Changes in the occurrence of rainfall-induced landslides in Calabria, southern Italy, in the 20th century

Abstract

Abstract.Only a few studies have investigated the geographical and temporal variations in the frequency and distribution of rainfall-induced landslides, and the consequences of the variations on landslide risk. Lack of information limits the possibility to evaluate the impact of environmental and climate changes on landslide frequency and risk. Here, we exploit detailed historical information on landslides and rainfall in Calabria, southern Italy, between 1921 and 2010 to study the temporal and the geographical variation in the occurrence of rainfall-induced landslides and in their impact on the population. We exploit a catalogue with information on historical landslides from June 1920 to December 2010, and daily rainfall records obtained by a network of 318 rain gauges in the same period, to reconstruct 448 493 rainfall events (RE). Combining the rainfall and the landslide information, we obtain a catalogue of 1466 rainfall events with landslides (REL), where an REL is the occurrence of one or more landslide during or immediately after a rainfall event. We find that (i) the geographical and the temporal distributions of the rainfall-induced landslides have changed in the observation period, (ii) the monthly distribution of the REL has changed in the observation period, and (iii) the average and maximum cumulated event rainfall that have resulted in landslides in the recent 30-year period 1981-2010 are lower than the rainfall necessary to trigger landslides in previous periods, whereas the duration of the RE that triggered landslides has remained the same. We attribute the changes to variations in the rainfall conditions and to an increased vulnerability of the territory. To investigate the variations in the impact of REL on the population, we compared the number of REL in each of the 409 municipalities in Calabria with the size of the population in the municipalities measured by national Censuses conducted in 1951, 1981, and 2011. We adopted two strategies; the first strategy considered impact as I REL = #REL / P , and the second strategy measured impact as R REL = #REL × P , where #REL is the total number of REL in a period, and P is the size of the population in the same period and geographical area. The analysis has revealed a complex pattern of changes in the impact of rainfall-induced landslides in Calabria in the recent past, with areas where I REL and R REL have increased, and other areas where they have decreased. Municipalities where I REL has increased are mainly in the mountains, and municipalities where R REL has increased are mainly along the coasts. The complexity of the changes in the frequency and impact of rainfall-induced landslides observed in Calabria suggests that it remains difficult and uncertain to predict the possible variations in the frequency and impact of landslide in response to future climatic and environmental changes.

Regional rainfall thresholds for landslide occurrence using a centenary database

“…Usually, a date between the start and the end of the rainfall event is selected (e.g. Gullà et al, 2012;Gariano et al, 2015b), and therefore, a unique combination of cumulated rainfall duration is calculated. The selection of the landslide event date is critical for this method as it can lead to an overestimation of the threshold, particularly if the end date of a long-lasting rainfall event is chosen.…”

“…The use of multiple rain gauges is a typical option for assessing rainfall thresholds (e.g. Caine, 1980;Gariano et al, 2015b;Peruccacci et al, 2017). The main advantage lies in the proximity of the rain gauge from the landslides, which provides a better relationship between rainfall and landslide triggering.…”

“…However discussion on the topic is scarce and different distances have been proposed for the same region. For instance, for the Calabria region (Italy) Vennari et al (2014) used 12 km as a limit, whereas Gariano et al (2015b) used 5 km.…”

Reply to Anonymous Referee #1

“…Wieczorek et al, 2000), the normalized event duration (e.g. Giannecchini, 2005) and the normalized antecedent rainfall (e.g. Aleotti, 2004).…”

Modelling landslide hazard under global change: the case of a Pyrenean valley

“…The review conducted on that topic, described in Gariano and Guzzetti (2016), indicates the increasing number of studies devoted to this issue. The future evolution of hydro-meteorological conditions implies that there will be some modification in the frequency of landslide occurrence (Brunetti et al, 2012;Vennari et al, 2014;Gariano et al, 2015) and also 15 in their seasonal temporality (Stoffel et al, 2014); these factors will evolve differently depending on the region of interest.…”