A proposal for rainfall scenarios triggering superficial landslides by analysis of autumn 2008 - winter 2009 events in Calabria

Precipitation is one of the most frequent causes for landslides triggering, especially superficial ones. It is well known that any phenomenon responds peculiarly to precipitation, but it is clear that periods of exceptional rainfall, as intensity and/or persistence, correspond to diffuse landslides. In the period November 2008 - January 2009, Calabria (Southern Italy) was interested by prolonged precipitation which caused widespread landslides that affected infrastructures, main and secondary, and urban centres. In the present work, the characteristics of precipitation are presented by the analysis of rainfall data recorded by the regional network managed by the Centro Funzionale Meteoidrologico (ARPACAL) of Calabria Region. Firstly the identification and characterization of individual rainfall events are presented. For each of the 115 stations considered, every single rain event was individuated and characterized, where "single event" means the sequence of rainy days preceded and followed by null data of rainfall. An event can be composed of a single day or even more consecutive days with rain heights unlike null values. In particular, for each event the distributions of accumulated rainfall event and of the daily maximum data have been obtained. Some historical rainfall events were selected and characterized in a similar way; these events (October 1951, December 1951, October 1953, November 1959, December 1972 - March 1973, September 2000) in Calabria have produced important hydrogeological disasters, with many losses and deaths. The comparison is done on a statistical basis, with the estimate of return period values of recent events, and by comparison of cumulative rainfall and the average daily rainfall. The comparison indicates that the scenarios of precipitation in 2008-2009 were heavier, in terms of cumulative rainfall, than the historical events taken as reference. Referring to the average daily rainfall, the historical events were instead heavier. The analysis of the landslide events and of characteristics of precipitation at a regional scale, in terms of cumulative rainfall and average daily rainfall, allow to propose triggering scenarios of superficial landslide events. This is an interesting result also for the activities of forecasting and prevention in the Civil Protection System.