Sediment Management and Monitoring, Tarbela Dam, Pakistan

Client: WAPDA, Tarbela Dam

Contract Period: 2013-2014

Dr. Morris was contracted as Individual Consultant at Tarbela Dam, Indus River, Pakistan, to analyze sedimentation issues and make recommendations for sediment monitoring. This critical 14,300 Mm3 reservoir had lost approximately 36% of its capacity to sedimentation since initial impounding in January 1975, and the sand-silt delta is approaching the area of the power intake.

Dr. Morris reviewed all available prior studies of the sedimentation problem, reviewed the data collection, sampling techniques and data available at the dam, assessed and identified the potentially feasible management strategies, and recommended the sediment data collection activities needed to support future sediment management decisions.

Sediment must inevitably be passed downstream of Tarbela dam by one of four options: (1) pass through turbines, (2) reservoir fills with sediment which passes over spillways, (3) annual reservoir emptying to flush sediment through new low level outlets, and (4) dredging. Only the last two options were considered feasible.

Cost estimates were compiled for both feasible options, relying heavily on data from prior studies. On a preliminary basis, dredging was identified as the lower-cost option due to: the low cost of power at the dam, by eliminating the need to halt power production during the flushing period and purchase more costly replacement power (if available), plus lower capital costs.

The most uncertain cost elements under the two options were identi­fied. For flushing, confirm the sediment concentra­tion and thus flushing duration required to achieve sediment balance. For dredging, confirm the rate of pipeline abrasion and its replacement cycle, which is a large component of the annual cost.

Detailed recommendations and equipment lists were prepared for sediment studies to answer key technical questions about the two identified options. In the case of flushing, monitoring of sediment concentration and transport across the top of the delta during drawdown was recommended to provide data for verification of the transport rates incorporated into the model used to simulate reservoir flushing. This would be accomplished using a fast jet boat as a sampling platform and depth-integrated sampling techniques. To better estimate dredging costs, a small test dredging project was recommended using pipelines of different diameters (flow velocities) and materials (steel, HDPE), to calibrate an abrasion-rate model and also confirm required transport velocity for Tarbela sediment.