Forecasting hazard scenarios and implications for the evaluation of countermeasure efficiency for large debris avalanches

The key objective of a slope failure study is to reduce deaths, injuries and property damage. Predictions of slope failures are mandatory to reduce hazard and vulnerability of elements at risk with appropriate warning policy and properly designed countermeasures. Numerical modelling has been recently used as an approach to back-analyse real events but few attempts have been carried out to forecast potential failure scenarios. Recently, a debris avalanche involving about 1.2 million m3 of debris and a nearby debris slide flow with failure volume of 0.1 million m3 occurred in the Italian Prealps from the accumulation of a paleo-landslide. Field surveys and monitoring revealed that the upper part of the slope above the largest recent landslide is unstable with a possible failure volume between 1 and 1.5 million m3. We conducted comprehensive fieldwork and analyses including geological and geomorphological surveys, compilation of a landslide inventory map, collection of rainfall records, study of triggering mechanisms and geotechnical properties of the failed materials, slope stability analyses and back-analyses of the runout phase of the landslide events. Through a series of slope stability analyses, the possible failure volume, sliding surface and topography are estimated, consistent with field monitoring data. Furthermore, with the help of calibration through back-analyses of the past landslide events, we assign a given degree of danger to slope elements with analogous conditions. We then numerically predict the runout patterns and mobility of the potential instabilities. The effectiveness of the planned passive countermeasures for potential zoning management is evaluated.