Landslides are one of the most dramatic natural hazards along with earthquakes and floods. For this reason, hazard and risk assessment has been the main aim of a large number of scientific papers (Corominas et al., 2014 and reference therein), focusing on geomorphological (Baek and Kim, 2015; Cardinali et al., 2002; Devoto et al., 2014) and multi-disciplinary or statistical approaches (Sterlacchini et al., 2007; Dai et al., 2002). The level of risk is generally defined as the intersection of hazard with the value of the elements at risk by way of their vulnerability (Crozier and Glade, 2006; Alexander, 2002). This assumption is generally based on a great number of variables; vulnerability of element at risk is closely related to the type of landslide, and frequency-based hazard assessment often relies on a few decades of knowledge of slope instabilities. Fortunately, previous years' measurements have been thoroughly collected thanks to GIS databases, web information sharing and a greater awareness of landslide risk. This attitude allowed some authors to calculate the costs of damages due to slope instabilities within many environments around the world: from 1972 to 2007, landslides and rockfalls cost EUR 520 million and caused 32 fatalities in Switzerland (Hilker et al., 2009), while in the United States a USD 1–2 billion expense in economic losses and about 25–50 deaths per year have been estimated (Schuster and Fleming, 1986), e.g., USD 9 million expense in only direct cost losses in Colorado during 2010 (Highland, 2013). Historical research indicates that more than 50 593 people died, went missing or were injured in 2580 landslides and floods in Italy, where 26.3 % of the 8102 municipalities have been hit by slope instabilities between 1279 and 2002 (Guzzetti et al., 2005b); economic loss related to the single destructive landslide at Ancona (Marche region) in 1982 was estimated at USD 700 million (Alexander, 1989). On the global scale, 2620 landslides were recorded during the 7-year period of 2004–2010, causing a total of 32 322 fatalities (Petley, 2012). Besides these historical data, the need for landslide damage prediction is very strong if we want to implement preventive measures against slope instabilities, even at a large scale. Within a small test site of about 20 km2 north of Lisbon (Portugal), cumulative risk expressed in direct costs for buildings and roads was calculated to be about EUR 5 million (Zêzere et al., 2008). In southern India, the triggering of many landslides hanging over 20 km long roads could cost from USD 90 840 to 779 500, with an average annual total loss estimated at USD 35 000 (Jaiswal et al., 2010). These expenses highlight how much people need protective measures against landslides and floods, which cause USD billions every year in damages and economic losses. This need can be summarized in the term “risk management”, referring to the full range of procedures and tasks that ultimately lead to the implementation of rational policies and appropriate measures for risk reduction (Crozier and Glade, 2006). One important task in risk management is the evaluation of benefits from preventive actions which can encourage authorities and population to invest money for preventing damage due to slope failures. To this end, the estimation of the most landslide-prone areas and of the effectiveness of possible preventive measures is needed.

Scientific literature offers a variety of different methods to assess risk and economic losses due to landslide events; both of these features represent the central topic when decision makers are called to act toward prevention, and thus an in-depth analysis is needed in order to obtain the best result with the least effort. The first problem to solve is the scale of analysis; many authors prefer a regional-scale approach, which necessarily leads to a qualitative or semi-quantitative analysis due to the impossibility of obtaining or considering data from every landslide (Von Ruette et al., 2011). In this kind of approach, the probability that a landslide could cause damage is accounted for through hazard or susceptibility maps, which are the start point of the analysis. These maps are next overlaid with information concerning elements at risk and their economic value, defined by maps of probability of direct monetary loss per year (Blahut et al., 2014) or by quantification of economic losses at municipal level (Pellicani et al., 2014). However, other authors focus on a slope-scale approach, in which information on landslide events and local features need to be wisely considered. Despite the fact that every event has to be evaluated one by one, this analysis usually allows quantitative assessment of landslide costs and losses; in this way, different alternatives can be thoughtfully weighted (Crosta et al., 2005) and local toolboxes for vulnerability assessment can be created (Papathoma-Köhle et al., 2015). The slope-scale approach can also help in the realization of a cost/benefit analysis, a topic rarely investigated in the scientific literature despite being of paramount importance, which consists of an economical comparison between the cost of prevention and the cost of the rebuilding what have been lost (Boonyanuphap, 2013; Crosta et al., 2005; Frattini and Crosta, 2006). Thus, given a specified landslide event which caused various damages, costs of rebuilding are well known, while costs related to a potential prevention plan depend on what type of preventive work is chosen and on what business company is selected. Every one of these last features has to be generally evaluated on a case-by-case basis.

In this work, we have considered the effects of an exceptional rainfall event that hit the Italian pre-Alps of the Vicenza province (NE Italy) in 2010 (Floris et al., 2012, 2013) to perform a cost/benefit analysis of landslide prevention vs. post-event actions. To achieve this goal, slope-scale (detailed) and large-scale (1 : 10 000) analyses (Lee and Min, 2001) were carried out. On the slope scale, numerical simulations and cost/benefit estimation were performed on a landslide reactivated by the 2010 rainfall event to define if preventive measures could avoid the reactivation of the instability and if they could effectively carry an economic benefit, as a result of an effective risk management methodology. On the large scale, a landslide susceptibility analysis was carried out in one of the most affected sectors (Marosticano Hills; Fig. 1) after the 2010 event to verify if instability phenomena could be foreseen and to estimate possible benefits from regional-scale prevention measures before the rainfall event.

In recent years, Italy has been hit by several exceptional rainfall events, causing damages to public and private buildings, infrastructures and activities. One of these events hit the province of Vicenza (Veneto Region) in 2010. This event lasted from 31 October to 2 November; the average of the rainfall measured by the 11 rain gauges located in the affected area was 336 mm and the maximum cumulative rainfall registered by one of the rain stations was 500 mm (Floris et al., 2012, 2013). In the following days, a great flood hit plain territories and 500 warnings of landslides, distributed over 20 municipalities, were received at the Soil Protection Division. Many of these slope failures affected the Marosticano area, a 110 km2 territory located in the northeastern sector of the province (Fig. 1). Here, landslides were classified as rotational/translational slides and earth flows (following the classification proposed by Varnes, 1978). These failures involved mostly silty-clay soils, i.e., the weathering products of Late Paleocene–Early Miocene extrusive magmatic rocks. Weathering of basic bedrock led to the typical geological and geomorphological environment within Vicenza's pre-Alps hilly belt, where basalt and tuffaceous rock outcrops are sporadic because a variable thickness of eluvial and colluvial deposits is present. The November 2010 event highlighted the partial lack of preventive and maintenance works, a soil defense attitude which has still to be acquired by authorities and population at the present time but is more needed today than in the past, because the frequency of exceptional rainfall events in Italy has increased in the last decades (Floris et al., 2013), with one damaging event about every 20 years (Floris and Bozzano, 2008); thus, the November 2010 event represents only one element of this developing trend. As a result, without any kind of soil protection, Vicenza's administration had to face EUR 300 million of remediation works and about EUR 1 billion of infrastructure and building losses. In this paper, we tried to estimate possible benefits from preventive actions before 2010 in the Marosticano area, where the costs for post-landslide remediation works amounted to about EUR 20 million.

On the slope scale, we chose to focus our attention on a roto-translational slide occurred in the Carrè municipality (Fig. 2); this landslide is located on an unstable slope which was affected by past and recent instabilities – including during the 2010 event – forcing authorities to demolish an old house and rebuild the main road, with a total direct cost of EUR 60 000. The landslide body lies above a basaltic bedrock and involves a few-meters-thick eluvium–colluvium layer. The landslide area covers about 5500 m2 (100 m length × 65 m width), with a 1 m high main scarp. Field data resulted in a supposed shear surface located within the first shallower meters (inside layer B shown in Fig. 3), where a decrease of strength occurred in silty-clay soils due to pore pressure rising. On the large scale, the entire territory of Marosticano hills was investigated.

In this paper, we dealt with detailed and large-scale analyses aimed to quantify possible benefits from landslide prevention: to this end we have considered the 2010 exceptional rainfall event that hit the pre-Alps sectors of the Vicenza province (NE Italy), triggering hundreds of instabilities.

On a detailed scale, we have considered the reactivation of the Carrè landslide after the 2010 event: it moved frequently in the past, destroying an old house and the provincial road. Numerical modeling demonstrated that a drainage trench, which was included in the project of post-event remediation works, could have been a good preventive measure to improve slope stability if applied before the landslide itself. Prevention costs were compared to those relative to remedial works, usually applied after the landslide occurrence. It was possible to define a saving of 30 % on the total amount (about EUR 60 000), surely a great economic improvement for local administrations (confirmed by cost/benefit analyses performed within the environment of Molvena and not included in this paper). It is important to note that in the analysis we did not include the tens of thousands of Euros spent on incalculable indirect costs, such as emergency actions and social costs due to inaccessibility of the road. Thus, if the municipalities of the Vicenza province had acted before the 2010 event, an important amount of money would have been saved and possibly reutilized for other purposes.

On a large scale, we performed a susceptibility analysis in the Marosticano hills, which were one of the most affected areas after the 2010 event; this analysis considered landslides occurring before 2010 along with the natural variability of geological, geomorphological and geotechnical features of soils involved in slope failures. This method allowed us to understand which factors are related to landslides occurrence and to point out the most susceptible sectors of the study area. The key index of this approach, called the “frequency ratio index”, provided classes of values within each factor which are more inclined to cause landslide events. The definition of slide and flow susceptibility maps, along with the obtained indexes, allowed us to give a solid basis to the observations related to the 2010 rainfall event: spatial analysis defined that areas near the roads and placed over basaltic and tuffaceous weathered bedrock were generally the territories more frequently hit by landslides, as effectively occurred during the November 2010 event. Comparing the results from susceptibility analysis with the spatial distribution of landslides triggered by 2010 rainfall event, we found that 50 % of the instabilities (including the Carrè and Molvena landslides) fall in areas classified as highly and very highly susceptible; hence such instabilities could be foreseen. Therefore, a part of the EUR 20 million expended for post-event actions in the Marosticano hills could be preserved by prevention measures. We could not get enough data on the cost of post-event actions for the foreseeable landslides; because magnitude of instabilities and damages were very similar all over the area (Floris et al., 2012) we can reasonably suppose that such costs can reach the 50 % of the total (about EUR 10 million). Taking into account results from slope-scale analysis, where we estimated a saving potential of 30 % from pre-event actions, up to EUR 3 million could be saved by regional landslide-hazard assessment.

Even if large-scale analysis helped us to roughly estimate possible benefits from landslide prevention in the study area, such approach cannot define where to act with preventive works; indeed, results of spatial analysis showed that the majority of the study area would need to be defended – regardless of money and time – in order to take care for all the most susceptible environments. Therefore, preventive works must be planned on a detailed scale with regards to those specific slopes which show instabilities occurred in the past. This is an important factor because landslides are frequently consequent upon partial or complete reactivation of existing landslide bodies, often triggered by rainfall (Floris and Bozzano, 2008, and references therein). Despite the limits of large-scale analyses, they are very low-cost investigations which can be easily performed by technicians of local and regional authorities and can be used as preliminary study to identify the most hazardous areas where to perform detailed surveys, supporting decision making and land use planning (Akgun, 2012).

In this work we performed quite simple analyses, which considered landslides occurred in the past (before and during the 2010 rainfall event), to point out possible benefits from landslide prevention on different scales. In order to perform forecasting analyses, a complete risk assessment has to be carried out to evaluate cost/benefit ratio of pre-event actions; more refined models recently proposed by some authors (Bordoni et al., 2015; Gioia et al., 2015) would be recommended to improve both spatial and temporal prediction of landslides and induced damages on slope and large scales. However, the adoption of more refined tools could affect the costs of landslide prevention assessment; therefore, a cost/benefit analysis is always a primary crucial step before choosing between the numerous investigation methods proposed in the scientific literature (Dai et al., 2002, and references therein; Guzzetti et al., 2005a), which often lead to very similar results (Othman et al., 2015) depending mainly on site characteristics and available data (Lagomarsino et al., 2015).

The case study we dealt with in this paper can effectively contribute to improve our awareness and knowledge on prevention benefits. It is real evidence which proves that avoiding landslide occurrence represents a sustainable policy to deal with the social side of risk mitigation. This methodology can also provide an economic point of view for the global landslide issue, giving authorities the appropriate tool to face this ever-growing problem. To this end, new-generation early warning systems should be developed for monitoring and preventing instabilities on local and regional scales (Manconi et al., 2015; Segoni et al., 2015); thanks to the new availability of free data from spaceborne sensors and of WebGIS low-cost solutions, such systems represent reliable and cost-efficient tools to reduce landslide risk (Stähli et al., 2015). Afterwards, prevention is effectively possible from the economic point of view to the architectural one and could represent an efficient way to defend every defenseless territory.