Flood Vulnerability Mapping of Lagos Island and Eti-Osa Local Government Areas Using a Multi-Criteria Decision Making Approach

Abstract

This study discusses the assessment and analysis of areas vulnerable to flooding in Eti-Osa and Lagos Island Local Government areas of Lagos state, using Geographic Information System (GIS), LIDAR and spatial modelling techniques. These areas require quality assessment of their level of vulnerability to floods, in order to take adequate measures and develop programs that will help prevent the impacts of flooding. A set of indicators influencing flooding, which are elevation, slope, flow accumulation and land use, were identified and used in the study. A vulnerability scale of (1 – 3) was developed, where 1 represents ‘low’, 2 represents ‘moderate’ and 3 represents ‘high’. The indicators identified were reclassified in the vulnerability scale. The flood vulnerability maps, in three different case scenarios, were created using the weighted overlay of the reclassified indicators identified. The weights used are determined by a Pairwise comparison method (Analytical Hierarchical Approach (AHP). The mapping method was implemented in ArcGIS environment. The vulnerability mapping results show that for a normal case scenario, 27.15% of the area have high vulnerability, 49.79% have moderate vulnerability and 23.06% have low vulnerability. However, for a worst-case scenario, 60.01% have high vulnerability, 37.71% have moderate vulnerability and 2.28% have low vulnerability. Buildings at risk and the extent of areas at risk were determined and relevant recommendations were made.