From Hydraulic Modelling to Urban Flood Risk

Urban floods cause high damages and thus mapping of flood risk is essential for protection and evacuation planning. To obtain such mapping, hydrodynamic modelling is central because it provides water depths and velocities everywhere and along the whole flood period. The case of the 1988 flood in Nîmes is presented as an example of the method to get the basis for evaluating flood risk. Once the hydro-meteorological event is selected, a lot of causes of uncertainty still exist, which are linked to either the flow processes in a complex environment or the simplifications of the 2-D models that are used for saving computation time. In order to gain information regarding the origin of such uncertainties, two series of laboratory experiments are described: one focuses on the error in the flow distribution at crossroad, the other one stresses the flow complexity in the exchange structures between the streets and the sewer network. Beyond the analysis of the causes of uncertainty, obtaining a detailed local assessment of flood risk requires a sensitivity analysis to cope with the associated uncertainty ranges; this analysis may be reduced if a relevant model is used, which means, for instance, that the processes described in the experiments are conveniently modeled.