WORLDBANK TECHNICALPAPER NUMBER 71 SECTORAL LIBRARY Reservoir Sedimentation WTP71 Public Disclosure Authorized Impact, Extent, and Mitigation September1987 K. Mahmood Public Disclosure Authorized Public Disclosure Authorized Public Disclosure Authorized *396 *M25 .-. FILE C RECENT WORLD BANK TECHNICAL PAPERS No. 20. Water Quality in Hydroelectric Projects: Considerations for Planning in Tropical Forest Regions No. 21. Industrial Restructuring: Issues and Experiences in Selected Developed Economies No. 22. Energy Efficiency in the Steel Industry with Emphasis on Developing Countries No. 23. The Twinning of Institutions: Its Use as a Technical Assistance Delivery System No. 24. World Sulphur Survey No. 25. Industrialization in Sub-Saharan Africa: Strategies and Performance (also in French, 25F) No. 26. Small Enterprise Development: Economic Issues from African Experience (also in French, 26F) No. 27. Farming Systems in Africa: The Great Lakes Highlands of Zaire, Rwanda, and Burundi (also in French, 27F) No. 28. Technical Assistance and Aid Agency Staff: Alternative Techniques for Greater Effectiveness No. 29. Handpumps Testing and Development: Progress Report on Field and Laboratory Testing No. 30. Recycling from Municipal Refuse: A State-of-the-Art Review and Annotated Bibliography No. 31. Remanufacturing: The Experience of the United States and Implications for Developing Countries No. 32. World Refinery Industry: Need for Restructuring No. 33. Guidelines for Calculating Financial and Economic Rates of Return for DFC Projects (also in French, 33F, and Spanish, 33S) No. 34. Energy Efficiency in the Pulp and Paper Industry with Emphasis on Developing Countries No. 35. Potential for Energy Efficiency in the Fertilizer Industry No. 36. Aguaculture: A Component of Low Cost Sanitation Technology No. 37. Municipal Waste Processing in Europe: A Status Report on Selected Materials and Energy Recovery Projects No. 38. Bulk Shipping and Terminal Logistics No. 39. Cocoa Production: Present Constraints and Priorities for Research No. 40. Irrigation Design and Management: Experience in Thailand No. 41. Fuel Peat in Developing Countries No. 42. Administrative and Operational Procedures for Programs for Sites and Services and Area Upgrading No. 43. Farming Systems Research: A Review No. 44. Animal Health Services in Sub-Saharan Africa: Alternative Approaches No. 45. The International Road Roughness Experiment: Establishing Correlation and and a Calibration Standard for Measurements No. 46. Guidelines for Conducting and Calibrating Road Roughness Measurements No. 47. Guidelines for Evaluating the Management Information Systems of Industrial Enterprises No. 48. Handpumps Testing and Development: Proceedings of a Workshop in China No. 49. Anaerobic Digestion: Principals and Practices for Biogas Systems No. 50. Investment and Finance in Agricultural Service Cooperatives (List continues on the inside back cover.) TO WORLD BANK TECHNICAL PAPER NUMBER 71 Reservoir Sedimentation Impact, Extent, and Mitigation K. Mahmood The World Bank Washington, D.C. The InternationalBank for ReconstrLction and Development IHE W\ORLD BANK 1818H Street,N.W Washington,D.C. 20433, U.S.A. All rights reserved Manufacturedin thc United Statesof America First printing September1987 TcchnicalPapers are not formalpLubLlicatiolns of thec World Bank.and af-CCil cLlated to encouragediscussion and commenitantd to commullicatethe resultsof thc Bank's work quicklyto the developmentcommunity: citatioln and the useof thesepap.er s shouldtake accountof their provisional chiaracter.The findlings,interpretatiois, and conclusionsexpressed in this paperare entirely thosc of thc autJor(s and shouldnot be attributed in any maninerto the VVorldBank. to its affiliated organizations,or to membersof its Boardof ExecuitvxeDirectors or the COUlnttriesthCe r epresent.Any m.aps that accompanythc text have beenp.repal-red solely for-the' Colnvlniece 1of readers; the designationsand presentationof material in themiido not implV thc expressiollof ally opinion whatsoeveroni the part of the WVorldBanik. its affiliates,or its Boardnor member countries conceriiing the legal status of any)Country. territory, city, or alca or of the authoritiesthereof or concerning thedelimitation (f its b5ouni;dariesor its national affiliation. Becauseof the informality and to presentthe resultsof research\ ith thc least possible delay, the typescript has not been prepared in accordance ith thc Procedures appropriateto formal printed texts, and tile Worldi1Bank accepts no) respionsibility for errors. The publication is supplied at a token charlge to defray part of the cost of manufactureand distrib1utioll. The most recent World Bankputblicationis are desciribedin the catalog 'fezv P'tiblications,a newv edition of which is issued in thle sprinig and fall of eacih year. Tle comnplete backlist of publications is shown ill the annual h1lex1oI'Plfbhaionwls. whichi contains an alphabetical title list and indexes of subiects, authors, anidCoulitries antd regions; it is of value principally to libiraries and institutiolnal purchasers. The latest edition of eacihof these is available free of charge fromii the Publications Sales Unit, Department F. The World Bank. 1818 H Street. N.W. Washington. D.C. 20433, U.S.A., or from Publications, The World Bank, oo, aJVCnuvLed'1na, 75 i 1o Paris, France. K. Mahmood is Professor of Enginicering at The George Washington University, Washington, D.C., and a consultant to the World Bank. Library of Congress Cataloging-in-Publication Data Mahmood, H. Reservoir sedimentation. (World Bank technicalpaper, ISSN 0253-7494 no. 71) Bibliography:p. 1. Reservoir sedimentation. 2. Water resources development. I. Title. II. Series. TD396.M37 1987 628.1'32 87-23003 ISBN 0-8213-0952-8 ABSTRACT The role of storage reservoirsin water resource developmentis described. It is pointed out that whereas the future demandswill require additions, the present capacity is being continuallyeroded by siltation. It is estimatedthat on a world wide basis the replacement cost of the capacity annually lost to siltation is around $6 billion. The world picture of erosion and sediment yield from drainage basins is reviewed to show that the world average yield at ocean level is a modest 500 ppm, but large variations exist and local values can be much higher due to natural conditions. Human actions and natural events that further effect sediment yields are illustratedwith case histories. Physical phenomenarelated to reservoir siltationare describedto provide a basic understanding of the problem. This is followedby a critical evaluation of currentlyavailable predictive methods. Finally, a fairly complete survey is presented of the design and operationalstrategies that can be used to alleviatereservoir siltation. Importantareas of research and developmentare identifiedand it is recommended that in view of the magnitudeof this problem, a concertedeffort should be undertaken. liii ACKNOWLEDGEMENTS This study has been sponsored by Agriculture and Rural Department of the World Bank. It was partiallysupported by U.S. National Science FoundationResearch Grant No. CEE-8313603. The writer expresseshis deep appreciationto Guy LeMoigne, Irrigation Advisor, the World Bank, for his encouragementduring this study. iv TABLE OF CONTENTS List of Figures vii List of Tables viii PREFACE ix I INTRODUCTION 1 II MAGNITUDE OF THE PROBLEM 5 III EROSION AND SEDIMENTATIONIN DRAINAGE BASINS 9 Weathering Processes 10 Erosion 11 Sediment Delivery Ratio 12 World Wide Rates of Erosion and Delivery 15 Human Impact on Sediment Yield 26 Impact of Natural Events 29 Measurementof Sediment Load 33 Special Considerations 34 IV RESERVOIR SEDIMENTATIONPROCESSES 35 Sediment Size 35 Entrainment 38 Suspension 42 Fine Material Load 43 Bed Material Load 44 Unit Weight of Deposits 45 Delta Formation 48 Fine Material Deposit 51 Density Currents 51 Erosion of Fine Material 55 V PREDICTIVE METHODS FOR RESERVOIR SEDIMENTATION 57 Trap Efficiency of Reservoirs 58 Spatial Distribution of Deposits 60 MathematicalModels 64 Evaluation 68 VI MITIGATION OF RESERVOIR SILTATION 71 Watershed Management 72 Debris Dams 82 Sediment Bypassing 83 Sediment Flushing 83 Sediment Sluicing 93 Density Currents 98 Sediment Dredging 100 v VII SUMMARY AND RESEARCH NEEDS 102 Summary 102 Research Needs 109 Sediment Yield 109 Sediment Diffusion in Deep Flows 110 Sediment Reentrainment 110 Density Currents 111 Empirical Methods 111 Mathematical Models 111 REFERENCES 113 vi List of Figures 3-1 Sediment Yield vs Drainage Area 27 4-1 Fall Properties of Sediment in Quiescent Water 39 4-2 Shield's Critical Shear Stress Diagram 41 4-3 Sediment Deposit in Reservoirs 47 4-4 Profiles of Typical Reservoir Delta and Nomenclature for Deposits 49 5-1 Brune's Curve for Reservoir Trap Efficiency 59 6-1 Schematic Catchment of River Jhelum at Mangla Dam 74 6-2 Measured Suspended Load for Jhelum River at Azad Pattan: 1979 and 1983 Data 78 6-3 Measured Suspended Load for Jhelum River at Karot: 1969 and 1979 Data 79 6-4 Measured Suspended Load for Kanshi River Near Palote: 1970 and 1981 Data 80 6-5 Measured Suspended Load for Punch River Near Kotli: 1966 and 1980 data 81 6-6 Design Operating Program for Roseires Dam - Median Inflow and Full Use of Storage 97 vii List of Tables 2-1 EstimatedAugmentation of Base Flow by by Storage Reservoirs 7 3-1 Annual Water and SedimentYield of World's Rivers 3-2 Annual Water and SedimentYield of World's Rivers, Drainage Area 21 3-3 Annual Water and SedimentYield of World's Rivers, by Unit