Slope instability following extreme precipitation events: analysis of predisposing factors in two study cases in Central Apennine, Italy

All over the world extreme precipitation events are generating increasingly significant problems in terms of slope stability. Central Italy, similarly, has shown in recent years an increase in the intensity and frequency of rainfall phenomena that can cause hydrogeological instability. The aim of this study is to propose a new methodology to assess landslide triggering thresholds in light of predisposing factors analysed using satellite indices. The procedure is mainly based on the evaluation of extreme rainfall events that occurred in the years following the extreme event under investigation, trying to find the connection between the slope dynamics and the geomorphological setting investigated in detail. At the same time, soil moisture and vegetation conditions in the area before, during and after each extreme event are also assessed using the NASA products, Landsat 7 and Landsat 8. The research focuses on an extreme precipitation event that occurred in a small area of central Italy in November 2013, which triggered some landslides. The evaluation of satellite indices of vegetation and moisture (NDVI and NDMI) and the analysis of geological and geomorphological context, allows to define landslide hazard. The results of the study made it possible to identify the daily precipitation between 150 and 200 mm as triggering threshold for two landslides in the investigated areas under low vegetation conditions. The geomorphological analysis showed that these landslides involved debris accumulated in specific slope and bedrock conditions. The type of landslides most susceptible to reactivations ruled by high-magnitude meteorological events in predisposed hydrogeological contexts have also been highlighted. However, the two studied landslides have a different genesis linked to different debris grain size, which induced, in one case, an earth flow, while in the second case produced a debris flow.