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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 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. |
