Modelling Urban Flood Inundation Under Climate Change Scenarios in Benin City, Edo State

Urban flood risk in Benin City is expected to intensify under projected climate change scenarios characterised by increased rainfall intensity and shifting seasonal patterns, yet flood inundation modelling incorporating future climate uncertainty has not been applied to this city. This study models urban flood inundation extents under current baseline and two climate change scenarios for Benin City using a coupled hydrological and hydraulic modelling framework. The Storm Water Management Model was calibrated and validated against observed flood extents from documented events in 2018 and 2021, achieving a Brier skill score of 0.71. Representative concentration pathway 4.5 and 8.5 climate projections for the 2040 to 2060 period, obtained through statistical downscaling of CMIP6 global climate model ensembles, were used to generate design storm inputs. Inundation extent and depth surfaces were generated for 10-year, 25-year, and 100-year return period events under each scenario. Results project significant expansion of flood inundation extents under both climate scenarios, with the RCP 8.5 scenario producing 27 to 41 percent increases in inundated area compared to baseline for equivalent return period events. Critical infrastructure including health facilities, schools, and major road sections located within projected expanded inundation zones are mapped and quantified. The study demonstrates that current drainage design standards in Benin City are inadequate to accommodate projected climate-intensified rainfall, even under the moderate RCP 4.5 scenario. Recommendations prioritise drainage upgrading in high-growth catchments and revision of Benin City's stormwater design standards to incorporate a climate change safety factor. Keywords: flood modelling, climate change scenarios, urban drainage, Benin City, stormwater management.