Sediment Exclusion at Trishuli Power Intake, Nepal

Client: Daelim Industrial Co., Seoul, South Korea

Contract Period: 2014-2015

GLM Engineering (PerĂș) was contracted as a sediment management consultant for the Upper Trishuli project in Nepal, a proposed 216 MW hydropower facility with 350 m gross head and 76 m3/s design flow. The project includes a 24m tall barrage-type dam with large radial gates on a river having 3% slope which transports material as large as boulders. The project is being designed to include settling basins to remove sand, and the principal design problems we addressed were minimization of sand and gravel entrainment by the intake, optimal flushing of sediment from the impounded reach, and hydraulic con­ditions at the trash rack together with exclusion of floating woody debris.

A review was performed of all prior studies, followed by site visits to advise on the physical modeling tests being performed in Kathmandu. Alternative intake configurations were tested, including both frontal and lateral configurations, as well as alternative gate configurations at the dam to facilitate sediment clearance from the intake area and to minimize the entrainment of sand and gravel. Schematic designs were prepared for incorporation into the hydraulic model and testing under a wide range of flows. There was considerable emphasis on the part of the client to minimize modifications to the original design concept for the intake, debris screen and gravel trap, none of which was performing satisfactorily, so as not to increase the project cost estimate.

After carefully observing the flow characteristics at the intake for alternative configurations, a design was developed and successfully tested, which incorporated several features which allowed the overall concept of the lateral intake to be retained, yet which yielded greatly improved performance in terms of sediment exclusion, which eliminated undesirable flow patterns at the trash rack, and which also improved gravel trap function.

This project is continuing.