Flood susceptibility assessment using GIS and an analytical hierarchy process in Ilorin Metropolis, Kwara, Nigeria

Flooding remains a major environmental challenge in Ilorin Metropolis, Kwara State, Nigeria. In this study, flood susceptibility across the metropolis was assessed using geographic information systems (GIS), remote sensing, and the analytical hierarchy process (AHP). ASTER Global Digital Elevation Model and Landsat 8 OLI/TIRS imagery acquired in January 2024 were used to derive nine flood-conditioning factors: elevation, slope, drainage density, topographic wetness index, stream power index, distance from streams, distance from roads, land use/land cover (LULC), and normalized difference vegetation index. The factors were weighted using the AHP and integrated through weighted overlay in ArcGIS. The resulting flood susceptibility map classified 0.8% of the study area as very low susceptibility, 9.3% as low, 47.1% as moderate, 30.8% as high, and 12.0% as very high. Thus, 42.8% of Ilorin Metropolis was classified as having high or very high flood susceptibility, particularly along the Asa River corridor and low-lying urban areas. Independent validation using 200 flood and non-flood reference points produced an overall accuracy of 96.5%, a sensitivity (recall) of 1.00, a specificity of 0.95, a precision of 0.895, and a Cohen’s kappa of 0.919. The results demonstrate the utility of the GIS-AHP framework for identifying flood-susceptible areas and supporting urban flood mitigation and land use planning in the Ilorin Metropolis.