Reconstruction of lithotechnical terrain units for the assessment of shallow landslides hazard in Oltrepò Pavese (northern Italy)

The prediction of the areas prone to rainfall-induced shallow landslides needs of terrain units which represent the basis for the susceptibility and the hazard zonation of the territory. Terrain units correspond generally to sub-sections of catchments, distinguishes each other by slope angle, altitude and hydrological parameters. Instead, the proneness of a hillslope towards shallow landsliding is strictly related also to the lithotechnical features of the superficial soils, which are affected by these slope instabilities. Thus, the distinction of the terrain units across a catchment should consider also these soil properties. This work has the objective to develop and apply a methodology for the definition of the main litho-technical terrain units across a particular area, aiming to represent an input predictor for the assessment of shallow landslides susceptibility and hazard. The methodology was tested in the Oltrepò Pavese area (Lombardy region, northern Italy), that corresponds to the northern termination of the Italian Apennines and is characterized by a significant predisposition towards shallow failures events as a consequence of severe rainfalls. The following parameters are measured as basic input for defining the terrain units: 1) soil lithotechnical profile, determined through the field evaluation of soil density, porosity, carbonate content, resistance to the penetration; 2) grain size distibution and Atterberg limits of the levels identified in a soil profile; 3) thickness of the soil profile. These parameters were measured in more than 200 points, corresponding to field surveys realized in an area of about 250 km2. Soil profiles with similar features were, then, grouped through a cluster analysis, which allowed to identify soil groups characterized by similar lithotechnical properties. A map of distribution of these lithotechnical terrain units were obtained, linking the features of the soil profiles with the distribution of the bedrock materials with geological features similar to those ones of the reconstructed terrain units. The terrain unit distribution were compared with the shallow landslides inventory map of Oltrepò Pavese area, in order to identify the most prone lithotechnical terrain units by means of a knowledge-driven technique based on the Frequency Ratio methodology. These results could represent the basis for a future implementation of other methodologies (deterministic or datadriven models) for the assessment of shallow landslides susceptibility and hazard at regional scale, that could take into account also for this terrain units. This work was made in the frame of the ANDROMEDA project, funded by Fondazione Cariplo.