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

Large, sediment-laden �oods 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-ows, and incision of glacial deposits. Unlike many other high mountain �oods, 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 �lls. 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 �ood 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 �ood and helps shed light on how such �oods can magnify hazard and reshape the �uvial landscape.

山区地形中携带大量泥沙的洪水可能会造成灾难性后果，并在地貌演化中起着重要作用。这些事件通常是一系列相互关联的过程，但由于缺乏泥沙运动的定量数据，对这种“灾害级联”的理解受到了阻碍。在此，我们利用一系列高分辨率卫星图像来量化2021年尼泊尔中部喜马拉雅地区梅拉姆奇河洪水期间的侵蚀和堆积情况，为这类事件提供了独特的泥沙收支情况。我们的分析显示，通过戈尔卡滑坡的再次活动、冲沟形成、泥石流以及冰川沉积物的下切，源头发生了大规模侵蚀。与许多其他高山洪水不同，广泛分布的侵蚀表明该事件并非主要由单一源头驱动，例如冰川湖或滑坡坝溃决。高泥沙供应量导致高海拔、低地势的冰川谷发生堆积，并引发对相关古堆积物的灾难性下切。当这些物质被搬运到下游时，它导致了河床进一步下切，进而导致周围山坡崩塌。再往下游，随着河流坡度减小，下游盆地的主河道拓宽了3 - 5倍，堆积了约5 - 20米。然而，梅拉姆奇河的沉积量不足以容纳大量的洪水物质，超过70%的物质从梅拉姆奇河被输送到下游的因德拉瓦蒂盆地。我们的泥沙收支情况为了解大规模洪水所涉及的一系列事件提供了罕见的视角，并有助于阐明此类洪水如何放大灾害以及重塑河流地貌。