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Located in a humid tropical monsoon region, Gin River Basin is frequently exposed to adverse flood impacts. Flood exposure assessments extending beyond traditional hazard assessments are crucial to understand these impacts. We examined flood hazard and exposure dynamics during three flood events in the basin, simulating flood hazard using the rainfall–runoff inundation model. Model parameters, including Manning’s roughness coefficients and soil depth were tuned through a trial-and-error approach using daily rainfall data. Two datasets with different spatial resolutions (∼300 m and 30 m) were used to identify exposed areas. Despite differences in hazard characteristics, the impact on human settlements remained similar, with household damages of 43.72% in 2017, 52.74% in 2016, and 76.65% in 2003. Coarser (∼300 m) land-use maps failed to capture smaller settlements in heavily damaged mid- and upstream areas, whereas high-resolution settlement maps identified them, predominantly located in agricultural and shrubland areas. These areas accounted for 69.3% of exposure midstream, 19.8% downstream, and 10.9% upstream, correlating with the heavy damage reported midstream. This highlights the importance of high-resolution settlement data for accurate flood exposure assessments, even in data-scarce basins. Socio-economic factors, disaster preparedness, and government support should be integrated into detailed flood risk assessments to develop effective mitigation measures.

Study region: Gin River Basin, Sri Lanka. Study focus: The individual and combined effects of climate and land-use changes on flood flow and inundation in the Gin River basin, Sri Lanka, were assessed. Downscaled, bias-corrected future precipitation projected by an ensemble of general circulation models (GCMs) under the Representative Concentration Pathway (RCP) 4.5 emissions scenario was input into a flood model developed using the Rainfall Runoff Inundation (RRI) model. Possible flood mitigation measures were identified based on flood simulations through the integrated assessment incorporating future climate and land-use projections. New hydrological insights for the region: The basin is projected to receive increased rainfall during the southwest monsoon (9.7 %) and second inter-monsoon (15.7 %) seasons, excluding March and April, in the future. Human settlements will expand in the downstream areas of the basin, while a significant share of agricultural land (27 %) in the basin will be converted into shrublands by 2050. High flows are predicted to increase by 16 % and 4 % at the upstream and downstream gauging stations, respectively, while mean river flow is expected to decrease by 25 % (upstream) and 34 % (downstream). In addition, a 3.5 % increase in annual maximum inundation extent is projected. However, the total inundation extent in the basin can be reduced by 1.3 % by regulating land-use changes, particularly the conversion of agricultural land into riparian forests.