Rainfall thresholds for shallow landslides occurrence in a prone area of Northern Italy

Rainfall-induced shallow landslides can provoke severe consequences to people, infrastructures, cultivations and environment. For these reasons, it is necessary assessing the spatial and temporal probability of occurrence of these phenomena in the most prone zones of a territory, for early warning system strategies and land planning. The most adopted method for the determination of triggering events are rainfall thresholds. Empirical thresholds consider only rainfall attributes, such as duration, intensity and cumulated amount, while physically-based thresholds take into consideration also soil attribute, representing the soil conditions at the beginning of an event, such as the soil saturation degree at the depth of the sliding surface. This work focused to develop hydro empirical and physically-based thresholds for the occurrence of shallow landslides, taking into account field rainfall observations and soil moisture data, retrieved by hydrometeorological monitoring stations datasets. Monitoring stations were placed in 2 test sites representative of the hilly area of northern Italian Apennines and provided hydrometeorological time series, collecting data every 10 minutes. Empirical thresholds showed a good capacity to detect True Positives (TP: 95%) but they resulted affected by a high percentage of False Positives (FP: 24%), while physically-based thresholds detected 100% of TP and only 7% of FP, confirming the importance of soil conditions at the beginning of the event. Physically-based thresholds are reconstructed through a data-driven technique, based on “random forest”, that allows to find the best pair of parameters chosen within rainfall cumulated amounts and mean soil moisture conditions between 1 and 7 days. The model is calibrated considering a time span of 11 years (2007-2018) and validated using data between 2019 and 2021. The methodological approach is testing in different catchments of Oltrepò Pavese hilly area (northern Italy), that is representative of Italian Apennines environment. This work was made in the frame of the ANDROMEDA project, funded by Fondazione Cariplo.