Flood susceptibility mapping in the Tongo Bassa watershed through GIS, remote sensing and frequency ratio model

Abstract

Flooding constitutes a major problem for the inhabitants of Douala City in general and those of the Tongo Bassa watershed (TBW) in particular. Faced with this situation, public authorities need to put in place measures to mitigate the vulnerability of populations to these disasters. This article aims to map flooding risk areas in the TBW using geographic information system (GIS), field data (historical flood points), remote sensing data (Sentinel II image) and frequency ratio (FR) model. Six independent variables linked to floods were considered (land use and land cover patterns, altitudes, slopes, drainage densities, distances from watercourses and soil types). The map of land use patterns was obtained following the supervised classification of a Landsat 8 image using ENVI 5.2 software. The other five (05) variables were automatically generated from a digital terrain model using ArcGIS 10.2 software. The map produced shows that 1.41%, 8.88%, 28.51%, 33.86% and 27.33% of the basin area are respectively delimited into very low, low, medium, high and very high flood vulnerability classes. High and very high flooding risk areas (those where flooding is most likely to occur) occupy more than half of the basin (61.19%). These areas are characterized by significant imperviousness, low altitudes, weak slopes, significant proximity to watercourses and clayey soils. Most of the houses in the basin (66.92%) are located in areas affected by these two levels of exposure (high and very high). With respective success and prediction accuracy rates of 89% and 96.78%, a certain confidence deserves to be placed on the map of flooding risk areas produced. The results of this study could serve as decision-support tools in the context of the various developments undertaken by the public authorities, to fight against flooding in this basin.