Evaluation Of Sediment Management Strategies For Large Reservoirs

Multipurpose large dams play a key role in the development of world by providing water for irrigation, flood control and hydropower. Tarbela is one of the world's largest earth and rock fill dam. Being multipurpose dam, it provides vital role for economic stability and social development of Pakistan. Tarbela Reservoir has lost its significant capacity due to sediment deposition. The objective of the study was to evaluate different options for evacuation of deposited sediments and reducing sediment inflows to Tarbela Reservoir through sediment modeling by HEC-RAS. Sediment flushing from existing power tunnels was evaluated in first option and found not feasible due to the downstream constraints and loss of 7848 MW hydropower from Tarbela and Ghazi Barotha. New sediment bypass tunnels were proposed on right bank of the dam to overcome the constraints in second option. Sediment modeling was performed by HEC-RAS to evaluate each scenario of sediment flushing with different parameters. The sediment balance ratio and long term capacity ratio was also checked for each scenario for technical evaluation and also economic analysis was performed. Most technical viable scenario was flushing for 90 days at reservoir drawdown level of 390 m with discharge of 5000m 3 /s. However, this scenario was not economically feasible as net present value was negative, internal rate of return was 3-4 %, and benefit cost ratio was found less than one. The 3 rd option, with under construction multipurpose Diamer Basha Dam on upstream of Tarbela Reservoir, was also evaluated on HEC-RAS. Results depicted that large amount of sediments were trapped in the upstream reservoir which ultimately reduced significantly the inflow of sediments and delta movement in Tarbela Reservoir. This option is recommended because it will enhance the life of Tarbela Reservoir and it will keep on providing multiple benefits for longer time.