Flood inundation and risk mapping under climate change scenarios in the lower Bilate catchment, Ethiopia

Future changes in temperature and precipitation may be exacerbated by climate change, and this could result in extreme hydrologic events like floods and droughts. The objective of this study was to assess how the future climate would affect flooding in Ethiopia’s lower Bilate catchment. In order to simulate flood occurrences and maps for various climate scenarios, two hydraulic models—HEC-RAS and a semi-distributed physically based HEC-HMS hydrologic model—were utilized. For the baseline (1971–2000), mid-term (2041–2070), and long-term (2071–2100) cycles under the RCP4.5 and RCP8.5 scenarios, the bias-corrected data of five climate models were used. The baseline and future period stream flow data were used to produce the flood events for the 50- and 100-year return floods. Flood inundation area and depth were computed using the HEC-RAS model. The flood risk map of the catchment was developed by combining the flood vulnerability and hazard indexes. Future predictions showed that the catchment's maximum and minimum temperatures and average annual precipitation would rise, which also increase the yearly stream would flow. According to the RCP4.5 and 8.5 scenarios, the intensity of future floods will rise and inundate an area ranging from 88.5 to 161 ha considering the 50 and 100-year flood. The worst case scenario under RCP8.5 is the possible inundation of 161 hectares of the studied reach that could be inundated by a 100-year cycle flood. Climate change will make flooding more difficult in the 2050s and 2080s, yet an extreme and riskier flood may happen in the 2050s under the RCP8.5 scenario. The results of this study could aid non-governmental and governmental organizations in their efforts to better control flood risk in the lower Bilate catchment.