Avalanches, Landslides and Rock fall: European scale
Vulnerabilities - Landslides in the past
Landslides contribution to all global natural hazards
Landslides are caused by tectonic (seismic), climatic and/or human activities. Landslides are a significant global hazard. From global data in the 1970s it was concluded that ca. 14% of total casualties from natural hazards are attributed to slope failure (19). The well-known and often used EM-DAT International Disaster Database suggests that landslides account for 4.9% of all natural disaster events and 1.3% of all natural hazard fatalities between 1990 and 2015. These lower numbers are probably underestimates. Studies have shown that the EM-DAT database underestimated the number of fatal landslide events by 1400% (20) or 2000% (21) and the number of fatalities by 331% (20) or 430% (21). For the most part this underreporting is associated with the perception of landslides as a secondary hazard, with the cause of death often being recorded in connection with the primary hazard (e.g. an earthquake) rather than the actual cause of the loss. 54% of these landslide events occurred in Asia (22).
A global dataset of fatal non-seismic landslides, covering the period from January 2004 to December 2016, shows that in total 55,997 people were killed in 4862 distinct landslide events (18). The majority of these events were triggered by rainfall (79%). South, Southeast and East Asia, and South and Central America combined contain 88% of all global rainfall-triggered landslide events. 16% of all non-seismic landslide events were not generated by rainfall but were triggered by mining, construction or illegal hill cutting. These human activities caused almost 7% of all landslide-related fatalities (18).
The data do not show a clear long-term increase or decrease in global landslide impact; rather, the record shows that the number of landslides varies strongly from one year to another on a global scale. The data show that landslide occurrence triggered by human activity is increasing, in particular in relation to construction, illegal mining and hill cutting. This supports notions that, on a global scale, human disturbance may be more detrimental to future landslide incidence than climate change. This is supported by other studies (23). Things are different in the mountains, however: there is a high confidence that glacial retreat and permafrost degradation will increase slope instabilities in high mountain areas in the long term.
No clear relationships with climate change
A global assessment of the impacts of climate change on landslide risk has been carried out based on a review of scientific publications on past, current, and future impacts of climate change on landslides (1). The majority (80%) of the papers in this review found causal relationships between landslides and climate change. From this review, the type, extent, magnitude, and direction of the impacts of climate change on landslide location, abundance and frequency are not completely clear, however. The effects of the warming climate on landslide risk, and particularly the risk to the population, also remain difficult to quantify. The Alps are the most investigated physiographic area, including the French, Italian, and Swiss Alps.
It is still hard to determine if and where landslide risk, and particularly risk to the population may increase (or decrease) in direct or indirect response to climate change. Whether areas are subject to (an increasing) landslide risk not only depends on temperature and rainfall, but on local geological conditions and non-climatic factors (including land use/cover, agriculture and forest practices) as well. Besides, natural and human induced drivers of landslides interact in a complex way, even in a “stable” climate (1). The direction, magnitude and effects of these non-climatic, interacting drivers may outweigh changes in landslide activity due to climate change (4). Besides, in many areas global warming will have an impact on land use and land cover, on agricultural and forestry practices, and on the economy. These changes may also change the activity and the rate of occurrence of landslides, and hence landslide hazard and risk (5). The incompleteness of old climate and landslide records also plays a role; this limits the possibility to evaluate the impact of the expected environmental and climate changes on landslide frequency, and to estimate variations in the associated risk (6).
It was concluded previously that impacts of climate change on landslides were in places likely minor, compared to impacts from human disturbance (6).
Vulnerabilities - Future projections
Higher risk of shallow, rapid-moving landslides
Intense rainfall events are a primary trigger of shallow, rapid-moving landslides such as debris flows, debris avalanches, rock falls, and also ice falls and snow avalanches in high mountain areas (7). These events are a primary cause of landslide fatalities (8). Given the fact that in some areas global warming is expected to increase both the intensity of rainfall events and the frequency of these events, it is to be expected that in these areas the total number of people exposed to landslide risk will increase. These areas include the Alps, the Himalayas and most of the American Cordillera, but also the Atlas Mountains in northwestern Africa, mountains and hills in southwestern Africa, the East Africa's Rift Valley and the Arabian Peninsula, the Carpathians in Eastern Europe, the Appalachians in eastern North America.
More heavy precipitation events will probably increase the number of landslides in parts of Europe that are susceptible to landslides because of their steep hills and geological subsoil. This is especially the case for a number of regions along the north side of the Alps: the Jura Mountains, the Vosges, the Black Forest, the Swabian Jura, the Bavarian Pre-alps, the foothills of the Austrian Alps and the Bohemian Forest (16). These regions are also important transport corridors for Europe’s road and rail network. More landslides may disrupt these corridors and cause a lot of damage. Heavy precipitation events can trigger landslides when they exceed a certain threshold: a certain amount of precipitation in 3 succeeding days including at least 1 day with extremely heavy precipitation (17). The potential increase of the number of these events was assessed for central Europe, based on an intermediate scenario of climate change (the so-called SRES A1B scenario) and a large number of climate model runs. The results of this study show that the frequency of landslide-triggering extreme climate events will slightly increase over time. At the end of this century, the aforementioned areas along the north side of the Alps are likely to experience substantially increased landslide activity compared to current climate conditions. The increase may be up to 14 additional landslide-triggering rainfall events per year, on average (16).
Published results on the impact of rainfall do not always point in the same direction, however. For Calabria (southern Italy) antecedent rainfall in the month before a landslide event appeared to play a key role to initiate rainfall-induced landslides (9). For the nearby Puglia region, variations in rainfall and temperature did not justify the observed increase in landslide events between 1918 and 2006, however (10).
Higher air temperature
In addition to the intense rainfall events, the projected increase in air temperature is also expected to affect the stability of rock slopes at high latitudes (particularly in the northern hemisphere and at high elevations, where permafrost exists that may reduce when temperature rises (11). A number of investigators have examined the effects of air temperature on debris flows and rock falls, chiefly in the European Alps, and found an increase in landslide activity related to an increase in air temperature (12). According to IPCC, there is a “high confidence that changes in temperature, glacial retreat, and/or permafrost degradation will affect slope instabilities in high mountains, and medium confidence that temperature-related changes will influence bedrock stability” (13). In high mountain areas, not only small-sized rock falls and ice falls, but also large rock slides and rock avalanches may become more abundant (14). At high latitudes, particularly in the taiga and tundra areas in the northern hemisphere, permafrost melting can initiate ground instability processes even in low gradient terrain, producing incised gullies that transform rapidly into wide badland areas.
Global warming will also change the time required for the snow to melt, and the frequency of rain-on-snow events, two known triggers of landslides. 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” (13).
Lower risk of deep-seated landslides
The degree of activity and the occurrences of new deep-seated landslides are expected to decrease (15). Extremely to moderately slow deep-seated landslides (including earthflows, mudflows, complex and compound slides) generally do not pose a serious threat to human life. Hence, their predicted reduced activity will not decrease landslide risk to the population significantly, but it is expected to contribute to reducing landslide impact and the related economic damage (1).
The literature review revealed that variations in rainfall totals influence mostly rockslides, mud flows and earth flows, at both the local and the regional scale, whereas variations in rainfall intensity affect, mostly directly, rock falls and debris flows/avalanches, in the short-term and at the local scale. Changes in the air temperature influence directly ice falls and avalanches, and have an indirect impact on rock falls (due to the formation and opening of fractures), and on deep-seated landslides (due to changes in the hydrological cycle) (1).
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 (2). 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 (1).
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 (1). Landslide monitoring and early warning systems (3) 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 (1).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.
- Gariano and Guzzetti (2016)
- Sidle and Chigira (2004), in: Gariano and Guzzetti (2016)
- Stähli et al. (2015), in: Gariano and Guzzetti (2016)
- Sidle and Dhakal (2002), in: Gariano and Guzzetti (2016)
- Van Beek (2002); Wasowski et al. (2010); Lonigro et al. (2015), all in: Gariano and Guzzetti (2016)
- Crozier (2010), in: Gariano and Guzzetti (2016)
- Stoffel et al. (2014), in: Gariano and Guzzetti (2016)
- Guzzetti et al. (2005b); Petley (2012), both in: Gariano and Guzzetti (2016)
- Polemio and Petrucci (2010), in: Gariano and Guzzetti (2016)
- Polemio and Lonigro (2015), in: Gariano and Guzzetti (2016)
- Huggel et al. (2012, 2013); Stoffel et al. (2014); Chiarle et al. (2015); Ravanel and Deline (2015); Paranunzio et al. (2016), all in: Gariano and Guzzetti (2016)
- Ravanel and Deline (2011, 2015); Stoffel and Beniston (2006); Paranunzio et al. (2016), all in: Gariano and Guzzetti (2016)
- Seneviratne et al. (2012), in: Gariano and Guzzetti (2016)
- Huggel et al. (2012, 2013), in: Gariano and Guzzetti (2016)
- Malet et al. (2005); Coe (2012); Comegna et al. (2013); Rianna et al. (2014), all in: Gariano and Guzzetti (2016)
- Schlögl and Matulla (2018)
- Guzzetti et al. (2008), in: Schlögl and Matulla (2018)
- Froude and Petley (2018)
- Varnes and IAEG Commission on Landslides (1984), in: Froude and Petley (2018)
- Kirschbaum et al. (2015), in: Froude and Petley (2018)
- Petley (2012), in: Froude and Petley (2018)
- Guha-Sapir et al. (2018), in: Froude and Petley (2018)
- Innes (1983); Glade (2003); Soldati et al. (2004); Imaizumi et al. (2008); Borgatti and Soldati (2010); Crozier (2010); Anderson and Holcombe (2013); Lonigro et al. (2015), all in: Froude and Petley (2018)