Eighty Years of Data Collected for the Determination of Rainfall Threshold Triggering Shallow Landslides and Mud-Debris Flows in the Alps

Identifying the minimum rainfall thresholds necessary for landslides triggering is essential to landslide risk assessment. The Italian Alps have always been affected by shallow landslides and mud-debris flows, which caused considerable damage to property and, sometimes, casualties. We analysed information provided from different sources carrying on the most thorough research conducted for this alpine area. Thousands of documents and reports of rainfall values recorded over 80 years by rain gauges distributed in Sondrio and Brescia Provinces define the mean annual precipitation (MAP)-normalized intensity-duration thresholds for the initiation of shallow landslides and mud-debris flows. The established curves are generally lower compared to those proposed in literature for similar mountain areas in Italy and worldwide. Furthermore, we found that landslides occurred primarily at the same time or within 3 h from the maximum peak of rainfall intensity in summer events and in a period from 0 to 5 h or later in spring-autumn events. The paper provides a further contribution to the knowledge framework on the rainfall conditions required for the initiation of surficial landslides and mud-debris flows and their expected timing of occurrence. This knowledge is crucial to develop better warning strategies to mitigate geo-hydrological risk and reduce the socio-economic damage.