Detection of flooding by overflows of the drainage network: Application to the urban area of Dakar (Senegal)

Abstract. With the recurrence of flooding in African cities, there is growing interest in the development of sufficiently informative tools to help characterize and predict overflow risks. One of the challenges is to develop methods that strike a compromise between the accuracy of simulations, the availability of basic data, and the shortening of calculation times to be compatible with real-time applications. The present study, carried out on the urban outskirts of Dakar, aims to propose a method capable of modeling flows at fine resolution (5 m²), over the entire area, and providing a rapid diagnosis of how the drainage network is operating for rainfall intensities of different return periods, while taking urban conditions into account. Three methodological steps are combined to achieve this objective: i) determination of drainage directions, including modifications induced by buildings, artificial drainage and storage basins, ii) application of a hydrological model for calculating flows at the outlets of elementary catchment, iii) the implementation of a hydraulic model for propagating these flows through the drainage network and a storage model for retention basins. The network overflow points are calculated as the difference between the calculated flows and the network’s capacity to evacuate them. Simulation results show that the stormwater drainage network is capable of evacuating runoff volumes generated by rainfall with a low return period (10 years), but seems to overflow for rainfall with a rare frequency (100 years), with overflow rates sometimes exceeding 18 m³/s. The model, built on the ATHYS modelling platform, also provides boundary conditions for applying more complex hydraulic models to determine the local impact of drainage network overflows on limited areas.