Evaluation of sliding susceptibility in volcaniclastic soils of Campania (Southern Italy) aided by GIS techniques

A method for evaluating the susceptibility to debris slides, due to critical rainfall sequences, is discussed. The method concerns the volcaniclastic soils of Campania (Southern Italy) covering carbonatic slopes. It is based on the concept that, for a spatially homogeneous soil cover and a spatially homogeneous triggering rainfall sequence, different values of threshold slope gradient exist for limit equilibrium conditions, depending on the continuity and planform curvature of the soil cover. The method derives from the analysis of the location of the source areas of the landslides originated on May 5, 1998, on the slopes of the Pizzo d’Alvano massif, when more than a hundred of almost contemporary debris slides were triggered, providing a consistent data set. In order to test the potential of the method for the spatial prediction of debris slide triggering also in other areas of Campania displaying similar geomorphology and soil cover, it has first been applied to that same massif to verify, although a posteriori, its ability to assess the sliding susceptibility of the volcaniclastic soils prior to the May 1998 event. Comparison of the sliding susceptibility resulting from the method and the locations of the actual May 5, 1998, landslide source areas provided good results: the susceptible areas, covering 15% of the studied area, overestimated the actual source areas, while 94% of the actual source areas fell into susceptible areas. Accordingly, a simplified version of the method has been applied to a much larger area, historically and also recently struck by debris slides – debris avalanches – debris flows. Although, in this latter case, a comparison of the resulting sliding susceptibility with the locations of actual landslide source areas could be carried out only for a limited number of recent events, the results have provided useful hints for analysing the risk associated to this type of landslides in Campania. While it has originally been developed with regard to the volcaniclastic soil covers of Campania, where the role of discontinuities is generally deemed as particularly relevant to the spatial distribution of the source areas of debris slides, the method discussed in the present paper could profitably be used (with suitable adaptation and calibration) for assessing the rainfall-triggered shallow landsliding susceptibility also in different, residual and/or colluvial soil, contexts.