Sediment Management at Kali Gandaki HPP, Nepal

Client: World Bank

Contract Period: 2012-2015

The 144 MW Kali Gandaki run-of-river hydropower plant, with a design flow of 141 m3/s and 115 m gross head, is currently the largest power plant in Nepal. The 44 m dam with radial crest gates impounds a 7.7 Mm3 reservoir, but the sediment load is so high it could fill the reservoir with sand during a single monsoon. Pondage volume of 3.5 Mm3 has been sustained since operation began in 2002 by sediment sluicing, drawing down the reservoir when flows exceed about 200 m3/s to pass sediment beyond the dam. However, turbine abrasion is severe despite having a desander for diverted flow.

Dr. Morris and Eng. Portalatín of GLM Engineering COOP, were both contracted by the World Bank at different periods to evaluate aspects of hydraulics and sediment man­age­ment at Kali Gandaki in support of a World Bank - financed project to rehabilitate the facility to provide better control of sedi­mentation, reduce turbine abra­sion, plus other benefits.

Work performed included review of existing conditions during site visits, review and analysis of both sampling techniques and the data collected, data analysis to identify operational patterns which contri­bute to higher abrasion rates, and pre­paration of recommendations on sampling, monitoring and operat­ions. A 41 km reconnaissance of the river above the dam, a zone accessible only by raft/kayak, to better document the characteristics of the available bed material supply.

Hydraulic numerical modeling was performed by HEC-RAS to evaluate the impact of sedimentation on flood levels affecting a flood-prone community at the upstream limit of the reservoir. Long-term sediment transport modeling by the SRH-1D model was performed to evaluate the potential for modifying the operating rule to enhance sediment management and reduce sediment accumulation adjacent to the upstream community.

This project is continuing, now focusing on review of construction plans being prepared to implement the previously-developed rehabilitation strategies.