A simplified phisically-based approach for the assessment of hazard related to shallow landslides in southern Piedmont (Italy)

After a critical analysis on the main methods of shallow landslide evaluation, a simplified method for the assessment of the shallow landslides hazard is proposed in the paper. The model takes into account both the rainfall infiltration in the soil and its influence on slope stability. For the infiltration analysis the model uses the Green-Ampt method that considers the downward advancement of a saturation front from the ground surface as a consequence of a given rainfall. The thickness of saturated soil (h) is used as input information for the stability analysis according to the indefinite slope method. The utilization of the Safety Factor equation allows us to relate the input rainfall data with the landslides triggering through the thickness of saturated soil, assuming very low values for cohesion and friction angle. The goal of the proposed approach is to investigate triggering conditions for shallow landslides in relation to specific rainfalls whose frequency is computable, allowing for the drawing up of landslide hazard maps. The model was tested in some areas of Monferrato and Langhe Hills (Southern Piedmont, north-western Italy), valuable vineyard territories characterized by a high frequency of shallow landslides. Shallow landslides (of depths generally lower than 50 centimetres) can only be triggered when a drastic decrease of friction angle and cohesion occurs. Values of 20° and 0.5 kN/m2, for the friction angle and cohesion respectively, in conditions of total saturation, seem to give good results in the comparison between model tests and actual landslide maps. At present, other calibration tests are being carried out in the Alto Monferrato area.