Analyse hydro-pluviometrique des crues du Gard et du Vidourle des 8 et 9 septembre 2002

Hydrological rain analysis during the floods of Gard and Vifourle River on 8th and 9th September 2002

Résumé

A la suite des crues des 8 et 9 septembre 2002 dans le Gard, une importante campagne de collecte de données hydrologiques a été conduite par des équipes de recherche dans le cadre de l’Observatoire Hydro-Météorologique Cévennes-Vivarais. Les données collectées ainsi que quelques exemples d’analyses hydrologiques sont présentés dans ce qui suit.

Abstract

On the 8th and the 9th of September 2002, the Gard region received rainfall amounts locally exceeding 600 millimetres over 48 hours. This extreme rainfall event induced the most important flood of the Gard river of the century and major floods of the nearby Vidourle and Cèze rivers. The peak discharge of the Gard river is estimated between 5000 and 6000 m3/s for a watershed area of 1855 km2. It also produced remarkable flash floods of some of the Gard, Vidourle and Cèze river tributaries. Peak flood discharges of some upstream watersheds (areas smaller than 100 km2) seem to have exceeded 20 m3/s/km2. An extensive post flood investigation has been conducted by research teams involved in the “Cévennes-Vivarais Mediterranean Hydro-Meteorological Observatory” (Delrieu et al., 2003) to collect information about the floods of most of the tributaries of the Gard river and to analyse the hydrological behaviour of the watersheds during this extreme rainfall event. Beside the analysis of the available rainfall data and the estimations of peak discharges on the basis of the high water marks, elements concerning the timing of the floods given by eyewitnesses have been gathered. Simple hydrological models have been used to guide the hydrological interpretation. This type of study aims at increasing the sparse knowledge about the dynamics of the watershed responses during flash floods which is absolutely necessary to improve the flood warning and prediction methods. The comparison of the observed behaviours of various watersheds also reveals the effective influence of the land use, the geology and the soil types on flash floods. The investigation procedure as well as some hydrological interpretation results are presented herein.