Réf. Delaloye & al. 2008 - P

Référence bibliographique complète
9th International Conference on Permafrost (June 29-July 3, 2008, University of Alaska Fairbanks). Recent Interannual Variations of Rock Glacier Creep in the European Alps. DELALOYE R., PERRUCHOUD E., AVIAN M., KAUFMANN V., BODIN X., HAUSMANN H., IKEDA A., KAAB A., KELLERER-PIRKLBAUER A., KRAINER K., LAMBIEL C., MIHAJLOVIC D., STAUB B., ROER I., THIBERT E. Vol. 1, p. 343-348.

Abstract: Recent interannual variations of rock glacier surface motion are compared for 16 landforms monitored for a few years in various parts of the European Alps. Large fluctuations have been observed particularly since 2002. Most investigated rock glaciers have shown a similar behavior whatever their location in the Alpine arc, their size, or their velocity. The observed interannual variations appear to be primarily related to external climatic factors rather than to internal characteristics. They are mostly well correlated with mean annual ground surface temperature shifts with a delay of a few months, reflecting the thermal wave propagation deeper into permafrost. Seasonal factors may also play a significant role: a lower intensity of winter ground freezing and/or a larger winter snow accumulation appear to facilitate a higher rate of rock glacier surface motion.

Mots-clés
Creep, European Alps, interannual variations, rock glaciers, surface motion.

Organismes / Contact
Department of Geosciences, Geography, University of Fribourg, Switzerland.
Institute of Remote Sensing and Photogrammetry, Graz University of Technology, Austria.
Institute of Alpine Geography, Joseph Fourier University, Grenoble, France.
Institute of Geodesy and Geophysics, Vienna University of Technology, Austria.
Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan.
Department of Geosciences, University of Oslo, Norway.
Institute of Geography and Regional Science, University of Graz, Austria.
Institute of Geology and Paleontology, University of Innsbruck, Austria.
Institute of Geography, University of Lausanne, Switzerland.
Institute of Geography, University of Bern, Switzerland.
Department of Geography, University of Zurich, Switzerland.
Cemagref, Grenoble, 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 (rock glaciers)    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland, Austria, France Alps 16 rock glaciers   2310-2890 m a.s.l. 2000-2007

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
The 1987-2007 mean annual air temperature (MAAT) in the Alps was on average 1°C to 2°C warmer than during the preceding decades. Since 2000, two extreme, warm climatic events affected the Alpine region: the heat wave of summer 2003 and the 2006/07 warm swell, namely 15 months of quasi continuous, large, positive temperature anomaly between April 2006 and June 2007 (except August 2006). The 2003 heat wave induced a rapid warming of MAAT towards values exceeding those of earlier and later years (except in the Austrian Alps), and the 2006/07 event resulted in an exceptionally high MAAT, which exceeded everywhere the 2003 value by 0.5°C to 1.5°C.
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Mean annual air temperature (12-month running mean) standardized at 2500 m a.s.l. (gradient: -0.56°C/100 m). Region 1: St. Christophe meteorological station, 1570 m a.s.l. (Météo-France). Region 2/3: Grand-St-Bernard, 2472 m a.s.l. (MeteoSwiss). Region 4: Säntis, 2490 m a.s.l. (MeteoSwiss). Region 5/6: Sonnblick, 3105 m a.s.l. (ZAMG - Central Institute for Meteorology and Geodynamics).

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
Rock glacier temperatures
The snow cover buffers the ground-atmosphere heat exchanges for a long part of year, varying annual snow cover conditions alter the behavior of MAGST in comparison with MAAT (mean annual air temperature): both are not evolving in a parallel way. Despite the effect of snow, a homogeneous MAGST behavior is observed at the regional scale (Delaloye & Monbaron, 2003). The larger variations of MAGST are also similar at the interregional scale in the Alps with, however, some differences in 2005-2006 (colder values in the western regions 1-3). Three periods of warmer MAGST occurred in 2001, 2003/04 and 2007, with the 2003/04 event as the warmest.

Both 2003 and 2006/07 warm climatic events did not affect MAGST in the same way. In 2003 a strong MAGST increase already had occurred during the winter due to earlier snow insulation (Vonder Mühll et al., 2007); a second phase of warming succeeded in summer and was caused by the heat wave. An extremely high MAGST value was reached by the end of 2003. In 2006/07, the situation was the reverse. Later and lesser snowfalls in early winter favoured the cooling of the ground surface in spite of the persistence of mild air temperature. The extremely high MAAT was strongly attenuated in MAGST.

Rock glacier velocities
Despite variable size, morphology, complexity of flow field, mean annual velocity, seasonal rhythm, etc., the compared rock glaciers have shown a rather homogeneous and synchronous behavior. Three phases of higher creep rate can be quoted in 2000/01, 2003/04, and 2006/07. They all followed immediately a period of warmer MAGST.

Every rock glacier reached an absolute or relative maximal creep rate in 2003/04 before a drastic drop occurred for most of them between 2004 and 2006 (up to -81%). The drop was generally stronger to the west (Regions 1-3) than to the east (Regions 5-6). Stationary or slightly increased velocities were observed everywhere in 2006/07.

The situation in 2000/01 was more contrasted. A peak rate of deformation occurred in Region 4 and was also almost reached in Region 1. Relative maxima are reported from Regions 3 and 6 (no data in Region 2) whereas no maximum is documented in Region 5. Neighboring regions did not display similar behavior in rock glacier movement.
Modélisations
 
Hypothèses
Neglecting small differences in interannual variations that may be due to either internal and topographical characteristics of the rock glaciers or to the variability of the seasonal rhythm or to a shift in the measurement date, the rather homogeneous behavior of Alpine rock glaciers allows us to state that: (1) the driving processes are likely to be the same for all observed rock glaciers; (2) there should be a common climatic control of the permafrost creep rate; (3) a dominant effect of active layer solifluction or surface boulder creep can be excluded, as corroborated, for instance, by borehole deformation measurements on the Trais Fluors rock glacier (Ikeda et al., 2008).

As the interannual changes of surface motion appear to be mostly correlated to the evolution of the MAGST with a delay of a few months (the warmer the MAGST, the larger the velocity) one can infer that they should be caused by a thermally induced process. The delay being not long enough for the surface thermal signal to penetrate deeply into permafrost, interannual changes would thus be primarily caused by shifts in the deformation rate of shallow permafrost layers mostly located (if existing) above the shear horizon.

Higher creep rates should also be caused by the development of a thick winter snow cover, which during early summer provides more meltwater to penetrate into a warm rock glacier system.

Changing interannual creep rates may finally be related to the intensity of winter ground freezing. Where data is available, the maximal activity of rock glaciers appears to be linked to lower freezing index values.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Rock glacier velocities are influenced by air temperature
The 16 observed rock glaciers are located in six distinct regions:
Region 1- Southwestern Alps (France): Laurichard rock glacier;
Region 2- Western Swiss Alps (Valais): Mille, Aget-Rogneux, Mont-Gelé B and C, Tsarmine, Becs-de-Bosson, HuHH1 and HuHH3 rock glaciers;
Region 3- Northwestern Swiss Alps (Bernese Alps): Furggentälti rock glacier;
Region 4- Eastern Swiss Alps (Upper Engadine): Büz North rock glacier;
Region 5- Western Austrian Alps: Ölgrube and Reichenkar rock glaciers;
Region 6- Central Austrian Alps: Weissenkar, Hinteres Langtalkar and Dösen rock glaciers.

Interannual velocities are surveyed by terrestrial measurements (geodetics or real-time kinematic GPS) with accuracy in the mm to cm range. The annual campaign at each rock glacier is carried out as closely as possible on the same date - ideally by late summer - in order to avoid an effect of potentially strong seasonal variations on the reliability of the data. Ten to more than 100 marked points, covering part or the whole of the rock glacier or disposed along longitudinal and/or transversal profiles, are surveyed. The compared value is then the mean horizontal velocity for all moving points with uninterrupted series.

There are no boreholes on the 16 observed rock glaciers, except a shallow one (6 m) at Trais Fluors. Ground surface temperature is nevertheless monitored on several of them. Variations of the mean annual ground surface temperature (MAGST) are used as a proxy for the permafrost thermal regime at shallow depth, even if the intensity of the winter cooling and summer warming (freezing and thawing index) should not be neglected.

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

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

(4) - Remarques générales
 

(5) - Préconisations et recomandations
 

Références citées :

Delaloye, R. & Monbaron, M. 2003. Snow effects on recent shifts (1998-2002) in mean annual ground surface temperature at alpine permafrost sites in the western Swiss Alps. Eighth Conference on Permafrost, Zurich 2003, Extended Abstracts : 23-24.

Ikeda, A., Matsuoka, N. & Kääb, A. 2008. Fast deformation of perennially frozen debris in a warm rock-glacier in the Swiss Alps: an effect of liquid water. Journal of Geophysical Research (in press).

Von der Mühll, D., Noeztli, J., Roer, I., Makowski, K. & Delaloye, R. (eds). 2007. Permafrost in Switzerland. PERMOS Re­ports 4 & 5 (2002/03 & 2003/04). Swiss Acad. of Sciences.