Réf. Fischer & al. 2006 - A

Référence bibliographique complète
FISCHER L., A. KÄÄB, C. HUGGEL, J. NOETZLI. Geology, glacier retreat and permafrost degradation as controlling factors of slope instabilities in a high-mountain rock wall: the Monte Rosa east face. Natural Hazards and Earth System Science, 2006, vol 6, pp. 761-772.

Abstract : Since the end of the Little Ice Age (about 1850), the hanging glaciers and firn fields of the Monte Rosa have retreated continuously. During recent decades, the ice cover of its east face experienced an accelerated and drastic loss in extent. Some glaciers have completely disappeared. New slope instabilities and detachment zones of gravitational mass movements developed and enhanced rock fall and debris flow activity have been observed. This study is based on multidisciplinary investigations and shows that most of the detachment zones of rock fall and debris flows are located in areas, where the surface ice disappeared only recently. Furthermore, most of these detachment zones are located in permafrost zones, for the most part close to the modelled and estimated lower boundary of the regional permafrost distribution, where presumably warm and degrading permafrost exists. Many detachment zones are situated in transition zones between orthogneiss and paragneiss. The formation of detachment zones mostly seems to be caused by a combination of different factors. The presence of lithological transitions zones seems to be a crucial factor for the development of instabilities in rock.

Mots-clés

Glacier retreat, permafrost degradation, geology, slope instabilities, detachment zones, permafrost models, Monte Rosa


Organismes / Contacts

Glaciology and Geomorphodynamics Group, Department of Geography, University of Zurich, luzfisch@geo.unizh.ch.


(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 Mass movements, glacial hazards

Rock falls, ice avalanches, debris flows


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

Italian Alps

Monte Rosa

East face

2200-4500m asl

1885-2005


(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
The reconstruction of the glacier retreat based on the first approach shows the continuous retreat of hanging glaciers and firn fields since the end of the Little Ice Age. Unlike the strong retreat of many valley-type glaciers since about 1850, the changes of the steep glaciers in the Monte Rosa east face were not very distinctive from this time until the 1980s. However, during the last few decades an accelerated loss in extent of the ice cover becomes evident. Some glaciers (or parts of glaciers) disappeared within only a few years and they seem to decay through mass wasting.
Observations

The results of the second approach reveal a slight but progressive deglaciation in the Monte Rosa east face since 1956 and in some parts of the face a drastic loss of the ice-covered area in the last 10-15 years. The analysis of the orthophotos also reveals an occasional increase in the extent of certain firn fields and some hanging glaciers. Together, the two methods give an overview over the glacier retreat and reveal the areas with the most pronounced changes in glaciation.

Modélisations

Applying PERMAKART, the lower boundary of the possible permafrost occurrence (where locally permafrost may occur) is modelled at an altitude of 2700-2800m a.s.l. The lower limit of probable permafrost (where continuous permafrost is supposed) is located between 2900 and 3200m a.s.l., depending on the exposition and inclination of the rock wall. The PERMAKART model tends to overestimate the permafrost distribution in steep rock and, hence, can be consulted for an indication of the maximum permafrost occurence in the Monte Rosa east face.

Based on the ROCKFROST calculations, the lower boundary of the permafrost distribution is estimated to be between 3000-3300m (Jungfrau) and 3200-3500m (Corvatsch) depending on the exposition and inclination of the rock wall. The two classes Jungfrau and Corvatsch correspond to inner and slightly colder northern Alpine climate conditions, respectively. For the lower limit of permafrost occurrence, the model results for northern Alpine conditions are regarded to be the more likely for near-vertical snow-free areas of such steep rock walls like the Monte Rosa east face. However, the ROCKFROST model might slightly underestimate the permafrost distribution and the uncertainties of this approach are in the range of ±2°C, which corresponds to roughly ±200m vertically.

Together, the models indicate the sensitive areas around the lower boundary of the permafrost occurrence that can be localised between about 3100 and 3600m a.s.l. This large range is mainly due to the different aspects found on the flank.

Hypothèses
Since the Little Ice Age maximum (about 1850), the lower permafrost limit is estimated to have risen vertically by about 1m/year (Frauenfelder, 2005).

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Permafrost occurrence regarding the air temperature, snow cover and altitude


Permafrost occurrence and rock temperature

To assess the permafrost distribution in the Monte Rosa east face and its possible linkage to the slope stability problems, 2 different models were applied aiming at estimating the lower boundary of the permafrost occurrence.

In a first approach the model PERMAKART was applied. It is based on the so-called “rules of thumb” to predict permafrost occurrences. PERMAKART primarily considers radiation effects as related to aspect, air temperature as related to altitude and snow cover as related to slope foot areas (long lasting snow cover caused by avalanche deposits). These empirical rules have been first fitted to the Monte Rosa region using the 0°C isotherm altitude and temperature gradient from the meteo station of Plateau Rosa/Testa Grigia at 3488m asl. Based on a MAAT of -5.8°C of the Plateau Rosa station and lapse rate of 0.57°C/100m, a 0°C isotherm altitude of 2470m asl results. The calculations were based on a 25m gridded DEM with a vertical accuracy of 4-6 m.

A second model has been deduced from rock temperature calculations and called ROCKFROST. On the basis of meteo data, energy fluxes were modelled and eventually the spatial distribution of mean annual rock surface temperatures was calculated for climate conditions in the central and northern Alps for a time period of 1982-2002. By approximating the relation between the elevation of the 0°C isotherm and aspect with a polynomial function for different slope values, the permafrost occurrence in steep rock can be assessed within a GIS. For these calculations the same DEM was used as in the first approach. Grid cells with a minimum slope of 45° were considered as steep rock.

The glacier extents for different years were reconstructed by 2 approaches using different data sets. Both approaches were conducted by digitising glacier contours for different years since the early 20th century.

In a first approach, glacier extents were reconstructed on the basis of field observations in summer 2003, various old oblique photos since 1885 and a historical topographic map. The procedure is mainly based on the visual comparison of different photos. The delineation of glacier extent in 1982 and 1999 was exclusively based on oblique photos. The glacier extent of 2003 was mapped during the field work and from oblique photos.

In a second approach, air-photos of the years 1956, 1977, 1988, 1999 and 2001 were orthorectified and map-derived ground control points.


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

Over the recent two decades, the mass movement activity in the Monte Rosa east face has drastically increased and new detachment zones of rock falls, debris flows and ice avalanches have developed. The detachment zones of rock fall are situated in the very steep rock walls above 3500m a.s.l. and are distributed over the whole Monte Rosa east face.

During fieldwork in the extraordinary hot summer 2003, rock fall events from different detachment zones could be observed almost every day. Even during the winter the gravitational mass movements – though reduced – take place. This indicates in some places a strongly reduced stability in the bedrock and shows that rock fall activity is not only affected by thawing and melting water during summertime, but rather by changed glacier, permafrost and bedrock conditions.

All detachment zones of rock falls and debris flows are situated in the upper part of the rock wall, between 3400 to 4100m a.s.l. The comparison of changes in glacier extent with current detachment zones shows that many rock fall and most debris flow events originate in recently deglaciated parts of the Monte Rosa east face. An important observation is the spatial shifting of the active detachment zones with decreasing glacier extent. This observation points out that changes in glacier extent might affect slope stability significantly due to drastic changes in surface and also subsurface conditions in the deglaciated areas. The major ice avalanches observed during recent decades occurred particularly in zones with significant glacial changes. But the reactions of hanging glaciers on a rise in atmospheric temperature are varying considerably.

In addition, a concentration of detachment zones at transition zones between orthogneiss and paragneiss becomes apparent. (The geology of the Monte Rosa east face is characterized by layers of two different lithologies: orthogneiss and paragneiss.) This indicates that the transition zones between the two lithologies could favour or cause instabilities because of the different geotechnical properties of the two lithologies. The detachment zones of ice avalanches are also situated in the upper part of the flank, but most of them cannot directly be related to the geological setting. In some zones, though, where ice avalanches are influenced by rock fall and debris flows, lithological transition zones may have an indirect impact on the glaciers.

Modélisations
 
Hypothèses

Modelling analyses suggest a probable linkage between permafrost degradation and the formation of detachment zones. Many detachment zones of the present rock fall and debris flow events and also some starting zones of ice avalanches are situated in areas of most probably warm permafrost at the lower boundary of permafrost occurrence. This fact confirms the assumption, that instabilities may be formed in part due to increased temperatures in warm permafrost occurrence, which in turn may lead to decreased shear strength in the rock wall and enhanced water pressure. However, some of the detachment zones of rock fall are situated at higher altitudes where occurrence of cold permafrost is predicted. This suggests that not all rock fall events are directly connected to changes in ground-thermal conditions. A rise in permafrost temperatures may also laterally influence the thermal regime of hanging glaciers and have a destabilizing effect on cold hanging glaciers. Rising temperatures can thereby induce higher ice temperatures and more percolating melt water at the glacier bed and thus increase the stresses at the front of steep glaciers.


Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Slope instabilities frequency Geology, glaciation and permafrost

The 3 factors were analysed separately to assess the recent changes and contemporary conditions and the results were compiled for a comparison with the current detachment zones of the ongoing mass movements. These analyses have been done visually by comparing the 3 analysed factors with the positions of the actual detachment zones.

The detachment zones were detected and analysed based on the combination of daily visual observations during fieldwork (summers 2003 and 2004), photogrammetry and oblique photos. Detachment zones, transfer channels and frequency of the gravitational mass movement processes were observed and recorded during fieldwork. The detachment zones have been classified in 2 groups, those from ice avalanche and those from rock falls/debris flows. The geological setting of the Monte Rosa east face was investigated during fieldwork and a geological map was compiled.


(4) - Remarques générales

 


(5) - Syntèses et préconisations

In the view of ongoing or even enhanced atmospheric warming and associated changes it is therefore very likely that the slope instabilities in the Monte Rosa east face will continue to represent a critical hazard source.

Référence citéés :

FRAUENFELDER, R.: Regional-scale modelling of the occurrence and dynamics of rockglaciers and the distribution of paleopermafrost, Schriftenreihe Physische Geographie, Glaziologie und Geomorphodynamik, University of Zurich, 2005.