A combined hillslope hydrology/stability model for low-gradient clay slopes in the Italian Dolomites

The temporal trend of displacement of a landslide on a low-gradient clay slope in the Italian Dolomites as a function of precipitation was simulated with a combined hydrological-slope stability model based on groundwater and surficial displacement observations. Piezometric records and lithological profiles from boreholes suggest the occurrence of preferential groundwater flow through a very permeable top layer of about 1 m thickness, which overlies a less permeable, compact clay layer penetrated by dead-end cracks. To simulate groundwater levels in this system, a linear reservoir model with two outlets was applied, where the upper outlet drains the permeable top layer, and the lower outlet drains the dead-end cracks in the clay layer. The reservoir was assigned a bottom boundary level varying in time. This was done to represent an inferred yearly fluctuation of a matric groundwater level due to regional flow, which controls the thickness of the unsaturated part of the clay layer absorbing water from the cracks. Groundwater output from the model was used in a visco-plastic stability model to simulate the velocity of landslide movement, taking into account a phenomenon of strength regain that occurs in the clays along the slip surfaces. Calibration and validation on 3 years of groundwater and displacement data showed that both models yield good results. Thus, good estimates of cumulative landslide displacement were obtained as a function of precipitation. The general concepts underlying the models offer possibilities for regionalization because from a geological point of view, this landslide is typical of many others in the Dolomites.