Extreme Rainfall, Anthropogenic Sediment Supply, and Backwater Ponding: Compounding impacts on Flood Hazard in the Kathmandu Valley, Nepal

On 27–28 September 2024, the Kathmandu Valley experienced unprecedented rainfall, exceeding the previous record for 24-hour cumulative precipitation in Kathmandu and surrounding regions. This extreme event triggered severe flooding, resulting in loss of life and the burial of buildings, roads, and other infrastructure beneath thick sediment deposits. The flood also damaged hydrological gauging stations, preventing the recording of peak flood levels during the event. Despite this limitation, the flood left distinct geomorphic and sedimentary evidence along the floodplain, including high-water marks on building walls and indicators of sediment thickness. These field observations were used to reconstruct flood heights and sediment deposition, enabling the preparation of a field-based flood inundation map. We compare the reconstructed inundation extent with numerical model outputs, including (i) a 1-in-100-year return-period flood scenario and (ii) a hydrological model simulation driven by rainfall recorded during the 2024 event. The results show that flood inundation during the 2024 event was significantly greater than predicted by both model scenarios. The residual flood height inferred from field evidence is attributed to compounding effects, particularly increased sediment supply associated with anthropogenic activities, notably mining waste. In addition, we document pronounced backwater effects at river confluences and along river reaches confined within gorge sections, which further exacerbated flood severity by enhancing sedimentation and reducing the river’s conveyance capacity. We conclude that the combined effects of backwater conditions and high sediment accumulation significantly amplified flood inundation. Our findings highlight that, in many high-mountain settings where sediment supply and extreme rainfall are increasing, these processes, particularly at tributary junctions, should be explicitly considered in future flood models.