La contribution des eaux souterraines aux crues exceptionnelles de la Somme en 2001 Observations, hypothèses, modélisation

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The 2000-2001 high water levels in the Somme catchment highlited the possible contribution of groundwaters to surficial floods, with considerations which could be extrapolated to a great number of sedimentary geological provinces displaying a plateau landscape, those mostly located in the northern half territory of France. Successively an interministerial Commission, the Regional Directorate for Environment of Picardie, and the General Council of Somme asked Brgm to analyse the underground geo-structures and seepage as part of the fluxes and exchanges which took place during 2000-2001 winter, and thus to create a forecasting tool based on mathematical simulations. A flood is integrating several components, ranking from the rainfall to the river flux, the relative weight of which being affected by the morphology, the surficial hydrology, land uses, as well as seasonal conditions such as frozen soils. One peculiar component is the groundwater outlet which may be fairly significant from a chalk body, such as the general substratum of the Somme watershed. The hydrodynamic rules which apply to seepage, allowed an extension of modelling techniques, commonly used in groundwater resources management, to the seasonal events computation in the Somme region, in order to partly provide an explanation of the groundwater contribution within the water balance. Several models were created, lumped up and deteministic ones. From july 2001 on, a complete rebulting of the geological scheme was performed and data series computed. The first deterministic model was available in december 2001. Both models showed that the successive sub-catchement contributions to the river flow were increasing from up- to downstream. Seasonaly, the total groundwatercontribution, close to 100% in low water periods, remains at a significant level, even at flood time : not less than 80%. The isotopie facies gave a confirmation of the relatively high contribution of groundwater into the river flow. The forecasting tool built up in 2001 helps computing the variations of flow at the city of Abbeville. It is operated every fortnight in winter and spring time (from november to may) from 2002 on. The deterministic model is operated once a year, a posteriori, to bring up the local relative contributions of groundwater into stream flow, over the last one year period.