Rainfall-induced infiltration, runoff and failure in steep unsaturated shallow soil deposits

In unsaturated shallow deposits, rainfall infiltration and runoff may cause either slope failure or erosion processes depending on the combination of rainfall intensity and duration. Consequently, different flow-like mass movements may occur, whose distinction is fully necessary for the management and mitigation of the posed risk. To provide a contribution to this topic, the paper proposes an engineering reference framework to evaluate the amount of both rainfall infiltrating the ground surface and runoff flowing as wash out and remarks are outlined as far as the time to runoff and the slope failure time. This framework is validated through a numerical parametric analysis based on seepage and slope stability analysis. The obtained results show that time to runoff, time to failure and runoff rates are strongly affected by soil water characteristic curves, soil initial conditions, rainfall intensity and slope angle. Furthermore, slope stability analyses show that time to failure can be either shorter or longer than time to runoff depending on soil mechanical parameters. Finally, it is outlined that the proposed framework provides more accurate estimates of time to runoff and runoff rates compared to simplified standard procedures.