Modelling urban stormwater drainage overflows for assessing flood hazards: application to the urban area of Dakar (Senegal)

Abstract. With the recurrence of flooding in African cities, there is growing interest in developing 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 modelling flows at fine resolution (25 m2) over the entire area and provide 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 to calculate flows at the outlets of elementary catchment; and (iii) implementation of a hydraulic model to propagate these flows through the drainage network and implementation of a storage model for retention basins. The modelling chain was built within the ATHYS platform. The network overflow points are detected if the difference between the calculated flows exceeds the network capacity to evacuate them. Examples are given by carrying out simulations using 10- and 100-year design rainfall. The model also provides boundary conditions to apply more complex hydraulic models to determine the local impact of drainage network overflows on limited areas. However, the capacity of the method still needs to be validated in further research by comparing it with accurate data from observed flood events.