The 2021 Melamchi Flood: A massive erosional cascade in the Himalayan Mountains of central Nepal

Abstract

Abstract Large, sediment-laden floods in mountainous terrain can have disastrous consequences and play important roles in landscape evolution. These events often unfold as a series of interconnected processes, but understanding of such “hazard cascades” has been hampered by lack of quantitative data on sediment movement. Here, we use a time series of high-resolution satellite imagery to quantify erosion and aggradation during the 2021 Melamchi Khola Floods in the Himalaya of central Nepal, providing a unique sediment budget for such an event. Our analysis reveals massive headwater erosion via remobilization of Gorkha landslides, gullying, debris-flows, and incision of glacial deposits. Unlike many other high mountain floods, the widely distributed erosion suggests this event was not primarily driven by a single source, e.g., glacial lake or landslide dam failure. High sediment supply caused aggradation in a high-elevation, low-relief glacial valley and triggered catastrophic incision into associated ancient fills. As this material was transported downstream, it caused further riverbed incision that in turn resulted in failures of surrounding hillslopes. Further downstream, as river steepness diminished, the main channel in the lower basin was widened by 3-5-fold and aggraded by ~ 5–20 m. However, deposition in the Melamchi Khola was not enough to accommodate the vast amount of flood material, and over 70% was delivered from the Melamchi Khola to the downstream Indrawati basin. Our sediment budget provides rare insight into the chain of events involved in a massive flood and helps shed light on how such floods can magnify hazard and reshape the fluvial landscape.