The city of Castillon du Gard, located in the South of France, is subjected to frequent floods, entirely coming from urban runoff. These phenomena result from intense hydrological conditions that are happening on a dense urbanized area, with no consideration of runoff flow management. In France, the flood risk management is nowadays taken into account only through river management. The state of France has developed official procedures for defining the flood risk coming from rivers and implementing the results into city land use plan through the PPRi (protection plan for flood risk). However there is still no law, neither reference documents, dealing with urban runoff risk. As a consequence, following many recent floods, particularly in 2002, the state decided to work in collaboration with the City of Castillon du Gard in order to develop a new approach that can allow the City to take into consideration urban flood in their land use plan that should be validated in 2014.
A pilot study was then realized to define the innovative approach that will be used as a reference in France. The main objective of the study was then to development the new approach and define the application conditions. The methodology used was based on a precise and integrated hydrological and 1D-2D hydraulics modeling on the entire city area. The complete existing storm sewer network and hydraulic structures were taken into account as well as all the houses and walls that can divert the runoff flow. PCSWMM 2D, that is a particular user friendly and complete hydraulic modeling software, was used to quantify and represent the flow on the area. A large number of scenarios, based on various flood conditions were realized to calibrate the model and then decide which flow level and flow constraints would be used as a reference for the city development. Besides better understanding the dynamic behavior of the flows, the obtained maximum flow depths and velocities everywhere in the City permitted to quantify the flood risk level everywhere and then to establish the accepted development conditions. A new grid for urbanizing the City was then proposed to be used widely in France.