Assessment of Sediment Load of Langtang River in Rasuwa District, Nepal

It is the headwater area of the Trisuli River in the Narayani River system, covering a total area of 353.59 km2 with elevation ranging from 3652 m a.s.l. to 7215 m a.s.l. The glaciers of this catchment cover an area of 137.5 km2 while the remaining area of 216.1 km2 is covered by rock and vegetation. A gradual increase in air temperature during the pre-monsoon season from March to mid-June is observed, followed by the monsoon season from mid-June to the end of September, which is dominated by positive air temperatures and more than 80% of annual precipitation falls during this season. Suspended sediment concentration (SSC) is calculated from water samples collected from the Langtang River catchment using the Depth Integration Technique (DIT). The frequency of sample collection varies depending on the season, with daily samples collected during the monsoon season, weekly samples collected during the pre- and post-monsoon seasons, and bi-monthly samples collected during the winter season. Stage height for discharge calculation is also collected on the same frequency and time period as the water samples. The diurnal variation in sediment content is studied by collecting samples twice a day, in the morning at 8 AM and in the evening at 4 PM. The annual sediment yield of the Langtang River is 109,276.75 tons, with 37.69, 11.52, and 5.54 tons of sediment transported per day in the pre-monsoon, post-monsoon, and winter seasons, respectively. The monsoon season contributes the highest sediment load, with a value of 872.86 tons per day, accounting for 83% of the total sediment transport. A diurnal cycle of sediment discharge is observed, with higher sediment discharge during the evening than the morning, reaching maximum values of 41.1 kg∙s−1 and 61.5 kg∙s−1, respectively. A clockwise hysteresis loop is obtained for discharge and sediment discharge, where sediment flux is higher in the early monsoon than in the late monsoon for a corresponding discharge. The study aims to provide knowledge on the amount of sediment being transported from the Himalayan region on a seasonal as well as diurnal basis. This information may help predict and prevent natural calamities and is crucial for predicting the feasibility of constructing hydropower plants downstream. The Langtang River catchment is a significant area for hydroelectric power generation, and the study's findings can contribute to the development of effective land and water management strategies in the region.