Hydrometeorological modelling for flash flood areas: the case of the 2002 Gard event in France

In the context of flash flood forecasting, this paper proposes a few advances in our understanding of the hydrometeorological processes and their associated modelling requirements that may be useful to introduce within an operational forecasting chain. The study is focused on the September 2002 storm that produced more than 600 mm of rainfall in < 24 h and triggered a series of flash floods in the South of France. This catastrophic event took 23 human lives in 16 distinct subcatchments. This paper proposes a combined detailed analysis of the meteorological event and hydrological simulations of the response of four small-ungauged catchments. The meteorological analyses are based on observations and results of simulation of rain fields obtained with the MesoNH model. These analyses explained the steadiness of the storms that led to a locally intense precipitation: the role of the orography and favourable synoptic conditions. The hydrological model is set up without any calibration and the soil parameter specification is based on an existing soil database. Radar rainfall estimations are used. Simulated specific peak discharges are found to be in agreement with estimations from a postevent in situ investigation. Based on the model results, a cartography of the dominant process is proposed for the four selected catchments.