Karst flash-flood forecasting in the city of Nîmes (southern France)

Abstract

In southern France, karst flash-floods may be the result of two, potentially cumulative, phenomena:−Floods from highly localized events that mostly occur during autumn and are locally known as Cevenol rain events;−Floods exacerbated by recent rainfall events that contributed to saturation of the aquifer before the storm event, thereby increasing runoff.In any case, flash floods occurring in a karst landscape are directly linked to the structure and hydraulic properties of the karst aquifer.A methodology was developed for the city of Nîmes for forecasting these dangerous events, based on the study and modelling of karst-aquifer response to rain events. This work was composed of: (i) Definition of how the Nîmes system functions, leading to a conceptual model; (ii) Modelling of this conceptual model; (iii) Definition of a tool for hazard management, presented as an abacus and tested on particular strong rainfall event.