Historical evolution of slope instability in the Calore River Basin, Southern Italy

Geomorphology 282 (2017) 74–84

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Historical evolution of slope instability in the Calore River Basin, Southern Italy Nazzareno Diodato a,b, Marcella Soriano b, Gianni Bellocchi a,c,⁎, Francesco Fiorillo b, Andrea Cevasco d, Paola Revellino b, Francesco Maria Guadagno b a

Met European Research Observatory, 82100 Benevento, Italy Department of Science and Technology (DST), University of Sannio, 82100 Benevento, Italy UREP, INRA, 63100 Clermont-Ferrand, France d Department of Earth, Environmental and Life Sciences (DISTAV), University of Genoa, 16132 Genoa, Italy b c

a r t i c l e

i n f o

Article history: Received 25 February 2016 Received in revised form 6 January 2017 Accepted 8 January 2017 Available online 12 January 2017 Keywords: Climate variation Environmental threats Land vulnerability Natural hazards

a b s t r a c t There is interest in knowing historical spatio-temporal patterns of landslide activity. However, this is challenging to reconstruct because it is difficult to obtain detailed records for past landslide activity. Here, we deal with hydro-geomorphological signatures, such as storms, downpours, floods, snowmelt and mass movement, to detect annual slope instability events (ASIEs) over historical times. In order to obtain ASIEs for each year, a monthly Instability Density Index (IDI) was used and then monthly values were summed up to obtain a yearly value. Classes of monthly IDI varying between 0 (no instability) and 4 (highest instability) were determined from historical documents. We present an application for the Calore River Basin, Southern Italy, using data from a 313-year long series (1701–2013 CE). After 1880 CE the information becomes more valuable with directly observed landslide frequency. By this cataloguing, 129 ASIEs were obtained. Their evolution shows slight instability during the 18th century. Uneven and greater slope instability occurred instead across the 19th century, when an important phase of deforestation coincided with intensification of agricultural activities. Slope instability events continued during the 20th century but their causes are mainly related to anthropisation and the effects of recent climate change. It was determined that stormy autumns until the 19th century and successive winter-times with enhanced snowmelt, may have driven the reactivation of widespread instability events. We also found that mountainous and hilly terrains have an acute sensitivity to climate change. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Earth-surface geomorphological systems tend to be in a state of dynamic equilibrium with external driving forces (Kosmas et al., 1999). Moderate changes in climate have consequences for geomorphological processes such as storms and floods, which can, in turn, produce slope instability including accelerated soil erosion and shallow landslides. However, responses of geomorphological systems to climate and environmental changes are complex and not yet completely understood (Knight and Harrison, 2012). Long-term studies may be very useful to understand the likely trajectory of climate–vegetation–landscape interaction in river basins (Mulligan et al., 2004). Indeed, chains of linkages between climate, land-cover and land-use produce complex geomorphic responses (Viles and Goudie, 2003). Evidence suggests that vegetation reacts to severe weather events depending on many environmental factors, as well as the functional diversity of plant communities (Jentsch ⁎ Corresponding author at: Met European Research Observatory, 82100 Benevento, Italy. E-mail address: [email protected] (G. Bellocchi).

http://dx.doi.org/10.1016/j.geomorph.2017.01.010 0169-555X/© 2017 Elsevier B.V. All rights reserved.

and Beierkuhnlein, 2008). Therefore, the geomorphological systems are dynamic since one or more of their components can be sensitive to changes in given circumstances and changes in one component can (and often do) trigger instability elsewhere in the system (Thomas, 2001). In this way, patterns of slope instability may manifest themselves in history by means of specific characteristics referred to as hydrological signatures such as storms, downpours, floods and rain or snow events (Jothityangkoon et al., 2001). However, as noted by Gutiérrez et al. (2010), records become limited as we go back in the time, and the damage caused by landslides associated with storms and floods are frequently ascribed to these other hazards. While historical landslide records are sparse, several attempts have been made to relate geomorphological responses to well-defined climatic periods (Grove, 2001; Soldati et al., 2004; Astrade et al., 2011) or specific rainfall events (Irigaray et al., 2000; Fiorillo et al., 2001; Guadagno et al., 2005; Fiorillo and Guadagno, 2007; Cevasco et al., 2014; Persichillo et al., 2016). In more recent times, climate variability has also been associated with the incidence of precipitation extremes, which are responsible for landslides and damaging hydro-geomorphological events (Larsen et al., 2010; Bollati et al., 2012; Stoffel and

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Huggel, 2012; García-Ruiz et al., 2013; Cevasco et al., 2015). Among these, mass movements are common in steep mountain regions where they often constitute a major risk for human activities (Revellino et al., 2013). In less steep areas landslides are also feared to cause fatalities, injuries, destruction of houses, infrastructure and properties, as well as reducing productive lands. The spreading of urban settlements and transportation networks into landslide-prone areas has been increasing the potential impact of slope instability (Brandolini et al., 2012; Di Martire et al., 2012). This is due to the evolution of the connectivity between physical and social systems, where greater potential is held for interactions among people, communities and ecosystems in the path of future risk events (Hufschmidt et al., 2005). Recent observations show a worldwide increase in both the frequency and intensity of heavy rainfalls, coinciding with widespread flooding and landslides in Europe (Diodato and Bellocchi, 2010; Saez et al., 2013). As climate-related events become more intense, with less recovery time between them, the damages incurred also increase (Stern et al., 2013). The most important extreme hydrological events to occur in Italy during the last millennium were critically analysed (Finzi, 1986; Camuffo and Enzi, 1992, 1995). Records preserved in archives and libraries constitute a valuable source for investigating past climate and hydrological changes (Enzi and Camuffo, 1996; Delmonaco et al., 2000; Polemio and Petrucci, 2012). More specifically, an overall evaluation of the use of European historical archives for landslide studies is presented in Brunsden and Ibsen (1994). A detailed national study of historical landslides and flood frequency was undertaken in Italy by the AVI (Vulnerable Areas in Italy) project. AVI is a comprehensive inventory of landslide and flood events along with their consequences, prepared and later used to define the geo-hydrological risk for the entire country (Guzzetti et al., 1994). The inventory of Guzzetti et al. (2005) was used to compile a database of floods and landslides that occurred in Italy between 1279 CE and 2002 CE and caused deaths, missing persons, injuries and homelessness. N 50,000 people either died, went missing or were injured in 2580 flood and landslide events. We also confirm that the societal landslide risk is larger in Trentino-Alto Adige (in the North) and Campania (in the South) than other Italian regions (Salvati et al., 2010). There are several limitations regarding the calculation of landslide frequencies at different temporal and land spatial scales, including: the subjectivity of historical reports due to different levels of experience, training and conscientiousness of the observer and objective constraints such as the limited time span of historical sources. Carrara et al. (2003) underline that the historical approach tends to underestimate landslide

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hazard largely when human structures are lacking. However, although the quality of historical evidence can be strongly dependent on recording procedures and available records, the approach based on historical research provides an indication of at least the minimum level of landslide activity in an area (Crozier and Glade, 2005). In many cases, historical information can also provide disaster risk managers with a sense of the potential consequences of a particular forecast and the types of actions that can be taken in response (Hellmuth et al., 2011). Once the available historical data are well organised, they can be used to promote common sensitivity with respect to hydro-geomorphological damages and as an instrument of knowledge (Cipolla et al., 2000). Historical research is also desirable to provide a new perspective on the study of landscape conservation and climate change, especially in agricultural river systems that are highly dynamic and controlled by complex climatic, geomorphic and ecological processes (Krishnaswamy et al., 2000). Investigating hydro-geomorphological events is particularly important in the regions of Mediterranean Europe as torrential and erosive rainfalls frequently occur in these areas where floods are associated with high landslide hazards (Fig. 1). Damages caused by floods and landslides in the 19th century were retrieved from some perceptive historians. For instance, in 1870, the plant scientist Giuseppe Antonio Pasquale wrote: “the distribution of rain was so inconstant in all months of the year and so unequal in its fall, that damage outweighed the benefits” (Palmieri, 2002). Other than the negative impact on the vegetative cycle (and therefore on the economy of southern Italy), storms also affected landscape stability. Within Mediterranean Europe, the mountain features, the absence of large plains and a highly irregular rainfall pattern are the main factors that make the Italian Apennines a highly unstable area from a geomorphological point of view. This paper focuses on the Calore River Basin (CRB), which is a large watershed (~ 3000 km2) located inland of the Campania Region (Tyrrhenian side of the Southern Apennines). This study compiles a catalogue of annual slope instability events (ASIEs) for rainfall-triggered events, drawing upon news reports, scholarly articles and other hazard databases to provide a landslide catalogue at the river basin scale. In this way, for the first time, the history of the slope instability of the CRB is reconstructed and the relation with climate– vegetation–landscape evolution is investigated. This catalogue not only provides a basis for future planning, but also aids rediscovering the history of the landscape and thus encourages the reconstruction of new values in the social environment and so become an educational tool for future generations. Moreover, a new methodology is employed to overcome the lack of direct knowledge of some slope instability in the

Fig. 1. (a) Rainfall-induced landslide hazard of the north-western Mediterranean region. Red circles indicate possible hot spots (modified from Jaedicke et al., 2014). (b) Map showing the location of rainfall-induced shallow landslides in Italy between 2002 and 2012 (modified from CNR IRPI, 2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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area, which represents a type of knowledge that will hopefully be transferred to other territories. 2. Related research In recent decades, an increased exploration of historical documentary data has provided important insights into past landslide occurrence at regional scale. This increased interest was primarily driven by the need to understand landslide dynamics and thus to reduce the economic and societal costs associated with their occurrence (Petley, 2011). Several authors focused on landslides in various locations of the Mediterranean area causing damage to crops and economic losses (Calcaterra et al., 2003; Tropeano and Turconi, 2004; Marchi and Tecca, 2006; Montrasio et al., 2012; Lazzari et al., 2013) and historical landslide inventories at national and sub-regional scales have been compiled (Devoli et al., 2007; Elliott and Kirschbaum, 2007; Raška et al., 2015). In the Calabria Region, Polemio and Petrucci (2010) found that despite the favourable climatic trend, landslides have not decreased over the period 1915 to 2008 as recent exploitation of landslide-prone areas has increased vulnerability. Similarly, for the Daunia Apennines, Wasowski et al. (2010) showed that the frequency of landslides in 2006 was 160% higher than in 1976, even though both years were comparably wet. Despite much current concern being focused on the present-day impact of human activities on the environment, there is much to be learnt from the study of past changes (Allison and Thomas, 1993; Brunsden, 2001). This is the concept behind studies that investigate longer geomorphological time-series, using various techniques, for reconstructing landslide temporal patterns in various countries or regions. For instance, Calcaterra and Parise (2001) presented a documentary-historical study on landslides in a sample area of the Sila Massif (Calabria Region, Italy) since 1700. Similarly, Porfido et al. (2009) collected data on flood dynamics, flooded area sizes, flood duration, damage level, number of victims and induced geological effects since the 16th century for an Amalfi site (Tyrrhenian coast of Southern Italy). A very detailed landslide inventory, including 2723 landslide events detected from 1799 to 2009, was compiled for the central west sector of the Basilicata Region (southern Italy) (Lazzari, 2011; Lazzari and Gioia, 2015). More recently, Giannecchini and D'Amato Avanzi (2012) gathered flood and landslide data for a specific catalogue spanning the period 1300 to 2009, thus obtaining a lot of information about the events occurring in the Versilia

River Basin (Apuan Alps area). For sites outside the Mediterranean, we can refer to the research of Devoli et al. (2007), who used historical documents to investigate landslide occurrence between 1800 and 1990 in Nicaragua and that of Claessens et al. (2006) who used soil redistribution modelling to reconstruct the incidence of high-magnitude/low-frequency landslides over the last 1000 years in New Zealand. 3. Study area The CRB study area extends ~ 3000 km2 and is located in Northern Campania, Southern Italy (Fig. 2a). Centred around 41° 11′ North and 14° 27′ East, the CRB lies at the transition between the central and the southern Italian Apennines, with elevation ranging from 50 to 1800 m a.s.l. The appearance of these landscapes, already naturally varied in articulation of forms, colours and textures, is further modified by the vegetation, originally established on top of the hills and then expanded significantly, especially along the main roads. Land-use is limited to shrub and mixed-deciduous forest vegetation and agricultural crops. The woody cover reflects the lithological and morphological contrast in the basin. For instance, chestnut trees and woods tend to cover the carbonate massifs, as opposed to the hills with sparse vegetation (grasses and shrubs) and some Mediterranean groves (Braca, 1994). Regarding topography, 27% of the land in the CRB is under 300 m a.s.l.; 36% of the land is hills between 300 and 600 m a.s.l., 23% included land between 600 and 900 m a.s.l. and 13% formed by mountainous areas above 900 m a.s.l. (Fig. 2b). The CRB includes the two provinces of the Campania Region Benevento and Avellino. It lies mainly in the Benevento province and a small part extends over that of Avellino, including 175 municipalities. The Calore River originates near the town of Montella in the Picentini Mountain sub-range of the Appenines, above Salerno. It flows north to Benevento then turns and flows into the Volturno River after a course of 108 km. The CRB is divided into upper, central and lower basins: the upper is the dendritic-linear part, the central is meandering part and the lower is the linear part. During its run to Benevento and then up until joining the Volturno, the Calore picks up a number of tributaries, the largest of which are the Tammaro and the Sabato Rivers. The climate over the CRB is of Mediterranean type, with average annual precipitation ranging from 650 to 2000 mm (1289Mean ± 218SD mm) and rainfall erosivity between 900 and

Fig. 2. Geographic location of study area: (a) Land-use spatial pattern across the CRB (bold line) (arranged from http://hydis.eng.uci.edu/gwadi). (b) Hillshade zoom showing the main river network.

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4119 MJ mm ha−1 h−1 y−1 (1497Mean ± 389SD MJ mm ha−1 h−1 y−1) for the period 1921 to 1988 (Buondonno et al., 1989). Runoff is very irregular and generally follows the storm pattern of autumn, winter and spring months. The CRB is frequently crossed by depressions generating over the Mediterranean Sea itself (Wigley, 1992) that, reinforced by continental northeasterly airflow, produces important precipitation (Barriendos Vallve and Martin-Vide, 1998). Weather is then subject to a variety of mesoscale circulations and in turn to intense precipitation events, including disastrously rainy years. Three principal types of extreme rainfall event are important in terms of their consequences on geomorphic processes. In addition, the antecedent rainfall (water storage before heavy rains) and the way in which the fluctuation of rains evolves during the precipitation event are crucial to these processes (Fiorillo and Wilson, 2004; Wieczorek and Glade, 2005). The first type of event is a heavy local downpour reaching 50–100 mm. The second type is associated with more continuous rains characterised by longer duration (i.e., one to five days) and broad territorial impact. In this case, the cumulated precipitation may exceed 100–300 mm but the mean intensity is usually below 10 mm h−1. The third type of extreme rainfall is represented by rainy seasons of long duration (from several weeks to months). In these cases, landslides are usually triggered by occasional spells of intense rain that exceed the storage capacity (Starkel, 2012). The sequence of changes in rainfall intensity is a major factor controlling the dynamics of the hydro-geomorphological events, as well as the soil permeability and the density of the vegetation cover. 4. Data and methods 4.1. Geomorphological characteristics and slope instability From a geomorphological point of view, the CRB is part of the subApennine hills. These are generally made up of highly erodible rock formations, including clayey-marl flysch and clays, giving the landscape some peculiar characteristics, such as: low energy relief (heights of a few hundred meters and medium-low steepness); highly developed hydrography (often with dendritic pattern) and low angle summit

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surfaces. The high erodibility of the rock types and the rising gradients of slopes contribute to the erosion processes over large areas (Diodato, 2006). The flysch is part of the hilly area located immediately east of the Campano Apennine (Fig. 3a). In these areas, agriculture is the prevailing land-use and the landscape is predominantly characterised by arable land, frequently bordered with hedgerows and woody crops such as vineyards, olive groves and hazels. The rocky summits and slopes, located along the river incisions, accommodate oak forests and deciduous trees. In the CRB, landslides were found to be predominantly shallow landslides. Landslides were inventoried to about 17% of the whole surface of CRB following the IFFI project (Monti et al., 2007). As can be seen in Fig. 3b, the landslides are located along the main river networks in four major clusters represented by sub-basins (Calore, Sabato, Ufita and Tammaro, whose boundaries are not defined in the map). These mass movements are generally visco-plastic, with velocities ranging from slow to fast and are characterised by displacements along one or more surfaces. Shallow landslides frequently occur with soil creep and gully erosion, set on clayey-marly flysch or clays (Argille Varicolori Fm.) with gentle slopes (b 10%) (Guadagno et al., 2006). In most cases the displaced material is channelled in erosion ditches where the accumulations can move rapidly along the hydrographic network (Tedeschi, 1976; Guadagno et al., 2006). Where lithological and structural conditions are substantially homogeneous, flow-type phenomena, which are prevalently earthflows (Fig. 4a), earthslides (Fig. 4b), plurisources (Fig. 4c) and coalescent flows (Fig. 4d), can develop. Statistical modelling (Donnarumma et al., 2013) showed that the location of landslides in the study area is related to specific slopes angles, with the largest occurrence of landslides falling within an interval ranging from 9° to 14°. Debris slides and flows are typically shallow movement types that can occur in steep terrain (N 25/30° gradient). Initiation or reactivation of landslides generally occurs after periods of rain of two months, with antecedent precipitation of 150–600 mm and triggering rainfall events of 20–70 mm (De Paola and Diodato, 1999). As the antecedent precipitation is greater, a lower threshold value of rainfall is necessary for the reactivation of the landslide. Kinematic phenomena involve limited areas, although anthropisation and changes in agricultural techniques can significantly increase the hazard.

Fig. 3. CRB: (a) Geolithological map of the CRB and surrounding areas, arranged from ISPRA-SGI (http://sgi1.isprambiente.it/GeoMapViewer). (b) Landslide inventory map of the CRB on a 1:500,000 scale, using data from Guadagno et al. (2006) and IFFI data Project (http://www.isprambiente.gov.it/en/projects/iffi-project/default).

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Fig. 4. Different types of slope instability affecting the CRB: (a) Sant'Andrea district (commune of San Giorgio la Molara, Benevento Province) in 2000; (b) Fontana del Piesco district (commune of Montefalcione, Avellino Province) in 2003; (c) Upstream of S. Giovanni River (commune of Circello, Benevento Province) in 2005; (d) Ginestra degli Schiavoni (Benevento Province) in 2013. (Source images: (a) and (b) are from archive of Department of Science and Technology at University of Sannio, Italy (http://www.dstunisannio.it); (c) and (d) are from Google Earth platform (https://www.google.fr/intl/en/earth, accessed on January 06, 2017).)

Indeed, in the past, several areas of the CRB were affected by landslide events of various type and intensity during the course of numerous road constructions (Budetta et al., 2005; Guadagno et al., 2005; Revellino et al., 2008). Unpaved roads are one of the most common types of man-induced disturbances. Such roads induce surface runoff and can alter subsurface flow on hillslopes and this can affect the magnitude and timing of surface runoff (Wemple et al., 2001). By exposing the soil surface and increasing and concentrating runoff, surface erosion can be greatly increased on each of the different parts of the road prism (MacDonald and Coe, 2008). Slope instability events have affected the socio-economic life of communities since historical times. This is testified in scattered papers from various periods of time, approximately describing the scenario of damage, the inquiries made or other actions taken (Budetta et al., 1994; Diodato, 1999; Guadagno et al., 2006; Revellino et al., 2010).

and Wills (2012). Data for the purpose of this paper were obtained from various types of document, including archival records, local chronicles, books and papers. In particular, local chronicles and notarial deeds provided information on hydro-geomorphological instability over the period 1701–1810 CE. Original sources (ENEA–SGA, 1987; Diodato, 1999), other edited versions (Gisotti and Benedini, 2000; Diodato, 2007; Diodato et al., 2008) and related updates were used for analysing the hydro-geomorphological record over timespan 1811–1920 CE. Instead, the AVI Project (Lolli et al., 1995) together with the GIANO Project and books were used for preparing the events catalogue of the last century (Fig. 5). These documents contain records of meteorological and environmental observations. The manuscript texts usually focus on the consequences of extreme meteorological phenomena. The impacts on the

4.2. Documentary proxy data and slope instability indices compilation Data and methodological problems associated with the use of written sources in hydroclimatological reconstruction within the European countries are summarised in Brázdil (1992), Camuffo and Enzi (1994), Rodrigo et al. (2000), Pfister and Bràzdil (1999), Xoplaki et al. (2001)

Fig. 5. Historical data sources used in this study.

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environment and population depended on various factors. As a consequence, it was not always easy to compare the severity of the hydroclimatic conditions, unless they were associated with specific effects. During rainy periods, floods and storms are strongly linked with landslides reactivation (Petrucci and Pasqua, 2008; Petrucci and Polemio, 2009). Landslide activities are themselves often considered to be a secondary hazard associated with storms, which could be a result in the under-reporting of landslide impacts (Petley et al., 2005). Accordingly, Gregori et al. (2006), Foscari (2010) and Pradhan et al. (2012) showed that storms and floods could be used as a suitable proxy to describe different classes of slope instability phenomena. Various indices were proposed to grade slope instability, especially in historical climatology (Pfister and Bràzdil, 1999; Pfister, 2001) and hydroclimatology studies (Diodato et al., 2008; Polemio and Petrucci, 2012). Here, we have adopted the ASIEs, which defines a period, shorter than a hydrological year, during which one or more landslides have occurred in one or more sectors of a study area. To obtain ASIEs over any year, the Instability Density Index (IDI) was assumed and then summed for each month. The method proposed uses IDI to account for classes of monthly slope instability index, varying between 0 (no instability) and 4 (highest instability), as extracted from historical documents. This methodology reduces the level of subjectivity inherent to historical sources via the ordinal (semi-quantitative) slope IDI. IDI values were retrieved from the hydro-geomorphological signatures such as storms, downpours, floods, rain-snowmelt and mass movement. Based on the above findings and experimental observations of the trend that landslide activity follows floods in Southern Italy (Foscari, 2013 personal communication, Petrucci et al., 2010), IDI values were coded as follows: IDI = 0, no landslides (storm with lack of floods); 1, isolated landslides (storm with single flood); 2 sparse landslides (storms, downpours with scattered floods); 3 diffuse landslides (pervasive storms, downpours with diffuse floods); and 4, widespread landslides (rainfall, diffuse snowmelt and floods). These numerical values were assigned to each month from September to August of any hydrological year and then summed to obtain ASIEs. This provided a way to identify historical periods over which events could have pulsed in the CRB using data from a 313-year-long series (1701–2013 CE). These long-term

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studies are important to understand how the CRB was affected by slope instability phenomena over time. Via the cataloguing process, we obtained 129 ASIEs, mainly concentrated in the last two centuries. 5. Results and discussion In order to make historical visualisation informative and improve understanding on geohazards, this paper considers a set of historical visualisations with the goal of learning what made them useful and what lessons we can apply to time-resolved visualisation of slope instability. While it is understood that some geological hazards are related to climate, in particular to long and extreme rainfall events, the complexity of this relationship is not yet understood in detail and the interactions are not always those expected. The historical research has revealed the presence of 129 instability events, which mainly occurred in the last two centuries. Since each event can include from tens to hundreds of landslides, it was difficult to estimate how many landslides occurred in each year. 5.1. Slope instability temporal pattern We observed that the recurrence of expected landslides might be explained fundamentally by iterated occurrences of storms and floods. A discrete time series reveals fluctuations in the slope instability that widens as time progresses (Fig. 6). In order to identify possible trends in discrete data, the time series was split into stationary components with ‘fast’ and ‘slow’ variability. A low-pass 11-year Gaussian filter was designed for this purpose following Førland et al. (1997). Interpolating Gaussian filters are popular for fitting data because they provide a temporal continuity to discrete data, a property that permits them to satisfy a desirable smoothness constraint. Overall, slope instability events seem to follow a progressively increasing trend. The major peaks of instability were detected around 1810, 1840, 1900, 1960 and 2000 CE (Fig. 6). It is noted from the slope instability event distribution that the geomorphological system is consistent with an adaptive recovery when one or more unstable actions occur. As a matter of fact, the data show an erratic pattern with

Fig. 6. Temporal pattern of ASIEs (grey histogram bars) with related smoothed-fluctuation by 11-year Gaussian filter (thick black curve) and cumulative deviation (dashed yellow traits) in the CRB from 1701 to 2013. Black arrow indicates the main change point in the landslides occurrence. Evolution of the climatic periods and main land cover and use are also depicted. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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considerable intra- and inter-decadal variations, especially in the 20th century, when further extreme values were identified. The 18th century represents a period with a slight shift in hydrogeomorphological perturbation, followed by a more irregular pulsing due to a new regime in the landscape structure, and possibly associated with an important phase of deforestation which began in 1800 CE. This is identified as an important change point (arrow in Fig. 6). A similar picture was depicted in 1816 by the engineer Nicola Bellino, who reported the situation of the Siano territory (Parise et al., 2002, p. 263): “by describing the frequent flooding episodes which had caused heavy damage to the inhabitants and to their properties, due to materials transported downvalley by water, Bellino remarks the importance of clear cutting practices in favouring the slope susceptibility to landslides, especially on the steeper slopes”. However, at the beginning of the 19th century, the CRB landscape was able to absorb progressive changes over time and it only becomes unstable when an internal threshold is crossed (Fig. 7). During the 20th century, the CRB landscape has seen progressive anthropisation. A conditional instability in the system appeared, implying a rapid and quasi-irreversible change due to perturbations in the controlling environmental processes. Although the phase of deforestation occurred in the 18th century had already passed, the expansion of mechanical agriculture is temporarily active, followed by the burgeoning of infrastructure construction and urbanisation processes in more recent times. We believe that slope instability beyond historical experience will continue to occur as the climate variability and extreme weather will continue to affect the region. However, what planners must really face is the trend of climate extremes that may take a long time to emerge from the background of internal climate variability. In order to investigate slope instability regime patterns in each month, we calculated the relative frequency of events over three climatic periods (Fig. 8): 1701–1800 CE (end of the Little Ice Age, Fig. 8a); 1801–1900 (Transition Phase, Fig. 8b); 1901–2013 CE (Warmer Period, Fig. 8c). In the first period (1701–1800), instability is concentrated in the autumn period. In the second phase (1801–1900), the landslide events are spread over all seasons with the highest concentration in autumn and the beginning of winter. In the last period (1901–2013), the instability tends to assume a bimodal form with a relative peak in autumn and a second peak moving towards winter-time. However, the period with the highest relative frequency is the Transition Phase. This

is likely due to the frequent intrusion of cold air masses in the Mediterranean area from northern and northwestern Europe, which generated cyclones and associated hydrological extreme events across the central Mediterranean region (Slonosky et al., 2000). 5.2. Interpretation and validation The link between vegetation and hillslope geomorphology was the subject of a number of studies over recent years; yet it still remains difficult to separate the combination of global factors from the local environmental history affecting slope instability (Marston, 2010). Historical sources can provide an important picture for inferring, particularly if reconstructed historical slope instability is similar to the present evolution. Only synoptic landscape observations in historical times can provide an idea of what kind of relationship existed between the landscape and geomorphological instability, when considering the phenomena in a more global setting. Hence, we take advantage of the observations handed down by some local historians about the state of the landscape of the CRB since 1800 CE. The 19th century seems to be the most important regarding environmental changes occurring in the CRB. One of the first historians to refer to the landscape condition at the beginning of the 19th century was Nicola Nisco that in his manuscript “The presents civil status of Benevento people”, dealing with the lowest part of CRB, wrote: “not only the manufacturing industry, but agriculture itself is abandoned in Benevento, so that those lands that could give two harvesting every year, they produce just one. This ailment is caused by the ploughing up of many lands and the destruction of many woody scrubs wearing the hilltops” (Zazo, 1986, p. 7). Also, the historian Marcarelli (1915) complained over massive deforestation and destruction of fallows with floods and accelerated erosion on the side of Taburno Mountain, for around half of 1800. The deforestation, ploughing and the cultivation of sloping land were, in this sense, the most obvious phenomenon of a transformation in the relationship between man and nature that will be fully complete in the following century. Only between the end of the 19th and the beginning of the 20th century is a significant increase of cropland areas manifested (Fig. 9). At that time, croplands had largely replaced the original woodland. As testified by an investigation of Ettore Manfren (Contoli and Palladino, 1971), the deforestation also occurred from 1870 to 1910 when over 7850 ha of woodland in the upper part of the CRB and 2821 ha in the

Fig. 7. Historical picture of the middle CRB dating back to the beginning of the 19th century, with the view of the Calore River and, in the background, the Benevento hills with the Camposauro Mountain, where the riparian vegetation covering the hills around the Calore stream is visible (anonymous).

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Fig. 8. Relative frequency (%) of monthly slope instability patterns in CRB over different periods: (a) 1701–1800 (End of the Little Ice Age); (b) 1801–1900 (Transition Phase); (c) 1901– 2013 (Warmer Period).

lower part were deforested. These lands were made more vulnerable as hydroclimatological extremes were revealed with geomorphological instability. The increase in landslide frequency reported for the Basilicata Region (southeastern part of the CRB) over the 20th century was interpreted as a result of land-use change (Oliver, 1993). These correlations suggest that the main driving force behind the progressive and, sometimes rapid, increase in landslide activity is the alteration of the landscape resulting in modification of the geomorphological system and detrimental effects on slope stability. Intensive crop cultivation began in the Mediterranean after 1800 (Pongratz et al., 2008). Cropland areas increased little before the 18th century. Much land was covered by forests, which spread from the mountains to the valleys below (Rovito, 2001). This could explain the limited spatial extension of slope instability in the CRB before 1700. Recent results indicate that in catchments undergoing a permanent change in forest cover it takes between 8 and 25 years for a basin to reach a new equilibrium (Brown et al., 2013). Further, local vegetation-environment feedbacks, which are common in different regions of the world, may also trigger a cascade of amplifying effects, propagating from local- to large-scale and possibly leading to critical transitions in the large-scale climate action (Rietkerk et al., 2011). As mentioned above, the rate of deforestation peaked in the period 1800–1870 and declined successively until disappearing in the 20th century. Thus, the peaks of slope instability detected in the 20th century seem rather to be related to the abandonment of conservation agricultural practices and urbanisation. The spread of tobacco growing, which occurred mostly after 1870, became, by its profitability, the main crop. This undermined the relationship between agriculture and the environment and lead to a radical simplification of the landscape. This left the agricultural landscape generally vulnerable to climate change (Diodato, 1996). Other major factors driving mass movements and floods in recent times in the CRB are summarised as follows: (i) the exposure of bare

ground by indiscriminate agricultural activities; (ii) reduction of water infiltration by the expansion of uncontrolled urbanisation and rapid construction of buildings and roads; and (iii) the channelling of rivers in combination with urbanisation. These factors were mainly related to the increase in artificial roads across the CRB and urbanisation, especially since 1970 in the Benevento district. Furthermore, increased atmospheric temperatures during the highpressure conditions characterising the Mediterranean climate in the dry summer time, coupled with an increase in the intensity of storms in the following wet season over recent years, increases the risk of summer fires as well as debris flows and floods in the autumn and/or in the following years (Bisson et al., 2005). Moreover, recent observations at several sites in southern Italy indicate that an increase in extreme rainfall events (Diodato and Bellocchi, 2010; Diodato et al., 2011a) is likely a major cause of the accelerated erosion affecting slopes within the CRB (Diodato et al., 2011b).

6. Concluding remarks Slope instability events were recognised as a problem in some parts of Italy in the 16th century (Buratto, 1976). To better understand the sensitivity of the system to external forcing, such as rapid climate changes, it is important to know how far present-day slope instability rates are typical of the long-term geological norm, to what degree they are climatically controlled, how they might be anthropogenically accelerated and if so, over which time period they have changed. Modern landslide timing is dominated by human influences. An improved understanding of the intrinsic complexity of mass-movement processes at any elevation, process flows and their relationships with (and dependency on) climate variability is crucial in planning appropriate and prospective measures to reduce the negative impacts of future events (Stoffel and Huggel, 2012). These cycles have changed over time,

Fig. 9. Evolution maps of the land-use pattern in southern Italy for 1700, 1800 and 1900 CE. The boundary of the CRB is also indicated. N.U. is unclassified (arranged from Ellis et al., 2010).

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especially on local and regional scales, where urbanisation and land-use changes assumed an important role. In reconstructing slope instability events, as well as the major climate and land-use changes that occurred in the CRB since the beginning of the 18th century, the present paper provides a baseline to assess the past and future changes. Over the CRB, about 60% of land was wooded in the 17th century while settlements and organised fields dominated the rest. The first land clearance took place after 1700 CE and progressed to forest removal during the expansion of cropland in the 19th century. Slope instability peaks detected in the 19th century seem to be predominantly related to deforestation activities, whereas those detected in the 20th century are due to both land-use and climate changes e.g., urbanisation, abandonment of the conservational agriculture practices, and increase of extreme rainfall events. Hilly and higher elevation sites clearly show that the sensitivity of these systems to climate change is likely to be important and that events beyond historical experience will continue to occur as climate change continues. However, major climate-related slope instability might result from both extreme and non-extreme events that overwhelm a social system, revealing the dangers of policy inaction (Stern et al., 2013). It is certainly true that economic development led to an increase in technical skills and higher potential funding for the government of the territory. It is also true that traditional forms of land management, which in the past guaranteed direct relationships and immediate connection between people and environment (Palmieri, 2002), have currently disappeared. The abandonment of farming practices and the uncontrolled expansion of urban development, especially from the 1960s to the 1980s, has increased the geo-hydrological hazard over many areas of the CRB. All this was made possible mainly by the lack of information on territorial hazards in urban planning and development, which is also a consequence of the lack of communication between the scientific community and public administrations. Although the results obtained in this study can be considered as quite useful, it must be taken into account that the methodology adopted has some limitations due to the low frequency of newspaper items relative to direct observations of landslides in historical periods. The reconstructions could be improved and updated as data searches and recording continues. Generalisation is also required to transfer proxy information into a form useful for application in process models (in turn applicable for future projection of landslide occurrence). Yet, other questions, uncertainties and methodological problems remain open, on which future investigations should focus: i) Until now, no case studies exist on slope instability during the Little Ice Age and successive times across the CRB. Future research should be conducted in closer cooperation with soil scientists, climatologists, ecologists, archaeologists, historians, anthropologists and sociologists. This would provide massive synergy efforts to investigate the impact of past land-use and landslide dynamics to understand socio-economical mechanisms. Many studies emphasise the importance of having multidisciplinary teams in which ecological and social scientists work together to improve ecosystem conservation and protection of the societies depending upon them (Downes et al., 2013). ii) The exact location of slope instability is hard to know and mostly based on proxy data. The location of distinct landslide events during rainy seasons or years is very difficult. The dynamics of landslides in small basins is very specific depending upon the internal configuration, such as land-cover, land-use and storm timing. Historical studies may only indicate land-windows where these events could have occurred. Therefore, modelling is an important tool to understand where geomorphic processes change took place over long time scales. iii) The current knowledge of past landslide activity prompts questions about the vulnerability of Mediterranean landscapes in a changing climate and environment. However, it is also important

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