Sustainable Sediment Analysis, Upper Karnali Project, Nepal

Client: GMR Energy, New Delhi, India

Contract Period: 2014-2015

The Upper Karnali hydropower project on the Karnali River in Nepal, is being designed with 900 MW of installed with a design discharge of 664 m3/s operating at 159 m gross head. The dam will extend 30 m above the riverbed and will be equipped with large radial gates to provide barrage-like operation. Without sediment management, the 8.9 km long reservoir having 18 Mm3 of gross storage, could be completely filled with sediment during a single monsoon. It is desired to manage sediment to maintain the intake reservoir in operation, to permanently preserve at least 7 Mm3 volume in the upper portion of the pool for daily power peaking (pondage), and to also use reservoir storage as a sediment trap to avoid the expenditure of approximately $500 M on underground sedimentation basins. The passage of woody debris management is also an issue at this location.

GLM Engineering (Perú) was contracted as an independent consultant to provide expert inputs on all phases of modeling (1- and 2-dimensional numerical modeling, and physical modeling), and to provide guidance on the config­ura­tion and operating rule for the headworks to best achieve the project’s objectives.

The envisioned project operation involves utilization of a sediment bypass tunnel. At low flows all water is diverted to power, but at high discharges the river flow in excess of power needs is bypassed.

This is an ongoing project. Activities to date have included two site visits, giving particular emphasis on data review and recommendations for any additional sediment monitoring, model­ing recom­mendations, determining the potential transport rate for coarse bed material (gravels and cobbles), and review of the results of two different physical models.

As part of this work we prepared a daily operating model to provide a preliminary analysis of alternative operating rules, the year-to-year variability in the rate of sediment yield and trapping, and the impact of alternative operating rules on power generation.