Integrated hydrological and hydrodynamic modelling of climate driven flood risk and socioeconomic impacts in the River Nyamugasani Basin in Uganda

Flooding increasingly threatens settlements, livelihoods and services in Uganda’s Rwenzori region, yet evidence for future-climate planning remains limited. This study assessed flood hazard, exposure, institutional damage and management priorities in the River Nyamugasani Basin (RNB). Bias-corrected rainfall from 11 CMIP6 models under SSP1-2.6, SSP2-4.5 and SSP5-8.5 drove HEC-HMS; annual maxima were fitted with the GEV distribution and passed to HEC-RAS 2D. Flood maps were combined with population, land-use and institutional data, and management options were ranked using household surveys and expert ratings. Under SSP5-8.5, the estimated 100-year discharge increased from 61.9 m 3 /s at baseline (95% CI 44.1–95.7 m 3 /s) to 126.0 m 3 /s (95% CI 113.9–140.7 m 3 /s). The corresponding inundated area expanded from 16.05 to 22.96 km², and the exposed population rose from 4862 to 6956. Direct physical losses to schools, health facilities and religious institutions were estimated at USD 1.91 million in constant June 2025 prices, with a combined sensitivity range of USD 1.09–2.69 million. Diversion channels and gabion revetments were the preferred structural measures, whereas flood forecasting and warning systems led the non-structural measures. Calibration attempts for both HEC-HMS and SWAT showed weak performance under the limited available observations. The final HEC-HMS discharge series was therefore generated using physically estimated parameters rather than a calibrated parameter set and could not be independently validated. Consequently, the discharge, inundation, exposure and damage estimates should be interpreted as scenario-based screening results rather than engineering-design values. Despite this limitation, the findings can support climate-informed floodplain zoning, prioritisation of critical-facility protection, expansion of forecasting and warning systems, and phased evaluation of structural flood-management measures.