Rainfall thresholds for possible occurrence of shallow landslides and debris flows in Italy.

In mountain regions worldwide, rainfall-induced landslides and associated debris flows erode slopes, scour channels, and contribute to the formation of alluvial fans that may harm humans and destroy buildings. Rainfall-induced slope failures are frequent and widespread in Italy, where individual rainfall events can result in single or multiple slope failures in small areas or in very large regions. Most of the harmful failures were rainfall-induced, and several were shallow slides or debris flows. In the 60-year period 1950-2009, casualties due to landslides were at least 6,349, an average of 16 harmful events per annum. The large number of harmful events indicates the considerable risk posed by rainfall-induced shallow landslides and debris flows to the population of Italy (Guzzetti et al. 2005a; Salvati et al. 2010). In this work, we exploit information on rainfall events that have resulted in shallow failures and debris flows to define regional thresholds for the possible occurrence of rainfall-induced shallow landslides and debris flows in three regions in northern, central, and southern Italy. Following a review of methods for establishing the dependence of landslide occurrence on rainfallmeasurements,we illustrate three catalogues of rainfall events that have resulted in slope failures in the Abruzzo, Calabria and Lombardy regions. Next, we present the method used for the definition of objective rainfall thresholds, and we apply the method to the available catalogues to determine new intensity-duration (ID) thresholds for possible occurrence of shallow landslides and debris flows in the three regions.We conclude by comparing the new thresholds to similar empirical thresholds proposed for possible landslide initiation in Italy.