Controls on shallow landslide size

Most distributed slope-stability models for shallow landslide prediction neglect the forces acting on the sidewalls of landslide scars. Back-calculations and field observations, however, show that lateral root strength is a primary control on size and location of shallow landslides in soil mantled hillslopes. Here we report a theory that estimates landslide width, assuming that root strength acts primarily through a perimeter boundary. The model predicts that landslide width increases with increasing root strength and decreasing slope, as larger masses of soil are needed to overcome resisting forces. Perhaps surprisingly, the drier the soil, the larger the landslide mass (and width), whereas the water table rise reduces the size needed for failure. The comparison of the model results with field data suggests that landslide size is controlled by the local patchiness of soil thickness, root strength and topographically-driven relative saturation.