Morphometric and landuse analysis: implications on flood hazards in Ilesa and Osogbo Metropolis, Osun State Nigeria

Abstract

This study assessed the morphometric, landuse and lithological attributes of five basins (Iwaraja, Ilesa, Olupona, Osogbo I and Osogbo II) with particular reference to flood hazards in Ilesa and Osogbo metropolis, Osun State Nigeria. Ilesa town is situated within Iwaraja and Ilesa basins while Osogbo metropolis spread across Olupona, Osogbo I and Osogbo II basins. Twenty-three morphometric parameters were computed from DEM-based vector data for the sub-basins using GIS techniques. Image analysis technique of unsupervised classification was adopted to generate landuse map for the sub-basins. Classification was based on six landuse classes that were identified on the satellite imagery and confirmed by field investigations. Statistical analyses of the classification results were undertaken. Also, lithology of the basin was analyzed. Morphometric characteristics of Osogbo I, Osogbo II and Olupona drainage basins have the tendency to trigger high impact floods while flood hazard is relatively low in Iwaraja and Ilesa basins. However, relief attributes suggest possible exposure to flash flood particularly along the banks of main channel of Ilesa basin. Landuse analysis indicates that more than 85 percent of Osogbo I, Osogbo II and Olupona basins are built up and consequently impervious, leading to low infiltration-runoff ratio. Less than 32 percent of Iwaraja and Ilesa basins are built up while more than 66 percent of the basins are covered by vegetation of varying degrees of thicknesses, leading to high infiltration-runoff ratio. Occurrences of heavily fissured quartz schist in Iwaraja basin also contribute to high infiltration potential in part of Ilesa town. Consequently, Osogbo metropolis is highly susceptible to flood hazards compared to Ilesa town where potential flood dangers are easy to manage.

Keywords: Morphometry, Landuse, Flood Hazards, Exposure, Urbanization