Flood risk maps by using hydrological analysis based on digital elevation models: Case study in Douala Cameroon

Douala is a port city, located at 30 kilometres from the Atlantic Ocean, offering good life opportunities which cause human mobility from other parts of the country. The aim of this study is to reveal whether the flood in the city of Douala is due to the sea level rise and to simulate flood risk zones. The study additionally aims to be a support for central and local administrators. In this study, the number of floods between 1980-2000 and 2000-2018 was examined by scanning the flood events in the city of Douala. Secondly, by using SAGA GIS and GIS, hydrological analyses on the DEM data of the city of Douala were made and the basin and drainage system were created. After that the risky areas were mapped by simulating the rise of the sea level up to 1 m and 3 m. As a result of the analysis of the flood news, it is discovered that there were few flood events from 1980 to 2000, but after and since 2000 almost every year, at least one flood event has been recorded. Flooding in the city of Douala is caused by both natural and human factors. The topographic and soil structure analyses of the city, has shown that most parts of the southern region are the most risky areas as they are very close to sea level (0.5 to 1 meter high). The northern part of the city is not a flood risk zone in the topographic point of view since it is higher than the sea level, but human-induced factors cause flooding in that zone. In topographic point of view, marshy places such as Bonaberi, Youpwe, and Daido can be submerged with rising sea level. However, as a precaution, these places were filled with soils and turned into a settlement. This measure disrupts the natural drainage system, causing the runoff water to accumulate. Thus, flooding is increasing due to anthropogenic reasons. The map of land use for the period between 1986 and 2017 showed that the settlement was 10,708 hectares in 1986; it increased to 24,726 hectares in 2017. Floods that have been increasing in the city of Douala in recent years are directly related to the population growth. Population growth comes together with the need of settlement and to satisfy this need people turn to areas with flood risk. The cause of population growth is the seek of better opportunities that provides the city. These people come from all corners of the country. Measures taken recently by the government shows its awareness about the flood risk.