MODELLING GROUNDWATER FLUCTUATIONS AND THE FREQUENCY OF MOVEMENT OF A LANDSLIDE IN THE TERRES NOIRES REGION OF BARCELONNETTE (FRANCE)

On the basis of measurements of hydrological parameters and field monitoring of a landslide in the Terres Noires in the basin of Barcelonnette (France), a hydrological model was developed, describing groundwater fluctuations in relation to precipitation. These groundwater fluctuations can be used as input to a stability model in order to assess the temporal frequency of instability of the landslide. The calculated groundwater fluctuations, which can forecast years with landslide incidents, were roughly calibrated against dated movements obtained by dendrochronological research. The hydrological system of the landslide can be understood through a three-layer sequence: a rather permeable colluvial top layer underlain by a less permeable colluvial second layer, both overlying the nearly impermeable in situ non-weathered black marls (Terres Noires). The mean Ksat value for the matrix flow in the top layer is 15·7cm/day and in the underlying layer 0·7cm/day. However, water fluxes in these layers occur by two types of groundwater flow: matrix flow obeying Darcy’s law, and more rapid gravitational flow through preferential flow paths, increasing the conductivity by a factor of 10 to 100, as cube method Ksat measurements revealed. The model shows long-term yearly fluctuations of the phreatic surface, with peaks at the end of winter, as well as at the beginning of spring, and minimum values during the dry summer period. These long-term fluctuations are explained by the high drainage capacity of the top colluvial layer and the relatively low vertical water fluxes within the underlying colluvia