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A significant part of Campania is extensively covered by volcaniclastic soils, deriving from the alteration of airfall-sedimented formations of layered ashes and pumices that were ejected by Campi Flegrei and Mt. Somma–Vesuvius during explosive eruptions. Where such soils cover steep slopes cut in carbonate bedrock, landforms depend essentially on the morpho-evolution of such slopes prior to the deposition of the volcaniclastic soils, because these are generally present only as thin veneers, up to a few meters of total thickness. Historical records and local literature testify that, in this part of Campania, landslides that originate on carbonate slopes covered by such soils and terminate at their foot or at gully outlets are frequent, following critical rainfall events. Such landslides can be classified as complex, occurring initially as debris slides, but rapidly evolving into debris avalanches and/or debris flows. The localization of the initial sliding areas (i.e. ‘‘sources’’) on the slopes depends on both the spatial distribution of characters of the soil cover and the spatial distribution of the triggering rainfall events. It therefore appears reasonable to separate the two aspects of the problem and focus on the former one, in order to attempt an assessment of soil sliding susceptibility in the event of landslide-triggering rainfall. In this paper, some results of the application of a method aimed at such an assessment are presented. The method, called SLIDE (from SLiding Initiation areas DEtection), is based on the concept that, for a spatially homogeneous soil cover and a spatially homogeneous landslide-triggering rainfall sequence, different values of threshold slope gradient for limit equilibrium conditions exist, depending on morphological characters of the soil cover, such as its continuity and planform curvature. The method is based on the assessment of (1) soil cover presence, (2) discontinuities within soil cover, (3) slope gradients and curvature, by means of good resolution DEMs. It has been applied to sample carbonate slopes of Campania, where landslides originated either repeatedly or recently. Results are encouraging, and a soil sliding susceptibility map of a large area, based on a simplified version of method, is also presented.

A significant part of Campania is extensively covered by volcaniclastic soils, deriving from the alteration of airfall-sedimented formations of layered ashes and pumices that were ejected by Campi Flegrei and Mt.Somma-Vesuvius during explosive eruptions. Where such soils cover steep slopes cut in carbonate bedrock, landforms depend essentially on the morpho-evolution of the slopes prior to the deposition of the volcaniclastic soils, because they are generally present only as thin veneers, up to a few meters of total thickness. In this part of Campania, landslides that originate within the volcaniclastic soil veneer "sandwich" lying on carbonate slopes and terminate at their foot or at gully outlets are frequent and often destructive, following critical rainfall events. Such landslides can be classified as complex, occurring initially as debris slides, but rapidly evolving into debris avalanches and/or debris flows. The localization of the initial sliding areas (i.e. "sources") on the slopes depends on both the spatial distribution of characters of the soil cover and the spatial distribution of the triggering rainfall events. It therefore appeared reasonable to separate the two aspects of the problem, and focus on the former one, in order to attempt an assessment of soil sliding susceptibility in the event of landslide-triggering rainfall. Some results of the application of a method aimed at such an assessment are going to be presented. The method, called SL.I.DE. (from SLiding Initiation areas DEtection), is based on the assumption that, in case of spatially homogeneous slide-triggering rainfall sequences and volcaniclastic soil cover, different slope gradient threshold values for sliding failure exist, depending on soil cover continuity and planform curvature. Such threshold values are considered as higher on planar slopes than on slopes with discontinuities within the soil cover, as well as on slopes that display upwards concavity in planform (hollows). The method is indeed geomorphological, but it is also essentially quantitative, as in its workflow only the delineation of soil cover existence, and of discontinuities within it, are subjective. Comparison between past landslides' source areas localization (although available with variable reliability) and SL.I.DE. method susceptibility provided encouraging results, especially regarding soil-covered sloping hollows. Where such a localization was available with adequate accuracy and the number of nearby landslides was large, the method appeared as capable of recognizing most of the actual landslides' source areas. At the same time, the method reasonably overestimated areas considered as susceptible to sliding with respect to actual source areas. Overestimation, however, should be considered as such only with respect to past landslides: areas deemed as susceptible to sliding, but not interested by past landslides, should also be carefully evaluated with respect to the possibility of future landslides. Developments of the method are being pursued, by analyzing slope areas that fall into more than one susceptibility categories. In practical terms, for instance, opening new slope tracks within volcaniclastic soils covering slope hollows at angles > 30° appears inappropriate with respect to rainfall-triggered potential local instability. Assessment of soil sliding susceptibility, while only a subset of the related hazard and risk assessments, can certainly be considered worth investigating, as planners and communities could certainly take advantage from knowledge regarding where landslides can originate in case of critical rainfall. This, together with analyses of potential paths and run-out of debris avalanches/flows, could suggest including also the feasibility of slope stabilization work within evaluations of risk-reduction cost-benefit options, as techniques for the stabilization of soil covers on the slopes are nowadays indeed available.

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.

In this paper the results of the application of a method for the assessment of volcaniclastic soil sliding susceptibility to sample areas of Campania are presented. The method, called SL.I.DE. (from SLiding Initiation areas DEtection), is based on the concept that, for a spatially homogeneous soil cover and a spatially homogeneous triggering rainfall sequence, there exist different values of threshold slope gradient for limit equilibrium conditions, depending on the continuity and planform curvature of the soil cover. A large part of Campania, which forms an arc-shaped belt that, from N of the Mt. Somma-Vesuvius goes clockwise to S of it, within a radius of some tens of kilometres from the volcano, is extensively covered by volcaniclastic soils that derive from the alteration of airfall-sedimented ashes and pumices. The geomorphology of the steep slopes they cover is largely independent of the soils, as the latter represent a thin veneer upon carbonatic bedrock. In this part of Campania landslides within such soils are frequent, following intense and/or prolonged rainfall events. Such landslides can be classified as complex ones, occurring initially as debris slides but rapidly evolving through liquefaction into debris avalanches and/or debris flows. The consequences of debris avalanches and flows at the foot of slopes and at gully outlets have too often been catastrophic, and in recent decades the risk associated to such landslides has been growing, due primarily to increased and often uncontrolled urbanization. While the localization of such landslides depends on both the spatial distribution of some characters of the soil cover and on the spatial distribution of the triggering rainfall event, it appears reasonable to focus on the former, in order to attempt to assess the soil sliding susceptibility in the event of a triggering rainfall sequence, as it begins to appear evident that landslide-triggering rainfall can indeed occur over very limited areas. To this regard, extensive real-time monitoring of rainfall on volcaniclastic soil covered slopes has not yet been implemented in Campania, as there exist very few automatic rain gauges on the slopes, and weather radars do not exist yet. Therefore, an assessment of the soil sliding susceptibility in case of triggering rainfall proves useful, as it allows to consider also soil stabilization directly at sliding-susceptible areas within the framework of the evaluation of the options for reducing the risk related to such landslides in the studied area. In order to test the effectiveness of the SL.I.DE. method at assessing where soil slides can originate in case of triggering rainfall, it has been applied to some slope areas where such landslides have occurred either recently or repeatedly in the past, providing data regarding the position of source areas that can be considered sufficiently reliable. Comparison between actual landslides’ source areas and areas that were identified as susceptible provided good results. In addition, a comparison with the results obtained by means of other published methods on the same areas is also provided.