Application of a rational model in Gis for flood risk assessment in Accra, Ghana

Abstract

Accra has been experiencing periodic flooding that affect properties and lives. The government seeing the dangers involved has commissioned institutions such as Ministry of Works and Housing, Town and Country Planning and City Engineers to identify flood risk zones and adapt measures that will help reduce flood damages. These institutions identify flood risk zones using conventional methods such as watermarks on buildings and reported cases in the news media. Works carried out by these agencies were not able to give details about potential areas that are likely to experience this extreme event. Hence there was the need to find a new method of identifying and mapping of potential flood risk zones. To determine flood risk zones in Accra and its environs a hydrological model (modified rational model) was integrated into the GIS platform, by the arithmetic overlay operation method, using operators such as addition and division. The results show that although the delineated areas experienced a same rainfall intensity of 140.2 mm the flood intensities of these areas could differ. The high flood risk zone covers 35.66 percent of the study area, whiles the low risk zone covers 26.85 percent. The areas that will potentially experience periodic floods with a given input of rainfall are mostly below the 350-meter contour