Evaluation of sediment management strategies for Tarbela Reservoir

Abstract

Multipurpose large dams play a significant role in the development of world by providing water for irrigation, flood control and hydropower. This study aims to evaluate different strategies for the evacuation of deposited sediments and reducing sediment inflows to Tarbela Reservoir by using HEC-RAS. Sediment flushing from existing power tunnels was found unfeasible due to the downstream constraints and the loss of 7848 MW of hydropower. New sediment bypass tunnels on the right bank of the dam to overcome the constraints were analysed. The sediment balance ratio, long-term capacity ratio and economic analysis were computed along with sediment modelling for different scenarios. The most technically viable scenario was 90 days flushing at 390 m drawdown level with a discharge 5000 m3/s. This scenario was not economically feasible as NPV was negative, IRR was 3-4 %, and the benefit cost ratio was less than one. The Diamer Basha Dam under construction upstream of Tarbela Reservoir was also evaluated. The results suggested a large volume of sediment would be trapped in the upstream reservoir which would ultimately lead to a significant reduction in the inflow of sediment and the delta movement in Tarbela Reservoir. This option was recommended due to its multiple benefits and potential capacity to enhance Tarbela Reservoir's life.