Urban flood hazard analysis using integrated modeling in the Manohara Khola Catchment, Nepal

Climate change is intensifying extreme rainfall events, increasing the risk of urban flooding, particularly in developing countries like Nepal. This study examines the lower Manohara Khola, a major river of Kathmandu Valley, to assess flood risk reduction under climate change. The study follows three main phases: synthetic storm generation, integrated hydrological–hydraulic modeling, and assessment of climate change impacts on extreme rainfall, flood discharge, and inundation. The HEC-HMS hydrologic model was combined with the two-dimensional HEC-RAS hydraulic model to simulate peak discharge hydrographs from the upstream mountainous (forested and hilly areas of Shankharapur Municipality) region and flood inundation in the downstream area, respectively. The integration of hydrological and hydraulic models significantly reduced computation time, as a standalone inundation model would need to simulate the entire basin, including the upper mountainous areas. The flood inundation model was set-up using the flood event of August 9–10, 2022. To assess the impacts of climate change, high-resolution (2 km, hourly) precipitation data from the Non-Hydrostatic Regional Climate Model (NHRCM) under the SSP5-8.5 scenario were utilized. Extreme rainfall values available for the historical (1981–2000) and future (2080–2099) periods were used to simulate and compare flood hazards. Rainfall intensity–duration analysis indicates an average 60% increase in extreme rainfall, ranging from 12% to 110%. Simulated peak discharges suggest substantially higher future flood risk, with the future 100-year flood nearly doubling the current level. Flood-prone areas expand under future climate conditions, shifting low-risk zones into medium- and high-risk categories. Areas with inundation depths exceeding 1.0 m increase from 3.5 km² (41%) to 4.1 km² (48%) for the 5-year event. Overall, both flood depth and inundation extent increase significantly with higher return periods.