Abstract
Abstract. In Italy, rainfall is the primary trigger of landslides that frequently cause fatalities and large economic damage. Using a variety of information sources, we have compiled a catalogue listing 753 rainfall events that have resulted in landslides in Italy. For each event in the catalogue, the exact or approximate location of the landslide and the time or period of initiation of the slope failure is known, together with information on the rainfall duration D, and the rainfall mean intensity I, that have resulted in the slope failure. The catalogue represents the single largest collection of information on rainfall-induced landslides in Italy, and was exploited to determine the minimum rainfall conditions necessary for landslide occurrence in Italy, and in the Abruzzo Region, central Italy. For the purpose, new national rainfall thresholds for Italy and new regional rainfall thresholds for the Abruzzo Region were established, using two independent statistical methods, including a Bayesian inference method and a new Frequentist approach. The two methods proved complementary, with the Bayesian method more suited to analyze small data sets, and the Frequentist method performing better when applied to large data sets. The new regional thresholds for the Abruzzo Region are lower than the new national thresholds for Italy, and lower than the regional thresholds proposed in the literature for the Piedmont and Lombardy Regions in northern Italy, and for the Campania Region in southern Italy. This is important, because it shows that landslides in Italy can be triggered by less severe rainfall conditions than previously recognized. The Frequentist method experimented in this work allows for the definition of multiple minimum rainfall thresholds, each based on a different exceedance probability level. This makes the thresholds suited for the design of probabilistic schemes for the prediction of rainfall-induced landslides. A scheme based on four probabilistic thresholds is proposed. The four thresholds separate five fields, each characterized by different rainfall intensity-duration conditions, and corresponding different probability of possible landslide occurrence. The scheme can be implemented in landslide warning systems that operate on rainfall thresholds, and on precipitation measurements or forecasts.
Highlights
Landslides are frequent and widespread geomorphological phenomena in Italy (Guzzetti et al, 1994; Guzzetti and Tonelli, 2004)
The new regional thresholds for the Abruzzo Region are lower than the new national thresholds for Italy, and lower than the regional thresholds proposed in the literature for the Piedmont and Lombardy Regions in northern Italy, and for the Campania Region in southern Italy
Reasons for discarding an event were manifold, including the fact that: (i) landslides were not induced by rainfall, and the relationship between landslide occurrence and rainfall amount was weak or inexistent, (ii) the area where landslides were reported was affected by wildfires recently, a condition known to alter the amount of rainfall that can initiate slope failures (Cannon and Gartner, 2005), (iii) geographical location of the landslide was not possible or exceedingly imprecise, and (iv) rain gauges were not present in the area where a landslide was reported, or rainfall measurements were not available for the event that triggered the landslide
Summary
Landslides are frequent and widespread geomorphological phenomena in Italy (Guzzetti et al, 1994; Guzzetti and Tonelli, 2004). On 1 October 2009, a high intensity rainstorm in the Messina area, Sicily, triggered more than 500 shallow landslides in an area of less than 60 km2 Both events caused casualties, and severe economic losses. Rainfall thresholds for the possible occurrence of landslides are defined through the statistical analysis of past rainfall events that have resulted in slope failures, and can be classified based on the geographical extent for which they are determined (i.e., global, national, regional, or local thresholds), and the type of rainfall information used to establish the threshold (Guzzetti et al, 2007, 2008). We apply the two methods to the catalogue to determine new intensity-duration (ID) thresholds for possible landslide occurrence in Italy and in the Abruzzo Region, central Italy We conclude by proposing a probabilistic scheme based on multiple rainfall thresholds for the forecast of possible landslide occurrence
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