Hydrological response of weathered clay-shale slopes: water infiltration monitoring with time-lapse electrical resistivity tomography

International audienceThis work presents an attempt to monitor water infiltration and subsurface flow within a clay-shale landslide using time-lapse electrical resistivity tomography (ERT). A rainfall experiment was carried out on a plot of 100 m2 at the Laval landslide in the Draix experimental catchments (ORE Draix, South French Alps) in order to characterize the spatial and temporal development of water circulation in the soil and to identify when steady-state flow conditions are reached. The experiment was conducted during 67 h with initial unsaturated conditions in the slope. The apparent electrical resistivity values were inverted with a time-lapse approach using several cross models. The results indicate a significant decrease in resistivity ( 18%) compared to the initial state in the rain plot. Downslope progression of negative resistivity anomalies is imaged suggesting that vertical and subsurface lateral flows have developed. About 21 h after the start of the rain experiment, a constant level of resistivity values is observed indicating that the hydrological system reached steady-state flow conditions. This observation is consistent with ground water level observations and chemical tracer analysis. Computed differences in time of steady-state conditions highlight possible preferential flows near the landslide toe. A hydrological concept of functioning of the slope is proposed, and apparent saturated hydraulic conductivity (Ks of 1Ð7 ð 10 4 m ð s 1) is computed from the steady-state times. This study demonstrates the potentiality of ERT monitoring to monitor water infiltration in clay-shale slopes and the high water transfer capacity of reworked clay-shale material