Debris flows involving ash-fall pyroclastic soils in mountain ranges around Somma-Vesuvius volcano (southern Italy): a special case of landslide hazard

During the last 25.000 years, ash-fall deposits, mainly derived from the explosive activity of Somma-Vesuvius, were varyingly distributed along mountain ranges that surround the volcano, according to dispersion axes of each eruption and to morphological conditions of slopes. In such a geologically fragile environment, debris flows represent the most important natural risk for towns located at the footslopes, as it is clearly testified by the high loss of lives in the last century. Along slopes, a complex stratigraphy consisting of ash-fall deposits alternated with pedogenetic products, results in a multilayered and extremely contrasting permeability surficial hydrogeological system. This physical feature, along with the varying distribution of pyroclastic soils on slopes, leads to conditions that can locally increase pore pressure and generate initial instability during heavy rainfall events. Ill show results both of researches carried out in the field and of hydro-mechanical modelling of initial landslides that have allowed the deterministic definition of triggering hydrological thresholds.