Forecasts, warnings and mobility facing Flash Floods: Is temporality a major problem?: The case of the September 2005 Flash Floods in the Gard region (France)

Résumé En

Flash floods are complex phenomena. The atmospheric and hydrological generating mechanisms of the phenomenon are not completely understood, leading to highly uncertain forecasts of and warnings. These events particularly strike people travelling. For each catastrophe, up to half of the deaths are road users. Warning to such violent and fast events is particularly difficult for motorists. In both the social and physical aspect of the problem, space and time scales involved either in hydrometeorology, human behavior and social organizations sciences are of crucial importance. Forecasters, emergency managers, mayors, first responders and road users, all have different time and space frameworks that they use to take emergency decision for themselves or their community. The integration of space and time scales of both the phenomenon and human mobility is therefore a necessity to better deal with questions as forecasting lead-time and warning efficiency. The aim of this oral presentation is to focus on the spatio-temporal aspects of flash floods to improve our understanding of the event dynamic compared to the mobility dynamic. The authors propose a framework of analysis to compare the temporality of: i) the forecasts (from Méteo-France and from EFAS (Thielen et al., 2008)), ii) the meteorological and hydrological parameters, iii) the mobility conditions at different scales. The September 2005 event is particularly interesting for such analysis. The rainfall episode lasted nearly a week with two distinct phases separated by low intensity precipitations. Therefore the vigilance bulletin where somehow disconnected from the local flood's impacts on road.