Hydrodynamic Coupling Method for Stormwater Studies in Suburban Catchments—Study Case of the Magnan Basin, Nice

Abstract

Stormwater management in suburban areas shows a developing interest due to the expanding cities toward their suburbs and the drained areas to their centers. Drainage network is thus highly expanding. Moreover, when the topography comes to add steep slopes to the situation, the level of difficulties in managing stormwater raises leading to flash floods in the downstream area with associated damages and casualties. Recent storm events on the French Riviera have recalled how quick and violent flash floods can be. The 17 km2 Magnan basin, located in the western part of Nice city, is drained by an urban river characterized by complex changes in the geometry along its watercourse. The river stream is first natural in the upstream part (for 9.5 km) and then flows to a 1-km long artificial riverbed along the first urbanized areas. In the denser urban area, it enters a piped underground section until its outlet to the Mediterranean Sea 2.5 km further. There is a need for more local knowledge regarding runoff and flood dynamic in order to improve preparation and crisis management. This chapter presents a method to answer the following questions applied to the above-mentioned specific case: how rainfall should be modeled over urbanized areas to better represent runoff processes? How to model flow interactions between surface runoff (an urban river) and the stormwater network? Are the modeled results improved by taking into account such processes and interactions? These three aspects can be summarized as the following problematic: how to improve the understanding, the modeling, and the monitoring of a specific suburban study case? To answer these questions, available high-resolution photogrammetric terrain data are used to build a detailed digital elevation model (DEM). The specific study case is divided into two main parts: (1) the upstream area drained to the open-surface part of the river where the hydrology processes are simplified within Mike SHE (DHI) software; and (2) the downstream area drained to the underground piped river where the drainage network is modeled with Mike URBAN (DHI) and the overland flow of the selected area is modeled with Mike 21 (DHI). Modeling tools are based on the shallow water equations (SWEs). An innovative modeling method will be presented and guidelines for further research given to help improving modeling tools for stormwater management.