density) and d{^cAJ / dt is the variation of instant storage within the segment (where A^, is the transversal section wetted by rill, p the density of the transported grains and c is sediment concentration by volume), then the law of mass conservation is: ^^ = 2e ^ 9(pA) _ dipcAJ

^Q^

This equation, already used by Culling (1963), Meyer & Wishmeier (1969), Bennet (1974), etc., is known as continuity equation. It is valid, with opportune variations, for diffuse and concentrated flows of both sediment and water. When sediment and water continuity equation are considered together, they assure the correct transfer of the flow and sediment among contiguous cells of any distributed model (see forward). Practically, the continuity equation, together with transport capacity, detachment and sedimentation equations, ensures the linking of the different processes during erosive rains.The same equation is used also for transferring sediment along a slope until the toe-slope or a draining channel is reached. Now that the main mathematical tools for describing sediment production are available, models can be assembled. Obviously existing models do not describe fields, slopes or basins in the same way. In particular, a model is a simplified representation of nature and its performances depend on the way in which it is structured, the equations it uses, etc. Chow (1972) divided hydrological models into eight categories.

56 We will quickly examine the most important ones. In order to understand differences between models let us exemplify considering a basin as the object to which an erosion model is applied. The model is lumped if the basin (or large portion of it) is treated as one unit. In this case, the basin characteristics are necessarily simplified because the unit is treated as a uniform homogeneous system. This strongly reduces the possibility of controlling what happens inside the basin and model outputs are necessarily broad approximations if the basin is scarcely uniform. Distributed models subdivide the basin into a large number of small cells. Sediment is routed from one cell into another (using equations like Eqn. 10) and erosion and sedimentation can be located inside the basin. In this case each cell describes a sufficiently uniform and homogeneous area so that parameter values have reasonable chances of being well representative of reality. In any case, since there are parameters that often vary significantly even inside a single cell, model equations should be able to deal with spatial variability, introducing stochastic behaviour within predictions. Usually this is not the case. Let us consider a simple example of a process that takes place when a given threshold is overcome. Concentrated overland flow can dig a rill only when its shear velocity is larger than a given value which depends on soil cohesion (Rauws & Covers, 1988). Unless the model is distributed and each square metre is represented in a single cell, several rills may develop inside each grid element. Spatial variability of soil cohesion will allow rill formation first in a spot, later in another. The model will not allow a progressive increase in rill numbers because it deals with each grid element as a whole, abruptly passing from a no-rill situation to the opposite one. This type of model, where situations are always well determined, is said to be ''deterministic" while the one that considers spatial variability is said to be ''stochastic". The equations chosen for representing processes and rates inside a model can be based on either empirical relationships between variables or physically based formulae and algorithms. Obviously, an empirical model must always be tested, sometimes even partially modified, before being used in areas too different from those where it was developed. Physically based models have a better chance of reasonably approximating reality when used elsewhere. Models presented in the years 1960—1980 are mainly deterministic and lumped, and use physically based equations only rarely. More recent models are distributed and physically based equations are frequently used. As an offset, however, it is more costly to feed data and to run them because of the increased number of parameters and the length of the their codes. Among the many water erosion models in use nowadays, EPIC (Srinavasan & WiUiams, 1996) and GLEAMS (Nicks, 1996) must be mentioned. New models are WEPP (Nearing et al., 1989), KINEROS (Woolhiser, Smith & Goodrich, 1990), EUROSEM (Morgan et al., 1996), LISEM (De Roo et al., 1996) and the MEDALUS series (Kirkby, 1996a,b). Let us try to understand what type of parameters are needed for assembling a model able to describe soil loss. We have seen that the main detaching agent is raindrop impact. This means that we need equations able to account for splash effect

57

correctly. This is usually achieved through a correct estimation of drop kinetic energy, drop interception by canopy cover, intercepted water subdivision in "permanently" held water, stem-flow and new drop production by canopy (see Brandt, 1989, 1990; Styczen & Hogh-Smith, 1986). The erosiveness of the storm is also due to overland flow detachment and transport. This means that the model must generate runoff, then make it flow and cumulate along the slope. This means that slope angle and length must be known. Also, all the soil and vegetation properties that influence water infiltration (e.g., porosity, aggregate stability, surface depression storage, etc.) or resist runoff flow (surface roughness) are needed. The Universal Soil Loss Equation (USLE, Wischmeier & Smith, 1975) is a simple model where those parameters are taken into account. This is a deterministic, empirical and lumped model. Storm erosiveness is found by means of 3 factors: 1) rain erosivity, where drop kinetic energy is multiply by a sort of magic factor, i.e., maximum rain intensity in half an hour; 2) slope length; and 3) slope angle factors. Effects of vegetation and management are accounted for with two other factors. The soil erodibility factor includes all the soil characteristics more or less relevant to define soil resistance to detachment and transport and soil hydrological behaviour. This model was obtained using data collected in field plots. It gives estimates of mean annual soil loss over a 20-year period and should be applied to uniform, homogeneous fields with no gully erosion. Its use for shorter periods is not correct and should be avoided. A model with opposite characteristics is EUROSEM, which is deterministic and distributed and uses many physically based equations. It was explicitly written in order to be run on single rain events. In between these two extremes, the WEPP model describes what happens between one event and the next. It can therefore be used for assessing erosion over longer periods, at least a few years, although maintaining a high temporal resolution. A model that can compete with both the previous ones is MIKE-SHE (Danish Hydraulic Institute, 1993), associated with the erosion functions of EUROSEM, because it is based on an excellent hydrological model with a continuous simulation over indefinitely long periods. In some cases erosion models, able to run over periods of thousands of years may be necessary. In this case previously listed models are not suitable because parameters, that are relevant for a given time span, can lose their meaning at other time scales and become sources of errors. A model for long term erosion evaluation was presented by Hallet & Putkonen (1994) for a study comparing the age of some moraines. Here the parameter of major interest is linked to topography, which is represented in terms of the length and slope inclination of moraines. In particular, a simple sediment flow model is applied which is expressed by means of the following equation:

58 S = a ( l + bx) — dx

(11)

where a and b are site-specific constants, x is downslope distance and z is the altitude. In this case precipitation does not appear any longer as an explicit term: the time interval considered is so long that the characteristics of single rainfalls totally lose their importance. Only mean precipitation amounts over very long times are important. This fact, though, is reflected only on a and b and is important only when it is necessary to compare environments with definitely different climatic histories. Equation 11, together with the mass conservation one (Eqn. 10) leads to the following expression: dz .1 — = a(\ dt

bx)

ab dx'^

dz_

(12)

31

where the storage variation term of Eqn. 10 is neglected since it is not significant in this case. The use of Eqns. 11 and 12 permitted Hallet & Putkonen (1994) to solve a conflict about the dating of some moraines. The conflict arose following dating carried out by means of cosmogonic isotope analysis and stratigraphy-based considerations. The first technique allows the age of exposition to the surface of a certain rock to be evaluated. Nevertheless one should also consider the surface rejuvenating processes due to erosion. 3.2.5.2 De Ploey's erosion susceptibility model From 1987 until 1992, De Ploey (1989, 1990) developed a rather interesting model with a wide range of applications and intrinsic simplicity. The model was developed starting from Eqn. 13 relative to erosion linked to the upstream movements of little water falls found in rills and gullies (Fig. 22). The basic idea was that of considering the volume V of the eroded material proportional to the total mechanical energy

original surface plunge pool

Fig. 22. Sketch illustrating a (fictitious) longitudinal gully profile: small falls and headcut retreat rate are the basic idea supporting De Ploey's (1990) model.

59 (potential and kinetic) of the water at each fall. This simplification resulted in the following equation:

V{T)= EJ'^q^it) gh^it) -

u-{t)

\dt

(13)

where: T, t = time; E = soil susceptibility to headcut erosion; g = gravity; Qi = discharge rate at the i-th fall; hj = fall of the i-th height. De Ploey (1989) presented a simplified version of Eqn. 13 which is valid for a single headcut. It can be derived from Eqn. 13 by simply substituting the integral and the summatory with a total height h and a total discharge QT, where Q is the mean discharge rate over the time span T. Also u must be changed with a mean flow velocity U over the period T. Using such simplifications it follows that:

V = EQT gh

U'^

(14)

Of course Eqn. 14 is an approximation but it contains two terms that represent two different types of erosion. Let us suppose that diffuse interrill and rill erosion are dominant. Then the term linked to potential energy is negligible with respect to the velocity term. Hence Eqn. 14 may be rewritten neglecting gh. The opposite is true when gully erosion is dominant or when landscape is characterized by badlands. Here gh dominates. In the former case Eqn. 14 needs a long series of assumptions mainly due to difficulties in handling mean flow velocity. On the contrary, the latter case can be easily applied, hence it will be treated in some detail in the following paragraphs. Equation 14 then becomes:

(15)

E = APTgh^

where h^^ is the depth of the gullies found in a catchment of area A (more precisely, A is the gully catchment or the area pertaining to a given gully system) and total annual precipitation P has replaced Q. In this case time units are years. If we consider that V = A^jh^ (where A^ is the surface occupied by gullies), after substitution in Eqn. 15, the following is obtained:

60 AG

(16)

Equation 16 is easy to handle as the ratio between the two areas is measurable both from aerial photographs and field surveys. The annual mean precipitation can be estimated with a certain accuracy (ratings with uncertainties around 10—15% can be obtained). It is therefore possible to assess erosion susceptibility for gullies once the rvalue is known. The same technique is valid for assessing badland areas, where the ratio between the two surfaces is close to 1. From the data examined by De Ploey (1992), it results that in the case of gullies and badlands E varies according to the values illustrated in Table 7. It should be noticed that none of the classical elements, such as inclination intervals, slope lengths and relief energies appears in Table 7. Although some of the elements linked to the pedotype appear, the effect of vegetation is dominant. 3.2.5.3 PSIAC model One of the methods allowing all effects of soil erosion to be evaluated (considered as the quantity of removed material and destined to sedimentation) in hydrographic catchments of a certain extension, located in prevalently mountainous environments and characterised by high energy relief, is that applied in 1968 by the US Pacific Southwest Inter-Agency Committee (PSIAC) in the south-western territory of the United States (Table 8). The factors that the PSIAC method adopts as fundamental for the calculation of the sediments produced in a certain area, and therefore for the assessment of potential erosion within its boundaries, are the following ones: 1) lithology: composition, cohesion degree, bedrock alteration and fracturing; 2) pedology, soil typology and physicochemical characteristics; 3) climate: temperature regime, but mainly frequency, duration and intensity of rainfall. 4) runoff: flow regime both along slopes and riverbeds; TABLE 7 Erosion susceptibility values for gullies and badlands (after De Ploey et al.. 1995) E values (s/m')

Domains

10""*—5• 10"''

Badlands in different climatic belts, cultivated land; steppe areas with savanna, woodland; degraded grasslands and savannas (often due to overgrazing).

3-10~^—10~^

Partly degraded steppe woodlands or savannas with duricrusts including laterite cappings; areas underlain by soils with a high absorption capacity (sandy soils, chernozems); areas with mixed farming: arable land, prairies and forested slopes.

10"^—3-10"^

Areas with occasional gullying, predominantly thalweg gullying; forested catchments with dispersed cultivation; perennial grasslands and savanna woodland with appreciable runoff coefficients during storm events.

61 TABLE 8 Scoring table of the PSIAC model Sediment yield level

Land factors 1. Lithology (A)

2. Soils (B)

3. Climate (C)

4. Runoff (D)

High

(10)

(10)

(10)

(10)

a) Marine shales and related mudstones and siltstones.

a) Storms of several a) High peak a) Fine textured flows per unit days duration easily desperded; area. with short saline-alkaline; periods of intense b) Large volume high shrink-swell of flow per unit characteristics. rainfall. area. b) Freeze-thaw b) Single grain silt-fine sand. occurrence.

High to moderate

(7.5)

(7.5)

(7.5)

(7.5)

Moderate

(5)

(5)

(5)

(5)

a) Rocks of medium a) Medium textured hardness. soils. b) Moderately b) Occasionally weathered. rock fragments. c) Moderately c) Caliche layers. fractured.

a) Storms of moderate duration and intensity, b) Infrequent convective storms.

a) Moderate peak flows. b) Moderate volume of flow per unit area.

Moderate to low

(2.5)

(2.5)

(2.5)

(2.5)

Low

(0)

(0)

(0)

(0)

a) Massive hard formations.

a) High percentage of rock fragments. b) Aggregated clays. c) High content of organic matter.

a) Humid climate with rainfall of low intensity. b) Precipitation in form of snow. c) Arid climate, low intensity storms. d) Arid climate, rare convective storms.

a) Low peak flows per unit area. b) Low volume of runoff per unit area. c) Rare runoff events.

(Continued)

62 TABLE 8 (Continued)

Land factors 5. Topography (E)

6. Ground cover 7. Land use (F) (G)

8. Upland erosion 9. Channel erosion (H) (I)

(20)

(10)

(25)

a) Steep upland slopes in excess of 30%; high relief; little or not floodplain development.

Ground cover a) More than 509^- a) More than 50% a) Eroding banks continously or at frequent does not exceed cultivated. of the area 20%: b) Almost all of characterized by intervals with large depths and long flow area intensively rill and gully a) vegetation duration, or landslide sparse; little or grazed, b) Active headcuts and no litter; c) all of area erosion. degradation in b) no rock in surrecently burned, tributary channels. face soil.

(15)

(5)

(5)

(17.5)

(17.5)

(10)

(5)

(0)

(10)

(10)

(10)

(25)

a) Moderate up- Cover not exland slopes ceeding 40%: (less than a) Noticeable 20%). litter. b) Moderate fan or floodplain development.

a) Less than 25% a) About 25% of a) Moderate flow depths, the area medium flow duration cultivated. b) 50% or less characterized by with occasionally rill and gully eroding banks or bed. recently logged. or landslide c) Less than 50% intensively erosion, b) Wind erosion grazed. with deposition. d) Ordinary road and other constructions.

(5)

(-5)

(-5)

(5)

(5)

(0)

(-10)

(-10)

(0)

(0)

a) Gentle upland a) Area comple- a) No cultivation, slopes (less tely protected b) No recent logthan 5%). by vegetation; ging. b) Extensive rock fragc) Low intensity alluvial plain. ments, litter; grazing. little opportunity for rainfall to reach erodible material.

a) No apparent a) Wide shallow chansigns of erosion. nels with flat gradient, short flow duration. b) Channel in massive rock, large boulders or well vegetated. c) Artificially controlled channels.

63 TABLE 9 Classes of probable sediment yield, resulting from the summation of scores assigned to each erosion factor (example from Southwestern USA, USDA, 1968) Values

Class

mVha/year

>100 75-100 50-75 25-50 0-25

I II III IV V

>14.29 4.76-14.29 2.38-4.76 0.95-2.38 <0.25

5) topography: slope inclination and orientation, relief energy; 6) ground cover: expressed as percentage of covered surface; 7) land use: both farming and nonfarming use; 8) upland erosion: areal and mass erosion in inter-fluves; and 9) channel erosion: prevalently channelised erosion in riverbeds. The method is divided into three main phases: I: Subdivision of the whole hydrographic catchment considered into subcatchments with dimensions compatible with the degree of territory complexity and with the scale on which the results are given. II: Attribution of a "weight'' within each catchment unit to each of the above listed factors, directly proportional to the role assumed in favouring the erosion process. Values ranging from 0 to 10 are given to factors 1, 2, 3, 4, whilst 5, to which a first rank role is recognised, varies between 0 and 20. As for 6 and 7, which could also exert a containment action of erosion, the corresponding interval is -10 to 10. Finally, values of 0 to 25 are attributed to factors 8 and 9, since they contribute most to the sediment production. Ill: Summation of the given factors allows the attribution of the subcatchment to one of the 5 sediment-producing classes and, therefore, to one erosion susceptibility class (Table 9). The upper and lower limits of these classes are also expressed in m%a/year, corresponding to intervals prefixed by the model on the grounds of quantitative data, obtained by means of direct measurements. The model is considered correctly applied when the sum of values attributed to 1,2, 3, 4, 5, 6, 7 factors equals that relative to 8 and 9, which therefore must be evaluated first. This operation permits the verification of an agreement between the potential causes of erosion ( 1 + 2 + 3 + 4 + 5 + 6 + 7) and the effects (8 + 9) directly observable in the field. Not always do the two totals coincide; in this case one should proceed to a new assessment of the causes or consider whether the gap between the values thus obtained is ascribable to the action of a particularly aggressive factor, which should therefore be identified. A confirmation of the broad area over which PSIAC can be applied is given by Bazzoffi (1984). He tested both USLE and PSIAC models in three different watersheds whose sediment yield was measured by means of related reservoir silting. The results clearly showed PSIAC values closer than USLE ones to real soil losses.

64 3.3 Landslide hazard* =^by Alessandro PASUTO and Mauro SOLDATI

3.3.1 Introduction Several papers and detailed reviews dealing with landslide hazard have been recently published (Hansen, 1984; Varnes, 1984; Hartlen and Viberg, 1988; Brabb, 1991; Corominas, 1992; Jones, 1992; Hutchinson, 1995). An important incentive has certainly been given by the International Decade of Natural Disaster Reduction IDNDR - (1990-2000), proclaimed by the United Nations', during which different working groups have been set up, such as the International Landslide Research Group, aiming at the preparation of a proposal for worldwide hazard maps (Brabb, 1993), and the International Geotechnical Societies' UNESCO Working Party on World Landslide Inventory which is contributing to the establishment of a World Landslide Inventory by suggesting standard terminology for landslide investigation (WPAVLI, 1990; 1991; 1993). Most of the authors above quoted feel that landslide hazard is generally neglected, more emphasis being given to other types of hazards such as seismic and volcanic hazards. It has been estimated that every year about 225,000 lives are lost because of natural events in general (Burton et al., 1978), among which several mass movements causing many casualties (for a more comprehensive review of these events see Eisbacher & Clague, 1984; Hansen, 1984; Brabb, 1991; Gares et al., 1994). The fact that landslide hazard is usually underestimated is even more unfortunate, since slope movements are usually more easily predictable and manageable than earthquakes, volcanic eruptions or hurricanes. Actually, besides high-magnitude mass movements which occur quite seldom, there is a huge number of medium to small sized landslides which are so widespread that the related cost for human society is even higher than that of catastrophic events. The losses due to lowmagnitude, high-frequency events is also generally increasing, especially in developing countries, because of human activity which, on the one hand, tends to increase landslide hazard (road cuts, quarries, etc.) and, on the other hand, favours vulnerability situations. Jones (1992) stated that landsliding is generally poorly understood by nonspecialists and for this reason underestimated in development programmes. In addition, Varnes (1984) noticed that a considerable proportion of damage and casualties generally related to earthquakes and hurricanes (including also volcanic eruptions) is actually to be ascribed to landsliding which occurs as an effect induced by these phenomena. A significant example is that of the 1970 earthquake in Peru, which claimed 70,000 lives, 20,000 of which in a debris avalanche detached from the north peak of Nevado Huascaran (Plafker & Ericksen, 1978). The correct interpretation of landslide hazard and consequently the optimisation of the interventions to be carried out in order to mitic^ate the loss of human lives and

'UN resolutions 42/169 and 44/236 (for further details see Scott. 1992).

65 economic assets, depends on some general considerations. First of all gravitational phenomena should be considered as an effect of natural landscape evolution. It is in fact the interaction of these phenomena with man's activities and structures that causes casualties and damage. Indeed, on the one hand, population increase has determined a greater need for natural resources and space available for human activities and, on the other hand, constant technological progress and the improvement of living standards have caused remarkable changes in natural environments; as a consequence, natural events such as landslides have had increasingly serious consequences on man's life and activities. Unfortunately, it should be pointed out that the terms Hazard and Risk are often used rather ambiguously. After a general definition of the various aspects of geomorphology given by Panizza (1987), Hartlen & Viberg (1988) specified that "hazard is confined to the expected occurrence of landslides, while risk involves the damage expected to be caused by landslides (lives lost, cost of damaged buildings or facilities etc.)". Therefore, it becomes clear that landslide hazard should be adequately assessed in order to give a feasible prediction of the level of risk. Varnes (1984) maintained that among all natural hazards, landslides are those most easily foreseen and mitigated. For this purpose this Author defines the most fundamental principles on which landslide hazard assessment is based according to the following statements. ~ 'The past and present are keys to the future". This means that, according to uniformitarian principles, areas characterised by geological, geomorphological and hydrological conditions which favoured or triggered landslides in the past, or are doing so in the present, are very likely to be subject to failures also in the future. Nevertheless, the absence of past and present failure does not mean that landslides will not take place in the future. Thus, detailed investigations on the physical environment and on the spatial and temporal occurrence of landslides are of primary importance in terms of landslide hazard assessment. However, the effects of human activities and climatic changes, which are variable with respect to what has been mentioned above, should always be taken into account. — "The main conditions that can cause landsliding can be identified". An understanding of the processes involved in slope movements is fundamental and present technology in most cases allows these causing conditions to be identified. — "Degrees of hazard can be estimated". This means that, once the factors and the processes involved are known, qualitative or semiquantitative assessments of their contribution to landsliding can be made. To the three statements above quoted, Hutchinson (1995) adds a fourth: — "The various types of landsliding can generally be recognised and classified, both morphologically, geologically and geotechnically". This Author emphasizes in particular the importance of geotechnical aspects which are very often neglected or overlooked in this kind of investigation. The definition of landslide hazard and risk should therefore be based on a precise knowledge of landsliding phenomena, in terms of type, mechanism and evolution characteristics as well as the main factors which have conditioned or could condition

66 their temporal and spatial development, including the rate of human activities in the area studied. This chapter deals with the methods and techniques which can be utilised for the assessment of landslide hazard. It should be underlined that the term landslide is used in its most general meaning, that is: ''the movement of a mass of rock, earth or debris down a slope under the force of gravity" (Cruden, 1991) and that only subaerial phenomena will be discussed. 3.3.2 Types and causes of landslides 3.3.2.1 General aspects When a study of mass movements is enterprised, which also aims to define landslide hazard, it is of paramount importance to use clear terminology which can be universally understood also by nonspecialists (IAEG, 1990). It is therefore important for these phenomena to be immediately identified and subdivided according to the geomorphological and kinematic characteristics directly observable. In scientific literature, there are several ''classifications"' regarding to slope movements which have been proposed since the beginning of this century. These often complied with the needs of various authors but more recently an effort has been made to use the least ambiguous terminology. Most recent classifications try to emphasize both the processes leading to the development of a landslide and the materials involved; others are based on morphology, landslide mechanism, deformation velocity, causes of movement, geometry of the failure area and the deposits, age, etc. This paper does not aim to describe the numerous "classifications" proposed but particular attention will be given to the two most commonly accepted, that are illustrated and subsequendy modified by Varnes (Varnes, 1958; 1978; Cruden & Varnes, 1996) and Hutchinson (Hutchinson, 1968; Skempton & Hutchinson, 1969; Hutchinson, 1988). Both authors use the type of movement as a discriminating element. Basically, what changes is the kind of approach: whilst the former tries to emphasize the aspects linked to the result of the movement, the latter analyses the conditions that lead to failure. Essentially, the scheme proposed by Varnes is easier to apply and requires less experience, whereas the one proposed by Hutchinson is more complex and based on an approach more from the engineering viewpoint. Varnes proposed his first subdivision in 1958. He distinguished gravitational phenomena as falls, slides, subdivided into glides (with translational slip surface) and slumps (with rotational slip surface), flows and complex. The first two types were further subdivided according to the material affected, which could be either rock or soil; flows were in turn subdivided on the basis of particle size distribution and the water content of the loose soil. Subsequent elaborations of this scheme were carried

'It should be specified that the use of the term "classitlcation" is to be discouraged when referred to a complex natural process which could hardly be inserted into the rigid scheme of an excessively detailed distinction (see: 1.3).

67 out in 1978 and 1996 and at the moment this subdivision is universally accepted. The main criterion used by the author in the identification of the landslide categories is the type of movement, whereas a further subdivision is made on the basis of the type of material. As for the type of movement, five categories are represented: falls, topples, slides, spreads and flows. On the other hand, the materials are divided into two types: rock and engineering soil; the latter is further subdivided into debris and earth. In this way a landslide can be described by means of two words: the first one describing the material and the second one the type of movement (Table 10). These types are distinguished as follows. Fall: Generally the mass is detached from a very steep slope along a surface on which little or no shear displacement takes place. It occurs mainly in the air and the phenomenon includes the material's free fall, saltation, bouncing and rolling. Topple: The movement is due to stresses which cause a toppling momentum around a rotation point situated below the centre of gravity of the rock mass affected. The phenomenon can evolve into either a fall or slide. Slide: The movement takes place by means of a shear displacement along one or more surfaces or along thin zones of intense shear strain. Either rotational slides (slumps), in which the movement occurs along a curved and concave failure surface, or translational slides, in which the mass moves along a plane or undulating surface, can be found.

TABLE 10 Landslide types (according to Varnes, 1978 and Cruden & Varnes, 1996) Type of movement

Type of material Bedrock

Engineering soils Debris

Earth

Fall

Rock fall

Debris fall

Earth fall

Topple

Rock topple

Debris topple

Earth topple

Slide rotational translational

Rock slump Rock slide

Debris slump Debris slide

Earth slump Earth slide

Lateral spread

Rock spread

Debris spread

Earth spread

Flow

Rock flow

Debris flow

Earth flow"

Complex

Combination of two or more principal types of movements (e.g., rock fall avalanche)

""Mudslide (British usage)

68 Spread: These are movements characterised by lateral spreading, typical of a jointed rock mass and often combined with subsidence occurring in softer, less competent underlying materials; sometimes it is not possible to recognise a basal slip surface nor well defined ductile deformation zones. Flow: The phenomena characterised by continuous movements in space fall within this class; in them the shear surfaces (or thin zones of distributed shear) are shortlived, closely spaced and not usually preserved. From the kinematic standpoint the movement could be compared to that of a viscous fluid. Varnes foresaw a sixth type of movement called «complex» which is the result of the combination of two or more of the five classes above illustrated. In this regard, the specification introduced by Dikau et al. (1996) seems opportune: ''... all landslides involve more than one type of movement either acting concurrently in different parts of the failure (compound' landslide) or evolving downslope or over time into a different process (i.e., initial failure to subsequent deformation and complex landslides)". In their latest revision of this ''classification", Cruden & Varnes (1996) stated: "the sixth type proposed by Varnes (1978), complex landslides, has been dropped from the formal classification though the term «complex» has been retained as a description of the style of activity of a landslide". Moreover, other attributes are introduced so that the definition becomes more elaborate but more information is thus available on the gravitational movement. The type of landslide is described by a series of adjectives placed in sequence which provide information about the activity, the rate of movement, the moisture content, the material and type of movement (e.g., active rapid wet rock slide). On the contrary, the classification proposed by Hutchinson (1988) is much more complex since it: "... pays regard primarily to the morphology of slope movements, with some consideration being given also to mechanism, material and rate of movement". It ranks landslides into eight large groupings (rebound, creep, sagging of mountain slopes, landslides, debris movements of flow-like form, topples, falls and complex slope movements), in turn further subdivided. Recently, a new subdivision (Table 10) which takes into account both Varnes's (1978) and Hutchinson's (1988) classifications has been proposed. It was developed within the framework of a EU funded project on landslides in Europe", is of simple application and appears very suitable for landslide studies in Europe (Dikau et al., 1996). As regards the causes of mass movements, they can be divided into two large categories (Terzaghi, 1950; see Fig. 23):

'Composite according to Cruden & Varnes, 1996. 'EPOCH Programme, 'The temporal occurence and forecasting of landslides in the European Community" (CT. 90 0025) (Casale et al., 1994; Soldati, 1996).

69

causes reducing material's shear strength

geological factors (llthology and tectonics)'

causes increasing shear stresses

'• morphological features {slope geometry, slope angle, aspect etc

climatic conditions . (precipitations, freeze-thaw i cycles etc.) I

removal of lateral support (undercutting, excavations etc.)

tectonic activity (uplifts, seismic shocks etc.)

Fig. 23. Internal and external causes of landslides.

— Internal causes: they lead to the failure of the slope without the intervention of perceptible changes on the surface; they are in fact causes which reduce the materiars shear strength. — External causes: they determine an increase of the shear stresses by means of more or less evident modifications of slope morphology. Naturally, the factors belonging to these two categories can be combined to varying extents so they do not fit in with any particular classification. Let us now consider more in detail the internal causes which condition the stability of a slope. 3.3.2.2 Internal causes Geological factors They are given by all the primary characteristics that make up a slope, first of all lithology which includes both the rock's mineralogical and textural composition and its structure. Among rock types more easily subject to disarrangement phenomena, clays are to be quoted, both those of primary origin and those derived from the meteoric weathering of volcanic materials. Argillaceous soils are particularly subject to instability phenomena such as flows. Also the structure of the formation involved is very important. For example, formations made up of alternating layers, such as flysch, where rock types showing ductile mechanical behaviour overlie brittle rocks, are usually subject to widespread and vast landslides. Also discontinuities, both primary (bedding planes) and secondary (schistosity and foliation planes), play an important role in the materials' geomechanical behaviour especially with regard to mass movements such as the attitude of layers (low-angle dip-downstream beds will generally be more prone to sliding than dip-upstream levels). Of course, all these intrinsic characteristics of the materials can undergo changes of varying intensity and rapidity during the stages of a slope's geomorphological evolution. In fact, certain kinds of processes which to some extent modify the materials' physicochemical characteristics can take place, causing a consequent decrease of shear strength. Some processes such as weathering, softening, hydration.

70 exsiccation, etc., may require a long time and induce changes of the shear resistance which in some cases can be considered cyclical. Others such as remoulding, liquefaction, fluidification, develop much more rapidly and can lead to the collapse of an entire slope. These processes can have both natural (precipitations and snow melt) and man-induced causes (deforestation, creation of reservoirs, diversion of watercourses, etc.). In any case the final result is a progressive deterioration of the slope leading to decreased cohesion of the materials that make it up. As a consequence, hydraulic conditions which in the past had caused only small movements could, if the weakening process continues, result in a slope failure (Fig. 24). Among geological factors, also the tectonic history of the slope should be considered. The presence of important structural discontinuities such as faults, fractures, cleavage, joints, slickenside or even folds, fault-folds, etc. induce an intrinsic weakness of the internal bonds thus making the slope particularly prone to landsliding. The control exerted by tectonic factors is particularly important in large landslides or deep-seated gravitational slope deformations (Dramis & Sorriso-Valvo, 1994). Morphological features They include first of all the slope geometry, i.e., its height, length, shape, slope angle and aspect. The influence of the slope gradient is quite evident with respect to stability; indeed, very vast and steep slopes are more predisposed to disarrangement; on the other hand, even low-angle slopes can be subject to particular types of landsliding such as, for example, flows. Climatic conditions Although there are several climatic parameters playing some role in the starting of landslides, the most important one is without doubt rainfall. Rainfall should be considered not only as the triggering pluviometric event (considering therefore only its intensity) but also as the amount of rain falling and accumulating during a certain

^

landslide

Time

Fig. 24. Progressive decrease of the factor of safety against time. Temporary decreases of the factor of safety without slope failure may be due to exceptional rainfall and snowmelt. Slope failure takes place when the factor of safety descends below 1 (modified after Terzaghi, 1950).

71 period (considering also the length of time). If the percolation velocity is low, the rainfall duration assumes predominant importance for the triggering of the movement; on the contrary, in soils characterised by high hydraulic conductivity the function played by rainfall intensity becomes of paramount importance (Campbell, 1975). The role of water mainly consists of an increase in porewater pressures caused by variations of the groundwater level. The cyclicity of these changes is a determining factor in weakening the internal bonds of the materials making up the slope. The influence of the various types of rainfall will of course be different according to the type of landslide. Therefore, intense and time-concentrated rainfall will be particularly effective in triggering spatially limited movements such as debris flows, soil slips, mud flows (see also 3.6.14), whilst prolonged precipitations could mainly give rise to slides and complex phenomena. Several studies analysed the relationship between rainfall and soil disarrangements in order to identify a pluviometric threshold above which landslides occur. At the present level of knowledge no agreement has yet been reached about this. Nevertheless, it seems that this threshold could be related to the mean annual rainfall of the area in question (Govi & Sorzana, 1980). Another important aspect concerns the type of precipitation. In fact, the time interval whose cumulative rain amount can cause disarrangements was defined as "effective rain period" (varying according to the authors from 15 to 40 days), whereas the maximum rainfall occurring in a time interval of 1 to 3 days before the starting of disarrangement processes, was defined as "peak rainfall" (Friz et al., 1986). Therefore, the ratio between these two magnitudes conditions the development and type of slope movements. Even if the specific literature is very exhaustive, there are nevertheless margins of uncertainties on the applicability of these correlations in terms of forecast. 3.3.2.3 External causes As regards external causes which result in an increase of shear stresses, those processes giving rise to changes in slope geometry, mainly with an increase of the gradient, should first be considered. Among these processes the following are particularly important: undercutting due to the erosion of a watercourse or to the excavation for road construction, the exploitation of quarries or open pits, etc. Slope geometry can change also because of phenomena taking place underground: for example, the formation of large cavities owing to mineral chemical solution or squeezing out of underlying ductile materials. Overloading can be another phenomenon inducing an increase of applied shear stresses. This can happen owing to both natural causes, such as soil imbibition by rain or snow melt, accumulation of materials from volcanic eruptions, avalanches, debris flow or other movements, types of vegetation, etc., or human activities as, for example, in embankment construction, buildings or other structures, in accumulation of tailings from mines and pits or in lacustrine basins or artificial reservoirs. Also the removal of lateral support exerting a counterthrust on the slope could be a cause of unbalance. For example, a glacier's withdrawal is one of the most frequent causes which determined vast postglacial landslides in the Alps. To this category also

72 tectonic phenomena such as activities along fault planes and subsidence should be ascribed, besides gravitational movements themselves. It is, in fact, obvious that the detachment of a portion of slope induces in the in-place material a redistribution of the stresses which can eventually cause a new movement. Another factor which can favour the development of mass movements is lateral pressure. It can be exerted by water percolating into joints or by ice; also swellings due to hydration could induce considerable lateral pressures as well as the mobilisation of residual tectonic stresses. Tectonic activity can be a direct cause of slope movements in the case of regional tilting or uplift. Also in this case the effect of the tectonic process takes place essentially by means of an increase of the slope gradient. Finally, superficial deformations induced by volcanic processes can increase the shear stress within a slope also locally. A particular discussion should be reserved for seismic shocks and other transitory stresses such as vibrations induced by operating machines, explosions, rock falls, etc. These events can cause both a reduction of the materials' shear strength and an increase, although temporary, of shear stresses. The former can be exemplified by phenomena such as liquefaction induced by sudden, cyclic increases of porewater pressures with a consequent instantaneous decline of the soil's resistance and the collapse of the slope. This happens most frequently in saturated cohesionless mediumfine materials such as sands, silts, clays, etc. The increase of shear stresses is, on the contrary, linked to both the accelerations to which the materials are submitted during vibrations and the temporary increment of the gradient angle in unstable slopes, since: ''... it is known that on metastable slopes a change of gradient angle by tens of seconds may cause the disturbance of their equilibrium and the movement of potential and stable landslides and collapses" (Solonenko, 1977). Finally, the effect of vegetation on slope stability may be twofold: sometimes it promotes stability by reducing the action of climatic agents on the slopes, protecting the soil from precipitations, wind and sunshine: it retains a considerable amount of rain water by means of leaves, branches, etc., and eliminates water through evapotranspiration; runoff and erosion decrease and there is an increase in shear resistance of the slope thanks to the roots (Prandini et al.. 1977). On the other hand, the presence of vegetation may originate negative effects by increasing the load on slopes, enhancing the action of wind forces on the trees and disjointing the soil through the action of tree roots which widen cracks and favour infiltration (Varnes, 1984). 3.3.3 Techniques for landslide investigations In order to assess landslide hazard correctly, it is necessary to carry out in-depth investigations on the spatial and temporal occurrence of mass movements in the study area. The approach to the study of landslides, according to the cases and the scale of investigation, could benefit from various methodologies and techniques, mainly

73

consisting of field surveys finalized to geomorphological mapping, indirect methods producing factor maps depicting areas with different potential landsliding and, lastly, geotechnical and hydrological investigations. When possible, a combination of various approaches would be the most suitable answer (Fig. 25). 3.3.3.1 Retrospective research Retrospective research is fundamental in order to define the importance, distribution and repetitivity of slope disarrangements in the territory examined. It can greatly benefit from historical archive research, since it was demonstrated that many gravitational phenomena (about 90%), even of large dimensions, tend to take place through reactivation processes in areas already involved in similar events in the past (Pasuto & Silvano, 1987; Govi, 1988). For this reason archive research is considered as a necessary preliminary step in the study of the temporal occurrence of landslides. An investigation carried out by Brundsen & Ibsen (1994) showed that most countries in Europe have extensive historical sources of great value in assessing the characteristics of geomorphological processes. These sources mainly consist of narrative in literature, prints, paintings, ground and aerial photographs, remote sensing series, newspapers, maps, scientific '- Bibliography - Archives - Radiometric dating - Dendrochronology - Lichenometry -- etc.

Retrospective research

Data base and GiS

TECHNIQUES FOR LANDSLIDE INVESTIGATIONS

- Storage, treatment and analysis of data - Link between data and models

Remote sensing

- Ground photographs - Aerial photographs - Satellite and radar imagery

Mapping

Direct mapping Indirect mapping • Land systems mapping

Geotechnical investigations and monitoring Fig. 25. Techniques for landslide investigations.

- Surface deformations - Subsurface deformations - Groundwater level

74 papers and technical reports. It should be emphasized that it is often difficult to gain access to and analyze data from most of the sources mentioned above, because they are not usually collected for scientific purposes. In addition, the data are generally discontinuous in space and time and are influenced by the perception filter of the recorder. Despite these difficulties, archive investigations often allow statistical analyses to be carried out. This is the case of the temporal landslide record of the Ventnor complex on the Isle of Wight, off the southern coast of England. The record shows increasing landslide activity over the present century, partly due to a higher number of events recorded, but also possibly related to increasing rainfall, at least for the recent period (Ibsen & Brundsen, 1996). In general pluviometric series are very significant for the evaluation of the temporal frequency and distribution of instability phenomena. In fact, as previously described, meteorological events are often the triggering cause of landslides. On the basis of the statistico-probabilistic analysis of historical files on climatic data it is possible to assess the return time of critical precipitations above which widespread mass movements are started. A frequent recurrence in rainfall time usually implies higher hazard levels (Govi, 1988). Examples of landslide inventories compiled by government agencies and research institutions are rather numerous nowadays. Nevertheless there have been successful attempts to set up archives on landslides by single researchers, even in remote times, such as the works by Almagia (1907: 1910) who collected several pieces of information on nearly 800 landslides affecting the Italian Apennines between the years 1103 and 1908. More recently, in a bibliographical study, Guida et al. (1979) collected references of 950 papers on landslides and hydrological hazards in Italy in the 1900-1978 period. Landslide inventories, based also on aerial photo-interpretation and field surveys and often resulting in maps, have become frequent since the Seventies in several countries (see also Brabb & Harrod, 1989). Among the others, significant examples relate to the Czech Republic and Slovakia (Nemcok & Rybaf, 1968; Rybaf, 1973); to some Italian regions (Carrara & Merenda, 1976; Guzzetti & Cardinali, 1989; Antonini et al., 1993) and to the United States (Brabb et al., 1972; Brabb, 1989). Alger & Brabb (1985) carried out thorough bibliographic and archive perusals on landslides in the United States, collecting over 6,500 pieces of information. Landslide inventories, though representing only a first step toward the definition of landslide hazard assessments or maps are, however, of great benefit from a planning viewpoint. For this reason there is a growing need to create landslide inventories based on the above mentioned kinds of information and various projects have developed within the framework of the International Decade of Natural Disaster Reduction. This is witnessed, for example, by the activity of the International Geotechnical Societies' UNESCO Working Party on World Landslide Inventory, aiming to contribute to the definition of the World distribution of landslides (WPAVLI, 1993). For this purpose, also the AVI project was set up, commissioned by the Italian Ministry of Civil Defence to the National Group for Prevention of Hydrogeological Hazards, aiming

75

to complete an inventory of the sites historically affected by landslides and floods in Italy and intended to be the first step towards the outlining and mapping of hydrogeological risk in Italy (Guzzetti et al., 1994). As for the Italian territory, an important contribution to the definition of landslide hazard and risk was given by the investigations carried out within the framework of the SCAI Project (Study on Unstable Inhabited Centres) of the National Group for the Prevention of Hydrogeological Hazards, promoted with the purpose of ranking and representing on maps the landslides affecting inhabited centres which had been declared either fit for restoration or relocation or those in which hazard conditions for public and private safety had been recognised. Reviews accompanied by geomorphological maps are already available for a few regions, such as Veneto (Bozzo et al., 1992), EmiliaRomagna (Annovi & Simoni, 1993), Piemonte (Luino et al., 1993) and Umbria (Felicioni et al., 1994). Retrospective research also has the advantage of landslide dating carried out by means of radiometric (usually '"^C), dendrochronological and lichenometric methods, as witnessed also by several authors. As regards the former, which is certainly the most widespread method, dating is carried out on samples of organic material (generally wood and peat) found inside the landslide bodies (Johnson, 1987; Corominas et al., 1994; Panizza et al., 1996b). At present, further perspectives are opening for chronometric investigations related to mass movements occurring in the Late Quaternary, thanks to the development of analytical techniques, such as Accelerator Mass Spectrometry (AMS), Thermally Ionized Mass Spectrometry (TIMS) and laser fusion, and new dating methods, such as Optically Stimulated Luminescence (OSL) dating and alpha recoil dating (R. Dikau, pers. commun., 1995). A significant advantage lies in the possibility of dating events directly; for example, the age of aspect of a landslide scarp can be obtained by determining cosmogenic nuclides produced in situ (see Nishiizumi et al., 1993). In addition, these methods enable the dating of materials other than organic ones. For example, OSL method may be used for debris flow, mud flow or rock avalanche dating, on the basis of the exposure of mineral grains to daylight. 3.3.3.2 Databases and GIS An important aspect of landslide investigations is the possibility to store, treat and analyse the spatial and temporal data available. With respect to the application of data capturing technologies, such as remote sensing, radar and Digital Terrain Models, computer tools are fundamental in guaranteeing the necessary links between data and models. In fact, temporal data can be used to define the occurrence of landsliding in terms of time series analysis or to evaluate spatial landslide patterns in terms of regional hazard assessments. These assessments consist of probability statements and landslide hazard scenarios concerning future slope stability (Dikau et al., 1996). A database generally stores attributes given to individual landslides in previous investigations. These attributes include a wide range of lithological, topographic, geomorphological and geothechnical information. There exist several code tables and checklists for the collection of data to be stored in databases (Carrara & Merenda,

76 1976; Pasek, 1975; Pasek et al., 1977; Kienholz, 1984: WPAVLI, 1990, 1991; Chacon et al, 1992). A significant example of a landslide database is that of the UK National Landslide Data Base, produced by Geomorphological Services Limited (GSL) for the Department of the Environment between 1986 and 1989, which results from a fully comprehensive study of literature and maps concerning landslides, though it is still not representative of the total distribution of mass movements in the UK. For each landslide record the database includes several attributes, which are displayed in the National Landslide Databank codes table (bedrock and superficial geology, type, age, etc.; see Ibsen & Brundsen, 1996). When the data on a landslide are complete, this database provides very useful information ranging from the geological aspects to the techniques used to investigate a landslide. Time attributes are a weak point of this database, since subdivision according to age is very limited; in fact these temporal aspects are unfortunately not always included in databases. Research carried out within a European Programme on Climatology and natural Hazards (EPOCH) dealing with the temporal occurrence of landsliding in the European Union (Casale et al., 1994) has provided data for the improvement, in terms of time attributes, of landslide databases of selected sites. Dendrochronology datings have been obtained for the Spanish Pyrenees (Corominas et al., 1994) and radiometric datings have been made in the area of Cortina d'Ampezzo (Dolomites, Italy) (Gasparetto et al., 1994; Panizza et al., 1996a). Furthermore historical archive data have been gathered for Germany, France and the southern coast of Britain. The EPOCH Project above mentioned has also contributed to the definition of temporal attributes of landslides in terms of activity, defining the type, the return period, the mode and the period of latest activity, besides the state of activity generally foreseen in databases (Flageollet, 1996; see Fig. 26). Geographical Information Systems (GIS) contribute to landslide research by storing, handling and analysing spatial and temporal data. These operations mainly aim to draw up landslide susceptibility and hazard maps, displaying the potential location and intensity of future mass movements, and to analyse landslide time series in relation to triggering factors. Geological maps, slope morphometry and landslide inventory maps make up the most important databases for these GIS analyses (Fig. 27). Through landslide susceptibility maps, which are quite easy to read for nonspecialists, decision-makers may be provided with a cost-effective policy option for coping with landslides. Examples of landslide susceptibility maps are provided by Varnes (1984) and Brabb (1984; 1993; 1995). However, Dikau et al. (1996) stress that though databases and GIS are potentially very powerful, they are just technical tools ''which cannot replace the necessity to improve our understand of the landslide process itself in space and time" and cannot obviate fieldwork. 3.3.3.3 Remote sensing Remote sensing is a very powerful tool for the study of landslides and the assessment of landslide hazard, especially because it is generally cost-effective and allows multitemporal reconstruction to be carried out. Remote sensing data can be used at

77

RETURN PERIOD

TYPE OF ACTIVITY

STATE OF ACTIVITY INACTIVE

PERIOD OF LAST ACTIVITY

MODE OF ACTIVITY

Prequaternary (> 2.000,000 yr BP)

STABILISED

long term (> 1.000 yr)

definitely or long term

RANDOM

SINGULAR (one event on record)

DORMANT —^ I— low frequency (100-1,000 yr)

temporarily or in a predefined period (i.e. 200 yr)

EPISODIC —> renewal failure (irregular evolution)

I — INTERMITTENT - nonseasonal - seasonal reactivation failure

ACTIVE

Pleistocene - Lower (700.000-2,000.000 yrBP) - Middle - Upper (10,000-700,000 yrBP)

PROGRESSIVE (increasing rate)

medium frequency (0-100yr) high frequency (1-10 yr) Holocene - ancient (3 000-10,000 yrBP) - ancient historic (200-3.000 yr 8P) - recent historic (1-200 yrBP) - present

- very high frequency (<1 yr)

(regular evolution)

SURGE (sudden in time scale measured)

-CONTINUOUS

I Fig. 26. Temporal terminology of landslides (modified after Flageollet. 1996).

various stages: for the recognition, classification and mapping of landslides; for monitoring the activity of landslides and for the prediction of future spatial and temporal distribution of landslides (Mantovani et al., 1996). Remote sensing may be based on satellite and radar imagery and on aerial photographs. Satellite imagery, with spectral reflectance bands in the visible spectrum and in the near infrared, are particularly useful in regional studies of large-scale instabihty ij.i;iiB»g»j:<.m<<^^y

GEOLOGICAL FACTORS (lithology, tectonics)

m.^mimmi^'^^^^^mmm.

SLOPE CHARACTERISTICS (slope angle, slope aspect etc.)

LANDSLIDE INVENTORY

LANDSUDE SUSCEPTIBIUTY AAAP Fig. 27. The contribution of GIS in landslide hazard mapping.

VEGETATION COVER LANDUSE ETC.

78 thanks to their wide geographical extent, while vertical stereoscopic aerial photographs are fundamental for landslide investigations ranging from a regional to a local scale (Hansen, 1984; Brundsen, 1993; Hutchinson, 1995). In both cases multitemporal covers are often available. Satellite imagery has been used in landslide studies since the midseventies, but mainly for scientific purposes; this is due to poor definition, which generally enables the user to detect only large mass movements, and to the lack of stereoscopic cover. According to Brundsen (1993): 'it is certain that there are satellites capable of recording the detail required but the results are not open to current work" because priority is given to security. However, the availability of satellite imagery with spatial resolution below 10 m and with stereo capabilities is expected in the next decade (Mantovani et al., 1996). Nevertheless several authors have used LANDS AT or SPOT images for the identification of mass movements, which are recognised indirectly through specific terrain conditions associated with landslides, such as changes in lithology, vegetation and soil humidity, and not through morphological evidence as is the general procedure with aerial photographs (Rib & Liang, 1978). In practice, for the inventory mapping of landslides and the assessment of slope instability, satellite imagery and large-scale aerial photography are used in combination, the latter for the study of representative sample areas and the former for extrapolation of the findings to wider areas (Mantovani et al., 1996). Infrared imagery provides valuable information for the identification of the existing landslides and of landslide-susceptible terrains. In fact, according to Rib & Liang (1978), they provide information on surface and near-surface moisture and drainage conditions as well as indicating the presence of bedrock at shallow depths enabling a distinction between loose superficial deposits and bedrock. Other remote sensing techniques, such as multispectral imagery and microwave radiometry, may be used in landslide investigation, but they have been applied in a limited number of cases. However, there are significant perspectives in advanced remote sensing techniques which may also give input data for CIS systems particularly regarding soil moisture and subsurface water concentrations (Brundsen, 1993). Systems such as AIRSAR (Airborne synthetic aperture radar) or AVARIS (Advanced airborne visible/infrared imaging spectrometer) can be very valuable for the study of shallow landslides and forested slopes. Within the coming decade there will be increasing availability of radar satellite imagery (ERS-1, ERS-2, JERS-1, Almaz, Radarsat), which, however, because of geometric distortion will give rather poor quality images in mountainous regions (Mantovani et al., 1996). Remote sensing techniques are, however, traditionally based on the use of vertical aerial photographs on various scales (generally ranging from 1:5,000 to 1:70,000) which can be monochrome, true colour, false colour and infrared. Aerial photographs are generally more easily available with respect to the above described images and show higher definition. The greatest advantages of using aerial photographs in landsHde studies are the following (Rib & Liang, 1978): — the use of aerial photographs gives a three-dimensional and overall view of a relatively large area;

79 — the boundaries and morphological features of existing mass movements can be identified; — hints of incipient movements (scarps, cracks, etc.) can be observed over a wide area; — surface and subsurface drainage patterns can be outlined: — relationships between topography, drainage patterns and natural and man-made features often become evident; — rock formations and superficial deposits can be observed in their undisturbed conditions; — geomorphological features can be identified even if a vegetation cover is present, which makes observation quite difficult during fieldwork; — continuity and recurrence of geological and geomorphological features can be defined; and — the temporal occurrence and development of landslides can be outlined by comparing photographs of different periods. With respect to the possibility of reconstructing slope evolution, interesting perspectives come from the application of analytical archive photogrammetry, which "represents a development of conventional photogrammetry made possible using computerized analytical methods" (Chandler & Brundsen, 1995). This technique enables precise spatial data to be obtained from a wide variety of historical and archive photographs, including oblique photographs. Two images of the same object and at the same time are needed even if taken from different positions and with different cameras. With this method, which involves the use of a graphics laboratory, it is possible to produce data location maps, contour maps, profiles, DTM's and other analytical illustrations. This technique has been developed and tested in the study of the Black Ven landslide in Dorset (England) using oblique and vertical aerial photographs taken at five intervals from 1946 to 1988 (Chandler & Brundsen, 1995). 3.3.3.4 Mapping Mapping is often a necessary step in landslide hazard studies. Maps may consist either of basic documents such as geomorphological maps and landslide maps or of landslide susceptibility maps and hazard zonation maps. Mapping can be defined as direct, when the landslide extent and distribution are recognised on a geomorphological basis with the aim of extrapolating slope stability assessments to the rest of the investigated area, or indirect, when the possibility of landslide occurrence is assessed in relation to the controlling factors, with or without consideration of landsUde distribution (Hansen, 1984). Direct mapping is usually obtained through aerial photo-interpretation and ground surveys, which vary proportionally according to the scale adopted for the study, together with bibliographical and archive investigations. The result can be inventory maps (see also section 3.1) which record landslide distribution over an area (Nemcok & Rybaf, 1968; Brabb et al., 1972; Rybaf, 1973; Carrara & Merenda, 1976; Radbruch-Hall et al., 1976; Conway et al., 1980; Alger & Brabb, 1985; Guzzetti & Cardinali, 1989; Antonini et al., 1993). They are generally the basis for further

80

detailed studies aiming to assess future failures. The information contained in these maps may also include landslide type, bedrock lithology, superficial deposits, slope angles, engineering geological zonation, etc. Geomorphological maps are another product of direct mapping. These are generally at a larger scale (less than 1:25,000) than those described above and form a very useful basis for landslide hazard studies since they include details on general slope morphology and related processes, and define the type of failure and degree of activity as well as the features associated with landslides (for a review of geomorphological mapping methods and legends see Tricart, 1965; Bashenina et al., 1968; Verstappen & Van Zuidam, 1968; Demek, 1972; Panizza, 1972; Demek & Embleton, 1978; Barsch & Liedtke, 1985; de Graaf et al., 1987; Cooke & Doomkamp, 1990; Pellegrini et al., 1993; Gruppo di Lavoro per la Cartografia Geomorfologica, 1994). Geomorphological mapping may be easily and usefully applied to the study of mass movements. There exist many significant examples of landslide-orientated geomorphological maps, such as that of Brundsen et al. (1975), carried out with aim of identifying landslide hazard in an area of Nepal where the construction of a road and a bridge was proposed, or that of Panizza (1990) where the landslides surrounding the town of Cortina d'Ampezzo (Italian Dolomites) are mapped in detail. An interesting attempt to include the temporal aspect in landslide mapping was made by Flageollet (1994) in the above mentioned Dolomite area on the basis of data and maps collected and elaborated by Panizza (1990) and Gasparetto et al. (1994). The temporal information reported on the computer-based landslide map of the area of Cortina d'Ampezzo is related to periods of activity during the Holocene and to the frequency and incidence (first time failure or removement failure) of the movements. Geomorphological maps may in many cases be too complicated to be read by nonspecialists. This is the reason why planners and engineers are generally provided with derivative maps which appear simpler from a purely graphic standpoint and take into account only those processes and landforms that are connected with slope instability. The product can be a geomorphologically-based landslide hazard map. Many examples, carried out according to different methodologies, are known in literature. The first example should refer to the French ZERMOS (Zones Exposees aux Risques de Mouvement du Sol et du Sous-sol) hazard maps which have been produced for several sample areas at a scale 1:25,000. The hazard (actual and potential) is assessed and rated taking into account distribution of landslides, evidence of instability, as well as geological, geomorphological and hydrological aspects (for further details on methodology and limits, see Antoine, 1977; Humbert, 1977). Another significant example is provided by Kienholz (1978) who carried out a study in the Grindenwald, an area of the Swiss Alps affected by mass movements and avalanches; first a geomorphological map was produced and from this a natural hazard map was obtained. The degree of hazard was assessed for a high number of previously defined geomorphological units using checklists. When the casual factors of landslides can be recognised, measured and mapped and the analysis of the distributions of the different factors controlling landsliding can

81 be made, indirect mapping can be carried out. An original method aiming at the elaboration of hazard susceptibility maps has been defined and tested in a few sites in Italy (e.g., Panizza et al., 1980; Carrara et al., 1987) and has been recently adopted by the National Geological Survey as the official procedure for the compilation of geological hazard maps (Amanti et al., 1992). This method includes both indirect and direct mapping procedures. In fact, this approach consists, on the one hand, of the analysis of the causes of instability (ascribable to both slope and fluvial processes) which are outlined in a series of thematic maps (geological, hydrogeological, vegetational, etc.) and weighted by means of specific procedures, with the aim of producing a first derivative map (integrated analysis map) depicting the distribution of potentially unstable areas. On the other hand, by means of geomorphological investigations, the analysis of the effects of instability is carried out in order to give an outline of landforms and processes, both active and inactive, referable to geomorphological instability. The cartographic documents produced during this phase are a geomorphological map and a second derivative map (map of geomorphological dynamics) showing the areas which are at present unstable. The comparison and critical discussion of these two derivative maps lead to the production of a "geomorphological hazard map", a concise document depicting present and potential unstable areas subdivided according to the causes of instability (see also chapter 3.9). A partly similar approach was used for research on landslide hazard in Tuscany (Italy) which led to the elaboration of several landslide maps, accompanied by geological information permitting the assessment, albeit qualitatively, of the ratio between the degree of stability attributed to an area and its structural characteristics. In these hazard maps the following categories are distinguished: highly hazardous areas, including active and dormant landslides, areas characterised by high potential instabiUty linked to the morphological aspects, areas prone to landsliding owing to lithological aspects, areas of medium stability and finally stable areas (Nardi et al., 1985, 1987; Dalian et al., 1991). Jones (1992) refers to a further approach. Land Systems Mapping, which is placed among indirect mapping techniques and is based on the subdivision of the study territory into limited areas (land systems, land units, land elements, passing from large scale to small scale subdivisions) in which predictable combinations of landforms, soils, vegetation and superficial processes take place. This approach, created with the goal of producing basic maps for rural planning (Christian & Stewart, 1952), has been extremely useful in landslide hazard assessment since it allows large portions of territory to be quickly classified providing a useful regional picture for data collection and storage (Savigear, 1965; Wilhusen, 1979; Styles et al., 1984; Kienholz, 1984; Cooke & Doomkamp, 1990). 3.3.3.5 Geotechnical investigations and monitoring Slope movements show great variety not only because of the different types but also from a kinematic and geometric point of view. Each landslide is characterised by its own evolutive story which necessarily conditions the types of instruments to be

82 installed, the number and location of measurement points, the frequency of sampling, etc. Standardisation of the methods of intervention and the choice of sensors to be used is therefore a difficult task to be achieved. The scheme of instrumentation should be based on in-depth geological and morphological information of the site as well as on an accurate back analysis of all historical and bibliographical data available. In order to obtain relevant results, a monitoring system should not only be the result of the sum of various technologies but above all of a preliminary analysis of the phenomena and should derive from adequate reflections on the data. Similarly, a set of sensors placed haphazardly inside and around a landslide body will produce a series of measurements which are not easy to interpret and compare (Angeli et al., 1994). Therefore it is important to plan a monitoring system correcdy, in order to get sufficient and, in particular, reliable data when the alarm for the population, the risk definition, slope modelling and stabilisation can be predicted. Methodological aspects The use of slope instrumentations and the planning and installation of more complex monitoring systems are necessary when the various problems concerning slope stability have to be faced. Moreover, these instruments produce a considerable amount of data which can be particularly useful in slope stability analysis and modelling with results that will mostly depend on the input data quality. Four different stages can be identified (Sowers & Royster, 1978): 1) Bibliographical analysis concerning the geological and geomorphological aspects in order to acquire a general view of the problem. 2) Boring, sampling, testing, etc., in order to better characterise the phenomenon studied from a geotechnical, hydraulic and kinematic standpoint. 3) On the basis of the data previously collected the areas which require further investigations are pointed out, thus returning to the second phase. 4) The last stage consists in the installation of a monitoring system, the assessment of its reliability and a constant control of the areas most exposed to risk. Monitoring can be used for different goals: on the one hand for investigations finalised to possible upgrading interventions on a slope affected by mass movements and, on the other hand, for safety and control works and for testing the effectiveness of certain operations aiming at increasing the level of civil defence in the event of landslides threatening inhabited centres. One should always bear in mind that: "...instrumentation is merely a tool, rather than an end in itself..." (Dunnicliff, 1995) and that installation should always be preceded by a careful analysis of the situation which will allow some hypotheses to be formulated on mass movement evolution. In this way, it will be possible to decide the kind of measurements to be carried out, which instruments to be used and the best planning for positioning sensors as well as choosing an appropriate number, the depth of installation and adequate recording systems (Wilson & Mikkelsen, 1978). The main goals concerning the use of instrumentation and monitoring systems are the following:

83

— — — —

to determine the geometry and depth of the shp surface; to measure both vertical and horizontal movements of the unstable rock mass; to assess the rate of movements (acceleration or deceleration); to record the changes of the water table, possibly with respect to the climatic trend of the area (mainly precipitations); — to provide an adequate alarm system in case of impending danger for public safety; and — to verify the effectiveness of the control measures. Naturally all these functions can be carried out in the presence of a slope movement which has already developed in its main aspects or as a precautionary measure when it is necessary to plan some engineering works or urban development which could induce conditions of instability in the surrounding territory. Sensors and instrumentation From the remarks previously made, it is evident that there are two main parameters which should always be known when studying, surveying and measuring a mass movement. These are: deformations, both superficial and deep, and groundwater pressure. Since these magnitudes are directly correlated to meteorological factors, on which also the functions of some sensors depend, such as rainfall, temperature, barometric pressure, etc., it is opportune to survey and record also these parameters. The characteristics of the main and most frequently used instruments suitable for monitoring the above illustrated parameters, will now be discussed; although it is not the aim of this chapter to provide with an exhaustive review of all types of instruments used for landslide monitoring which are better illustrated in more specific texts and articles (e.g., DunnicHff, 1988). Monitoring of surface deformations Superficial slope movements are normally monitored in order to determine the changes in superficial morphology of the landslide area, the spatial extension of the deformations and, if possible, the tendency to retrogression or enlargement and, finally, the size of the movements. Quick, generally very economical methods are used in emergency situations and when the amount of movements is considerable. They consist of metre tapes or rods placed across the most active cracks or, if the movement affects a rock mass, in glass telltales cemented across each crack. These simple instruments can provide useful information on the movement's mechanism and direction. More precise measurements can be obtained by means of strain-gauges which can be made of threads, rods or tapes (the former being used also for surveying displacements at depth). This kind of apparatus is usually fitted out with electric transducers which transform the measurements provided by the instruments into proportional electric impulses which can be continuously recorded. Tiltmeters measure instead the rotational component of a point's displacement. Topographic precision instruments can survey displacements of points considered in movement with respect to fixed ones with a precision which depends on the

84 distance between the instrument and the surveyed point and which is usually around 15 mm. Also precision levels can be utilised; they are generally suitable for monitoring settlements at the crest of the slope or along the intermediate benches on the slope face (Kobold, 1961). For the control of horizontal movements a precision theodolite equipped with an infrared diastimeter is generally used. In this case, a system of local bench marks should be set out, both inside and outside the landslide body whose position is periodically checked from positions considered fixed. This method offers very accurate precision and measurements can be carried out even over long distances, although temperature and barometric pressure should always be taken into account since they could induce errors in the values read by the diastimeter. An important contribution to the measurement of superficial displacements is nowadays offered by the GPS (Global Positioning System), which is based on the use of 18 satellites, positioned along different circular orbits at about a height of 20.2 km, and one or two receiving stations on the Earth's surface. In this way, both relative positions of two points (up to distances of about 1 km the precision is 5 mm) and absolute altitude measurements (in this case the errors are around 10 cm) are obtainable. The development of this system has undergone considerable delay owing to the interruption of the US Space Program which had planned sending into orbit several other satellites which could be used for this purpose. As a consequence, at present use is limited to some specific times of the day. Also the relatively high costs and difficulties in the data elaboration have reduced its potential. Another method applied is terrestrial photogrammetry, especially for the monitoring of excavation fronts, rock movements or whenever the direct access to the slope is not possible. This method is based on the use of a phototheodolite which allows successive stereophotographs to be taken from a fixed station; these pictures can be compared in order to evaluate the displacements of fixed targets or to indicate movements wherever these might develop. From the stereoscopic model it is possible to obtain a Digital Terrain Model by means of sophisticated calculation programmes, thus enabling the operator to get quantitative evaluations. The accuracy of this method is not very high and is proportional to the distance between the phototheodolite and the slope; precision measures around 20-30 mm over distances of about 100 m can be achieved. Monitoring of subsurface deformations Measurements of displacements in the subsoil are of paramount importance for identifying the depth of a slip surface and assessing the volume of the rock mass involved in the movement, the presence of one or more deformational horizons and the amount of their movements. The methods for monitoring displacements in depth require the use of instruments which should be positioned inside boreholes, whose location and depth should be decided on the basis of accurate preliminary investigations. The instruments most frequently used for these measurements are the following.

85 The inclinometer is generally used for evaluating the depth of the failure surface (Angeli et al., 1989) and the movements along this surface; it measures the changes of inclination in a tube in two mutually perpendicular near vertical planes. The readings are performed at prefixed depths inside the tube and allow the change in slope at various points to be determined. The instrument records the temporal deformation trends of the tube and provides information on the amount and direction of the movement and on the depth of the slip surface. Borehole extensometers work in the same way as those utilised for superficial movements. In this case, though, the wire should be adequately protected by a sheathing from direct contact with the soil and anchored well beneath the failure surface at the bottom of piezometric or inclinometric tubes. These devices can be used to monitor movements perpendicular to the axes of the boreholes inside which the instruments have been installed. Precision is not less than 1 mm. Inside the same borehole it is possible to install several wires anchored at different depths in order to identify also the position of the movement surface. Monitoring of acoustic emissions generated within the rock mass can give useful information on the degree of activity of the mass movement and, if correctly placed, also on the depth of the failure surface. This method is still being tested but it will probably give positive results. Sensors are used such as geophones which transform the elastic wave propagating through the ground (determined by the acoustic emission) into an electric signal proportional to the emission amplitude. The system is most effective when the amplitude of the signals is high, that is for rocks and cohesionless soils rather than for cohesive soils. Monitoring of groundwater pressure Groundwater level plays a very important role in slope stability and therefore the monitoring of this magnitude is extremely useful, especially if related to meteoric inputs and displacements. When a landslide occurs, porewater pressure should be measured in proximity to the slip surface, thus enabling effective stress analysis to be performed. Groundwater pressure is generally monitored by means of open standpipes, vibrating wires, pneumatic or electric piezometers. Monitoring of climate (weather) The monitoring of meteorological parameters in the study area is very important considering the influence that they exert on the evolutive dynamics of slope movements. Correlations of these magnitudes with hydrogeological and kinematic ones gives a better understanding of landslide mechanisms and can also produce useful forecasting elements, especially in the case of an automatic warning and alarm system. The parameters most frequently surveyed are: — rainfall and snow-water equivalent, measured by means of a tipping bucket raingauge, which can be provided with a warmed collection funnel in order to measure the water-equivalent of snowfall; — temperature;

86 — snow depth, measured by ultrasonic snow gauge; and — barometric pressure. In addition, also wind intensity and direction can be surveyed, as well as sun irradiation and air humidity. All these sensors are usually connected to a data logger for automatic acquisition. 3.3.4 Landslide hazard assessment In the previous chapters a definition of landslide hazard was given. It refers to the possibility that a landslide of a certain magnitude takes place in a particular time interval and in a specific area. This definition shows that there are two precise domains in which landslide hazard can be assessed: the temporal and spatial domains. Therefore, an evaluation of landslide hazard should answer two main questions (Hartlen & Viberg, 1988): 1) Where will the landslide(s) occur? 2) When will the landslide(s) occur? The spatial aspect defines the areas where instability processes are more likely to develop in a certain length of time, the temporal one should in some way lead to a forecast of the moment when instability phenomena could occur in a certain area. At this point it is opportune to emphasize the difference between landslide forecasting and landslide prediction since, according to Jones (1992): "the former is focused on identifying the location, magnitude and timing of specific events, normally based on monitoring procedures, so that emergency actions may be achieved efficiently whereas the latter is spatially extensive and concerned with establishing zones of different level of threat by emphasizing the magnitude-frequency characteristics of recorded landsliding and probability of slope instability" (see also Flageollet, 1989). The spatial aspect can in turn be considered from two different points of view which necessarily imply the application of diverse investigation methodologies. On the one hand there is the hazard originating in a single slope movement or in a limited portion of slope; in this case the investigations will tend to concentrate on a specific site and to assess in particular the subsoil's characteristics (Hutchinson, 1995). On the other hand there is the assessment of landslide hazard on a regional scale which, owing to the different methods of investigation utilised, will give more importance to the aspects concerning the superficial morphological characteristics of the area examined, without studying the subsoil's characteristics in depth, or doing so only in specific points. Several authors have proposed diverse methodological approaches in order to define landslide hazard. Usually the main goal is to produce maps able to classify all existing landslides and to zonate also the areas at present stable by rating them with different degrees of hazard susceptibility, assessed on the basis of geological, geomorphological, morphometric and geotechnical aspects, independently of the investigation scale (Meijerink, 1988).

87 In a broad sense, the techniques apphed can be subdivided into quahtative evaluation (relative mapping) and quantitative assessment (absolute methods) techniques. As for qualitative hazard evaluation, several Authors have proposed different investigation methodologies. Some of them concern mapping methods, like those used, for example, in the ZERMOS project in France, by Kienholz (1978) in the Grindelwald area in Switzerland or by Seijmonsbergen (1992) in the Vorarlberg region in the Western Austrian Alps. These approaches are mostly based on the use of direct survey in the field and the drawing up of several mapping products which should be compared in order to come to a qualitative evaluation of landslide hazard. Moreover, other qualitative approaches based on the application of statistical methods have been proposed. According to these, the area to be analysed should be subdivided into grid cells and would require as input data a landslide distribution map and a land-unit map. A second stage envisages the identification of the most significant slope stability controlling parameters which are afterwards stored as coded terrain attributes (e.g., geology, morphometry, hydrology, etc.) in a database. The statistical elaboration of all these data for each of the grid cells identified will lead to the definition of the areas affected by different landslide susceptibility. For example, Carrara & Merenda (1976) used grid cells of 200 x 200 m in an area of 600 km^ obtaining a slope stability hazard map from the statistical analysis of more than 30 attributes. The recent application of GIS methods was shown to be particularly fruitful in assessing landslide hazard, especially since it is possible to undertake the study on different scales, from regional to site investigation scale; each scale requiring its own input maps and mapping unit attributes. If the regional scale is chosen, the data concerning the geological and geomorphological features are put together with other parameters linked to slope stability. For each of the above listed parameters, the relative contribution to slope failure is estimated, thus a zonation of the study area in terms of landslide susceptibility can be carried out. Statistical treatments, such as multiple regression or discriminant analysis, have been recently incorporated in GIS recording systems (Carrara et al., 1991) and ''favourability functions" have been derived (Chung & Fabbri, 1993). However, these methods are dependent on the scale of the discretisation of the slope system and do not generally take into account the failure mechanism so they do not always completely explain the slope failure distribution. As regards landslide hazard assessment on a detailed site investigation scale, absolute or numerical methods are more and more utilised. Such methods, based on deterministic approach, express landslide hazard, through stability modelling, in terms of safety factor or of probability that a certain event might take place. The required input data are: depth of the failure surface, soil strength parameters, slope angle and porewater pressure. It is possible to combine the use of GIS with these deterministic models following diverse approaches, among which the use of an infinite slope model, which calculates the safety factor for each pixel or the selection of a number of slope profiles or pixels from the GIS which are subsequently exported to external

88 slope stability models (Van Westen, 1994). An improvement of the climatic input data for the deterministic models was obtained through the development of general circulation models (GCM's) which have been recently used to deduce regional-scale climatic features. Downscaling of GCM's is made by means of a statistical treatment which interrelates the characteristic patterns of regional climate parameters and large-scale atmospheric flow (Von Storch et al., 1993). Rainfall occurrence obtained through this procedure can be used as input data for hydrological and slope stability models. It must, however, be emphasized that the quality of the results of GIS analysis and modelling is always strictly connected with the precision and number of input data for each parameter considered; therefore even if extremely powerful technologies and computer-based media are available, the reliability of landslide hazard assessment depends very much on the experience and ability of the scientists and technicians involved in archive analysis, remote sensing, ground survey, mapping and monitoring. It is particularly important that landslide specialists provide planners and decisionmakers with valuable and reliable landslide hazard assessments; this may help the incorporation of these assessments in land planning policies, which have been quite limited so far. This could give a further contribution to invert the widespread tendency of many countries to face the landslide problem only in terms of postevent clearance and remedial measures instead of adequate prevention planning poHcies.

3.4 River hazard 3.4.1 River erosion Sometimes with the term river erosion extremely different morphogenetic processes linked to the action of a watercourse are defined; this can lead to some confusion in describing phenomena which differ in their origin and evolution. It is therefore of fundamental importance to distinguish the following types: — erosion in the strict sense; — cavitation; — abrasion; — degradation. With the term river erosion in a strict sense the removal and loading of detrital material from a riverbed bottom and banks is intended. With the term river cavitation the mechanical action of water only on riverbed sides and bottom is intended. With the term river abrasion the mechanical action following the impact of detrital materials transported by the river current is intended. With the term river degradation the whole complex of morphological phenomena linked to the presence of a watercourse is intended. For example, the fragmentation

89 of the riverbed rocks owing to physical or chemical processes, such as gelivation or hydrolysis, determined by the river's water; or mass movements occurring on the river banks following the undermining of the slope's toe, etc. 3.4.1.1 Erosion in the strict sense In order to have material load transport, the current's kinetic energy must exceed the forces of gravity, inertia, friction and cohesion which tend to keep detrital particles in a motionless state. Erosional processes in a strict sense and those derived from transport and deposition are a function of several variables: on the one hand the current's velocity, density, turbulence, load, etc.; on the other hand the detritus's dimensions, bulk unit weight, shape, etc. The ratio between the current's velocity and the dimensions of individual particles is represented in Sundborg's diagram (Fig. 28): in abscissa the particles' diameters are shown whilst in ordinate the current's velocity pattern is represented. It is seen that for each detrital element there is a critical velocity above which the element is removed and loaded, i.e., eroded; similarly there is a critical velocity below which the element can no longer be kept in motion, i.e., transported and, therefore, it is deposited. This shows that the velocity necessary for removing a particle from the riverbed bottom seems to be greater than that necessary for maintaining its transport. Moreover it can be observed that the dimensions of the material transported in suspension increase with velocity, to the detriment of those transported at the bottom.

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90 3.4.1.2 Cavitation Mechanical actions of this kind are due to tiny bubbles of water vapour formed by sharp variations in hydric pressure. Sudden velocity increases and turbulent movements can determine a pressure decrease along the sides of the riverbed, with consequent water vapourisation. These bubbles behave as detrital elements and their constant formation and destruction gives rise to an extremely rapid sequence of impacts on the sides with consequent disgregation of the rocky material. A similar effect is created by the violent impact of water at the foot of a waterfall. The mechanical effects of cavitation do not seem to be very important in the general frame of river erosion. 3.4.1.3 Abrasion The mechanical processes linked to detrital particles carried by the watercourse depend on the quantity and size of the latter. The action of clayey and silty materials is neghgible. Sands mainly produce a smoothing action on rocky riverbeds which leads to progressive wearing but, nevertheless, is unfavourable to the various phenomena of geomorphological erosion since polished rocks are more resistant than those with discontinuous, rough and irregular surfaces. Coarser materials collide individually with the riverbed walls causing disjointedness, divarication and disgregation which are a function both of the current's impact energy and of the riverbed rocks' cohesion. 3.4.1.4 Degradation The presence of the watercourse determines a set of degradation phenomena along both the river banks and the slopes overhanging the watercourse. During minimum-flow periods, the riverbeds are subject to weathering and physicochemical alteration processes linked both to climate and lithology: in colder regions a rocky bed can undergo gelivation processes whilst in humid regions hydrolysis, solution, etc., prevail. The alternations of emersion and submersion from the water can cause the breaking up of rocks subjected to repeated cycles of wetting and desiccation. These and other processes can reduce the rock resistance to river abrasion and cavitation or lead to a real fragmentation with consequent production of detritus which is afterwards rearranged and removed by the current in the form of erosion in a strict sense. River-bank erosion processes (in a wide sense) can accelerate and provoke slope degradational phenomena, such as mass movements or, in any case, induce disequilibrium conditions. River processes taking place within a given time, including maximum and minimum flow phases, can be summarised in the following way. At the beginning of the highwater period, the river-bottom detrital materials are subject to rearrangement and erosion in a strict sense; afterwards, with the progressive increase of water volume and particle load the velocity at the bottom of the river decreases. Thus, bottom erosion in a strict sense stops while abrasion and disarrangement can occur along the banks, with consequent bank erosion and possible slope degradational

91 phenomena. On the contrary, the decrease of a major flow causes sedimentation of materials along all the inundated bed. Finally, when the flow goes back to normal levels and is no longer loaded with debris, bottom erosion in a strict sense resumes, affecting the newly deposited material. This kind of erosion continues also during the minimum-flow period. After all, the balance of a maximum-flow event is sedimentation, possibly accompanied by river bank erosion and slope degradational phenomena; on the contrary, the balance of a minimum-flow period is a cutting into freshly deposited alluvial sediments. 3.4.1.5 The action of mountain streams A particular reference must be made to the degradational processes caused by mountain streams which, among the various types of watercourses, are particularly energy-rich geomorphological agents. The main characteristics from which their geomorphological effectiveness is derived are the following (Tricart, 1977): — they show riverbeds with a usually high slope angle; — they show a various and irregular course, with frequent changes of slope angle and variations of riverbed width; — they flow on extremely heterogeneous material, ranging from more or less cohesive bedrock to load-assumed detritus and large boulders fallen from the surrounding slopes. They are moreover characterised by very irregular regimes, more or less accentuated according to the climate peculiarities: major highwaters which noticeably modify the watercourse's morphological features and sometimes also those of the surrounding territory; generally extreme minimum flows which, on the contrary, exert no influence whatsoever on the riverbed. During major highwaters, caused by sudden and violent storms, waters convey a high load of debris such as mud, stony fragments and vegetal remnants, thus making up a high density and velocity mass, therefore characterised by noticeable energy. Slope angle and width changes determine frequent and successive episodes of sedimentation and transport of other material, with load substitution processes. Overflows from the ordinary stream bed and bank erosion actions take place, with consequent undermining of the slope toes, degradation and eventually triggering of mass movements. Downstream, at the end of the course, the load of the mountain stream is unevenly spread at the base of the relief, owing to the twofold effect of slope angle and spreading decrease and loss of water which is no longer confined between the narrow flanks of the mountain valley. In this way proluvial fans are formed which are a transitional form between slope and alluvial fans. The dynamics of these processes, intermediate between river and slope processes, can cause hazardous erosion and inundation effects, with even disastrous consequences for buildings, farming activities and sometimes human lives.

92 3.4.2 River instability'^ *by Franca M A R A G A '

3.4.2.1 General overview The dynamic action of waters moving in a stream channel and materials carried by them can produce important changes in the environment: they interfere with a territory's socioeconomic development, where land reclamation for production activities or urban settlements has directly influenced the fluvial system. Such changes can create geomorphological hazard situations under the variable impulse of floods both in mountainous regions and in the plain, with morphogenetic processes characterised by different dimensions and lengths of time. The processes are sometimes occasional, often repetitive and in any case interdependent on the effects produced along the watercourse in spatial and temporal succession, with long-term consequences for the whole hydrographic network. The extreme diversity of the processes makes a river hazard difficult to assess using prediction criteria, since this would be defined by local, natural and man-induced factors in periodical adjustment, so as that give rise to conditions of hydrodynamic equilibrium during the propagation of a flood wave. Even in short periods of time, the evolution of the composite physical fluvial system, defined by the channel and by its flood plain, can be identified as a "dynamic metastable" (Schumm, 1977) composite system of equilibrium, with a recurrent step of evolution during overflow events. The velocity of hydrodynamic parameters conditions the intensity of the activated process and therefore the "pressure" of the consequent effects on the territory for the same length of time. An area subject to flooding, for example, faces a higher hazard if the flood rises suddenly with a high overbank flow speed. The hazard of watercourses is mainly connected with occasional and exceptional changes of their water stages occurring during extra-bank flow rather than with the changes periodically generated by ordinary flow, the latter being more frequent but less noticeable. The channel can be considered a fluvial form constantly adjusting itself to changes occurring at times along its course in the short and long term, whilst the alluvial plain behaves as a form in occasional adjustment, over long periods of time. Usually the river hazard is recognized when flooding events take place in the populated flood plains, since the in-channel flooding, although more frequent, is also more limited and therefore more easily controllable and manageable. Their repetitiveness in time can nevertheless progressively lead to a threshold situation and trigger catastrophic effects. In any case man-built infrastructures, including flood-protection works, can be severely damaged owing both to riverbed in-channel highwater flows and to extra-bed flows: flooding, erosion and deposition inside and outside the riverbed are the

^This work was done with the support of CNR-IRPI (Torino) services and includes the collaboration of E. Beretta for the archives of aerial photographs and historical data, C. Novellone for the internal library, P.G. Trebo for the photo reproduction and E. Viola for the cartographic drawings.

93 processes defining watercourse hazard; the rate of the phenomena conditions the intensity of the effects produced on the territory (Fig. 29). An incorrect perception of river instabihty phenomena (Shumm, 1994) can lead to erroneous assessments of hazard conditions (Govi, 1980; Sear et al., 1994; Piegay and Lama, 1995; Simons and Downs, 1995), especially in forecasting progressive effects linked to flood wave propagation. In particular, with embanked rivers in a large natural flood plain, man forgets the

Fig. 29. Bridge on the River Po which collapsed owing to bottom erosion during the November 1994 flood (at the top a 1981 picture (courtesy of C. Falco); at the bottom the riverbed lowering is marked by the sill along the piles' alignment).

94 tendency of a protected area to be submerged by flood waters and the possible breaking of an embankment causes disastrous floods (Govi & Maraga, 1995). On the contrary, geomorphological memory preserves on the territory the physical traces of the abandoned river forms that condition the overbank flows. Shapes and sizes of geomorphological forms have to be considered as the integrated product of the natural actions of in-channel waters, geological factors, meteorological conditions and man's activities. In particular, the width of the major riverbed formed on alluvial deposits has been shown to conform in its dimensions to the hydrological history of the watercourse, whilst the channel pattern reflects the sedimentary conditions of the cutthrough alluvial deposits. Figure 30 shows three typological situations of physical flooding systems that are representative of: a) aggradational single-thread system; b) deepening single-thread channel system, composed of several flooding levels; and c) multithread channel system. The presence of embankments on the alluvial plain of single-thread systems confines the highwater flows within artificial levees whose breaking can cause vast inundations depending on the amount of flooding waters and the extension of the low lands. In particular, in the aggradational single-thread system, the flood determined by the breach of the embankment is transversally propagated according to the gradient of the plain along directions with no possibilities of return downstream into the former river channel and the flooding currents go out of their original water-

levee overtopping stage

^^f^ flooding inundation by breaching

Fig. 30. Topographic sections of flood prone areas with trends of inundation caused by embankment breaching with a small flood corridor in a meandering channel: a) prograding alluvial plain along the river axis with a large flood corridor in a wandering channel; b) alluvial plain with incipient river terracing; c) torrent flood plain.

95 course. In the erosional single-thread system and muhithread system, the currents transfer downstream according to the system's dominant flow direction. In the specific situation of confluences embanked in an alluvial plain between an aggradation tributary system and an erosive one, the geometrical situation of the alluvial plain and of the connected hydrographic junction can increase the risk conditions (Maraga, 1986). Indeed, the embankment of the main channel can create a dam effect obstructing the downstream propagation of a flood coming from a tributary (Figs. 31, 32). The comparison between the two pictures emphasizes the twofold damming role of the river embankments, that of flood propagation confinement and that of drainage obstruction of flooding waters coming from the tributary. Often anthropic factors determine an acceleration of the evolutive fluvial system, as in the case of channelisation which produces a higher velocity of flood wave propagation, with effects such as producing worse damages at longer intervals. The geomorphological instability of fluvial territories, affecting both the riverbed and the alluvial plain, depends on the propagation patterns of the highwater waves along the river course and their frequency. If the frequency of floods is defined by meteorological events, the wave propagation patterns, under the same hydrological conditions, reflect the conveyance of the fluvial physical system, defined by the geometrical parameters of the hydrographic network and channel cross sections. With flood waves exceeding the ordinary channel discharge, the alluvial plain behaves as a temporary waterway whose boundaries can correspond with natural scarps in a mountainous territory or terraced valley floor, or with structural embankments raised for the containment of highwaters in the open plain. During the overbank flooding a river's hydrosystem is composed of at least two patterns of wave propagation: the main channel and its flood prone area. Since the fluvial physical system is open to many variables linked to interdepending natural and anthropic factors, its answer to flood dynamics and velocity of adjustment varies according to local conditions. Hydrodynamic variations leading to flood events can generate hazard processes of different magnitude according to the scale of the event: channel, alluvial plain or hydrographic network. Flood recurrence times differ according to whether they are confined within the channel or flooding the alluvial plain. In the first case they usually show a return time falling within a 5-year period and can cause variations along the bank lines; on the other hand in the second case they show return times of a few decades and can produce variations in the fluvial course or in the topographic surface of the alluvial plain. Finally, flood events with secular or multisecular recurrence generally produce disastrous effects. Fluvial instability implies three types of geomorphological modelling: — planimetrical variations of the bank line due to channel form mobility with flood plain loss or addition (Fig. 33); — altimetrical variations of the topographic surface due to flooding with scouring or deposition; — textural variations of the drainage network due to new erosional processes of the

96

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channel with drainage increasing. The probabiUty of these processes taking place within a given territory depends on the morphological features of the watercourse at the flooding stages. 3,4.2.2 Main channel shaping The main runoff riverbed can be subject to instability processes at the channel cross section during highwater flows which have the capacity to mobilise the sediments

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Fig. 32. Inundation of inhabited centres in the mid-Po Plain: owing to the presence of embankment (1), the tributary flooding waters cannot be drained. The inundation was produced by the River Taro during the secular flood of November 1982 and affected a 70 km" developed area downstream. Rightward flow (mosaic of vertical aerial photographs, cone. S.M.A. no. 429. 27 Oct. 1995).

98

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Fig. 33. Planimetric mobility of the riverbank line with 0.5 km range of soil erosion and 1 km on acquisitions along the River Po wandering channel downstream of the confluence with the R. Ticino 400 km from the outlet in the 1955-1988 period (courtesy of F. Dutto. modified).

from the river bottom and banks, thus generating transport of unconsoHdated deposits. The frequency of these discharges defined as formative, generally shows return times of around two years and corresponds to flows occupying the full width of the riverbed cross section (Riggs, 1976; Caroni & Maraga, 1985; Wharton et al., 1989; Kennet & Wahl, 1984). The magnitude of the phenomenon is linked to sediment transport and depends both on the duration of the formative discharge and on sediment availability. The water level, sediment particle size distribution and the local riverbed gradient, all contribute to the sediment entrainment, whilst the availability of a sediment depends on the sedimentological conditions of the riverbed. The riverbed shape reflects the sediment transport mechanisms which are variable according to the particle size distribution of the alluvial deposits. In gravelly riverbeds, bed load and debris flow takes place in the multithread system, whilst selective transport takes place in straight channels and in transitional channels. The entrainment of alluvial materials and their deposition inside a riverbed occurs by means of erosional or depositional processes and bed forms construction whose size and distribution reveal either cutting or filling effects in the riverbeds. In the first case, risk situations are generated owing to the possible presence of works within the riverbed or to destabilisation of the structure following selective bottom erosion. In the second case, risk situations are more likely to be found with works located in the fluvial territories surrounding the riverbed owing to lateral bank erosion and consequent soil removal. In fluvial stretches where the river bottom is in proximity to the bedrock, lateral erosion of the bank is the only possible action during the floods. Instability phenomena in the main discharge channel produce modifications affecting both the bank line and the fluvial axis; they characterise fluvial activity both in the short term on a segment scale, within the evolution of the riverbed forms, and in the long term on a catchment scale, as part of the hydrographic network evolution. 3.4.2.3 Channel pattern mobility Channel pattern mobility expresses the freedom of a watercourse to accomodate its

99 hydraulic geometry parameters with the variation of water levels, according to the local and temporary hydrodynamic equilibrium conditions during flood wave propagation. This mobility occurs on the bank lines with different patterns in relation to the channel type which, in turn, is different on the various river stretches of variable lengths. Finally, cross section parameters are related to the slope change of the longitudinal profile. Form mobility in space is always related to a channel pattern and coincides in time with formative discharges flowing near the bankfull stage. The formative discharge activating sediments mobility is greater in the multithread system than in the simple or composite single-thread system since it depends on the width of the modelling form. Therefore, under the same flow rate conditions, a single-thread form is more mobile than a multithread one, since the former concentrates water flows into one channel, and produces variations linked to the bank line, whilst the latter distributes water flows into a network of smaller interlacing channels, with the effect of reducing the water stage peaks and thus the sediment movement. The most unstable form in the single-thread system is the transition form defined as wandering (Neill, 1983) or transitional (Carson & Griffiths, 1988), which corresponds to an irregular-meandering channel with a gravelly riverbed. This form may be interpreted as a stage of evolution from the multithread system to the singlethread one in gravelly sediments induced by selective transport which can be activated owing to an insufficient sediments supply (Cunge, 1985; Maraga, 1989, 1992). In the alluvial plains on which an artificial floodway zone has been imposed, the flood water levels derived from the presence of embankments are higher than those of a free flood plain and increase sediments mobilisation within the riverbed. The instability of this transitional form usually affects bank scarps made up of gravelly sediments in the lower part and sandy ones at the top and is produced by the mobilisation of gravels. The phenomenon is often associated with a forking of the flow water in correspondence with transversal or median gravelly bars forming incipient pavement. The formation of a new main channel can be determined by the occlusion of the former channel with an inversion effect in the cross section: this is a typical phenomenon occurring in torrential watercourses (Quesnel, 1957). In this case the position of lateral erosion advances according to occasional rather than repetitive geometrical characteristics, affecting in time all the bank line over few kilometers onto the plain (Dutto & Maraga, 1993). When the wandering channel corresponds to a channelisation phase in the alluvial plain with a planned floodway, the meander chute cutoff effect is then possible, with upstream occlusion of the meander bend by episodical deposition on its talweg (Figs. 34, 35). In the case of exceptional flooding, the form's instability appears enhanced in the flow forking processes which, during the propagation of a flood wave, can reconquer the previously abandoned ancient courses of the original multithread system. 3.4.2.4 Channel network adjustment Throughout the hydrographic network the fluvial system is integrated with the slopes

100

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by means of watercourses of progressively decreasing order, in relation to the diminishing area of the feeding catchment basin. The network is considered as a set of inter-connected fluvial segments, with a structure controlled by the principles of energy dissipation transporting both water and sediments downstream (Rodriguez Iturbe et al., 1992; Ohmori and Shimazu, 1994). The instability phenomena of the main runoff channel, of any kind or size, induce variations along the network and therefore modify the propagation velocity of the flood waves. The network's geometrical variations can occur according to the two following patterns: — temporarily, during catastrophic flood events caused by overbank flows controlled by the distinctive features of the alluvial plain along directions not corresponding

101

Fig. 35. Wandering channel form originated by the evolution of an original multithread system in the River Po at the confluence with the R. Tanaro. The activation of abandoned channels induced a diversion (a) of the secular flooding of November 1994 towards the inhabited centres of the left side, thus producing two embankment breaches (b): meander lobe with water reflux (c); (1) embankment (Cone. S.M.A. no. 107, 24 Mar. 1980).

to the direction of the channels axis; — permanently, owing to the stretching and shortening of single segments, or to an increase of their density due to the new formation of first-order segments or changes of the flow path; the latter are localised at the confluences and head points of lower-order watercourses. The causes of instability which produce permanent effects on part of the network, also induce adjustments on other sectors of the basin over long periods and with flow rates even smaller than those that caused the initial sudden occurrence of the phenomenon. 3.4.2.5 Headward erosion First-order watercourses are involved in debris flow phenomena with the triggering of small superficial landslides at their heads or on the lateral slopes, induce in the

103 3.4.2.7 Flooding The velocity of propagation and the height of the wave at peak discharge are directly responsible for the magnitude of the effects producible in highwater conditions. The presence of flood-prone areas around the main channel, however, produces a wave attenuation with water spreading by overbank flows onto the flood plain. The longitudinal and transversal components of velocity and the organisation of flows within this complex system of the channel and its flood plain have been investigated during the last few years by means of physical laboratory models (Sellin, 1995) and applications to case-studies along selected river courses (Archer, 1989; Magillian, 1992; Caroni et al., 1994; Myers & Lyness, 1994). During a flood event, a watercourse shows wave transfer patterns which differ according to the size and planform of the riverbed, which can be particularly marked if multithread and single-thread channel systems are compared (Caroni & Maraga, 1994). This is shown by the flood wave dynamic behaviour which affects both the propagation velocity wave and the amount of flooded, according to the geomorphological characteristics of the hydrosystem. In the multithread system, the propagation pattern shows a direct variation with

Fig. 37. Alluvial fan instability in Alpine territory: a) block deposition in apical sector with channel avulsion; and b) fine-grained sediment deposition in distal sector with erosion processes. Case of an alluvial fan remobilised during September 1981 flood in the lower Val d'Aoste, North-Western Alps, in the Torrent Renanchio at the confluence with Dora Baliea, rightward flovs (cone. S.M.A. no. 429. 27 Oct. 1995).

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the flow rate increase, whilst in the single-thread system a marked decrease during overbank flows is recorded (Fig. 39). This means that with equal water volumes a multithread segment can convey the flood flows downstream without noticeably modifying the wave's shape, whilst a single-thread segment can cause a attenuation of the peak discharge. The flooded area increases progressively during the rising of the wave in the multithread system together with water levels; in the single-thread system, on the contrary, the areas show a moderate increase when overflow begins, due to the expansion of the waters in flood-prone areas. Such propagation behaviour of the flood wave is justified by a fluvial hydrosystem's propensity to distribute the high flows in more or less large flooded areas whose extent varies according to the corresponding widths of the channel system; the wave is thus attenuated with peak discharge mitigation effects relative to the floodprone area available. The multithread system allows several channels to be activated, even simultaneously, offering a wide space to the in-channel flows, so much so that it can limit, if not exclude, overbank flows. The single-thread system, on the contrary, is composed of two separate hydrodynamic phases: within the river channel and/or in flood-prone areas, with overflow discharge that can occasionally exceed the inchannel flows, in accordance with the local geometrical conditions of the flood plain. The geomorphological setting of theflood-pronearea conditions the aforementioned mitigation effects of thefloodwave peak. Indeed, a terraced development at differences

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in level of even some decimetres can define a natural order of zoning with subsequent degrees of flood control, corresponding to areas of subsequent dissipation. Uniform development on a given level, however, creates continuous dissipation by overland flow, unless intervention works are carried out to create artificial floodway zones. Figure 40 shows the development of flood-prone areas corresponding to given flood stages in the cases of an aggradating or deepening single-thread system and in the case of a multithread system, referred to case-studies of flood-prone areas in the Po Plain, extending respectively over 70, 60 and 20 km^ (Maraga, 1990).

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106 3.4.2.8 Inundation The aspect of fluvial hazard which most affects the physical arrangement of a territory and has the most serious effects on its management is the inundation of the natural ground surface. This is in direct relation to the extension of the areas susceptible to the instability phenomenon, to the recurrence interval and to the rapidity with which the inundating process is exhausted. As a consequence of the inundation produced by extraordinary water volumes, the following effects can be determined on a territory: — of temporary nature, concerning the water flooding, where the extension and the persistence of the inundation depends on the plain morphology, considering also any man-made structures; — of permanent nature, concerning the erosion or sedimentation forms in the inundated areas: the prevalence of one or the other is controlled by the slope of the surface affected. The suddenness of an inundation phenomenon affects the development of morphological processes and determines the degree of seriousness of any damage produced by the event. This usually occurs on a mountainous valley floor or alluvial fan environment, where peak waves propagate along flood plain slopes with gradients similar to those of the channels. On the wide plains, on the contrary, where the watercourse is laterally confined in an artificial flood way zone and the inundation characterised by sudden overflow can be determined by a breach in the embankment. A long recurrence of inundation due to breaching of embankments concerns the great alluvial plain of the River Po (Govi and Turitto, 1993), with historical records quoted in chronological lists and in official articles of the Ministry for Public Works: the events listed started from the eleventh century. The location of the inhabited centres directly affected by floods is shown in Fig. 41 (Govi and Maraga, 1994). The worst most recent flood struck the lower territories of the Po Plain in November 1951, claiming 80 lives. This flood was exceptional owing to the extent of the inundated area (1,000 km^) and was caused by three successive breaches occurring within 3.5 h along a total of 3 km of embankment, facing two inhabited centres: the flood sediments covered over 8 km" of urban and farming land, with sand deposits up to 3 m thick near the breaches. In the nineteenth century the Po Plain was already defended from floods by means of a continuative system of embankments along a 400 km course from the mouths of the Po upstream and its management was already aroused interest in the civil engineering field (Comoy, 1860). A peculiar geomorphological criteria of setting the embankment lines affected the flood way configuration which is very irregular as a consequence of the watercourse's evolution: the embanking lines are near the riverside in the case of aggradational single-thread system or far away from the riverside in the case of erosional single-thread system, with average widths of the embanked flood-plain area of, respectively, 0.7 and 4 km. This reflects the primary protection system laid down centuries ago, in accordance with the contemporary morphological situation of the watercourse.

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Fig. 42. Evidence of flood plain splay with local scour (arrow: a) after a secular flood; circle shows the break in the main railbed whose layout crosses a meander bend cut off over one hundred years ago. River Tanaro during the flood of Nov. 1994 (cone. S.M.A. no. 429. 27 Oct. 1995).

109 The presence of abandoned riverbeds near the floodway is associated with the hazard of breaches caused by water infiltration under the embankment, since in these places the alluvial deposits are characterised by a coarser and more heterogeneous particle size than the surrounding sediments, thus making up a preferential site for through flow processes. The abandoned riverbeds, even at a considerable distance from the watercourse, control the overland flows. For example, in the mid Po Plain a historical flood of 1705 occupied the surrounding land over a 20 km wide area, distributing its waters along channels parallel to the valley axis, in accordance with the traces of abandoned riverbeds dating from the Bronze Age onwards (Castaldini & Piacente, 1995). During the more recent flood of November 1994, which affected the upper Po Plain basin, serious damage to man's infrastructures was recorded just in correspondence with ancient river courses which had been activated by greater inundation currents during the propagation of the highwater (Fig. 42). In the presence meandering river course, controlled by the uninundable terrace scarps, such as in meanders cut deeply or confined by embankments, the riverbed mobility is in marked contrast with the relative staticity of the imposed natural or man-made limits. The scarps and the embankments are always subjected to the erosion caused by the meanders' evolution within the floodway zone. In the case of embankments, the meanders traces convey the overbank currents straight onto the embankments and generate breaches (Fig. 43). As one of the river processes linked to the modelling of a watercourse which can induce a flood hazard situation due to the breaching of an embankment, bank erosion should be mentioned. It is produced by both the downstream migration of meander bends and the deposition of sediments |>;i.5|-;;:'| embanked floodway zone

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110 within the riverbed, with the consequent formation of middle bars or islands along straight courses or downstream of confluences. The assessment of the inundation probability, based on the flood recurrence, should take into account, for a given investigation territory, the morphological changes of the riverbed and alluvial plain as well as the development of man-made infrastructures. Previous instability phenomena, caused by inundations, could no longer represent hazard situations, whilst new hazard conditions could be generated owing to the mobility of the fluvial system.

3.5 Marine hazard 3.5.1 Geomorphology 3.5.1.1 Coastal environment A coast is a stretch of varying extension between land and sea. It comprehends the shoreline which has a changeable morphology in space and time, according to short and long-term fluctuations caused by sea-level variations due to several causes: from the flux and reflux of waves and tides to the regional or worldwide changes of the sea level caused by tectonics, eustatism, etc. Among the processes that shape a coast, those involving waves are the most important. There are various ways in which waves break a shoreline. First of all, if a coast is not a sheer drop from a cliff to the sea, the waves approaching it lose some of their propagation velocity owing to friction on the shallow sea floor. In other words, if the wave approaches the coast in an oblique direction with respect to the isobaths, the wave portion that first reaches the shallow bottom undergoes a slowdown due to friction. Therefore, wave fronts tend to arrange themselves parallel to the isobaths, i.e., the direction of the wave energy tends to assume a direction normal to the coast. This phenomenon is named wave refraction (Fig. 44). As a consequence, wave energy is concentrated towards headlands and promontories whereas it is dispersed in bays and inlets. Besides undergoing a diversion of their fronts owing to wave refraction, when waves come in contact with a shallow sea bottom a deformation of their oscillatory motion also occurs with overturning of the crest into the hollow in front of them creating plunging breakers. The water is thrown forward and goes up the coast as far as its transport energy allows. Afterwards, the water returns down the shore to the sea under the force of gravity, with a more or less disorderly and forceful landward flow, called backwash. Although the waves hit the coast, although with reduced energy owing to friction and with a direction diverted by refraction, they are also subject to a process of reflection with an angle equal to the angle of incidence. As a result of the interference between the two wave systems, direct and reflected, a fairly complex composite wave is produced. The incident wave, to some extent diverted by refraction, the reflected wave and

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Fig. 44. Wave refraction, due to the concentration of wave energy on headlands and to the dispersion around bays, has an important influence on erosional and depositional processes affecting coastlines.

the backwash all have different angles and directions and determine a composite landward and seaward movement with a zig-zag trajectory, subparallel to the shoreline, called longshore current. Coastal regions, in particular those in direct contact with the sea, are characterised both by high humidity, owing to the constant presence of sea water and by scarceness of vegetation, owing to the saltiness of the air. These two characteristics create very particular conditions, quite different from those of other environments where abundant humidity is accompanied by luxuriant vegetation. In this way processes of concentrated and widespread runoff take place, as well as landslides and solifluction, owing to littoral rock and debris impregnation. Moreover, the presence of sea water is a cause of important and widespread phenomena of physical and chemical weathering of rocks: salt cracking, humidification, frost shattering, solution, hydrolysis, hydration, etc. Wind, besides being a wave generator, is an important agent which transports sea water towards territories not otherwise reached by the waves and also sand along the coast or to the inland in the form of littoral dunes. Tides are an important modelling agent. These periodical variations of the sea level along a stretch of coast (intertidal zone) are the cause of several processes: salt cracking, humidification-exsiccation, etc. They also broaden the zone subject to wave motion. Finally, with the rising and lowering of tides, conspicuous horizontal movements of masses of water, called tidal currents, take place. These are very important agents of sea-floor erosion, sediment transportation and deposition.

112 especially near harbours, bays and, in particular, lagoons and estuaries, where they add to the effects of fluvial currents. Rock characteristics, such as cohesion, fissuring, frost shattering susceptibility, permeability and cementation can favour littoral marine degradation to varying extents. Also particular tectonic arrangements may condition a coast's degree of stability; for example, a dip-downstream attitude can facilitate rock slides. Animal and vegetal organisms are a direct or indirect cause of several physical and chemical processes when alive: roots widening joints, animals such as date mussels digging through rocks either mechanically or by secreting acid substances, coastal plantations damping the wave motion, corals and calcareous algae building atolls and reefs, micro-organisms producing carbon dioxide and thus causing karst phenomena, etc. 3.5.1.2 Coastal processes Similarly to river erosion, diverse morphogenetic processes linked to the action of littoral sea waters are defined by the term coastal erosion. Like the categories previously defined regarding watercourses, four types of coastal erosion can be distinguished: Coastal erosion in the strict sense, means the removal and loading of detrital material from a sea shore. Cavitation means the mechanical action of the waves only against the coast. Abrasion means the mechanical hitting action of debris thrown by the waves against the coast. Coastal degradation is a whole complex of morphological phenomena linked to the presence of the sea. For example, the weathering of rocks owing to humidification and exsiccation or salt cracking processes determined by marine water; or mass movements following undermining activities at the toe of a sea cliff, etc. The load of marine waters is made up by organic and inorganic materials in solution and by detrital elements of the most varied dimensions. Only a small portion of these is derived from actions of direct coastal erosion; in most cases they are conveyed to the sea by watercourses, glaciers, winds, etc. Part of this detrital material undergoes wearing due to the movements of the sea, or is transported and finally abandoned off-shore or transferred from one point of the coast to another. As happens with fluvial currents, transportation trends and conditions depend on the one hand on marine water velocity, turbulence, density and temperature and, on the other hand, on the dimensions, nature, bulk density and shape of the materials. Solid matter can be transported in solution, in suspension, hy floating, saltation, rolling and dragging along. Marine currents transport solid matter exclusively in solution, suspension or by floating; tide or longshore currents may assume noticeable rates of velocity and turbulence and thus can transport also by rolling, dragging or saltation. The latter is typical of plunging breakers which have considerable kinetic energy. Backwash can transport even coarse materials, by means of rolling, dragging and saltation.

113 The deposition of the detrital load transported by sea waters may occur owing to a decrease of a current's energy, as happens with fluvial currents. In certain cases the deposition takes place owing to the interference of two currents having opposite directions, following the ''algebraic" annulment of their kinetic energy, as in the case of two opposite and contrary wave fronts meeting as an effect of wave motion refraction, or caused by the interference between coastal breakers and backwash current, or between tide ebb and fluvial current, etc. The movements of the sea, in particular wave motion, rearrange and distribute the grains of solid matter which are progressively deposited along the coast: in this way littoral sands seem to be "washed", that is deprived of finer silty and clayey elements. These finer particles are, however, present in fluvial deposit sands whose patterns of sedimentation and transportation make their removal more difficult. On the other hand, coarser elements can mix with littoral sands since, once they have been deposited by waves, they can hardly ever be removed by backwash. 3.5.1.3 Cliffs A marine cliff (Pig. 45) is a rocky bluff, generally without vegetation, in contact with the sea. It has a steep or even vertical slope angle and is created by the direct or indirect erosive action of the sea. Marine cliffs are found in competent rocks but also in sands and clays. On the other hand, not all steep or rocky coasts are cliffs: there are cases of false cliffs which have not been modelled by the sea but by other phenomena, such as scarps along littoral faults or subaerial erosion coasts submerged owing to transgression movements. A distinction should be made between live cliffs, at present in straight contact with the sea and subject to marine erosion, and dead cliffs, no longer active and separated from the sea by coastal deposits or an erosional shore platform. Erosion, in a broad sense, and the dismantling and retreat of a cliff all depend on both littoral abrasion and cavitation and the removal of debris, as well as the degradation phenomena due to the sea. Moreover, the processes depend on the type of wave motion, tide extension, rock resistance, the topographic features of the shoreHne, the steepness of the submerged scarp, the quantity and particle size

Fig. 45. Schematic example of a cliff profile.

114 distribution of the detrital materials transported, etc. An essential role in a cliffs morphological evolution is played by several characteristics: a difference in the degree of competence gives more or less steep profiles; mineralogical composition may favour to varying extents chemical weathering and therefore a scarp's degradation and retreat; the degree of homogeneity or the alternance of different lithological types conditions a cliffs shape; the strata's tectonic attitude or the presence of joints determines more or less steep and indented coastlines. The retrogradation mechanism of a cliff might be schematically simplified as follows. The starting point is given by a cliff of unspecified origin in direct contact with the sea: the processes of weathering increase the number of fissures and cause a certain fragmentation of the rock; at the same time the actions of cavitation and especially abrasion effected by coastal breakers dig a notch in correspondence with the mean sea level; after a certain lapse of time the fall of a portion of rock can occur in correspondence with the steepest and most fissured stretch of sea cliff The fallen debris which accumulates at the cliff's toe for a time protects the scarp from the attack of coastal breakers. The latter in the meanwhile gradually reduce the rock fragments by means of abrasive processes linked to the waves' landward and seaward motion; the smallest detrital fragments are progressively removed, owing to the erosive effect of the waves and in particular longshore currents. As the accumulation covering the cliff's toe is gradually dispersed, meteoric degradation may continue to produce and/or increase rock fissuring in the scarp. Moreover, the breakers' actions can dig a new notch and reestablish the morphological situation described at the beginning: thus, a retrogradation process may start all over again. Retrogradation of sea cliffs may occur with extremely varying rates: from imperceptible to 10 cm/year. Therefore, with other conditions being equal, the evolution of cliffs depends on the erosion processes which break up and demolish the rocky walls, those which determine a reduction and removal of the debris and finally the weathering process, and in particular the actions of marine waves and currents. Nevertheless, an important role seems to be played by the more general movements of the sea, i.e., fluctuations of its level in either a transgressive or regressive sense. Indeed, the retrogradation 6f a marine cliff leads to the formation of a wave-out erosional platform (Fig. 45) whose progressive extension obstructs the coastal breakers. The latter gradually lose their erosive power as they come to attack the cliffs toe since friction with the platform disperses a considerable portion of the waves' initial energy. If the general movements of the sea or, rather, the relative movements of sea and coast tend to a transgression, the evolution mechanism of sea cliffs does not stop, but the cliff is more and more subject to retrogradation, leaving at its toe an erosional shore platform which is gradually submerged by the transgressive sea. If on the contrary the movements are regressive, then the sea cliff cannot be lapped by the waves any longer and thus turns in time into a "dead cliff. In some cases cliffs have vast and irregular cavities of varying dimensions: notches, caves, arches, etc. Their origin can be due to physical or chemical degradation processes or to differential marine erosion of lithological types of varying resistance or rock portions having different degrees of fracturing. In limestones the

115 origin is often related to karst solution phenomena, simultaneous with or preceding coastal erosion processes. Sometimes the waves' action can isolate large rocks and stacks from the cliff. Usually a jagged and irregular coast with bays and headlands is subject to a regulation of its shoreline: in fact, owing to wave refraction beaches are formed on bayheads whereas promontories are eroded, with consequent attenuation of the embayed shoreline. Subsequently, baymouth bars, spits and erosion platforms develop further reducing wave energy towards the coast thus favouring littoral sedimentation. The latter takes place according to processes and landforms mainly governed by longshore currents. In some places, however, especially in correspondence with more or less resistant rocks, selective littoral erosion occurs which progressively forms and accentuates a more or less regular coastline. 3.5.1.4 Beaches The term beach is given to a littoral belt made up of present and recent cohesionless marine deposits. Seaward it is bordered by the low water zone called offshore and landward by sand dunes and the first bedrock outcrops (Fig. 46). A beach is subdivided into areas the first of which is called mshore. This is the portion ranging between the mean high and low-tide levels and the permanently emerged zone. Generally the inshore contains a low tide terrace and a surf zone. The latter corresponds to the steepest portion, where surging waves and backwash movements usually take place. The emerged beach is often longitudinally divided by a berm into two parts: the backshore and \ht foreshore. The berm is formed by an accumulation of materials abandoned by coastline breakers, beyond the backwash limits of influence, where water goes back to the sea mainly by underground flow. It can be increased also by the wind, depositing sandy material from the foreshore. Coasdine breakers of particular intensity may erode the berm portion facing the sea. The backshore is made up of present littoral deposits and only exceptionally can it be invaded by sea waters; in some places it contains shallow lagoons and on its surface houses and farmland can be found. In the offshore and inshore if the detrital material is not subject to violent actions by the waves, it is removed and is moved slightly forwards, in correspondence with backshore

foreshore

Fig. 46. Schematic example of a beach profile.

inshore

offshore

116 crests, and backwards, in correspondence with wave troughs: typical oscillation ripples can be observed. Along the surf zone, seaward current ripples are found if the backwash movement prevails; on the contrary, if the advancing movements are dominant, landward current ripples are formed. The study of these detailed seaforms can be applied to reconstruct the dynamics of coastline waters. On the other hand, when the plunging breakers are rather forceful, the grains of material are bluntly raised by turbulent movements: some are thrown on the emerged beach and others slowly fall down onto the sea floor. Backwash currents can remove the finest particles and carry them off-shore; they can also easily transfer those materials which, raised by the wave motion, stay for a while in suspension near the sea floor. The debris, once removed, transported, deposited and again removed, undergoes a continuous alternating landward and seaward movement, with trajectories variable in space and time, depending upon the waves' direction and energy, the force of backwash, the direction of longshore currents, etc. Afterwards, it will be deposited in lower turbulence, lower current velocity zones or in points where a compensation of two opposite currents occurs. The general dynamics of a beach is usually characterised by an alternance of two actions: an action of prevailing accumulation of debris and an action of erosion in a strict sense of the material. The alternance rhythms of these phases are extremely variable: daily, several days and so on up to completely seasonal. Indications of the degree of stability of this type of coast are derived from the equilibrium between supply and removal of detrital material. If the balance is equal over a certain time, then the beach may be considered stable. On the other hand, beaches can be affected either by progradation or retrogradation, whether the supply of material or its removal prevails. Particles are selected by waves according to their dimensions, weight and shape. The coarser ones, which are usually heavier, are abandoned on the beach, whilst the finest ones can be taken back, transported and deposited off-shore. The material can also be thrown by plunging breakers beyond the high tide limit or, in any case, beyond the sea level at that moment: if backwash does not take place in this stretch of beach, because the breakers' water goes back to the sea through underground passages made up of detrital permeable layers, even the finest debris can remain on the beach. As a consequence, the dryer and the more permeable a beach is, the more the process of sedimentation, and therefore of progradation is accentuated: this fact can easily be observed on Mediterranean beaches in summer. On the contrary, during rainy periods, especially with high and frequent waves, the beach sediments are saturated with water and therefore become practically impermeable; as a consequence, all the water goes back to the sea through the subsoil, with velocity directly proportional to the beach gradient. Moreover, rain water can add to sea water, thus increasing the erosion power in a strict sense and the transportation of backwash waves. To all this, also the removal and undermining action of plunging breakers should be added, with the increased likelihood of debris being taken on load by the waves, since the energy necessary to keep a detrital element in movement is lower than that required for its removal from the state of immobility.

117 For these reasons many beaches are subject to erosion activities in winter: on the one hand great plunging breakers and backwash currents attack and demolish the beach and shoreline bar, on the other hand they convey to the sea part of the debris that make up beach deposits. Generally in a prograding beach the prevailing sedimentation stage alternates with phases of partial erosion and marine and aeolian rearrangement. The debris accumulation on the seaward face of the shoreline bar causes progressive advancement of the crest, with consequent widening of the backshore. The beach slope gradient mainly depends on the size grade of its grains: fine sands make up very gentle seaward slopes whereas coarse sands or gravels build quite steep slopes: — 2° for 0.12 mm diameter sands: — 8° for 0.50 mm diameter sands; — 12° for 2 mm diameter sands; " 15° for 5 mm circa diameter gravels; — 20° and more for pebbles of 64 mm and over in diameter. It should be considered, though, that the slope angle depends also on the energy of the wave motion which, in turn, is partially linked to the longitudinal trend of the coastline, its irregularities and the extension of the sea front. Preferential places for debris accumulation and therefore for beaches are the sea stretches facing river mouths, because of the abundant supply of materials, or the bays and the areas between islands and coasts, owing to wave refraction. Sand dunes can be found landward, beyond the berm: they are accumulation landforms due to wind action and are similar to those of arid regions. With the latter they have in common a vegetation-lacking environment which favours wind deflation processes. The debris supply area is the beach whereas the accumulation area usually corresponds with the backshore or with inner stretches of coast, where the energy of the wind blowing from the sea is lower whereas the presence of vegetation progressively increases. Seaward, opposite the beach, longshore bars can be found on the sea floor: they are long and narrow submerged deposits, subparallel to the coastline. The patterns that lead to their formation are complex, various and different. In some cases the first stage of their formation could occur in neutral zones, where waves and currents are compelled to deposit part of their load because of the interference of oppositedirection energies. In other cases the deposit could be the result of progressive slowdown of the waves owing to friction with the sea floor. This would lead to a loss of energy with consequent sedimentation of the coarser elements taken on load after the dismantling of headlands. In other cases they could be remnants of ancient berms submerged following marine transgression. Bayhead beaches are formed due to the decrease in wave energy owing to the phenomenon of refraction at inlets. The same phenomenon can also give rise to a neutral zone between an island and the coast, following the collision of two opposite wave energies, with consequent deposition of debris: the progressive sedimentation can then build up a junction between an island and dry land with the formation of a

118 tombolo. Hooked spits are the result of littoral drift motion owing to a leeward wind loss of part of the load transported following a decrease in energy. Cuspate forelands are formed following the collision of two opposite directions of currents carrying debris which is thus accumulated. Littoral barriers mainly derive from the emersion of a shorebar following the progressive availability of material. Lagoons are small bodies of water separated from the sea by littoral barriers or tombolos and are in direct and active connection with the inland fluvial network, since they are linked to the open sea by means of inlets, which are the starting points of channels going through the lagoons. In the lagoon there are marshy areas which are nearly always emerged and covered by grass and shrubs, called schorre, and others emerging only at low tide called slikke. In lagoons the reflux waters of tidal currents are added to the fluvial ones, thus freeing the bottom of the channels from debris. A lagoon's life depends on many conditions: the amplitude of tides, the flow rate and sediment load of influent streams, the dimensions and shape of the lagoon itself, the width of the mouths, the direction of marine waves and currents, etc. Within the same lagoon a live part is distinguished from a dead inner part. In the latter the tidal currents arrive much reduced, the channels are transformed into closed-system channels where shores prevail over slikkes. When abandoned to natural processes, which tend to obstruct their mouths and to fill their basins with debris, lagoons slowly subject tend to fill up going through the stages of dead lagoon, coastal marsh and swamp. 3.5.1.5 The evolution of coastlines A coastline, whether marine cliff or beach, can show either a positive or negative balance depending on whether the supply, that is the sedimentation phase, is superior or inferior to the drawing, i.e., the erosion and removal phase of the materials making up a coast. Coastline supply can be derived from two different activities: — those resulting from the subaerial modelling processes of the mainland (especially fluvial, colluvial and aeolian); — those due to either the direct or indirect action of the sea (cliff dismantling and retrogradation, erosion in a strict sense, erosion of the continental shelf and biological contributions, such as shells of mussels). Also the drawing of solid matter can be the result of two different activities: — elements taken on load near the coast, transported by currents and abandoned on another coast or towards the open sea; — elements removed by aeolian deflation and dispersed over the open sea or far inland. The negative balances of coastlines, or the erosion of a coast in a broad sense, i.e., when removal actions prevail over supplying ones, have various causes: natural and man-induced. Among the former wave refraction, causing the erosion of promontories, has already been mentioned. Other factors can play an important role, such as: climatic

119 changes which modify the trends and amount of erosion and sedimentation in the mainland and can also change the winds' trend; changes in the path of watercourses near their mouths with consequent modifications, haltings or new supplies of material along coastlines. Finally, eustatic, isostatic and tectonic marine transgressions should be quoted since they lead to noticeable, although extremely slow, invasions on the mainland by the sea. In most cases the latter are relatively slow phenomena, especially compared with human life, and hardly ever can they be adequately contrasted. On the contrary, the case of phenomena caused by man's actions is different: in the last few centuries and with increasing intensity up to the present, many coastline areas of the Earth have suffered deep changes in their natural equilibria, as a consequence of man's various activities. This has led to various degrees of alteration which can be defined in terms of impact (see chapter 1.5). 3.5.1.6 Continental shelf and slope The connection between dry lands and sea floors takes place all over the Earth's surface along a stretch of territory defined as continental margin. Therefore, all continents are bordered by a continental margin which develops from the shoreline to the abyssal plain according to the following morphological units (Fig. 47): a) continental shelf; b) shelf break: c) continental slope: and d) toe of the slope. Whereas the origin and the structural attitude of continental margins reflect the geodynamic features of the continents, the forms that characterise them show constant elements derived from the homogeneity of the geomorphic processes (erosional and sedimentary) which modelled them. For this reason, continental shelves show the following characteristics: mildly sloping erosion or accumulation surfaces up to a depth of 120 to 200 m, shelf margins made up of a deeply convex surface formed by the prograding accumulations of fine sediments; the slope is a high-gradient surface linking the margin with the BEACH

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a - submerged beach b - posidonia prainie c - beach rocks d - late quaternary deposits e - olocenic deposits f - slumpings area g - slope deposits area

Fig. 47. Schematic example of a continental shelf profile.

\

^ \ g

120 bathyal plain and, finally, the toe of the slope where the materials mobiHsed by gravity forces along the slope surface accumulate. Usually the bathyal plain is found at a depth of 2,000 m and goes as far as 4,000 to 5,000 m.coastal hazard The surface of connection between the continental shelf and the bathyal plain, with a difference in height of a few thousand metres, is essentially affected by gravitational geomorphic dynamics. The terrigenous sediments, which accumulated on top of the slope also in the periods of maximum sea-level lowering in periglacial epochs, are normally found in unstable conditions, giving rise to slumping even over very wide areas. Moreover, landslide events repeated in time in areas characterised by high terrigenous intake from the continent produce turbidity currents along the slope, with high linear erosive energy. This phenomenon gives origin to deep cuts called canyons, and can also be favoured by particular lithological and structural conditions. Therefore, the canyon system of a slope reflects the morphodynamic and sedimentological structural characteristics of the whole continental margin. A landslide originating at a canyon's head on the edge of the shelf produces a turbidity current which can run for several kilometres along the whole cut, as far as the toe of the slope. Eventually the current reaches the bathyal plain where the materials in suspension progressively lose their energy and are deposited, forming large fans. Even without the presence of canyons, an accumulation of fine sediments is nevertheless produced at the base of the scarp making up the toe of the slope. 3.5.2 The hazard"^ *by Paolo ORRU' and Antonio ULZEGA'

3.5.2.1 Coastal hazard The problems related to coastal hazard are of considerable environmental importance because of their implications regarding risk: in fact, human activities now take place along most coastlines, sometimes with continuity. A coastal environment which for many significant reasons can be used as an example is that of the Mediterranean Sea (Fig. 48). This sea is a semiclosed body of water localised at midlatitudes and not subject to extreme meteorological and climatic events. Since the most remote times its coasts have been a location favoured for human settlements. Over the centuries this has led to the creation of harbours, to urban and industrial centres, to hydraulic land reclamations, to the excavation and discharge of materials and so on in places mainly chosen according to their physiographic characteristics. Usually these areas have also coincided with situations subject to rapid geomorphological evolution, such as fluvial estuaries, peninsulae, coastal plains, lagoons, etc. Settlements, linked initially only by sea, subsequently required the progressive construction of communications routes of growing importance along the coasts: first roads and then railways. Along coasts the highest levels of hazard are due mainly to the following conditions:

'A. Ulzega has also written paragraph 3.5.1.6.

121

Fig. 48. Location of quoted examples: 1) Capo Altano, cliff in rotational slide; 2) Baia del Poetto in Cagliari, active cliff and beach erosion processes: 3) Baia di Carbonara, littoral erosion processes; 4) Muravera coastal plain, inundation processes in coastal plain and lagoon; 5) Venetian lagoon and R. Po delta, lagoon earth filling processes; 6) Ria di Olbia. fluvial progradation processes in a harbour area; 7) Gulf of Asinara, submerged paleocliff with rotational slide; 8) Capo Caccia. submerged paleocliff with rock falls; 9) Strait of Messina, unstable submerged paleocliff owing to neotectonics; 10) Gulf of Orosei, active canyon; 11) Gulf of Gioia, canyon's headward retrogradation processes; 12) Gulf of Taranto, scarp deltaic processes; 13) Strait of Corynthus. scarp landslide processes owing to neotectonics; and 14) Channel of Sicily, sediment dynamics due to dragging currents,

1) 2) 3) 4)

active geodynamics owing to neotectonics, volcanism, seismicity and subsidence; geological-structural attitude favourable to instability; high fluvio-deltaic and littoral sedimentation; high meteo-marine energy.

3.5.2.2 Hazard affecting cliffs With respect to marine cliffs, the hazard is linked to retrogradation, whose evolution velocity is function of: — energy of incident wave motion; — geological-structural conditions; — seismo-tectonic activity. In terms of geomorphological risk, the situations of highest hazard are found where mass movements associated with retrogradation affect constructions on the cliff top's

122 surface; as for people's safety, the risk consists in the possibihty of sudden falls from unstable scarps. For example, the case of the Sella del Diavolo promontory, east of the township of Cagliari in Sardinia, is illustrated. It corresponds to a small horst made up of a Miocene sequence of sandstones which give way towards the top to marly-arenaceous Umestones and eventually to Tortonian calcareous reef banks. The coast shows a tectonically-derived cliff which is morphologically active owing to the continuous undermining effect at its toe caused by sea waves. In different epochs, gravitational phenomena of varying intensity have taken place, such as slumps, rock detachments and partial falls of the slopes. In proximity of the most unstable part of this promontory there is a busy tourist harbour and a beach which is very crowded during the summer. This has led to situations of serious risk which lately have caused the loss of human lives. In order to mitigate the undermining process at the base of the sea cHff, the Ministry of Civil Defence of the Italian Government has constructed a vast cliff which has locally reduced the direct risk but, at the same time, has triggered erosional processes in the nearby beach, with noticeable damage to seaside-resort activities. In some places, also nonactive marine cliffs, that are no longer undermined at their toe by sea waves can show conditions of potential instability, given by: — dormant deep-seated gravitational slope deformations; — fault surfaces. The geomorphological risk is often associated with works which owing to load changes or modification of the internal runoff, alter the scarp's conditions of equilibrium. Deep-seated gravitational deformations consequent to Pleistocene variations of the sea level have affected the Capo Altano sea cliff, in south-western Sardinia, as illustrated in the block diagram of Fig. 49. At present, the direct sea action does not affect the sea cliff; nevertheless the recent construction of a major coast road on the slip surface of an ancient landslide has determined its reactivation, thus causing the complete destruction of the work itself. Transport infrastructures and residential and industrial settlements along coasts affected by uplift neotectonic movements, are particularly subject to risk, as shown by the accelerated retrogradation processes along the Italian coasts of Liguria, Tuscany and Calabria. 3.5.2.3 Hazard affecting beaches Along most of the Mediterranean coasts, a general increase of the erosional processes derived from the present rising of the sea level can be recorded. This phenomenon is particularly evident on beaches, where even minimal variations of the sea level can induce large-scale effects. Inevitably, these phenomena are related also to vulnerability as the natural process of erosion affecting a beach can determine risk conditions for the works constructed in the high-beach active zone. Moreover, the concomitance of a natural event with unsuitable use of the beaches can exacerbate and accelerate erosional processes.

123

Fig. 49. Capo Altano, south-western Sardinia — block-diagram showing the fossiHsed coastal rotational slide from the "Eolianiti di Funtana Morimenta" (Middle Pleistocene). The recent construction of a road embankment (a) in correspondence with the slip surface reactivated the slide which caused a partial destruction of the works. Lithology: 1) marlstones. sandstones and conglomerates (Oligocene); 2) rhyolitic tuffs (Oligo-Miocene); 3) ignimbritic lavas (Oligo-Miocene); 4) aeolianites (Middle Pleistocene); 5) aeolianites and colluvial deposits (Upper Pleistocene); 6) conglomeratic beach-rocks (Holocene); 7) sandy colluvial deposits (Holocene-Present); 8) sands and gravels from present beaches (modified after Orru & Ulzega, 1984).

Finally, the construction of engineering works on the shore line could itself determine immediate or subsequent impact effects, such as the long-shore dispersion of a beach's sandy materials, and therefore trigger accelerated erosional processes. In any case, even if erosional processes do not coincide directly with the works, the loss or degradation of a beach always means serious economic consequences. As an example, the case of the Poetto beach, in southern Sardinia, is now reported. This beach originated during the Versilian emersion of a 10 km long littoral barrier closing a complex lagoon system of Tyrrhenian-related backshore. The full area is no longer provided with the feeding of solid materials owing to the complete channelisation of watercourses, the presence of quarries in the riverbeds and the construction of reservoirs upstream. Moreover, since the Second World War the beach has been subjected to the removal of several million cubic metres of sand used for the building industry in both its submerged and dry portions. Over the past fifteen years a general acceleration of erosional processes has been recorded; they have produced a 70 m retrogradation of the shoreline and a general lowering of the beach profile. This new arrangement allows the coast roads and seaside resort areas to be reached by the most intense sea storms. With marine meteorological conditions of low energy, sandy residual deposits witnessing the presence of paleobeaches can be preserved, although in a precarious

124 equilibrium. This is the case of Spiaggia del Riso near Villasimius, placed at the south-eastern extremity of Sardinia, where the particle size and chromatic characteristics of its exclusively quartz grains made it famous as a seaside resort. The recent construction of a large tourist harbour nearby has altered the beach's dynamic equilibrium, causing total removal of the sand. The extremely rapid erosional process occurred after sea storms during two winter seasons, with the formation of an erosion shore about 4 m high over the Pleistocene clastic substratum, whose retrogradation is still taking place and is threatening the structures of a local resort (Fig. 50). 3.5.2.4 Hazard affecting lagoons Another problem regarding coastlines of great environmental and economic importance is found in lagoons, marshes, deltas or estuary wetlands. In these areas the development of aqua-culture activities is in contrast with the need to preserve biotopes of international importance. Wetlands in general are subject to filling processes which are taking place more or less rapidly according to the management trends of surrounding territories. In the lagoons of Veneto, in the northern part of the Adriatic Sea, a variation in the hydraulic regime, with prevalent transportation of fine sediments by the affluent rivers, is favouring the development of eutrophic conditions leading to the overproduction of biogenic material. Moreover, a further alteration of the ecological balance of these environments as been caused by the intake of waste substances from high-density urban settlements and farming activities. This new tendency has reduced

Fig. 50. Villasimius coast, "Spiaggia del Riso" after a year from the construction of the new tourist harbour. Legend: 1) granites with basic dykes: 2) quartz sands in a highly oxidised silty matrix (Versilian); 3) present sands; a) emerged abrasion shelf: b) transgressive pebble deposit; c) erosion shoreline subject to accelerated retrogression shows some pedogenetic levels; d) original shoreline.

125 and in some cases ruined the productivity of these lagoons as food resources. The same problems, with varying levels of gravity, are involving the Adriatic lagoon system of the River Po delta which makes up one of the most significant natural environment of the Mediterranean. In the Mediterranean morphoclimatic domain the filling up processes affecting coastal wetlands are often caused by occasional high-water events. In the coastal plain of the Flumendosa River, in south-eastern Sardinia, the inadequacy of the slopes and the man-induced degradation of the vegetal cover owing to fires, sheep grazing, etc., produces great quantities of detrital materials. Dismembered slope materials are therefore loaded up and transported towards the sea in concomitance with catastrophic floods which periodically occur at the end of every dry season. The detrital mass carried by the river passes through the coastal plain, damaging farming activities and transport infrastructures and is finally dispersed into the lagoons. In this way a devastating effect on the biological balance in fish-breeding plants is produced, accompanied sometimes by the total destruction of high value food resources. The lagoon systems are goverened by extremely complex equilibria between physical and biological processes. Sometimes interventions of hydraulic engineering aiming at a rational water exchange with the sea, produce deep changes in the natural equilibria instead with consequent diffusion of salt water into the environment. This is the case of Cabras lagoons, in centre-western Sardinia, whose fish productivity used to be among the highest in Europe but has now fallen to extremely low levels. 3.5.2.5 Hazard affecting continental shelves On a continental 5/z^//hazard conditions are defined as follows: 1) rapid progradation of deltaic apparatuses; 2) high sea-floor dynamics of sediments; 3) active geodynamics. Moreover, on a continental shelf the impact of human settlements is felt to a greater extent, such as the dumping of industrial waste and the input of urban sewage. An example of the former can be found in north-eastern Sardinia, where rias coast morphologies are frequent, i.e., derived from fluvial valleys submerged by the sea. In this area the Olbia ria is located which is occupied in its innermost part, 4 km from its outlet, by an important tourist harbour and industrial port as well as by the city of Olbia itself. Access is made through a narrow, 100 m wide and 10 m deep, navigable channel. The ria itself was utilised as an anchoring and trade landing place since the Punic period (sixth century B.C.), whereas in Roman times it was the most important port in Sardinia. At present the ria is affected by accumulation processes of fluvial sediments resulting from the increase of solid load in the Rio Padrongianu, whose delta is placed at the southernmost tip of the inlet. While a small portion of sediments is taken in load by littoral drift and transported towards the outer mouth, the high content of organic particles from the sewage waters of the city of Olbia and concentrated in the marine environment amalgamates with most of the fluvial sediments, thus hindering their dispersion. Therefore the filling processes of the ria

126 act directly on the access channel to the harbour, causing a serious risk for maritime traffic and compelling port authorities to undertake continuous, extremely expensive dredging and reclamation operations (Fig. 51). In the Mediterranean marine prairies made up of Phanerophytes {Posidonia oceanica) are the main element controlling the trophic state of the sea environment and a fundamental regulation element of the geomorphic processes taking place in the upper continental shelf and submerged beaches. The extension of marine prairies depends on the general, climatic-morphological conditions of the environments in which they are found and reacts prompdy to global changes. It should be emphasized that at present the prairies investigated show critical stability conditions and even small actions from the outside can produce rapid retrogradation processes in its lower and upper limits or widespread degradation. The external man-induced actions are brought about by mechanical demolition in the case to create anchoring points and by the practice of trawl-net fishing (Fig. 52). Moreover, urban and port settlements along the coasts give rise to covering processes produced by the sedimentation of suspension particles, whereas chemical degradation is caused by industrial and agricultural products spilled over the territory. The maintenance of optimal quality for the marine environment is the best way of preserving very important resources for

Fig. 51. Aerial photograph of the southern shore of Ria di Olbia (north-eastern Sardinia). 1) Progradating delta of Rio Padrongianu; 2) dredged canal for access to Olbia's commercial and industrial harbours.

127

Fig. 52. Side scan sonar image of the continental shelf of the Gulf of Orosei (centre-eastern Sardinia) showing the damage caused by trawl-net fishing on the Posidonia oceanka marine prairies. Legend: 1) canal filled by sand ripples; 2) furrows due to mechanical erosion; 3) sea-floor profile (modified after Orrii & Ulzega, 1987).

Mediterranean countries, regarding both tourism and fishing activities. Among the risk situations connected to the degradation of sea prairies there is the reduction of fishing stocks owing to the lack of adequate nurseries, as well as the increase in the extension of submerged beaches, with consequent degradation of the dry beaches used for seaside resort activities. At varying depth on Mediterranean continental shelves, rock scarps, which evolved during the Quaternary glacio-eustatic regressive and transgressive phases into sea cliffs, are often preserved. At the moment of their morphogenesis, these cliffs were subject to coast dynamics which produced rock falls, rotational slides, deepseated gravitational slope deformations with basal accumulations of both landslide masses and debris fans. Their submersion with respect to the present sea level stopped the undermining processes at the foot of the cliffs but did not nullify their instability. For example, in the gulf of Asinara in Sardinia at a depth of about 70 m there is a 20 m high paleocliff, which developed in Miocene calcarenites with monoclinal attitude. Its front is affected by a succession of rotational slides alternating with debris fans (Fig. 53). Another example is found on the shelf in north-western Sardinia, off the coast of Alghero, where at a depth of 80 to 120 m a paleoshore of structural origin, with alternating high cliffs and inlets, is present. This landform is made up of Mesozoic carbonatic rocks and on the walls of the cliffs shear surfaces, open fractures and heaps of unstable blocks are preserved. These structures are all related to large rock falls (Fig. 54).

128

Fig. 53. Block-diagram of the submerged cliffs of Gulf of Asinara (northern Sardinia). 1) Miocene calcarenites; 2) coarse clastic sediments and blocks: 3) sandy and silty sediments; F) presumed fault; a) sharp rock edge and detachment crown; b) rotational slide rock body; c) accumulations at the toe of the rock edges; d) rock edges buried by fme sediments; e) layer's heads bounded by sharp edges; f) cuts buried by fine sediments (after Ulzega et al., 1986).

In these situations of potential risk in conditions of tectonic stability, engineering projects, such as the construction of platforms for the exploitation of hydrocarbons or the laying of submarine pipelines, should start only after carrying out a careful and precise check of the geomorphological conditions of the sea floor. A different problem is given by paleocliffs in tectonically active areas, as emphasized by the example of the straits of Messina. The sandstones and conglomerates of the Messina Formation (Middle-Upper Pleistocene) are intersected by marine abrasion shelf at present dismembered into several portions found at varying depths, up to 90 m. The neotectonic movements and the seismogenetic activity keep the rotational sliding surfaces of the continental shelf edge active. Moreover, the cover of the superficial sediments is affected by gravitational sliding producing detachment niches at the top and landslide accumulations at the toe. The channel between Sicily and the Italian peninsula is affected by the presence of various works such as gas and oil pipelines, electric and telephone cables (Fig. 55). These engineering works are subject to continuous monitoring because of the high geomorphological and sedimentological dynamics of the straits of Messina, whereas the planning and constructing of major communication infrastructures (road and railway bridges or tunnels) is made extremely complex by the same problems. 3.5.2.6 Hazard affecting continental slopes The passage from continental shelves to deep sea floors usually takes place along an

129

Fig. 54. Block-diagram of the buried cliffs of the Alghero coast (north-western Sardinia). 1) Mesozoic limestones and dolostones; 2) bioclastic sands and gravels: 3) silty sands; a) 75—80 m deep abrasion surface; b) step-like edges and outmost boundary of buried abrasion shelves; c) closed depression filled by fme sediments; d) fractures; e) mega-ripples fields; f) presumed faults; g) 20-30 m high subvertical edge; h) canyon; i) riverbeds; 1) fan-deltas; m) buried abrasion shelf (after Ulzega & Orrij, 1990).

inclined connection surface on which important geomorphological processes occur. On the continental shelf break and slopes the main cause of hazard is given by gravitational instability, related to progradation of sediments on the break, seismic and volcanic activity and the dynamic evolution of canyons. Sediment mobilisation due to gravitational energy, takes the form essentially of slumpings, turbidity currents and debris flow. The transportation of sediments from the slope to the bathyal plains assumes extremely high hazard values especially in correspondence with tectonically active margins. An example of canyon dynamics in a situation of tectonic stability is offered by the continental shelf of western Sardinia, deeply cut by the heads of several canyons. Among them, the Gonone canyon shows retrogradation due to regressive erosion which leads the head itself to move upstream up to a depth of 50 m, at a distance of 200 m from the high sea cliffs of the coast (Fig. 56). The shallow depth of the

130

1:25000

Fig. 55. Block-diagram of the Strait of Messina, Sicilian side near Mortelle. 1) Poorly cemented sandstones and conglomerates of the Messina Formation (Middle-Upper Pleistocene); 2) cross-bedding littoral sands (Holocene); 3) shelf's silty sands and sandy silts (Holocene-Present); a) marine abrasion surfaces disjointed by neotectonic activity: b) gravitational slide surfaces; c) detachment crowns of superficial sediments in boundary zone; d) basal landslide heaps; e) methane pipelines (after Orru et al., 1994).

canyon head has allowed its dynamics to be studied and the evolutional process in course to be defined by means of scuba diving investigations; among these phenomena the linear movement of the shelf sediments, the block slides of present biogenic formations and Pleistocene beach-rocks and the formation of wide slumping surfaces are studied. The construction of a submarine pipe for the dumping of urban sewage is now being planned; its outlet will be placed in correspondence with the canyon head. If on the one hand the active processes of sedimentary dynamics will favour the disposal of sewage, on the other hand active gravitational sliding is a risk for the work itself and special foundations and continuous maintenance interventions are necessary. Another example referring on the other hand to a tectonically active area is offered by the Tyrrhenian coast of Calabria, where the continental shelf is of such limited extent that in some cases the canyon heads reach the proximity of the shoreline. A series of step-like normal faults affect the whole continental margin favouring the formation of sliding surfaces. In these morpho-structural conditions a great industrial harbour is being constructed whose entrance is placed exactly between the two heads of the Gioia Tauro Canyon. At the moment of its construction, the materials resulting from the digging of the port basin, about 5.5 million cubic metres, were thrown into the sea directly on the canyon's margin. On June 12, 1977 at

131

Fig. 56. Block-diagram of the Gonone canyon (centre-eastern Sardinia). 1) Mesozoic limestones and dolostones; 2) stratified sediments (Miocene-Pliocene:'); 3) basalts (Quaternary); 4) deltaic and lagoon silts with peat (Upper Pleistocene); 5) paleoriver deposits; 6) sandstones and beach conglomerates (Upper Pleistocene); 7) organogenous silty sands (Holocene); 8) littoral sands (Holocene); a) canyon linear cuts; b) heads subject to erosion with cropping out of bedrock; c) edge progradation; d) slumpings; e) basaltic plateau with emission centre; f) paleodelta; g) beach-rocks; h) paleolagoons; i) paleocliffs with structural control; 1) lithothamnos plain; m) Posidonia oceanica sea-prairie; n) active sea cliff with Pleistocene and Holocene caves and breaker furrows; o) delta; p) lagoon; u) submarine pipeline. Detail of the canyon's head; 1) basement; 2) red algae bioconstructions; 3) ripple-mark sands; 4) silts; a) sea rocks; b) biogenic reef subject to landsliding; c) gravitational sliding furrows of organogenous coarse sediments; d) gravitational sliding of fine sediments, slumpings (after Orrij & Ulzega. 1987).

7:20 h, a submarine landslide was triggered in the dumping area whose current of turbidity, travelling at a speed of 15 17 km/h. caused at 8:12 h the rupture of a cable at a depth of about 600 m. At the same time, on the surface a lowering of the sea level by about 2 m first occurred, soon followed by a 5 m high wave which hit the harbour structures and the coast thus causing the demolition of one of the quays and other widespread serious damage (Fig. 57). The subsequent study of the Gioia Tauro canyon, carried out by scuba divers and submarine explorations along its slopes and axis up to a depth of 1,000 m, clearly showed that the event could be foreseen, due to a situation of great geomorphological risk. A similar event took place on October 16, 1979 in Nice where the waste materials derived from the extension works of the

132

Fig. 57. Gioia Tauro canyon (southern Calabria), a) Schematic topographic map of the canyon showing its two heads near the piers off the Gioia Tauro industrial harbour; b) longitudinal profile of the canyon showing the flow time of the turbidity current generated by the landslide (after Colantoni et al., 1992).

airport were dumped into the sea at the head of the Var-canyon. Among marginal areas, in the continental slope geomorphological dynamics is noticeably reduced owing to the limited availability of sediments and the decrease of slope gradients and, therefore, of gravitational energy. In any case, at the depths pertaining to these environments, 200—2,000 m, works and activities of economic interest are rare. Fig. 58. Block-diagram of the western margin of the Gulf of Taranto (north-eastern Calabria). 1) Submarine fan of the R. Trionto with linear flow channels along the whole scarp: 2) submarine fan of the R. Crati with top flow channels and basal mass transportation lobes; 3) scarp furrows showing retrogressive heads cutting through the submerged beach.

133

134 Nevertheless, situations of hazard can be determined in particular conditions, for example in tectonically active areas with margins in compression, where a very narrow margin with nearly nonexistent shelf corresponds to the rapid uplift of the dry lands. A significant example of geomorphological evolution of this type is observed along the southern margin of the Gulf of Taranto, in the Ionian Sea: where neotectonics has produced a great increase of energy in the slopes of the emerged reliefs, these give way without interruptions to the continental slope, with an anomalous accumulation of materials in correspondence with the coast, which give origin to complex sedimentary bodies in conditions of instability along the slope. Migration of the sediments towards the deep sea floor occur by means of linear movements in cone-shaped structures, the formation of lobes in prodeltaic environment, fan-delta or directly on the upper slope, where the narrowness of the shelf hinders littoral drift. In these conditions, along the whole margin in correspondence with the heads of the inflow channels, retrogradation niches are formed which directly affect the submerged beach: this creates situations of risk, first of all for maritime structures but it can also cause sudden withdrawal of the shoreline, with hazard for the roads running along the coast and for seaside resorts (Fig. 58). Another example of continental margin characterised by an important geodynamic activity is offered by the Hellenian arc. The activity in this case consists of high

Fig. 59. Channel of Sicily, about 20 nautic miles north of Scherki shoal. Side scan sonar image showing the sea floor at a depth of-400 m; (a) sand ripples; pipeline subjected to both burying and undermining processes: the darkest echoes (b) represent rock block heaps (c) lain down in order to stabilise the pipeline.

135 seismicity, with shelf uplift and slope instability. Moreover, active faults affect poorly cohesive evaporitic rocks, situated in the bathyal plain and at the base of the continental slopes. Owing to these contributory causes successive landslides triggered along the Corinth slope caused the repeated rupture of underwater electric and telephone cables. In morphological high places channelised in correspondence with straits, strong currents are usually created, due to the compression of the flux lines of large water masses; this phenomenon takes place with a preferential direction and triggers tractive currents flowing with a velocity of some metres per second on the sea floor. The displacement of sediments gives rise to parabolic dunes with high translation velocity. In the western Mediterranean, this phenomenon was observed in the Straits of Messina and in the Channel of Sicily. The latter is a body of water stretching from Cape Bon in Tunisia to Cape Feto in southern Sicily. The morphology of the channel includes Adventure Shoal to the north and Scherki Shoal to the south, in turn separated by an axial zone structured by a horst-graben system forming ridges and parallel channels with a NW-SE direction. On the bottom of the channels intense dynamic processes affect the coarser sediments which migrate towards the Pantelleria Basin to the south in the shape of parabolic dunes up to 20 m long and 3 m high. Along this path, three parallel methane pipelines which are the main energy link between North Africa and Europe were constructed. The movement of parabolic dunes can cause the undermining at the base of the pipelines which, in order to protect them, are weighed down with a cover of rock blocks and by regular periodical inspections (Fig. 59).

3.6 Glacial and periglacial hazard* *by Alberto CARTON

3.6.1 Introduction The situations of environmental risk in high mountain areas are mainly linked to the presence of ice and snow masses. Moreover, there are other phenomena that progressively lead towards gravitational movements in a strict sense; they are mainly due to the action of ice, snow and melt water. Since the areas where these phenomena take place are often remote and with a very scarce human presence, the resulting risk is often underestimated. In fact, the consequences often affect territories located even at noticeable distances from the place where a certain phenomenon has occurred. In the particular case of an alpine environment, the effects can be felt as far as the valley floor. As a consequence, in a period when mountain exploitation for sport and tourism purposes is very intense both in summer and winter, the hazard no longer concerns the single user but rather permanent anthropic structures or small and big sites alike, which are often very busy with people, although only seasonally. The hazard rate of certain phenomena is in certain cases very high owing to their

136 suddenness which is accompanied by a poorly defined or totally lacking series of premonitory signs linked to several variables and not always completely known or adequately taken into account. The prevention of these types of processes is moreover under-evaluated since within the field of natural hazard phenomena those regarding the presence of snow, and even more of ice masses, certainly do not represent a high percentage. Finally, in the present phase of retreat or immobility of the glacial fronts, also ice dynamics seems to be underestimated or neglected by most people, apart from specialists and some other frequent visitors of the mountains. On the contrary, one should consider that climatic changes occurring even in a short time (e.g., little Ice Age) can produce deep variations in the ice mass which in turn could affect recently developed areas or predispose future hazard situations. Hereafter, the hazard phenomena (Fig. 60) that can be found in a mountain environment are listed: — ice rock avalanches; — supraglacial debris fall/slide outside the lateral moraine; — ice fall from snout of glaciers (ice avalanches); — rapid advance of snout of glaciers (surges); — emptying of internal water-pocket; — emptying of proglacial lake; — emptying of ice-dammed lake; — debris flows caused by bursts of dammed ice;

Fig. 60. Typical example of periglacial hazard: in the foreground the destructive and accumulation effects of an avalanche; in the background an avalanche cone (photo M. Panizza, Caucasus, June 1974).

137 — — — — — — — — — —

debris flows caused by bursts of ice-marginal lakes; debris flows resulting from bursts of moraine dammed lakes; debris flows resulting from bursts of subglacial and englacial water pockets; high water events connected with rapid glacier ice melting; lahars; hazards caused by mountain climbing activities; mass falls due to glaciopressure; avalanches; advance of rock glaciers; debris flows caused by short and intense precipitations on scattered glacial deposits or by talus deposits. Before considering the series of phenomena that take place in the mountain environment, the formation of debris flows should be emphasized in particular, since they are a recurrent phenomenon among those connected with glacial dynamics. Their formation is mainly determined by sudden releases of large amounts of melt water as well as by the abundance of unconsoHdated sediments which are usually found near the unstable glacier margins in the form of thick moraine ridges or as scattered glacial deposits or, more generally, as ice-contact debris. Moreover, the constant retreat of the glacial fronts, starting from the middle of last century (which coincided with the maximum Holocenic expansion), has uncovered very vast surfaces, at present covered by large amounts of detritus not yet fixed by the vegetation and often lying on polished rocks. On the same surfaces a hydrographical network deriving from a glacier is found, characterised by extremely variable and unforeseeable flow-rates and capable of loading remarkable quantities of loose glacigenic sediments. As regards the waters more specifically, it should be remembered that the particular land surface arrangement (high relief energy, deep valleys, etc.) emphasizes, from the hydraulic standpoint, the effects of surface running waters, conferring them with high energy: their flow is often confined in narrow, steep and uneven spaces (gorges, troughs, canyons). In cases when glaciers release sudden amounts of water, some torrents are compelled to run in valleys which are inadequate with their new occasional flow; as a consequence, an anomalous raising of their level takes place with the depositing of various types of debris located on slope portions usually untouched by the flow of watercourses. 3.6.2 Ice-rock

avalanches

In some morphological and structural situations, ice and rock avalanches involving large volumes of material can occur. This phenomenon affects at first only portions of rocks that by sliding drag along also ice masses in their movement (Fig. 61). The so-formed melange is extremely mobile and can run very long stretches. The presence of ice, both in the form of blocks and/or melt water, modifies the mechanical behaviour of the landslide body by reducing the shear strength angle, making it very fluid and providing a "lubricated" sliding surface. More seldom the phenomenon can

138

Fig. 61. Schematic example of supraglacial rock fall outside the lateral moraine.

take place when a suspended glacier lies on fractured, easily degraded rock as a result of the infiltration of large amounts of water repeatedly turning into the solid state. Even seismic events can trigger ice-rock avalanches. The consequences of such a phenomenon can be both of direct and indirect type. An ice-rock avalanche can directly affect works connected with man's activities or create barriers to the hydrographical network with the consequent formation of temporary impoundment areas. In the latter case the heap's morphological features and dimensions can change with time, following the melting of the entrapped ice: variations will thus be a function of the amount of ice involved in the fall. The development times of the phenomenon are extremely short and prediction is almost impossible. Only in some cases can the starting of isolated rock falls foreshadow the event. Usually the rock portions affected by this kind of disarrangement are located in inaccessible places or at least in areas which are not normally frequented and are therefore not normally kept under direct observation. When a slope's proneness to movement is detected, it will be possible to set up a traditional monitoring system on the unstable rock mass if the consequences are assessed as negative for human activities. Ice-rock avalanches of large proportions are reported in the specific bibliography

139 regarding Peru: among catastrophes of this kind perhaps they were the most disastrous. From the top of Mount Huascaran in 1962 a portion of the ice cap detached (3.5 million cubic metres). A second avalanche of about 300,000 m \ detached from just below, followed the previous one. The great volume of ice dragged along a large quantity of rock and subsequently hit and removed a portion of a glacier. Eight villages were submerged and destroyed. Following an earthquake in 1970 an ice-rock avalanche was formed from the same Mount Huascaran; the towns of Ranrahirca and Jungay were devastated. Also in the Caucasus several cases of ice-rock avalanches were recorded. At the toe of Kazbek, from the Defdoraki glacier, rocks, mud and ice fell into the underlying valley in 1766, 1785, 1808, 1817, 1832. In the central Caucasus, the valley of Guenal Dom was devastated in July 1902 by two ice-rock avalanches coming from a glacial cirque bordered by the Giumaraykhkn, Kazbek, Marly Knokh and Fombal Fziti peaks. In the Alps, on the Simplon Pass, in 1901 on the flank of the Fletschorn a large volume of stones was detached and fell on the Rossbode glacier thus forming a gigantic avalanche rated at 2^3 million cubic metres. In literature other cases of ice-rock avalanches are described on the Italian side of Mont Blanc in Val d'Aoste (Val Ferret and Val Veny) in 1717, 1728 and 1920. In Switzerland in the Valais in 1714 and 1749 a vast amount of rock below the Glacier de Diablerets slipped perhaps owing to the glacier's melt water percolating within the rock fissures. 3.6.3 Supraglacial debris fall/slide outside the lateral moraine This phenomenon can take place during a glacier's accretional stage with a prevalently vertical development. At times the ice thickness increase can reach or overcome the height of the lateral/frontal moraine. The event takes place preferentially in the glaciers whose top is covered with abundant supraglacial debris (blackglacier) or when large boulders which have fallen on the glaciers are conveyed towards zones more favourable topographically to this type of phenomenon. The thickness increase of a glacier tongue is not necessarily determined by an expansion phase, especially when glaciers with abundant floating moraine are considered. In the latter, in fact, ablation is strongly inhibited by the debris cover and in difficult situations of frontal and superficial ''disposal" the mass can react by swelling up. Once it has reached the top of the moraine, the coarser detritus can slide outwardly as far as the scarp's toe (Fig. 62). The effects produced by this kind of event are normally of small entity and fall in the category of risk case histories only if they directly involve anthropic works (usually roads and paths). The phenomenon occurs suddenly when it actually takes place but its preparation develops slowly and is easily controlled. Usually only modest amounts of debris or more often isolated boulders are involved. Premonitory signs are given by a series of small slides along the morainic bank's outer scarp or by the proximity of large boulders to the glacier margins. When the debris' coarser fraction shows instability,

140

as-;

'^<^^^

Fig. 62. Schematic example of supraglacial debris slide outside the lateral moraine.

it will be possible to remove it artificially, using the same techniques applied for the removal of rock blocks along slopes. The use of explosives for the crumbling of very large blocks must be carried out with care, since an unforeseen modification of the moraine ridge's upper margin could become an element predisposing future disarrangements of various kinds (side lobes, breaching, etc.). It is obvious that the opportunity to intervene must be assessed as a function of the anthropic work affected since in some cases (such as a partial obstruction of a little frequented secondary mountain road) it could be economically more convenient to remove the debris at the end of the movement rather than take a preventive measure once any possible hazard has been pointed out. The deriving consequences are almost exclusively of a direct type: a road's partial obstruction, impact with a few buildings, etc. In any case, they are not events that could noticeably interfere with the hydrographic network. Given the little damage usually caused by this kind of phenomenon, a case history worthy of note does not exist: the commentary of the event is usually known only at a local level and does not constitute an interesting piece of news for the general public. In the Italian Alps a secondary road, intensely frequented during the summer, passes by the imposing frontal moraine of the Miage glacier, the largest of Mont Blanc. A series of pulse episodes occurring there this century (Mortara & Sorzana, 1987) has noticeably swollen the glacier's surface to the point of inducing the fall of

141 blocks from the summit of the lateral moraine onto the road underneath, along a 200300 m long front (Dutto & Mortara, 1992). 3.6.4 Ice fall from snout of glaciers (ice avalanches) The fall of great portions of ice is typical of those glacial tongues showing a suspended front (slope gradient over 30"", Eisbacher & Clague, 1984) or lying on rocky bedrocks showing particular morphological features. The fall of the front can cause an ice avalanche owing to both a net detachment of the tongue's foremost part and a slide or topple of seracs (Fig. 63). The detachment of the glacier's front takes place when the weight of the portion that is going to slide overcomes the sum of the frictional and cohesion forces at the bottom with the rest of the mass. The detachment usually occurs in correspondence with a crevasse and can be caused by a thinning of the ice mass due to the effect of traction towards the lower part of the valley. This is caused by a net difference between the glacier's advancing velocity and that assumed by the frontal part; the latter being imposed by the weight of the mass located along the steepest stretch. Other times the phenomenon occurs during an ice fall characterised by crevasses going nearly as deep as the bed rock, thus reducing the connection surfaces with the rest of the mass. This type of event can be favoured and regulated not only by the force of gravity but also by the air temperature which, following the ice melting, can form water films at the bottom of the glacier (Heim, 1895) acting as a lubricant with consequent velocity increase and friction decrease. Other rare cases of ice avalanche detachments can take place in those glacial bodies lying isolated and suspended in niches located on steep slopes. If the ice is collected within a concavity, in particular climatic conditions, a water layer can form between rock and ice, capable of detaching the ice mass from the bedrock and perhaps diminishing the ice weight on account of a hydrostatic thrust. Even earthquakes can cause the fall of glacial fronts or ice falls.

Fig. 63. Schematic example of ice avalanches.

142 Often if the ice volumes or the quantity of seracs involved in the fall are considerable, an ice-block avalanche can form which is sometimes mixed with debris collected during the movement (Alean, 1985). The distance run by the ice mass can be very long, since the slide takes place between media having a low frictional coefficient (ice on ice or ice on rock). Premonitory signs can be: an increase of ice fall activity at the glacier's toe, with repeated and frequent detachments, observation of sudden movements of large ice monoliths or sudden openings of crevasses. In situations of potential or actual hazard, the zones can be monitored by means of photogrammetry techniques or remote systematic control by using benchmarks located on points presumably involved in the movement. Moreover, wherever possible, cables fixed to the ice and connected to strain meters can be used, in order to assess the extent of the opening of crevasses or the ice mass dilation. This second type of control implies a series of logistical problems since the anchoring points of the steel cables and the topographic signs are placed in positions which are often inaccessible and/or covered with snow for several months a year. Moreover, traditional topographic control implies other problems, often linked to the distance of the fixed stations (necessarily placed on opposite slopes in a panoramic view) and the maintenance of the sights' verticality. Probably the application of the G.P.S. could give a useful contribution by systematically measuring the position of the prefixed benchmarks and in assessing the thickness variation of the ice body. The damage produced can be of different extent. If the ice-avalanche stops at the toe or near the glacier, where human settlements or works are not usually present, then the damage can be limited. In this case the subject involved will be the mountain-climber, and the phenomenon will fall within the case histories of mountain accidents. On the contrary, the entity of the damage can be relevant if within the radius of action of these phenomena ski pistes or ski-lifts are present. On the other hand, if the distances run are noticeable, the mass can affect large valley portions and stretch as far as the location of human settlements or involve areas of high daily frequency (the summer months and the warmest hours are usually the most favourable periods). The phenomenon can interfere with the hydrographic network only for short periods; the possible barriers will progressively lose volume with time as a result of ice melting. One of the events of this type that most struck public opinion took place on 30 August 1965 when a large portion of the Allain glacier's front crashed into the huts and the service road of the Mattmark building yard. The building yard had been placed in that area for the construction of a dam which, among other purposes, should have defended the valley from another hazard connected with the presence of the glacier, i.e., to avoid the frequent and catastrophic depletions of lake Mattmark (a glacial barrier lake with intermittent regime linked to the glacier's advancements). The tragedy created a stir not only for the number of casualties but for the region in which it happened which, by tradition, has taken great care regarding this kind of problem. One of the worst tragedies in the history of alpinism happened in 1990, again as

143 a result of a serac detached from a glacial front. In the Pamir range, at the border between Tadzikistan (former Soviet Union) and China, an earthquake caused an icefall which triggered a huge avalanche. In September 1895 in the Bernese Oberland a portion of the Altels ice tongue (4 miUion m^) broke at 3,100 m and fell to the Gemmi valley floor. During the previous summer a vast crevasse had formed in the stretch subsequently affected by the detachment. Miraculously unscathed was the village of Randa in 1973 (Zermatt valley, Valais). 500,000 m^ of ice detached from the front of the Bis glacier, fortunately in several sequences. Prior to this, however, this glacier had created several problems. In 1977 on Mont Blanc about 200 people in the village of La Tour were threatened by an ice-fall from the Tour glacier. In 1949, from the same glacier, some 450,000 m^ of ice had detached. The village of Eggen was totally transferred following its destruction in 1957 due to the fall of an ice mass from the Hochmatten glacier, located on the western side of Breihorn, on the Simplon massif. 3.6.5 Rapid advance of glacier snouts (surges of glaciers) Some catastrophes finding their origin in glacier phenomena are also linked to exceptional, sudden and rapid advances of glaciers' snouts, also called surges of glaciers. Such phenomena have been observed in the North American glaciers in Greenland, in Alaska, in Antarctica, in Karakorum and in the Italian Alps. Evidence of ancient advances have been recorded also in the Swiss Alps. This mechanism has not been completely understood, although it seems to depend on the dynamic behaviour of the glacier rather than on strictly climatic causes. Various authors have taken interest in this subject: Embleton, Flint, Kasser, King, Paterson, Post, Robin, Sharp. According to some of these researchers, the phenomenon is connected with the glacier's mass balance (Kasser, 1970, in Adrian, 1975), but this would not be sufficient to explain the velocity of the mechanism. Robin (1969, in Adrian, 1975) proposed three causes, actually linked to the glacier's dynamics: 1) stress instabilities; 2) temperature instabilities; and 3) bed-friction instabilities. Stress instabilities in a layer of ice can be caused by recrystallisation of individual portions of the glacier under strain; the stress concentrations can propagate as waves and can thereby produce unusually high rates of flows (Jonas & Mulles, 1969; Robin, 1969; Robin & Barnes, 1969; Palmer & Nye, 1972, in Adrian, 1975). Also the warming up of cold glaciers can produce a rapid and sudden advance (Lliboutry, 1969, in Adrian, 1975). Nevertheless, temperature instabilities are not considered as a primary cause, since the phenomenon has been observed at the same time in temperate glaciers. Probably the main cause is given by a sudden change of the friction conditions at the base of the glacier. A sudden decrease in the bed-friction can in fact be produced when the thickness of a water layer at the bottom of the glacier exceeds the size of the bed obstacles or following the detachment (super-

144 cavitation) of ice (Weertman, 1962; 1969; Lliboutry, 1964, in Adrian, 1975). Campbell & Rasmunsen (1969, in Adrian, 1975) demonstrated that a friction decrease of only 5% is sufficient to cause a surge. In some cases are the advance velocities really exceptional: Embleton & King (1971) recorded advances of some 350 m per day, Post (1960, in Adrian, 1975) measured a movement of 6.5 km in no more than 9 months for an Alaskan glacier. More generally, according to Patterson (1969, in Adrian, 1975), during a surge a glacier can move with a velocity up to 100 times greater than its normal one. The rapid advance of a glacier snout causes both direct and indirect types of hazard. The cases where an advancing glacier snout occupies territories where anthropic infrastructures are present fall within the first type. Those where a surge triggers other natural phenomena such as ice mass instability or damming of confluent valleys, belong to the second type of hazard. For example, it seems that the Huascaran ice-avalanche of 1962 (Morales, 1966) and the Mattmark accident (Eisbacher & Clague, 1984; Brockamp et al., 1967) were caused by glacial surges. The still scanty knowledge of the triggering causes of surges as well as the various theories so far produced show that the problem has not been completely solved. For this reason it is extremely difficult to foresee and/or monitor events of this type. Considering that each individual glacier tends to assume with time the same attitudes, at present the only reliable way to hypothesize future events consists in analysing their behaviour in the past, by means of historical analyses and field surveys (geological glacial studies). Case histories related to this phenomenon are numerous and scattered all over the world. Since the mechanism is not well-known, sometimes the event is ascribed to other causes. Besides the cases quoted above, another surge phenomenon is worthy of mention since it was a very spectacular case for the Italian Alps, although it did not have direct consequences for the territory: the advance of the Solda glacier snout (Ortles-Cevedale Group) which underwent an advance of over 1,200 m (Desio, 1967) in only one year (1817-1818). 3.6.6 Emptying of internal water-pockets, proglacial lakes and ice-dammed lakes Some of the glaciers' melt waters fill superficial endoglacial or subglacial cavities, forming lakes or water pockets which owing to different causes can quickly release large amounts of water. Water reservoirs are frequent at the margins of glacial tongues in correspondence with depressions between ice and rock, between ice and moraine ridges and between ice and ice (Fig. 64). Other basins can be formed following the damming of a valley by a glacial tongue. Water pockets are also present within the glaciers and probably correspond to the most hazardous situations since their existence is not known beforehand. Other small bodies of water can be found between ancient end moraines or be formed by the damming of a valley by a lateral moraine. The latter cases, though, seem to show a low hazard degree since they are not directly involved in glacial dynamics. It is obvious that the most important hazard phenomena connected with the

145

Fig. 64. Schematic example of internal water-pockets.

depletion of water reservoirs of glacial origin almost exclusively affect areas occupied by temperate glaciers undergoing intense melting with a runoff occurring both at the surface, at some depth and at the bottom of the glacier. The water pockets forming inside a glacier or at bottom depressions can evolve in different ways. The former are "water bubbles" migrating according to the movements assumed by the glacier which will exist until the surface of the cavity within which they originated is breached. This can occur as a result of the formation of a new outlet due to a glacier's physicodynamic variations (crossing of a crevasse area, collapse of ice portions supporting the pocket, etc.) or a contact with a glacier's margin. The latter can be depleted owing to the removal of temporary obstacles in the subglacial flow. In any case, the water volumes made available through endo- and subglacial runoff vents (Jackson, 1979) will emerge at the glacier's margins, usually in extremely short times. The depletion of epiglacial basins is also linked to the glacier's physicodynamic variations, resulting from the modification of the supporting topographical surface or the opening of endoglacial runoff ways (crevasses). In the first case the water runoff will be prevalently superficial.

146 The other ponds placed at the margins of the glacial tongues (Fig. 65), especially those located in depressions between ice and morainic ridges, or outside them, can suddenly release their water as a result of a dam's failure or cuts in the moraine. In the first case the cause can be the pressure exerted by the water on the ridge, which causes a syphoning or a settlement (Costa, 1988), or a partial failure in correspondence with ice-cored points (Haeberli & Epifani, 1986). In the second case a mild saddle in the summit profile of a containment ridge, in concomitance with a sudden raising of the pond's water level (snow and ice melting, intense rainfall, etc.), can act as an overflow and eventually become deeper on account of linear erosion. In these cases, as well as the highwater wave, a large quantity of material directly removed from the ridge itself is mobilised from the onset, thus giving way to a debris flow. When the basin is formed due to damming by a glacial tongue, the water release can occur owing to a failure or settlement of the tongue or its retreat (Clague & Mathews, 1973). Typical of the latter phenomenon is a repetitivity normally linked to ice fluctuations (Vivian, 1975; Tokmagambetow et al., 1980). The sudden release of large volumes of water following the opening of an outlet gives origin to phenomena of noticeable torrent activity, ranging from disastrous floods to the mobilisation and transport of considerable amounts of debris (debris flows); the latter is usually abundant on the spot, in the morainic ridges or on wide surfaces recently freed from the glaciers following the progressive and continuous ice retreat since the middle of the nineteenth century. The entity of the effects will depend on the amount of water released, the release time, the sloping of the channel

Fig. 65. Schematic example of proglacial lake at the margins of the glacial tongues.

147 which collects the running waters and its capacity to take the loose debris. In specific literature these events are named with such terms as debacle (French), glacial burst (English), jokulhlaup (Icelandic). As regards subglacial or endoglacial water pockets, it is practically impossible to foresee their existence and consequently to prevent their effects. If studies of glacial geology show the tendency of a certain glacier to have this type of event, it might be worth trying to identify the existence of these water basins by means of geophysical methods (for example, georadar, radio-echo-sounding, etc.). These investigations, though, are usually expensive and impracticable and seldom produce clearly interpretable results. In any case, if the location of a cavity is known, it could be drained by a tunnel. Other premonitory signs can be looked for, such as anomalous lowering of the lake's level, increase of flow-rate and turbidity in the pro-glacial torrent, formation of new springs and little streams outside the morainic ridges. Experience teaches us that in these cases rather than forecasting, the best remedial measure would be prevention, which can be carried out by means of appropriate modelling and stabilisation of the pro-glacial streams' riverbeds and protective works against the occurrence of debris flows. Prevention from the damage caused by natural water reservoirs located within depressions between ice and rock, or between ice and morainic ridges, or between ice and ice, due to damming by a glacier tongue, can still be carried out effectively when the accumulated water volume is not excessive. Tunnels or canals may be dug through the ice, rock or debris in order to control the level of the dammed lake. If these measures are not appliable on account of logistic difficulties, it will be necessary to construct flow-control weirs and debris containment dams downstream of the lake. In some cases, in order to avoid the formation of large impoundments, it is useful to prepare a deep and stable cut, properly coated with brickwork, in order to avoid the collapse of detritus and removal of material which would certainly trigger debris flows in concomitance with highwater conditions. In order to identify and prevent possible settlements or siphoning in morainic dams with ice-cores, their presence could be ascertained by means of thermic flow analysis in the ground with BTS measurements (Bottom Temperatures of the winter Snow cover). The realisation times are extremely rapid, since a dam's failure (be it of ice or debris alike) takes place in short times and at unexpected moments. Only when alarm systems are installed is it possible in most cases to avoid the loss of human lives. In the high mountain environment, this type of phenomenon is one of the most frequent and several examples have been recorded in the specific bibliography. Cases of flooding linked to glacial breaches and debris flows determined by the collapse of ice dams have been described in nearly all the temperate glacialised regions of the world: in the Himalayas, in the Karakorum, in the Caucasus, in Norway, in the Alps, in the Peruvian Andes (Cordillera Blanca and Hua Yuashe), in the western part of North America, in Alaska and in Iceland (Clague & Rampton, 1982; Helbing, 1940; Kerr, 1934; Liss, 1970; Mason, 1929, 1935; Mathews, 1965; Post & Mayo, 1976; Stone, 1963; Thorarisson, 1939; Young, 1980).

148 Some cases are particularly worthy of note, such as the disastrous event that took place on March 13, 1941 in the Cordillera Blanca owing to an earthquake which caused the sudden depletion of two glacial lakes and triggered a debris flow which destroyed part of the town of Huaras and killed thousands of peoples. In the Argentinian Andes, in the Juncal group, (near Mendoza) the Nevado glacier dammed a valley, forming a lake that twenty years later (January 1934) was suddenly depleted. In the Karakorum the Shyok glacier dammed the valley fed by the longest glacier of the chain, the Shiachen (75 km). The lakes thus formed were extremely large and chronicles reported floods connected with them in 1835, 1839, 1842 and 1927. Also the disasters which occurred several times in numerous Alpine valleys should be mentioned. On the Italian Alps the Santa Margherita lake, repeatedly dammed by the Ruitor glacier, was repeatedly depleted since the second half of the fifteenth century. At the end of the XIX century the lake dammed by the Tongue of the Cevedale glacier was suddenly depleted through a tunnel in the ice. In 1979 on Mt. Rosa the Locce lake was depleted and gave origin to a debris flow which destroyed a chairlift. In Val de Bagnes (Martigny, Valais, Switzerland) a vast iceavalanche fan obstructed the course of the River Dranse and as a consequence a barrier-lake was formed. In the Otz valley, in the Tyrolean Alps, from 1600 until 1890 — during the Little Ice Age — barrier-lakes linked to the numerous glacier advances were formed over 12 times: several jokulhlaup and ice floods were caused following these valley obstructions. Large-sized debris flows were formed from the Mattmark lake (Saas valley, Valais, Switzerland), once again dammed by a glacier's tongue and its moraines. In the Tete Rousse Glacier (Arve valley. Haute Savoie, France), a water pocket was suddenly depleted in 1892, thus freeing about 200,000 m^ of water and carrying ice and debris downhill. Some of these events, quite remarkable and recurring in past centuries, have at present reduced or exhausted their hazard, on account of the noticeable retreat of several glaciers after the Little Ice Age. In other cases, effective interventions have definitely averted the danger (Welchner, 1773; Stotter, 1846; Richter, 1892; Finsterwalder, 1897; Liestol, 1956; Hoinkes, 1969, 1972; RothUsberger & Allen, 1970; Rothlisberger, 1974, 1979; Nijasov, 1975; Lichtenhahn, 1979; Bachman, 1984; Eisbacher & Clague, 1984). 3.6.7 Highwater events on the valley floor connected with the presence of glaciers In the valleys of catchment basins where numerous glaciers are present, particular meteorological events, especially of the thermal type, can contribute to increasing exceptional highwater phenomena. At present it seems that few data or specific studies on this topic are available, although some significant events occurring during recent years show the decisive role played by glacial masses (Rey & Dayer, 1990). A sudden temperature increase causes a raising of the freezing level at high altitudes. It follows that if the areas affected by the phenomenon are occupied by glaciers, vast ice portions usually placed above the snow limit, would be found in the ablation zone. Moreover, precipitation which at these altitudes is normally snowfall,

149 turn nearly exclusively into rainfall, with consequent rillwash and accelerated melting phenomena. The release of large volumes of melt water in short times is a consequence of events of this type. The increased water availability will then feed the main watercourses which will show flow-rate values well exceeding the seasonal averages. Unfortunately nothing can be done to avoid such events: at the most the population can be alerted and preventive measures can be taken by means of monitoring the high-altitude temperatures and comparing them to the average seasonal trends. An example of this could be the disastrous flood of 1987 in Valtellina (Smiraglia, 1987a,b) and also in the higher Val d'Ossola, Northern Italy. During the days of the flood, the temperature values recorded at the stations located at higher altitudes (2,100-2,350 m a.s.l.) were superior to OX. This fact led to a raising of the 0°C isothermal line to around 3,400 m. Since the main glaciers in Valtellina have their fronts at altitudes of 2,200—2,500 m, an intense and rapid melting of the winter snow took place in most of them, thus reducing the drainage basins and depleting the snow accumulated over several years and perhaps the ice itself. 3.6.8 Lahars Also the rare and yet occurring combination between glaciers and volcanoes can be an element of hazard in mountainous areas. In active volcanoes, whose summit is above the snow limit or is covered by a snow cap, the occurrence of mud flows caused by the sudden melting of snow and ice in direct contact with incandescent lavas is quite possible. These flows, called lahars, can interfere with the hydric network and obstruct riverbeds or go as far as developed areas. The mobility of these events is high since a vast amount of fine and cohesionless pyroclastic material suspended in the melt water is removed. The occurrence of such a phenomenon can only be foreseen on the basis of volcanic activity monitoring. In hazardous situations defence works could be planned, although they are difficult to carry out since the event as well as the volume of material involved are unforeseeable data. The most recent example of this phenomenon took place in 1985 in the Colombian Andes. On the Nevado El Ruiz volcano (5,400 m a.s.l.), whose top was covered by an ice cap, melting and mobilisation of ice and snow followed a small eruption, with the formation of some lahars, the largest of which destroyed the city of Armero, claiming 20,000 lives (see chapter 3.8). 3.6.9 Hazards derived from mountain climbing A final kind of hazard, although limited and involving only mountain climbers, is derived from the instability shown in some occasions by the ice surface. Alpinist literature quotes innumerable cases of situations with often tragic outcome, when the collapse of an ice bridge, the opening of a crevasse hidden by snow or the failure of

150 a snow ledge caused the loss of human Hves. Only prudence, experience and compliance with the basic rules of rock and ice climbing can prevent and reduce this type of hazard. Another recent activity linked to the presence of glaciers is given by the practice of summer skiing. Also in this case a strict compliance with the rules set out by the public boards in charge can prevent accidents, considering also that in the last few years a thinning of glacial masses has taken place as a result of the raising of the freezing level. As a consequence, many crevasses have formed, together with an increase of the morainic cover and a reduction of utilizable areas. 3.6.10 Mass falls owing to confluence glaciopressure In case histories, also the phenomena derived from slope instability (usually consisting in mass movements such as falls) accompanying the maximum glacier retreat can be mentioned. In the Dolomite region some landslides which took place in the Late Glacial and Post-Glacial were observed; they are characterised by failure surfaces along slopes at the outlet of valley narrows, upstream of which there is a confluence of two or more valleys which once were occupied by glaciers (Fig. 66): they were triggered by a tensional discharge following the loss of pressure previously exerted on the rocky slopes by two or more glaciers merging in a valley narrow (Panizza, 1973). Even if the fall of large slope portions can directly affect human settlements or obstruct a whole valley, with the negative resulting consequences, the extremely high hazard degree assumed by this type of phenomenon is purely theoretical. Indeed, the long time span from the withdrawal of the glacial network to the present (some 12,000 years) has produced nearly total exhaustion of these events. 3.6.11 Assessment of the hazard derived from the presence of ice masses Relying on more or less accurate case histories, some authors have tried to parameterize some aspects of the hazard phenomena derived from the presence of ice masses by identifying their types, the ways they are triggered, their development and propagation, in order to set out forecast and prevention models (Haeberli et al., 1989; Laenen et al., 1987; Church, 1988; Dutto & Mortara, 1993). The parameters taken into account for this kind of analysis are usually the development times which each event can assume, the propagation distances and the volumes of the materials involved. From an analysis carried out on a certain number of cases (about 90) occurring in the Italian Alps, the following data have emerged (Dutto & Mortara, 1993). Falls of glacial fronts and landslides of ice and rock are the events that develop in very short times: from a few seconds to several minutes. The depletion of endoglacial lakes requires times ranging from minutes to hour, whilst for barrier-lakes and proglacial lakes times of a few hours to several days are necessary. The advances of glacial fronts and the debris falls from the glacier's margins require much longer periods (months to years). On the contrary, the propagation distances of instability phenomena are maximum

151

I ^2405

Fig. 66. Late and Post Glacial landslides in the Cortina d'Ampezzo area (Dolomites, Italy) due to confluence glaciopressure in valley narrows (arrows show valleys once occupied by glaciers) (modified after Panizza, 1973).

for the depletion of barrier-lakes and proglacial lakes, followed by mass movements involving ice, depletion of endoglacial lakes, advances, falls of the fronts and debris falls outside the moraine. As for the volumes of material involved, in the specific cited case, the first place is occupied by movements of rock and ice, together with the depletion of barrierlakes (triggering of debris flows and jokulhlaup). Extremely variable volumes characterise the falls of glacial fronts, whilst the debris falls from the moraine involve minimum amounts of material (Fig. 67). The combined analysis of these parameters results in particularly interesting observations related with the fact that the hazard scale varies according to the parameter considered. For example, the depletions of endoglacial lakes are to be considered extremely hazardous on account of their rapid evolution, even if the water volume can be less than that coming out from barrier-lakes. The latter are usually

152

volume

Fig. 67. Development times, propagation distances and debris volumes (water ± ice ± snow ± rock ± glacial and alluvial deposits) of the various instability phenomena. The most frequent ranges of values are shown (instability phenomena with propagation times, distances and debris volumes can take place also outside the areas identified) (modified after Dutto & Mortara, 1993).

characterised by slower depletions but with longer propagation distances. The same authors (Dutto & Mortara, 1993) tried to give a semiquantitative assessment of the hazard derived from the phenomena considered (scarce; average; high) by drawing up a classification based on the repetitivity of single events, the perceptibility of possible premonitory signs and the relative hazard, defined as the sum of the hazards derived from the development times, the propagation distances and the volume of the materials involved. 3.6.12 Avalanches The phenomenon of avalanches has been known since the mountains have been covered with snow and among the hazards recurring in the mountains, is the best known and most observed (Fig. 60). Probably it is about avalanches that most forecasts and prevention measures are made. Moreover, it is one of the few phenomena for which specific research agencies have been established, with the purpose of prevention and elaboration of thematic maps showing the areas more or less affected by the phenomenon. For the study of avalanches mathematical models have been developed based on the analysis of numerous parameters. The consultation of any modem treatise on the subject emphasizes the quantity of physical, mechanical

153 and meteorological data considered in this type of study and the complexity of their treatment. The vast knowledge about it is due to the fact that the people living in mountain valleys are obliged for several months a year to put up with a kind of event affecting much wider territories than those occupied by glacial masses. Once the only kind of defence available for mountain people against this hazard was exclusively passive: buildings constructed in traditionally safe areas, Hmited winter movements, scarce presence of human structures at high altitudes. Afterwards, the construction of communication ways in any possible place as well as the development of tourism and winter sports determined both a permanent and seasonal transfer of people toward high-hazard zones. A more active defence from avalanches therefore became necessary so their mechanisms had to be studied. The formation of avalanches due to spontaneous detachments takes place on the snow masses when either an increase of stresses (agent forces) or a reduction of resistance forces is observed. The former is mainly due to the accumulation of fresh snow (precipitations, wind), the latter can be derived both from destructive snow metamorphism (passage from star-shaped crystal to spheroidal crystal) and to constructive metamorphism (rapid growth of angle-shaped crystals) or to the temperature increase (formation of lubricating levels owing to pore saturation performed by melt water). Among accidental causes, the fall of debris and snow ledges, thunder, explosions, passage of animals or skiers should be mentioned. Avalanche subdivision varies according to the criteria applied. According to the type of movement, the first distinction is made between powder snow avalanches and flow avalanches. The difference between these two types is based on the velocity, the height of the mass in movement, the density, the effect of topography and the flow trends (Scheiwiller, 1986) (Table 11). Other subdivisions take into account the type of detachment, the position of the sliding surface, the ground morphology and the snow conditions (humidity) (Haefeli & Quervain, in Fraser, 1970; Fraser, 1970; Roch, 1980) (Fig. 68). TABLE 11 Comparison between the characteristics of powdery-snow avalanches and those of skimming avalanches, according to Schiwiller (after Casati and Pace, 1991)

Typical velocity Thickness of the mass in movement Density Effect of topography

Trends of flow

Flow avalanches

Powdery snow avalanches

30—60 m/s

-100 m/s

<5—10 m -100-300 kg/m' The avalanche is channelised in small valleys The roughness of the slide surface is a very important characteristic Granular flow

-100 m - 5 kg/m' None; avalanches follow the highest slope-angle lines The roughness of the slide surface is not an important characteristic Turbidity current

154

Fig. 68. Schematic subdivision of avalanches, according to HaefeH & Quervain (in Frazer, 1970), modified.

From the point of view of forecast it is possible to utilise a third subdivision based on the possible existence of a delay time between the meteorological event

155 which causes the avalanche and its effective detachment. From this standpoint "immediate" avalanches are distinguished from ''delayed" avalanches. The former, with which the term meteorological avalanches is also associated, are formed after a sudden variation of meteorological parameters (e.g., heavy rainfall, marked warming up), whilst the detachment of the latter is ascribed to the slow transformations within the snow. In this second case it is, therefore, very important to know the local characteristics of the snow cover. It is obvious that in reality it is extremely difficult to make a distinction between immediate avalanches and delayed ones, but this permits an understanding of how the aspects concerning the forecasting are conceptually different and also shows the data available. Avalanches often cause the loss of human lives and damage to property. To give an idea of the gravity of the phenomenon it is sufficient to quote that in the 1965—1985 period nearly 2,000 people lost their lives in the Alps owing to these events. Those usually involved in this type of hazard are the inhabitants of villages, workers employed in construction works on mountain slopes, tourists and military troops operating in high-mountain areas. As for infrastructures, the artifacts and constructions involved in these phenomena are mainly houses, communication routes and their infrastructures (bridges, underpasses, etc.). The consequences can be very serious, especially when human lives are lost: a person cannot resist for long under an avalanche heap, especially because of the adverse thermic conditions. The type of aid, the promptness of the intervention and the location of the person in difficulty is therefore the most important condition for success (Fig. 69). As regards damage to infrastructures, besides the cases when constructions (such as houses, hotels, etc.) are directly struck, entire inhabited centres can remain isolated for several days, even when not directly involved with logistic difficulties linked to the electricity supply, the disruption of communications, the food supply, etc. The perfect knowledge of a territory based on complete historical and geomorphological studies, aimed at the reconstruction of the past history of avalanches by means of chronicles and evidences gained from direct surveys in the field, provide extremely useful information. Since these phenomena show a systematic recurrence and usually take place in concomitance with analogous climatic conditions, it is of paramount importance to carry out a series of environmental surveys. For this purpose the various boards in charge of the study of avalanches have already equipped a set of sample areas in which specialised operators record meteorological and nivometrical data. In particular, the stability of snow is part of the general problem of mechanics governed by three groups of strictly interdependent parameters: meteorological, nivological (superficial and internal) and local parameters. The first two act also as indicators and together with a series of "visual" observations (e.g., the triggering of small avalanches at high altitudes) are used for prevention purposes (Table 12). Meteorological parameters show extreme variability due to the different altitudinal belts and to the important relief role which "generates microzones" characterised by different climates owing to the "different orographic protection". The superficial

156

1.0

0.8

^ 0.6 n OS

n o 0.4

0.2

0

1

2

3

4

5

hours Fig. 69. Probability of survival for a person buried by an avalanche in function of the length of time, according to Roch, 1980.

nivological parameters are continuously and directly surveyable (also automatically) whilst the internal ones are more difficult to obtain, since they can be recorded only through manual operations at ground level (digging of trenches). Local parameters, if known and correctly taken into account, can be considered from time to time as constants. The latter noticeably interact with meteorological parameters and, as a consequences, with the nivological ones. The forecast of avalanche risks is still an open problem, based on models using mainly deterministic and statistical approaches. Even if a large amount of objective data is available, practical experience teaches that a valid forecast is also greatly based on the operators' experience and "well-founded confidence" in their work, blending the physical data with personal "intuitions" and local knowledge. Still within the framework of risk forecast, numerous attempts to lay out "decision-making charts" (flow charts) have been made, but the latter have shown some effectiveness and applicability only in simple cases, where no more than five variables (32 possible combinations) are taken into account. In the winter of 1993—1994 the need for avalanche reports according to standardised scales, led to the drawing up of a single scale, with five levels of

157 TABLE 12 UNIFIED SCALE OF AVALANCHE HAZARD (Modified after Bassetti, 1994) (Valid from the 1993-94 winter season in France, Switzerland, Austria, Germany, Italy and Spain) Hazard scale

Stability of the snow cover

Probability of avalanche detachment

1) Low

The snow cover is usually well consolidated and stable

Detachment is possible only with high overload^ on very few extremely steep'* slopes. Only small spontaneous avalanches are possible (so-called discharges)

2) Moderate

The snow cover is moderately consolidated on some slopes; in the remaining ones it is well consolidated

Detachment is probable with high overload^ especially on the steep'' slopes shown. Large spontaneous avalanches should not be expected

3) Considerable

The snow cover shows a weak to moderate consolidation on many steep'' slopes

Detachment is probable with weak overload^ especially on the steep'* slopes shown. In some situations medium-sized spontaneous avalanches are possible and, in single cases, even large ones

4) High

The snow cover is poorly consolidated on most steeps slopes

Detachment is probable already with weak overload*" on most steep slopes. In some situations many medium-sized spontaneous avalanches should be expected and occasionally large ones

5) Very high

The snow cover is in most cases poorly consolidated and unstable

Many medium-sized spontaneous avalanches should be expected even on moderately steep" slopes

"In the Avalanche Report the following parameters are described in a more detailed way: altitude, aspect, pattern of the ground, etc. ^Overload: high) e.g., compact group of skiers, snowcat vehicles, use of explosives; low) single skier, ohneski excursionist. Definitions = steep slopes: slopes with angle over 30°; extremely steep slopes: slopes with unfavourable characteristics concerning inclination, ground pattern, proximity to crests and soil roughness; spontaneous: without man's intervention; exposition: cardinal point to which the slope is facing.

hazards, valid for all the countries whose territories fall within the Alpine range (Table 12). Defence against avalanches is carried out by means of various types of interventions that can act either directly or indirectly on the snow cover or the avalanche itself. In areas characterised by a high hazard level, both rigid and elastic containment works, placed in adequately separated lines, are constructed with the aim of stabilising the snow cover. For this purpose snow nets, racks and bridges made of steel cables, aluminium, wood and concrete are anchored to the ground with various devices. These works stand out of the ground at heights of a few metres and are usually inclined at about 15° downhill with respect to the perpendicular slope line. Another system of defence is given by the modification of the snow deposit by using

158 barriers that reduce or accelerate the wind velocity as necessary. The deflectors, usually made of steel-wire and nylon nets, slow down the wind velocity by conveying the snow accumulation into nonhazardous areas. On the contrary, the accelerator barriers which exploit the "Venturi effect" increase wind velocity in the most critical points, thus avoiding snow deposition. Temporary hazard situations can be solved in the short term by means of artificial impulses on the snow cover by triggering a series of small explosions. In this way large accumulations of snow are avoided and therefore the fall risk is reduced. Other protections made of earth, stone, walls or concrete can be installed; they mainly consist of channelising or diverting works, suitable to contain the avalanches in their natural slide paths or compelling them to take small diversions. The problem connected with this type of defence is given by the dimensions of the artefact, usually constructed on the basis of previous events or estimates. Especially on low-angle slopes or flat land it may be useful to slow down the avalanche velocity to avoid the involvement of risk areas. In this case it is possible to construct appropriate works such as dams, earth and stone barriers, butresses and braking wedges. To protect roads, railways and inhabited centres, purpose-planned works, based on recent engineering technologies, have been constructed in recent years, giving excellent results. Transport networks are usually protected not only by defence works placed along the slopes but also with tunnels, walls and canopies. Large earthworks or wedge-shaped concrete embankments are located so as to protect building sites, power lines, groups of buildings or entire inhabited centres. Besides all this series of interventions aimed at preventing and avoiding the triggering of avalanches, mention should be made of the natural and effective "antidote" for avoiding or mitigating the formation of avalanches in proximity of the tree line: that is, the presence of woods. They absorb part of the snow from the ground, supporting the snow cover with tree-trunks and prevent the formation of shear fractures. Forestation and deforestation can, therefore, alter the evolution of a slope in some cases. The developing times of such a phenomenon are fast (Fig. 70) and the triggering is quite sudden. It is possible to forecast that sooner or later there will be snow detachment in a certain area, although the precise moment is not foreseeable. Only the distance between triggering zone and strike area (structure, person, etc.) can in some cases allow people a few moments to find shelter or run away. The situations derived from the movement of fast powdery avalanches (>50 m/s), during which destructive pressure waves preceding the arrival of the avalanche itself are determined, are particularly critical. As for communication routes, some alarm systems capable of blocking the transit in correspondence with systematic avalanche paths have been set up. It is practically impossible to make a choice and quote disastrous examples of avalanches owing to the great number of cases occurring every year. The press, some periodicals and specialised reviews notify the amount of damage caused to people and property every year during the winter season. The hazard of this phenomenon is so serious that specific thematic maps have had

159 150

h 500 km/h

Powder avalanche h 400 b 100 y U

8

h 300

0)

x: o c m

I

Surface avalanche h 200

50

100

m/s 0 cm

Bottom avalanche

^

O O

50

100

150

200

250

Height of snow in cm Fi^ 70. Relationship between avalanche velocity and height of snow (modified after Roch, 1980).

to be drawn up in order to give the most complete picture of the phenomenon. These maps have been compiled by official research boards and/or military corps, on the basis of historical investigations, field enquiries, geomorphological surveys, analyses on the state of vegetation and information deduced from the analysis of aerial photographs. Usually the areas which have already been struck by avalanches are shown on these documents and subdivided according to the type of source used for data collection. Also the zones exposed to avalanches, those considered hazardous and the locations of their tracks are traced on these maps. The avalanches, progressively numbered and named as regular, periodical and exceptional, are described in single cards. For each episode, the fall frequency, slope orientation, detachment, slide and accumulation areas, historical and statistical records plus any other useful element, are indicated. Figure 71 shows an example of avalanche map. The relative legend is that utilised by the "Experimental Centre for the study of snow, avalanches, Alpine meteorology and hydrogeological defence" of the Forestry Department of the Region Veneto, Italy (Fig. 71). 3.6.13 Rock glaciers Another source of hazard in high-mountain regions is given by the presence of rock glaciers, especially those that are still active. They are frozen bodies of detritus which in the form of lobes or tongues slowly descend en masse along slopes or valleys. Their lengths range from a few hundred metres to over one kilometre. Their presence

160 Intersection of partcular areas for photointerpretation arxl field survey

FORC i> P6R0SA

Fig. 71. Map of probable location of avalanches in a Dolomite area (Municipality of Auronzo di Cadore, Belluno, Italy). (From: Centro Sperimentale Valanghe e Difesa Idrogeologica, 1986, modified).

is strictly linked to rigorous thermic conditions, since they are remnants of a discontinuous permafrost. In the Alps they are usually found above 2,500 m and usually affect slopes and valleys where the mean annual temperature is below -1—2°C (Haeberli, 1985). Rock glaciers are considered a geological hazard almost exclusively from the engineering point of view. Their movement and/or deformation velocity, although high, can always be kept under control and therefore will not cause direct damage to people. Most active rock glaciers move at surface velocities ranging from 0.5 to 1 m/year"'. In the Yukon (Hughes, 1966) velocities up to 250 cm/year"' were recorded, in Switzerland (Chaix, 1943) up to 131—161 (surface) cm/year'; usually they vary between 73 and 5 cm/year ' (Giardino, Shroder & Vitek, 1987), yet a 5 to 10 cm/year"' velocity is sufficient to destroy most of the structures built on rock glaciers. Rock glaciers can cause three principal types of damage to structures: damage resulting from gross active movements, from other deformation processes and by sloughing material. Specific bibliography considers the hazard of this phenomenon comparable to that of landslides, moisture-sensitive soils or active faults. Surface deformation can cause damage to structures owing to differential bedrock settlements. The latter can be determined by partial ice-melting within the rock glacier or by removal of the finer fraction by means of running waters during thaw.

161 Deformations were observed also in active and nonactive rock glaciers, following artificial cuts or excavations. Besides generating disturbances in the thermic regime inside the detrital body, this type of activity triggers en-echelon tension cracks parallel to the cut and surface distortions behind the cut face. Owing to the above mentioned causes, also compression-hnked deformations are associated with the settlements of surface layers (surface layer compression is a short-term phenomenon and would occur very quickly after or during appHcation of structural loading). Even structures placed in the proximity of rock glaciers can be subjected to damage. Large boulders can collapse from the front or more seldom from the sides in a few minutes, owing to the different velocity assumed by the surface with respect to the deeper parts (surface velocity is usually greater than that at depth). Pylons, power lines, posts, ski lifts, radio repeaters and any other prevalently vertical structure can be slowly tilted. In these cases use of supports and relevelling operations must be considered. Also transport infrastructures such as roads and railways require constant maintenance in order to maintain their original direction (if they are constructed on rock glaciers) or reconstruction works if transit has been obstructed by the advancing front. Pits and tunnels that go through these landforms are subject to considerable deformation, owing to tangential stresses, and to high pore water pressure exerted by melt water. In particular, tunnels and pipelines should have directions which comply with the flow trend of the rock glacier. A good prevention norm means avoiding the positioning of towers, pylons, power lines, etc. on active rock glaciers which should not be intercepted by roads, railways, tunnels, pipelines or other structures. Also the forms considered inactive can at times give some problems. Correct procedures would require a field survey to be planned, accompanied by a series of investigations aimed at verifying the presence and the rate of activity of rock glaciers. When interference with these forms is inevitable, all possible construction devices and remedies should be put into practice in agreement with the geotechnical expertise developed during construction in areas subject to permafrost (Biyanov, 1976; Andersland & Anderson, 1978). In rare circumstances rock glaciers can generate barriers along valleys or form narrowings. Sometimes the latter are considered for dam abutments. In the specific literature it is recorded on that rock glaciers have been used as abutments for dams to retain mine tailings in Colorado. Warning signs of instability are: the formation of a surface microrelief, fannelshaped depressions and longitudinal, meandering troughs. The analysis of thermic infrared remote sensing imagery can reveal the presence of permafrost inside the structures considered. A check of aerial photographs taken in different periods will allow the eventual advance and velocity of the front to be recorded. Geophysical borings (in particular geoelectrics), BTS measurements and possible drillings could give evidence of the presence and type of ice inside (ice-cored or ice-cemented rock glacier). Geomorphological hints of activity are given by the bulging shape of the whole mass, the steep front with a bank of rocks at the toe, the surface ondulating and modelled in a series of grooves.

162 Foster & Holmes (1965) reported the case of blocks rolling from an active rock glacier in Alaska up to a distance of 26 m. In the La Sal Mountain (Utah), the same authors observed mass failures on the front of a rock glacier from an angle of 52° to 40°, with debris moving 81 m down the valley. During the construction of a railroad in Colorado, nearly 100 years ago, rock glaciers were encountered. Several active rock glaciers were cut through by mining-tunnels in Colorado (Brown, 1925). Giardino (1983) points out that in one of these (Mt. Mestas) remarkable deformations occurred. The collapse of an adit and raising in Camp Bird near Ouray, Colorado, were ascribed to deformations of an active rock glacier (Griffiths, 1976 in Giardino et al., 1987). 3.6.14 Debris flows Debris flows are flows of sediments and water that go down to a valley floor along traditional water paths (torrential riverbeds) or along slopes through new routes. This type of event, which develops following the sudden influx of large amounts of water on loose soils is particularly frequent in high-mountain regions. Heavy precipitations, snow melt water, phenomena such Sisjokulhlaup on glacial, torrent and slope deposits are the cause of the phenomenon. The volumetric ratio between water and sediments and the different dimensions of the debris itself create a differentiated series of movement types known as earth flows, mud flows, grain flows and debris avalanches (Fig. 72). Usually the debris flow deposits are poorly selected and are recognised on the ground because they make up typical banks and lobes. In most cases the debris flow deposits are not very specific and are recognised on the ground because they form typical banks and lobes. According to the volume of material involved and especially of the amount of water, they run over distances ranging from a few tens of metres to several kilometres, with velocities up to 30—40 m/s. In a high-mountain environment debris flows are found along the flanks of valleys, troughs and torrent riverbeds, mobilising the unconsolidated sediments covering the slopes and the deposits obstructing the bottom of valley floors. Some can also be periodic and recur in the same areas after a certain number of years, perhaps in relation with the time necessary for the formation of debris to substitute that carried away during the previous event. Small-sized debris flows which develop along the top portions of slopes or canyons usually mobilise debris fans or cones and can be hazardous, especially for road networks: footpaths, trails, muletracks, mountain roads, etc. A greater hazard is found with those channelised within torrent beds which generate alluvial fans or are inserted in them. Alluvial fans are very often highly populated areas owing to the favourable conditions which they offer to man's settlement compared with valley floors. The areas most at risk, i.e., those that can be subject to debris flows and floods, are the apical ones and those located along the torrent axis. In other situations, debris flows, regardless of their dimensions, can create barriers across streams resulting in the formation of impoundments. The phenomenon is quite frequent at confluences, when the mass of debris transported suddenly loses energy

163

WATER river

FINE DEBRIS

str earn

COARSE DEBRIS

Fig. 72. Types of debris movements connected to the volumetric ratio between water, debris and the debris different grain-size (modified after Haeberli and Naef, 1988).

owing to widening of the hydraulic section and is deposited. The Hfe and dimensions of the impoundment will depend exclusively on the topographic conditions, the debris particle size characteristics and, most of all, the erosive capacity of the dammed watercourse. The forecast possibilities are practically nonexistent, since it is not possible to know in advance the quantity of water involved nor the length of time it will remain. As a consequence in mountain regions most of the areas covered by loose debris are potential sources of mud flows. Prevention measures can be either passive or active. In the first case the development of areas considered hazardous should be avoided, in the second, efforts should be made to reduce the probability of debris flows. As regards slopes and canyons, the first measures to be taken should attempt to stabilise debris with afforestation techniques or the construction of weirs, secondly accumulation materials that could turn into flows should be removed. The nearer the first intervention is to the debris zone of origin (middle and higher parts of the basins), the more effective it will be. On large alluvial fans prevention will be carried out by means of defences (embankments, barriers, groins) to divert or channelised the debris.

164 On the basis of a series of investigations aimed at the study of debris flows in the Alps during the 1987 floods it was realised that apart from occasional events, this type of phenomenon normally takes place over wide areas of territory (Haeberli & Naef, 1988). The event is the result of the interaction of various main factors that are: — the presence of loose debris in canyons, fans and areas in front of glaciers (following the retreat of the glaciers themselves, the snow fields and the permafrost); — the presence of a large amount of melt water often due to a delay in thawing, because of late snow falls; — the presence of loose material saturated with water following heavy spring precipitations; — the raising of thermal zero. Moreover, the slope angle of the debris surface should not be neglected, the presence of watercourses fed by melt water at the edges of the deposits and a stagnation of groundwater next to the surface (rock and/or permafrost). On the basis of these considerations it is obvious that the knowledge and monitoring of these factors can be useful for the prevention of the phenomenon. 3.6.15 Final remarks By analysing the series of phenomena that take place in high mountain regions (glacial and periglacial morphoclimatic environment) and the connected hazard, experience has shown that in many cases it is extremely useful to know about the behaviour and past history of a phenomenon. Reports and historical records not always register all the events; moreover the most ancient ones could not be recorded or those that happened in areas far from the presence of man. The growth of ancient mountain settlements and their transformation into villages, the exploitation of mountains for tourist-sport purposes both in summer and winter and the need to construct new communication routes, has led to the occupation of areas that not long ago were the absolute domain of phenomena linked to mountain evolution dynamics. Knowledge of the ''geological" history of these areas is therefore possible only by means of a correct and exhaustive analysis of the landscape and its forms. For hazard assessment in mountainous environments it is therefore extremely necessary to perform a basic geomorphological-type study aimed at recognising single events occurring in the recent geological past, by considering the surface deposits and the signs left in the morphology and vegetation cover. By identifying, defining and dating these traces it is possible to reconstruct both the sequence of the events, in a relative and often absolute chronology and their patterns of occurrence. From these reconstructions the areas exposed to risk and the frequency of the phenomena can be recognised. Moreover it is possible to identify portions of territory that at first sight would seem to show no hazard but which in remote times underwent various degrees of damage. In order to carry out all the programme listed before, it is necessary to use more "information markers" which would make use of analyses performed on topographic maps, in the field and on aerial imagery.

165 Another element to be taken into account for hazard assessment is the phenomenon's return-time: for this purpose it is necessary to specify temporally the various events recorded on the ground. In a high-mountain environment the dating techniques of dendrochronological, lichenometric and '"^C type on organic remnants such as peat deposits, fragments of wood, buried soils, etc., are particularly effective and useful.

3.7 Geomorphology and seismic risk 3.7.1 Seismic risk Like the definitions shown in chapter 1.6, seismic risk may also be considered as the "product" of the seismic hazard and the social and economic vulnerability of a given area. Therefore, it will be defined as the probability of an earthquake of a preestablished magnitude occurring within a given number of years, with specific consequences for the environment. Thus, seismic risk is defined not only by the expected seismic event itself, but rather, by the setting of this event in the geological and physical-geographical conditions of the area affected, in the density of the population, in the conditions of existing buildings and constructions, in the type of economy, in the level of preparation for and knowledge about seismic events on the part of the population, in the presence of aid facilities and in the efficiency of the civil defence network. In fact, the factors making up the definition of seismic risk may be summarized according to the outline shown in Fig. 73 (Panizza, 1991). In a strict sense, seismic hazard connected with the earthquake and the seismotectonic characteristics of an area, is distinguished from seismic susceptibility; the latter corresponding to a hazard induced by the physicogeographical situation of the area considered. Seismic hazard should refer to the types, features, mechanism, and earthquake characteristics y

seismic hazard

f/

Magnitudo J focus area dimensions ] hypocentre \^etc.

seismotectonic f seismicity \rorj characteristics | tectonic structures j r ^ ^ L

SEISMIC RISK

<

seismic susceptibility

vulnerability

\

(

lithobgy geomorphology hydrogeology etc.

exposure economy social organization program plan

Fig. 73. Outline witli the factors converging into the definition of seismic risk.

166 phenomenology of earthquakes, that is, to the energy they propagate, to the depth at which they occur, to their recurrence according to time sequences that may even be relatively brief, to the dimensions of the focus area, etc. This type of hazard is also linked to seismotectonic characteristics or, in other words, the relationship between the focus and faults identified on the surface by geological methods or inferred at depth through seismological studies. As far as seismic susceptibility is concerned, the term refers to local, geological, morphological, hydrological and other factors, of both the surface and the substratum, which may amplify or reduce seismic vibrations or create situations of precarious geomorphological equilibrium. The geomorphological studies applicable to seismic risk assessment can be divided into two sectors (Fig. 74): a) morpho-neotectonic investigations conducted to identify active tectonic structures; and b) geomorphological and morphometric analyses aimed at identifying the particular situations that enhance or reduce seismic susceptibility. These two sectors will be discussed in the following chapter. 3.7.2

Morpho-neotectonics

Morpho-neotectonics is that part of Structural Geomorphology that deals with the "relations between landforms and recent tectonic movements" (Panizza and Piacente, 1978b).

TECTONIC STRUCTURE I morphoneotectonics

[^"^

(SEISMOTECTONICS^

SEISMIC SUSCEPTIBILITY (local conditions)

V'

•-.

EARTHQUAKE CHARACTERISTICS

SEISMIC HAZARD (earthquake expected)

^y^

rfsURFACE EFFECTsYi

'^^

IMIHIHIMIHIIII^

W

liimniHiiwiiiiHiwiiiiniiniiHiimtwiiMiiiiiiiwitiHniiiiiiHiHje

"GRAVITATIONAL" EFFECTS (mass movecDflfTts)

^

-•

meVtodoiogica} way 'nxxphoneottctonks • • surface faults'

VULNERABILITY

"•'•"«•» methodological way toc^ conditions- mass movements'

Fig. 74. Two different methodological Geomorphology contributions for seismic risk assessment.

167 In this multivariable natural system, recent tectonic movements are considered as an internal variable within the system, whereas all the other geomorphological causes (climatic conditions, azonal agents, morpho-selection, etc.) are considered external variables, that is, outside the system (see Panizza & Piacente, 1978b). Viewpoints are not unanimous regarding the chronological interval defining the "neotectonic" time period. This time period may encompass an age that ranges from the Miocene to the Present, to time segments limited to the Quaternary alone or even only the latter parts of it. For example. Oilier (1988) gives the following four definitions of Neotectonics: 1) tectonic movements during the present and the recent past; 2) tectonic movements during the Neogene and Quaternary'; 3) legal use related to so-called ''capable faults"\ 4) the tectonic activity that created the present topography. Morpho-neotectonic research is based on the concept that tectonic movements have brought about changes in the Earth's surface. The identification of such changes can therefore allow the neotectonic movements to be traced back to their origin and thus permit an appraisal of the likelihood of their continuing or of others occurring in the future. With investigations of this type and thus through the identification of active tectonic structures, geomorphology contributes to the assessment of an area's seismic hazards. Another consideration to be made, concerns the relations between morpho-neotectonics and morpho-selection. In fact, the latter proves to be inversely proportional to the former. In other words, the more recent and intense the morphoneotectonic activity is, the less time is available for morpho-selection to manifest itself. For example, in a recent fault scarp, the selective differentiation on the various rock types present is generally not very prominent. Several morphological features make up "peculiarities" from a strict morphogenetic viewpoint which can be signs of neotectonic deformations (Panizza et al., 1978). In Fig. 75, some particular kinds of slopes, valleys and hydrographic networks resulting from tectonic movements are illustrated. For example, a hnear ridge can be the result of a fault scarp evolution or correspond to its upper part or show a regional uplift. An altimetric ridge discontinuity can be determined by a crest-transversal fault. A particular definition is that of "capable fault" (see Caggiano, 1979): a) A ''capable fault'' is a fault which has exhibited movement at or near the ground surface at least once within the past 35,000 years or movement of a recurring nature within the past 500,000 \ears (United States Atomic Energy Commission, 1973). b) A fault which has been active during the Late Quaternary (International Atomic Energy Agency, 1972). A scarp can be associated with a vertical fault or an areal uplift: on its surface triangular or trapeziform facets can be produced. Reverse slopes and areas affected by landslides and particular erosion forms can be linked to faults. Valleys with simple or double river bends can reveal the presence of a horizontal fault. A movement of this kind could also produce fluvial barbed or upstream confluences. Irregularities found in some valleys, such as overhanging confluences or truncated valleys can be linked to faults subject to various movements. Relief landforms of this kind or similar features can therefore be an indication of recent movements and after having been

168

Fig. 75. Schematic examples of particular morphostructures resulting from neotectonic movements: a) fault scarp and counterslope (Bastardo basin, Tevere valley, Italy; after Gregori, 1988); b) fault scarp with facets; c) fault and double river bends; d) fault and landslides.

selected, verified, deepened and quantified, they can give substantial evidence of them. It is in any case opportune to remember that the morphological evidence of a neotectonic movement is connected with its velocity and with the velocity of geomorphic processes (which, in turn, are related to lithology, climate, etc.)Morpho-neotectonic studies are generally organised according to the following phases, in increasing order according to the degree of detail provided. (1) Bibliographical research: the collection, processing, and interpretation of bibliographical data that indicate or infer the existence of neotectonic displacements. It is undertaken primarily in order to track down information on the distribution of Plio-Quaternary deposits as well as tectonic elements and any tectonic activity. This research should be consolidated in the form of a map of the Plio-Quaternary deposits and tectonic elements. Another aim of bibliographical research is to collect data that may be utilised for the specific objectives of the study. Thus, it is useful, for example, to carry out studies on the seismicity of the area in question and elaborate the data into a map of seismic epicentres as well as studies on vertical movements of the ground, both recent and in progress, to be synthesized in the form of isolines relating to the whole of the movement and studies on geothermal or volcanic phenomena, etc. (2) Regional remote sensing: satellite image interpretation for the identification and selection of lineaments of natural origin and their possible neotectonic significance. Each selected lineament is individually described and reproduced on a map on the same scale as that used for the map of Plio-Quaternary deposits and

169 tectonic elements. The comparison between these two maps allows their different elements to be qualified and subdivided into three categories: 1) Tectonic elements that are probably active. 2) Lineaments that may be linked to possible active tectonic elements. 3) Tectonic elements that are probably inactive. For example, the faults taken from bibliographical analysis, which are superimposed on Plio-Quatemary deposits and coincide with a selected lineament taken from remote sensing, should be inserted in the first category. The second one includes the selected lineaments not corresponding to tectonic bibUographical elements which, nevertheless, show sufficient evidence and congruence of signs of morpho-neotectonic features. In the same category also morpho-neotectonic signs overlapping on Plio-Quatemary deposits are indicated. In the third category all the tectonic elements that do not correspond to any selected lineament and that do not overlap Plio-Quaternary deposits are indicated. In any case, it is obvious that among the examples above described there are several intermediate cases which will have to be individually analysed. Also the coincidence with lineaments of seismic epicentres of superficial earthquakes is important. (3) Aerial photograph interpretation: listing and selection of geomorphological evidence of tectonic activity and related lineaments. The first operation to be carried out consists in an aerial photograph-based census of all the neotectonic geomorphological indications, their selection and representation on topographical maps according to the legend shown in Fig. 76. The second procedure consists of the identification, guided by geological data, of landforms resulting from inactive tectonics or rather, from passive structures. These are then eliminated leaving only lineaments with hypothetical neotectonic significance. The lineament direction, length and location are indicated on the data sheet, together with aerial photographs on which were identified, the geological deposit involved, relevant bibliographical references and any possible correspondence with the elements drawn from phases (1) and (2). All relevant geomorphological features must be reproduced with observations as to their quantity, quahty, congruence, distinctness and freshness. During the same phase of aerial photo-interpretation itineraries for field checking are chosen and subsequently represented on topographical maps, together with notes on the specific field-checks that are to be carried out. (4) Field survey: verification of the neotectonic hypotheses and precise geometrical, geological and geomorphological specifications of any existing tectonic structure. Field records should include explanatory notes, geological sections, plans, photographs, geological maps or other types of maps, depending on the case in hand. A data sheet should be compiled for all lineaments, reporting in particular the geological characteristics of both the bedrock and the superficial deposits, as well as their interrelationships. Geomorphological features should be specified and any other supplementary datum recorded. Every lineament must then be interpreted from a neotectonic point of view. If it corresponds with a fault, the type and direction of movement and, if possible, the amount of neotectonic displacement should also be

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Stated on the data sheet. All analyses performed in the field or on laboratory samples should also be specified. It is extremely important to carry out accurate investigations before associating tectonic causes to displacements or foldings, even if they are evident and affect Quaternary deposits. For example, neotectonic studies near Lake Garda in northern Italy (Castaldini & Panizza, 1988) have permitted to distinguish landforms linked to tectonic causes from gravitational processes or collapses derived from melting of dead-ice blocks or "glacio-tectonic" phenomena, etc.

171 On the basis of the above specified morpho-neotectonic studies, a subdivision of the lineaments according to the following categories is established (Panizza, Castaldini et al., 1987). 1) Active tectonic element: recorded displacement and/or deformation of rocks and/or significant forms. 2) Tectonic element supposed to be active: so defined on the basis of numerous qualified and congruent geomorphological indications, or of other sources. There are no visible rock displacements and/or deformations nor significant landforms. The presence of an inactive fault or tectonic deformation is in any case ascertained from geological evidence. 3) Tectonic element supposed to be inactive: so defined on the basis of scanty, non qualified and noncongruent geomorphological indications, or of other sources. There are no visible rock displacements and/or deformations nor significant landforms. The presence of an inactive fault or tectonic deformation is in any case ascertained from geological evidence. 4) Inactive tectonic element: displacements and/or rock deformations are not ascertained. The presence of an inactive fault or tectonic deformation is in any case confirmed. 5) Qualified lineament: with numerous, qualified and congruent geomorphological indications or of other kind, but showing no outcrops capable of confirming displacement and/or deformation. 6) Unqualified lineament: with unquahfied or noncongruent geomorphological indications and showing no outcrops capable of confirming displacement and/or deformation. 7) Lineament not corresponding to tectonic elements: so defined when showing numerous and/or qualified and/or congruent geomorphological indications, but without any ascertained displacement and/or rock deformation. It should be noted that the term "significant" is used for rocks and forms to indicate that their age lies within the neotectonic interval considered. The elements derived from morpho-neotectonic studies are listed in a specific thematic document called morpho-neotectonic map (Fig. 77). These morpho-neotectonic studies contribute to the definition of a neotectonic synthesis, together with investigations in other fields of Earth Sciences: for example, the results of seismo-tectonic investigations, or those concerning the main fracture directions of rocks. The synthesis document is a neotectonic map, like the one shown in Fig. 78. Also in this sector of Geomorphology, as for the treatment of all geomorphological phenomena, the temptation to indulge in schematizations must be resisted as these could lead to inappropriate deductions and interpretation errors due to the geomorphological convergence of landforms which are outwardly similar but in fact are genetically different. Remaining in the field of morphoneotectonics, the significant example could be quoted of asymmetrical valleys which show tilting phenomena in one direction or the other, according to the erodibility of the rocks, the rate of tectonic movements and the intensity of fluvial erosion processes. Another example

172

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Fig. 77. Examples of morpho-neotectonic map. Legend: 1) classified lineaments; 2) field check-points; see Fig. 76 for morpho-neotectonic symbols.

is offered by a series of fluvial terraces distributed on various levels: the latter can show several orders corresponding to different stages of evolution or to the same order involved in various tectonic displacements. 3.7.3 Geomorphology and seismic susceptibility As mentioned above, there are particular geomorphological situations that can condition seismic susceptibility. In other words, these situations can offer local responses to seismic acceleration, causing mitigation or amplification of earthquake intensity. In order to calculate seismic acceleration, the following variables should be known: — magnitude, frequency of occurrence and distance of seismic events; — material sequences for each point of the terrain, as well as geotechnical data such as density and seismic wave velocity;

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— groundwater levels; — topographic conditions. There is specific literature on the various geomorphological, geological, geotechnical, hydrogeological situations, among others, that can influence seismic effects in a negative or positive manner. Only specifically geomorphological parameters will be discussed in the following. Those of a geological type (lithology, contacts between different rocks or deposits, presence of fractures, etc.), geotechnical type (particle size distribution of the deposits, degree of consolidation, mechanical characteristics, etc.), hydrogeological type (presence and depth of the water table) or others will not be discussed here and specific works should be consulted on these topics. It should be mentioned that research in this sector of applied sciences is still at an initial stage, in which the results have not yet permitted the indication of precise methods of evaluation, model procedures or seismic effect calculations, unless the circumstances are relatively uncomplicated. In most cases, qualitative assessments may be formulated, based mainly on the consequences of recent earthquakes and the interaction of their effects.

174 The most important geomorphological situations that can condition seismic susceptibihty, are the following: ~ Slope angle. ~ Debris. ~ Morphology. ^ Degradational slopes. — Paleolandslides. — Underground cavities. 3.7.3.1 Slope angle It is well known that, in a static situation, with other variables being equal (lithology, moisture, etc.), slope instability increases with the slope angle. In other words, slope stability is inversely proportional to slope angle. In the dynamic situation of an earthquake, the instability of a slope and consequently that of any buildings on it will be greater than in a static situation. For the towns located in the Campania and Basilicata regions affected by the earthquake of the 23 November 1980, the Italian National Research Council (CNR) Finalised Project "Geodynamics" (1983) adopted a correction coefficient (Sp) for the buildings and structures located in seismic areas, which depends on the slope angle, as follows: 5/7=1 + 1.5/ where / is the mean slope angle expressed as a percentage. The ^/^-coefficient, which expresses the increase in seismic activity, is multiplied by the seismic coefficient C, which defines the force employed for the seismic testing of constructions (C = 0.1 for the earthquake regions in the first category; C - 0.07 for those in the second category). In any case, the CNR Finalised Project '^Geodynamics" (1983) advises against construction under the following conditions in the case of slopes with an mean slope gradient /° greater than the established limit. This limit depends upon the type of rock or deposit, as shown in Table 13. 3.7.3.2 Debris The slope deposits made up of cohesionless materials, as in the case of talus, morain, TABLE 13 Slope angle limits for several types of terrains, according to stability during seismic shocks /° limit Incoherent debris (including cataclastic debris), with water table depth >10 m Incoherent debris (including cataclastic debris), with water table depth <10 m Marls and very compact clays Semi-coherent sands and clays (that may also be mylonitic), with water table depth >10 m Clays of compact texture, but very "tectonized" and/or fissured

20° 10° 30° 25° 10°

175 TABLE 14 Increases in seismic intensity for some types of debris, compared to granite Rock types

Seismic intensity

Granite Pebbles and gravel Sands Clays

0 1—1.6 1.2-1.8 1.2-2.1

alluvial deposits, etc., generally give rise to geomorphological situations that can amplify seismic susceptibility. More specifically, increases in seismic intensity can be linked to three factors: a) surface slope angle of the debris; b) its particle size characteristics; c) thickness of the deposit. The debris slope angle has already been considered in the more general case of the last section and the reader is referred to it for details. Particle size distribution can significantly enhance seismic intensity. It is outlined in Table 14, modified from Medvedev (1965) with some simplifications. Some cohesionless rock types are compared with compact granite, under conditions where there is no water infiltration. This infiltration can further increase seismic intensity up to values of 3—4, approximately. The thickness of the deposit also plays a part in amplifying seismic intensity. In the case of deposits thicker than 5~10 m, Siro (1985) indicates critical thicknesses on the basis of the most probable frequencies of the arriving vibrations and the average propagation velocities of the transverse waves, which determine the amplifications of seismic intensity. For velocities of transverse waves from 200 to 600 m/s, the critical thicknesses resulting are shown in Table 15, along with the proposed corresponding coefficients of amplification ranging from 1.5 to 2.5. 3.7.3.3 Morphology An amplification of seismic intensity resulting in an increase in damage to buildings has often been observed in correspondence with particular morphological situations

TABLE 15 Critical thickness of debris, compared with the velocity of transverse seismic waves, with ampHfication coefficients ranging from 1.5 to 2.5 Average velocities (m/s)

Critical thickness (depth in m)

200 300 400 500 600

8-40 11-60 15-80 19-100 23-120

176 such as crest lines, terrace borders, scarp edges, or abrupt variations in slope gradient. These amplifications appear to be due to phenomena involving the concentration of seismic rays that are reflected as a result of their different inclination angles, with respect to the vertical, corresponding to sudden topographical changes. Studies aimed at investigating this topic (Castellani et al., 1982) indicate amplifications as great as 3.5. Similar amplification phenomena have been observed in correspondence with buried morphological structures, such as paleoriverbeds or fossil terraces or forms of erosion masked by superficial deposits. There are also cases involving seismic ray concentration phenomena, due, however, to refraction phenomena where the angles of incidence are shown to differ from the vertical. In the intervention projects planned for the towns of the Campania and Basilicata regions (southern Italy) affected by the earthquake of November 1980 (CNR Finalised Project "Geodynamics", 1983), 10-20m reserve zones were advised in correspondence with rocky crests, isolated summits, terrace edges and border areas of steep scarps. 3.7.3.4 Degradational slopes The amount of weathering on slopes and therefore the degree and type of rock degradation taking place can affect seismic susceptibility. For example, in the Friuli earthquake of 1976 (northern Italy), there were numerous debris falls from the slopes of the Tagliamento river valley and those of its tributaries. The debris ranged from small or isolated rock fragments to large boulders and rock falls of vast proportions (Fig. 79). The survey and mapping of these mass movements by Govi and Sorzana (1977) revealed that most of the rock

Fig. 79. Rock fall caused by earthquake in 1976, Braulins (Friuli, Italy) (Photo M. Panizza, May 1976).

177 falls occurred prevalently in places where there had previously been other gravitational events. Geomorphological surveys have revealed the different sources of the debris accumulations and their different ages of formation. Observations on the area led to the indication that the phenomena, occurred prevalently in calcareous rocks showing intense tectonic fracturing, which are subject to strong frost weathering. This type of disturbance was much less frequent in the case of the earthquake of southern Italy in 1980, even though there are slopes in the area made up of calcareous rocks, some of which have been intensely fractured by tectonic activity. Frosty weathering is lacking in this area or, in any case, it is much more limited compared with the Alps. Frost action exerts a slow and progressive degradation of the rocks. In the case of seismic shock, the fragments detached are not only unbalanced debris, which would have fallen in any case on account of frost action and gravitational processes, but also include material that is still partially attached to the rock or on slopes lower than those with detached fragments. It is as if the seismic shocks complete a sort of "cleaning operation" on the slopes, eliminating the weathered material. Studies aimed at the assessment and identification of areas subject to this type of seismic susceptibility must consider not only the sector beneath slopes or scarps undergoing degradation processes, which can be affected by rock falls or other landslides, on the basis of gravitational mechanisms alone. They must also consider those areas that can be reached by boulders thrust by strong seismic shocks beyond the gravitational fall limit or set into a rolling motion and movement because of the so-called "vibrating table" effect. An investigation on the limits of maximum advance of fallen boulders following the 1976 Friuli earthquake, was carried out by Onofri and Candian (1979). By means

Fig. 80. Slope parameters considered by Onofri and Candian (1979) for an investigation on the limits of maximum advance of fallen boulders following the 1976 earthquake in Friuli (Italy).

178 of a statistical analysis on 98 cases of rock fall, the authors have set up two diagrams for the definition of the boulders' maximum advance limits for each single slope angle (Fig. 80). In the first diagram (Fig. 81) the parameters taken into account are: H (altitude of fall or vertical height between the detachment point and the arrest one) and D (projection on the horizontal plane of the distance between A and B). These parameters totally prescind from the morphological pattern of the slope on yi = D

m 1400

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300

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Fig. 81. Diagram of the limits of maximum advance of fallen boulders following the 1976 earthquake in Friuli (Italy) prescinding from the morphological pattern of the slope (after Onofri and Candian, 1979).

179 which the boulders fall. In the second diagram (Fig. 82), also the influence of the slope morphology on the path followed by the boulders is considered. The parameters taken into account are: the ratio between the area Ap subtended by the slope profile, with respect to the horizontal plane, and the square number of the fall height (H'); and the 0^ angle (inclination of the energy line). For each of the two diagrams the limits of confidence of 80, 90 and 95%, respectively, have been calculated. 3.7.3.5 Paleolandslides Earthquakes sometimes set into motion older landslides (see 3.7.4). The villages of Calitri, Bisaccia, Senerchia and other localities in southern Italy affected by the November 1980 earthquake, are emblematic examples of this. The landslides occurred in mainly clayey rocks, in flysch facies, sometimes chaotically arranged or highly tectonised. They affect stretches of land several kilometres long with a transport involving hundreds of millions of cubic metres of material. A slide affecting the village of Calitri in the province of Avellino started to move a few days after the seismic event, involving about 23 million cubic metres of material. Its sliding surface was calculated at a depth of over 100 m. The movement corresponds to a partial reactivation of a pre-existing rotational slide (Hutchinson & Del Prete, 1985) evolving downslope into earth flows that reached the Ofanto river, modifying its course. Cotecchia (1986) maintained that pore-water pressure, which increases noticeably in concomitance with seismic vibrations in clayey and flyschoid rocks in tectonic contact with water-saturated calcareous strata, must have played a very significant role in triggering into motion the paleolandslides of this region. It has been demonstrated (Agnesi et al., 1983) that earthquakes lead to an increase in the overall size of landslide bodies, yet without significant modifications of their shape and typology. 3.7.3.6 Underground cavities Earthquakes in calcareous areas with underground karst processes can cause the fall or collapse of cave roofs or the removal of debris from rock cavities. The consequences on the surface are depressions or subsidence of the ground with the resulting danger of collapse for the buildings located above. limrt of predictabiWy

95<Mi •

limft of pnedtctabiNty

90% •

limit of predictability

80% •

Fig. 82. Diagram of the influence played by the slope morphology on the maximum advance of fallen boulders following the 1976 earthquake in Friuli (Italy) (after Onofri and Candian, 1979).

180 3.7.4 Earthquake-triggered mass movements'^ *by Doriano CASTALDINI

3.7.4.1 Worldwide examples Earthquakes are considered to be a major triggering cause of mass movements in many geological materials. One of the first earthquake-triggered landsHde was documented as early as 372—373 B.C. when Helice, a Greek city on the northern coast of the Peloponnese, sHd into the sea after having been razed to the ground (Marinatos, 1960; Seed, 1968). Many other examples occurred in historical times such as the mass movements produced by the earthquake at New Madrid (Missouri) in 1811, at Kansu (China) in 1920, at Chait (USSR) in 1949, in California in 1957, in Alaska in 1964 and in Japan in 1978 (see Hansen and Franks, 1991). Data from 40 historical worldwide earthquakes were studied by Keefer (1984) in order to determine the characteristics, the geological environment and hazards of landslides caused by seismic shocks. One of the main results obtained by Keefer (1984) was to define relative levels of shaking that triggered landslides in susceptible materials. Four types of internally disrupted landslides — rock falls, rock slides, soil falls and disrupted soil slides — are started by the weakest shaking. More coherent materials with deep-seated slide surfaces, require stronger shaking while lateral spreads and flows are triggered by even stronger shakings, but the strongest shaking is probably required for highly disrupted rock avalanches and soil avalanches. Such topic was further investigated by Keefer and Wilson (1985, 1989). At a more general level, many detailed investigations have been carried out all over the world in order to assess the relationships between earthquakes and mass movements (see Plafker et al., 1971; Nilsen & Brabb, 1975; Solonenko, 1977; Youd, 1978; Harp et al., 1981; Perissoratis et al., 1981; Sorriso-Valvo, 1986; Wue Cai & An Ning, 1986; Geh et al., 1988; Rymer & White, 1989). In more recent times a conference on slope stability in seismic areas was organised (Faccioli and Pecker, 1992); a special session on "Seismicity and Landslides" was held during the 16th International Symposium on Landslides (see Bell, 1992) and landslides certainly triggered by earthquakes have been described in Japan (Yamagishi et al., 1993) and in California (Jibson et al., 1994). More specifically, as reported by Bell (1992), a huge landslide in Iran, occurring during the 21 June 1991 Manjil-Roodbar earthquake, with M 7.3 magnitude, was analysed (Anvar et al., 1992). This slide started the day after the earthquake and continued for several weeks threatening the blockage of a road used for emergency operations and the filling of a large reservoir of a dam along a downstream river. In Crozier (1992) a methodology for determining paleoseismicity is discussed and demonstrated for three earthquake-triggered landslides in New Zealand, while according to Naumann & Savigny (1992) the five major rock avalanches that have occurred in SW British Columbia since the end of the last glaciation are at least likely to have been triggered by high pore-water pressure increase caused by seismic shock.

181 Other authors, from a general point of view, show how the effects of an earthquake on a slope depend on several natural factors (such as the characteristics of the shock, the slope angle, the properties of the rocks and soils involved, the porewater pressure, etc.) as well as irrational human activities (such as deforestation, inadequate land use, etc.) contribute to further deterioration. After analysing the relationships between earthquakes and mass movements on a world-wide scale, Cotecchia (1987) remarks that the behaviour of a slope affected by an earthquake depends on the nature of the ground motion, the slope geometry and its composition. Indications of the likelihood of mass movements under nonseismic conditions do not always hold good for landsHdes triggered off by earthquakes. Ground shaking can produce accelerations within the soil mass, accompanied by a system of stresses that can completely alter the strength of the materials. Thus when considering the response of a slope to accelerations that vary in amplitude and direction, the dynamic properties of the ground cannot be ignored (Cotecchia, 1992). Moreover, it is opportune to remember that Cendrero & Dramis (1994), in a general survey on landslides in Europe, noticed that in areas subject to seismic influence, both the size of individual landslides and the total areas affected tend to be somewhat greater, but no significant differences exist in typology and morphometry between recent earthquake-triggered movements and older, presumably climate-triggered ones. This suggests that seismicity does not change the style of landscape evolution, but it probably increases its rate. This is confirmed by the fact that only 5% of the area affected by landslides after an earthquake corresponds to first-motion movements. The remaining ones are reactivations of older movements. 3.7.4.2 Examples in Italy One of the first reports on earthquake-triggered landslides in Italy is by Vivenzio (1788) who described the geomorphological effects produced by a catastrophic earthquake which struck Calabria (southern Italy) in 1783 (Cotecchia et al., 1969). Several other historical records and oral traditions exist about gravitational movements triggered by earthquakes in Italy (see, for example, Oddone, 1915 and 1931; Govi, 1977; Govi & Sorzana, 1977; Genevois & Prestininzi, 1981; Serva, 1981; Dramis et al., 1982; Pellegrini & Tosatti, 1982; Crescenti et al., 1984; Zecchi, 1987; Murphy, 1993; Mazzini, 1994). The characteristics of some mass movements connected with the two major earthquakes occurring in Italy in the past 20 years (Friuli, north-eastern Italy, and Campania and Basilicata, southern Italy) will now be examined. The earthquake that occurred in Friuli on May 6, 1976, with a main shock of magnitude 6.4 and intensity IX-X (Mercalli scale), produced several surface effects over an area of more than 1,600 km' (Weber & Courtout, 1978). The most striking surface effects were given by the numerous landslides activated by the earthquake (Govi & Sorzana, 1977). They mainly consisted of rock falls and, to a lesser extent, block-slides involving rock volumes of up to 100,000 m \ especially on calcareous slopes with high relief energy. Fractures, visibly opened as a result of the earthquake, appear to outhne many of the gravitational forms (Govi, 1977).

182 The results of the above mentioned investigations on the relationships between the landslides and the Friuh earthquake are here given in detail: — the photo-interpretative method for the survey of landslides connected with an earthquake was found to be adequate for rapidly acquiring an overall picture of the effects triggered on the slopes by seismic shocks; — most of the landslides triggered by the Friuli earthquake were rock falls which took place mainly where these phenomena had already occurred in the past; — important factors influencing the landslides at an equal distance from the epicentre, were the weakening of the rocks by intense tectonic fracturing and the steepness of the slopes controlled by lithological and structural conditions. Further studies (Girardi et al., 1981) carried out in the area struck by the 1976 earthquake, have emphasized that the whole area along the front of a pede-Alpine overthrust shows evidence of the close connection between seismic events and largescale landslides ascribed to Late Pleistocene. The latter appear to be rock slides along bedding planes or faults. The latest high-intensity earthquake in Italy (with a main shock of 6.8 magnitude and intensity X (Mercalli scale) happened on November 23, 1980 and struck a vast area in southern Italy, especially in Campania and Basilicata regions (Deschamps & King, 1983; Westway & Jackson, 1987). The most evident surface effects of the earthquake were mainly due to mass movements of various types, in relation with the lithological and structural characteristics of the bedrock (Cantalamessa et al., 1981; Cherubini et al., 1981; Genevois & Prestininzi, 1981; Cotecchia, 1982; Maugeri et al., 1982; Agnesi et al., 1983; Crescenti et al., 1984: Cotecchia & Del Prete, 1984; D'Elia et al., 1985 and 1986; Hutchinson & Del Prete, 1985: Cotecchia, 1986; Carton et al., 1987; Fenelli et al., 1992; Bisci & Dramis, 1993). These events mainly occurred immediately after the main shock, their movement becoming completely or almost exhausted in a short period. Block-slide phenomena, bordered by earthquake-induced fractures were fairly frequent. The huge landslide 3 km in length that occurred in a pelitic-arenaceous bedrock at San Giorgio in Molara was of this type. From eye-witness accounts and historical data, it seems that this has only moved in association with earthquakes (Genevois & Prestininzi, 1981; Dramis et al., 1982). At Trevico, Dramis & SorrisoValvo (1983) observed lateral spreading in an area of conglomerates resting on clays. The event consisted of the deepening, amounting to some decimetres, of a small graben-like depression (50 m long, 15 m wide and 2 m deep) located on a hill top. For this form too, testimonies from local inhabitants refer to activation corresponding to previous earthquakes and no disturbances within aseismic periods. Another huge mass movement showing recurrent activity and accompanying seismic events involved the whole town of Bisaccia in Campania, even if the displacements were limited. This landslide has been described also recently by several researchers: Crescenti et al. (1984), D'Elia et al. (1985), Esu et al. (1985 and 1987), FeneUi (1988), Fenelh & Picarelh (1990), D'Elia (1991), Fenelh et al. (1992), Bisci & Dramis (1993).

183 3.7.4.3 Study methodologies in Italy At present various studies on the evaluation of landslide hazard in seismic areas are being carried out in Italy. For example, the investigations in north-western Tuscany require detailed geological and geomorphological surveys (1:5,000 scale) in order to draw up a stability map. This will show active and dormant landslides, as well as areas potentially exposed to mass movements due to their morphological and lithological features (see D'Amato Avanzi et al., 1993). An interesting study project on earthquake-triggered landslides is being carried out by Genevois (1994). It is schematically represented in Fig. 83 and is here summarised. The existing data bank phase is based on bibliographical research on earthquakes (consulting seismic catalogues for historical, instrumental and macroseismic data and a consequent selection of the study areas on the basis of isoseismal maps, e.g., minimum VII of intensity (MCS scale), and on the distribution of epicentres and landslides (references of catalogues and historical archives, requests to public authorities such as Council Boards, Districts, Regions, research agencies and Universities; detailed reviews of scientific articles and magazines). The phase of identifying earthquake-triggered landslides implies the correlation and assessment of the data acquired in the previous phase. The identification of earthquake-triggered landslides is carried out by means of correlations that can be of direct or indirect type. With relation to the direct correlation, the method is based on the identification of landslides occurring simultaneously with the seismic event {direct landslides) or certainly set into motion some hours or days after the seismic shock {indirect landslides). Indirect correlation is carried out on the basis of the morpho-structural and/or topographic characteristics (very low gradient angle) of prehistoric landslides or on the basis of documented simultaneity between seismic event and mass movement. In the latter, with temporal concomitance a 4—5-day time-interval is intended, between earthquake and landslide, although obviously some problems remain when the date of the earthquake or of the subsequent landslide is not known. The evaluation of the correspondence between earthquakes and landslides can be good (if direct or indirect correlation is io\\nd)\ fair (if the interval is a little longer than the time span or if a careful location of the landslide is not possible, e.g., more than one in the same area, or an approximate date of the earthquake) or no correlation (time interval too long, uncertain location of the landslide, very ancient landslide, too vague a date for the earthquake). The next step consists on the compilation of landslide cards. This phase of the research is based on a collection of data (geological, geomorphological, hydrogeological, tectonic and neotectonic, geotechnical, geomechanical, climatic, seismological) and other possible integrations (geological and geomorphological surveys, photogeological analysis, in situ investigations, laboratory tests, landslide classification, statistical data analysis). The final content is given by a complete geological model, that is a geotechnical-geomechanical model, an isoseismic-line map

184 (^HISTORICALAND BIBLIOGRAPHICAL RESEARCHES^ _[ LANDSLIpis")

(EARTHQUAl
T-

QSELECTION OF THE AREAS9

C

PRELIMINARY CORRELATION X

/^

f DIRECT I temporal agreement

)

1 INDIRECT temporal window - surface effects

LANDSLIDE CARD: ^ GEOLOGICAL, GEOMORPHOLOGICAL GEOTECHNICAL, SEISMICAL CHARACTERIZATION

GENERAL DATA BANK OF EARTHQUAKE TRIGGERED LANDSLIDES

QVALIDATION )

PRELIMINARY CORRELATION BETWEEN GEOLOGICAL CATEGORIES AND SEISMIC PARAMETERS

f PARAMETRIC STATIC I ANALYSES PARAMETRIC PSEUDOSTATIC ^ ANALYSES J r DYNAMIC ANALYSES ^ NEWMARK 1 METHOD

r REM. DEM METHODS

EARTHQUKES-LANDSLIDES CORRELATION X

EVALUATION OF POSSIBLE SEUS ON SAMPLE AREAS Fig. 83. Scheme of a study project on earthquake-triggered landsHdes (after Genevois, 1994).

(when possible) and a data file of all the parameters collected. The following stage implies a series of evaluation tests based on criteria of choice (hthology, kinematic mechanism, first-time slide or re-activated slide, seismic stress

185 and related parameters), static analyses (back analysis, parametric studies, finite and distinct elements solution), pseudostatic analyses (Sarma's method or others, parametric studies such as Kh, r^^, c, O) and dynamic analyses (Newmark method, methods simulating time history of induced stresses and assessing pore-water pressure and strain development). On the basis of the investigations so far described, the analysis of the correlation between geological and geotechnical macro-categories (lithology, slope aspect and angle, mechanical behaviour) and earthquake parameters (intensity, magnitude, distance, directivity) are defined. This logical sequence is based on the combination that a multivariate analysis of the factors involved in post-earthquake-induced landslides may be useful, if not essential, in finding the relationships between these two phenomena. This knowledge should therefore be considered as the starting point of any evaluation of possible future scenarios in any area of particular interest. The ending topic of this research will be, as a consequence, the analysis of the probable behaviour of sample areas subject to possible future earthquakes. Many earthquake-induced mass movements show a more or less direct connection with other surface effects of seismic activity, in particular ground fractures and faults (see, for example, CavaUin et al., 1977; Carton et al., 1978; Bollettinari & Panizza, 1981, Fig. 84; Carmignani et al., 1981; Cinque et al., 1981). These features may be preserved and, therefore, paleoseismological studies may detect and date them, thereby permitting extension of knowledge beyond the local historical threshold (up to 2,000 years in the most fortunate cases). In recent times, it has been possible to survey directly, with scientific methods, surface effects of strong earthquakes (Panizza, Castaldini et al., 1987; Vittori et al., 1991; Galli and Meloni, 1993) also comparing them with the historic and instrumental seismicity of an area. The above mentioned papers illustrate criteria which may aid in the identification of areas suitable for paleo-earthquake investigations. Historical seismicity and the presence of well developed geomorphic tectonic features are recognised as the most reliable criteria for identifying active faulting. 3.7.4.4 Conclusions The real causes of earthquake-triggered mass movements are not completely understood. Nevertheless, according to existing studies and taking into account the research experience in Italy, it is possible to express some final remarks. First of all, mass movements induced by seismic events are mainly grouped into five types: rock slides, debris slumps, rock avalanches, rock falls and soil slides. The typology and dimensions of earthquake-triggered landslides are related to both the geological environment of the site and the characteristics of the seismic shock. For example, inertial forces due to rapid loading may cause falls and toppling in rock masses, whereas a number of documented cases of landslides due to liquefaction of saturated loose sand highlight the role of pore-water pressures induced by seismic shaking. In clayey deposits, several factors can interact in a complex pattern, thus producing variable effects. Sometimes it is possible to recognise long-term

186

Fig. 84. Surface fault near San Gregorio Magno (Salerno. Italy) caused by the November 1980 earthquake (the photograph was taken by M. Panizza immediately after the event).

displacements as the delayed consequence of pore pressure build-up induced by cycl IC loading. Many of these phenomena (mostly the largest ones and deep-seated gravitational movements) show atypical recurrent demobilisation (stepwise activity), being reactivated only as a consequence of very strong seismic shocks; it could, therefore, be quite difficult to recognise them a long time after the last seismic event. Moreover, It can be argued that many landslides (both dormant and active) owe their first activation to a strong earthquake in the past. Many mass movements (mostly slow ones) are started only some days after the

187 main shock. It has also been observed that quite frequently earthquake-induced landslides reach the valley bottom, thus causing displacement of the watercourses or, sometimes, their damming. Earthquake-triggered mass movements are also connected with other ground effects such as fracturing and faulting. With reference to the studies on these phenomena, as already observed by Bisci & Dramis (1993), it is often possible to identify areas affected by earthquake-related mass movements also starting from fast survey (such as photo-interpretation), even if more particular investigations are the only way to provide more precise information about seismic influence on landscape evolution. Detailed geomorphological survey and mapping of earthquake-prone areas is of basic relevance for land management, since it constitutes the main tool for assessing potential slope instability in case of seismic shocks. In this framework, in order to determine the evolution of unstable slopes, geological, structural, geophysical and paleoseismological analyses must play a major part in any investigation to be carried out. Extremely detailed studies can also rely on the monitoring of earthquake-induced landslides. Once more, the importance of considering earthquake-induced mass movements (and, more generally, surface effects) in the evaluation of global seismic hazard and risk should be emphasized, since what might seem a marginal aspect can quite often be a major factor in determining the real local situation. Finally, it is opportune to point out that seismicity does not change the style of landscape evolution but probably increases its rate.

3.8 Geomorphology and volcanic hazard The evaluation of the hazard determined by volcanic eruptions depends upon several factors. First of all, on the type of activity. Basaltic lava effusions, for example, are characterised by slow and intermittent flows and generally by low-intensity explosions. Therefore, as far as risk for the population is concerned, these eruptions are usually slow enough to permit a safe escape but all that cannot be removed like houses, crops, etc., are in most cases destroyed. In some cases attempts to divert lava flows have been made. In the case of Mt. Etna, for example, in 1669 some men equipped with metal bars managed to shift part of the lava by reducing the speed of the main flow. More recently, in 1983, a partial diversion was achieved by means of explosive charges. Even more recently, in 1992, in order to divert a lava flow which entered a subterranean canal and reemerged along the slope threatening an urban centre located downhill, the inlet of the canal was obstructed with boulders transported by helicopter, thus diverting and slowing down the flow (Fig. 85). Similar other attempts were carried out in Hawaii in 1935, 1942, 1955 and in 1960, when several walls were constructed near Kilauea which partly succeeded in confining a lava flow. Explosive eruptions, are on the other hand, much more dangerous, in particular those that produce clouds of incandescent ashes and avalanches and pyroclastic

188

Fig. 85. Scheme of the interventions carried out for the defence of Zafferana, during the Etna eruption (Sicily) of 1992.

flows. Whilst effusive eruptions seldom reach high velocities, incandescent clouds can attain a velocity exceeding 100 km/h and exert an enormous destructive power. They run down valley floors and can overwhelm also hills and small reliefs, levelling the morphological features of the volcano's lower slopes and covering distances of up to dozens of kilometres. In terms of risk, whole towns can be destroyed and probably the best known example is that of Pompeii, wiped out by the eruption of Vesuvius in 79 A.D. This ancient Roman town was completely buried within 2 days under the volcanic ash fall-out, which killed over 15,000 people. Even more catastrophic was the case of St. Pierre in Martinique which was destroyed on 8 May 1902 by a vast explosive cloud coming down from Mt. Pelee, claiming 30,000 lives. In these cases the only way out would have been a preventive evacuation of the inhabitants. Also the starting up of pyroclastic ejecta movement on volcanic slopes causes a very serious hazard in some areas of the Equatorial belt, with the formation of the so called lahars. The latter are high-density mud flows usually determined by intense rain fall, which run down along the valleys at the toe of the volcano, causing far greater damage and casualties than the eruptions themselves. In other cases, such as in Indonesia, the phenomenon can be caused by a sudden depletion of a crater lake. Recently, on 13 November 1985, the Nevado del Ruiz volcano (Colombia) started an explosive activity with modest ejection of ash and pumice: the eruption was sufficient to melt the volcano snow cap, triggering a vast mud flow which struck the town of Armero killing 25,000 people. For the defense against some kinds of lahar connected

189 with crater lakes, containment walls or lake-draining tunnels have been adopted in Indonesia. Other parameters for the evaluation of volcanic hazard are given by the event characteristics, the quantity of erupted material, the transport mechanism, eruption energy, magma fluidity, etc. (see Barbed & Gasparini, 1977). Also the reconstruction of the history of volcanic effusions brings important contributions for the hazard assessment of various kinds of volcanoes, both in terms of extension and direction of the phenomena (Sheets & Grayson, 1979). The most up to date knowledge of volcanic risk is based on the activity associated with the most recent and best known eruptions, such as Mt. St. Helens, about which a full book has been written dealing with warning signs and patterns of behaviour to be assumed in these circumstances (Saarinen & Sell, 1985). As for the forecasting of eruptions, the observation of several volcanoes has allowed various phenomena to be considered as signs warning of an imminent eruption. They are: the melting of snow caps, the disappearance of crater lakes, the drying out of wells and springs, the withering of the surrounding vegetation and the flight of animals and birds. Nevertheless, the modem methods of forecasting are based on instrumental measurements such as (see Oilier, 1988): the monitoring of earth tremors linked to the magma ascent; tilt measurements of swellings or tumescences caused by lava pushing inside the volcano; temperature measurements of crater lake, hot spring and fumarole waters; measurements of the composition of gases erupted from craters or fumaroles; measurements of changes in local gravity and magnetic fields linked to lava movements at depth. Still in the field of forecasting, methods based on the recurrence of eruptions and on the relationship with other events such as sunspots or moon phases have also been elaborated. For a detailed knowledge of modem indicators of volcanic forecasting, see Tazieff & Sabroux (1983) and Decker (1985). The contribution of Geomorphology in volcanic hazard assessment deals essentially with the influence that the local conditions of a relief can have on the direction and velocity of flows. Valley cuttings and depressed areas, directly connected with the emptive vents, are the most favourable flow and accumulation routes for the magmatic advance. Obviously the high gradient slopes correspond to the most risky areas for human settlements when other hazard conditions remain equal. The collection of all these data for each volcano can lead to the drawing up of maps showing a territory's volcanic hazard potential. An example of this kind is offered by the lava invasion hazard map (Fig. 86). Moreover, climatological and meteorological studies can indicate the areas most subject to possible invasion of volcanic ashes or other products linked to wind intensity and direction (see Sheets & Grayson, 1979).

3.9 Geomorphological hazard maps Several examples of this kind of mapping can be found in literature, with applications in many parts of the world, showing the most varied geomorphological stability

190 MAP OF THE NEVADO DEL RUIZ VOLCANIC HAZARD

PYROCUSTIC FALL

^V^i_>-

Type1595A.D.

'^Z^

Fig. 86. Example of a volcanic hazard map: Nevado del Ruiz. Colombia (modified after Parra, Cepeda andThouret, 1986).

conditions. In this paper the situations directly derived from the author's experience will be described, with reference to the Italian territory which, unfortunately, offers a great variety of geomorphological hazards: from landslides to soil erosion, from badlands to river erosion, from subsidence to beach degradation etc. (see Panizza & Piacente, 1978a; Panizza et al., 1980; Gruppo Geografia Fisica e Geomorfologia CNR, 1987; Amanti et al., 1992). The author's founded conviction is that in order to create a geomorphological hazard map it is necessary to operate keeping separate and afterwards comparing, on the one hand, the causes of instability (natural context and human activities) and, on the other hand, the effects of instability (geomorphological history and dynamics) (Fig. 87). It is opportune to keep "causes" separated from ''effects" on account of the following considerations: — the logical simplicity of the operative system; — the possibility of measuring/verifying the role of the various instability parameters on the basis of unstable forms actually observed; — a multidisciplinary approach with the possibility of applying concrete contributions from various specialists in Earth Sciences.

191 analysis of the

CAUSES

natural context human activities

geomorphological history

geology hydrology climate morphology land use

present geomorphological dynamics

etc. GEOMORPHOLOGICAL DYNAMICS

GEOMORPHOLOGICAL INSTABILITY Fig. 87. Logical scheme for the determination of geomorphological

instability.

3.9.1 Causes of hazard Among the causes that favour or prevent instability, the following could be cited: geological, hydrogeological, topographic, climatic, and land use. Included among man-induced causes are all human activities, such as hvestock-rearing, agriculture, engineering works etc., as they can all modify the environment. The analysis of all these parameters which favour instability could be shown on an Integrated Analysis Map (lA). Both the natural and anthropic contexts will also indicate the limits within which geomorphological instability may develop, taking into account that some parameters remain constant (e.g., lithology, the force of gravity, etc.) and others vary (e.g., human activities, neotectonics, etc.). In other words, it will be possible to define the range of variability, e.g., for predictable landslides in a certain area, over a specified time period, both in terms of volume and quantity: it will thus be possible to estimate that in the course of 10 years, one to five landslides could occur, with a maximum volume of 100 m"* each. As another possibility, one could predict, for example, that in one year the soil loss due to rill wash on a particular slope will reach 100 tons over 1 km". In order to elaborate an Integrated Analysis Map, it is therefore necessary to overlap and compare the various basic elements that are the predisposing hazard causes within a given territory. The instability degree of a certain portion of land is always the result of the interaction of two or more causes acting through complex

192 mechanisms and the influences of the different factors involved may either be summed or multiplied. As a consequence, the rating of each single cause without taking into account its interconnection with all the others, is to a certain extent an abstract and incomplete operation. This is particularly true in areas characterised by recent orogenesis, like in Italy, where poorly cohesive and highly deformed rocks outcrop. The same areas are subject to uplifting phenomena and are therefore affected by extremely intense geomorphological dynamic processes. Moreover, if unfavourable rainfall regimes are associated with these factors, then the most hazardous instability conditions will coexist and interact. In this framework, it is nevertheless difficult to ascertain the relative importance of each of them. The characteristics of the materials ascribable to their composition, texture, structure, stratigraphy, pedology, alteration, etc., all belong to the geological causes together with their tectonic history and that of the territory where they crop out. Particular importance should be given to the distinction between active tectonics and tectonics with a passive role only. It must be emphasized that within each geological body, the zones characterised by a different degree of chemicophysical alteration should, as far as possible, be identified and mapped separately. This natural process is in fact of paramount importance since in time it causes a progressive decline of the intrinsic resistance of the materials making up a slope. Also erosional processes due to rillwash or superficial mass movements are largely controlled by the rate and type of rock and soil weathering (Carson & Kirkby, 1972; Coates, 1980). As for hydrogeological causes, the role of water on slope instability can be considered inferior only to the force of gravity: this applies to the actions ascribable both to superficial hydrology and to percolation, storing and migration of water underground. Various phenomena are connected to the action of water, such as: runoff and the deriving processes of sheet, rill and gully erosion, solifluction related to rock impregnation, subsidence, river erosion or sand and silt liquefaction, when cohesionless materials are subjected to seismic shocks, etc. Among topographic causes various aspects of slope geometry are included: slope angle, altitude (relief energy), length and shape of the slope, all are morphological elements that control instability in different ways and to different extents (Demek ed., 1972; Carson & Kirkby, 1972; Cotecchia, 1978). One should also consider that thanks to the recent developments of computer technology the basic data of the existing topographical maps have been acquired in automatic and semiautomatic ways. Moreover, it has been possible to automatically produce a wide range of documents representing the above listed morphological factors over wide areas. As regards climatic causes, it is well known from literature (see, for example, Derbyshire, 1976; Starkel, 1976) that a large number of slope instability phenomena take place under the effect of rainfall. For example, in some kinds of landslides (soil slip, debris flow) mostly affecting the superficial cover, their constant association with pluviometric events often of high intensity leads not only to the identification of the threshold values beyond which the displacement occurs but also to the assessment of the return times of extreme precipitations. The problem may become more complex when mass movements involving large rock volumes are considered.

193 These landslides often affect a considerable thickness of the bedrock and can respond with a certain delay to a precipitation, owing to the geotechnical characteristics of the materials and the groundwater patterns of circulation. In these cases an important role is attributed to the length of the precipitations, rather than their intensity. The problems related to land use causes are mainly determined by farming, grazing and forest-management activities, that is by the kind of vegetal cover which, to various extents, protects the soil (see 4.2). This action can take place in the following ways: by reducing the mobilisation and transport processes due to the actions of gravity, wind, water, etc.; by favouring the percolation of rainwater and therefore limiting the runoff; by determining a barrier effect against all processes of physical degradation; by intercepting large amounts of rainwater for their biological activities, etc. In particular, an analysis of the vegetal species should be carried out as well as a subdivision of the territory according to their level of ''impedance", that is the power of protecting soil from erosion. Also the slope management and intervention techniques should be considered (e.g., terracing or other farming methods). 3.9.2 Effects of hazard The analysis of effects is based on a geomorphological investigation that offers an overall picture of the forms and processes of instability, both past and present, this should result in a Geomorphological Dynamics Map (GD). Such an analysis should also provide a reconstruction of the geomorphological history of the most disastrous events which have occurred in the area. A study of geomorphological dynamics is concerned both with forms of erosion and forms of accumulation. These two types of landforms may be considered separately or in their interaction. The study of past events should lead to a better understanding of the magnitude, frequency and type of phenomena which reflect geomorphological instability. A comparison of present-day processes with past geomorphological events will lead to an estimate of an evolutionary trend for instability. For example, a history of cliff retreat could be compared with present-day rates of coastal erosion or the recurrent flooding of a plain could be compared with present-day rates of fluvial sedimentation. The identification of the most frequent and significant disarrangement phenomena, and therefore also of their determining causes, allows the investigations on the predisposing elements previously illustrated to be concentrated on them. The effects of geomorphological hazard are essentially traceable back to geomorphological dynamics processes and their deriving forms, distinguished also according to their state of activity. The latter can be subdivided into the following. Active landforms: including those produced by ongoing processes at the time of surveying and those due to processes not taking place at that time but recurring in short cycles (frequent, seasonal, etc.). Dormant landforms: corresponding to features characterised by geomorphological evidences or witnesses (direct, historical, etc.) of activity within the present morphoclimatic system which have objective possibilities of reactivation since they have not completed their evolution. These landforms can furthermore be subdivided

194 according to their recurrence, that is the length of their inactivity intervals (e.g., periods shorter than a century or longer). Inactive landforms: including both those ascribable to morphoclimatic conditions different from the present ones (e.g.. Pleistocene glacial landforms) and those which, although being produced by the present morphoclimatic system, have accomplished their evolution and cannot change any further (e.g., terraced alluvial riverbed no longer reached by flow waters). Figure 88 illustrates a simplified legend referring to the most frequent phenomena among geomorphological hazard effects on the Italian territory. Mapping of this kind is easily deduced from a detailed geomorphological survey. 3.9.3 Synthesis of survey By analysing the list of the geomorphological hazard effects (see Fig. 88) it is possible to deduce some of them correctly by considering the overlapping of predisposing causes (such as landslides and solifluction). Other geomorphological hazard effects, although depending on several causes, show their frequency in space and time (when, for example, a watercourse shows a tendency to deepen its riverbed). In other situations an evaluation of the hazard effects can be rather difficult and uncertain and a reliable forecast of their occurrence could be made only with difficulty (e.g., certain processes of man-induced degradation). As a consequence, when a hazardous area has to be recognised and properly assessed by means of the reciprocal influence of the "causes", not all the "effects" Hsted in the corresponding legend will be considered, but mainly those deriving from the following types of slope movements: — falls and topples; — slumps; — slides; — flows, solifluction, gelifluction: As an example, for the landslides of the first group mainly the geological, hydrogeological, climatic and topographic causes will have to be considered. As for flows, solifluction and gelifluction, the geological, hydrogeological, climatic and land use related causes will have to be taken into account. As regards other geomorphological processes, such as sheet, rill and gully erosion, the deductions of potential hazards can be based only on a limited number of causes (e.g., lithology and land use) and on a hypothesis of extension or not of the ongoing phenomena (effects). The latter can be obtained from measurements and monitoring of the process itself. After the first two phases of survey and data acquisition regarding the geomorphological effects and of definition and assessment of the predisposing causes, it is necessary to compare them. In fact, only the comparison and critical discussion of the two documents (map of integrated analysis and map of geomorphological dynamics) will lead to the elaboration of a synthesis document (Geomorphological Hazard Map) showing both the areas actually unstable and those potentially unstable, subdivided

195

ACTIVE

DORMANT

degradationai or landslide scarp

erosional river scarp

^fV~

NON ACTIVE

erosional marine or lacustrine scarp erosional glacial or cryogenic scarp karst scarp man-made scarp trench or crack deep-seated defomnation rock fall or topples

(^I^

slide

(uZ^

slump flow, solifluctlon or gellfluction small landslide debris flow

(2Z^ r^

^

^

(^I^

deepening riverbed gully "calanco"

'Z^J^-l.A

a>*

r-*-'-'-'-j-*'*^

rill or sheet erosion doline

CS

CH)

doline field man-induced degradation red

orange

Fig. 88. Legend of the most frequent phenomena among geomorphological hazard effects in Italy.

according to the instability causes. This procedure, however, implies problems of different kinds. In fact, data collected in a sufficiently objective way (instability processes) are sometimes related

196 a priori to a series of parameters that interact in time and space and from one area to the other in various ways. Their real role in causing instability conditions is not easily ascertainable. For this reason, an analysis system concerning the relationships between causes and effects should be provided. Such a system should be "open" and reproduceable. For every study area it should therefore be possible to identify the type and number of causes playing a role in the various instability processes, by applying a statistical model relating every instability parameter to the unstable forms and processes actually observed in the field. Let us consider the work on Monte Santa Giulia (Panizza et al., 1980) in order to illustrate an example of implementation of a document of this kind. The map of geomorphological hazard was obtained by overlapping the two analysis documents: the map of integrated analysis (lA) and the map of geomorphological dynamics (GD). This operation led to the following four different combinations. 1) Indications of instability in the GD which do not find evidence for the same process in lA. This means that the present hazard phenomenon has led to a modification of some of the predisposing parameters (e.g., slope angle, vegetation, etc.). 2) Indications of instability in the lA without instability overlapping for the same process in the GD. It is therefore deduced that the hazard process has not yet taken place. 3) Coincidence of unstable sectors for the same process, deduced both from the map of integrated analysis (lA) and from the map of geomorphological dynamics (GD). This shows that potentially unstable areas (lA) are developing this tendency at present (GD) since they are in effect subject to degradation, and also that the ongoing instability finds optimal conditions for further development. 4) Overlapping of unstable sectors as a result of different processes. This can show that the present degradational processes tend to develop and evolve according to phenomena that are different from those deduceable from the combination of various causes. Any discrepancy between the two maps can obviously be ascribed to a lack of sufficient data for the lA or to missing points in its elaboration. As a conclusion, it is opportune to emphasize that for the elaboration of this geothematic map two operations are extremely delicate: — the parametrisation, quantification, subdivision into various classes (for example, slope angle classes); and — the evaluation and attribution of their weight in triggering instability conditions (e.g., whether a certain kind of lithology or of precipitation is more destabilising). To solve the problem correctly, it is necessary to carry out, every time and for each area, a calibration and assessment on a "regional" basis, avoiding generalisations that could not have any practical or scientific foundation.

197 4 MAN AS GEOMORPHOLOGICAL AGENT* *by Sandra PIACENTE

4.1 An approach to the problem Man is a geomorphological agent who was originally multizonal, but has progressively become azonal up to the present day. Unlike other agents, such as water, ice, wind and volcanoes, he is not limited or localised but, on the contrary, is less and less conditioned by environmental variables. Compared with other living creatures, Man has a great capability of movement and adaptation: his activities on the environment are a result of his technological development and are guided by economic, social and cultural needs. Man transforms, corrects and modifies natural processes by increasing or decreasing their rate of action and by causing the rupture of certain equilibria which Nature will try to reconstitute in different ways. With the passing of time this modifying action has assumed increasingly widespread and intense patterns. The perception and awareness in scientific terms of Man's geomorphological activity is very recent even if some scientists had already hinted at the effects on the natural environment of an economy centred on intense farming in the colonial period of North America. It was the American geographer Marsch (1864), in the middle of the last century, who recognised the disturbing influence on the natural environment exerted by Man, considered as an agent modelling the Earth's surface. In the same period, the great French geographer Reclus (1871) admitted that although the action of Man can improve the value of the natural environment, it can also contribute to its degradation. In particular, he stated that: 'It is the improvidence of the inhabitants, and not so much the geological constitution of the soil, which is the principal cause of the devastating action of streams". For example, in the mountains of Provence during the course of centuries, the destruction of the vegetation cover related to the expansion of farming land and pastures has contributed to the destruction of the original landscape. Subsequently, the Russian scholar Woeikof (1842—1914) reconsidered the ideas previously expressed by Reclus and Marsch and ascribed some climatic variations to the practice of deforestation. At the beginning of the twentienth century the German writer Friedrich connected the term Raubwirtschaft, that is devastation, to Man's activities. In fact, he believed that: "The destructive exploitation of resources leads of necessity to foresight and to improvements, and that after an initial phase of ruthless exploitation and resulting deprivation human measures would, as in the old countries of Europe, result in conservation". In 1901 the Italian geographer De Marchi, in his "Trattato di Geografia Fisica" maintained that: ''Man modifies to his advantage vegetation, reliefs, hydrography and the coasts" and concluded by saying that: "with the diffusion of civilisation and the discovery of some new dominant forces of Nature, Man's geographical action is becoming increasingly intense, widespread and uniform to the point that it is already possible to forecast that it will

198 become a main morphological agent". Still in those years (1915), the German scholar Fischer published the book ''Der Mensch als geologischer Faktor", where a quantitative evaluation of the volume of natural material used by Man for the construction of roads and buildings is made. In 1922 the British writer Sherlock in one of his works deals with the various aspects of anthropic activity on the environment. In 1929 the French agronomist Aufrere in his book on agriculture ''Les rideaux etude topographique" dedicated a chapter to Man as ''geomorphological agent". In 1935 the volume entitled "Man as a modeller of Earth", by the German author Fels, was published. In 1951, Mensching, another German writer, made an interesting correlation between the accumulation of silty material in the Weser valley and deforestation on the river's upstream slopes in historical epochs. In 1953 the French author Tricart used the adjective ''erosion agent" referring to Man. In 1956 the proceedings of an international symposium on ''Man's role in the changes on the Earth's face" were published. In 1959 the Italian geologist Gortani in his "Compendio di Geologia", in the chapter dedicated to the causes of mass movements attributes to Man's actions such as large quarry excavations, slope cuttings, deforestation and farming practices, the responsibility for some of these movements; although he considered these activities as "occasional causes which accelerate the failure of an equilibrium already compromised". In 1968 the American National Academy of Sciences established a special commission in the field of geological sciences in order to study this new problem in depth. In 1972 this commission published the volume "Earth and human events". In 1971 the Russian Gerasimov and other authors pointed to some of the effects produced by the exploitation of natural resources in the former USSR and placed human impact on Nature in a historical context. In 1973 the Italian geomorphologist Panizza dedicated a chapter of his textbook "Elementi di Geomorfologia" to anthropic degradation. In 1979 Castiglioni, another Italian geomorphologist, dedicated a full chapter of his volume "Geomorfologia" to the forms caused or influenced by Man's activities. Still in 1978, with the work by Haigh, Anthropic Geomorphology is rightly recognised as a branch of Geomorphology. In the last few decades various authors from different countries have dealt with anthropic geomorphology; among them worthy of note are: Gregory & Walling (1979); Goudie (1981); Drew (1983); Neboit (1983); Nir (1983). The environment is a natural system which has reached a delicate balance over a long period of time; this system works according to complex and interdependent mechanisms which have not yet been fully understood. Man is not outside nor is he above Nature. What makes him different from the other living creatures is free will, i.e., his capability of choosing, which consists in behaviour and responsibilities not so much towards the outer world but rather towards himself, since it is Man himself, with his history, who started environmental problems. From this fact a complexity of relationships between knowledge and economics, development and technology,

199 progress and conservation is derived. The identification and, above all, the evaluation of changes brought about by Man on the physical environment is an operation which is more complex than it might seem at first. In this kind of study one fact is certain: the changes which have affected the environment as a consequence of Man's activities during this century are far more widespread and radical than those occurring during the previous thousand years of human presence on Earth. It is not easy to distinguish and separate ''natural" forms from ''artificiaf ones. There is a law governing reciprocal actions and the unity of geographical actions according to which it is not possible to consider facts and phenomena separately. The various components of Earth's surface are to be considered not according to unilateral relations but as reciprocal and complex. There is an action of the physical environment on Man and a reaction of Man towards the environment (Fig. 89). Moreover, it should be remembered that Man's influence on morphogenesis cannot be easily exemplified since it cannot be reduced to a simple acceleration of natural dynamics. In some cases Man's settlements and activities can cause the worsening of erosional processes in an unstable environment, such as mountainous areas, where the extremes of the climate are accompanied by high slope gradients. In other cases the works of Man can cause disarrangements in an otherwise stable environment. This happens, for example, in tropical regions, where the destruction of rain forests and savannahs has given rise to new forces, associated with very intense rainfall, forces which were

Fig. 89. Example of reciprocal relationships between Man and the environment: the village of Namche Bazar (Himalaya) was built within a glacial cirque modelled by man-made terraces (photo M. Panizza, March 1975).

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once mitigated by the thick tree canopies. In this case, more than an acceleration of erosion, the introduction of a new, more aggressive morphogenetic system is produced which is founded on a different hierarchy of the phenomena involved (Neboit, 1983). The analysis of anthropic activities cannot take into account only "what" Man has done but also "how" and for "how long", since human action is discontinuous in space, in time and in intensity. Moreover, it cannot leave out of consideration the social context and the historical dimension within which it took place (Neboit, 1983). A correct assessment of Man-induced morphological effects becomes possible only by means of a systematic study carried out according to the following procedure (Nir, 1983): — historical approach, in order to identify Man's interventions on landforms; — geomorphological approach, in order to assess the amount and extension of the geomorphological processes observed; — socioeconomic approach, in order to investigate the dynamicity of human activities by considering economic and social-structural parameters; and — planning approach, in order to unite and integrate the various points of view. Also the mental attitude of whoever is going to evaluate the interactions between Man and the environment is a fundamental aspect to be considered. In fact, from a position of submission to Nature, which is typical of preindustrial societies and pioneer settlement areas, Man can pass through a vision of harmony with Nature, in which human systems occupy a well defined place in the natural order, and eventually develop a concept of dominance over Nature, operating in the firm belief that artificial interventions can enhance economic and social benefits. A final consideration should be made about the planetary level of this problem: only a clear picture of the facts and interventions on a worldwide scale can lead to some solutions. This is particularly true today, since industrial society has involved the whole planet, with all its physical and social realities.

4.2 Man's activities and their geomorphological consequences 4.2.1 General aspects Man's interventions take place essentially on that thin layer of the Earth's surface which makes up the interface between atmosphere and lithosphere where most energy exchanges and complex phenomena take place. Hardly ever are these phenomena confinable within preconstituted and rigid schemes, but they can nevertheless be summarised as follows (Castiglioni, 1979): — artificial forms, directly modelled by Man's activities; — works aiming to divert, correct or upgrade natural processes; — modifications of natural phenomena, indirectly resulting from Man's activities. The intensity and extension of these activities depend upon four factors: — demographic, that is the numerical consistency of the individuals practicing any

201 activity which is reflected on natural dynamics; — historical: length of time of human presence and activity; — technological and cultural: capacity to exploit the environment according to one's own purposes; — socioeconomic: demand for better living conditions and consumer goods. It may be useful to remember that Man appeared in East Africa about three million years ago, probably in Tanzania, Kenya and Ethiopia. His presence in Europe goes back to some one and a half million years ago, whilst his arrival in the British Isles and Japan took place much later. Australia and North America were probably inhabited up to 30,000 years ago. After the expansion of the agricultural revolution (some 10,000 years ago), a considerable increase of the population occurred which can be assessed as about 200 million people by the time of Christ and to 500 million around the year 1650. Subsequently, the industrial revolution and the development of science and technology, especially of medicine and related disciplines, together with the colonisation of new lands led to a real explosion of the population which reached 1,000 million in the mid nineteenth century, 2,000 million at the beginning of the twentienth century and has at present exceeded 5,000 million people. It is obvious that the great increase of anthropic density, together with the development of culture and technology, is the main cause of the transformations of natural environments. The historical order of the effects of Man's intervention on the physical world can be schematised as follows: — consequences of hunting; — consequences of animal-farming; — consequences of agriculture; — consequences of natural resources exploitation: — consequences of engineering works. The characteristics of these activities and their main consequences on the environment, even when negative, are here discussed (Piacente, in Panizza, 1988). 4.2.2 Consequences of hunting This human activity, which can with no doubt be reputed the most ancient, is certainly the least disturbing with respect to the physical aspects of the environment. It was the first activity enterprised by Man for his survival and has always been present in the history of mankind as food resource, for the supply of clothes and ornamental materials and as a sport. From the geomorphological standpoint it may imply the deforestation of some areas in order to create clearings and hunting trails. It can cause limited soil creeping as a consequence of periodical or continuous passage along slopes; nevertheless, if it is properly managed, this activity can contribute to the conservation or the upgrading of areas in precarious conditions of stability.

202 4.2.3 Consequences of animal-farming Not only is the breeding and farming of domestic animals one of the earliest of Man's activities but also one of the most persistent anthropic actions. It was introduced some 10,000 years ago, when the forced concentration of animals and people in oases and along watercourses accustomed them to live in symbiosis: thus the domestication of animals began and therefore the practice of breeding them. Apart from the destruction of the original vegetation, obtained by means of fire and wood cutting, the activity of animal-farming implies: the fertilising and irrigation of soil, in order to favour the growth of grass and the removal of stones, so as to make the transit of animals easier. Nevertheless, the real problem is not so much animal-farming but rather overfarming, i.e., the excessive concentration of animals per surface unit. One of the most serious effects of overfarming is the change in the pedological profile: a soil's "A" horizon, degraded and enriched in manure, is subject to changes in its composition and structure. The repeated and regular action of animals' hooves causes not only soil compaction and therefore the decrease of its permeability, but also the soil's continuous mobilisation which can eventually lead first to the detachment, and the destruction after, of the tree cover thus triggering widespread soil creep (Fig. 90). What happened in the Sahel during the severe drought of 1970 can be quoted as an example. The concentration of people around large cities, with the consequent increase of animal-farming in these areas, led to the degradation of herbage and to the consequent disappearance of edible species and proliferation of weeds. Moreover,

Fig. 90. Creep phenomena due to over-grazing in Ra Stua (Cortina d'Ampezzo, Dolomites. Italy) (photo M. Panizza, July 1995).

203 the use of wood-bearing plants including their roots as fuel caused the progressive drying up of the vegetation around towns and villages, leaving portions of bare soil which would be easily removed by the actions of water and wind. 4.2.4 Consequences of agriculture The alteration or destruction of the natural vegetation in order to plant other, more useful species doubtlessly causes chemical, biological and physical modifications of the soil, as well as microclimatic changes. In fact natural vegetation, which represents the biological answer to the totality of the environmental parameters - soil, climate and morphology - absorbs a greater quantity of morphogenetic energy than the artificial vegetation. Indeed, the latter favours the effectiveness of the degradation processes, with even greater effects with those types of farming practices which periodically leave the soil bare (Fig. 91). The degradability of cultivated lands is favoured by monoculture which is typical of latifundism. This practice is based on periods of absence of the vegetation on large slope portions, thus favouring the areal development and the articulation of degradation processes. Instead with the introduction of polyculture, where each allotment was worked by a single owner, it was possible to produce a greater variety of crops, with nonsimultaneous periods of absence of vegetation cover. In countries with ancient rural civilisation. Man introduced measures of mitigation of erosional phenomena such as terracing, which makes farming operations easier. Along watercourses running in plain areas, it has been a long practice, especially

Fig. 91. Example of rapid tendency toward deserification following a long drought period (1970) and intense soil exploitation (east of Dakar, Senegal) (photo M. Panizza, May 1984).

204 in Europe, to cultivate crops in the outer portions of riverbeds. In this way a reduction of the flow section is determined and as a consequence runoff takes place at higher velocity and with an increased erosive power during high-water periods. In the history of agriculture there have been at least six great technological innovations which by causing remarkable increases of the soil's productivity have also implied evident interferences in the soil's natural dynamics. The first one was irrigation, already applied 6,000 years ago in the plain of the rivers Tigris and Euphrates; the second one was the use of animals for working the land; the third one was the exchange of farming products between the old and the new world; the fourth one was the invention of the internal combustion engine, the fifth one was given by fertilisers and the sixth one by chemical pesticides (Orr, Roberts & Lansford, 1981). Irrigation, obtained by means of artificial draining, induced great changes in the hydrographic network which can be summarised as follows: regulation of the watercourse flow rates obtainable by means of new hydric elements such as floodways and reservoirs; augmentation of sedimentation processes with consequent raising of the soil level and increase of evaporation; storing of water stocks with relative interference in the sedimentation, current flow and erosion patterns; changes in the flow rate owing to the construction of dams; alterations of the dynamic equilibrium following the construction of harbours and moles. Moreover, irrigation may induce microclimatic changes, especially in the rate of humidity. Canal and drainage systems have influenced the alluvial plain landscapes in many countries like, for example, England and Italy. The use of animal work for ploughing has intensified the exploitation of slopes. The consequences of ploughing depend on the angle between furrow and slope directions. Vertical ploughing, from the slope top to the toe, is the most simple method but also the most hazardous, especially if hydric circulation has not been considered. Furrows can in fact evolve into concentrated runoff streams. Furthermore, on mostly clayey slopes the practice of ploughing transversally to the gradient, according to the contour lines, could favour water stagnation, absorption, percolation and plasticisation phenomena with consequent hazard conditions owing to solifluction or earth flows. The exchange of farming products between the old and the new world has not induced particular geomorphological phenomena, apart from those cases where a radical conversion in farming methods was introduced with considerable alterations along the slopes. The introduction of mechanical machinery has implied a more intense exploitation of farming lands: tractors and disk-ploughs have contributed to triggering more marked erosive phenomena. The ploughshare towed by oxen goes down into the soil at the most 15 cm, whilst mechanical ploughing can create furrows up to 50—60 cm in depth. Moreover, since mechanical machines require much more space for their movements, many shrubs and hedgerows have been destroyed, thus increasing erosive processes. The use of chemical fertilisers and pesticides can alter the soils' pedological characteristics to a certain depth with consequences also for morphogenetic processes.

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Nevertheless, there are also some positive effects of farming on the environment: the systematic conservation and re-establishment of farming soils along slopes may improve the defence against erosion, as well as the maintenance of paths and cart-roads, the conservation of clearings inside woods, the management of hydric systems, etc. 4.2.5 Consequences of resource exploitation The rise of the world population and the consequent increase of needs is the fundamental element which has created an enormous increment of resource exploitation. The consequent need to satisfy the ever-increasing demand for natural resources inevitably determines several environmental problems. For example, the increased density of people, houses, industrial plants and transport infrastructures intensify the production of pollutants released in the air, water and soil. Moreover, the construction of buildings, underground structures, communication routes, sport and leisure facilities and the dumping of waste materials and garbage in certain areas could cause the destruction of the vegetation and create conditions predisposing accelerated erosion and mass movements. The drawing of water and gas from the subsoil for industrial and domestic purposes can cause lowering of the water table or land subsidence and increase the hazard of aquifer pollution. The century-old struggle of Man for the exploitation of natural resources has always been accompanied by the search for instruments and forces capable of enhancing his capacity of carrying out works. For an extremely long time this possibility was guaranteed by the physical work of Man himself and of the animals that he was progressively domesticating. The prime source of energy was food, provided for both Man and animals by the vegetables, which are the only living beings capable of absorbing energy by exploiting directly the Sun's heat and light. Subsequently, great discoveries such as fire, the wheel, the lever, the arch, the pulley, etc. enhanced and made Man's and animals' work easier. An important stage in the history of human work was the capacity to utilise great and small physical forces found in the environment, such as wind and the movement of water in rivers. Subsequently, the history of mankind progress was more and more marked by the discovery of steam and fuel engines capable of reducing energy waste and increasing work production. The discovery and later use of fossil fuels, like natural gas and oil which could produce caloric powers much superior to coal's, was a major step forward. Even more important was the discovery that all bodies possess enormous energy powers at an atomic level. Today's world often considers with perplexity and fear this energy source, which probably Man is not yet capable of knowing and controlling. Notwithstanding this, the experience and studies still in progress point to the fact that atomic power may be one of the most effective solutions for our energy needs. Similarly, also the discussion on the research and exploitation of the so called "powers at nil risk" is particularly heated. Hydroelectric power plants, solar panels, wind mills and tide power plants are the most interesting alternative solutions. It

206 should nevertheless be emphasized that these so called ''mild powers" show two considerable drawbacks: the first is the poor energy yield and the second the high cost necessary for their generalised exploitation, at least at the present state of the art. The problem concerning the progressive depletion of traditional energy sources has triggered a confrontation, and sometimes a clash, between two opposite tendencies of thought. According to some people, if the present rhythm of development is maintained the complete exhaustion of energy reserves will soon be reached: in these conditions, if a catastrophe is to be avoided the only solution is to return to a preindustrial situation. This is the position of some ecologists, at least the most radical ones, according to whom technological development is a synonym of environmental catastrophism. On the contrary, according to others, mankind will manage to solve its own problems of growth and development by means of studies and investigations producing new technological conquests, as has already happened several times in history. Human welfare and mining and energy resources in general have always been so intertwined that the various stages of Man's history were named after the most available minerals: stone, bronze and iron ages. Notwithstanding the progressive depletion of the richest ore deposits, the constant improvements of exploitation technologies have allowed the quarrying of ever increasing amounts of raw materials to be carried out at more and more competitive prices. Nevertheless, the main problem is not so much given by the progressive dwindling of the available materials but rather by the high human and environmental costs resulting from intense and constant exploitation. Destruction of the original landscape, slope instability, river and groundwater pollution are the most serious impacts of Man's misuse of the natural resources. At this stage, the negative effects of Man's activities spread over larger and larger areas threaten to exceed the direct benefits. In particular, the mining industry contributes to the atmospheric heating and pollution since it is one of the main consumers of power. The extraction of minerals causes particularly serious environmental consequences in developing countries since most of the world's ore deposits are concentrated there. It should be pointed out that with the term '^minerals" a great variety of raw materials directly extracted from the soil and subsoil is meant, including: metals such as aluminium, iron, copper; substances for industrial use such as lime, sodium carbonate; lithic materials such as sand, gravel and energy-producing elements such as uranium, or compounds such as coal and liquid and gaseous hydrocarbons. If the excavation of mineral resources continued at present rhythms, the environmental consequences would be devastating. Between 1750 and 1900 the consumption of minerals doubled, whilst from 1900 to the present day the increase has been 10-fold. It should furthermore be pointed out that today new materials have appeared, such as plastics, ceramics and high technology compounds, which compete with metals. As a consequence, the consumption of minerals is growing more rapidly in developing countries than in industrialised ones. The mining and working of ore deposits has created in many countries vast degraded areas. Environmental damage is caused by several factors such as: site

207 morphology, amount of quarried material, depth and extension of ore deposit, chemical composition of the extracted material and of the surrounding soil and rock, techniques used for quarrying and working the material, management and disposal of tailings and rock waste. A significant example of this is offered by the Panguna copper mine, in Papua New Guinea, which dumped 600 million tons of contaminated metal waste into the Kawerong river. It has been calculated that each year the digging out of noncombustible materials from the subsoil causes damage to half a million hectares of land. This sort of damage is often irreversible, at least on a human life scale, as witnessed by the dramatic changes in the Bolivian landscape after over 500 years of mining activities. The geological and, in particular, geomorphological problems linked to the exploitation of natural resources are innumerable. As an example, the various mining activities carried out on the Earth's surface can be quoted. Whether rock materials or soils are involved, quarrying activities always have considerable impact on the original relief, since they imply changes in surface morphology, with digging out of material from one area and its accumulation in another. In the last hundred years the number and extent of these interventions have multiplied, especially since the introduction of more and more powerful mechanical machines for earth movements. As material is excavated there is an inevitable accumulation of waste which often makes up real "man-made reliefs". Also garbage heaps, tipping areas and other industrial refuse make up semipervious surfaces which compromise both the drainage patterns and the potability of water. An example of irreversible environmental degradation is given by the consequences of quarrying activities in riverbeds (see 2.1.2). Moreover, the negative effects are not limited to the exploitation period but can persist and in some cases worsen if the quarrying area is abandoned without adequate upgrading operations for proper reuse. Slope stability problems arise not only in the quarrying areas but also in the heaps of accumulated waste materials. Also cases of soil and water pollution occur if quarries or soil pits are utilised as uncontrolled dumping sites. These human activities can also cause the alteration or destruction of geomorphological assets (impact). 4.2.6 Consequences of engineering works These consist in the construction of roads, bridges, dams, buildings, reservoirs, hydraulic works, dumping sites, harbours and coastal structures, tourist facilities, etc. Changes usually occur in the hydrographic network together with floods, landslides, slope instability, reduction of soil permeability, microclimatic variations, air, water and soil pollution, etc. (Fig. 92). One of the first changes brought about in the environment by the construction of roads, houses and other artificial structures is the transformation of natural and farming terrain into urban soil. In geomorphological terms, Man's activity in urban areas determines the interruption of natural processes governing the formation of soils and the introduction of a new ''anthropic soil" made up of asphalted areas, gardens

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Fig. 92. Phenomena of intense erosion along the banks of an artificial lake formed after the construction of a dam near Torun (Poland) (photo M. Panizza, October 1970).

and various kinds of constructions (Nir, 1983). Urban development is indeed characterised by the sealing up of natural soil with the formation of impermeable surfaces. Urban soil is subjected to totally artificial drainage, with a higher density than natural drainage. In urban areas water percolation is much reduced, resulting in a higher flood hazard. In 1850 only 50 towns worldwide had more than 100,000 inhabitants, whereas only three had more than 1,000,000 people. In 1973 over 1,000 towns had more than 100,000 inhabitants and nearly 200 more than 1,000,000 dwellers. Also the construction of airports and harbours causes widespread changes in water drainage systems, especially regarding the vast extent of the surfaces involved. Slope cutting for road and other constructions always determines variations in stability conditions and in groundwater flow patterns, with waterfall effects in areas upstream of the cutting, greater production of detrital materials downslope, destruction of the vegetation, etc. Intense tourist and sport development of high mountain areas has introduced another disturbing effect, thus compromising the already difficult balance of these areas even further with the felling of trees in order to create ski pistes, cableways and ski lifts. It is not so much the modest extension of deforested areas that can decrease slope stability but rather the fact that these wood cutting activities are mostly perpendicular to the valley floor and rather inclined, thus favouring the concentrated flow of surface water and the formation of avalanches.

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Hydraulic works can produce important changes in the natural evolution of rivers, mainly by reducing the intake and transport of debris to the sea. For example, river barriers, besides modifying the water runoff trend, are very effective traps for solid matter. Moreover, in order to mitigate the devastating effects of floods, a retention of debris which otherwise would have reached the sea is carried out by means of constructing flow-control weirs and basins with free overfall and reclaiming marshy areas in some alluvial plains. Man's action along sea coasts can become particularly detrimental (impact). Also in Italy, since the beginning of this century, the excavation of loose materials from sand dunes, frontshores and beaches, which make up three indissoluble elements of the same system, has been carried out in a disorderly manner, with neither planning nor control. In this way, an already precarious natural equilibrium, where easily erodable and transportable materials are subject to the action of high-energy agent such as wave motion, was further compromised. The demolition of dunes, precious sand deposits defending the inland territory and giving consistency to coasdines, has produced damage which is mostly irreparable. The construction of roads along beaches can cause disastrous effects: once the beach has been deprived of its upper portion its profile undergoes an alteration. Moreover, vertical walls hit by breakers during sea storms exacerbate the turbulence of backwash, increasing the drawing of material from low beaches. Major harbour works and the construction of shipping canals interfere considerably with the transit of material, hindering its transfer onto the beaches. Also dumping solid waste materials offshore, such as building yard rubble, can cause diversions of shore currents, with consequent depletion of material from the beach (see 3.5.2). Furthermore, urban development or any other situation characterised by high population concentration, increases the possible destructive effects (risk) of natural phenomena and the presence of an urban environment is responsible for climate changes, with the well known consequences of an increase in the greenhouse effect. Finally, it should be pointed out that engineering works can also produce positive effects on the environment or exert a specific defence function from natural hazards such as flood controls, improve the flow of watercourses, stabilise slopes, protect coasts. It is nevertheless indispensable to foresee with scientific exactitude all possible repercussions of human interventions on the environment system, lest an engineering work, although planned and constructed for the defence against some type of event, may have negative effects and trigger other kinds of hazard.

4.3 Conclusions In its relationships with Man, the environment should be studied according to a global approach centred on the concept of a system, by considering that: — a system represents a generalisation or, rather, a model and therefore an abstraction from the real world and its complexities;

210 — each system depends on the relationships existing between its variables; — the functioning of a system depends on the existence of forces and energy sources; — each natural system cannot be considered as an isolated system, since both matter and energy can be transferred; — a natural system always tends to evolve. What changes the state of a system are the processes that take place inside it; — in order to assess the energy of a system it is necessary to know not only the initial and final states but also the intermediate ones; — the natural system is very different from the anthropic one in terms of space, time and reactions: there are various degrees of complexity, diversity and state. Even environments which seem to be stable often give this impression only because they have been insufficiently studied and documented. According to these definitions, the natural system is the synthesis of the natural context (both physical and biological) of Man's activities (from historical heritage to artistic records, economic aspects, social conditions, etc.) and their position within a cultural perspective. This means, above all, recognising a continuity between Nature, history, traditions, culture, socioeconomic activities and development prospects. It means expressing the Man-Nature relationship in terms of continuous evolution and reciprocal incentives in time and space. In this way, also the dualistic view opposing "natural" and "anthropic" contexts would be overcome. Each single variable of this system, although characterised by its own function and evolution, should therefore be considered within the framework of the functions and dynamics of its own territory. If the evolutionary trend of the elements making up a system is not congruent their own evolutive tendency, congruent with the whole system's, then these elements are as yet unbalanced, although eventually they will reach some form of equilibrium. The achievement of this equilibrium can take place either through slow dynamic processes easily assimilated by other elements or particularly rapid and sudden ones with serious repercussions on all the system. Finally, another aspect should not be neglected: the factors making up a society are given by the physical environment and its history, but society often evolves autonomously, owing to economic and political causes. Therefore, society itself makes up a system, which sometimes appears to be unconnected with the main natural system. This extraneousness of the social system is obviously more apparent than real and is basically due to its evolution times which hardly ever coincide with those of the natural system. The most significant parameters of the environmental system can be schematised as follows. Scientific — in the environmental system each element has its own specific meaning and role in global dynamics. The knowledge of all its components is still insufficient; each research and result should be documented and stored for future investigations. Ethical — every species has the right to live in a suitable environment. Aesthetic — each landscape form and aspect, each animal and plant is beautiful

211 and significant and, as such, should be preserved with all its original prerogatives. Biodiversity — the diversity of species should be maintained and protected. Stability of the physical environment should not be compromised, since the life and development of the diverse living species depend on this parameter. Recreational value — the conservation and improvement of the environment is of value also from the recreational and leisure point of view (excursions, parks, photosafari, etc.) and therefore has also considerable social and economic implications. Economic value — natural and biological assets can make up a considerable source of immediate revenue and cultural value if properly managed as resources. Heritage for future generations — This is one of the fundamental reasons for preserving the environment and living species. The environment has a limited capacity to adjust to the ever growing changes caused by man's activities without undergoing irreversible transformations, especially where Man's presence is intense and long-lasting. Indeed, in some places these limits have already been exceeded. The study of the development-life quality ratio can be faced from two different standpoints: either starting from environmental management and the technologies used for assessing impact, or starting from analytical evaluation processes of the effective impacts as they are studied in order to better define specific measures for their correct management. In the past few years, the evolution of public management has brought about new social demands with a growing awareness of the social and environmental costs of technological development. A totally new phase of political commitment aiming at the satisfaction of previously unknown social demands has begun. Therefore static and habitual models must be overcome in order to take advantage of intercultural and pluralistic dialectic views. In this way a dichotomy has been created: on one side scientists, economists and managers maintain that decisions should be taken according to rational and scientific arguments; on the other side sociologists and anthropologists support the idea that governments should provide political solutions taking into account the conflicts between society and technology with a general and long-term view. The history of the last twenty years has been marked by scientific and social reflections upon these problems and the conflicts created by Man's inadequacy in defining and measuring "risk", which is always rediscussed and redefined by social processes directly involving the role of science. In fact, while for some risks the political measures are consolidated or at least outlined, for others they are still to be defined. Among them, the artificial transformation of the territory, the concentrations of carbon dioxide and ozone, urban congestion, the ever growing noise and psychological and social impacts of the urban environment should be quoted. Moreover, there is today a misleading tendency to consider the problem of risks out of its historical context, without considering what happened in the past. By directing most of his energies towards scientific-technological and economic knowledge, Man has not given adequate thought to the management of the problem. In such a complex world, so full of uncertainties, where roles and responsibilities are more difficult to

212

identify and past certainties are no longer valid, the crisis of self-confidence should be transformed into an awareness of Man's own limits in order to promote pluralism of ideas and the search for new perspectives and models, since a culture concentrating only on a single element impedes the understanding of changes. Man in fact is provided with a vast cultural heritage and lives in an environment which is not only the result of physical, chemical and biological processes, but also cultural and social ones. Within a context of global planning, it is therefore indispensable to find an agreement on the priority criteria for environmental management, in order to optimise the scanty resources still available. This implies that systematic analyses must first be made at a global level aiming to negotiate these problems which are bound to be more and more urgent in future. At the same time, it is necessary to develop studies and techniques directed to the decrease or at least optimisation of energy consumption, the increase of agricultural production, support for recycling projects, reclamation of degraded territories, etc. Obviously, on an international scale environmental problems are more difficult to solve than domestic ones. There are two reasons for this: a single authority capable of formulating and applying the most opportune political measures does not exist; the possible solutions must consider a balanced ratio between costs and advantages which in most cases varies from one country to another. Moreover, some countries could have more urgent national problems to face and therefore less economic potential for intervening in international problems. When ecological degradation extends its effects beyond national boundaries or solutions to environmental problems require interventions on an international scale, then the codification of adequate rules and norms clashes with greater difficulties. The concept of the existence of a common heritage at a planetary level was already recognised some time ago by several governments (for example, the treaties on Antarctica and the oceans). Nevertheless, the concept of clear common responsibilities in the conservation and management of natural resources is a very recently acquired notion. The awareness that environmental problems should be faced on a planetary scale has given rise to institutional innovations both at a national and international level. International Organisations such as the European Union, the African Union Organisation and Pan-American Nations Organisation have extended their cooperation also to environmental issues. Moreover, several specialised public and nongovernmental agencies and associations are involved in problems concerning the environment in a broad sense: sea pollution, management of nuclear and toxic waste, protection of species in danger of extinction, conservation of historical monuments, etc. There are three great categories of issues which require international solutions: 1) When resources are shared between different countries (for example, a watercourse or a sea). 2) When certain resources, although belonging to a single country, represent a value for the world community (for example, the rain forests or the various habitats of particular species).

213 3) Other environmental resources cannot be divided according to a national criterion since they are a common asset for all mankind (for example, the atmosphere or the oceans). One of the most representative examples is that of international watercourses which have become the object of negotiation between most European countries and North American. Over 40% of the world population lives in fluvial basins interesting more than one nation. Although international legislation is still insufficient on this issue, two basic principles have been established by international organisations: each country has the clear obligation not to cause serious damage to the countries with which it shares the watercourse; the rights on the waters must be equally divided between the partner states. An example of successful agreement, achieved in 1960 with the help of the World Bank, is that relative to the River Indus catchment basin, equally shared between India and Pakistan. Among various difficulties, the different prospects of developed and developing countries should not be underestimated. The former are undergoing a qualitative transformation of consumer goods, due to the substantial stability of the population and saturation with primary goods: in these conditions a decrease in environmental stress can be foreseen for the near future. Moreover, great technological development offers technologies which are more compatible with respect for the environment. Last but not least, the growing awareness of public opinion towards environmental problems. In developing countries, however, the population continues to grow with the prospect of doubling in the next 50 years (e.g. Asia). An urgent increase in the production of fundamental goods is therefore required since in most of these countries the standard of living is at the limit of survival. In these cases the protection of the environment is no longer the main issue but, on the contrary, is considered as a hindrance to the development of agricultural and industrial production. All this is accompanied by obstacles presented by the inadequacy of infrastructures, too expensive depuration technologies and strong cultural and social barriers. In order to achieve positive results, the problem should be faced through constant dialogue between scientists and politicians and also through the indispensable involvement of public opinion, by means of information and education programmes. Another aspect should also be emphasized: if on the one hand Man uses and models the Earth's surface, on the other hand he is becoming more and more vulnerable to natural phenomena: floods, landslides, volcanic eruptions, earthquakes which in the past had only a local effect, now hold worldwide dimensions since all social realities are linked by strictly interdependent economic, energy and communication systems. Although it is now undoubtedly certain that Man's presence and actions have negative repercussions on the environment, nevertheless there is no need to be excessively alarmed. Some remarkable changes that Man has brought about in Earth's natural phenomena, behaving like a real geological agent, are neither sudden nor recent, but they are rather the cumulative product of different practices carried out over a long period of time. Since the effects of Man's activities on the environment

214 in the near future are not totally foreseeable, the present challenge consists in assessing correctly not only the "how" and "how much" of human actions but rather the capacity for assimilation and reaction of the whole natural system. It should be remembered, in fact, that the natural system has its own dynamism within such vast temporal and spatial dimensions that Man's activities often exert effects whose importance is only apparent. Nonprofessional simplifications, preconstituted schemes and superficial media information are unfortunately more and more frequent. This kind of approach should be avoided as it brings about bewilderment and confusion in public opinion, creating obstacles for a correct attitude to environmental issues. The environment system should therefore be analysed with scientific exactitude in all its components and interactions, so that each intervention of Man can take into account all possible repercussions and consequences once they have been properly assessed.

5 VULNERABILITY AND GEOMORPHOLOGICAL RISK

5.1 Vulnerability to hazard With reference to the subject defined in section 1.6 and considering natural risk in general, it is possible to state that the severity of the risk depends on the hazard degree, the vulnerability degree and the relationship between them. It was shown how a territory's vulnerability is given by the concurrence of various components (Fig. 93): exposure, economy, planning and social organisation. It is not within the author's competence nor within the goals of this volume to provide a specific discussion on the sectors concerning engineering, economics, political and social sciences or whatever could be included in the definition of vulnerability. Nevertheless, for some of the components shown in Fig. 93, it was considered opportune to show in any case the main aspects of the relationships with hazards: this is the case of exposure, economy and planning. Among the other topics that can be of more interest to a geomorphologist, environmental education, which is included in social organisation, was dealt with in most detail (see 5.4). This interest is of two kinds: the first is directly professional and involves teachers, researchers and technicians, the second, on the other hand, concerns the social role that any

Exposure VULNERABILITY <

Economy Planning

.Social organisation Fig. 93. Scheme of the environmental components of vulnerability'.

215

low

exposure VULNERABILITY

high

Fig. 94. Scheme of risk severity in relation with vulnerabiUty exposure and hazard probabihty.

environment operator could and should assume not only in the contingent risk moments, but especially in the daily practice of a territory's use and management. As for exposure. Fig. 94 connects this vulnerability component to the probability of a hazard's occurrence. With reference to the definition given in section 1.4, a hazard is seen to be linked to the probability of its occurrence in time, as well as its magnitude. It is obvious that the higher the hazard and vulnerability values, the higher will also be the severity of the risk. Moreover, if the value of one of the two factors is low, as a consequence, the severity of the risk is relatively low: like, for example, a low-vulnerability area like a desert related to a high landslide hazard, or an area with low landslide hazard, like a plain, related to a highly populated area. In both cases the geomorphological risk connected with landslides will be relatively low. Again in the case of exposure, considered as a component of vulnerability, a certain scale of values is taken into account. In the first place human life is found, including as a consequence in terms of risk not only the death of people but also any kind of injury, disease and stress. At a lower level of vulnerability the aspect of goods can be considered, with its consequences of property damage and economic

216 loss that a particular hazard can cause: in this case not only is the way of constructing buildings, structures and infrastructures of fundamental importance, that is, the types of construction and the materials used, but it is also essential to consider where these works are constructed, for example, in situations of geomorphological instability. Exposure to vulnerability is considered also with regard to the environment, including both biological (fauna and flora) and abiological components (for example, geomorphological assets: see 2.2) since they can both undergo destruction or damage. As regards another component of vulnerability, i.e., the economy, the higher vulnerability of industrial areas has already been pointed out: factories, infrastructural buildings, communication routes, etc., can receive considerable material damage when a hazard occurs. Furthermore, a concentration of workers, employees, students, etc. makes the percentage of human lives that can be lost or injured even higher. On the contrary, in farming areas the risks concerning material damage or loss of human lives are generally more limited. As for planning, the development of new housing, new activities and new infrastructures, as well as the expansion of existing ones, should take into account the vulnerability degree of the new constructions and settlements, relating them with the type, frequency and magnitude of the geomorphological hazards; that is, their risk should be properly assessed. The population increase and greater leisure time give rise to more and more unregulated urban developments, the construction of second homes and the pursuit of recreational facilities. The increasing need for land leads to the development of previously avoided hazard-prone areas or of coasts and mountains which are potentially more hazardous. Therefore, the evaluation and detailed zonation of the potential hazard zone are indispensable for adequate planning policy, together with the application of rigorous norms and restrictions on construction activities. Unfortunately such rulings always appear to be in contrast with the ever increasing competition for land and with commercial interests, and are therefore difficult to apply. Vulnerability should also be considered in relation to the degree of social organisation present, i.e., environmental education, civil defence, and prediction and monitoring techniques. Where people know and safeguard their territory, where the bodies governing intervention are efficient, and where advanced warning systems of major instabihties exist (e.g., river-spates, sea storms etc.) and finally, where such a level of social organisation is high, there will certainly be lower levels of vulnerability.

5.2 Risk mitigation As previously said, risk mitigation can be achieved either by reducing the hazard or by reducing vulnerability (Fig. 95). As regards geomorphological hazard reduction, there are two possibilities: process modification or hazard resistance.

217

RISK mitigation

LiA-TADf^ J i.HAZARD reduction

process modification

[ hazards resistance (active protection) % /... K .r-r^ A r^M .-xX . structural measures (passive protection) VULNERABILITY reduction v^ K / [ prediction, forecast

Fig. 95. Scheme of risk mitigation.

In the case of process modification, action should be taken regarding the causes of the geomorphological phenomenon: for example, in a process of solifluction water percolation into the ground could be eliminated; or in a process of stream erosion the water velocity could be reduced. Nevertheless, with some causes, such as those of a meteorological nature, it is difficult to intervene, considering the present stage of even the most sophisticated technologies. In the case of hazard resistance, it is opportune to prearrange particular measures to protect the environment subject to a geomorphological process: for example, the location and resetting of vegetative ground cover against soil erosion; or the construction of defence works against bank erosion, such as lateral walls built along riversides. This type of hazard reduction can also be defined as ''active protection" against the risk. Nevertheless, when some kinds of geomorphological hazards are to be faced, like certain mass movements, it seems difficult to find effective countermeasures, unless very high economic costs are accepted which in some cases might not be justified by the intervention works planned. Proceeding now to vulnerability reduction, with reference to exposure, there are two possible choices (Fig. 95): structural measures or prediction and forecast. Structural measures can be defined as "passive protection" against the risk: that is, specific construction norms for buildings, infrastructures, etc. should be adopted in areas considered as subject to some form of geomorphological instability. These specific engineering measures are applied considering for each case the entity and type of hazard as well as the expected lifetime of the structure. Nevertheless, this passive protection can hardly be comprehensive. In any case, the assessment of the magnitude and probability of the hazard and the evaluation of costs and benefits will allow the entity of the risk to be identified. As previously stated in section L6, it will be possible to choose the guarantees and the levels of protection which means making a political choice: the higher the risk threshold adopted, the more onerous and expensive it will be. As previously underlined, another way to reduce vulnerability is offered by the possibility to predict and forecast a geomorphological hazard. By means of these operations, risk mitigation is assuming more and more importance and a greater likelihood of results thanks to scientific and technological advances. An example is given by weather forecast, increasingly facilitated by remote sensing communications and computer elaborations. Another example can refer to mass movements which can be identified by means of monitoring systems and anticipated by simulation models of the geomorphological process.

218 5.3 Prediction and forecast It is important to define the terms "prediction" and "forecasting" and to point out the differences. In Uterature there are numerous, more or less analogous definitions, among which the most recent are given by Flageollet (1989), concerning the most specific geomorphological aspects, and by Smith (1992), concerning more general environmental aspects. For other definitions the bibliography quoted by the above mentioned authors may be referred to. The term prediction shows the probability of recurrence in a certain area of an event which has already occurred. It is based on statistical methods derived from historical data, catalogues, long-term monitoring operations, etc. Predictions are usually long-termed and can be both regional (e.g., earthquake-related landslides or meteorological events) and local (for example, the amount of erosion on a slope or along a watercourse). As for the recurrence of geomorphological events, for some of them (e.g., landslides) it is sometimes possible to establish the temporal occurrence of the phenomena and therefore a prediction, especially on regional terms (see Casale et al., eds., 1994). In other cases, such as those connected with man's activities, it is much more difficult to make predictions, unless regular and repetitive actions are taken into account (e.g., the systematic exploitation of a natural resource). The term forecast refers on the contrary to an indication of the timing, location and magnitude of an impending hazard. Forecasting can therefore take place only on a precise site and requires a detailed knowledge of the phenomenon, especially by means of monitoring systems. Specific examples are illustrated by Gasparetto et al. (1994) and Mantovani et al. (1994) for some landslides in the Eastern Dolomites. In some cases, in connection with the monitoring systems for hazard forecasting, there is also an "alarm system" which alerts the public of an impending risk. An example of an alarm system is offered by Fig. 96 showing the Tessina landslide, in the Eastern Dolomites. For the study, prediction and forecasting of natural hazards, including geomorphological ones, simulation models are often applied. They are a simplification and scale reduction of a phenomenon and are used with ever increasing frequency thanks to the development of instrumentation and the use of computers. The models that are most frequently used in Geomorphology are essentially of two types: physical and mathematical. Physical models are based on the laboratory reproduction of the physical characteristics of the phenomena studied: for example, the lithological, slope-angle, meteorological, etc. characteristics of a landslide are reproduced in scale in order to assess the evolution of a prototype and transfer the results to the real case. Mathematical or digital models make use of a computer receiving a certain number of data inserted by means of prearranged or specifically developed programmes and elaborated in order to simulate a natural phenomenon. These models can be applied though only in limited situations and there are elements of uncertainty connected with the problems of modelling the geomorphological process itself, of ensuring the correct representation of the choice of model assumption, of the weight

219 P4

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Fig. 96. The Tessina landslide alarm system (Belluno, Italy).

to be given to all different data categories, of the simulation of time, magnitude and frequency factors, etc. Mathematical models can be subdivided into deterministic, probabilistic and optimization models (Anderson ed, 1988). For a more detailed knowledge on these subjects the following articles are advised: Brundsen (1980), Thornes (1983), Woldenberg ed. (1985), Anderson ed. (1988) as well as those quoted in them.

5.4 Environmental education* *by Sandra PIACENTE

In recent years there has been considerable progress and advance in scientific and technological knowledge together with a growing interest and, in particular, an emotional involvement of public opinion. This has not been supported, however, by an adequate level of general education or by suitable educational strategies aimed at the general public. This situation is rooted in a rapidly and radically changing reality, mainly caused by man's impact on the environment, which has not been followed by proper development plans and environmental education programming. Indeed, the present situation shows that the knowledge of natural phenomena is generally inaccurate and scattered.

220 The knowledge of the Earth's evolution and dynamics are lacking at all levels: social, political, economic and educational. No effort is made to give a careful analysis of physical reality but, on the contrary, there is a dangerous tendency to improvise and elaborate partial and superficial notions on phenomena related to particularly catastrophic events, without pursuing a correct understanding of the causes and processes determining the phenomena and considering only the most evident aspects, which are certainly not the only ones. It is now evident that any strategy of intervention involving the natural environment becomes feasible only through a radical change of people's mentality. The general attitude of interpreting and using the surrounding environment according to one's personal needs should therefore change: this implies a participation and an independent ability to assess a situation which can be achieved only by means of an adequate and correct knowledge of the problems. This should take place with the conviction that knowing and learning compel people to assume a different mental attitude, i.e., an active role with respect to the environment in which they live, showing the way towards a new way of making, receiving and transmitting cultural values. The concept of evolution, i.e., the continuous dynamism of any natural system, is fundamental to any knowledge of the environment. A dynamic approach to this study can be correctly achieved at various, even extremely elementary levels, by breaking down a system into its basic components. In this way the knowledge of the real world will no longer be based only on superficial and factual data but will give the necessary importance to the continuous changes which often exert decisive influence on our lives, our culture, our economy as well as conditioning our future. The actual environment should be considered as the result of relations between natural and human elements, which can be ascribed to simple and complete systems whose variables have contributed to transform the Earth. The correct attitude to the environment means developing research which is historically-based in the widest possible sense, since this will allow the individuation of the disturbing factors which in various epochs interrupted the balance of this system forcing other elements to adapt. A disturbing factor, which often seems exceptional, when occurring in a sudden and striking way, will instead appear to be neither exceptional nor perturbing after an investigation of this type, since it will be seen that it was already present within the system, especially if the environment was receptive. The knowledge and identification of the most probable ''exceptional" events will allow them to be included in this system and the other elements to be prepared for coexistence. Furthermore, the actual dynamics of society should be considered because though it originated and evolved in the physical environment, society has developed independently into a system which appears as external to the main system, owing essentially to political and economic factors. This extraneousness of the social system is more apparent than real, basically because its rhythms of evolution are hardly ever comparable with those of the physical system. In any case, apart from shortcomings, inaccuracies and sensationalisms, in recent

221 years all mass-media have undoubtedly contributed in divulging new knowledge and creating a public conscience: the concern for environmental problems has become commonplace. This reversal of this trend of public mentality is the most noticeable and tangible result of years of images, articles, round tables and debates. Public opinion has matured new ideas and convictions: society cannot rely any longer just on aid, voluntary service and good-hearted feelings. It should rather pursue well defined political guidelines and prevention-type initiatives leading towards risk mitigation. Nowadays, at the end of the twentienth century, the protection of the environment is mostly considered as dynamic conservation, i.e., to increase in value all those aspects that make the environment significant and unique on account of its characteristics, origin, history and, especially, the life and activities that take place in it. From this viewpoint each environment, even the apparently most insignificant, should have its true value properly recognised and therefore protected, since in any case it contains original and specific features: the correct identification and knowledge of its distinctive characters are of paramount importance. This type of dynamic conservation, including all the initiatives and works capable of promoting and protecting a certain environment in order to make it directly accessible to the general public, should necessarily be put into practice by means of forms of knowledge and multiple management. Indeed, only through the understanding of its past, even the most remote one (such as its geological history), of its evolution mechanisms and equilibria, is it possible to identify those aspects and values, often unknown or forgotten, neglected or damaged, capable of providing correct elements for assessment and planning. Therefore the outcome is the definition of natural assets (such as geomorphological assets). If these are properly identified and understood, they are no longer seen from a merely aesthetic or spectacular point of view, nor do they belong only to the narrow field of scientific knowledge, but become a stable cultural heritage, an integral part of the formation and therefore of the education of all those that benefit from these assets. Knowledge is the basis on which a rational framework can be built in order to create a policy of correct use of the environment, i.e., conservation and improvement, with appropriate interventions both for protection and at a cultural, promotional, social, touristic and economic level. However, such integrated planning should not be limited to emphasizing acquired knowledge and expertise, but should aim at a deeper study of ideas previously just guessed at, preferably using local forces and resources, involving both young and old in a responsible role: the former in the educational and professional phases, the latter in the conservation and transmission of knowledge and values by recalling the past to prepare for the future. Of course each operation of cognitive research already has an intrinsic value, independently of any possible use that could be made of it, but in the case of environmental and cultural assets, knowledge nearly always triggers operative proposals which often promote ideas and opinions, and thus assume considerable educational significance. In the past few years the demand for knowledge regarding conservation and the

222

correct defence of Nature have become a clear social need, since people have become aware that these are the essential basic conditions not only for a better quality of life but also for the survival of the human race. Area management therefore becomes the usual field preferred for a comparison between individual and general interests, assuming that each hypothesis and research proposal should be realised not so much for giving immediate answers but mainly for asking the right questions correctly, which would support new ideas and incentives. A correct conception of balanced relationships between man and Nature is based upon two essential parameters: research and information. The former is developed by means of integrated and coordinated studies in the different sectors of natural, human and social sciences; the latter is developed by giving the citizen or, even better, the future citizen, adequate means to obtain knowledge. Indeed, the knowledge of environmental dynamics is a necessary requirement for understanding the problems resulting from equilibrium, unbalances and development and for trying to outline, at least at a cognitive level, forecast scenarios. Naturally, such a complex and absorbing project cannot be faced in an episodic or occasional way, with the incentive, for example, of particularly disturbing phenomena (such as violent earthquakes, volcanic eruptions, landslides, floods, pollution, etc.) but should be part of a well-constructed and continuous programme of information and education, with well defined scientific elements and essential data: it is not necessary to know "a lot" but ''the little" that is known should be correct. It often happens, in fact, that an extremely accurate knowledge is possessed on specific scientific topics whilst very little is known of the mechanisms and interactions that regulate Earth dynamics. It is therefore clear that an Environmental Education programme of this type should find an appropriate ground: in our opinion school education is the right area in which a stable and lasting intervention should be practiced. For the great majority, this kind of knowledge is received only or essentially in the period of compulsory education and in any case notions acquired at school are nearly always the starting point for wider and deeper studies. Also conferences, debates, television broadcasting and newspaper articles in general can accomplish the task of popularisation and spreading information very well, but only if they manage to avoid the temptation of following fashions or being sensational at all costs. Information should help in the perception, identification and, at least, in the partial understanding of natural phenomena. One should have the awareness that in order to face a natural phenomenon correctly it is necessary to approach it scientifically and not emotionally, that is, present it as a problem remembering that hypotheses must be formulated by means of direct observation and data acquisition. Indeed, only after the collection and analysis of data is it possible to verify the hypotheses and therefore draw some conclusions. Especially today, in a global historical, political, economic and cultural context, disciplinary problems have no reason to exist and should be solved with any possible means to obtain knowledge, in a close interaction between physical and biological variables. This complexity inevitably arouses some difficulties also caused by the spatial overlapping of elements

223 having different temporal evolution. Moreover, as regards the environment, it is clear that, besides forecasting, it is necessary to insist on the possibility of controlling the phenomena, or at least of some phenomena. In many cases in fact the forecast and understanding of events can be achieved, but they cannot yet be controlled. Environmental Education should have as a final goal a closer approach to Nature, not only by means of immediate instruments such as the eyes, the touch, the feelings and emotions, but also by means of the instruments of knowledge and conscience.

6 GEOMORPHOLOGY AND ENVIRONMENTAL IMPACT ASSESSMENT

6.1 Introduction When a project is undertaken there are several effects on the environment in which it is inserted. These effects can be of various types: quantitative and qualitative, direct or indirect, short or long-term, permanent or temporary, single or cumulative, positive or negative, etc., and include also those that can occur during the construction and decommissioning phases (see Wathern ed., 1988). Seldom have these environmental problems been a condition or a part of planning. Indeed, the aim of the impact procedures is to ascertain beforehand, by means of analytical approaches, whether the changes brought about in the environment permit the re-estabilshing of an acceptable balance in the use of environmental resources both for the defence of public health and people's living conditions. The feasibility of a project can be assessed on the basis of the results of these studies, in relation to the impact foreseen on the environment and the identification of risk levels. If the result is negative, studies will have to show the modifications which will have to be made to the enterprise so that it can be compatible with the environment, or possible alternatives that could reduce its negative effects. The importance of a precautionary evaluation of a project's consequences, has led to the definition of systematic and integrated procedures based on various technical, administrative, scientific, social, etc., parameters. Environmental Impact Assessment (EIA) is a procedure for assessing the environmental implication of a decision to implement policies, plans, programmes and projects. Thus, the scope of an EIA system could encompass all these undertakings at all levels of government. Lee & Wood (1978) have suggested a tiered comprehensive system applied to a sequence of action categories and administrative levels (Fig. 97). However, the vast majority of environmental impact statements in different countries have been related to projects. The first country that officially introduced the procedure of impact evaluation was the United States of America: in 1969 a law (NEPA = ''National Environmental

224

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Policy Act") established the principles of environmental policy in that country. It introduced a series of norms for the preventive evaluation of impacts: the ''Environmental Impact Statement" (Council of Environmental Quality, 1973). Afterwards, other countries, in particular France, the United Kingdom, Canada and Australia, adopted similar procedures. On 27 June 1985 also the European Community acknowledged this need and adopted a Directive making environmental assessments mandatory for certain categories of projects. This Directive is applied to projects that owing to their nature, size and location can determine a considerable impact on the environment. Two kinds of project categories should be taken into account in this evaluation: those that in any case produce serious effects on the environment and therefore necessarily imply an Environmental Impact Assessment (EIA); those that may have serious environmental consequences according to circumstances: for the latter category the compulsoriness of the EIA will have to be established for each individual case. As regards the impact studies, it is necessary to identify those environmental components which are not only particularly significant to the environment itself but also sensitive to the work that is going to be carried out in a certain territory; these are named environmental indicators. They are employed both in the description and interpretation of the environment, in the planning of the location of a certain project

225

and within the radius of influence of the project itself, as well as in the impact identification and evaluation phase, concerning the influence produced on the indicators. For identifying and assessing impacts, surveying and monitoring systems are used which refer to environmental indicators and transfer the data into adequate control lists or cause-effect matrices (see Leopold et al., 1971). A subsequent phase will consist in attributing a weight, that is an environmental value to each indicator, in order to assess impact and identify a priority order of importance. This allows corrective operations to be carried out in order to reduce or compensate the impact effects and assess the compatibility of the planned work with the environmental characteristics of the territory. Here it is not the case to go deeply into the complex general themes of EIA, regarding which the reader is referred to the above mentioned authors and their relative references, but in the following chapters the problems concerning Geomorphology more specifically will be dealt with. Also the conceptual and methodological bases used for some research of an European Union contract' (see Marchetti et al., 1995; and more in particular: Bollettinari, 1995; Panizza, 1995; Rivas et al., 1995; see also the final publication: Panizza et al., 1996) will be illustrated.

6.2 Concepts First of all, research must be carried out to identify the three main groups of geomorphological components, which may be treated differently (see Panizza, 1995): processes, landforms and raw materials (Fig. 98).

HAZARD (if hazardous, e.g. for landslides)

PROCESS — (e.g. cliff evolution)

> O -i

O X

LANDFORM (e.g. sea beach)

-^

ASSET (if valuable, e.g. for social value)

RESOURCE (if used, e.g. as sea-side resort)

RAW MATERIAL — (e.g. littoral deposits)

->

ASSET (if valuable, e.g. for economic value)

RESOURCE (if used, e.g. as sand quarry)

a. cc

O

o UJ

o

Fig. 98. Conceptual basis of the relationships between two of the geomorphological components (landforms and processes) and a project.

'Human Capital and Mobility Contract ERBCHRXCT 930311 (Coordinator: M. Panizza).

226 Figures 99 and 100 show in synthesis the conceptual bases of the simpHfied relationships between the above mentioned geomorphological components and the project. In particular, the processes which when hazardous are geomorphological hazards, may interfere with a project, which is always characterised by specific vulnerability and cost (Fig. 99). The activity of these geomorphological processes may produce damage for the project, that is to say a risk for the project. This is the case of a landslide (geomorphological hazard) which may damage a motorway (project). In this situation the natural component of the environment shows an active role and the project a passive role. Furthermore, particular landforms which if valuable are geomorphological assets, characterised by specific fragility and value, may be affected by a project (Fig. 99). The effects on geomorphological assets deriving from the implementation of a project make up a direct impact, which causes environmental damage for the same assets. This happens, for example, when the construction of a road ruins a glacial cirque. In this case the natural component of the environment has a passive role with respect to the project which plays an active role itself. Nevertheless, it should always be remembered that there are some processes that can be considered as beneficial (e.g., a particular kind of erosion which may be considered as an educational example). However, these processes produce a landform and then can be included in it. The same considerations can be made for particular raw materials which if valuable constitute geomorphological assets: the interferences with a project may produce a risk or a direct impact (Fig. 100). landforms

environmental damage

GEOMORPHOLOGY

| processes

damage to the project a = in active position p = in passive position

Fig. 99. Conceptual basis of the relationships between two of the geomorphological components (raw materials and processes) and a project.

227 raw materials

GEOMORPHOLOGY ^ processes Geomorphological b hazards j

damage to the project a = in active position p = in passive position

Fig. 100. Types of thematic maps for Geomorphology and EI A studies.

Also in this case there are some materials that can be considered as hazards (e.g., salty soils meta-stable sands). However, these materials are the result of particular processes and therefore can be included in the latter.

6.3 Methodology Research should be carried out following the scheme below (Panizza, 1995). 1) Types of Projects. 2) Investigation phases. 3) Mapping. 4) Indicators. 5) Evaluation of Hazards and Assets. 6) Evaluation of Impacts l.s. 7) GIS Methods. 6.3.1 Types of Projects The norms of the European Union for EIA's studies cover essentially the following types of projects of the 1st category, here subdivided into groups with analogous characteristics for geomorphological research: a) — refineries; — chemical plants, plants for coal aeration, asbestos mining processes, etc.; — thermic and nuclear power plants; b) radioactive and toxic waste plants; c) transport infrastructures (motorways, railways) and airports;

228 d) commercial harbours and navigation lines. Owing to the connections with Geomorphology, also the following projects from the second category can be taken into account: e) tourism infrastructures; f) land use change; g) mining activities. 6.3.2 Investigation phases The investigation phases can be defined as follows, with progressive increases of detail and scale. I = Reconnaissance — general evaluation — site selection: general studies on a small scale in a large area. II = Site and specific investigations: verification of the technical suitability of the selected sites. III = Detailed planning of engineering works: research and detailed measures. IV = Monitoring. V = Mitigation. There are moreover three steps normally followed for the development of a project: 1) design; 2) operation: — construction; — functioning; and 3) decommissioning. EIA investigations should be carried out in Phases I and II so that the results can be included in the detailed planning and design of the project (Phase III). This planning phase should include proposals of monitoring during construction and recommendations for mitigation measurements to be included. As construction proceeds, more information will become available, particularly about the materials and the subsurface conditions. Also a failure of the design may occur. In carrying out diagnosis and remedial measures it may be necessary to modify the predicted EIA. The procedures to be followed will, however, be the same. During the functioning operation it is recommended that the degree of success of the predictions and impacts is evaluated as a guide to maintenance routines and future design practice. In the decommissioning operation it will be necessary to design a new EIA, in order to predict the effects of the abandonment or dismantling of the plant (e.g., pollution, uncontrolled mine drainage, etc.). 6.3.3 Mapping The types of thematic maps used for these EIA studies are summarised in Fig. 101. They are both base maps and derived maps, to be used at different scales according to the investigation phases: for example a small or medium scale map for phase I, a detailed one for phases II or III.

229 3.3.a = geomorphological map 3.3.b= geotechnical map 3.3.C = hydrogeological map

}

}•

base

3.3.x = hazard map (from: 3.3.a - 3.3.b - 3.3.c) 3.3.y = landforms - assets map (from: 3.3.a) }^ derived 3.3.Z = raw materials - assets map (from: 3.3.a - 3.3. b) J Fig. 101. Conceptual and methodological scheme of the role of Geomorphology (landforms and processes) for the EIA and a project.

As for thematic base mapping, it is possible to make reference to various geomorphological, geotechnical and hydrogeological maps, well-known in literature. As for thematic derived mapping it is possible to make reference to some examples of stability and hazard maps made by some groups of researchers; however, examples of thematic maps concerning geomorphological assets are rarer. The derived thematic maps can be integrated with data from literature, archives, databases, etc. During investigation phase I more general maps like morphometric and morphographic maps can be used. For example, with respect to the hazard maps (concerning geomorphological processes) references may be made to the contents of chapter 3.9. On the other hand, as regards mapping of geomorphological assets, reference should be made to the contents of chapter 2.3. 6.3.4 Indicators In the specific literature various definitions of the term indicator have been given since this word may be applied to all the variety of environmental aspects, ranging from geological indicators to urban ones, to quote just two. Although varying considerably from one author to another, involved in Environmental Impact Assessment, all the definitions of this term show the need to emphasize in a significant and summarised way an environmental phenomenon. In the specific case, a geomorphological indicator should describe a situation, a geomorphological process and, according to the situations, also its evolutive trend and its susceptibility to an external intervention. Therefore appropriate indicators should be selected for each assessment investigation in order to acquire an exhaustive but concrete picture of the specific situation; at the same time, all those not strictly necessary should be omitted, in order to avoid accumulating an excessive burden of data which would make the work unmanageable from an economic point of view. Impact is expressed by the changes affecting an environmental unit following the implementation of a project. These changes will be emphasised just by the indicators which depend both on the environment and on the project. Example. Let us consider an arenaceous slope subject to natural erosion producing

230 sandy debris. The detritus accumulates in a riverbed at the bottom of a slope and is subsequently carried as far as the sea and deposited along the coast. It is obvious then that any human intervention on the slope will alter the pre-existing equilibrium. In fact, if the slope is totally or partially covered with concrete (project), erosion will either stop or decrease and therefore the production of detritus will tend to zero. As a consequence, the river will no longer receive the input of material that determined its pristine balance and the energy previously dissipated in the load and transport of the sandy sediments will be mainly directed to fluvial erosion activities. Supposing the riverbed is made up of clayey rocks, the material removed will no longer contribute to the replenishment of the pre-existing beaches, contrary to what happened when the transported material was made up of sand. Consequently the beaches, thus deprived of adequate solid supply, will retreat. This example is shown just to quote some of the processes that take place when the slope conditions undergo changes. Also other processes could occur, such as interference with the groundwater or undercutting at the slope toe. In such a case the process indicators to be found are: one in the fluvial system and the other in the coastal one. In order to evaluate indicators for hazards and assets, it is important to consider the scale and the density' of particular hazard processes and/or geomorphological assets. The lifetime of the project must be considered in order to evaluate the hazardous processes which can be at risk for the project or can be induced by the project. It is therefore necessary to prepare a list of indicators for hazards and assets. From this point of view, the frequency of the processes must be evaluated and compared with the lifetime of the project. According to the main groups of geomorphological components, the investigation phases and the scale of investigation, the indicators will be subdivided into three groups: A. Morphometric and morphographic. B. Hazards. C. Resources. 6.3.5 Evaluation of Hazards and Assets The evaluation of hazards should be developed using different methods: 1) Direct measures (e.g., on scarp retreat). 2) Mechanical models and calculations (e.g., geotechnical measurements). 3) Crossing of ''causes" (e.g., overlapping thematic maps). 4) Statistical approach of ''effects'' (e.g., recurrence of landslides). The first method consists of direct measures of some indicators. In the case of an active cliff the measure of the risk for a project on the top of the cliff itself, may be represented by the direct measure of the retreat of the slope or the variation in the slope angle, etc. The second method is developed using geotechnical measures and engineering models. For example in the cliff case geotechnical parameters and engineering models may be used in order to evaluate the retrogradation of the cliff.

231 The third method takes advantage from the crossing of different thematic maps in which the indicators are considered. For example, in the case of the above mentioned chff the evaluation should be estimate by the overlapping of different maps such as lithological, morphometric, vegetational, etc. The fourth method requires a good knowledge of many parameters in a large area. On the basis of statistical behaviour a forecasting of hazardous events can be evaluated. For example, in the cliff case, the retreat of the slope can be predicted on the basis of the general retreat along all the coast. The evaluation of assets is described in 2.3. The hazards and assets will be related to the following groups of morphogenetic units: 1) Weathering. 2) Slope: a) soil erosion/sedimentation; and b) mass movements. 3) Periglacial (including special mass movements). 4) Glacial. 5) Fluvial. 6) Coastal. 7) Aeolian. 8) Karst. 9) Subsidence. 10) Groundwater. 6.3.6 Evaluation of Impacts Starting from Figs. 98 and 99 it is possible to indicate a conceptual and methodological scheme of the role of Geomorphology for the EIA of a project, with the specification on how the active and passive elements combine in giving different types of impact is. (broadly speaking) (Cavallin et al., 1994). Figure 102 takes into account the geomorphological components landforms and processes. Beside the consequences in terms of risk and direct impact shown in section 6.2, a project during its implementation, functioning and decommissioning, may produce induced hazards, i.e., hazards which did not exist in the area before the introduction of the project. These induced hazards may give rise to three kinds of induced direct and/or indirect effects: direct risk, indirect risk and indirect impact (Fig. 102). Direct risk can be delineated as the effect on the project of a hazard induced by the project to itself; in this case a reflexive action takes place. For example, the construction of a road may cause the instability of the slope where this road is built, thus endangering the project. Indirect risk consists in a hazard induced by a project which damages the surrounding settlements. This is the case of a landslide induced by a road cut, which endangers a village located in the vicinity of the project.

232

INDIRECT RISK

environmental damage

K

""T^ on the surrounding settlements

RISK T

damage to the project a = in active position p = in passive position r = in reflexive position

Fig. 102. Conceptual and methodological scheme of the role of Geomorphology (landforms and processes) for the EIA and a project.

Finally, indirect impact refers to the effects of hazards induced by a project on geomorphological assets existing in areas surrounding the same project. For example, the filling of a lake, which is considered a geomorphological asset, due to a landslide triggered by the construction of a road. The same considerations can be made by taking into account the geomorphological components processes and raw materials. For the evaluation of the different types of impact l.s. mentioned above, for each type of project (6.3.1) and for each investigation phase (6.3.2), the main research instruments are the maps (6.3.3) and the indicators (6.3.4). 6.3.7 GIS techniques Geomorphologists have long recognised the importance of morphometric studies. The availability of altitude data in digital format, and the possibility of preparing and analysing Digital Terrain Models (DTM) may be important tools for quantitatively analysing topographic elements (see Pike, 1993). Software packages specifically designed to produce high fidelity DTM are now available (e.g., Carla et al., 1987; Carrara, 1988) and to produce derivative maps such as slope, aspect and so on. The principal aim is to design and apply a structured method for EIA studies in the field of geomorphology, including data collection, updating, modelling and analysis, using GIS techniques to automate and optimise the decision-making processes. Part of the required parameters for an evaluation of the impact on assets consists of subjective analytical expertise, weight setting, application of nonspatial indices, etc., that cannot be applied directly in a GIS. Nevertheless, some of the criteria need a spatial definition a priori, such as all phenomena occurring in the study area or in

233 the area of influence where GIS apphcations can be partially or completely included. In most cases, though, it becomes evident that a GIS represents a convenient tool for interpreting the data necessary for the assessment in terms of feasibility analysis, robustness of the impact measurements, sensitivity and comparability analysis with spatially referenced information. Much work is still required, however, to bring spatial data analysis, geomorphology and EIA into the realm of routine procedures using widely accepted and significant standards (Patrono, Fabbri & Veldkamp, 1996).

6.4 Quantification of impact* *in collaboration with Mauro MARCHETTI

6.4.1 Raw materials The methodology proposed by Rivas et al. (1995) is presented below. It has been used in the already cited Human Capital and Mobility Contract. The types of direct impact on geomorphological raw materials which can take place as a result of different activities are: — consumption as a consequence of direct extraction; — sterilisation as a result of activities which make the resource unusable; — permanent sterilisation; — temporal sterilisation; and — degradation due to pollution which can alter the properties of the material. In order to quantify the impact on existing raw materials in a given area, the following parameters have to be considered: V: total volume of the deposit (m^); v: % total volume affected; u: % of useful material; P: price ($m'^). The price of a resource is usually directly related to its quality, rarity and exploitability. R: reversibility of action (dimensionless: 1 to (-1)). Obviously the degree of reversibility depends on the type of project; we can consider the minimum reversibility (-1) for those projects which imply the irreversible covering (construction on top) of the deposit for an unlimited time (nuclear power station, urban areas, roads, dams, etc.); reversibility near to 0 is applied to activities such as forestry, agricultural uses or holiday villages, where the technical measures for decommissioning are more feasible (-0.01 times years of implantation of the activity); -0.5 for legal limitations, such as National Parks, etc. When the project does not limit the use of the resource, the value of reversibility is 1; a: relative abundance or rarity is the affected resource volume or area divided by the total resource volume or area (dimensionless); the geographical area used as reference could be the region, province, or the area within a certain distance of the project; A: accessibility is a measurement obtained by the division of the number of potential

234 users (inhabitants) within a certain radius of the resource and the distance (D, in metres or km) of the resource from the nearest road (H/D); the quotient A^JA^^^, will always be <1 since accessibility after a project will never be less than before. The direct impact on raw materials (/,) can be expressed as a potential monetary loss by means of the following expression: A "''"'"'''' A

I = V'vuP'R'A'

postproject

The total direct impact on raw materials {Ij^) would be:

i- 1

where n is the number of outcrops. 6.4.2 Landforms Here an investigation methodology set up and applied in the already quoted Human Capital and Mobility Contract research is illustrated. It represents a synthesis of some methodological proposals and a combination into a minimum approach to assess Quality and Impact on geomorphological landforms after a project (Marchetti, Panizza and Patrono, 1996). In order to evaluate the direct impact on landforms, it is necessary to assess the quality of each of them. Here only the scientific attribute, as defined in paragraph 2.2, will be considered, since it is peculiar to Geomorphology. The scientific quality^ of each landform (Q) can be considered as depending on two main characteristics: intrinsic scientific value (V) and degree of presentation (C). These may be related as follows: G = V* C Intrinsic scientific value can be defined from the geomorphological standpoint as: V = L ir G + L ic O + L ir P - L ir E g

a

p

e

where G, O, P and E are the characters of the landform (see § 2.2) and in particular: G = model of geomorphological evolution; O = object used for educational purposes; P = paleogeomorphological example;

235

E - ecological support. In the last equation they are (usually) equal to 1, unless one of them is put in evidence (then >1) or, on the contrary, is disregarded (then <1). L^, L^, Lp and L^ are the levels of interest of the landform (considering here its rarity) (see § 2.2) and particularly the following weights: L worldwide =1 L super-regional = 0.75 L regional = 0.5 L local = 0.25 no interest =0 The condition of preservation ( Q of a landform is connected to its conditions at the survey moment and can be quantified as 0 < C < 1 (1 = well-preserved). For the assessment of the direct impact the procedure is as follows: The degree of damage (D) to a landform can be calculated as a consequence of the implementation of a project. Due to the possible variations from case to case, is is better to consider its value in a continuous range: 0 < D < 1. As a suggestion, for the example that follows: 1 = no damage, well-preserved; 0.75 = some deterioration with loss of some minor elements; 0.5 = degraded owing to some human activities which hide part of the features; 0.25 = several human activities which deteriorated the characteristics of the landform; 0 = characteristics of the landform destroyed. The direct impact on a landform (//) can be expressed in terms of reduction of scientific quality, as:

/ = Q - e * / ) = G*(l-^) and if normalised: ^ ' "

^ ^Q-Q^D) Q

^ Q^

{\ - D) Q

The deterioration is compared to the quality of the most representative or ideal situation found in the area. The total direct impact on landscape (ly^) can be calculated as:

'

n Q -*^max

where n is the number of landforms considered as geomorphological assets in the studied area and Q^,^, = Max {Qj,..., QJ.

236 The indices of impacts here presented consider the impacts as a loss in terms of quaUty; in particular the closer to 1 are /, and Ij^ the greater is the damage to the landforms themselves. Other relevant parameters to be taken into account are visibility and reversibility, which are, in the case of landscape, directly related to mitigation procedures. 6.4.3 Processes A methodology proposed by Bollettinari (1995) and applied in the already quoted Human Capital and Mobility Contract is here illustrated. This methodology is founded on the principles which were presented in paragraph 3.9 and is used for the assessment and mapping of the geomorphological hazards. According to this methodology, each unstable area is catalogued and accompanied by a detailed list of the processes in action and/or the causes favouring instability. Significant parameters for the quantitative evaluation of hazard, or the environmental components which cause instability, should be established for all the territory affected by the project. The factors favouring environmental degradation (F) are the lithological, hydrological and climatic conditions as well as the indicators related to possible processes taking place in the study area (/). Whereas the former offer on the whole a picture of the actual or potential causes of instability, the latter show the effective processes in action at the moment of investigation. Each parameter chosen is considered separately from the others and quantitatively assessed through the attribution of a weight for each unstable area thus identified during the first phase of the research. The weight is attributed by the operator on the basis of the knowledge acquired during field investigations, with reference to previously fixed standard values. The sum of the scores attributed to each parameter (F and /) gives for each area the assessment of hazard (V^J. F^ ^ F, ^ ... ^ F 1

2

- L ^ I, ^ ... - I /!

1

2

= V n

px

(for area x). On the other hand, the sum of the hazard's values obtained for each area gives an appraisal of the global hazard for the territory examined {V^TOT) before the accomplishment of the project. V px + Vpy + ... + Vpz = y pTOT rnr

The same procedure for the attribution of weights to each parameter chosen must be repeated with reference to the hypothetical situation which would be determined after

237 the completion of the project. The value thus obtained {VpTor) provides information on the hazard conditions affecting the territory investigated following the accomplishment of the works (or gives information on territory instability after the project has been carried out. It should always be kept in mind that a V'^TOT ^^^ue greater than V^joj shows a worsening of stability and therefore emphasizes the presence of impacts). The difference between the value {VfyTor) obtained from the evaluation of the global hazard after the implementation of the project and the one calculated in the real situation (that is before the execution of the works) gives the quantitative measurement of the modifications induced in the geomorphological environment, that is the induced hazard (see § 6.3): H =V

- V pTOT

^pTOT

A further stage of the research implies that the impact value should be again recalculated taking into account the hypothetical final situation existing after the accomplishment of possible mitigation measures capable of eliminating or mitigating the recognised impacts. Hazard {V\TOT) ^i^l have to be recalculated in these hypothetical final conditions using the above described method. Finally, from the difference between the hazard after the accomplishment of mitigation measures and the hazard assessed after the implementation of the project, the value of residual induced hazard will be obtained: / / = V" r

'^

pTOT

_

y pTOT

The data thus obtained are summarised in a specific thematic map which can be easily consulted even in subsequent phases of the project.

7 CONCLUSIONS

As a conclusion of this book, in the prospect of a cultural union with and in the hope of an integration between environmental values and humanistic disciplines, I would like to mention the interrelationships between geomorphology, or more in general geoscience, and cultural resources and, in particular, archeological, historical and architectural ones. I think that this topic is particularly important today since we are living in a period where the desire to preserve and emphasize these examples of cultural heritage is particularly felt as well as the awareness that, following natural events or human impacts, these cultural assets could be definitively lost. These relationships may be summarised briefly as follows (Panizza & Piacente, 1990, 1991).

238 1) Environmental risks: from hazards that are earth-related (erosion, earthquakes, volcanoes, subsidence, etc.), water-related (floods, sea-storms, changes of level of the water table, water pollution, etc.), air-related (hurricanes, tornados, air pollution, etc.) and hazards related to living beings (biological infestation, human impacts, degraded areas, etc.). 2) Causes and effects: on the one side, the environmental factors that have led to the construction and the type of the cultural resource in question; on the other, forecast of the consequences on the environment of the various types of conversion possibilities, both structural and those regarding its usage. 3) Environmental education: informative and educational programmes for an integrated knowledge of the environment in which the cultural resource is situated and promotion of environmental awareness and consciousness. As regards geological risks, i.e., earth-related risks, it is sufficient to cite the following as examples in Italy. Orvieto, Agrigento, Volterra, etc. for slope degradation. Numerous cities known for their artistic importance in Friuli and Irpinia that were damaged by the recent earthquakes of 1976 and 1980. Naples and Catania for volcanic hazards. Venice and Ravenna for subsidence-related problems. The example of Florence alone is sufficient to recall a historic city that has recently experienced serious damage to its artistic heritage as a consequence of river flood. Together with other artistically endowed cities, Venice can serve once again as an example to indicate the environmental factors that have conditioned a certain type of urban design and architecture. At the same time, Venice itself is at risk of being stifled and destroyed by an extremely intense tourist pressure in its historic centre, and by irrational industrial development in the surrounding areas. A strong emphasis on an interdisciplinary approach is required for such complex research which intends to focus on the functional-historical lines of urban development and to interpret the comprehensive series of events of each archeological, historical and architectural monument that make them up. This emphasis will have to be strong enough to stimulate the joint efforts of various specialised fields (geomorphology, archeology, petrology, anthropology, environmental sciences, natural sciences, economics, the arts, social sciences, etc.). Only the coordinated efforts of these specialised fields will permit an understanding of the historic structures within the framework of their specific morphology and dynamics. Therefore it will be possible to offer, to those persons responsible for making decisions, the means to choose those agencies most capable of evaluating the functional end economic benefits of each individual project. Assessments of the economic and social yield of the preservation and restoration projects, the criteria for reconstruction and conversion choices, the indications regarding the type of use most suited for these cultural resources or parts of them, will also have to take into account the above-mentioned relationships. The results of that must promote environmental consciousness, by means of studies that start from the analysis of the most remote causes, such as geological factors, and eventually arrive at the most current causes, such as socioeconomic implications. In fact, this cultural approach, which is based on the knowledge of the

239 components that have determined and regulate to date the evolution of the various characteristics of an area, represent the only proper instrument to create an awareness of the environment and the indispensable premise to its correct use.

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263

Biography

Mario Panizza, professor of Applied Geomorphology and Head of the Earth Science Department at Modena University (Italy), doctor ''honoris causa'' in Geography at Cluj University (Romania), president of the European Centre of Geomorphological Hazards in Strasbourg (France), and member of the Executive Committee of the International Association of Geomorphologists. He has carried out scientific activity and professional work in numerous countries around the world.

Professor Mario Panizza

265

INDEX Abrasion 42, 88, 90, 112, 113 Accelerated erosion 37, 205 Active fault 185 Active landform 193 Active tectonic element 171 Aeolian erosion 53 Aerial photograph 78, 161, 169 Aesthetic 28, 210, 221 Agriculture 201, 203 Alluvial plain 95, 99, 106 Alluvial-fan 102 Animal-farming 202 Archeology 238 Aspect 16 Asset 9, 26, 31, 221,230, 232 Assets map 229 Atomic power 205 Avalanche 136, 137, 141, 152, 156, 160, 162, 208 Avalanche hazard 156 Avalanches 23 Azonal 197 Backshore 115 Backwash 110 Badland 59 Bank erosion 91, 110 Bathyal plain 120 Bayhead beach 117 Beach 115, 122, 209 Biostasis 36 Black-glacier 139 Block-slide 181 Bottom erosion 91, 93, 98 Breaching 140 Calanco 51 Canyon 120, 129, 137 Capable fault 167 Cavitation 88, 90, 112, 113 Channel erosion 63 Channel pattern 98, 99 Classification 2, 66 Cliff 113, 121, 127, 128

Climate 193 Coastal degradation 112 Coastal erosion 42, 112, 114, 122 Coastal hazard 120 Coastline 118 Conservation 11 Continental shelf 119, 125, 126, 129 Continental slope 119, 128 Corrasion 49 Creep 54 Culture 26, 210, 212, 221, 237, 238 Cuspate foreland 118 Dam 22 Database 33, 76 Debris fall 176 Debris flow 71, 75, 101, 129, 136, 146, 162 Debris slump 185 Degradation 90 Dendrochronology 75, 76 Desertification 36 Digital Terrain Model 84 Digitalisation 34 Dormant landform 193 Drop impact 44, 55 Dune 117, 135 Dynamic conservation 221 Earth flow 179 Earth material 11 Earthquake 139, 141, 148. 165, 173, 177, 179-181. 183, 185, 222 Ecological support 27 Economic value 211 Education 27. 213, 219 EIA (Environmental Impact System) 7, 28. 223, 225-228, 231, 233. 234 Electric energy 19 Endogenetic 1 Environmem 4. 5, 10, 28, 135, 197, 201, 205, 206, 209-211, 220, 223, 225, 226, 236, 238 Environment 6 Environmental 7

266 Environmental catastrophism 206 Environmental education 214. 219. 238 Environmental indicator 224 Environments 14 Equilibrium 210 Erosion 37 Erosion agent 198 Erosion factor 38 Erosion hazard 37 Erosion pavement 54 Erosional platform 114 Exaration 49 Exogenetic 1 Fall 67 Farming 204 Fault scarp 167 Flood 92, 93, 95, 100, 102, 106, 222 Flood plain 95, 103, 108 Floods 98 Flow erosion 46 Flow 68 Forecast 217, 218, 222 Forecasting 86, 223 Foreshore 115 Geographical Information System 32. 75. 87. 232, {see also GIS) Geological erosion 37 Geology 2 Geomorphological 191 Geomorphological assets map 229 Geomorphological convergence 4 Geomorphological dynamics map 193. 196 Geomorphological hazard 6, 7, 35, 92, 190. 193, 194, 216. 217. 226 Geomorphological hazard map 194 Geomorphological indicator 229 Geomorphological map 21, 31. 80 Geothermal energy 25 GIS 232 Glacial hazard 135 Glacier 25 Glacier's dynamic 143 Glaciopressure 25, 150 Global hazard 236 Global Positioning System 84 Groundwater pressure 85 Gully erosion 51, 57, 59, 60

Hazard 5. 230 Hazard map 189. 191. 194. 229 Hazard probability 215 Hazard reduction 216 Hazard resistance 217 Headward erosion 101 Historical archive 73. 183 Historical records 164 Human activity 5 Hunting 201 Hydrogeology 192 Hydrographic network 99, 102 IMPACT 6. 7, 13, 15, 28. 119, 125, 198, 206. 209, 211. 223. 225. 228, 231, 234, 237 Impact 9 Impedance 193 Inactive landform 194 Indicator 225. 229. 232 Innovation 11 Inshore 115 Instability 35. 190. 191 Integrated analysis map 191, 196 Inundation 91. 106, 107 Isoseismic-line map 183 Jokulhlaup 147. 162 Karst 179 Lagoon 118. 124 Lahar 149. 188 Land use 63. 193 Landslide 22. 135 Landslide causes 68. 70 Landslide dating 75 Landslide hazard 64. 66. 81, 86, 87 Landslide hazard assessment 65 Landslide inventories 74 Landslide investigation 72, 73 Landslide susceptibility 87 Lateral spreading 182 Lava 187 Lava flow 187 Littoral barrier 118 Longshore bar 117 Longshore current 111 Man 6. 26. 65. 119, 126, 197, 220 Map 19, 80

267 Map of geomorphological assets 31 Map of slope aspect 18 Mapping 79 Marine hazard 110 Meander 99, 100, 105 Men 234 Meteorological parameters 85 Microclimatic changes 203 Mitigation measurements 228 Model 3, 55, 58, 60, 63, 88, 183, 209, 218, 230 Monitoring 81, 84, 218, 228 Morpho-neotectonic 166, 169 Morpho-neotectonic map 171 Morphoselection 49 Mountain climbing 149 Mud flow 149 Natural equilibrium 209 Natural resource 207 Natural system 210, 214, 220 Nature 198, 222 Neotectonic 167, 173, 183 Neotectonic map 172 Newmark method 185 Newspaper 222 Normal erosion 37

Raubwirtschaft 197 Raw material 7, 8, 13, 19, 28, 206, 225, 232. 233 Reflection 110 Refraction 118 Remote sensing 76, 168 Reserve 8 Reservoir 21 Resource 5, 8, 14, 16, 19, 25, 27, 36, 126, 197. 205, 206, 212, 218, 223, 230, 234. 237 Retrogradation 116 Reynolds' number 38 Rhexistasis 36 Rill erosion 57, 59 Rill flow 46 RISK 6, 7, 9, 26, 65, 98, 121. 122, 130. 135, 156, 164, 188, 205, 209, 211, 214-216, 223, 226, 228, 230, 231, 238 Risk mitigation 8, 217 River degradation 88 River erosion 88, 90, 94 River hazard 88, 92 River transport 98 Rock avalanche 75, 180, 185 Rock fall 176, 180, 181, 185 Rock glacier 159 Rock slide 180, 185

Offshore 115 Paleoseismology 185 Pedogenesis 15, 16, 36 Pedology 60 Periglacial hazard 135 Permafrost 160, 164 Photogrammetry 79 Piping 50 Planning 216 Plunging breaker 110,116 Pollution 206, 212, 222 Population 201, 213 Prediction 86, 138, 217, 218 Progradation 116 Proluvial fans 91 Pseudokarst 50 Quatemar 3 Quaternary 2, 127, 168 Rain erosion 43

Sarma's method 185 Scenic 28 Schorre 118 Seismic activity 174 Seismic amplification 175 Seismic hazard 165, 187 Seismic risk 165, 187 Seismic shock 72, 174, 176, 177, 180 Seismic susceptibility 165, 172 Slide 67 Slikke 118 Slope angle 18, 19, 117, 174, 176-178, 181 Social system 220 Soil 14. 36, 43, 50, 53, 55, 60, 202, 205, 207 Soil avalanche 180 Soil erosion 36-63, 36, 38, 60, 63 Soil slide 185 Soil vegetation 38 Solar energy 18 Solar energy map 19 Solar radiation 16

268 Spit 118 Splash 44,49 Spread 68 Stokes' equation 40 Subsurface flow 49 Suffosion 50 Surface deformation 83 Surface erosion 37 Surface fault 185, 186

Tectonic fracturing 182 Television 222 Terrace 13, 176 Thematic maps 228 Tides 111 Tombolo 118 Topography 192 Topple 67 Transport capacity 47, 48 Tunnel erosion 50, 51

Upland erosion 63 Vegetation 202, 203, 208 Vibrating table 177 Vibration 175 Volcanic eruption 222 Volcanic hazard 187 Volcanic risk 187, 188 Von Karman's universal constant 39 Vulnerability 5, 7, 64, 165, 214, 215, 226 Vulnerability reduction 217 Wandering 99 Water 19 Water bubbles 145 Water erosion 40, 42, 55 Water pocket 144 Wave refraction 110, 111 Weather 85 Wind 53, 111, 118, 205 Wind erosion 53 21-130 01-210