Inundation and Hazard Mapping on River Asa, using GIS
The prevalence of flooding in Nigeria has underlined the need for rational and accurate mitigation of the menace. The major part of River Asa in Ilorin is traditionally identified as a flood prone area of Kwara State. Most of the flood management strategies in this region have been geared towards ‘preventing’ flood by an attempt to contain the river. No attention is paid on forecasting on the extent of flood for a particular return period due to insufficient hydrological record. Preparation of a comprehensive flood hazard map for this river would be the one of most crucial steps for implementing non-structural remedial measures. Therefore, this research was aimed at producing an inundation and hazard map using a Geographical Information System across River Asa. Thirty (30) years of rainfall data obtained from Kwara State meteorological station was used to derived hydraulic discharge of the river using Institute of hydrology method of model. Log Pearson Type III model was for flood frequency distribution. The elevation data was obtained by extracting the contour lines on the Federal Survey maps. Digital Elevation Model (DEM) was generated from the contour lines of the catchment by interpolating contour values using GIS. The floodplain map was obtained by analysing the combined factors of rainfall, land use variation, DEM and slope, being the major factors contributing to the flooding of an area. The actual catchment area contributed to Asa dam discharge is 537.30 km² and soil infiltration capacity is medium with value of 0.35 because more than 55% of soil formation is stone basement formation. The built up area within the catchment has increased by 14.353 km² (23.89%) with an annual change of 1.3km² between 1976 and 2004. Log Pearson Type III model gave a discharge ranges from (6306 – 10,458) m3/s for return period between (2-50) years. The flood risk map showed that 56% of existing structure will be at risk at discharge value of 50 years return period. The study produced inundation and hazard map of River Asa using Remote sensing data. The map will help state government in making decision to mitigate flooding.