Influence de l’urbanisation anarchique sur le risque d’inondation : cas du bassin versant de la rivière Mutimbuzi

Abstract

With a population which has a growth rate of 6 to 7 %, due to the high birth rate of Burundi and the rural exodus, the city of Bujumbura in the western part of Burundi records a rapid build-up expansion associated with the regression of its land cover. Based on the analysis of Landsat images of 2000, 2016 and 2022; land cover dynamics were highlighted through supervised classification. Data show that the built-up area of Mutimbuzi river watershed recorded an increase of 0.32% between 2000 and 2016, 2.46 % between 2000 and 2022 and 2.14% between 2016 and 2022. In order to promote environmental protection, our work has four chapters. The first contextualizes the influence of uncontrolled urbanization on flood risk. The second chapter is devoted to the bibliographical review; The third chapter presents the morphological, climatological and hydrological characteristics of the Mutimbuzi river watershed. As for the fourth chapter, it is a question of evaluating the influence of uncontrolled urbanization on the risk of flooding using the HEC-HMS and HEC-RAS software. Urbanization is spreading toward the mountainous area located in the eastern part of the city. Using the HEC-HMS software, we assess the flow rates of runoff floods in the Mutimbuzi river watershed considering variable rains’ return periods for different land use situations: the situation in 2000, the situation in 2016, the situation in 2022, the situation assuming urbanization up to an altitude 1000m (A1000), the situation assuming urbanization up to an altitude 1250 m (A1250) and the situation assuming urbanization up to an altitude 1500m (A1500). The results show that the situation A1500 for a rainfall with a return period of 25 years leads to a higher flow than the one generated by the situation 2000 for a hundred- year rainfall. This highlights the rigor of the risk of flooding resulting from an urbanization towards the heights of Bujumbura city. By entering peak flows data into the HEC-RAS model, we determined the areas of floodplains and assessed the vulnerability to the risk of flooding for communities living downstream. By analyzing with ArcGIS the results found in HEC-RAS, we proposed to limit urbanization to an altitude of 1000 m in order to allow the Mutimbuzi river to evacuate a flood of a return period of 20 years with possibility to limit the risk of flooding. In the same way, the urbanization up to 1250 m of altitude will greatly increase the flood zone, as a tenyear flood flow will be able to generate overflows.