Evaluation of Flow Speed in Urbanized Areas and Flood Hazard Mapping in Flood Risk Prevention Schemes

For the elaboration of flood risk prevention schemes (“Plan de Prévention du Risque Inondation” in France), the characterization and cartography of flood hazard can be based on different approaches. From historical approaches, consisting in the reconstitution of flood limits and heights from historical information and testimonies for some well-known major events to hydro-geomorphological methods, aiming at defining the limits of different envelops that the flood can mobilize, based on topographical and terrain analysis, or numerical modelling, with different modes of representation of the floodplain and varying degrees of precision. In urbanized areas, where the most vulnerable places reside due to the density and variety of the stakes exposed to the flood, classical hydraulic modelling technics such as 1D, 2D or simple 1D/2D coupled models often have important limitations regarding the complexity of the flow, due to the modification of the river streams and the presence of numerous artificial obstacles in the floodplain. The case study aims at showing the gain of integrating small-scale urban obstacles, like buildings, in the model, both as elements deflecting the flow, increasing the flow speed and hazard in the streets, and as places that can be flooded, taking into account the volume stored in the buildings. The study compares different approaches and shows the gain in terms of precision for the stakeholders of the building and using large models for integrating small-scale elements that can produce accurate results, usable both for the production of hazard maps and in the context of flood management plans.