Avalanches, Landslides and Rock fall Italy
Vulnerabilities Italy - Slope destabilization in the past
Worldwide, climate change is affecting the stability of mountain slopes (31). Shrinking glaciers, thawing permafrost, and decreasing spring snowpack worsen the mechanical conditions of rocks and soils (32). In the European Alps, the number of slope instability events is expected to increase. The response of slope stability to climate change is complex, however (33). Slope stability is especially affected by precipitation and temperature (30).
A climate signal in recent events
358 slope failure events have been studied that have occurred at heights above 1500 m in the Italian Alps between 2000 and 2016. The study was aimed at disentangling the relations between climate change and slope instability in high-mountain areas. The events cover the entire Italian Alpine region, which is 27.3% of the European Alps, and all types of landslides, debris and mudflows, glacial lake outburst floods, and ice avalanches. The slope failure events were related to the climate history of each event of slope instability by using data from 131 automatic weather stations in Northern Italy (30).
47% of the events were associated with temperature anomalies, 38% with a combination of temperature and precipitation anomalies, and 7% solely with a precipitation anomaly. Whether temperature or precipitation is the main trigger depends on the type of event. 47% of the debris and mudflows could be related to precipitation anomalies compared to 27% of the landslides. Large slope events appear to be much more sensitive to temperature anomalies (33% of the events) than to precipitation (4%). Small-volume events are more closely related to both precipitation and temperature anomalies than large events (30).
Whether temperature or precipitation is the main trigger also depends on season and elevation. The role of precipitation decreases (and that of temperature increases) from spring (41%) to winter (0%), and from the low (42%) to the high (5%) elevations. During summer, precipitation is as important as temperature. Summer events sharply prevail, whereas spring and autumn ones are almost balanced; winter events are the clear minority. The concentration of case studies in summer is partly due to a higher frequentation of high-mountain areas in this season (30).
Main impact of high temperatures
Overall, a high occurrence of positive temperature anomalies in the lead-up of a slope failure stands out, associated or not with heavy precipitation. This supports the hypothesis of a role of climate warming in destabilizing slopes at high-elevation sites in recent years. Long-term temperature anomalies may be responsible for permafrost thawing in depth (34). Short-term temperature anomalies affect near-surface dynamics: in permafrost environments, temperature variations and short-term extremely warm conditions could affect rock stability within hours through rapid thawing processes (35).
Looking at past climate trends and future projections, the authors of this study expect that, for the Italian Alps, slope instability driven by positive temperature anomalies, and the effect of intense short-term precipitation (36), will become more and more important. Processes related to long-term precipitations will lose relevance.
Vulnerabilities Italy - Landslides in the past
In Italy, landslides are a serious threat to the population. In Italy from 1950 to 2015, 661 fatal landslides occurred causing 4105 fatalities, including 1910 estimated fatalities caused by the Vajont landslide (9 October 1963), the most destructive slope failure in Europe in historic time. In this period, the frequency of low magnitude events (with one or two fatalities) has remained unchanged, and the magnitude of the most destructive events has decreased. The reduced magnitude of the most destructive events is related largely to the increased availability of information, and to improved monitoring and warning systems (25). Different numbers are mentioned by (16): according to (16) 1279 persons were killed and 1731 were injured by landslides in the period 1954-2013 (16).
Rainfall is the primary trigger of landslides in Italy (17); other causes are earthquakes and human activities. The impact of rainfall on the occurrence of landslides was studied for Calabria, southern Italy, for a dataset of landslides and daily rainfall records in the period 1921-2010. The dataset consists of 1466 rainfall events with landslides: 49.7% in winter, 42.4% in autumn, 5.7% in spring and 2.2 % in summer. The analysis shows that landslides were triggered at lower rainfall volumes in the period 1981–2010 than in previous periods, indicating that the vulnerability of the territory to landslides has increased (18). On the other hand, the frequency of major catastrophic and catastrophic landslides and (flash) floods has decreased since 1971, be it that Calabria has suffered from damaging effects in recent decades even though rain did not reached extreme characteristics (21).
The complexity of the changes in the frequency and impact of rainfall-induced landslides observed in Calabria suggests that it remains difficult and uncertain to predict the possible variations in the frequency and impact of landslide in response to future climatic and environmental changes (18).
The 13 largest landslides that occurred between 1991 and 2003 caused 2,584 deaths (1); this number does not agree, however, with the data above by (16). According to the EM-DAT database, the largest landslides cost the country 1.2 billion dollars. A first attempt at estimating the direct costs of increased risks of floods and landslides in three regions in Italy (Lombardy, Calabria and Lazio) indicates the value of land at risk of floods at approximately 103 million Euros, and at risk of landslides at 187 million Euros (1).
Heavy rainfall caused a landslide that destroyed numerous houses in the village San Fratello in southern Italy in February 2010. A part of the village was about to slide of a hill. 1300 residents had to be evacuated. In 2009 the area was also hit by a landslide that killed tens of people (2).
In Italy, landslides increased substantially during the second half of the 20th century, mostly because of urbanization and agricultural land abandonment (4). It is estimated that as many as half of Italian cities are at risk from such events (5).
Vulnerabilities Italy - Rock falls in the past
Permafrost and in general cryosphere degradation might play a role in the growing number of slope failures at high elevation in the Alps that has been documented since the beginning of the 21st century (20). A study of 41 rock-slope failures in 1997 - 2013 at high elevation (above 1500 m a.s.l.) in the Italian Alps showed that temperature is a key factor contributing to slope failure occurrence. Surprisingly, slope failures are not only associated with relative warm conditions, accelerating snowmelt and permafrost thaw, but with relative cold conditions as well (19).
For a catchment in the Italian Alps, changes in slope stability in time and space have been analyzed and linked to changes in climate (44). The research focused on small- to medium-sized rockfalls, slides, and debris flows. The initiation of these mass movements was derived from aerial photographs and orthophotos from various sources spanning from 1945 to 2019, additional field campaigns in 2019, and results from a previous study (45). Large mass movements were not taken into account since the response of these large-scale phenomena to changes in temperature and precipitation may take several years to decades (46). Small- to medium-sized events, however, may reflect a more direct reaction to environmental change and be more easily ascribed to climatic extremes (47). More than a thousand initiation points of mass movements were detected in the period 1945-2019, the oldest mainly in the lower part of the catchment, and the more recent ones at increasingly higher elevations. The scientists concluded that small- to medium-sized rockfalls now occur up to 300 m higher in these mountains than 70 years ago. Slope instability has shifted toward higher elevations over time, matching observations made in other studies in the Alps (48). The driving processes are mainly permafrost thaw and frost cracking, where a more frequent thawing and freezing of slope surfaces increases the fractures in the rock until parts of the rock fall. The important role of these processes was also clear from the high number of rockfalls in response to extremely high temperatures registered in spring 2007. According to the authors, the frequency of rockfalls has increased since the 2000s. The authors also found an increase of the number of debris flows triggered by rainfall events since 2010, most likely due to warmer and wetter summers.
Vulnerabilities Italy – Overview future projections
As for the potential climate change scenarios for Italy, one can expect (3,6):
- an increase of mud/debris slides, these phenomena being considered as those with the highest component of induced risk linked to the expected increase in extreme meteorological events;
- a general decrease in deep slide phenomena as a consequence of the precipitation decrease, both on an annual and a seasonal scale;
- a general decrease in the mean river flow rates, especially in the plains, with a consequent decrease in hydraulic dangerousness;
- increase of collapsing landslides, concerning the Alpine chain heights, due to progressive temperature warming and ice melting; also, generalized risk of rock falls in the Apennine region because of more frequent and sudden temperature changes, especially in winter;
- glacial lake outburst due to glacier melting in the Alpine area. The projected glacier retreat in the 21st century may form new potentially unstable lakes. Probable sites of new lakes have been identified for some alpine glaciers (9). Rock slope and moraine failures may trigger damaging surge waves and outburst floods from these lakes;
- increase of flash-flood events for mountain and pedemountain belts of Alps and Apennines, that could occur especially in central and northern regions, as a consequence of more frequent intense precipitation combined with negative effects related to growing urbanization, land-use change, wild fires, and scarce maintenance of rural areas and forests.
Effects of these phenomena spread widely over the territory, with a high induced risk for the population.
Vulnerabilities Italy – Landslides in the future
An assessment for Umbria, Central Italy
To assess landslide hazard several countries have developed early warning systems. These systems are generally based on a threshold of rainfall that triggers landslides (23). Rainfall itself, however, does not trigger landslides; it’s the impact of rainfall on destabilization of the soil that counts (24). Infiltrating rainfall can significantly increase the soil pore pressure and thus induce slope movement (22). The trigger for landslides, therefore, should be a combination of rainfall and soil saturation degree.
This combination of rainfall and soil saturation degree is at the base of the early warning system for shallow landslides of the Civil Protection Centre of Umbria Region, Central Italy. At higher rainfall intensity, this threshold is reached at a lower soil saturation degree. Vice versa, when soil saturation is already high less rainfall is needed to reach the threshold. Currently, landslide hazard is forecast up to three days in the future with this early warning system. The Umbria region is prone to landslides, and specifically to rainfall-induced landslides. More than 500 shallow landslides triggered by rainfall events have been recorded during the period 1990-2013 (22). A test of this early warning system showed that the system identified more than 86% of a selection of more than 230 occurred landslides in this period, thus illustrating the potential to use this system as a tool to evaluate the impact of climate change on landslide hazard (22).
It is to be expected that climate change affects shallow landslide occurrence through changes in rainfall volume and intensity, and temperature, evaporation and soil moisture. For the Umbria Region, this impact was evaluated by feeding projections of climate change for the periods 2040-2069 and 2070-2099 into the early warning system. The results were compared with current landslide risk (reference period 1990-2013). The projections were made with five (global) climate models and a high-end scenario of climate change (the so-called RCP8.5 scenario) (22).
According to the results, the annual number of landslide events will increase by 30% and 45% for 2040-2069 and 2070-2099, respectively, despite a reduction in mean annual rainfall and soil moisture content. This increase is due to an increase of rainfall intensity and hence rainfall amounts in the hours prior to landslides. The number of landslide events increases during the cold-wet season (from October to March), when soil moisture conditions do not change that much from current conditions but rainfall intensity strongly increases. During the warm-dry season (from April to September) the number of landslide events may decrease or stay more or less the same because the effect of reduced soil moisture due to higher temperatures offsets the effect of rainfall intensity increase (22).
In the central Apennines of Italy, and in similar areas in the Mediterranean region, the expected increase in rainfall intensity, coupled with a general lack of maintenance of old debris flow controlling structures, may increase the frequency of large debris flow events with catastrophic consequences in areas that are currently considered at low to moderate landslide risk (25).
An overall increase in landslides occurrence was projected for the Esino river basin (Marche Region, Central Italy) throughout the twenty-first century, based on 208 rainfall-triggered landslides over the period 1990-2012 combined with future projections of changes in these threshold conditions. These projections were based on three regional climate model simulations driven by an intermediate (RCP 4.5) and high-end scenario of climate change (RCP 8.5). The increase in landslides occurrence was particularly marked for the RCP 8.5 scenario (28).
Peloritani Mountains, Sicily
For the Peloritani Mountains area in Sicily, projections based on three regional climate models show a tendency of a decreasing landslide hazard. Between now and 2100 return periods may increase up to a factor of 2.5 or 3.5, respectively for an intermediate (RCP 4.5) and high-end scenario of climate change (RCP 8.5). This change is due to the projected increase of the interarrival time between rainfall events, combined with a prevalent decrease of rainfall event duration and depth (29).
The zone of warm permafrost (mean annual rock temperature approximately -2 to 0°C), which is more susceptible to slope failures than cold permafrost, may rise in elevation a few hundred meters during the next 100 years (8). This in turn may shift the zone of enhanced instability and landslide initiation toward higher-elevation slopes that in many regions are steeper, and therefore predisposed to failure.
IPCC conclusions in 2012
In 2012 the IPCC concluded that there is high confidence that changes in heat waves, glacial retreat, and/or permafrost degradation will affect high mountain phenomena such as slope instabilities, mass movements, and glacial lake outburst floods, and medium confidence that temperature-related changes will influence bedrock stability. There is also high confidence that changes in heavy precipitation will affect landslides in some regions (10). There has been an apparent increase in large rock slides during the past two decades, and especially during the first years of the 21st century in the European Alps (11) in combination with temperature increases, glacier shrinkage, and permafrost degradation.
There is medium confidence that high-mountain debris flows will begin earlier in the year because of earlier snowmelt, and that continued mountain permafrost degradation and glacier retreat will further decrease the stability of rock slopes. There is low confidence regarding future locations and timing of large rock avalanches, as these depend on local geological conditions and other non-climatic factors (10). Research has not yet provided any clear indication of a change in the frequency of debris flows due to recent deglaciation. In the French Alps, for instance, no significant change in debris flow frequency has been observed since the 1950s in terrain above elevations of 2,200 m (12). Processes not, or not directly, driven by climate, such as sediment yield, can also be important for changes in the magnitude or frequency of alpine debris flows (13).
IPCC conclusions in 2019
The IPCC concluded in 2019 that there is high confidence that the frequency of rocks detaching and falling from steep slopes (rock fall) has increased within zones of degrading permafrost over the past half-century, for instance in high mountains in Europe (37). Available field evidence agrees with theoretical considerations and calculations that permafrost thaw increases the likelihood of rock fall (and also rock avalanches, which have larger volumes compared to rock falls) (38). Summer heat waves have in recent years triggered rock instability with delays of only a few days or weeks in the European Alps (39).
In the European Alps, avalanche numbers and runout distance have decreased with decreasing snow depth and increasing air temperature (40). In the European Alps and Tatras mountains, over past decades, there has been a decrease in avalanche mass and run-out distance, a decrease of avalanches with a powder part since the 1980s, a decrease of avalanche numbers below 2000 m, and an increase above (41).
Future projections mostly indicate an overall decrease in snow depth and snow cover duration at lower elevation, but the probability of occurrence of occasionally large snow precipitation events is projected to remain possible throughout most of the 21st century (37). An overall 20 and 30% decrease of natural avalanche activity in the French Alps is estimated for the mid and end of the 21st century, respectively, under a moderate (A1B) scenario of climate change, compared to the reference period 1960 – 1990 (42). The overall number and runout distance of snow avalanches is projected to decrease in regions and elevations experiencing significant reduction in snow cover (43)
Avalanches involving wet snow are projected to occur more frequently during the winter at all elevations due to surface melt or rain-on-snow (42, for the French Alps).
In summary, there is medium evidence andhigh agreement that observed changes in avalanches in mountain regions will be exacerbated in the future, with generally a decrease in hazard at lower elevation, and mixed changes at higher elevation (increase in avalanches involving wet snow, no clear direction of trend for overall avalanche activity) (37).
Both physical constructions (“hard” measures) and “soft measures” such as sustainable land management and forest harvesting can prove cost-effective against landslides (25).
Existing single (e.g., a retaining wall, a check dam, a drainage) or multiple (e.g., a system of retaining barriers or a set of drainages in a slope, a set of check dams in a catchment) defensive structures may require modifications to adapt to the new, predicted climate conditions. Defensive structures may have been designed for a specific type of failure (e.g., a slow moving deep-seated landslide) and as a result of a change in climate a different type of landslide may be triggered (e.g., a very rapid soil slip or debris flow). In this case, the presence of structural defensive measures gives a false sense of safety (26). From a literature review of landslide risk under climate change it is recommended that all structural slope defensive measures be checked to evaluate their efficacy in the new or predicted climate conditions (25).
In the central Apennines of Italy, between 1930 and 1960, complex systems of check dams were installed and catchments were reforested to reduce torrential phenomena and debris flow events. The combined effect of the containment capacity of the check dams and the lowered erosion capacity of rainfall produced by the forest cover has reduced the frequency and intensity of the debris flow events. Today, the check dams are filled by large quantities of materials, and most of the forests are not, or are poorly maintained. An increase in rainfall intensity, or in the number of intense rainfall events, may reactivate the debris flows causing the collapse of single or multiple check dams, with potentially catastrophic domino effects that can mobilize volumes of material larger than historically recorded. Lack of maintenance of the forest worsens the situation, increasing the potential magnitude of the debris flows. The long-term, basin-scale effort conducted between 1930 and 1960 to mitigate the debris flow hazard is in jeopardy due to the predicted climate changes in this area. The efficacy of the entire defensive system provided by the sets of check dams and the forest should be evaluated considering the projected climate conditions, and their meteorological and environmental consequences. Similar situations are found in other parts of the Apennines, in the Italian Alps, and elsewhere in the Mediterranean area and in Japan (25).
The effect of soft measures may be restricted because long-term land planning frequently does not consider climate change and the related environmental and societal consequences. In Italy for instance, River Basin Authorities have prepared basin-scale, landslide (and flood) hazard and risk assessment and management plans largely ignoring the effects of the predicted climate and environmental changes. This limits the future effectiveness of the plans that may even be counterproductive. In places, the plans ignore or underestimate the risk posed by specific landslide types and particularly the types that are expected to increase in response to the predicted climate changes (e.g., very to extremely rapid soil slips and debris flows). In these areas, mitigation actions and adaptation strategies based on the existing risk assessments may be misleading, inadequate, or incorrect (25). Landslide monitoring and early warning systems (27) are a different type of effective non-structural defensive measure that can greatly reduce landslide risk, and particularly the risk to the population. The ability of existing networks of meteo-hydrological sensors to measure variables relevant to landslide early warning may be reduced when climate changes more rapidly (25).
Soft adaptation measures in Italy include (6):
- A law on the implementation of hydro-geological protection. It requires the authorities responsible for hydrological basins management to detect risk areas, set prevention plans and establish regulations to avoid additional risk due to anthropogenic factors. It is also the legal basis for the identification and funding of urgent preventive measures.
- A government directive on the establishment of an integrated warning system at the national and regional level that includes (1) the monitoring of hydro-pluviometric data and water availability, (2) a group of national experts in seasonal weather forecasting and climatology with the aim to update the scenarios for the next three-month period, (3) the implementation of a network of centres for data processing, supporting decision-making for civil protection and warning for hydro-geologic and hydrologic risk, (4) the promotion, financing and coordination of technical and scientific initiatives aimed at widening the knowledge base on extreme weather events and at applying it to the development of early-warning, evaluation and real-time monitoring tools, and (5) the implementation of a national Radar Plan for nowcasting.
The response to the expected increase in the frequency and gravity of extreme events should be to restore and set back in security major hydro-geological risk areas. Security regulations for buildings in rivers’ expansion areas and in areas at risk of landslides and avalanches need to be applied. Low vegetal cover areas need to be reforested with the aim of mitigating the climate warming effects and adapting the territory to induced risks (soil defence, desertification) (3).
Urgent preventive measures which had been funded up to 2006 amounted to 447,36 million Euro for flooding risks and 667,88 million Euro for landslide risks (7).
In southern Europe the risk of landslides is reduced through revegetation on scree slopes, which enhances cohesion and slope stability coupled with improved hazard mitigation (14).
Adaptation strategies - AdaptAlp
According to AdaptAlp, a project of the six Alpine countries on natural hazards in the Alpine region, the ten most significant actions required at this time to prepare for the risks caused by global warming in the Alps are (15):
- Improve public preparedness and personal responsibility by encouraging participation in emergency planning. To properly inform the public, risk management plans must address both emergency preparedness and early warning systems.
- Incorporate climate change adaptation into spatial planning. A few examples to create a sustainable regional development that is less vulnerable to natural hazards are: financial incentives, establishing hazard zones, setting appropriate construction standards of buildings and infrastructure in risk areas, keeping endangered spaces free of development, and performing hazard assessments through the use of hazard mapping.
- Involve local stakeholders in a risk dialogue. The dialogue includes meetings between important stakeholders, such as land and real estate owners, as well as those responsible for infrastructure and the public sector.
- Encourage cross-border networking on integrated risk management.
- Encourage a ‘common language’ and harmonised procedures when developing and using hazard maps.
- Increase the size of flood plains, floodwater conduits and basins. Governments need to consider multiple uses of the same land and consider strict legal binding instruments that ensure a priority for flood retention areas is given.
- Think of flood risk management in terms of an entire river basin to find solutions that are sustainable. Horizontal and vertical cooperation between all levels of government and the private sector are essential.
- When planning for natural hazard risks consider all the environmental risks within a defined area. Natural hazards—floods, droughts, landslides—generate risks that are interrelated and so should be addressed jointly.
- Use risk-management tools to explore the social and economic consequences of various adaptation measures. Risk planning tools allow for the integration of a wide range of strategies that reduce the risks of natural hazards, including spatial planning instruments, technical protection structures, specific protection measures for individual buildings and early-warning systems.
- Support the collection and interpretation of local climate change data.
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Italy.
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- Ministry for the Environment, Land and Sea of Italy (2007)
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- IPCC (2012)
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- Paranunzio et al. (2019)
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- Gariano and Guzzetti (2016); Stoffel and Huggel (2017), both in: Paranunzio et al. (2019)
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- Hasler et al. (2012), in: Paranunzio et al. (2019)
- Gariano and Guzzetti (2016), in: Paranunzio et al. (2019)
- IPCC (2019)
- Gruber and Haeberli (2007); Krautblatter et al. (2013), both in: IPCC (2019)
- Allen and Huggel (2013); Ravanel et al. (2017), both in: IPCC (2019)
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- Fischer et al. (2012); Ravanel et al. (2010); Ravanel and Deline (2011), all in: Savi et al. (2021)