Abstract
In the last decades the Valtellina valley (northern Italy) has suffered from several catastrophic rainfall -induced shallow landslide events inducing debris flows. The growing of urban settlements has driven population to colonize areas at risk, where prediction and prevention actions are nowadays a challenge for geoscientists. Debris flows are widespread in mountain areas because occurring along steep slopes covered by loose regolith or soil coverings. Under such conditions, heavy rainfall events might cause slope instabilities due to the increase in pore water pressure depending on hydraulic and geotechnical properties as well as thicknesses of soil covers. Despite the initial small volumes, debris flows hazard is significant due to the sediment entrainment and volume increase of the involved material, high velocity and runout distance. In such a framework, predicting timing and position of slope instabilities as well as paths, volumes, and velocity of potential debris flows is of great significance to assess areas at risk and to settle appropriate countermeasures. In this work, back analyses of debris flows occurred in representative sites of the Valtellina valley were carried out with the aim of understanding their features and providing a methodological basis for slope to valley scale susceptibility mapping. Numerical modeling of slope stability and runout was completed allowing the identification of the detachment, transport, and deposition zones of previously occurred landslides, including other potentially unstable ones. Results from this study emphasize issues in performing distributed numerical modeling depending on the availability of spatially distributed soil properties which hamper the quality of physicsbased models. In the framework of hazard mapping and risk strategy assessments, the approach presented can be used to evaluate the possible runout phase of new potential debris flows recognized by geomorphological evidence and numerical modeling. Furthermore, analyses aimed to the probabilistic assessment of landslide spatial distribution, related to a specific value of rainfall threshold, can be considered as potentially applicable to multi -scale landslide hazard mapping and extendable to other similar mountainous frameworks.