Flood Risk Assessment of Osse Watershed, Southwestern Nigeria

Flooding is a natural disaster that occurs as part of the earth's biophysical processes and can be disastrous owing to human activities and climatological variables. Flood risk maps are an important tool for determining flood susceptibility in flood-prone areas. This study focused on a flood risk assessment in Osse Watershed, which is located in Ondo, Edo, and Ekiti States, Nigeria, and has experienced floods regularly to varying degrees. Flood risk zones were calculated using spatial analysis in a GIS based on six relevant physical parameters: slope, elevation, drainage density, land use, rainfall amount, and soil permeability. During the Analytical Hierarchy Process (AHP), the relative relevance of physical components was assessed in a pairwise matrix to obtain weight values. The expert-knowledge method was used to derive a second scenario flood risk map. Flood risk zones have been plotted based on their relative weights using the weighted overlay method in ArcGIS. The flood risk areas were identified based on ‘very low risk’, ‘low risk’, ‘moderate’, ‘high risk’, and ‘very high-risk’ categories. The findings of the first scenario indicated that approximately 65% of the total study area is at very high risk of flooding, while the second scenario, using the AHP calculator, showed that about 59% of the study area is at high risk. The paper recommends adopting nonstructural flood mitigation measures, including forecasting, land use controls, relocation, insurance, and awareness campaigns. It further recommends an integrated basin-wide approach to flood management and research incorporating validated flood records and socioeconomic vulnerability indicators