RAINFALL THRESHOLD REGIONALIZATION: COULD IT IMPROVE LANDSLIDE SUSCEPTIBILITY ZONATION?

(G. Crosta - Dipartimento Scienze della Terra, Univerisy of Milan, Italy)

Rainfall, soil properties and morphology are major factors in controlling slope stability in colluvial and detritic materials. A long series of events involving soil slipping has been collected throughout Northern Italy with the aim of evaluating the possibility for rainfall threshold regionalization. Data from different locations, regarding soil properties, triggering rainfalls and local conditions, have been analysed by means of a slightly reviewed method originally presented by Pradel & Raad and based on Green & Ampt infiltration model. Triggering events can be generally discretised between Piedmont, prealpine and alpine phenomena on the base of the model, suggesting the adoption of more precise rainfall thresholds with respect to those presented in the literature and derived by a lower bound fitting. Intensity vs. rainfall duration plots put in evidence the results allowing to introduce limits connected to soil permeability and thickness, and showing the relative importance of antecedent precipitations. Furthermore, local contributions as a consequence of anomalous water discharges from obstructed culverts reveal the role played by anthropic structures on the triggering of such phenomena. On the basis of the ample available data base, probabilistic determination of a factor of safety can be reached for different locations by adopting Monte Carlo techniques. Eventually, the effect of local bedrock and topographic irregularities has been included in the analysis by introducing the effects of different seepage and saturation conditions.