Estimation of Suspended Sediment Load and Spatial Distribution of Sediment Yield in the Marsyangdi River Basin, Nepal

Abstract

Himalayan river basins are characterized by exceptionally high sediment loads driven by active tectonics, glacial influence, steep terrain, and intense monsoon precipitation, yet quantitative estimates of spatially distributed sediment generation remain scarce in Nepal. This gap constrains the sustainable planning and operation of run-of-river hydropower projects, where excessive sediment loads increase turbine abrasion, maintenance costs, and production losses. This study applied the Soil and Water Assessment Tool (SWAT) to estimate sediment yield and sediment concentration in the Marsyangdi River Basin, Nepal, a 2,944 km² glaciated tributary of the Gandaki River system hosting three operational run-of-river hydropower stations with a combined installed capacity of 189 MW. The model was calibrated for 2012–2014 and validated for 2015, reproducing streamflow with very good performance (NSE = 0.864, R2 = 0.874 in calibration; NSE = 0.830, R2 = 0.867 in validation) and sediment load with satisfactory performance (NSE = 0.556, R2 = 0.608 in calibration; NSE = 0.516, R2 = 0.617 in validation). Simulation for 2012–2024 indicated that about 89% of total basin sediment generation originates from five southern sub-basins characterized by higher orographic precipitation, whereas northern rain-shadow sub-basins contribute only marginally. Average annual sediment export at the basin outlet was estimated at 9.42 Mt/yr, with mean sediment concentration of 1,385 mg/L. About 73% of the annual sediment load occurred during the monsoon months, with concentrations exceeding the 1,500 ppm operational threshold on 95 days a year, highlighting a distinct period of elevated hydropower operational risk and the need for targeted monitoring and watershed management.