Réf. Kääb & al. 2005 - A

Référence bibliographique complète
KÄÄB, A., REYNOLDS, J.M. & HAEBERLI, W. (2005): Glacier and permafrost hazards in high mountains. In: Huber, U.M., Bugmann, H.K.M., Reasoner, M.A. (eds.), Global Change and Mountain Regions (A State of Knowledge Overview). Springer, Dordrecht. 225-234.

Abstract: Glacier- and permafrost-related hazards represent a continuous threat to human lives and infrastructure in high mountain regions. Related disasters can kill hundreds or even thousands of people at once and cause damage with a global sum on the order of 100 M€ annually.

Mots-clés
Climate change, debris flow, flood, glacier, ice avalanche, permafrost.

Organismes / Contact
Department of Geography, University of Zurich-Irchel, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. phone +41-1-635 51 46, fax +41-1-635 68 48, kaeaeb@geo.unizh.ch
Reynolds Geo-Sciences Ltd, 2 Long Barn, Pistyll Farm, Nercwys, Mold, UK-Flintshire, CH 7 4 EW

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
  Glaciers and permafrost Torrential events, Glacial hazards, Mass movements  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
World          

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses

For those rivers fed largely by ice melt, reduction in glacier volumes will have a particularly strong impact on dry-season river flows, and on the provision of downstream water.

Glacierized mountain areas would be among the most heavily affected parts of the world in the event of accelerated future warming. Due to the complex interactions of the different variables of the energy balance in such areas, potential future changes can only be estimated very roughly. Empirical methods and energy balance considerations indicate that a large fraction (about one-third to one-half) of the presently existing mountain glacier mass on earth could disappear over the next 100 years with anticipated atmospheric changes. With an associated upward shift of the equilibrium line by some 200 to 300 meters, yearly thickness losses of 1 to 2 meters would have to be expected for temperate glaciers, and many low-latitude mountain ranges would lose major parts of their glacier cover within decades. The consequences would include changes in hazard situations, but also in the water cycle and in landscape evolution.


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
   

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
On steep slopes, freshly exposed or thawing non-consolidated sediments can become unstable, resulting in debris flows and landslides of varying magnitudes. Once one event has occurred in a particular valley, the remaining slopes may become destabilised even further. The risk of secondary damming of rivers by debris flows also needs to be considered. In places of pronounced glacier retreat, changes in stress distribution and surface conditions of rock walls in deeply cut glacier troughs could induce large mass instabilities. The general tendency is towards a shifting of hazard zones with considerable changes in the processes involved and a widespread decrease in the stability of high-mountain slopes.

It is predicted that both the number and size of glacial lakes will increase as climate changes. Marked changes in glacier extent due to climate change may be accompanied by both the formation and disappearance of ice- and moraine-dammed lakes, and steep hanging glaciers may become less stable. On the other hand, steep glacier tongues with their present-day potential for large ice avalanches could disappear.

Re­vegetation of deglaciated terrain is slow and leaves morainic deposits unprotected against erosion over extensive time periods of several decades and more. On steep slopes, freshly exposed or thawing non-consolidated sediments can become unstable, resulting in debris flows and landslides of varying magnitudes. Once one event has occurred in a particular valley, the remaining slopes may become destabilised even further. The risk of secondary damming of rivers by debris flows also needs to be considered. In places of pronounced glacier retreat, changes in stress distribution and surface conditions of rock walls in deeply cut glacier troughs could induce large mass instabilities. The general tendency is towards a shifting of hazard zones with considerable changes in the processes involved and a widespread decrease in the stability of high-mountain slopes.

The instability of rock slopes can also be connected to glacier variations. Glacier retreat leads to stress redistribution within adjacent valley flanks, which can cause mass movements such as rock-slides. For example, late-glacial ice retreat was associated with a large number of landslides and these events have also been observed during the present glacier retreat since the end of the Little Ice Age. Changes in the thermal regime of cold rock walls and related effects on rock stability are still poorly understood processes but are of increasing concern in view of recent catastrophes.


A widespread risk in high mountains is related to accumulations of loose sediments on steep slopes, which represent potential sources of debris flows. Such debris accumulations can occur in the form of moraines, moraine dams, or steep valley flanks uncovered by retreating glaciers. In other words, they are strongly connected to glacier development. Another important factor in this context is permafrost, which influences the stability and hydrology of debris slopes. Whilst the trigger mechanisms of these frequently unexpected debris flows often remain unclear (e.g. melting dead ice, permafrost-hydrology interactions) and are therefore difficult to predict in individual cases, the respective hazard potential seems to be connected to the presence of permafrost and its changes.

Many of the largest known glacier catastrophes are characterised by hazard combinations and/or process chains. For instance, if ice and/or rock avalanches enter natural or artificial (i.e. reservoir) lakes, they are able to trigger flood waves and, as a consequence, can lead to overflowing and breaching of natural (or artificial) dams with corresponding flood and debris flow disasters. Ice avalanches are a special risk in the winter season when the run-out distance increases considerably due to strongly reduced friction on snow. In addition, ice avalanches are able to trigger large snow avalanches, thereby greatly enhancing the avalanche volume. Of much more importance than the direct impact of glacier length variations is the indirect risk of triggering glacier floods and ice avalanches. Advancing glaciers are able to dam rivers and create lakes. Manyknown glacier floods have their origin in such ice-dammed lakes. On the other hand, retreating glaciers often leave behind moraine-dammed lakes. These moraines can be breached resulting in floods and also represent an important sediment source for debris flows. Glaciers retreating or advancing over a topographic break in slope have a greatly increased risk of ice breaking off their tongue (cf. the 1965 catastrophe of Allalin Glacier, Swiss Alps). Shock waves related to ice- and rock-fall impacts are able to destabilise glaciers and other high mountain terrain.

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales

It is predicted that both the number and size of glacial lakes will increase as climate changes. Coupled with increasing rural development and investments in infrastructure, particularly in hydropower, the vulnerability of mountain communities to outburst floods is growing rapidly. Furthermore, for those rivers fed largely by ice melt, reduction in glacier volumes will have a particularly strong impact on dry-season river flows, and on the provision of downstream water for hydropower, irrigation and potable water supplies. Whilst catastrophic floods (too much water too quickly) are a very palpable hazard, so too are “soft” hazards, such as reduced glacier water during the dry season. Consequently, hazards in high mountains must be considered in relation to water resource management and cannot be seen in isolation.

In 1979, an outburst flood from the Lago delle Locce breached the lateral moraines of Belvedere Glacier and caused a severe debris flow, which among other things destroyed a chair lift. To prevent further lake outbursts, the lake level was lowered and controlled by an artificial channel. In summer 2001, the glacier started an untypical surge-type movement with glacier speeds increased be one order of magnitude compared to previous decades. As a consequence, the glacier became heavily crevassed and its tongue advanced over the Little Ice Age moraines, destroying forest and tourism infrastructure. In spring 2002, a so-called supraglacial lake of 3 million m3 developed on the glacier, which became a severe flood risk for the village of Macugnaga within a couple of weeks. The Italian Civil Defense Department and the scientists involved initiated emergency actions. These included continuous lake level monitoring, evacuation of certain parts of the village of Macugnaga, an automatic alarm system, the installation of pumps and detailed scientific investigations. From summer to autumn 2002 the lake lowered naturally. (Haeberli et al. 2002; Kääb et al. 2003).


(5) - Syntèses et préconisations
The general tendency is towards a shifting of hazard zones with considerable changes in the processes involved and a widespread decrease in the stability of high-mountain slopes. Special measures are needed to ensure the structural stability and durability of installations for tourism, transportation and telecommunication in permafrost areas. Similarly, detailed hazard assessments must be undertaken routinely and regularly to avoid damage to hydropower installations due to the impact of glacier-derived floods, which can cost many tens of millions of Euros. If, in fact, environmental conditions in high-mountain regions were to evolve beyond the range of Holocene and historical variability, hazard assessments may become increasingly difficult because estimates of hazard potential based on empirical data from the past (historical documents, statistics, geomorphological evidence) will not be directly applicable under new conditions.

Under such circumstances, the concept of sustainable development in the highest belts of cold mountain areas becomes questionable, because large-scale climatic forcing would by far outweigh any local environmental influences. The main challenge would, in fact, be to adapt to high and accelerating rates of environmental change (Haeberli and Beniston 1998). Empirical knowledge would have to be increasingly replaced by improved process understanding, especially concerning runoff formation and slope stability. Robust numerical models would have to help with the design of hazard mitigation measures at high altitudes. The intensive research on glacier hazards carried out in Switzerland during the past decades (e.g. Haeberli et al. 2001) can illustrate possibilities and limitations of hazard assessments and mitigation.

The above-mentioned recent catastrophes clearly demonstrate the key issues with respect to assessing and mitigating glacier and permafrost hazards:
-   The large potential for hazard assessment based on remote sensing and numerical modelling has to be fully exploited, and knowledge has to be transferred to affected regions in the second and third world;
-   Scientifically objective criteria need to be developed to assess the hazard potential of glacial lakes and other glacial and periglacial hazards;
-   Scientists should work towards a greater transfer of information and improved communication between the scientific and political communities to raise the awareness and willingness of the responsible authorities to use the available information and knowledge basis on glacial and periglacial hazards;
-The impacts of environmental change on hazard potential need to be continually monitored and a rapid transfer of this information is critical for the successful mitigation of hazards in highly sensitive high-mountain environments.

Référence citées dans l'article :

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Davies, M.C.R., Hanza, O. and Harris, C. (2001). The effect of rise in mean annual temperature on the stability of rock slopes containing ice-filled discontinuities. Permafrost and Periglacial Processes, 12(1), 137-144.

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