Flood hazards characterization using multi-criteria decision and flood frequency analysis in Osun River Basin, Nigeria

Abstract Flooding is regarded as one of the world's most dangerous natural disasters with great highly devastating social, economic and environmental impacts. This study employs the use of a GIS-based multi-criteria decision approach (MCDA) and flood frequency analysis to assess the flood potential zones and magnitudes in the Osun River basin. Six flood causative factors (soil type, elevation, slope, drainage density, distance from the river, land use land cover) were considered and integrated into the Geographical Information System using analytical hierarchy process (AHP) and weighted overlay with a consistency ratio of 0.04. The output was classified as having a flood potential ranging from very low to very high. HEC-HMS hydrological model was used to simulate previous potential flood discharges from 1981 to 2020 within the river basin. A basic descriptive analysis was performed to understand the hydrological characteristics of the basin from the previous records. We analysed the flood frequency from the simulated stream peak flow using the Gumbel frequency distribution method. The results from the analysis showed that 11% of the study area is highly prone to flooding. The moderately prone zones cover more area (82%) and 7% of the area is not prone to flooding. The peak discharge for the simulation period ranged from 531.5 to 1846.8 m3/s. The peak discharge (1846.8 m3/s) at the basin has a 41-year recurrence interval. Using the Gumbel’s extreme value distribution method, the calculated discharge flood lies within 1117.43 m3/sec to 1858.51 m3/sec for 5 years to 150 years’ return period for the Osun River basin