METODOLOGIE DI ANALISI E DEFINIZIONE DI SOGLIE DI EVENTI PLUVIOMETRICI CHE HANNO INNESCATO COLATE RAPIDE E FRANE SUPERFICIALI IN UN’AREA DELL’ARCO ALPINO NORD-ORIENTALE

Debris flows and shallow landslides, due to the high velocity, in recent decades have caused the most damage in Italy both in terms of casualties that economic losses. The triggering is in almost all cases related to intense rainfall events. For these types of landslides one of the main risk mitigation measures is the adoption of early warning systems based on rainfall thresholds that identify the critical amount of precipitation for landslide initiation. The aim of this research is the development of objective, repeatable and exportable methodologies for the identification, analysis and characterization of rainfall events responsible for the triggering of shallow landslides and debris flows and the definition of empirical rainfall thresholds. The study area is the province of Trento (6,208 km2), located in the north-eastern Alps, and characterized by complex orography, with 70% of the area at an altitude above 1,000 m. 260 debris flows and shallow landslides with known date of occurrence in the study area have been extracted from the Italian Landslide Inventory (Progetto IFFI) and descriptive statistical analysis related to the main landslide controlling factors have been performed in order to assess the representativeness of the sample with known date of activation respect to the total debris flows and shallow landslides of Trento province. An objective and rigorous statistical methodology has been defined for the identification of the beginning of the triggering event based on the critical duration, that is the minimum dry period duration separating two stochastically independent rainy periods. The critical duration has been calculated for each rain gauge of the study area and its variability during the months of the year has been analysed. An analysis of the rainfall spatial variability in a neighbourhood of the landslide detachment zone has been then carried out through the examination of the Monte Macaion radar maps during some summer convective events, the comparison of rainfall records of rain gauges located in a 10 km buffer around the landslide, and the calculation of the Pearson's correlation coefficient between pairs of neighbouring rain gauges. An automatic procedure for the identification and characterization of the triggering rainfall event has been developed, which provides in output the event duration, cumulated rainfall and average intensity, the maximum rainfall height for fixed durations (from 5 minutes to 96 hours), the cumulated antecedent rainfall (from 1 to 30 antecedent days), the maximum return period of the event, the duration, rainfall amount and intensity associated with the maximum return period. The following rainfall thresholds have been then calibrated with the frequentist approach: cumulated event rainfall-duration (E-D) and average intensity-duration (I-D), which represent the rainfall event in its entirety, and rainfall amount-duration and intensity-duration associated with the event maximum return period (ETR-DTR and ITR-DTR), which consider the most critical portion of the event therefore responsible for the landslide triggering. In the absence of information about the landslide time of activation, the end of the triggering event has been identified using two criteria: the rainfall peak intensity and the last registration of the day. The relationship between rainfall thresholds and some environmental landslide controlling factors (i.e. lithology, land use and elevation) has been analysed and finally the contribution of antecedent rainfall has been evaluated. The main outcomes of the research are: the good applicability of the methodology adopted for the objective identification of the beginning of the triggering event, the low representativeness during convective summer events of the rainfall information recorded at the nearest rain gauge with respect to the precipitation over the landslide source area, the influence of the two criteria for the identification of the end of the event on the thresholds coefficients