2 14. 8 5 M E HODS OF COMPUTING SEDIMENTATION IN LAKES AND RESERVOIRS A contribution to the International Hydrological Programme, IHP - II Project A. 2.6.1 Panel Stevan Bruk, Rapporteur Unesco, Paris lO:£/q.C?SSrf& llsn-. ^2>l Unesco, Paris ~-F::\r>Y J^TCRX'AT.'OfvAL REFERENCE C7.X~RF: ; aAAi. vAT::v.'v ;;;;r/ METHODS OF COMPUTING SEDIMENTATION IN LAKES AND RESERVOIRS A contribution to the International Hydrological Programme, I HP - II Project A. 2.6.1 Panel r- ••••' — - -,-•• „ _ Stevan Bruk, Rapporteur '; t^'^S-^' -^::V WA'r--R SUPPLY | A.\U b -.' -n„. ,,._.,, ,....•"""••" ' ^s Hague .SKI 3iS£ February 1985 The Yugoslav National Committee for the International Hydrological Programme contributed to the printing Of this book Unesco'1985 Printed by the Printing Department of the Jaroslav Cemi Institute for the Development of Water Resources P.O. Box 530,11000 Belgrade, Yugoslavia Preface Although the total amount of water on earth is generally assumed to have Remained virtually constant, the rapid growth of population, together with the extension of irrigated agriculture and industrial development, are stressing the quantity and quality aspects of the natural system. Because of the increasing problems, man has begun to realize that he can no longer follow a "use and discard" philosophy ~ either with water resources or any other natural resources. Asa result, the need for a consistent policy of rational management of water resources has become evident. Rational water management, however, should be founded upon a thorough understanding of water availability and movement. Thus, as a contribution to the solution of the worlds water problems, Unesco, in 1965, began the first world-wide programme of studies of the hydrological cycle - the International Hydrological Decade (IHD). The research programme was complemented by a major effort in the field of hydrological education and training. The activities undertaken during the Decade proved to be of great interest and value to Member States. By the end of that period, a majority of Unesco's Member States had formed IHD National Committees to carry out relevant national activities and to participate in regional and international co-operation within the IHD programme. The knowledge of the world's water resources had substantially improved. Hydrology became widely recognized as an independent professional option and facilities for the training of hydrologists had been developed. Conscious of the need to expand upon the efforts initiated during the International Hydrological Decade and, following the recommendations of Member States, Unesco, in 1975, launched a new long-term intergovernmental programme, the International Hydrological Programme (IHP), to follow the Decade. Although the IHP is basically a scientific and educational programme, Unesco has been aware from the beginning of a need to direct its activities toward the practical solutions of the world's very real water resources problems. Accordingly, and in line with the recommendations of the 1977 United Nations Water Conference, the objectives of the International Hydrological Programme have been gradually expanded in order to cover not only hydrological processes considered in interrelationship with the environment and human activities, but also the scientific aspects of multipurpose utilization and conservation of water resources to meet the needs of economic and social development. Thus, while maintaining IHP's scientific concept, the objectives have shifted perceptibly towards a multidisciplinary approach to the assessment, planning, and rational management of water resources. As part of Unesco's contribution to the objectives of the IHP, two regular publication series are issued: "Studies and Reports in Hydrology " and 'Technical Papers in Hydrology ". Occasionally, documents such as the present one are produced by special arrangements with IHP National Committees or other national entities associated with the production of the basic report and are made available for distribution from the IHP headquarters at Unesco, Paris. Foreword This technical report was prepared conforming to the decision of the Bureau of the Intergovernmental Council of the International Hydrological Program, which in 1981 appointed a rapporteur (S. 3ruk) and two co-rapporteurs (Fan Jiahua nad H.E. Jobson), under Project IHP.II.A.2.6.1. In preparing the report, the rapporteurs were joined by the representatives of the International Commission on Continental Erosion (J. McManus) and of the International Training Center for Water Resources Management (CEFIGRE) (J. Evrard). The terms of reference were "to review methods of computing sedimentation in lakes and reservoirs and to prepare a report on recent developments." The report was to provide a summary of recent developments in the study of sedimentation in lakes and reservoirs and in practical computation methods. It had to include recommendations on appropriate ways to prevent or reduce silting of reservoirs during their operation. Finally, the report had to try to identify further research needs and suggest possible action within the framework of the third phase of the IHP (1984-1989). The contents of the report and the drafts of its chapters were discussed in the meetings in Paris, July 1981 and September, 1982. The six chapters were contributed by the following members of the group: Chapter 1 — Technical and Economic Impact of Reservoir Sedimentation by J. Evrard, Electricit6 de France, Beziers Chapter 2 - Physical Processes of Reservoir Sedimentation by J. McManus, University of Dundee, Dundee, U.K. Chapter 3 — Field Measurements by H.E. Jobson, U.S. Geological Survey, Mississippi, U.SA. Chapter 4 - Methods of Preserving Reservoir Capacity by Fan Jiahua, Institute of Water Conservancy and Hydroelectric Power Research, Beijing, China Chapter 5 — Prediction Methods by S. Bruk, "Jaroslav Cerni" Institute for Development of Water Resources, Belgrade, Yugoslavia Chapter 6 — Conclusions and Recommendations by writers of all chapters, collected and edited by S. Bruk Mr. B.R. Payne from the International Atomic Energy Agency contributed to Chapter 3. The drafts of all chapters were circulated among the members of the group and mutually commented. The manuscripts were reviewed by Dr. McManus for English editing and Mr. H£. Jobson took care of bringing the report into a camera—ready form. Mr. Zhang Youshi, Senior Program Specialist in the Department of Water Sciences of Unesco acted as Technical Secretary to the project and contributed to the coordination of the efforts. International organizations, such as the World Meterological Organization, International Atomic Energy Agency, International Commission on Irrigation and Drainage, International Association of Scientific Hydrology and its Commission on Continental Erosion, as well as the International Training Center for Water Resources Management - CEFIGRE, were kept informed about the progress of the project. The contents of the report were discussed at the first meeting of the rapporteurs at Unesco, Paris, between June 29 and July 1,1981, and later revised at the 2nd meeting also held at Unesco, Paris, from September 20-24,1982. The report was completed and submitted in a preliminary form for discussion at a special session of the 2nd International Symposium on River Sedimentation, Nanjing, October 11-16,1983. In the final report, the authors responded to the comments and suggestions made during the discussions at Nanjing. A proposal of the follow—up activities worked out at the meeting in Nanjing is included in the last chapter of the present report. Belgrade, February, 1985 Stevan Bruk, Rapporteur Table of Contents Page INTRODUCTION Scope of the Report j. The Adopted Approach " Points Emphasized in the Report u CHAPTER 1 - TECHNICAL AND ECONOMIC IMPACT OF RESERVOIR SEDI­ MENTATION 1.1 PHYSICAL AND BIOLOGICAL IMPACTS 1 1.1.1 Influence on the Reservoir 1 1.1.2 Influences on the River Reach Upstream From the Reservoir 2 1.1.3 Influences on the River Downstream From the Dam 2 1.1.4 Influences of Sediments on the Aquatic Biocenosis 5 1.2 IMPACT ON THE SAFETY OF THE PROJECT 8 1.2.1 Influences of Sediment Deposits on the Bottom Outlets 8 1.2.2 Influence on Gates and Valves 8 1.2.3 Influences on the Dam 8 1.2.4 Influences on the Supervision of the Dam 8 1.3 ECONOMIC AND SOCIAL IMPACTS 9 1.3.1 Consequences of the Reduction of Storage 9 1.3.2 Influence of the Reduction of the Water Surface 10 CHAPTER 2 - PHYSICAL PROCESSES OF RESERVOIR SEDIMENTATION 2.1 INTRODUCTION 11 2.2 NATURE OF THE SEDIMENT SUPPLY 12 2.3 MORPHOLOGICAL EFFECTS OF RIVER IMPOUNDMENT 14 2.3.1 Aggradation Above the Reservoir 18 2.3.2 Degradation Below the Reservoir 19 2.4 MOVEMENT OF SEDIMENTS IN RESERVOIRS 19 2.4.1 Wind Effects 20 2.4.2 Solar Heating 24 2.4.3 River Input 25 2.5 GEOMORPHOLOGY OF LAKE FLOOR DEPOSITS 31 2.6 SHORELINE EROSION 32 Plage CHAPTER 3 - FIELD MEASUREMENTS 3.1 FIELD MEASUREMENT OF SEDIMENT INPUT AND DISCHARGE FROM LAKES AND RESERVOIRS 41 3.2 RATES OF ACCUMULATION 45 3.2.1 Surveying Methods 45 3.2.2 Dating of Sediment 50 3.2.3 Remote Sensing 52 3.3 SEDIMENT DENSITY 54 3.4 PARTICLE SIZE 55 3 J WATER CURRENTS 56 3.6 RESIDENCE TIME IN RESERVOIRS 58 CHAPTER 4 - METHODS OF PRESERVING RESERVOIR CAPACITY 4.1 METHODS OF MINIMIZING SEDIMENT DEPOSITION IN A RESERVOIR .. 65 4.1.1 Reduction of Sediment Inflow by Soil Conservation 65 4.1.2 Trapping and Retention of Sediment by a Vegetative Screen 71 4.1.3 Bypassing of Heavily Sediment-Laden Flows 75 4.2 METHOD OF MAXIMIZING SEDIMENT