A Combined Pipe and Overland Flow Model to Support Urban Flood Risk Management: Case Study of the Espartes Watershed

Located in the Alpes-Maritimes (France), the Espartes watershed (1.6 km2) is characterized by a strong artificialization of both its surface and its hydrographic network. Nearly 50% of the surface of the basin is urbanized, and nearly 50% of the length of the hydrographic network is either canalized or artificialized. By decreasing infiltration and increasing flow velocities, such artificialization leads to frequent flooding in different parts of the basin. In this context, decision-makers need effective tools to assess the current flood risk, as well as the impact of potential urban developments on it (e.g. enlarging the diameter of pipes, creating retention basins, or facilitating infiltration). We model the Espartes watershed using MIKE + (DHI, 2023). MIKE + solves the 2D and 1D Saint–Venant equations for respectively overland flow and stormwater flow in the collection system. First, we calibrate the model on measured discharges, available in November and December 2017. Then, we run simulations with design-rainfall forcings, for the return periods of 10 and 100 years. These simulations allow us to evaluate the hazard of flooding under the current state of urban planning. By implementing different mitigation scenarios in the model, the next step in this work will be to quantify their influence on flood risk in order to provide the technical knowledge needed for decision-making.