Future Flood Hazards in Nepal: Projecting Magnitude Changes and Targeting Nature-based Mitigation.
Date:
Sim Reaney, Ivo Pink and Callum Pearson
Flood impacts in Nepal are increasing, and are projected to intensify under climate-change over the 21st century. To help mitigate this increased hazard, two of the key questions are ‘how much will the flood magnitude and frequency change?’ and ‘If we are to use catchment-based mitigation approaches, including nature-based solutions, where in the catchment should they be implemented?’ We present the results of simulation modelling in the Karnali catchment and flood-source risk mapping in the East-Rapti. Within the Karnali catchment, we used the SPHY hydrological model, coupled with 12 CMIP6 GCM ensemble members, to simulate changes in flood magnitude and frequency. This modelling was undertaken within a GLUE-based uncertainty-estimation framework. Within the East-Rapti, we mapped the most likely effective locations for implementing nature-based solutions with SCIMAP-Flood. This tool is based on catchment connectivity, land-use, rainfall patterns, and floodwater travel times to the impacted communities. The simulations for the Kanali catchment show a projected increase in the 1% annual exceedance probability flood magnitude of + 40% (P10/90: 33/48%) (medium-emissions) and + 79% (P10/90: 54/82%) (high-emissions) for 2060–2099, with rainfall-runoff contributing ≥ 90% of the additional flood water, as opposed to groundwater or glacial-melt. For the East-Rapti catchment, the SCIMAP-Flood results indicate that spatial targeting of mitigation measures in irrigated and rainfed agriculture, and the prevention of deforestation or the removal of shrubland, would be the most effective approaches. If these actions were in the upper catchment above Hetauda, or upstream of Manahari, they would have the most effective reduction in the flood peak. Overall, these tools improve our understanding of the flood hazard generation by quantifying changes in flood magnitudes and frequencies, analysing the contributions of runoff components, and mapping the critical source areas of floodwater generation. When combined, they can provide effective measures for developing flood risk management strategies.
Presented at AOGS 2026, Fukuoka, Japan