Flood Vulnerability Assessment in Lagos City Using Multi – Criteria Analysis

Flood disaster is considered as a major natural hazard due to its devastating effects on the affected area. Geographical Information System (GIS) is integrated with Multi-Criteria Decision Analysis (MCDA) method and is used to analyse the flood vulnerable areas. The actual aim of this is to provide a more flexible and accurate decisions to the decision makers in order to examine the effective factors causing flood. The study considers eight factors that contribute to flood within the Oke-Afa water catchment in Lagos State, Nigeria which include rainfall, stream order, distance from stream, elevation, land use, slope, soil, and geology. Multi-criteria evaluation approach which involves ranking method and pair-wise comparison method (Analytical Hierarchical Process-AHP) was used to calculate the weights of each factor. Using AHP the weightage derived for the factors are 0.38%, 0.21%, 0.16%, 0.10%, 0.06%, 0.04%, 0.03%, 0.02% respectively for rainfall, stream order, distance from stream, elevation, land use, slope, soil, and geology. The study also made use of Landsat satellite image of 2014 and topographic map of Lagos state to classify the different land use types within the area. The digitized topographic map covering the study area was used to generate the Digital Elevation Model (DEM) and contour map respectively. At the end of the study a flood vulnerability Map of the vulnerable areas within Oke-Afa was generated with a view to assisting decision makers on the menace posed by the disaster.