The Catastrophic Flash-Flood Event of 8–9 September 2002 in the Gard Region, France: A First Case Study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory

Abstract

International audienceThe Cévennes–Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8–9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600–700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked space–time structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5–10 m3 s−1 km−2) and reached locally extraordinary values of more than 20 m3 s−1 km−2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented

Two‐way one‐dimensional high‐resolution air–sea coupled modelling applied to Mediterranean heavy rain events

“…The MCS stayed over the same region for about 24 hours, leading to surface rainfall totals reaching very extreme values (Figure 4(b)): 691 mm in 24 hours was recorded in the Gard region. This flooding resulted in more than 20 fatalities (Delrieu et al, 2005).…”

Improvement of resilience of urban areas by integrating social perception in flash-flood risk management

“…The flow peaks are reached within a few hours, thus giving little or no advance warning to mitigate flood damage (Borga et al, 2007;Borga et al, 2008). This hydrological response leads to the occurrence of a typology of floods known as flash floods because of their rapid onset, i.e., within six hours of rainfall (Ogden et al, 2000;Delrieu et al, 2005;Marchi et al, 2010;Hapuarachchi et al, 2011;Naulin et al, 2013;Ballesteros-Canovas et al, 2015).…”

Accounting for rainfall systematic spatial variability in flash flood forecasting

“…Distributed hydrological models show significant improvements after including the local aspect of precipitation (Michaud and Sorooshian, 1994;Zoccatelli et al, 2010); and in particular, an increase in the likelihood criterion (Nash-Sutcliffe) of up to 30% (Zoccatelli et al, 2010). Studying the particular case of the flash-flood event of 8-9 September 2002 in the Gard region, Anquetin et al (2010) and Delrieu et al (2005) found that the distribution of the spatial forcing particularly influences the flood dynamic when it interacts with the spatial distribution of the soil properties before saturation.…”