Rainfall erosivity and sediment dynamics in the Himalaya catchment during the Melamchi flood in Nepal

Rainfall erosivity is an indicator of rainfall potential to cause soil erosion. The Melamchi extreme flood occurred on June-15 and recurred on July-31, 2021 in Nepal. During these flooding events, a large volume of sediments were eroded, transported and deposited due to the high rainfall erosivity of the basin. In this study, the temporal and the spatial distribution of rainfall erosivity within the Melamchi River Basin was estimated and further linked to sediment discharge and concentration at various sites along the river segments. The daily rainfall data for the event year 2021 of the entire basin were used. Validation was performed by post-flooding grain size sampling. The result showed that rainfall and rainfall erosivity exhibit pronounced intensity within the Melamchi River basin, particularly at Sermathang and Tarkeghang, both located in the middle section of the basin. The average annual rainfall in the Melamchi region was 3140.39 mm with an average annual erosivity of 18302.06 (MJ mm)/(ha h yr). The average daily erosivity of the basin was 358.67 (MJ mm)/(ha h) during the first event and 1241 (MJ mm)/(ha h) for the second event. In the upper section of sampling, the sediment size ranged from 0.1 mm to >8 mm and was poorly graded. However, the lower region had smaller sediment ranging from 0.075 mm to >4.75 mm and also well graded. The smaller size (<1 mm) sediment passing was much higher in the Chanaute (78%) and Melamchi (66.5%) river segments but the larger size (>100 mm) sediments were passed relatively higher from the Kiwil (8.20%) and Ambathan (8.39%) river segments. During pre-monsoon and monsoon seasons, the highest sediment concentration was found to be 563.8 g/L and 344.3 g/L in Bhimtar and the lowest was 238.5 g/L and 132.1 g/L at the Ambathan, respectively. The sediment concentration during the pre-monsoon was found to be higher than the sediment concentration during the monsoon season in the Melamchi River. The more erosive regions in the basin were associated with the presence of highly fractured rock, weathered rocks and a thrust (weak) zone. The higher rainfall erosivity at upstream and the higher sediment concentration at downstream during flooding events have coincided well in the basin. Thus, the estimation of rainfall erosivity at the catchment scale and its influences on sediment concentration in the river are crucial for erosion control measures during flooding times in the Himalaya.