Assessment and classification of climatic aggressiveness with regard to slope instability phenomena connected to hydrological and morphological processes

A pluviometrical data analysis was carried out for 292 Italian weather stations in order to identify an index able to classify the climatic aggressiveness with respect to some instability phenomena (quick flows, soil erosion and shallow landslides). Several climatic variables were calculated such as the average annual rainfall, the two-year return time maximum rainfall amount for 6-, 24-h, 1-, 5-day and monthly durations, the occurrence probabilities of potentially erosive phenomena, Canuti’s precipitation coefficient relative to landslide triggering and the Fournier index calculated either in the original form or with the modification proposed by Arnoldus. The latter was elaborated from the mean monthly rainfalls over the whole observation period (MFI) and as average of the individual indices calculated from the monthly dataset of each year (MFIavg). The study of the relationships among the analyzed quantities outlined the high versatility of Fournier index, particularly the modified form, which appeared closely related to all the investigated variables; therefore it may be used as a suitable synthetic way to describe different features of the pluviometric regime of a given location. A classification framework based on two threshold values of MFIavg and of the corresponding MFI is also proposed.