From rainfall to slope instability: an automatic GIS procedure for susceptibility analyses over wide areas.

The present paper proposes an automatic procedure in a GIS environment for the analysis and prediction of large-scale landslides due to rainfall events. It runs, for each unit cell, an hydrological balance based on the Curve Number method, CN (Soil Conservation Service 1972 and 1975), computing the evolution of groundwater as a result of precipitation and then checks the overcoming, or not, of limit equilibrium conditions of the land in the domain of interest. The mathematical model was implemented in the free and open source GIS GRASS. For any sequence of consecutive days of rain, according to the conditions of humidity prior to the time-history under study, the hydro-geotechnical model allows (1) the determination of the oscillations of the phreatic table, (2) the part of saturated soil and (3) the slope stability analysis, by taking into proper account the pore pressures build-up. The results of the procedure are returned in raster format, allowing an easy and intuitive interpretation of the land mass sensitivity to meteoric actions. This procedure was applied on an extensive kinematic phenomenon surrounding the city of Santo Stefano d'Aveto (Liguria, Italy). The realized maps of landslide susceptibility are in excellent agreement with what is evident on site.