Sensitivity of flash-flood simulations on the volume, the intensity, and the localization of rainfall in the Cevennes-Vivarais region (France)

This paper suggests a new method to investigate the signature of the spatial structure of the rainfall on flash-floods events. The rainfall structure is analyzed considering three of its characteristics: (1) the areal rainfall over the basins, (2) the rainfall intensity statistical distribution, and (3) the convective cells geographical localizations within the catchments. The analysis is done by performing hydrological simulations forced by uniform rainfall pattern, fully spatially distributed rainfall patterns, and spatially broken down rainfall patterns. Two contrasted severe flood events that occurred within the Cevennes-Vivarais region (France) are studied as applications using actual data. A distributed hydrological model-based approach was applied on 20 catchments ranging from 50 to 2240 km(2). For one event, the method suggested that the accurate geographical storm cell localization is needed to obtain accurate discharges simulations. For both events, the method allowed to show that the accurate areal rainfall estimation for each intermediate catchment was not of larger importance than an accurate sampling of the rainfall intensities spectrum.