La franosità nel bacino del Torrente Versa (Pv): evoluzione nel tempo e nello spazio

The two main purposes of this work are the definition of the present slope stability conditions of the Versa valley and the prediction of its future evolution. Firstly, a new geological map of the area has been produced in order to better understand the existing correlations between lithostratigraphic sequences, structural setting and landslide susceptibility. The principal active slope process is represented by a generalized solifluction of the colluvial covers that gradually accumulate at the toe of the slopes. In addition, there are more areal defined slope instability processes such as landslides (mainly earth flows and earth slides) and areas affected by high erosion rates. These processes are generally due Io particular local geological and structural conditions or to the evolution of the solifluction. Studies on slope instability in the catchment have been carried out for the first time in 1978, after the 1976 flood and repeated in 1992. The collected data convey an overview of the evolution of the area over a significant period of time. It has been noticed a 34% reduction of the landslide number and a 28% increase of the area affected by slope instability, which in turn covers about the 24% of the catchment area. Many of the instability phenomena identified in the 1978 survey show clear symptoms of reactivation, particularly retrogression of the rear scarp and coalescence of neighboring landslides. All the collected data have been inserted in the computer and processes via a GIS software (ILWIS). In the resulting maps each pixel is characterized by appropriate values for each of the factors contained in the definition of landslide hazard. The superimposition of the different factors (slope inclination and orientation, lithostratigraphy, structure, hydrogeology, land use) to the map of the geomorphological processes allows to obtain the critical combination of factors contributing to the occurrence of each landslide. Once these factors have been identified, it is possible to conduct a systematic research on the area. It is then possible to detect slopes that have been unstable in the past and where the natural morphological characteristics have been altered artificially (especially with agricultural works) and areas that at present are not affected by active instability phenomena but that are highly prone to landsliding.