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Avalanches, Landslides and Rock fall France

Vulnerabilities- Avalanches

With regard to avalanches and landslides, the impacts of climate change remain uncertain (1).

A change in avalanche hazards in connection with climate change is uncertain. In general, it is assumed that the possible change would follow the snow cover evolution. A decrease in avalanche hazards is likely at low and medium altitudes. Yet, heavy precipitation events might counterbalance this trend by triggering general avalanche situations (2).

Vulnerabilities - Alpine mass movements

The degradation of permafrost in steep slopes is a major factor for the reduced stability of rock walls and the rock fall pattern. Increased precipitation might lead to more frequent and extended slope instabilities in the future. In particular, the changes of intense precipitation could impact the shallow landslides (through the surface water runoff and stream actions), while the changes of long-term precipitation could impact the deep landslides (through underground water action). On the other hand, the possible future decrease of summer precipitation may have a positive effect by reducing the deep and shallow landslides activity (2).

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 (3). 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.

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 (4). Rock slope and moraine failures may trigger damaging surge waves and outburst floods from these lakes.

Vulnerabilities - Landslides

The Alps

Projections of climate change between 2010 and 2100 have been translated into future debris flow activity for two Alpine catchments, one in France (Barcelonnette Basin) and one in Italy (Fella River catchment) (11). These projections are based on a number of regional climate models (driven by general circulation models) and both an intermediate and high-end scenario of climate change (the so-called RCP4.5 and RCP8.5 scenarios, respectively). From the model outcomes two meteorological proxies for debris flow activity were derived and translated into future debris flow activity: one using 1-day rainfall amounts and one being representative of short-lasting convective rainfall systems. The 1-day rain-proxy was chosen as precipitation is a dominant trigger of debris flows. As rain gauges are very local and often non-catchment representative, the second proxy was used to mimic the mesoscale atmospheric conditions leading to heavy local convective precipitation. All of these projections show either an increase or little change in the number of days with debris flows between now and 2100 (11).

An inventory of 2966 landslides in the French and Swiss portion of the European Alps over the period 1970–2002 (10) shows that the majority of landslides recorded occurred during the spring (March/April/May, 29 %) and summer (June/July/August, 36 %), with the lowest numbers recorded during autumn (September/October/November, 15 %) and winter (December/January/February, 20 %).

It is hard to predict changes in landslide occurrence from projected changes in precipitation due to climate change since changes in precipitation patterns are hard to predict at the small-scale. As an alternative approach, changes in weather types can be used as a predictive tool for landslide events under differing future climate scenarios. One should be aware, however, that other factors (such as rainfall persistence, geology and topography) play an important role in the occurrence of landslides as well (10).

For the Alps, the main trigger of debris flows is high intensity, short duration rainfall (12). Under future climate change, it is likely that increases in extreme rainfall will alter debris flow frequency (13). Previously, a limited number of climate change impact studies focused on debris flows, with inconsistent results: some indicating less debris flows in the future (14), others more (15), or concluding that accurate quantification of changes in the number of debris flows is not possible (16). 

The Pyrenees

The risk of landslides in mountainous areas may change in the future especially as a result of climate change and changes in land cover. The link with climate change is the effect of changes in moisture content in the subsoil due to changes in precipitation intensity and volume, and a shift from snowfall to rain. Changes in land cover can affect slope stability through different processes: on the one hand, the roots of plants and trees increase slope stability; on the other, vegetation adds weight to the slope and more weight increases the risk of a landslide. Vegetation also influences moisture content of the subsoil (24).

To make things more complicated, there are several types of landslide. The sensitivity of landslides to changes in climate or land cover may depend on especially the size and depth of the landslide (25). Shallow landslides are generally governed by shorter-duration rainfall, and therefore by changes in the intensity of rainfall. In contrast, deep-seated landslides may be affected by changes on a much longer term, such as changes in the monthly rainfall, seasonal snow or groundwater.

How important is climate change when compared to changes in land cover for future changes in the risk of landslides? This was studied for a high-elevation valley in the French Central Pyrenees. Future projections were assessed for four different scenarios of land use, leading to more or less forest cover, and for two scenarios of climate change, a moderate (the RCP4.5 scenario) and high-end scenario (RCP8.5). These projections refer to the near future (2021-2050 period) and far future (2071-2100 period). The results were compared with the situation of 1981-2010 for reference (24).

The results show that for this case climate change has a stronger impact on landslide risk than changes in land cover. Climate change may periodically increase the water table in the subsoil such that slope stability decreases. This effect even dominates over the slope stabilizing effect of more forest cover. This is particularly significant for the high-end scenario of climate change. refore, even if future forest growth leads to slope stabilization, the evolution of the groundwater conditions will lead to destabilization (24).

According to this study, landslide hazards in this part of the Pyrenees are projected to occur 1.5 and 4 times as often in the near and far future, respectively, under the high-end scenario of climate change. The increase of landslide frequency in areas prone to landslides is higher for the shallow landslide type than for the deep landslide type (24).

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 (5). 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 (6) 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 (5). 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 (7). 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 (8).

IPCC conclusions in 2019

Rock fall

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 (17). 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) (18). Summer heat waves have in recent years triggered rock instability with delays of only a few days or weeks in the European Alps (19).

Snow avalanches

In the European Alps, avalanche numbers and runout distance have decreased with decreasing snow depth and increasing air temperature (20). 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 (21).

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 (17). 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 (22). The overall number and runout distance of snow avalanches is projected to decrease in regions and elevations experiencing significant reduction in snow cover (23)

Avalanches involving wet snow are projected to occur more frequently during the winter at all elevations due to surface melt or rain-on-snow (22, 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) (17).

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 (9):

  • 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.

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for France.

  1. ONERC (2007/2009)
  2. ESFR ClimChAlp (2008b), in: Castellari (2009)
  3. Noetzli and Gruber (2009), in: IPCC (2012)
  4. Frey et al. (2010), in: IPCC (2012)
  5. IPCC (2012)
  6. Ravanel and Deline (2011), in: IPCC (2012)
  7. Jomelli et al. (2004), in: IPCC (2012)
  8. Lugon and Stoffel (2010), in: IPCC (2012)
  9. AdaptAlp
  10. Wood et al. (2016)
  11. Turkington et al. (2016)
  12. Schneuwly-Bollschweiler and Stoffel (2012); Stoffel et al. (2014); Van den Heuvel et al. (2016), all in: Turkington et al. (2016)
  13. Winter et al. (2010), in: Turkington et al. (2016)
  14. Jomelli et al. (2009), in: Turkington et al. (2016)
  15. Stoffel et al. (2014), in: Turkington et al. (2016)
  16. Melchiorre and Frattini (2012), in: Turkington et al. (2016)
  17. IPCC (2019)
  18. Gruber and Haeberli (2007); Krautblatter et al. (2013), both in: IPCC (2019)
  19. Allen and Huggel (2013); Ravanel et al. (2017), both in: IPCC (2019)
  20. Teich et al. (2012); Eckert et al. (2013), both in: IPCC (2019)
  21. Eckert et al. (2013); Lavigne et al. (2015); Gadek et al. (2017), all in: IPCC (2019)
  22. Castebrunet et al. (2014), in: IPCC (2019)
  23. Mock et al. (2017), in: IPCC (2019)
  24. Bernardie et al. (2021)
  25. Crozier (2010), in: Bernardie et al. (2021)

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