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
In Italy, landslides are a serious threat to the population (Guzzetti, 2000; Guzzetti et al., 2005a, b; Salvati et al., 2010), with 3025 persons killed (1279), missing (15) or injured (1731) by landslides in the 50-year period 1954–2013. In Calabria, a region in southern Italy, landslides have killed 38 and injured 150 people in the same period (POLARIS, 2015). Of the 8103 municipalities in Italy, 656 (8 %) have experienced landslides with casualties (deaths, missing persons, injured people). The number of the municipalities increases to 1531 (19 %) if evacuees and homeless people are considered. In Calabria, 46 of the 409 municipalities (11 %) have experienced landslides with casualties in the period 1950–2013. Considering evacuees and homeless, the figure increases to 150 municipalities (37 %).
Rainfall is the primary trigger of landslides in Italy (Guzzetti et al., 1994; Trigila et al., 2010). To model the relationship between rainfall and landslide occurrence, a standard approach consists in the joint analysis of landslides and rainfall records to determine empirical rainfall thresholds for possible landslide occurrence (Reichenbach et al., 1998; Guzzetti et al., 2007, 2008). The approach assumes a stationary (in time) relationship to link landslide occurrence to rainfall measurements (Reichenbach et al., 1998; Guzzetti et al., 2007). However, rainfall conditions that have resulted in landslides in the past may change over long periods, or may vary in the future due to environmental and climatic changes, including changes in rainfall intensity and frequency, and in the pattern of the meteorological triggering events. Variations in the frequency and intensity of the rainfall events will affect the frequency of rainfall-induced landslides (Crozier, 2010). An increase in the frequency and the intensity of extreme rainfall events was observed in several geographical regions in the world (IPCC, 2013), including Italy (Brunetti et al., 2002). Temporal variations in the rainfall conditions that result in landslides can jeopardise the definition and the application of empirical rainfall thresholds for the prediction of landslide occurrence (Guzzetti et al., 2007, 2008).
Only a few studies have investigated the geographical and temporal variations in the frequency and distribution of rainfall-induced landslides (e.g., Reichenbach et al., 1998; Mathie et al., 2007; Polemio and Petrucci, 2010; Chiang and Chang, 2011; Lollino et al., 2014; Stoffel et al., 2014), and the consequences of the variations on landslide risk (Guzzetti et al., 2005b; Salvati et al., 2010). Lack of information limits the possibility to evaluate the impact of the expected environmental and climate changes on landslide frequency, and the related risk. In an effort to fill this gap, we exploit detailed historical information on landslides and rainfall in Calabria, southern Italy, in the 90-year period 1921–2010 to study the temporal and the geographical variations in the occurrence of rainfall-induced landslides and in their impact on the population, and we investigate some of the natural (i.e., rainfall) and human (i.e., population density) causes of the changes.
In the literature, the analysis of the effects of climatic and environmental
changes on landslide activity is performed using modelling or empirical
approaches (Crozier, 2010). The modelling approach investigates variations
in the stability/instability conditions of single landslides driven by
records of rainfall and/or pore pressure measurements, and attempts to
predict variations in the stability/instability conditions of slopes using
synthetic, future rainfall records obtained from downscaled global climate
models (Buma and Dehn, 1998, 2000; Dehn and Buma, 1999; Dehn et al., 2000;
Comegna et al. 2013; Rianna et al., 2014). The empirical approach exploits
records of landslide occurrences to determine variations in the activity or
the frequency of the landslides, and can be separated in two groups
depending on the period covered and the tools used to construct the records
of the landslide occurrences. An approach exploits palaeoenvironmental
evidences to construct landslide records and to analyse periods of
increased/decreased landslide activity. Adopting this approach and
exploiting
Another approach consists in the comparison of catalogues of historical landslide occurrences to records of river discharge or rainfall measurements. Reichenbach et al. (1998) used a catalogue of historical landslide and flood events in the Tiber River basin, Italy, and records of mean daily discharge at different gauging stations in the same catchment between 1918 and 1990, to determine regional hydrological thresholds for landslide and flood occurrence. Stoffel et al. (2014) analysed changes in the frequency and seasonal distribution of shallow landslides in Piedmont, NW Italy, from 1960 to 2011, and identified two periods of increased landslide frequency that they attributed to an increase in the mean annual temperature. Polemio and Petrucci (2010) studied monthly rainfall (and temperature) records in Calabria from 1921 to 2006, and determined that landslide occurrence did not decrease in the region in their observation period, despite a decrease in the monthly total rainfall.
A difficulty in the application of the empirical historical approach lays in the availability of accurate records of historical landslide events. In many areas, accurate and sufficiently complete records of rainfall measurements are available for long (multi-decadal) periods. However, landslide records are not commonly available for comparatively long periods (Guzzetti, 2000; Guzzetti et al., 2005b). Further, the completeness and quality of the landslide records varies with time (Petrucci and Pasqua, 2008; Petrucci and Gullà, 2010), depending on the abundance and type of the historical and recent sources, and the skill of the investigators (Lang et al., 1999; Guzzetti et al., 1994, 2005a).
A limited number of global catalogues lists information on landslides and
floods, including (i) the
In Italy, abundant information exists on historical landslides. An inventory of historical landslides was first prepared in the framework of the AVI – Aree Vulnerate Italiane (an acronym for Areas Affected by Landslides or Floods) national project to cover the period 1917–1990 (Guzzetti et al., 1994), and was next updated to cover the period 1900–2002 (Guzzetti and Tonelli, 2004). Guzzetti (2000) prepared a catalogue of historical landslides with direct human consequences from 1279 to 1999. The catalogue was revised and extended by Salvati et al. (2003, 2010) and by Guzzetti et al. (2005a, b). Recently, Brunetti et al. (2015) compiled a catalogue of 1981 rainfall events that have resulted in at least 2408 shallow landslides in Italy in the period 1996–2012. In addition, a number of regional and local catalogues of landslide events exists in Italy, including catalogues for e.g., the Emilia-Romagna (Emilia-Romagna SGSS, 2015), Umbria (Salvati et al., 2006), and Calabria (Petrucci and Versace, 2005, 2007; Petrucci et al., 2009; Palmieri et al., 2011) regions.
We base our analysis of the possible variations in the rainfall-induced landslides in Calabria on two sources of information collected in the region, including: (i) a database of daily rainfall measurements, and (ii) a record of occurrences of rainfall-induced landslides. Both sources cover the 90-year period 1921–2010. Our approach relies on the construction and analysis of three catalogues: (i) a catalogue of landslide events, (ii) a catalogue of rainfall events, and (iii) a catalogue of rainfall events with landslides.
We first define a landslide event (LE) as the occurrence of one or more landslides in a given municipality and in a given date (day, month, year). In the literature, no clear definition exists for a rainfall event (RE), and no common criteria exist to single out RE from rainfall records (Melillo et al., 2015). In this work, we define a rainfall event (RE) as a continuous sequence of rainy days (i.e., days with cumulated daily rainfall > 0 mm) preceded and followed by at least 1 dry day (i.e., a day with no measured rainfall, Gullà et al., 2012). We further define a rainfall event with landslides (REL) as the occurrence of a LE during or immediately after an RE. To single out the individual REL, we use two criteria. First, the geographical distance between the LE and the location of the rain gauge where the event is determined shall be < 5 km. Where two or more rain gauges meet this criterion, we select the rain gauge closest to the landslide. Second, the date of the LE must be between the start and the end dates of the RE, or no more than 1 day after the end of the RE. The starting date of the REL corresponds to the start date of the RE. The end date of REL is (i) the day when the rainfall-induced landslide occurred (if the landslide occurred between the start and the end dates of the RE) or (ii) the end date of the RE (if the landslide occurred in the day following the end day of the RE).
Our analysis also relays on the definition of empirical rainfall thresholds
for possible landslide occurrence in Calabria. To define the rainfall
thresholds we adopt the method proposed by Brunetti et al. (2010) and
modified by Peruccacci et al. (2012), where the threshold curve is a power
law relationship linking the rainfall duration
Our study area is Calabria, a region in southern Italy that extends for
15 080 km
A number of studies have investigated variations in the rainfall patters and trends in Calabria. Ferrari and Terranova (2004) revealed a reduction in the annual and the winter amounts of rainfall for two overlapping periods (1920–2000 and 1960–2000), and Caloiero et al. (2011) recognised an augmented trend in the summer rainfall. Caloiero et al. (2008) showed that short-duration rainfall events were most frequent in November between 1921 and 1960, and in October between 1961 and 2000. Brunetti et al. (2012) observed a marked decrease in the cumulated annual rainfall in the period 1916–2006, particularly in the E (Ionian) side of the region, and attributed the decrease to a negative trend in the monthly rainfall in the autumn–winter period, whereas in the summer the tendency is toward an increase of the cumulated rainfall.
To compile the catalogue of landslide events (LE) we used different sources of information, including local and national newspapers, web sites, reports from national and regional agencies and public offices, and post-event field surveys (Petrucci and Versace, 2005, 2007; Petrucci et al., 2009; Palmieri et al., 2011). Each record in the LE catalogue lists: (a) a LE identification number, (b) the date (and time when available) of occurrence of the landslide(s), (c) the geographical location of the landslide(s), (d) a short description of the landslide(s), (e) an indication of whether a “single” or “multiple” (two or more) landslides were reported, and (f) qualitative information on the size (“small” or “large) of the reported landslide(s).
Overall, the catalogue lists information on 7600 LE from June 1920 to December 2010 (on average 84 LE per year, 7 LE per month). Not all the records in the catalogue contain all the information. Information on the geographical location of the landslide(s) consists in the geographical coordinates of the site(s) where the landslide(s) has (have) occurred (available for 23 % of the LE), or in the geographical coordinates of the centroid of the municipality where the landslide(s) was (were) reported (available for 77 % of the LE).
Figure 2a shows the number of LE in each of the 409 municipalities in Calabria. The average number of LE per municipality is 17, with 146 municipalities with less than 10 LE, and three municipalities (Torre di Ruggiero, Simbario, Spadola) with only one LE. Four municipalities experienced more than 100 LE, including Catanzaro (142 LE), Cosenza (120), Reggio Calabria (117), and Scilla (104). We note that the municipalities of Reggio Calabria, Catanzaro and Cosenza (Fig. 1a) host the three largest and most populated cities in the region. The two adjoining municipalities of Scilla and Bagnara Calabra, along the SW coast of the region, together were affected by 190 LE, most of which occurred along the SS18 national road and the national railway connecting southern Calabria to Central Italy (Diodato et al., 2011; Petrucci and Pasqua, 2013; Iovine et al., 2014).
Figure 2b portrays the temporal distribution of the LE. The average number of LE per year is 67. For 15 years in the catalogue, less than 10 LE were recorded. We consider these years as characterised by low landslide impact. For 17 years in the catalogue, 100 or more LE were recorded. We consider these years as years with a high landslide impact, including 2009 (492 LE) and 2010 (499). The 3 single days with the largest number of reported LE were 30 November 1933 (87 LE), 3 October 1996 (67), and 13 January 2009 (52). The cumulated number of LE (black line in Fig. 2b) measures the completeness of the historical catalogue (Guzzetti, 2000; Wood et al., 2015). In the early period 1910–1950, the cumulated curve exhibits a slope lower than in the later period 1950–2010. We interpret this as an indication that the catalogue is more complete in the recent part (after 1950) and less complete in the older part of the series.
To obtain the catalogue of rainfall events (RE) we used rainfall measurements captured by a network of 318 rain gauges in Calabria (Fig. 1a) between 1 January 1920 and 31 December 2010. On average, the single rain gauges operated for 47 years, with 13 rain gauges (4.0 %) that operated for the entire 90-year period, and three rain gauges (0.9 %) with only 2 years of measurements. Figure 1c shows the number of rain gauges per year in the observation period. From the mid-1920s to 2010, at least 150 rain gauges were operational every year. The largest number of rain gauges (> 200) was available in the 1930s, and between 1950 and 1975, whereas a minimum number of gauges was available in the early 1940s, during the Second World War. A decrease in the number of the available rain gauges was observed in the most recent years (from 2000), and is due to the replacement of old rain gauges with new, automatic gauges, many of which were located in places different from the old gauges.
We adopted strategies to consider and mitigate the effects of the
heterogeneity inherent to the rainfall measurements. We obtained the
database of rainfall measurements used in the study from the “Centro
Funzionale Multirischi” of the Environmental Protection Agency of Calabria
(
Exploiting the rainfall information available to us, and adopting the
approach proposed by Gullà et al. (2012), we reconstructed 448 493
rainfall events (RE) in the 90-year observation period. This is an average of about 4893 RE
per year, with a maximum of 7760 (1.7 %) RE in 1964. For each RE we
determined (a) the start and the end date of the event, (b) the rain gauge
where the RE was detected (with an event referred to a single rain gauge),
(c) the event duration (
Summary statistics for rainfall events with landslides
(REL) in Calabria for the entire catalogue, and for different periods and
landslide subsets. Legend: #REL, number of rainfall events with
landslides;
Figure 2c shows the number of RE per rain gauge. A reference area
based on Thiessen polygons was assigned to each rain gauge. The average
number of RE per rain gauge is 1240, with a maximum of 2822 RE for the
Catanzaro rain gauge. For 84 rain gauges, located mainly in the N part of
the region, more than 2000 RE were reconstructed. Figure 2d shows the
number of severe RE (
Figure 3a shows the number of severe RE per year in the 90-year observation period. The average value per year is 28, and was exceeded 43 times, of which 15 times in the 1921–1950 period, 18 times in the 1951–1980 period, and 10 times in the 1981–2010 period. The 1951–1980 period was characterised by the largest average number of severe RE per year (318). The maximum number of severe RE (609) was recorded in 1954, and the minimum (79) in 1922. In six years (1930, 1933, 1940, 1954, 1973, 1996) the number of reconstructed severe RE was larger than 500.
Using the method presented in Sect. 3 we reconstructed 1989 REL
in Calabria between October 1921 and December 2010. The REL have an average
duration
Figure 2e shows the number of REL per municipality. The average number of REL in a municipality is four, the minimum is zero (95 municipalities, located chiefly in the N part of the region and along the SW coast), and the maximum is 47, in the Catanzaro municipality (Fig. 2e). More than 30 municipalities experienced 10 or more REL in the investigated period. Figure 2f portrays the number of REL per year. On average, 16 REL occurred every year with a maximum of 139 REL in 1973. The 3 days with the largest number of REL were 18 February 2010 (39 REL), 29 February 1956 (33), and 27 November 1959 (30). The month with the largest number of REL was January (337), followed by November (255), and February (254). The decades with the largest number of REL were those between 1950 and 1959, between 1970 and 1979, and between 2000 and 2009. Inspection of the cumulated number of REL (black curve in Fig. 2f) reveals that in the early period 1910–1950, the rate of the REL is lower than in the later period 1950–2010. This is a result of the different completeness of the information of the LE (Fig. 2b).
Figure 3b shows the number of REL per year in the 90-year observation period. For six years more than 60 REL were recorded: 1973 (135 REL), 2009 (101), 1953 (92), 2010 (78), 1954 (76), and 1996 (72). The average value (16 REL per year) was exceeded 28 times: twice in the 1921–1950 period and 13 times in each of the following 30-year periods. The intermediate (1951–1980) and the recent (1981–2010) periods were characterised by average number of REL per year equal to 24 and 20, respectively. In both cases the values are larger than the 90-year average (16).
Maps show the number of rainfall events with landslides
(#REL) per municipality in Calabria in the
Top: Maps show the number of rainfall events with
landslides (#REL) per municipality in Calabria in
Our catalogue lists 1466 severe REL in Calabria from October 1921 to
December 2010, of which 534 (36.4 %) have triggered “single” landslides
and 932 (63.6 %) “multiple” landslides. We acknowledge uncertainty in
the classification of an REL as having triggered a “single” landslide.
Indeed, a “single” landslide REL may have triggered multiple landslides
and the information may not be available to us. Conversely, REL with
“multiple” landslides are certain, because they have triggered two or more
landslides. We note that REL with “single” landslides have average and
maximum cumulated event rainfall
We investigated the spatial and temporal distributions of the REL listed in our catalogue, and their temporal variations. For the purpose, we segmented the catalogue considering (i) the four seasons, (ii) two seasonal periods (i.e., “dry” and “wet” period proposed by Vennari et al., 2014), and (iii) three consecutive, non-overlapping 30-year periods (1921–1950, 1951–1980, 1981–2010).
Figure 4 shows the geographical distribution of the total number of
REL in each municipality for the four seasons. Most of the REL occurred in
winter (728, 49.7 %) and autumn (622, 42.4 %), with only 84 REL
(5.7 %) in the spring and 32 REL (2.2 %) in the summer (Table 1). The geographical distribution of the REL occurred in the autumn
(Fig. 4a) and winter (Fig. 4b) seasons are similar, with
REL in the winter exhibiting average and maximum values of
Figure 5 shows the geographical distribution of the total number of
REL in each municipality for the “dry” period from April to October
(Fig. 5b) and the “wet” period from November to March
(Fig. 5a) adopted by Vennari et al. (2014) to determine seasonal
rainfall thresholds for possible landslide occurrence in Calabria. REL in
the “dry” period (307, 20.9 %, Table 1) occurred mostly along
the E (Ionian) side of the region, whereas REL in the “wet” period (1159,
79.1 %) were distributed throughout the region, with longer rainfall
duration
Figure 6a, b and c show the number of REL per
municipality in three periods: 1921–1950 (old period), 1951–1980
(intermediate period), and 1981–2010 (recent period). Of the 1466 REL in
the catalogue, 143 REL (9.8 %) occurred in the old period, 720 (49.1 %)
in the intermediate period, and 603 (41.1 %) in the recent period
(Table 1). The spatial distributions of the REL in the intermediate
and the recent periods are similar, with a larger number of REL affecting
the SE part of the region in the recent period. Figure 6d,
e and f portray the monthly distributions of the REL in
the three periods. In the old period (Fig. 6d) REL occurred between
October and March, with the majority of the REL (51, 35.7 %) in November.
In the intermediate and the recent periods, the REL were also most abundant
between October and March. In particular, in the intermediate period 1951–1980, REL were equally distributed in the autumn (322, 44.7 %) and the
winter (329, 45.7 %), with peaks in October (145, 20.1 %) and November
(170, 11.6 %, Fig. 6e). In the recent period, the majority of the
REL (369, 61 %) occurred in winter, with a distinct peak in January (162
REL, 26.9 %) (Fig. 6f) and only 86 REL (14.3 %) in October and
November. In the three considered periods, the REL exhibited similar ranges
of rainfall duration
Using the three catalogues of landslide events (LE), of rainfall events (RE), and of rainfall events with landslides (REL) in Calabria in the 90-year period 1921–2010, we first investigate the changes in the yearly distribution of rainfall events with landslides. This is followed by an analysis of the changes in the rainfall conditions that have resulted in landslides in Calabria, measured by empirical rainfall thresholds for landslide occurrence. Lastly, we compare the variations in the number and distribution in time of landslide events with the variations in the density of the population in each municipality in Calabria, to analyse variations in the landslide impact and risk to the population.
Three key aspects of our analyses need to be addressed. First, our catalogues consider only rainfall-induced landslides that were noticed (and recorded) because they have caused damage to public or private properties, or to the population. Thus, the catalogues, and particularly the landslide catalogue, are not complete. Second, an underestimation in the number of landslides in the old period of the series (1921–1950) is expected, due to the reduced availability of the sources of information. For this reason, we conduct our most relevant analyses comparing the intermediate (1951–1980) and the recent (1981–2010) periods, which we consider equally complete for statistical purposes (Fig. 6). Third, the daily time scale of the rainfall series used for our analysis is not sufficient to determine rainfall thresholds for short duration (< 24 h) events. Thus, the thresholds defined in this work are not adequate for rainfall durations < 24 h, and cannot be used in landslide warning systems. However, we maintain that the thresholds are adequate to analyse variations in the landslide rainfall triggering conditions in Calabria.
Average (mean) and maximum (max) values of cumulated event
rainfall
Maps show the number of rainfall events with landslides
(#REL) in each municipality in Calabria in the three 30-year periods,
To determine if the yearly distribution of REL has changed in the 90-year
observation period, we analysed the histograms shown in Fig. 6d,
e, and f, and we computed for each month the ratio
REL
Cumulated event rainfall-rainfall duration (ED) thresholds, at 5 % exceedance probability, for rainfall-induced landslides in Calabria, for the entire catalogue and for different subsets. #REL is the number of rainfall events used to define the threshold.
To ascertain whether the rainfall conditions for possible landslide
occurrence have changed in Calabria in the 90-year observation period, we
defined cumulated event rainfall–rainfall duration (ED) thresholds for all the
REL in the catalogue, and for different subsets. Table 3 lists the
equations and the ranges of validity of the established 5 % ED thresholds.
Except for the validity range, only minor differences exists in the
parameters controlling the thresholds defined for the entire catalogue
(
Inspection of Table 3 reveals that the
Considering the seasonal periods, the threshold defined for autumn
(
Figure 7a compares the ED threshold defined in this work for Calabria
(
Figure 7b shows that the ED thresholds for landslide occurrence in
Calabria are different for the three 30-year considered periods
(
To investigate the variations in the impact of landslides on the population
of Calabria, we compared the number of REL with the size of the population
in each of the 409 municipalities in the region. For the purpose, we used
population data available from national Censuses conducted by the Italian
National Institute of Statistics (ISTAT –
Top: Maps show density of population in the 409
municipalities in Calabria for three 30-year periods,
Figure 8a, b, c portray the changes in the
density of the population in the 409 municipalities, for the three
considered periods. The number of municipalities with a low (
To evaluate the impact of REL on the population, we adopt two strategies.
The first strategy considers impact as
Adopting the first strategy, we calculated I
Population did not change (increased/decreased) evenly in Calabria in the
investigation period (Fig. 8a, b, c). For this
reason, we computed the ratio between the number of REL and the number of
people in each municipality, for the three considered 30-year periods
(Fig. 8d, e, f). In the old period (1921–1950), 75 % of the municipalities (305 out of 409, Fig. 8d) did
not experience REL (
Using the second strategy, we calculated
The second case consists of 30 (7 %) municipalities (orange areas in
Fig. 9, covering 1008 km
Map showing variations in
The third case consists of 147 (36 %) municipalities (yellow areas in
Fig. 9, 5886 km
For 87 % of the municipalities where
Overall,
We recognise that multiple factors have conditioned the results of our analyses and the discussion. These include (i) the completeness of the landslide catalogue, (ii) the reliability and homogeneity of the rainfall records, (iii) the length (90 years) of the catalogue, (iv) the number (3) and length (30 years) of the segmentation periods of the catalogue, (v) the modelling tools and parameters used to single out the RE and the REL, and (vi) to determine the rainfall thresholds. More sophisticated statistical techniques may also be used to analyse the catalogues of landslide events, of rainfall events, and of rainfall events with landslides (Rossi et al., 2010; Witt et al., 2010). For these reasons, we acknowledge that our results are preliminary.
We have investigated geographical and temporal changes in the occurrence of 1466 rainfall-induced landslide events in Calabria, in the 90-year period 1921–2010.
Our work revealed that the rainfall conditions that have resulted in
rainfall-induced landslides in Calabria have changed in the observation
period. We found that less cumulated event rainfall (
We investigated the changes in the influence of rainfall events with landslides on the population of Calabria, adopting two complementary strategies. The analysis revealed a complex picture. The impact of REL on the population has increased in 37 municipalities covering 15.5 % of the total area and hosting 18.5 % of the population, and has decreased in 47 municipalities covering 17.8 % of the total area and hosting 20.8 % of the population. The risk posed by REL to the population has increased in 30 municipalities covering 6.7 % of the total area and hosting 24.0 % of the population, and has decreased in 47 municipalities covering 39.0 % of the total area and hosting 19.1 % of the population. Overall, 42.5 % (57.5) of the regional population has experienced an increased (decreased) level of landslide impact or risk.
We note that the observed changes in the impact of rainfall-induced landslides on the population of Calabria in the investigated 90-year period are due to changes in the number of the events (a largely natural component) and to changes in the number of the exposed elements (a largely societal component). Several of the municipalities with an increased landslide risk between the intermediate and the recent periods have experienced a similar number of REL (hazard has remained about the same) in the two periods, but a large increase in the size of the population (exposure has increased) in the recent period.
Overall, our research revealed a complex picture of the changes in the impact of rainfall-induced landslides in Calabria in the recent past, with areas where the impact and risk to the population have increased, and other areas where the impact and risk have decreased. The geographical pattern of the variations is diversified (Fig. 9), revealing the complexity of the interactions between the natural (climate) and the human induced (population, land cover) factors that control the frequency of the rainfall-induced landslides, and the intensity of the consequences. We argue that there was nothing special or peculiar in the natural and societal landscapes in Calabria. We therefore hypothesise that the same complexity and variability exists in similar surrounding regions, in Italy and in the Mediterranean region. The hypothesis can be tested using the same methodology experimented in this paper for the joint analysis of historical landslide, rainfall and population information, anywhere adequate information is available.
Finally, we stress that the complexity of the temporal and geographical pattern of the variations in the frequency and impact of rainfall-induced landslides observed for the recent past in Calabria suggests that it will be difficult and uncertain to predict the possible variations in the frequency and impact in response to future climatic and environmental changes.
The “Centro Funzionale Multirischi” of the Agenzia Regionale per
l'Ambiente della Calabria (ARPACAL,