Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Ravanel & al, 2013 - A

Référence bibliographique
RAVANEL l., DELINE P., LAMBIEL C., VINCENT C., 2013 Instability of a high alpine rock ridge: the lower arête des Cosmiques, Mont Blanc massif, France. Geografiska Annaler: Series A, Physical Geography, 95, 51–66. doi:10.1111/geoa.12000

Abstract : Rockfalls are dominant in the rock slopes and rock ridge morphodynamics in high mountain areas and endanger people who pass along or stay there, as well as infrastructure that host them (cable cars, refuges). Risks are probably greater now because of fast permafrost degradation and regression of surface ice, two consequences of the atmospheric warming of the last decades. These two commonly associated factors are involved in the instability of rock slopes by modifying the mechanical behaviour of often ice-filled rock fractures and the mechanical constraints in the rock masses. This paper examines over 15 years the instability of the lower Arête des Cosmiques on the French side of the Mont Blanc massif. Its vulnerability is due to the presence of a high-capacity refuge on its top (3613 m a.s.l.). In 1998, a part of the refuge was left without support when a collapse of 600 m3 occurred immediately below it. Since this date, reinforcement work has been carried out in this area, but the whole ridge has been affected by around 15 relatively shallow rockfalls. Through a multidisciplinary approach, this article assesses the role of the cryospheric factors in the triggering of these rockfalls.

Mots-clés
 high alpine rock slopes, rockfalls, permafrost, glacier shrinkage, hazards, mountain infrastructure, Mont Blanc massif

Organismes / Contact

Authors / Auteurs :

  • RAVANEL l., EDYTEM Lab., Université de Savoie – CNRS, Le Bourget-du-Lac, France & TIK Lab., Swiss Federal Institute of Technology, Zurich, Switzerland
  • DELINE P., EDYTEM Lab., Université de Savoie – CNRS, Le Bourget-du-Lac, France
  • LAMBIEL C., IGD, Université de Lausanne, Switzerland
  • VINCENT C., LGGE, Université J. Fourier – CNRS, Saint Martin d’Hères, France

(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
  permafrost fonte du permafrost/chute de blocs  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
France Massif du Mont-Blanc Arrête des Cosmiques      

(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
 

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

 


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 

A large number of the rockfalls which have affected the LAC over the last 15 years seem to result from permafrost activity/degradation and loss of glacier buttressing or from their combination.

As suggested by the occurrence of rockfalls mainly during or at the end of hot periods in summer, degradation of the ice has likely participated in the triggering of a large part of these rockfalls. Geotechnical properties of bedrock containing ice depend on temperature (Davies et al. 2001; Gruber and Haeberli 2007): when it increases, the shear strength decreases. The rock temperature increase can be caused either by heat conduction, heat advection or a combination of both (Hasler et al. 2011). In the latter case, it is often the water from snowmelt or storm rainfall that transfers heat to depth, as efficiently as the rock is fractured.
The strength of ice depends at the same time on its temperature, hydrostatic pressure, and conditions of its formation, but also depends on the rock materials it contains: dirty ice observed at the LAC is probably of poor geotechnical quality.

The sharp temperature difference that exists between the NW face of the LAC in cold permafrost and the SE face in warm permafrost likely creates a strongly negative horizontal gradient directed from the warmer to the colder side of the ridge.

At last, an important influence on rock temperature in the LAC could be the influence of the refuge itself on rock temperature.

Une part importante des chutes de blocs qui ont affecté la partie basse de l’arrête des Cosmiques, sur les 15 dernières années, apparaît comme conséquence de la dégradation du permafrost, du renforcement de la perte de volume du glacier, ou de la combinaison de ces deux éléments.

Ainsi que le suggère l’observation de chutes de blocs au cours, ou à la fin, de périodes estivales particulièrement chaudes, la dégradation de la glace apparaît comme un élément important du dans le processus de déclenchement de ces chutes de blocs. Les propriétés géotechniques de la glace présente dans le « bedrock » dépendent des variations de température. Lorsque cette dernière augmente, les forces de cisaillement diminuent. L’augmentation de la température de la roche peut être causée par la conduction thermique, par la convection thermique, ou par la combinaison des deux processus. Dans le dernier cas, l’eau issue de la fonte ou des phénomènes orageux peut-être vecteur de transport de chaleur, et ce d’autant plus lorsque la roche est fracturée. La force/qualité de la glace dépend à la fois de sa température, de la pression hydrostatique, et des conditions de sa formation, mais dépend aussi des matériaux qu’elle contient. La glace sale « dirty Ice » observée dans la partie basse de l’Arête des Cosmiques possède probablement une qualité géotechnique moindre.

La différence importante de température qui existe entre la face NW de l’arête des Cosmiques (permafrost froid)et la face SE (permafrost chaud) est vraisemblablement à l’origine d’un fort gradient horizontal négatif, depuis la partie chaude vers la partie froide de l’arête.

Enfin, la présence du refuge pourrait avoir une influence importante sur la température de la roche au niveau de la partie basse de l’arête des Cosmiques.

Modélisations
Hypothèses
 

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

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

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