Réf. Ravanel & al. 2010 - A

Référence bibliographique complète

RAVANEL, L., ALLIGNOL, F., DELINE, P., GRUBER, S., RAVELLO, M. 2010. Rock falls in the Mont Blanc Massif in 2007 and 2008. Landslides, Vol. 7, 493–501.

Abstract: Due to a lack of systematic observations, the intensity and volume of rock falls and rock avalanches in high mountain areas are still poorly known. Nevertheless, these phenomena could have burly consequences. To document present rock falls, a network of observers (guides, mountaineers, and hut wardens) was initiated in the Mont Blanc Massif in 2005 and became fully operational in 2007. This article presents data on the 66 rock falls (100 m3V50,000 m3) documented in 2007 (n=41) and 2008 (n =25). Most of the starting zones are located in warm permafrost areas, which are most sensitive to warming, and only four rock falls are clearly out of permafrost area. Different elements support permafrost degradation as one of the main triggering factors of present rock falls in high mountain areas.

Mots-clés

Rock falls - Permafrost - High alpine environments - Mountains - Mont Blanc Massif

 

Organismes / Contact

• Laboratoire EDYTEM, Université de Savoie, CNRS, 73376 Le Bourget-du-Lac, France (Ludovic.Ravanel@univ-savoie.fr)
Glaciology, Geomorphodynamics and Geochronology, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland

 

(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

 Température

 Permafrost

 Eboulements et chutes de blocs en haute montagne

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 France, Italie

 Massif du Mont-Blanc

 

 

 

 2007-2008

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

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

The possible presence of permafrost is estimated from an approximate model of the mean annual ground surface temperatures (MAGST) of the massif, carried out using an energy balance model (TEBAL; see Gruber et al. 2004b). MAGST values are not presented because rock wall surface temperatures are modeled for the period 1982–2002 based on meteorological data from Corvatsch and Jungfraujoch stations (Switzerland) and not from the Aiguille du Midi station (where air temperature is the only data measured, since only February 2007); only a qualitative index of the probability of existence of permafrost for each sector is proposed. Permafrost is considered unlikely, possible, and likely when MAGST is >1°C, between 1°C and −1°C, and <−1°C, respectively.

 

(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

This article presents an inventory and first synopsis of rock falls having a volume >100 m3 in the Mont Blanc Massif during 2007 and 2008.

Developed since 2005, a network of rock fall observers in the Mont Blanc Massif surveys for the first time and as exhaustively as possible the rock instability in high alpine steep rock walls. In 2007 and 2008, 66 events were observed and documented. Most of the starting zones are located in warm permafrost areas (0 to −5°C; see Noetzli et al. 2003), which is most sensitive to warming. For several rock falls, massive ice has been observed in the detachment zone; this supports the relevance of the thaw of the ice, which fills fractures in high alpine rock walls (“ice-cemented”).

Permafrost conditions seem today more and more important because warming is thought to be a mechanism through which climate controls rock wall stability and, consequently, natural hazard in mountain areas. Thus, to study the role of permafrost degradation in rock fall triggering, subsurface rock temperature has to be modeled for each rock fall scar. A standard statistical analysis of the distribution of rock walls, according to elevation and aspect, is in progress. It is complemented by historical research to characterize the recent evolution of the frequency and volume of rock falls, which is essential to argue that global warming is affecting rock fall triggering through permafrost degradation.

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

 

Rock falls, as most of the instabilities in rock slopes, are usually related to existing fractures (mesoscale and fine-scale fracturation is poorly studied in the Mont Blanc Massif), along which a rock mass is destabilized by a triggering factor. The permafrost degradation could be an important one. Only four events (6%) are clearly out of the permafrost area. The 41 events (61%) that occurred where permafrost presence is likely could be related to permafrost degradation (active layer formation, active layer thickening, or warming at depth). Historical studies that are currently developed (e.g., Ravanel and Deline 2008) support this. They point out a clear evolution with a strong correlation between rock fall occurrences and the warmest periods over the last 150 years (see also Evans and Gardner 1989). It is to note that the years 2007 and 2008 have, respectively, the seventh and eighth highest mean annual temperatures in Chamonix for a century (MétéoFrance data) and probably for at least 500 years (see Casty et al. 2005). About 90% of the events took place during summer, i.e., the hottest period of the year. Massive ice has besides been observed in about 12 scars. This observation largely corroborates the ice-filled fractures thawing. Bonding of ice-filled fractures and its reduction or loss during degradation can be related to a combination of ice/rock interlocking and ice/rock adhesion (Gruber and Haeberli 2007). Moreover, many events have originated from ridges and spurs, possibly due to more rapid thaw in such geometries (Noetzli et al. 2007). Two of the three main events, the Tour des Grandes Jorasses and the Tré-la-Tête events, occurred in September, i.e., when the active layer (i.e., the top layer of the permafrost that thaws during the summer) is almost the deepest (see Gruber et al. 2004a). The parameter “permafrost” could also explain the development of collapses in cold and deemed stable north faces. The average altitude of scars on north-facing slopes is indeed well smaller (3,090 m a.s.l.) than the one of the west-facing (3,270 m a.s.l.) and especially the ones of the east-facing (3,390 m a.s.l.) and southfacing slopes (3,370 m a.s.l.). This asymmetry is consistent with the temperature distribution at and below the surface of steep rock walls (see Noetzli et al. 2007). However, there is no clear trend regarding the orientation of the rock walls affected by the most important rock falls: among the six events with a volume ≥5,000 m3, three have affected south faces, one a west face, one a NE face, and one a north face. Several years of observations are probably necessary to establish a relationship between aspect and volume of the scars.

Mean annual air temperature of 2007 and 2008 at the Aiguille du Midi are quite the same (−7.5°C and −7.8°C, respectively) as the summer temperatures and cannot explain the significant difference in number of events between 2007 and 2008. Only the April mean temperature was really  higher in 2007 (−5.8°C) than in 2008 (−12.2°C). So, the thawing period should have begun earlier in 2007 than in 2008. Concerning precipitations, summer 2007 has  been largely wetter than summer 2008. With higher air temperatures, percolating water in fractures could have more degraded permafrost by advection of heat, in complement of slower heat conduction from the surface (see Gruber and Haeberli 2007). This may explain, at least in part, the difference in number of events (45 rock falls in 2007, 21 in 2008).

First, data were collected in 2005 through observations made by a small number of Italian and French mountain guides. Since 2007, the observer network is operational with about 30 French and Italian guides and additionally several hut keepers and rescue teams. The Swiss and SW sides of the massif are not surveyed. In addition, educational posters in huts and a website (http://edytem.univ-savoie.fr/eboulements) invite mountaineers to send their own observations. A form is filled for each observed rock fall or its deposit, with the characteristics of the event: date, location, weather and snow conditions, and volume. For each year, data on identified events have been verified and completed on the field by the beginning of autumn by one of the first authors to ensure a good homogeneity of the recorded data. Furthermore, for 2007, the number of rock falls that formed supraglacial deposits has also been checked using aerial photographs at 1:20,000, dated September 16, 2007. For the 2 years, this checking phase has not revealed rock falls that were not reported by the observer network, even in less frequented areas of the massif.

For each event, scar elevation, slope angle, and aspect of the affected slopes are calculated using GIS […]. Deposits have been mapped on the field or from aerial photographs for 2007, even for the smallest rock falls which usually produce deposits of several hundreds of square meters. […] The collapsed volumes and the maximum scar depths have been computed from the dimensions of the scars, surveyed on the field with Laser Technology TruPulse 200 laser rangefinder or, when impossible, from altitudes reported on scar photographs. Thus, the maximum depths are sometimes unknown, often given a minima, and the uncertainty on volumes may reach 25%.

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

Références citées :

Casty C, Wanner H, Luterbacher J, Esper J, Böhm R (2005) Temperature and precipitation variability in the European Alps since 1500. Int J Climatol 25:1855–1880

 

Gruber S, Haeberli W (2007) Permafrost in steep bedrock slopes and its temperature-related destabilization following climate change. J Geophys Res 112:F02S18. doi:10.1029/2006JF000547

 

Gruber S, Hoelzle M, Haeberli W (2004a) Permafrost thaw and destabilization of Alpine rock walls in the hot summer of 2003. Geophys Res Lett 31:L13504

 

Noetzli J, Hoelzle M, Haeberli W (2003) Mountain permafrost and recent Alpine rock-fall events: a GIS-based approach to determine critical factors. In: Philipps M et al (eds) Proceedings of the 8th International Conference on Permafrost, Zürich, Switzerland, pp 827–832

 

Noetzli J, Gruber S, Kohl T, Salzmann N, Haeberli W (2007) Three-dimensional distribution and evolution of permafrost temperatures in idealized high-mountain topography. J Geophys Res 112:F02S13. doi:10.1029/2006JF000545

Ravanel L, Deline P (2008) La face ouest des Drus (massif du Mont-Blanc): évolution de l’instabilité d’une paroi rocheuse dans la haute montagne alpine depuis la fin du petit âge glaciaire. Géomorphologie 4:261–272