Nile Basin Reservoir Sedimentation Prediction and Mitigation

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2 Nile Basin Reservoir Sedimentation Prediction and Mitigation BY Kamaleldin E. Bashar ElTahir Osman ElTahir Sami Abdel Fattah Alnazir Saad Ali Muna Musnad Ishraqa Osman S. Coordinated By Dr. Ahmed Musa Siyam UNESCO Chair in Water Resources-Sudan Scientific Advisor Dr. Alessandra Crosato UNESCO-IHE 2010

3 Produced by the Nile Basin Capacity Building network (NBCBN-SEC) office Disclaimer The designations employed and presentation of material and findings through the publication don t imply the expression of any opinion whatsoever on the part of NBCBN concerning the legal status of any country, territory, city, or its authorities, or concerning the delimitation of its frontiers or boundaries. Copies of NBCBN publications can be requested from: NBCBN-SEC Office Hydraulics Research Institute 13621, Delta Barrages, Cairo, Egypt nbcbn-sec@nbcbn.com Website: Images on the cover page are property of the publisher NBCBN 2010

4 Project Title Knowledge Networks for the Nile Basin Using the innovative potential of Knowledge Networks and CoP s in strengthening human and institutional research capacity in the Nile region Implementing Leading Institute UNESCO-IHE Institute for Water Education, Delft, The Netherlands (UNESCO-IHE) Partner Institutes Nine selected Universities and Institutions from Nile Basin Countries. Project Secretariat Office NBCBN-SEC office, Hydraulics Research Institute Cairo - Egypt Beneficiaries Water sector professionals and institutions in the Nile Basin Countries Short Description The idea of establishing a Knowledge Network in the Nile region emerged after encouraging experiences with the first Regional Training Centre on River Engineering in Cairo since In January 2002 more than 50 representatives from all ten Nile basin countries signed the Cairo Declaration at the end of a kickoff workshop was held in Cairo. This declaration in which the main principles of the network were laid down marked the official start of the Nile Basin Capacity Building Network in River Engineering (NBCBN-RE) as an open network of national and regional capacity building institutions and professional sector organizations. NBCBN is represented in the Nile basin countries through its nine nodes existing in Egypt, Sudan, Ethiopia, Tanzania, Uganda, Kenya, Rwanda, Burundi and D. R. Congo. The network includes six research clusters working on different research themes namely: Hydropower, Environmental Aspects, GIS and Modelling, River Morphology, flood Management, and River structures. The remarkable contribution and impact of the network on both local and regional levels in the basin countries created the opportunity for the network to continue its mission for a second phase. The second phase was launched in Cairo in 2007 under the initiative of; Knowledge Networks for the Nile Basin. New capacity building activities including knowledge sharing and dissemination tools, specialised training courses and new collaborative research activities were initiated. The different new research modalities adopted by the network in its second phase include; (i) regional cluster research, (ii) integrated research, (iii) local action research and (iv) Multidisciplinary research. By involving professionals, knowledge institutes and sector organisations from all Nile Basin countries, the network succeeded to create a solid passage from potential conflict to co-operation potential and confidence building between riparian states. More than 500 water professionals representing different disciplines of the water sector and coming from various governmental and private sector institutions selected to join NBCBN to enhance and build their capacities in order to be linked to the available career opportunities. In the last ten years the network succeeded to have both regional and international recognition, and to be the most successful and sustainable capacity building provider in the Nile Basin.

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6 1 INTRODUCTION Introduction Statement of the Problem Purpose of the Study Long Term Objective Short Term Objectives Significance of the Study Research Questions and Hypothesis BACKGROUND Preamble Reservoir Sediment Transportation and Deposition Types of Reservoir Sedimentation Sediment Impacts Impact of Sediments on Hydro-plant Equipments Impact of Sediments on Cooling System Impact of Sedimentation on Power Intake Blockage and the Practiced Sediment Removal Measures Sediment Removal Activities in Sudan Reservoir Sedimentation previous Studies Trap Efficiency METHODOLOGY Introduction Inventory of Dams in Sudan Selection Criterion of Case Studies Selected Case Studies ROSEIRES RESERVOIR Introduction The Study Area The Data Roseires Reservoir Operations Sediment Accumulation Siltation Rate Trap Efficiency Storage Capacity Variation and Silt Deposited Trap Efficiency Accumulation Rate Impacts of Sediment in Hydropower Plants Impact of Sediment on Cooling Systems Impact of Sedimentation on Power Intake Blockage and the Practiced Sediment Removal Measures References ASWAN HIGH DAM (AHD) Introduction Previous Sediment Studies at AHD... 30

7 5.3 Traditional Approach for Sedimentation Analysis GIS Approach for Sedimentation Analysis Methodology Sediment Deposition Mapping Conclusions and Recommendations References List of Research Group Members LIST OF FIGURES FIGURE 2-1: LONGITUDINAL CROSS SECTION OF RESERVOIR SEDIMENTATION... 6 FIGURE 2-2: TRAP EFFICIENCY CURVES DUE TO BRUNE FIGURE 2-3: TRAP EFFICIENCY CURVE FOR RESERVOIRS, CHURCHILL (1948) FIGURE 2-4: COMPARISON OF ROSEIRES RESERVOIR TRAP EFFICIENCY DATA WITH THAT OF BRUNE S (SIYAM, 2005) FIGURE 4-1: LOCATION OF THE ROSEIRES DAM AND RESERVOIR WITHIN THE BLUE NILE IN ETHIOPIA AND SUDAN FIGURE 4-2: VARIATION OF STORAGE WITH TIME AND RESERVOIR LEVEL FIGURE 4-3: VARIATION OF STORAGE WITH TIME AND RESERVOIR LEVEL FIGURE 4-4: VARIATION OF TRAP EFFICIENCY OF THE RESERVOIR WITH YEARS OF OPERATION FIGURE 4-5: VARIATION OF % SILTATION RATE WITH TIME AND RESERVOIR LEVEL FIGURE 4-6: VARIATION OF THE SILTATION RATE WITH AREA IN 2005 AND FIGURE 5-1: CROSS SECTIONS OF AHDR FOR YEARS 1964, 1998, AND FIGURE 5-2: RESERVOIR BED ELEVATION (2003) AND SEDIMENT DEPOSITION (2003) LIST OF TABLES TABLE 2-1: RESERVOIR SEDIMENTATION IN SOME DAMS IN THE NILE RIVER AND NEIGHBOURING RIVERS AND ADDITIONAL STUDIES CARRIED OUT... 4 TABLE 2-2:DIFFERENT SEDIMENT REMOVAL EQUIPMENTS USED IN EVACUATING SEDIMENTS... 9 TABLE 2-3: COEFFICIENTS FOR CLAY, SILT AND SAND (KG/M3) TABLE 2-4: K VALUE FOR RESERVOIR OPERATION 2 (USPR, 1982) TABLE 2-5: ASSUMED COMPOSITION OF DEPOSITED SEDIMENT IN ROSEIRES RESERVOIR TABLE 3-1: INVENTORY OF OPERATIONAL DAMS IN SUDAN TABLE 3-2A: INVENTORY OF DAMS UNDER CONSTRUCTION: MERAWI DAM TABLE 3-2B: INVENTORY OF DAMS UNDER CONSTRUCTION: HEIGHTENING OF THE ROSEIRES TABLE 3-3: INVENTORY OF PROPOSED DAMS TABLE 3-4: RESERVOIR SEDIMENTATION IN SOME DAMS IN THE NILE RIVER AND NEIGHBOURING RIVERS AND ADDITIONAL STUDIES CARRIED OUT TABLE 4-1: STORAGE CAPACITY TABLE 4-2: ACCUMULATED SILT VOLUME DEPOSIT FOR DIFFERENT SURVEYS TABLE 4-3: ROSEIRES RESERVOIR TRAP EFFICIENCY % TABLE 4-4: SILTATION RATE FOR DIFFERENT SURVEYS (MM3/YEAR) TABLE 4-5: SILT DEPOSITED PER YEAR AS A PERCENTAGE OF STORAGE CAPACITY (2005, 2007) TABLE 4-6: EQUIPMENTS DEPRECIATION DUE TO SILTATION IN ROSEIRS RESERVOIR TABLE 4-7: COST OF ENERGY DUE TO COOLING SYSTEM BLOCKAGE TABLE 5-1: NAMES AND LOCATIONS OF THE HYDROGRAPHIC SURVEY STATIONS IN AHDR... 31

8 This report is one of the final outputs of the research activities under the second phase of the Nile Basin Capacity Building Network (NBCBN). The network was established with a main objective to build and strengthen the capacities of the Nile basin water professionals in the field of River Engineering. The first phase was officially launched in After this launch the network has become one of the most active groupings in generating and disseminating water related knowledge within the Nile region. At the moment it involves more than 500 water professionals who have teamed up in nine national networks (In-country network nodes) under the theme of Knowledge Networks for the Nile Basin. The main platform for capacity building adopted by NBCBN is Collaborative Research on both regional and local levels. The main aim of collaborative research is to strengthen the individual research capabilities of water professionals through collaboration at cluster/group level on a well-defined specialized research theme within the field of River and Hydraulic Engineering. This research project was developed under the Cluster Research Modality. This research modality is activated through implementation of research proposals and topics under the NBCBN research clusters: Hydropower Development, Environmental Aspects of River Engineering, GIS and Modelling Applications in River Engineering, River Morphology, flood Management, and River structures. This report is considered a joint achievement through collaboration and sincere commitment of all the research teams involved with participation of water professionals from all the Nile Basin countries, the Research Coordinators and the Scientific Advisors. Consequently the NBCBN Network Secretariat and Management Team would like to thank all members who contributed to the implementation of these research projects and the development of these valuable outputs. Special thanks are due to UNESCO-IHE Project Team and NBCBN-Secretariat office staff for their contribution and effort done in the follow up and development of the different research projects activities.

9 This research is a follow up-in depth- of the Phase I completed research titled Assessment of the Current state of the Nile Basin Reservoir Sedimentations Problems. The main objective of this research is to present an overview of the reservoir sedimentation problems in the Nile Basin. Some specific objectives were carried out e.g. Determination of optimum reservoir operation policies, providing guidelines for the assessment of remaining capacity and useful life of reservoirs, assessment of the suitability of the selected models, creating database and data inventory of the operated, on going and proposed dams. Two case studies were selected. The selection criteria based on the availability of data, suitability of the site research problems and contribution of the case study to the general understanding of the research problems. One case study was selected from Sudan at Rosiers reservoir and the other case study was from Egypt at Aswan High Dam. For Rosiered reservoir case study, secondary data was collected from the dams operation unit in the Ministry of Irrigation and Water Resources. Bathymetric surveys carried at Roseires reservoir (1976, 1981, 1985, 1992, 2005 and 2007) were composed and used to estimate the sediment accumulation, sedimentation rate and trap efficiency. The 1966 data was used as the base line information and all other surveys were compared to it for storage and sedimentation estimation. Remote sensing and GIS techniques were applied in Aswan High Dam case study. Different surface maps were generated with different subsequent survey to illustrate the seasonal sediment transport characteristics of the system. Based on the results of the GIS applications, identification of the major zones ranging from low to high sediment deposits, accumulation of sediment deposits over the years, associated depths, and their geographical distributions were recognized. The conclusion achieved can be summarized as follows: - A- Roseirs Reservoir A-1. Storage capacity losses It is found that the average annual silt deposited in Roseirse reservoir since first filling is about 273 Mm 3, that means the storage capacity of the reservoir have been decreased by 37% from the install capacity A-2. Trap Effecieny A relationship between observed trap efficiency and years of operation was found. The trap efficiency for the reservoir follows linearly the square root of time and is inversely proportional to it. It is projected that the trap efficiency of Roseires reservoir after 100 tears will be in the order of 14%. A-3. Accumulation Rate It is observed that the siltation rate has been dropped from million cubic meters per year to million cubic meters per year at 467 reduce level and from million cubic meters per year to million cubic meters per year at 481 reduce levels. A-4 Socio Economic Impact The cost due to loss of efficiency in energy generation and repair shutdown was estimated. It is found that the silt loads which inflow to power intakes cause wear to all water ways and result in decrease of its life span. The energy lost due to cooling system blockage in Roseirse hydro plant: B- High Aswan Dam By comparing sediment deposition maps of year 1999, 2001 and 2003 resulted from applying GIS techniques, it is concluded that a significant sediment deposition was achieved since more than 80% of the deposit thickness was more than 2.5. The larger changes from year 1999 to year 2003 occurred in the wider entrance of the reservoir

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11 AHDR: GIS: GPS: MOIWR: NEC: RS: SI: Aswan High Dam Reservoir Geographic Information System Geographic Position System Ministry of Irrigation and Water Resources National Electricity Corporation Remote Sensing sedimentation index. : Trap Efficiency UTM: Universal Transverse Mercator

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13 1 INTRODUCTION 1.1 Introduction Reservoir sedimentation is a severe threat to the optimal use of water resources in many river basins. The Nile Basin is no exception in this respect. Some of the reservoirs in the basin have already silted up substantially (see below) and also new reservoirs like the Merowi dam and dams in Ethiopia and Uganda will be subject to sedimentation and hence storage losses. Good prediction of the future reservoir sedimentation is important, but even more important is to find optimal mitigation methods to reduce sedimentation after some years in which inevitable sedimentation is taking place. A number of reservoirs elsewhere for example is now regularly flushed and no further storage loss is experienced. The Nile Basin Capacity Building Network for River Engineering (NBCBN-RE) was initiated in year 2000, with the objective to create an environment where the professionals from the water sector sharing the River Nile resources can exchange their ideas, practice and learn from each other to reach a common vision to develop the water resources of the basin efficiently. Within this framework the phase II is now starting and the present proposal was developed in line with the broad objectives of the NBCBN to materialize the exchanging and learning processes. The network has created very good opportunities for the exchange of knowledge and information between engineers and scientists on different technical issues and reservoir sedimentation is one of the most important in this respect. Hence this proposal outlines a research plan on Reservoir Sedimentation prepared for phase II ( ) by the River Morphology cluster. In the first phase of the NBCBN-RE project, the cluster completed a research report titled Assessment of the Current State of the Nile Basin Reservoir Sedimentation Problems, which presents an overview of reservoir sedimentation problems in the Nile Basin. For phase II and according to this proposal a follow up in-depth research is proposed for a four year period from October 2006 to The planned research is based on the outputs and recommendations of phase 1 report as well as the output of the bridging workshop, May It is split up in two parts, in line with the planning of Phase II of the NBCBN-RE project. 1.2 Statement of the Problem Worldwide Reservoir Sedimentation is a serious problem and considered as salient enemy. The graduate loss of capacity reduces the effective life of dams and diminishes benefits for irrigation, hydropower generation, flood control, water supply, navigation and recreation. On one hand sediment deposition propagates upstream and up tributaries, raises local groundwater table, reduces channel flood capacity and bridge navigation clearance, and affects water division and withdrawals. On the other hand, the reduction of the sediment load downstream can result in channel and tributary degradation, bank erosion and in changes of the aquatic habits to these more suited to a clearer water discharge. As for the River Nile, there exist a number of dams that have been constructed during the last century. For some others designs have either been completed or at feasibility stages as well as potential sites do exist for new dam projects to come. The existing reservoirs especially those on the Blue Nile and the River Nile are seriously affected by sediment deposition at unexpected rates. Currently, for some reservoirs, costly sediment control measures are being practiced. Decision of postponing the problem by heightening the dam has been taken and unresolved Nile Basin Capacity Building Network ( NBCBN ) 1

14 situation is waiting for those in design and planning stage. Also the Aswan High Dam, though large, it is facing the problem of 100% trap efficiency. Further, in Sudan, Ethiopia and Uganda new dam projects are underway or have been proposed. Data pertaining to sedimentation, current adopted operation polices and practiced control measures in these reservoirs in general are far from being comprehensive and/or optimised. In addition the impact of theses polices and measures on new dam/reservoir projects or vies-versa need a thorough understanding and special tackling as usually the consequences propagate in both upstream and downstream directions. It is expected that launching joint research, within the basin, will contribute to the common understanding of the siltation characteristics of the reservoirs in the region, help in defining the most suitable assessing tools (models), and in selecting the most appropriate combat measures. The outcome is vital piece of information for both dam owners and users as operation polices can be modified to prolong the useful life of reservoirs for future generation. 1.3 Purpose of the Study This study has two objectives long term and short term. The short-term objectives should be realized in the Phase II studies Long Term Objective 1. Determination of optimum operation policy to combat sedimentation problems in the selected reservoir. 2. Guidelines for the assessment of remaining capacity and useful life of reservoirs 3. Assessment of the suitability of the selected models. 4. Publishing papers to disseminate knowledge gained in regional and international peer-reviewed journals Short Term Objectives 1. Completion and creation of database with respect to the I. Sediments and water in flows and out flows in the exciting reservoir. II. Sedimentation rates III. Adopted operation policies. IV. State of water quality V. Socio-economic and environmental impacts. VI. General information about dams and reservoirs. VII. Identification of problems and mitigation mechanism. VIII. Inventory of proposed dams. 2. Exploring study in the use of satellite imagery to determine reservoir sedimentation 3. Simulation with a numerical model of sedimentation processes in some selected reservoirs to explore the use of models to improve the understanding of sedimentation processes in reservoirs, including the effect of some mitigation methods 4. Development of new mitigation measures to combat reservoir sedimentation measures and testing them with the numerical model. 5. Building capacity in the field of reservoir sedimentation prediction and mitigation. 1.4 Significance of the Study The reservoir storage is used for water supply, irrigation, power generation and flood control but in the Sudan, creeping problem of sedimentation causes several implications. First, the lost storage capacity has an opportunity cost in the form of replacement costs for construction of new storage since the present level of supply is to be maintained. Second, there are direct losses in the form of less hydropower production capacity Nile Basin Capacity Building Network ( NBCBN ) 2

15 available, less irrigated land to produce food and reduced flood routing capacity. Finally the fully silted reservoirs created a decommissioning problem that has both direct and indirect costs. For example in Roseires reservoir, regardless of the all efforts done by National Electricity Corporation (NEC) and MOIWR, the sediment problems are still in growing, The adverse effects of these problems can be clearly reflected on the socio-economical part to the all users of the dam. The average cost of sediment removal may reach more than 626,000 million US$ per year. Currently the average annual volume of sediment removed from the hydropower intakes is estimated to be 125 Mm 3 /year at an average cost of 5 US$ per cubic meter. From this point of view, this study attempted to promote the deep investigation in the reservoir sedimentation problems, obtaining all the necessary data, searching for the modern silt removal techniques worldwide used, pointing the appropriate analytical tools to provide reasonably reliable results, investigating the existing reservoir operational rules and policies and come up with recommendations that can provide the decision makers with appropriate solution. 1.5 Research Questions and Hypothesis The research questions to be answered include: 1. How much storage capacity lost in the reservoirs under consideration? 2. At what rate is the loss of capacity taking place? 3. What is the current trap efficiency of the reservoirs under consideration? 4. What are the available techniques to de silt the reservoirs and their relative costs? 5. What are the socio-economic impacts of the sedimentation? 6. Can the new techniques such as hydro suction be used in our reservoirs? Nile Basin Capacity Building Network ( NBCBN ) 3

16 2 BACKGROUND 2.1 Preamble Causes of reservoir sedimentation along with the range of problems caused were discussed in research report Assessment of the current state of the Nile basin reservoir sedimentation problems prepared in Phase I. Table 2.1 gives an overview of to what extent some reservoirs in the Nile basin and neighbouring basins have been affected by reservoir sedimentation. For some reservoirs the sedimentation has seriously affected the available storage. As stated in the first report the overall objective of Reservoir Sedimentation research (Group I) is develop methods to manage efficiently and economically reservoir sedimentation in the Nile basin. Research carried out in the first phase has achieved the following results. Overview of existing reservoir sedimentation problems in the Nile Basin. Estimation of sediment deposition and distribution on five reservoirs on the Nile system and one neighbouring river system. Investigation of socio-economic impacts of reservoir sedimentation problem. Critical examination of control measures, practices, successes and failures. Design and early start of database on reservoir sedimentation. Identification of future research needs. Identify cross-cutting themes for future joint research work between and among research groups/clusters. Identified capacity building needs for the Nile Basin researchers in river engineering in general and reservoir sedimentation in particular. Table 2-1: Reservoir sedimentation in some dams in the Nile River and Neighbouring rivers and additional studies carried out Reservoir Country Period Storage considered losses Angereb Ethiopia % Additional studies carried out under Phase I Koka (Awash R.) Ethiopia % Roseires Sudan % Economic studies of reservoir sedimentation. Khashm El-girba Sudan % Study of the socio-economic impact of reservoir sedimentation. Aswan high Dam Egypt % It is relevant to summarize below the results of the investigation of socio-economic impacts of sedimentation in Roseires reservoir. In this study, two aspects were studied, the impact of reservoir sedimentation in the rates and availability of water for crop production and power generation. Nile Basin Capacity Building Network ( NBCBN ) 4

17 i Irrigation and crops production: Assuming that a volume of water is equivalent to the silted volume in Roseires reservoir to irrigate cotton and wheat crops, economic gains were determined by comparing cost production with the income value from sale of products. Results have shown that it is justified to have a sediment control facilities, with a capital cost as high as the cost of the total project, if only the active storage of the reservoir would have been preserved. ii Hydropower generation. The study has assumed a volume of water equal to the silted volume passing through the turbine for power generation. Results have shown that Roseires sedimentation lead to losses in the energy sector, 25% of these losses is due to the cost of annual dredging. Other losses are due to the reduced generation efficiency due to the passage of high silt concentration. The net economic revenue from sale of energy was also calculated. The second study demonstrated clearly that it is better to invest in mitigating measures to keep sufficient storage in existing reservoirs rather than to abandon a dam site and built a new reservoir. Recommendations for the next phase were given as well. According to the Phase I report follow-up research should mainly focus on the last four achievements of phase I research, namely: Completion of the database started in phase 1. Follow-up research on reservoir sedimentation taking into account the identified needs. Participation in cross-cutting research projects on the impact of climate change on reservoir sedimentation, flood management and managing water scarcity. Provide training service on reservoir sedimentation to concerned professionals in the Nile basin based on the applied research experience of the group. In depth study for a real life problem (e.g. Roseires reservoir), detailed studies will be specified for proper solution. 2.2 Reservoir Sediment Transportation and Deposition The reservoir sedimentation involves entrainment, transport and deposition. They originate from the catchments area, river system and settle in reservoirs. As a river enters the reservoir, its cross section of inflow is enlarged due to the effect of the backwater curve. Thus it causes a decrease in the water flow velocity; subsequently the sediment carrying capacity of water is reduced too. The major part, or all, of the sediment transported will deposit in the u/s part of the reservoir influenced by the back water curve. Reservoir sedimentation undergoes different processes of transportation and settling of sediment. This causes the reservoir to possess different kinds of deposition at different positions. These differences are controlled by the effects of the sediment particle size, hydraulic condition and sediment transportation methods in the reservoir. Due to different behaviour of sediment particles in transportation and deposition, they have different impacts on the reservoir sedimentation pattern and storage losses. Thus, it is important to treat each type separately, so as to understand how they are deposited and transported in the reservoir. This is hardly needed in analyzing the reservoir sedimentation problem and providing the best measures. The rate of the reservoir sedimentation and form of the deposition is affected by the rate of sediment transport and the method of its deposition in reservoir. Sediment particles are transported by different mechanism depending on the sediment size and the water sediment holding capacity. Due to existence of different kinds of sediment particle in the stream inflow, several transporting and depositing kinds occur in the reservoir. In general, the river sediment is divided in two major parts; bed-load and suspended load. They exist in the stream inflow at different ranges and different quantity with respect to the time and space. The increase or decrease of any type of sediment has direct reflection on the deposition pattern in the reservoir (Nzar, 2006). Nile Basin Capacity Building Network ( NBCBN ) 5

18 2.2.1 Types of Reservoir Sedimentation The river flow usually carries a wide range of the sediment particle sizes and they are transported either as a bed load or as a suspended load. In general, the bed load material (coarse sediment particles) move near the bed and start to deposit in the beginning of the reservoir entrance in the form of the delta as shown in figure (2.1). The suspended sediments (fine sediment particle with lower settling velocities) are transported deeper into the reservoir either by non stratified flow forming a uniform deposition at the middle of reservoir, or by stratified flow depositing at lower part of the reservoir forming a muddy lake. Generally the suspended load is divided in two parts; one comes from the bed of the river, and the other load from the catchments area as wash load. Batuca and Jordaan (2000) have classified the reservoir sedimentation based on the location of deposition into three categories, with inclusion of the sedimentation in backwater reaches as a part of the reservoir sedimentation. The position of each type of reservoir sedimentation can be seen in the longitudinal profile of the reservoir shown in figures 2.1 which are classified as Back water deposition, Delta deposition and Bottom set deposition. Figure 2-1: longitudinal cross section of reservoir sedimentation Back water Deposition This type of deposition occurs in the river reach before entering the reservoir. After changing the water level in the river by the effect of back water curve, the velocity of water will be reduced. Subsequently a small part of the coarse sediment will deposit in this region till it reaches the reservoir delta deposition. It is considered as a transition between the original river bed and delta formation as shown in figure (2-1). In theory, the backwater deposit should grow progressively, into upward and downward direction of the river, because it extends with changes of bed forms. However this growth is limited, because the stream adjusts its channel by eliminating meanders, forming a channel having an optimum width-depth ratio or varying bed form roughness. These factors make the stream transports its sediment load through the reach with evolution done in one direction (Nzar, 2006). The backwater deposition is not fixed, but it is fluctuated and advanced toward the reservoir and delta. As a result of the variation of the reservoir water surface and water flow velocity, the backwater sediment is reeroded, transported toward the reservoir and contributes in the formation of the delta. Delta formation Delta formation is caused by rivers that enter a reservoir, lake, or sea. The process involves deposition of sediment of large sand sizes (bed load) due to the reduction of stream sediment holding capacity. Nile Basin Capacity Building Network ( NBCBN ) 6

19 Mainly, the change of the water level and the expansion of the inflow cross section in the reservoir are considered to be the most important reasons to diminish the water velocity and continuity of sediment movement in the stream at the delta reach. Therefore the deposition happens in this place at the beginning. The deltaic deposition takes place along and across the reservoir and its basin (in the main river reach and over the flood plain as well) (Betuca and Jordaan, 2000). From the observation of the reservoir sedimentation, the delta formation may contribute the majority of the sedimentation in the hydrologic ally small reservoir. While for the large reservoir the delta constitutes only a small part of sedimentation (Fan and Morris, 1997). Due to the small volume of shallow part at head of reservoir, the deposition and formation of delta even with small volume will be problematic from the standpoint of upstream aggradations. The longitudinal cross section of the delta can be divided in two zones, the top-set bed and front-set bed which are different in surface slope and deposition texture as given in figure (2-1). According to the US Bureau of Reclamation (1986) the slope of top-set of the delta is in the range from 100% to 20% of the original bed slope, which was found to be based on observation of 31 reservoirs in the United States. For design purposes 50% slope is acceptable. Hence: - S topset = 0.5S river Based on the same survey data done by US Bureau of Reclamation (1986) the slope of front set can be have the relation:- S frontset = 6.5S topset In some cases the delta formation takes the major part of the reservoir sedimentation. For example in Glenmore reservoir at Canada, about 10 percent of its total water capacity was lost in the year 1968, with about 70 percent of the deposits that occurred in the delta area (Fan and Morris, 1997). The advancing shapes of the delta formation toward the reservoir are different. They are affected by hydraulics and geometric shape of the reservoir inlet. This results in different advancing speed in delta propagation, subsequently having different impacts on reservoir sedimentation. Sloff (1991) has indicated that the parameters which affect the shape of delta formation are namely, Slope of the valley, length and shape of the valley, sediment particle size and its distribution, and Reservoir operation and capacity of inflow ratio. According to the empirical criterion which was developed by Zhang and Qian (1985), there are two major type of delta formation. The 1st are the Wedge-shape deposits; in which the front reaches to the dam wall and sediment site are uniformly distributed in the basin. The second are the delta-shape deposits; in which the front does not reach to the dam wall and sediment site are non-uniformly distributed in the basin. According to the experimental investigation made by Chang (1982) in the laboratory, the delta formation starts with the deposition of the bed load at the channel mouth. The suspended load is deposited rather uniformly over reservoir bottom (Nzar, 2006). Bottom-set bed depositions Bottom deposition of the reservoir is formed by transporting and depositing the fine sediment, which is carried by the water to the middle and end of the reservoir in suspension stage. This type of deposition is mainly composed of clay and silt fraction, which are transported in the reservoir water body either by the turbulent suspension or by turbidity currents. Its deposition starts beyond the delta up stream the dam wall site. The shape and configuration of the deposit is affected by the process of transporting and depositing of suspended material. There are two main ways of transporting fine sediment into the reservoir body. First one is by suspension action of the sediment particle. In this case they travel beyond the delta toward the reservoir body either by the action of electro-magnetic of small particles or by turbulence action of flowing water. The Nile Basin Capacity Building Network ( NBCBN ) 7

20 second way is by gravity action on the sediment-laden water which enters to the bottom of the reservoir in the form of turbidity current. Depress flow Woods and light material that come with flow cause many troubles and interruption during dams operation. They are quite dangerous to gates and especially to running turbines when the protecting screens are broken under the heavy pressure of the accumulated materials. The best method to get red of depress is to direct these floating materials towards the spillways to pass downstream. However, since the depress (wood) come from the upper catchments then it would be better to treat the problem there by improving and protecting the environment of the wood source. For more information on this topic consult the reference (Nzar, 2006). 2.3 Sediment Impacts Impact of Sediments on Hydro-Plant Equipments The presence of sediment in water flowing through the turbine causes wear on all water ways components by its abrasive impact. The higher the sediment content and the harder minerals it contain the greater the wear. The wearing of the water ways results in poor performance of the plant than its optimum. This result in loss of energy production during operation and ultimately the loss becomes so high that the plant has to be shutdown to allow for the equipments to be repaired to its original performance. In such conditions there is cost due to loss of efficiency in energy generation, repair shutdown. Moreover silt loads which inflow to power intakes cause wear to all water ways and result in decrease of its life span Impact of Sediments on Cooling System In spite of using effective water filtration systems to cooling water system in Roseirse hydro plant such as screens, cyclones and double filters silt occasionally cause blocking to the coolers and results in rising temperature in oil coolers for bearings and air coolers for generator. These resulted in tripping or isolating units due to high temperature and this leads to decrease in generated power from the plant Impact of Sedimentation on Power Intake Blockage and the Practiced Sediment Removal Measures The Blue Nile, which is virtually unregulated in Ethiopia, carries a very heavy silt load (up to 3 million tones per day) during the flood season, with the result that the Roseirse Reservoir below minimum operating level become effectively to fill with sediment within about 10 years of first impounding. large quantities of sediment both in suspension and as bed load were being transported through the reservoir during July to august period. Blue Nile river catchments are rich with forest and farms, so the inflow from tributaries carries very heavy loads of debris in addition to sediment load. It is anticipated that debris consisting of grass, roots, and logs of all sizes need to be cleaning by special equipments. Since the time when the silt deposits first reached the dam, the operation of the existing power station has been shown to be liable to serve disruption by the effects of trashes and sediments inundating the intake screens during the rising spates of the flood. Deposition in reservoir has restricted the flow within the main river channel by the formation of levees and silt banks. The main channel of river is located in front of the deep sluices area. Flow to the spill ways and power station intakes has tended to keep open a subsidiary channel to these two structures which run from the deep sluices along the face of the dam. The effect of silt deposition in the front of the spillway have created deeper channel 40 to 50 meters wide adjacent to the spillway where water will flow from the deep sluice channel to the power plant intakes. It s also possible that more submerged logs will be moved by these higher velocities to the power plant. Nile Basin Capacity Building Network ( NBCBN ) 8

21 From the dam site, one can clearly identify three islands of silt located in back water upstream of the dam. This is regarded as the delta deposits on the east embankment side and the biggest one in the west embankment near the old power station intakes. Sediment deposition influences the physical aspects of water quality and the aquatic system of the streams.the accumulation of sediment disturbed the operation of the dam for flood control, power generation and irrigation. The Ministry of Irrigation and Water Resources (MoIWR) has been responsible for all dams and reservoirs in the Sudan before the newly formed Dams Implementation Unit. The Roseirse dam administration is responsible for both operation and maintenance of the dam and reservoir. During the flood seasons, Roseirse reservoir deposits large quantities of sediments in different area inside the reservoir. The accumulation of sediment increase yearly and this could result in block of the water intakes. In the case of Roseirse sedimentation emergency conditions prevailed in august 1980 with the intake to the power plant blocked with trash and debris which severely reduced power generation. Large quantities of sediment both in suspension and as bed load were being transported through the reservoir during the flood season. The sediment combined with the trash blocks completely the trash screens at the power plant intakes. Roseirse hydro power plant administration tried to decrease the effects of the power intakes blockage by removing the accumulated debris and sediment in the front area of the power intakes by using trucks cranes. Also they use a debris boom to stop debris to reach the screen of the power intakes moreover they installed trash rake cranes for trashing the floating debris. The above efforts improve the performance of the power plant during the flood season. 2.4 Sediment Removal Activities in Sudan Due to very critical situation during the rainy season since 1975, the situation becomes so urgent that an instant solution had to be found. In September 1980 based on efforts of PEWC and MOI, teamwork from USAID (Agency for International Development) visited Roseirse reservoir to study the sedimentation problems at Roseirse Dam project. The team recommended using special equipments for sediment removal based on a full bathymetric surveys and direct site observation. In 1984 a joint project was initiated by USAID, the Netherlands Development Cooperation Programme and the Sudanese authorities. The donors of this project supplied the necessary grab dredgers, dump barges, tugs and survey vessels. They dredging sediment from the morning area in the front of the power house and send the dump barges to the main river channel for evacuation. In the flood season after opening the deep sluice gates the evacuated sediment flushing by high current of river flow. The Dump Barge dredger now is practice to carry out dredging in advance of the flood season (in summer season) in order to open out the approach channel to the power intakes, and this has improved the situation during the flood season by letting the incoming sediment trapping in the morning area before reaching near the power intakes. A debris boom consisting of assembled sections of oil barrels about 100 meters in length was installed upstream from the west abutment of the spillway. During the flood season special type of pump (Toyio pump) used to suction slurry water from the front area of the power intakes and delivered to down stream of the dam over the concrete body. Table 2.2 below shows the different sediment removal equipments using for evacuation sediments from Roseirse reservoir. Table 2-2:Different Sediment Removal Equipments Used in Evacuating Sediments NO- Equipments Install year Notes 1 Trash rake (gantry) 1983 Improve efficiency of generation 2 Adebris boom (oil barrels boom) 1983 Isolating floating debris to reach the power intakes 3 American Crane 1983 Cleaning sediment &debris in front of Nile Basin Capacity Building Network ( NBCBN ) 9

22 model 7530 power intakes. 4 Front-end loader 1983 To handle trash and sediment loading on top of dam. 5 Dump barge 1987 Summar season(moiwr) 6 Toyio pump Flood season (MOIWR) 7 Suction Dredger 2007 Flood season (NEC&MOIWR) 2.5 Reservoir Sedimentation Previous Studies The Blue Nile has been known from earliest recorded times to bring down considerable amounts of silt in its flood time, renewing the fertility of intermittently flooded areas along its banks each year. The silt material originates mainly from heavy erosion in the upper catchment area in Ethiopia, where the slope of the river is steep. As a result of this high silt load, the reservoir operation is unavoidably accompanied with reservoir sedimentation. In order to up-date the level content relationship in Roseires reservoir lake cross-section surveys were carried out in different years. Data of the bathymetric surveys carried out on the Roseires during the years 1976, 1981, 1985, 1992, 2005 and 2007 were used in this research study. The reservoir is operated in accordance with the regulation Rules (1968). These rules are designed primarily to meet irrigation demands and provide a stipulated flow at Khartoum with production of hydro-electricity regarded as secondary to these requirements. The system of operation divides the year into three main periods: (i) The flood before filling when the reservoir is held at a low level to reduce siltation (ii) The filling period, when the reservoir is filled according to detailed program. (iii) The period of shortage when the storage within the reservoir is used to supplement the natural river flows to meet the requirements of irrigation and minimum flow to Khartoum. The flood season starts from early June and the filling starts in September. The aim of the operation is to maintain the level of the reservoir at m required for the power station. However, if the floods are above normal, the level in the reservoir will rise to the level required to pass the discharge. For the maximum recorded flood, Roseires reservoir attained m R.L. Filling is carried out on the falling flood and the rules are complicated by the need to delay filling as long as possible to reduce siltation, yet to ensure filling every year. The starting date for filling varies from year to year according to the flow at Ed Deim upstream of Roseires reservoir and then follows a day by day program The starting date for filling lie between 1 st September and 26 th September and filling is completed within 45 days. Details of the amount to be taken into storage are specified each day of the filling period When the natural flow at the river is sufficient to meet the irrigation demand, minimum flow at Khartoum and all evaporation losses, the reservoir is held at the retention level. During the shortage period, when the natural flow in the river does not supply the irrigation demand and other needs, a balancing operation is carried out by controlling the amount of water released from the reservoir storage. Apriority is given to the irrigation demand taking into consideration that the storage is not exhausted before 10 th of June. 2.6 Trap Efficiency Reservoir trap efficiency is defined as the ratio of deposited sediment to total sediment inflow for a given period within the reservoir economic life. Trap efficiency is influenced by many factors but primarily is Nile Basin Capacity Building Network ( NBCBN ) 10

23 dependent upon the sediment fall velocity, the detention-storage time, flow rate through the reservoir and reservoir operation. The relative influence of each of these factors on the trap efficiency has not been evaluated to the extent that quantitative values could be assigned to individual factors. The detention-storage time in respect to character of sediment appears to be the most significant controlling factor in most reservoirs (Siyam, 2005) Trap efficiency estimates are empirically based upon measured sediment deposits in large number of reservoirs mainly in U.S.A. Brune (1953) and Churchill (1948) methods are the best known ones. Brune (1953) has presented a set of envelope curves for use with normal ponded reservoirs using the capacity inflow relationship of reservoirs. These curves are reproduced in Figure (2.2). They are not recommended for use in computing T.E of de-silting basins, flood retarding structures or semi-dry reservoirs. Churchill (1948) developed a trap efficiency curve of settling basins, small reservoirs, flood retarding structures, semi-dry reservoirs or reservoirs that are frequently sluiced. The essence of Churchill s method is contained in a graph relating the percentage of sediment that passes through a reservoir to a so-called sedimentation index SI. The latter is defined as S / (3.1). 1 S (3.2) Where = retention time and = mean velocity of water flowing through the reservoir see figure (2.3). (A.Taher,1999). Figure 2-2: Trap efficiency curves due to Brune Nile Basin Capacity Building Network ( NBCBN ) 11

24 Figure 2-3: Trap efficiency curve for reservoirs, Churchill (1948). General guidelines for using these two methods were given by Murthy (1980). He recommended using the Brune method for large storage or normal ponded reservoirs and the Churchill method for settling basins, small reservoirs, and flood retarding structures, semidry reservoirs or reservoirs that are continuously sluiced For a given reservoir experiencing sediment deposition, its trap efficiency decreases progressively with time due to the continued reduction in its capacity. Thus trap efficiency is related to the reservoir remaining capacity after a given elapsed time (usually considered from the reservoir commissioning date. As trap efficiency is influenced by reservoir operation, it is important to closely examine the reservoirs in order to make judgment on their impact on trap efficiency. There are four main operation periods for Roseires dam reservoir. During the rising flood, the reservoir drawdown attains the level of 467 R.L which is the lowest operating level. Over this operation period, minimum sediment deposition is expected despite the large quantities of sediment inflow which may approach 3 M ton/day. This is particularly true after many years of continuous operation of the reservoir where a well defined channel, capable of transporting almost the whole sediment inflow past the reservoir during the drawdown period, was developed naturally (Siyam, 2005) The reservoir filling period commences after the flood peak has passed. According to the reservoir operation rules, filling may start any time between the 1 st and the 26 th of September each year depending on the magnitude on the flow at El Deim gauging station. From past experience, filling normally starts within the first ten days of September when the suspended sediment concentration is still relatively high at about 2500 mg/l. The filling period usually continues for nearly two months. Due to the gradually rising water level and the relatively high suspended sediment inflow, significant sediment deposition is expected during the filling operation period. In contrast, during the third and fourth operation stages (maintaining full retention level and reservoir emptying), sediment deposition is insignificant due to the exceedingly small sediment and inflow quantities. From the above description, only operation filling period is of importance as far as reservoir sedimentation and trap efficiency are concerned in Roseires reservoir. Therefore this is taken in consideration when estimating the trap efficiency using either Brune or Churchill method. Over the filling period, the water level at 474 m R.L is considered for the computation. The reservoir content at this mean level is used together with Nile Basin Capacity Building Network ( NBCBN ) 12

25 an annual inflow of 50x109 m3 to estimate the trap efficiency using both methods. The results are compared with measured values for the years when reservoir surveys were made. The measured trap efficiency is computed from the following equation. 6. (%) (( v0 v) /( x140x10 )) (3.3) Where,. = trap efficiency after T years of operation v = original reservoir volume, m3 0 v = volume remaining after T year of operation = average specific weight of deposited sediment over T years (t/m3) is calculated from the following equation (Miller, 1953) / 1 x Ln i i Where Where i the initial is value of and is given by cl cl sl sl cl, sl and sa cl, sl and sa sa sa 3.5 are fractions of clay, silt and sand respectively of the incoming sediment. are coefficients of clay, silt and sand respectively which can be obtained from the table (2.3), (USPR, 1982) for normally moderate to considerable reservoirs drawdown (Reservoir Operation 2) which is the case for Roseires reservoir. Table 2-3: Coefficients for Clay, Silt and Sand (kg/m 3 ) Clay Silt Sand The compaction Coefficient K is found similarly from the table (2.4). Table 2-4: K Value for Reservoir Operation 2 (USPR, 1982) Clay Silt Sand The composition of deposited sediment in Roseires reservoir differs widely. A reasonable approximate composition assumed in this study is given in the table (2.5) below. Table 2-5: Assumed Composition of Deposited Sediment in Roseires Reservoir. Clay Silt Sand 25% 45% 30% For theses assumed values γi = t/m 3 and K = t/m 3. Comprehensive field measurement core sampling programmer of deposited sediment in a number of major and minor canals in Gezira Scheme was made in 1989 (HRL,1990). The mean value of γi for a depth below bed level varying from 80 mm to 500 mm was t/m 3 which is very close to the adopted value for Roseires reservoir considering that the Gezira Scheme draws its water from the Blue Nile. Nile Basin Capacity Building Network ( NBCBN ) 13

26 Brune s method is certainly the most widely used one to estimate reservoirs trap efficiency. Siyam (2000) has shown that Brune s curve is a special case of amore general trap efficiency function given by the following equation:. (%) 100 exp( / ) (3.6) Where, in addition to the already defined terms, is a sedimentation parameter that reflects the reduction in the reservoir storage capacity due to the sedimentation processes. Siyam (2000) demonstrated that Eq.(3.4) with values of = , and describes well the upper, median and lower Brune s curves respectively as depicted in Figure (2.3). Shown in the Figure Brune s data for semi-dry reservoirs ( = 0.75), and in the case of a mixer tank where all the sediment is kept in suspension (ß = 1). Shown also in the figure Roseires Reservoir data fitted by Eq (3.6) with = which was the mean of the individual ß values resulting from fitting the observed trap efficiency data with Eq. (3.6). Figure 2-4: Comparison of Roseires Reservoir Trap Efficiency Data with that of Brune s (Siyam, 2005) Figure (2.4) shows the success of the method to limit reservoir sedimentation via reduction of the reservoir level during flood. It is observed from Figure (2.4) that Roseires reservoir data fall between Brune s data for normally ponded and semi day reservoirs. This is because Roseires reservoir belongs to neither type. According to Roseires reservoir operation rule, the reservoir is ponded at full retention level for about only 2 months in the years. Considerable drawdown precedes the pondage stage to reduce the reservoir sedimentation; while gradual drawdown follows the pondage stage in order to satisfy downstream requirements. Nile Basin Capacity Building Network ( NBCBN ) 14

27 3 METHODOLOGY 3.1 Introduction The objective this study is to assess the sedimentation and its effects in some selected reservoirs in the Nile basin. The study will explore the use of satellite imagery and GIS to determine reservoir sedimentation. Simulation models of sedimentation processes will be used in some selected reservoirs to explore the use of models to improve our understanding of sedimentation processes in reservoirs, including the effect of some mitigation methods. The study aims at the development of new mitigation measures to combat reservoir sedimentation and testing them with the numerical model. In this chapter an overview of the dams in Sudan, a brief description of the criteria used to select the case studies and the selected case studies will be given. The methodology used to achieve each objective is described in its respective chapter. 3.2 Inventory of Dams in Sudan The inventory of dams in Sudan is categorized into three groups namely operational dam, Dams under construction and proposed dams. The inventory includes some basic information on dam s characteristics. Table 3.1, 3.2, 3.3 and 3.4 give the inventory of operational dams, dams under construction and proposed dams respectively. A. Operational Dams Dam Name Sennar Dam Gabel Awlia Dam Khashm Elgirba Dam Roserois Dam Time of construct ion Table 3-1: Inventory of operational dams in Sudan Minimum Water Level (m) a.m.s.l Maximum Design Capacity (Mm 3 ) Reduced Capacity due to sedimentatio n (Mm 3 ) No. Of Sluice Gates (8.4*2m) & 72 (3.4*3m) (4.5*3m) *10 3 2* Nile Basin Capacity Building Network ( NBCBN ) 15

28 B. Under Construction Dams Table 3-2a: Inventory of Dams under Construction: Merawi Dam Dam Type of Dam Concrete face rockfill Dam crest elevation 301 masl Overall length along crest 8.6 km Lowest bedrock elevation below dam 218 masl Maximum Dam hight above bedrock 83 m Reservoir Length Surface Area Full Supply level (FSL) Low Supply level (LSL) Full Supply Storage Volume Live storage volume Low Level Sluices Number of Gates & Type Size of Gates (width height) Gate silt elevation Discharge capacity at FSL Overflow Spillway Number of gates & types Size of Gates (width height) Gate silt elevation Discharge capacity at FSL Power Facility Number and type of turbines Turbine unit rated output Rated head (net) Maximum Head (gross) Minimum Head (gross) Total Turbine Discharge at Rated Head Generator Unit PF Synchronous Speed (50 Hz) Total installed generator capacity 170 km 724 sq km 298 masl 290 masl milliards 4.88 milliards 6 Radial 10 m 8 m m m3/sec 4 Radial 12 m 14.7 m m m3/sec 10 Kaplan 110 MW 41.6 m 53 m 38 m 3029 m 3 /sec 138 MVA RPM 1242 MW Transmission Line Length 520 km (via Atbara) Transmission Voltage 500 KV Number of circuits 2 Conductors per circuits mm 2 Capacity per circuits 994 MW Nile Basin Capacity Building Network ( NBCBN ) 16

29 C. Proposed Dams Table 3-2b: Inventory of dams under construction: Heightening of the Roseires Max Reservoir level 490 m a.m.s.l Estimated surface area 600 km 2 Estimated Heightening 10 m concrete dam Length of embankment dam 25 km at the two embankment Table 3-3: Inventory of proposed dams Feature Data 1. Kajbar Dam and Reservoir Location of dam Near the village of Soba, about 120 km downstream of Dongla Crest elevation of dam m a.s.l Full supply level (FSL) 213 m a.s.l Maximum flood level m a.s.l Downstream level during floods (DSL) 207 m a.s.l Reservoir storage capacity 360*10 6 m 3 Headrace and tailrace canal Headrace canal length 250 m Headrace canal width 160 m Invert elevation canal 200 m a.s.l Forebay powerhouse m a.s.l Tailrace canal length 280 m Tailrace canal width 180 m Tailrace canal invert elevation 194 m a.s.l Canal outlet powerhouse m a.s.l Powerhouse with erection bay Location Right bank Type Surface Turbines 6 Kaplan runners Rated head 15 m Rated output 34.7 MW each Spillway dam Location River channel Type Combination of overflow dam and gated spillway Spillway gates 20 fixed wheel gates, each 10 m*9.5 m Design flood m 3 /s Concrete and embankment dams Total length of dam 2201 m Dam on right bank, height 4 m/23 m Overflow dam in river channel, height 22 m Non- Overflow dam on left bank height 20 m Power and energy Installed capacity 208 MW Annual average energy 1046 *10 6 KWh 2. Shereik Dam and Reservoir Location of dam Near Shereik village, about 290 km upstream of Merwe Dam Crest elevation of dam 347 m a.s.l 349 m a.s.l Full supply level (FSL) 343 m a.s.l 345 m a.s.l Downstream level during floods (DSL) 340 m a.s.l 342 m a.s.l Reservoir storage capacity 2.2 *10 9 m *10 9 m 3 Headrace and tailrace canal Headrace canal length 800 m Headrace canal width 550 m Nile Basin Capacity Building Network ( NBCBN ) 17

30 Invert elevation spillway canal 320 m a.s.l Invert elevation powerhouse 325 m a.s.l Tailrace canal length 600 m Tailrace canal width 550 m Tailrace canal invert elevation 320 m a.s.l Powerhouse with erection bay Location Right bank Type Surface Turbines 6 Kaplan runners 6 Kaplan runners Rated head 18 m 20 m Rated output 52.5 MW each 61.5 MW each Spillway dam Location Right bank Type Combination of overflow spillway and low level sluices Spillway gates 4 radial gates Spillway gates for low level sluices 2*13=26 radial gates Design flood m 3 /s Embankment dam Total length of dam 3614 m 3630 m Dam on right bank, height 23 m 25 m Dam in river channel height 45 m 47 m Dam on left bank height 27 m 29 m Power and energy Installed capacity 315 MW 369 MW Annual average energy (for DSL=340 m a.s.l) 1630 *10 6 KWh 1810 *10 6 KWh 3.3 Selection Criterion of Case Studies In a workshop during the first phase it was agreed that a case study in each of the interested Nile basin countries should be named. The selection of case study should be based on the following: Data availability Easiness of accessibility Suitability to the sited research problems Contribution of the case study to the general understanding of the research problems Based on these criteria five reservoirs were selected two in Ethiopia, two in Sudan and one in Egypt. 3.4 Selected Case Studies Table 3-4: Reservoir sedimentation in some dams in the Nile River and Neighbouring rivers and additional studies carried out No. Reservoir Country Period considered Storage losses 1 Angereb Ethiopia % 2 Koka (Awash R.) Ethiopia % 3 Roseires Sudan % 4 Khashm Sudan % El-girba 5 Aswan high Dam Egypt % Nile Basin Capacity Building Network ( NBCBN ) 18

31 4 ROSEIRES RESERVOIR 4.1 Introduction This chapter is dedicated to discuss the case study of Roseires reservoir in Sudan. Roseires dam is known to lose one third of its reservoir by sedimentation. In the following subsections the case of the Reservoir will be discussed. 4.2 The Study Area Roseires reservoir is located in Sudan and situated along the Blue Nile reach between the dam site and the Ethiopian border. The dam is located in the vicinity of the formerly Damazin Rapids, approximately 6 km upstream the Roseires and some 500 km south of Khartoum. This dam was built in the year 1966 for multipropose irrigation, fisheries and hydropower (Gibb, 1996). The watershed of the Roseires reservoir is located between longitudinal lines ( ) north and longitude lines ( ) east. The soil properties of the study area are clay layers covered with hilly forest at Eldeim then surround by poor Savanna in Roseires and Damazin. The climate is hot in summer with rains but is cold in winter. The temperature is between (27 46 c). The annual average rain fall is 700 mm and usually falls between June to October in Damazin and 1500 mm in Eldeim. Rainfall increases gradually upon going South and decreases towards the North till it is almost dry (Ministry of agriculture in Blue Nile State, 2008). Figure (4.1) shows the location of the reservoir within the Blue Nile system. Figure 4-1: Location of the Roseires Dam and reservoir within the Blue Nile in Nile Basin Capacity Building Network ( NBCBN ) 19

32 4.3 The Data This study uses secondary data available from the dams operation unit in the Ministry of Irrigation and Water Resources. The bathymetric surveys carried at Roseires reservoir (1976, 1981, 1985, 1992, 2005 and 2007) were collected and used to estimate the sediment accumulation, sedimentation rate and trap efficiency. The 1966 data was used as the base line information and all other surveys were compared to it for storage and sedimentation estimation. 4.4 Roseires Reservoir Operations Since the trap efficiency is influenced by reservoir operation, it is important to closely examine the reservoirs in order to make judgment on their impact on trap efficiency. The roseires reservoir filling period commences after the flood peak has passed. According to the reservoir operation rules, filling may start any time between the 1st and the 26th of September each year depending on the magnitude of the flow at El Deim gauging station. From past experience, filling normally starts within the first ten days of September when the suspended sediment concentration is still relatively high at about 2500 mg/l. The filling period usually continues for nearly two months. There are four main operation periods for Roseires reservoir. During the rising flood, the reservoir drawdown attains the level of 467 R.L which is the lowest operating level. Over this operation period, minimum sediment deposition is expected despite the large quantities of sediment inflow which may approach 3 M ton/day. This is particularly true after many years of continuous operation of the reservoir where a well defined channel, capable of transporting almost the whole sediment inflow past the reservoir during the drawdown period, was developed naturally (Siyam, 2005). Due to the gradually rising water level and the relatively high suspended sediment inflow, significant sediment deposition is expected during the filling operation period. In contrast, during the third and fourth operation stages (maintaining full retention level and reservoir emptying), sediment deposition is insignificant due to the exceedingly small sediment and inflow quantities. From the above description, only operation filling period is of importance as far as reservoir sedimentation and trap efficiency are concerned in Roseires reservoir. Therefore this is taken in consideration when estimating the trap efficiency using either Brune or Churchill method. Over the filling period, the water level at 474 m R.L is considered for the computation. The reservoir content at this mean level is used together with an annual inflow of 50x10 9 m 3 to estimate the trap efficiency using both methods. The results are compared with measured values for the years when reservoir surveys were made. 4.5 Sediment Accumulation Sediment accumulation in the reservoir is calculated using the bathymetric survey data collected from the Dams Directorate of the Ministry of Irrigation and Water Resources. The base line was taken as the design storage capacity of the reservoir at the different levels in The storage capacity in the different bathymetric surveys compared to that of 1966 at different level enables estimation of sediment accumulation rates. Thus, the comparison between accumulated silt volumes deposited between the different surveys is obtained. This work is done using spreadsheet analysis in excel. The accumulated volume of deposited sediment Vd can also be calculated Empirically from the following formula V d 6. /100 * 140x10 * T / Where V d = accumulative volume of deposited sediment, m 3 Nile Basin Capacity Building Network ( NBCBN ) 20

33 . = trap efficiency after T years of operation (%) T = years of operation = average specific weight of deposited sediment over T years (t/m 3 ) calculated from Miller, 1953 formula 4.6 Siltation Rate The average silt deposit per year for the different reduced levels is calculated by dividing the sediment accumulated by the corresponding number of years of operation. The percentage of silt deposited is obtained by the following calculation: %age silt deposited per year = V A / d / N /. Where: V = Volume of silt in the given range in m 3 A = Average surface area of the reservoir at the middle of given levels in m 2 D = difference between given levels in m. N = number of years of operation. 4.7 Trap Efficiency Reservoir trap efficiency is defined as the ratio of deposited sediment to total sediment inflow for a given period within the reservoir economic life. Trap efficiency is influenced by many factors but primarily is dependent upon the sediment fall velocity, the detention-storage time, flow rate through the reservoir and reservoir operation. The relative influence of each of these factors on the trap efficiency has not been evaluated to the extent that quantitative values could be assigned to individual factors. The detention-storage time in respect to character of sediment appears to be the most significant controlling factor in most reservoirs (Siyam, 2005). Trap efficiency estimates are empirically based upon measured sediment deposits in large number of reservoirs mainly in U.S.A. Brune (1953) and Churchill (1948) methods are the best known ones. For a given reservoir experiencing sediment deposition, its trap efficiency decreases progressively with time due to the continued reduction in its capacity. Thus trap efficiency is related to the reservoir remaining capacity after a given elapsed time (usually considered from the reservoir commissioning date). The measured trap efficiency is computed from the following equation: V 0 V. (%) 6 *140*10 Where, T.E. = trap efficiency after T years of operation V 0 = original reservoir volume, m 3 V = volume remaining after T year of operation = average specific weight of deposited sediment over T years (t/m 3 ) is calculated from the following equation (Miller, 1953) / 1 * Ln i Where i the initial is value of and is given by: i cl cl sl sl sa sa Nile Basin Capacity Building Network ( NBCBN ) 21

34 Where P cl, P sl and P sa are fractions of clay, silt and sand respectively of the incoming sediment while cl, sl and sa are coefficients of clay, silt and sand respectively which can be obtained from the tables prepared by USPR, 1982 for normally moderate to considerable reservoir drawdown (reservoir operation 2) which is the case for Roseires reservoir. The essence of Churchill s method is contained in a graph relating the percentage of sediment that passes through a reservoir to a so-called sedimentation index SI. This method is given by:. 1 S S V Where T = retention time and V = mean velocity of water flowing through the reservoir (Taher, A., 1999). Brune s method is certainly the most widely used one to estimate reservoirs trap efficiency. Siyam (2000) has shown that Brune s curve is a special case of a more general trap efficiency function given by the following equation:. (%) 100 exp( V / ) Where, in addition to the already defined terms, is a sedimentation parameter that reflects the reduction in the reservoir storage capacity due to the sedimentation processes. Siyam (2000) demonstrated that the above equation with values of = , and describes well the upper, median and lower Brune s curves respectively. Brune s semi-dry reservoirs ( = 0.75), and in the case of a mixer tank where all the sediment is kept in suspension ( = 1). The Roseires Reservoir data was fitted with = which was the mean of the individual values resulting from fitting the observed trap efficiency data. 4.8 Storage Capacity Variation and Silt Deposited The variations of the reservoir storage capacity and silt contents with elevations calculated from the bathymetric surveys years, 1976, 1981, 1985, 1992, 2005 and 2007 are shown in the following subsections. Table 4-1: Storage Capacity R.L (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) Table (4.1) shows the decrease in the storage capacities with time at all reduce levels. Figures 4.2 a and b show the variation of storage with reduce level in the specific survey years and the variation of the storage Nile Basin Capacity Building Network ( NBCBN ) 22

35 with time at specific reduce level. It can be seen that after forty one years of operation ( ), the total capacity of the reservoir have been reduced to million cubic meters and million cubic meters have been lost in the last two years ( ). a b Figure 4-2: Variation of storage with time and reservoir level As the initial capacity below reduced level 467 was established to be 638 million cubic meters the loss of capacity below this level was 97.8% of the initial storage. The expected total capacity at design stage of the reservoir at level 490 m was 7.4 Mm 3. However, due to the loss of capacity found now at level 481 m which amounted to 1.92 Mm 3, the expected capacity after the heightening project implementation will be 5.48 Mm 3. Table 4-2: Accumulated silt volume deposit for different surveys R.L (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) (Mm 3 ) Table (4.2) Shows the accumulated silt deposited at different reduced levels in the different years of survey. It can be observed that there is an increase in the silt deposit with time at all reduced levels. After forty one years of operation ( ), the accumulated silt volume deposit of the reservoir has amounted to million cubic meters. About 14 million cubic meters have been added in the last two years ( ) i.e. about 1%. Figure (4.3) depicts the variation of silt deposited with time and reduced level. Figure 4-3: Variation of storage with time and reservoir level Nile Basin Capacity Building Network ( NBCBN ) 23

36 4.9 Trap Efficiency From Roseires reservoir resurveys summarized above, the observed and computed trap efficiency values with Brune s and Churchill s methods are given in table (4.3). Figure (4.4) shows graphically the variation of the trap efficiency with time. Table 4-3: Roseires Reservoir Trap efficiency % Years of re-survey T (Years) Observed Brune s methods Churchill s methods Figure 4-4: Variation of trap efficiency of the reservoir with years of operation From Figure (4.4) it can be seen that the observed trap effeceincy is invesely propotional to the square root of operation time. This figure may be used to estimate subsequent trap efficiency of Roseiers reservoir. From the figure, the projected trap efficiency after 100 years of continuous operation will be about 14% if conditions remain the same in the mean time. However, for the heightening of Roseiers dam as planned, some modifications should be done for this relationship. Also the long term impacts of the changing in the present operation rules on reservoir trap efficiency is unpredictable. It is generally believed that the volume of deposited Sediment from the 1992 resurvey as given in Tables 4.1 and 4.2 was over estimated. Making use of the results of the later resurvey in 1995, it is expected that the trap efficiency in 1992 to be close but higher than its observed value in 1995 due to the relatively short time in between the two resurveys. From Table (4.3) both Brune s and Churchill s methods overestimated the trap efficiency values. The failure of these methods may be attributed to their structures as they consider only few factors. In the earlier years of the reservoir life, the rate of sediment deposited was high as reflected in the relatively high observed trap efficiency values. The deposition rate, however, decreased progressively with time as witnessed from the gradual drop in observed trap efficiency from 45.5% in 1976 to 26.2% in This trend was not reflected in the computed trap efficiency values using both Brune s and Churchill s methods which remained fairly constant over the years of observations. Nile Basin Capacity Building Network ( NBCBN ) 24

37 4.10 Accumulation Rate Table (4.4) contains the average silt deposited per year for the different reduced levels. As depicted in figure (4.5) it can be seen that there is a decrease in siltation rate with time at all reduced levels. This phenomenon can be explained by the fact that as time passes a decrease in the reservoir storage capacity occurs; flow velocities for the same discharges are increased; the sediment carrying capacity of the flow being the limiting factor of sediment transport is in turn increased. The siltation rate has dropped from million cubic meters per year to million cubic meters per year at 467 reduce level and from million cubic meters per year to million cubic meters per year at 481 reduce levels. Table 4-4: Siltation Rate for different surveys (Mm3/Year) R.L (m) (Mm 3 /Year) (Mm 3 /Year) (Mm 3 /Year) (Mm 3 /Year) (Mm 3 /Year) (Mm 3 /Year) Years Figure 4-5: Variation of % siltation rate with time and reservoir level The volume of silt deposited in the area impounded by the given reduced levels, the average surface area of the reservoir at a given reduced level and the corresponding estimate of % silt for years 2005 and 2007 are shown in table (4.5). Table 4-5: Silt deposited per year as a percentage of storage capacity (2005, 2007) Years Level (m) Silt Vol. A(x10 6 m 2 ) %age Silt A(x10 6 m 2 )Area (Mm) of Silt Vol. %age of Silt (Mm) Nile Basin Capacity Building Network ( NBCBN ) 25

38 % Silt River Morphology Research Cluster 2010 As expected, siltation rate is generally heavy below the minimum draw-down R.L maintained during the flood period which is 467. Siltation rate is small above this minimum draw-down level. There is no increase in the percentage silt deposited in the ranges Figure (4.6) show the variation of the siltation rate for a given area in the reservoir in 2005 and Interval (m) Figure 4-6: Variation of the siltation rate with area in 2005 and Impacts of Sediment in Hydropower Plants The cost due to loss of efficiency in energy generation and repair shutdown was estimated. Moreover silt loads which inflow to power intakes cause wear to all water ways and result in decrease of its life span. Table 4.6 below shows the decrease of life span to Roseirse plant equipment due to use of slurry water Impact of Sediment on Cooling Systems Table 4.7 below shows the energy lost due to cooling system blockage in Roseirse hydro plant: Table 4-6: Equipments Depreciation Due to Siltation in Roseirs Reservoir TURBINE PARTS NORMAL LIFE OPERATION LIFE LIFE DESTRICTION DEPRECIATION DAMAGE % PART COST EURO DAMAGE COST INTAKE SCREEN U\S MENT- GATE INTAKE CONTROL GATE D\S MENT- GATE THE PENSTOCK SPIRAL CASE STAY VANES GUIDE VANES RUNNER BLADES TURBINE HUB RUNNER CHAMBER WALL DRAFT TUBE THE SHAFT SEAL TURBINE SHAFT Nile Basin Capacity Building Network ( NBCBN ) 26

39 TURBINE GUID BEARING P.I.PS FOR SHAFT SEALING LKG WATER PUMPS DE-WATERING PUMPS WATER COOLING VALVES DE-WATERING VALVES WATER PIPES WATER FILTERS SYCLONES THRUST BRG COOLER TURBINE BRG COOLER GOVERNOR OIL COOLER GENERATOR AIR COOLER WATER FLOW GUAGE WATER PRESURE GUAGE TOTAL Table 4-7: Cost of Energy Due to Cooling System Blockage NO- SEASON OUTAGE HOURS ENERGY LOST MWH ENERGY LOST COST ($) Nile Basin Capacity Building Network ( NBCBN ) 27

40 TOTAL TOTAL COST = (EURO) = *1.5 = $(AMERICAN DOLLAR) 4.13 Impact of Sedimentation on Power Intake Blockage and the Practiced Sediment Removal Measures The efforts of sediment removal systems are very costly. The cost of 1 m 3 of sediment may reached 6$ in addition to the power loss due to shutting down of units under sediment removal process. From the operation history complete blockage of the power intakes took place during flood seasons of 1975, 1983 and again in 2002 but partial blockage happen in most of the years during the flood season. The average annual silt deposited in Roseirse reservoir since first filling is about 273 Mm 3. Referring to the last reservoir survey in 2007 the storage capacity decrease by 37% from the install capacity and this is reflect the very critical situation in this reservoir References Agarwal, K.K. and K.C. Idiculla. (2000): Reservoir sedimentation surveys using Global Positioning System, Central Water Commission, Ministry of Water Resources, R.K.Puram, New Delhi Nazar, A. R. (2006): Exploratory Study of Reservoir Sedimentation by 2D and 3D Mathematical Modeling, MSc Thesis WSE-HERBD Gibb and Coyne ET Bellier (1996): Roseires Dam, Ministry of Irrigation, Hydro- Electric power Republic of the Sudan and Gibb and Coyne ET Bellier. Siyam, A.M. (2005): Assessment of the current state of the Nile Basin reservoir sedimentation problems, Nile Basin Capacity Building Network (NBCBN), River morphology Research Cluster, Group1. Taher, A. S. and M.R.M. Tabatabai (1999): Assessment of Reservoir Trap Efficiency Methods. Nile Basin Capacity Building Network ( NBCBN ) 28

41 5 ASWAN HIGH DAM (AHD) 5.1 Introduction Reservoirs are considered as a vital source of water supply, provide hydroelectric power, support diverse aquatic habitat, and provide flood protection. Reservoirs offer many benefits to the communities including flood control, water supply, fish, and hydropower. Determining the impacts of sediment on the reservoir operations is critical to maintain current operations and planning for future needs. Sedimentation within the reservoirs is the main problem that could reduce the reservoir capacity and sequently affecting its economic life. Proper management of the reservoir requires determination of current reservoir volumes and sedimentation rates. Current trend towards a more efficient management of reservoir is using the application of Geographical Information System (GIS). GIS is used for importing, analyzing, modelling, visualizing, and reporting information for the reservoir and gives functions of spatial data management, mapping and analysis to assist decision-making. Mapping the reservoir bathymetry has been used to define reservoir bed characteristics. Repetitive bathymetric mapping can help in determining the sedimentation rates, scour and deposition of bed material, and the effectiveness of dredging. It was estimated that more than 134 million ton per year is deposit in the Aswan High Dam Reservoir (AHDR) and has altered reservoir volume. It was recognized that the potential ability for sedimentation has an effect on the capacity of the reservoir and the need for data to assess the status of sediment deposition. These management goals require a thorough knowledge of the reservoirs' characteristics including their volumes. A research dealing with the investigation of sediment deposits in (AHDR) using geographic information systems based application is presented. AHDR is the second largest man-made reservoir in the world. It extends from the southern part of Egypt to the northern part of Sudan, about 500 km length. Annually bathymetric survey for the reservoir using Differential Global Positioning System (DGPS) was conducted. Spatial data were collected from aerial photographs, bathymetric data, and satellite images corresponding to the study area. The research was performed on a number of stages. These stages are: survey planning, survey execution and storage, data preparation and pre-processing, spatial data and attributes data creation, database building, and the results presentation and analysis. The results included a detailed GIS database from which contour maps, color-by-depth hill shade maps, and surface difference maps of the reservoir bed elevations were generated. Surface difference maps were produced with each subsequent survey to illustrate the seasonal sediment transport characteristics of the system. Based on the results of the GIS applications, it was identified the major zones ranging from low to high sediment deposits. The accumulation of sediment deposits over the years, associated depths, and their geographical distributions was recognized by GIS capabilities. The research utilized the Aswan High Dam Reservoir-related data to develop GIS application in order to meet the following three objectives: - A comparison between the traditional and the GIS approaches for sedimentation analysis in the reservoir. - A definition of sediment deposition patterns by taking the advantage of GIS capabilities to produce sediment deposition mapping for the reservoir bottom. - An evaluation of usage of GIS technique for estimating the lateral and longitudinal distribution of deposited sediment in reservoir. Nile Basin Capacity Building Network ( NBCBN ) 29

42 5.2 Previous Sediment Studies at AHD Since 1964, as a result of the construction of the Aswan High Dam (AHD), virtually all of the annual million tons of sediment load carried by the Nile River has been deposited behind the dam. The main concern has been for the effect of accumulation of these huge sediment deposits on the reservoir's storage capacity and hydropower production capability. Early prognostications depicted that sedimentation in the reservoir would limit its storage capacity within a few years after the construction of the AHD (Sterling, 1972; George, 1972). Eighteen years after the closure of the river at Aswan, it seems that these predictions were highly exaggerated. Research findings provide an up-to-date quantitative assessment of the temporal and geographical distribution of sediments in the reservoir. This information is considered essential for the development of more realistic predictions of the effect of sedimentation and appropriate management of the reservoir. 5.3 Traditional Approach for Sedimentation Analysis Traditional methods of sediment deposition analysis in the Aswan High Dam Reservoir were based on the comparison of the surveyed cross sections in different years to estimate the sediment volumes of the reservoir. The total amount of deposited sediment was evaluated by assuming gradual distribution to the amount of sediment between each two successive cross sections. Reservoir volumes were calculated by measuring the area of the cross sections and lengths between these sections. The volume was calculated as the sum product of the mean area of every two successive sections and the length between them. From a comparison of the year 2003 reservoir volume with the year 1964 (original volume), It was estimated that more than 5.2 billions tons of sediment deposit in the reservoir. In addition, the deposition thickness for each cross section of the reservoir was obtained from year 1964, to year 2003 as shown in Table 5.1. It was concluded that the traditional method has provided adequate results and accurately represents the sediment volume and it can represent the longitudinal distribution of sediment deposition along the reservoir. However, the traditional method would never be used for mapping as it lacks a visual component to represent the extent of sediment deposition. 5.4 GIS Approach for Sedimentation Analysis The advantages of the new technologies such as GPS and GIS have been taken into consideration in order to create methodology of sedimentation analysis in reservoirs by offering a detailed characterization of sediment deposition distribution. Nile Basin Capacity Building Network ( NBCBN ) 30

43 Table 5-1: Names and Locations of the Hydrographic Survey Stations in AHDR 5.5 Methodology The bathymetry survey of years 1999, 2001, and 2003 were selected only for the reservoir area between the cross section number (3) at Km and the cross section number (24) Km upstream Aswan High Dam which contain the major sediment deposition, see Fig In addition, this area has the availability of data in formats, which can be used to create digital surfaces of the reservoir bottom. A Geographic Information System utilizing the ArcView3.2 software was used in this application. The approach calculates the sediment volumes over entire reservoir area by comparing these digital surfaces. Through this comparison, sediment distribution can also be analyzed. The Universal Transverse Mercator (UTM) coordinate system was used in the application to allow for utilization of data from other resources. All data sources such as aerial photographs, satellite images, and other data have the same coordinate system and projection to create the base map for the reservoir. The high quality baseline data lead to accurate sediment investigations. Nile Basin Capacity Building Network ( NBCBN ) 31

44 Figure 5-1: Cross sections of AHDR for years 1964, 1998, and Sediment Deposition Mapping The sediment deposition maps of years 1999, 2001, and 2003 covering the same geographic area were produced and compared to identify changes in the reservoir bed elevations and the deposition in the reservoir. The presentation of this analysis was images color coded by the amount of change. Areas shaded with green indicate of the significance of sediment deposition. Areas of no overlapping data or erosion remained white, see Fig It was detected that more than 80 percent of the deposition thickness was more than 2.50 meters. The larger changes from year 1999 to year 2003 occurred in the wider entrance of the reservoir. These maps were generated volumes based on a more accurate method that used data for the entire reservoir and not just data from a few cross sections. The accuracy of these maps may be affected by the density of the data coverage. Nile Basin Capacity Building Network ( NBCBN ) 32

45 Figure 5-2: Reservoir Bed Elevation (2003) and Sediment Deposition (2003) 5.7 Conclusions and Recommendations Reservoir sedimentation may strongly affect the economy of hydropower schemes. The research indicated that the combination of hydro acoustics, GPS, and GIS are capable of producing bed elevations maps comparable in accuracy and quality to traditional surveying method. A key difference between the traditional and GIS analysis approaches is that the GIS approach calculates sediment volumes over the entire reservoir area by comparing digital surfaces, whereas the traditional approach applies an average area method to calculate volumes based on a limited number of cross sections. A future benefit of the GIS analysis approach will be the ability to view time perspective of sediment change and support automated sedimentation analysis. When additional surveys are performed for the Aswan High Dam Reservoir, new sediment depth grids can quickly be created to represent sediment depositions with respect to prior surveys. However, certain issues and problems were recognized during this study. The amount of the surveyed data of the reservoir is not enough to cover the entire reservoir due to the huge area. It is recommended to complete bathymetric survey for all the remaining areas of reservoir in future. Nile Basin Capacity Building Network ( NBCBN ) 33

46 5.8 References 1. Abdel-Fattah, S., A. Amin and L. Van Rijn, (2004), Sand Transport in Nile River, Egypt, Journal of Hydraulic Engineering, American Society of Civil Engineers, ASCE, Vol. (130), No. (6), P Adongo FG (1975) Phosphorus removal efficacy of Phragmites mauritianus (Kunth) constructed wetland in Jinja, Uganda. M.Sc. thesis, D.E.W. 015 IHE Delft, The Netherlands. 3. Barbanti A, Bergamini MC, Frascari F, Miserochi S, Ratta M and Rosso G (1995) Diagenetic Processes And Nutrient Fluxes At The Sediment-Water Interface, Northern Adriatic Sea, Italy. Marine Freshwater Research Vol. 46 No.1 pp George, C.J. (1972) In: The Careless Technology: Ecology and International Development (ed. by Farvar & Milton). The National History Press, Garden City. 5. Maguire, D.J., (1992), Geographical Information Systems: Principles and Applications, Long man, England 6. Mau, D.P., and Christensen, V.G., (2000), Comparison of Sediment Deposition in Reservoirs of Four Kansas Watersheds: U.S. Geological Survey Fact Sheet 7. Morris, L.M., and Fan, J., (1998), Reservoir Sedimentation Handbook: New York, McGraw-Hill, p Mostafa, M.G. (1978) Sediment processes in the Nile River. United Nations Development Program Report no EGY/73/024, New York. 9. Mostafa, G. (1987). Reservoir Sedimentation, Post Graduate Course in Sediment Transport Technology, proceeding Vol. 2 Ankara, Turkey 10. Sheppard, S.R., (2000), Visualization Software: Bringing GIS Applications to Life, GeoEurope, Issue Roberts, J. D., Jepsen, R. A., and James, S. C. (2003). Measurements of sediment erosion and transport with the adjustable shear stress erosion and transport flume, J. Hydr. Eng., ASCE, Vol. (129), No. (11), Van Rijn, L. C. (1984a). "Sediment transport, Part I: bed load transport" J. Hydr. Eng., ASCE, Vol. (110), No. (10), Van Rijn, L.C. (1984b). Sediment transport, Part II: Suspended load transport, J. Hydr. Eng., ASCE, Vol. (110), No. (11), Van Rijn, L. C. (1990). Principles of fluid flow and surface waves in rivers, estuaries, seas and oceans, Aqua Publications, Amsterdam, The Netherlands. 15. Sterling, C. (1972) The Aswan disaster. In: Our Chemical Environment (ed. by G.Giddings & J.B. 508 Scot E.Smith et al.) 16. Wagner, H.L. (1976) The Landsat Interactive Grey Map and Level Slice System. Master of Science Thesis, the University of Michigan, Ann Arbor, Michigan. Nile Basin Capacity Building Network ( NBCBN ) 34

47 List of Research Group Members Name Country Organisation Dr. Ahmed Musa Siyam Sudan UNESCO-CWR Dr. Kamaleldin E. Bashar Sudan UNESCO-CWR Eng. ElTahir Osman ElTahir Sudan MPPPU- Kh. State Ms. Semunesh Golla Ethiopia MoWR Dr. Sami Abdel Fattah Egypt Hydraulics Research Institute Eng. Nindamutsa Astere Burundi IGEBU, Burundi Mr. Musenze Ronald S. Uganda Makerere University Prof. Ahmed Salih Sudan MoIWR-HRS Mr. Alnazir Saad Ali Sudan NEC Eng. Muna Musnad Sudan UNESCO-CWR Prof. Sami Omer Sudan Nilein University Ms. Ishraqa Osman S. Sudan UNESCO-CWR Shokry M.A.Abdelaziz Egypt Stuttgart University Scientific Advisor: Dr. Alessandra Crosato Senior Lecturer in River Morphology UNESCO-IHE, the Netherlands Full Profiles of Research Group Members are available on: The Nile Basin Knowledge Map Nile Basin Capacity Building Network ( NBCBN ) 35

48

49 Reservoir sedimentation is a severe threat to the optimal use of water resources in many river basins. The Nile Basin is no exception in this respect. Some of the reservoirs in the basin have already silted up substantially and also new reservoirs like the Merowi dam and dams in Ethiopia and Uganda will be subject to sedimentation and hence storage losses. Good prediction of the future reservoir sedimentation is important, but even more important is to find optimal mitigation methods to reduce sedimentation after some years in which inevitable sedimentation is taking place. A number of reservoirs elsewhere for example is now regularly flushed and no further storage loss is experienced. The existing reservoirs especially those on the Blue Nile and the River Nile are seriously affected by sediment deposition at unexpected rates. Currently, for some reservoirs, costly sediment control measures are being practiced. Decision of postponing the problem by heightening the dam has been taken and unresolved situation is waiting for those in design and planning stage. Also the Aswan High Dam, though large, it is facing the problem of 100% trap efficiency. Further, in Sudan, Ethiopia and Uganda new dam projects are underway or have been proposed. Data pertaining to sedimentation, current adopted operation polices and practiced control measures in these reservoirs in general are far from being comprehensive and/or optimised. In addition the impact of theses polices and measures on new dam/reservoir projects or vies-versa need a thorough understanding and special tackling as usually the consequences propagate in both upstream and downstream directions. It is expected that launching joint research, within the basin, will contribute to the common understanding of the siltation characteristics of the reservoirs in the region, help in defining the most suitable assessing tools (models), and in selecting the most appropriate combat measures. The outcome is vital piece of information for both dam owners and users as operation polices can be modified to prolong the useful life of reservoirs for future generation. Publisher: The Nile Basin Capacity Building Network, 2010