Flood Vulnerability, Social Risk, and Economic Damage Assessment in the Lower Karnali River Basin, Nepal

ABSTRACT: The lower Karnali River basin is one of Nepal’s most flood-prone regions, yet it has lacked an integrated risk assessment. This study presents the first comprehensive analysis combining physical vulnerability, social vulnerability, and economic damage assessment for the corridor. Utilizing a validated 2D HEC-RAS model across eight return periods (2–500 years), we quantified impacts on 23,929 buildings, 477 km of roads, and 16,065 hectares of paddy cropland using locally calibrated depth-damage curves. Those calibrations were mainly focused on three building typologies, two road surface classes, and paddy crops at the maturity stage. A Social Vulnerability Index (SVI) was developed for 27 wards, integrating demographics, healthcare access, and education. Findings indicate total economic damages range from NPR 395 million (2-year) to NPR 3538 million (500-year), with buildings consistently accounting for the largest share (41–42%). Madhuwan Ward 6, Geruwa Ward 1, and Rajapur Ward 7 emerged as the highest combined flood risk hotspots through the integration of social vulnerability and physical hazard. The results prove that social infrastructure investment, particularly in healthcare, serves as a direct flood risk reduction measure. This research provides a spatially explicit evidence base to guide targeted mitigation, land-use policy, and social protection in the basin.