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Landslide-risk scenario of the Costa Viola mountain ridge (Calabria,
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Southern Italy)
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Oreste G. Terranova1, Stefano L. Gariano2,3, Claudia Bruno1, Roberto
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Greco1, Annamaria D. Pellegrino4, Giulio G.R. Iovine1*
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1) CNR-IRPI (National Research Council, Research Institute for Geo-hydrological
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Protection), via Cavour, 6, 87036, Rende (Cosenza), Italy.
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2) CNR-IRPI (National Research Council, Research Institute for Geo-hydrological
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Protection), via della Madonna Alta, 126, 06128, Perugia, Italy.
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3) University of Perugia, Department of Physics and Geology, via A. Pascoli, 06123,
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Perugia, Italy.
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4) Regional Basin Authority of Calabria, Cittadella Regionale, Germaneto, 88100,
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Catanzaro, Italy.
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* corresponding author, tel. +39 0984 841 419, e-mail
[email protected]
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Landslide-risk scenario of the Costa Viola mountain ridge (Calabria,
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Southern Italy)
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The study area of the Costa Viola mountain ridge is strongly exposed to shallow
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landslides triggered by rainfall. Starting from a susceptibility map, recently
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published by the same team, the related risk has been assessed, limitedly to
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landslide sources. The hazard has first been evaluated by considering the
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recurrence periods of triggering events. Economic values and physical
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vulnerabilities of the types of elements at risk have been assumed by considering
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land-use, and effects of similar events in Southern Italy, respectively. The
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1:30,000-scale risk map has been obtained by combining the above-mentioned
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layers. Low risk prevails in the study area, whilst highest values are concentrated
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along the coast, where villages and main infrastructures are located. The expected
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annual damage exceeds 570 Million € (about 68% pertaining to low-risk cells).
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Risk maps obtained through similar simplified GIS-based procedures may play a
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crucial role in accurate land-use planning and effective decision-making.
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Keywords: shallow landslide, susceptibility, hazard, physical vulnerability,
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potential worth of loss, risk, logistic regression
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1. Introduction Shallow landslides are among the most damaging natural hazards in terms of
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economic losses and casualties. Such phenomena are characterized by rapid dynamics
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and are widespread worldwide. As for other types of dangerous phenomena, in addition
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to direct damage – e.g., those caused to buildings, transport infrastructures, agricultural
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crops, water supply networks, and sewerage and communication systems – indirect
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damage should be considered: for instance, conspicuous financial losses may occur due
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to impossibility of using assets not directly affected by the landslides. Events
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characterized by activation of widespread shallow landslides may hamper the economic
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and social development for years after their occurrence. Furthermore, the same area
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may be affected by recurrent landslide events due to its intrinsic exposition to
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predisposing (e.g., steep slopes, lithology) and triggering factors (e.g., intense rainfall,
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seismic shocks).
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Aiming at assessing hazard and risk, the evaluation and mapping of spatial triggering
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probability of shallow landslides – i.e., “source susceptibility” – must first be
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accomplished. In case a significant propagation downslope of the landslide source is
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expected (e.g., in case of soil slip-debris flow transition), the “propagation
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susceptibility” must also be evaluated (Iovine, 2012). The occurrence of shallow
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landslides is usually strictly related to external triggering factors: therefore, hazard
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evaluations can be performed based on recurrence probabilities of the causal factor
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(e.g., rains or earthquakes), which can be estimated by statistical analyses of temporal
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series. Finally, when vulnerability and economic values of the elements exposed to risk
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are known, a further, multidisciplinary step toward total risk assessment can be
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undertaken (Varnes and IAEG Commission on Landslides and Other Mass Movements
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on Slopes, 1984; Dai et al., 2002; Gaprindashvili and van Westen, 2016).
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Due to its geological history, and to present climatic and seismic characteristics,
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Calabria (Southern Italy) is mostly prone to shallow landslides (Carrara et al., 1982;
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Pellegrino and Borrelli, 2007). In fact, prolonged rainfall and rainstorms frequently
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occur; earthquake catalogues include several high-magnitude events occurred in the past
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centuries; weathered and tectonized terrains diffusely crop out along steep slopes cut by
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the hydrographic network on actively uplifted massifs. In Calabria, shallow landslides
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are mostly triggered by severe rains (Terranova and Gariano, 2014; Vennari et al.,
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2014). Frequency and intensity of rainstorms increased in recent years, despite a
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reduction of annual rainfall amounts, due to occurrence of medicanes favoured by the
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interaction between warm sea and rugged orography. A recent investigation allowed to
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identify numerous landslide events that affected either small areas or large part of the
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Calabrian territory in the last decades (cf. Iovine et al., 2014, and references therein).
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In this study, an example of rainfall-induced shallow-landslide risk evaluation at source
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locations is proposed for the Costa Viola mountain ridge (Calabria). The analysis is
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based on “source susceptibility” evaluations, given the limited runout generally
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observed in historical cases occurred in the study area. Recurrence periods of the
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triggering events were obtained through hydrological analyses of annual maxima of
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daily rainfall. Economic values of the elements at risk were assumed based on land-use.
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Physical vulnerability was obtained by taking into account types of existing elements
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and intensities of expected phenomena, together with effects of similar events occurred
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in Southern Italy. The resulting risk map (at 1:30,000 scale) has been obtained by
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properly combining, in a GIS environment, the above-mentioned layers.
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2. Study area
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The Costa Viola mountain ridge extends approximately 82 km2 along the SW
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coast of Calabria. The area includes short high-gradient drainage basins (the widest
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thereof are named Favazzina and Sfalassà), which flow into the Tyrrhenian Sea between
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the villages of Scilla and Bagnara Calabra (Figure 1). The Palaeozoic basement of the
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area is mainly made of middle-high grade metamorphic rocks and acid intrusive rocks,
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strongly tectonized and deeply weathered (Figure 2). The sedimentary cover (Late
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Miocene to Holocene deposits) unconformably lies on the basement, and is made of:
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conglomerate, arenite, clay, and marl with subordinate evaporite (Late Miocene-Middle
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Pliocene); sand and conglomerate, generally terraced (Middle Pliocene-Middle
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Pleistocene); alluvial and coastal gravel, sand, silt and clay (Late Pleistocene-
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Holocene).
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At the base of the mountain ridge, a NE-SW trending fault system is to be found. Such
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tectonic structures belong to the Calabrian-Sicilian Rift Zone, an active regional fault
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system that permitted the uplift of the Costa Viola with respect to the Tyrrhenian basin
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since Middle-Pleistocene (Tortorici et al., 1995). In the area, seven orders of marine
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terraces were recognised along the Tyrrhenian coast (Tortorici et al., 2002 – cf. Figure
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2b), as a result of the combined effects of regional uplift and eustatic sea-level changes.
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Extensive flat areas characterize the study area, bordered by steep slopes and cut by
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deep canyons. Short high-gradient torrents, characterised by ephemeral discharges and
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flash floods, drain the western slope of the ridge. Along the densely urbanized coast,
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steep cliffs delimit outcrops of narrow and discontinuous deposits (alluvial/colluvial and
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marine).
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In the area, average yearly rainfall varies between 600 and 900 mm·y−1 (Terranova,
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2004; Terranova and Iaquinta, 2011). At the Scilla rain gauge, located by the coast and
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characterized by the longest available data record (from 1939 to 2015), annual amounts
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range from 507.2 mm to 1270.6 mm (782.3 mm on average), mainly concentrated
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between October and March (ca. 74% of the annual amount). The maximum daily
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rainfall (147.8 mm) was recorded at Scilla-Solano on November 1, 2015.
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The study area is scarcely urbanized. The only villages are Bagnara Calabra and Scilla:
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the first is located in the widest portion of the above-cited coastal deposits; the ancient
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settlement of Scilla lies on a small rocky outcrop facing the sea, whilst its recent
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hamlets mainly developed by the coast. The main infrastructures are mostly to be found
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in the coastal sector: here, the railway, the highway (A3 “Salerno-Reggio di Calabria”),
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and the southern Tyrrhenian state road (SS.18) extend in a narrow strip.
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In the study area, shallow landslides are mainly represented by debris slides and earth
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slides (often called “soil slips”, sensu Campbell, 1975; see also Cruden and Varnes,
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1996), that commonly show a limited run-out (ten metres in average). Subordinately,
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rock falls and debris flows are also to be found, characterized by greater run-out and
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destructive power; larger slope movements of various types are also present. As a
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whole, the average run-out of the mapped landslides is ca. 50 m.
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In the last 75 years, several slope instability events affected the study area (on average,
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ca. 0.6 events per year, cf. Figure 3), mainly related to debris slides, rock falls and,
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subordinately, debris flows, that damaged rural areas and the secondary road network.
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In the last 20 years, a remarkable increase of frequency of damaging events can be
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appreciated (up to ca. 1.3 events per year). On 12 May 2001 and 31 March 2005, in
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addition to numerous debris slides that stroke the innermost portion of the study area,
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debris flows severely hit the urbanized areas and the main transportation infrastructures,
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causing train derailment (Bonavina et al., 2005). As a rough underestimation of the
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economic loss produced by shallow landslides (except for debris flows) since 2001, the
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funds for remedial works allocated by either national or regional authorities amount to
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ca. 9.5 Million € (Basin Authority of Calabria – unpublished data).
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Despite evidence of hydraulic erosion and shallow instabilities on the slopes, the return
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periods (T) of the rainfall recorded at the Bagnara Calabra and Scilla rain gauges in the
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last two decades are rarely notable, presumably due to a different (“unlucky”) location
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of the rain gauges with respect to the rain cell centres. For instance, those related to the
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events occurred in 2001 and 2005 are not worthy of note (T < 2y for daily maxima);
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low values (T~3y) also characterize rainfall recorded from 2008 to 2010, with T Tr are listed for
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Scilla and Bagnara Calabra: a marked spatial variability of rainfall can be appreciated
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on some cases; values of T range from 10 to over 100 y. More in detail, the historical
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maximum occurred in 1988 (i.e., 2-3 years before the flight used for validation), whilst
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maximum daily rainfall recorded in 1948 e 1951 have T = 20.4 and 24.4 y, respectively.
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Annual maxima of daily rainfall from 1947 to 1955, and from 1974 to 1988, may be
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considered responsible for the slope movements recognized in the 1955 and the 1990
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flights (i.e., those employed for calibration and validation), respectively. As a result, the
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susceptibility map obtained by taking into account the calibration set of shallow
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landslides can be related to triggering rains with about 20 y of return period, therefore
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depicting a hazard scenario (H) with T = 20 y (cf. H in Figure 5).
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The hazard was derived from the original (unclassified) susceptibility map. In
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theoretical terms, the probability of occurrence, P, of an event (e.g. a rainfall able to
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trigger a given type of landslide) can be obtained from its return period, T, as: P 1 / T .
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Hazard related to an event of given intensity can also be computed from the classical
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formula H 1 1 1
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(cf. e.g. Gostelow, 1992). Nevertheless, information needed for such type of evaluations
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is commonly inadequate in landslide analyses at regional scale. Hence, either
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approximated or empirical approaches must be adopted. In the present study, hazard
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was merely obtained from susceptibility values, by considering the ca. 20 y return
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period of the triggering rainfalls responsible for the landslide population used for
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calibration: susceptibility values were, then, adopted as a proxy for hazard with respect
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to a scenario of 20 y, being assumed as representative for the considered temporal
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window.
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Economic values (W) of the elements at risk were assumed (Table 5, second column)
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depending on land use. Values were assigned based on market surveys (mainly for
P
N
, in which N is the extent of the considered temporal window
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buildings), construction costs for transport infrastructures, and asset values compared
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with results of market surveys for agricultural and urban uses. Note that the adopted
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economic values span in a range of four orders of magnitude.
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By taking into account previous events occurred in the same environment (Bonavina et
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al., 2005; Iovine et al., 2014), the potential propagation of the soil slips in form of debris
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flows (run out) could be neglected. In addition, based on intensities of expected
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phenomena and on effects of previous shallow-landslide events, occurred in Southern
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Italy in the past decades (Sarno, May 1998; Soverato, September, 2000; Messina,
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October 2010 – cf. Antronico et al., 2002; Sorriso-Valvo et al., 2004; Iovine et al.,
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2009; Ardizzone et al., 2012), physical vulnerabilities (V) were assumed for the types of
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elements at risk recognized in the study area (e.g., buildings, roads, agricultural crops –
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cf. Table 5, third column). Vulnerability values, determined by expert knowledge, range
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from 0.1 to 0.9.
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Accordingly, the potential worth of loss ( WL V W ) could be computed (Table 5, last
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column; cf. WL in Figure 5). By considering the variability in space and time of land use
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in the study area, values of WL were ranked into 4 classes: WL< 1; 1≤WL
