Rainfall thresholds for the possible occurrence of landslides in Italy

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 threshCorrespondence to: M. T. Brunetti (mariateresa.brunetti@irpi.cnr.it) olds 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.

Rainfall intensity–duration conditions for mass movements in Taiwan

“…However, more recently, mathematical and/or statistical criteria have been used to objectively determine the lower boundary. Guzzetti et al (2007) were the first to propose the Bayesian inference method for determining I-D thresholds; Brunetti et al (2010) proposed a frequentist approach; and Saito et al (2010a) adopted the quantile-regression method.…”

“…We therefore determined thresholds by the frequentist statistical method and the random sampling method, which are statistically robust approaches. Brunetti et al (2010) suggested the frequentist statistical method for the definition of objective empirical-rainfall thresholds. Following this method, the standard I-D threshold was defined as 5 %.…”

“…Among these, the relationship between rainfall intensity (I) and duration (D) is most commonly used to estimate rainfall thresholds (e.g., Caine 1980;Guzzetti et al 2007Guzzetti et al , 2008Brunetti et al 2010;Saito et al 2010aSaito et al , 2014. Several studies have also found that peak rainfall intensity and the initiation of mass movements are often concurrent (Aleotti 2004;Chien-Yuan et al 2005;Guzzetti et al 2007Guzzetti et al , 2008Dahal and Hasegawa 2008;Saito et al 2010aSaito et al , 2010b.…”

Debris flows in the eastern Italian Alps: seasonality and atmospheric circulation patterns

“…For the estimation of parameters α and β, we applied the frequentist approach proposed by Brunetti et al (2010). Note that the frequentist method allows identifying I -D thresholds at different levels of exceedance probabilities (see for example Brunetti et al, 2010;Peruccacci et al, 2012). In this work, we adopted a 5 % exceedance level, which means that the probability of a debris-flow-triggering rainfall event (I -D pair) to be under the estimated I -D threshold is less than 5 %.…”

“…The multiplier (α) and exponent (β) parameters are constants and are estimated by fitting the power-law model to the empirical data. For the estimation of parameters α and β, we applied the frequentist approach proposed by Brunetti et al (2010). Note that the frequentist method allows identifying I -D thresholds at different levels of exceedance probabilities (see for example Brunetti et al, 2010;Peruccacci et al, 2012).…”