Hydrological post event survey after the autumn 2014 floods in the Cévennes region in France: results and first hydrological analyses

Abstract

International audienceDuring the autumn 2014, an exceptional succession of intense rain events and associated flash floods have hit a large territory extending from the Hérault to the Gard and Ardèche regions in south east of France. These events caused 17 casualties and estimated damages ranging from 550 to 600 billion euros (FFSA). On several upstream watersheds, the reported floods seem to be the largest observed from human memory, and may therefore become reference events for flood risk prevention. A post event survey was organised within the Hymex framework in order to document the first 3 events which occurred successively from the 15th of September to the 15th of October. A total of 64 peak discharge values were estimated, enabling a detailed description of observed hydrological reactions. The analysis of this dataset confirms the very significant peak discharge levels, which remain however significantly below the envelope curves and the discharge values observed during some other historical events in this region (for instance the 2002 flood). A detailed hydrological analysis of this dataset based on rainfall runoff simulations is now in progress. It is worth noting that some of the affected areas were already hit by the 2002 and 2008 events, for which post event surveys were already conducted: therefore, a comparison of the hydrological reactions for these three events will be possible. The proposed communication will include the presentation of the characteristics and consequences of the main flood events, the dataset obtained from the post event survey, and the first lessons derived from the hydrological analysis