Groundwater induced flood estimation in extreme rainfall context; methodological approach applied to Somme basin (France)
Résumé
Climate change is expected to modify hydrological cycle, and extreme events would be more frequent. Increasing extreme precipitation (frequency and intensity) projected in northern Europe and continental Europe (IPCC WG2 AR5 chapter 21) would result in an increase of flood
risk. In chalky basins located in northern France (Somme catchment) and the United Kingdom (Pang and Lambourn catchments), groundwater flooding, with return periods superior to 100 years has already occurred in winter 2000/20101. The estimation of groundwater contribution
to flood peak under extreme rainfall context remains difficult because of a lack of appropriate methods and data. Nevertheless, this estimation is a decisive element for policy makers to assess and manage the flood risks.
In the Somme basin, a probabilistic approach to develop «extreme» climate scenarios based on cumulative rainfall return period of 1000 years and hydrodynamic modeling of the groundwater and rivers system were used. A statistical extreme value analysis using a long rainfall time series
allowed defining rainfall events of return period of 1000 years. Correlative analysis rainfall-discharge of the Somme river at the watershed outlet (Abbeville station) performed with cumulative rainfall over n days show that the best correlation is obtained with a total of n=180 rainy days. In addition, estimating the river discharge of return period 1000 years was made by fitting a probability distribution for extreme events (GEV law) Several scenarios were simulated with the hydrodynamic model of the Somme basin; they differ in the temporal distribution of daily rainfall over the period of 180 days, the groundwater level at the onset of the rainy millennial episode, and the value of base flow of the river at the start of the rainy millennial episode.
The results highlight the rainfall distribution importance on the maximum discharges and the influence of the water table and the base flow of the river at the onset of the rainy period. It is also clear that for such climatic events, the groundwater contribution to the Somme river discharge remains important, between 65% and 72% of the flow according to the scenarios.