Réf. Ravanel & Deline 2011 - A

Référence bibliographique complète

RAVANEL, L., DELINE, P. 2011. Climate influence on rockfalls in high-Alpine steep rockwalls: The north side of the Aiguilles de Chamonix (Mont Blanc massif) since the end of the 'Little Ice Age'. The Holocene, Vol. 21, n°2, 357–365, doi: 10.1177/0959683610374887

Abstract: Rockfalls fundamentally affect the morphodynamics of high mountain rockwalls, and represent a great danger for both people and infrastructures, but still are poorly known. By comparing old, recent and new photographs, in addition to geomorphological field data, we propose an inventory of the rockfalls that occurred since the end of the ‘Little Ice Age’ on the north side of the Aiguilles de Chamonix (Mont Blanc massif), ranging in volume from 500 to 65 000 m3. These 42 rockfalls occurred after 1947, of which > 70% during the last two decades, with a maximal frequency during the warm summers, especially in 2003. Average elevation of scars (3130 m a.s.l.) close to the lower modelled permafrost limit, and the topography (e.g. spurs) of the affected rock faces enhancing lateral heat fluxes, suggest that a climatically driven permafrost degradation has triggered many of the recent rockfalls in high-Alpine steep rockwalls.

Mots-clés

High mountain - Mont Blanc massif - Permafrost - Photographs - Post-‘Little Ice Age’ period - Rockfall - Rockwall

 

Organismes / Contact

• Université de Savoie, 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

 Temperature

 Permafrost

 Mass movements

 Rockfalls

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 France

 Massif du Mont-Blanc

 Aiguilles de Chamonix

NW > North side

 

 1862-2009

 

(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

The rockfalls on the north side between 1862 and 2009

Our rockfall inventory allows the building of the rock face evolution on the north side of the Aiguilles de Chamonix since the end of the LIA. If the oldest photographs show large scars formed prior to 1862 (e.g. on the west face of the Petits Charmoz), rockwalls have not experienced significant changes between the end of the LIA and the late 1940s. The first two rockfalls occurred not before 1947, on the broad west face of the Aiguille de Blaitière, and on the north face of one of its secondary peaks. The largest of the documented rockfalls (total volume: 65 000 m3) occured on 10 September 1947, a very large slab detached along a major fracture between 2900 m and 3150 m a.s.l., marking the west face with a long and clear streak; thus small rockfalls shaped the edges of the scar until September 1952. In the late 1940s, two other rockfalls affected the west face of the Grands Charmoz, by the fall of a small pillar (7000 m3) on the first tower (3265 m a.s.l.) of the NW ridge, by the collapse of a small pile of blocks from the terraces in the middle of the face. Finally, 5000 m3 of rock collapsed in 1954 from the Plaques Burgener area, on the west face of the Aiguille du Grépon.

There were no further rockfalls until the 1970s (18 000 m3) at the base of the NW face of the Aiguille du Peigne (c. 2615 m a.s.l.), with its irregular scar (5000 m3, h =140 m), which affected the upper part of the West face of the Aiguille des Pélerins in 1976. In 1980, the Carpentier pillar – one of the main pillars on the west face of the Aiguille des Grands Charmoz – was affected by a large collapse (20 000 m3, h ~ 150 m). During the early 1980s, three other rockfalls occurred on the north face of the Pointe des Pélerins (12 000 m3), the Aiguille des Pélerins, and the upper part of the Frendo spur on the North face of the Aiguille du Midi (18 000 m3, h = 90 m).

In the 1990s, rockfalls affected the Rognon des Nantillons (25 000 m3), the top of the Aiguille du Fou (4000 m3), the north (50,000 m3) and west faces (7000 m3) of the Aiguille de Blaitière, and the north face of the Aiguille du Midi (three rockfalls ranging in volume from 1500 to 3500 m3). The large 1995 Aiguille de Blaitière rockfall completely removed the overhang resulting from the small 1947 rockfall at the foot of the North face – both collapses controlled by a large fracture, parallel to the surface – while another rockfall affected a part of the top overhang of the 1947–1952 scar on 21 August 1995.

During the decade 2000–2009, 19 collapses occurred in the study area. Among them, a dozen rockfalls occurred on the north face of the Aiguille du Midi, ranging in volume from 1000 to 13 000 m3. In only the summer of 2003, 12 collapses happened on the Aiguilles de Chamonix, small in volume except for a rockfall (15 000 m3), which involved the whole summit of the northern secondary peak of the Aiguille du Fou, and another rockfall (30 000 m3) at the bottom of the west face of the Aiguille du Plan. Four rockfalls occurred during the summers 2007, on the north face of the Aiguille du Midi (10 000 m3 & 3000 m3), and 2009, on the north face of the Pointe des Pélerins and on the Rognon du Plan (4000 m3 & 7000 m3).

Because our rockfall inventory is based on objective data (series of photographs), and not on testimonies or reports, it avoids the bias that would result from a recent increasing awareness of the phenomenon producing better documentation. Moreover, the redness of the rock coating on granite faces is directly linked to the surface exposure time: light grey scars remain visible for more than millennia in the Mont Blanc massif (Böhlert et al., 2008). The sharp contrast in colour of the scars on the rock faces and the high quality of the photographs allow a very detailed analysis of the faces and guarantee an exhaustive inventory of the rockfalls.

Our reconstruction of the rock face evolution on the north side of the Aiguilles de Chamonix since the end of the LIA can then be compared with the climate evolution during the same period.

Coupling between climate and rockfalls

Evidence of permafrost in the Aiguilles de Chamonix rockwalls come from (i) the monitoring of surface rock temperatures at the Aiguille du Midi since 2005 (Deline et al., 2009); (ii) the  modelling of the MAGST; and (iii) the presence of numerous hanging glaciers, characterised by cold-based ice (Gruber and Haeberli, 2007). Climatically driven, rockwall permafrost is sensitive to air temperature. Despite cooling periods during the 1930s, 1960s and 1970s, the mean annual air temperature (MAAT) in the Alps increased by over 1.5°C from 1906 to 2005 (Beniston, 2005), against c. 0.74°C on a global scale (Intergovernmental Panel on Climate Change (IPCC), 2007). The Chamonix-Le Bouchet weather station has collected data since ad 1876 but in a continuous way only since 1934; but MAAT in the Alps has been lower between 1860 and 1930 than after 1930 (Beniston et al., 1997; Casty et al., 2005). Important permafrost degradation is unlikely to have happened before the second third of the twentieth century.

The first period of rockfalls (six events), from the late 1940s to the early/mid 1950s, has been, together with the last two decades, one of the warmest since one century. 1947 had the highest temperature anomaly of this period, with three heatwaves during the summer (one was extreme), and sharp variations of summer precipitation, with the second wettest month in July (136 mm) of the decade and the driest month in August (57 mm). The two largest rockfall (R7 and R10) occurred during and at the end of this summer, when the weather pattern may have triggered the warming of the permafrost well below the active layer, as suggested by the maximum depth of R10 scar (c. 10 m). Both rockfalls could have been prepared – if not triggered for one of them (R7), for which the exact date is unknown – by the two Valais earthquakes in 1946 and the Vallorcine earthquake on 30 August 1947, as two rockfalls (R4 and R5) which occurred in the cold but seismic year 1954.

From the 1970s to date, rockfall frequency has increased. If the precise date for one of them (R21) is not known, another (R23) occurred during the heatwave of the 1976 Summer – the fifth warmest summer in Chamonix since 1860 – likely a result of thickening of the active layer as in 2003 (Gruber et al., 2004b). Several rockfalls took place during the early 1980s, particularly in 1982–1983, with fairly hot summers: summer 1983, with two rockfalls (R17 and R37), was the fourth warmest summer since the end of the LIA. The 29 other rockfalls (70% of inventory) occurred during the two last decades, characterized by an accelerating global warming: two of the hottest summers of the study period, and the seven warmest winters of the period 1934–2009 took place in the 2000s, and seven of the eight hottest years in Chamonix since the end of LIA took place during the last 15 years. During the past two decades, 16 of the 22 precisely dated rockfalls occurred during three of the seven hottest years, and 12 during the 2003 summer heatwave – the hottest summer recorded in Chamonix, in the Alps and in Europe since the first systematic measurements of temperatures (Beniston, 2004; Schär et al., 2004). Because there was no heavy precipitation or earthquake during the summer 2003, temperature-driven thickening of the active layer in rockwalls is the only factor that can explain this increased rockfall occurrence (Gruber et al., 2004b; Schiermeier, 2003).

Glacial debuttressing could have played a role for rockfalls (R6 and R20), the only one detached from the foot of rockwalls, while c. 60% of the rockfalls have affected ridges, spurs and pillars, i.e. areas where the topography accelerates permafrost degradation because of the propagation of strong lateral heat fluxes from rockwalls well exposed to solar radiation (Noetzli et al., 2007). Some collapses (e.g. R12, R13, R16, and R23) may thus be linked to a deepening of the active layer resulting from energy fluxes from warmer rockwalls. Half of the 2003 collapses occurred in the Sector 3 (Aiguille du Midi). This may result from the shrinking or even disappearance of a part of the ice-snow cover under the effect of the summer heatwave, with effects in terms of formation of an active layer (Fischer et al., 2006) and mechanical detachment of rocks.

(…) The relationship between cumulative frequency and volume of the rockfalls over the periods 1862–2009, 1862–1990 and 1990–2009 shows that the last two decades, marked by an acceleration of global warming, have experienced a rockfall frequency much more important than during the period 1862–1990 (…).

Observations

 

Modélisations

 

Hypothèses

By the end of the twenty-first century, several scenarios predict that global MAAT increase will range from 1.8°C to 4.0°C (IPCC, 2007). In the Alps, this temperature increase may reach 4°C to 5°C, with extreme summer temperatures exceeding the current ones by 6–8°C (Beniston, 2003). The 2003 heatwave provides an overview of the kind of summer that could affect the Alps regularly towards the end of the twenty-first century, with a number of hot days being multiplied by five by 2070–2100 (Beniston, 2004). At the same time, the mean winter temperature would increase, with probably a weaker seasonal regeneration of the permafrost. The relationship between cumulative frequency and volume of the rockfalls over the periods 1862–2009, 1862–1990 and 1990–2009 shows that the last two decades, marked by an acceleration of global warming, have experienced a rockfall frequency much more important than during the period 1862–1990, suggesting that frequency of rockfalls at high elevation should increase if the present trend continues.

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

 Frequency / Intensity (volume)

It is a well-known fact that the evolution of alpine glaciers and climate are closely related (Vincent, 2002). The relationship between climate and rockfalls in high mountain rockwalls is not so clearly understood, even though rockfalls may have strong impacts on people, infrastructures, and landscape (Haeberli et al., 1997). This misapprehension is partly explained by a lack of rockfall data, while back analysis of past events is fundamental to assess the role of rockwall permafrost degradation, seismicity and glacial debuttressing in triggering rockfalls in high mountains (Evans and Gardner, 1989) – whereas the geological structure makes the rock face more or less prone to rockfall.

The term ‘permafrost’ refers to any subsurface material at 0°C or less for at least two years. In rockwalls, ice can form or melt in clefts, while air temperature evolves (Gruber and Haeberli, 2007; Haeberli et al., 1997; Harris et al., 2009). Because the strength of the ice lowers as its temperature rises, especially when approaching the fusion point (Davies et al., 2001; Fish and Zaretsky, 1997), rock slope stability can decrease by three processes, with different consequences in terms of volume and frequency: (i) formation of an active layer (i.e. superficial layer with positive temperatures during each summer season) by heat conduction; (ii) deepening of the active layer during one season or several years, by heat conduction; (iii) deep thawing in cleft matrix by heat advection (heat supply by percolating water) – that may be delayed by decades, centuries, or millennia (Gruber and Haeberli, 2007).

Based on a large corpus of documents, Ravanel and Deline (2008) showed a strong correlation between the warmest periods of the last 150 years and the occurrence of the main rockfalls onthe West face of the Drus (Mont Blanc massif), probably related to permafrost degradation. To validate this first result, the authors analysed numerous photographs about the North side of the Aiguilles de Chamonix since the end of the ‘Little Ice Age’ (LIA; c. 1860).

Characteristics of the high-Alpine north side of the Aiguilles de Chamonix: The granitic Aiguilles de Chamonix form a part of the NW side of the crystalline Mont Blanc massif, cut into panels by two main sets of subvertical fractures and faults (N0° and N40°–60°). Consequently, the north side shows steep and fractured rock walls. It forms a 5 km long sequence of peaks from the Mer de Glace to the Glacier des Bossons, regularly exceeding 3500 m a.s.l. and 1000 m high, with an area of 4.12 × 106. Based on their visibility from Chamonix (which determines the availability of photographs), three sectors are distinguished: Sector 1, from the Aiguille des Grands Charmoz (3445 m a.s.l.) to the Aiguille du Plan (3673 m); Sector 2, around the west face of the Aiguille du Plan; Sector 3 fits in the north face of the Aiguille du Midi (3842 m). Some of the rockwalls are ice/snowcovered, with small (e.g. upper Glacier de Blaitière) or large (e.g. north face of the Aiguille du Plan) hanging glaciers.

The “method to reconstruct the morphodynamics of the north side” is explained in the study.

 

(4) - Remarques générales

 

 

(5) - Syntèse et préconisations

Considered individually, the role of permafrost in triggering rockfalls in high-Alpine steep rockwalls is extremely difficult to establish. This role is nevertheless strongly supported by evidence from the analysis of the 42 documented rockfalls (ranging in volume from 500 to 65 000 m3) on the north side of the Aiguilles of Chamonix since 1862: (i) all rockfalls stem from the permafrost area; (ii) a very good correlation between rockfalls and the hottest periods within the study period exists: 70% of the 42 rockfalls took place during the last two decades, characterized by the acceleration of global warming; (iii) the maximal frequency of these collapses occurred during the heatwave of 2003, with no heavy precipitation or earthquakes during this period: this can be only explained by the degradation of permafrost; more generally, warm summers (1947, 1976, 1983, 2003) are periods of rockfall triggering; (iv) the average elevation of scars (3130 m a.s.l.) is close to the lower permafrost limit, where degradation is potentially more active; (v) convex topography (ridges, spurs, and pillars), characterized by heat fluxes from well-exposed rock faces, seem to be prone to collapse likely because of a faster permafrost degradation.

A climatic control over topography is thus detectable in topographic data and triggering of most of the recognized rockfalls in the Aiguilles de Chamonix since the end of the LIA is probably controlled by current degradation of permafrost.

Within the actual context of global warming prediction for the twenty-first century, rockfalls are expected to occur more frequently.

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