Numero 2-2008
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Géomorphologie : relief, processus, environnement, 2008, n° 2, p. 87 La répartition du pergélisol dans trois éboulis alpins du massif de la Dent de Morcles (Valais, Alpes suisses) Permafrost distribution in three alpine talus slopes of the Dent de Morcles massif (Valais, Swiss Alps) Kim Pieracci*, Christophe Lambiel*, Emmanuel Reynard* Résumé Des mesures de la résistivité électrique du terrain (sondages à la verticale et « traînés ») associées à des mesures thermiques de la sur- face du sol (température basale du manteau nival hivernal ou Bottom Temperature of the winter Snow cover et enregistrements en continu), ont été effectuées afin de déterminer la répartition et les caractéristiques du pergélisol dans trois éboulis, constitués de frag- ments calcaires, situés à proximité de la limite inférieure du pergélisol discontinu dans le Massif de la Dent de Morcles (Suisse occidentale). Les résultats obtenus indiquent que les résistivités électriques maximales et les températures de surface les plus froides se situent systématiquement dans la partie inférieure des éboulis étudiés alors qu’en direction de l’amont, les résistivités décroissent et les températures augmentent. On en déduit que la probabilité de détecter du pergélisol augmente dans le secteur distal. Cependant, les trois éboulis étudiés révèlent des résultats très différents. Si un pergélisol à forte teneur en glace occupe toute la pente dans le cas de l’ébou- lis des Charmettes, le pergélisol est incertain dans celui du Col du Fenestral. Enfin, la présence de pergélisol est possible dans la partie aval de l’éboulis du Creux du Bouis. Cette dernière configuration est typique, si l’on se réfère à d’autres études effectuées sur les ébou- lis alpins. Dans ce cas, des circulations d’air complexes à l’intérieur de l’éboulis, associées à d’autres facteurs comme la redistribution de la neige par les avalanches et une granulométrie grossière, en sont vraisemblablement la cause. Mots clés : pergélisol, éboulis (calcaire), prospection géoélectrique, Alpes (Suisse). Abstract Sub-surface geolectrical prospecting (vertical soundings, electrical mapping), associated with thermal measurements of the ground surface (Bottom Temperature of the winter Snow cover, continuous thermal measurements) were carried out to determine the distribu- tion and characteristics of permafrost in three limestone talus slopes situated near the lower limit of discontinuous permafrost in the Dent de Morcles Massif (Western Switzerland). The highest electrical resistivities and the lowest temperatures of the ground surface are systematically located in the lower part of the studied talus. In the upper part, resistivity decreases and temperature increases. This suggests that the lower part of the talus probably contains more permafrost than the upper part. However the three investigated talus are quite different. Ice-rich permafrost seems to be present throughout the slope of Les Charmettes talus. In the col du Fenestral talus, permafrost is uncertain whilst permafrost may possibly be present in the lower part of Creux du Buis talus. The presence of permafrost in the lower part of talus is typical of permafrost distribution in alpine talus. This distribution can be explained by complex air circu- lation, associated with other factors such as the snow redistribution by avalanches and the high macro-porosity of the talus. Key words: permafrost, (limestone) talus slope, DC resistivity, Alps (Switzerland). Abridged English version negative thermal anomaly at the foot of the slopes. The ano- Although the geomorphology, sedimentology and strati- maly has first been attributed to redistribution of snow by graphy of talus slopes have been extensively investigated avalanches (Haeberli, 1975). Recent studies (Lerjen et al., (Francou, 1988), the thermal regime of talus has still been 2003 and Luetschg et al., 2003) stress the importance of gra- poorly studied. Nevertheless, several authors (Haeberli, nulometry and graded bedding, which facilitate the overcoo- 1975; Lerjen et al., 2003; Delaloye, 2004; Luetschg et al., ling of the lower part of the talus slopes. At low altitude, se- 2004; Lambiel, 2006) have demonstrated the presence of a veral authors have demonstrated that the overcooling of foot * Institut de Géographie, université de Lausanne, Anthropôle, CH-1015 Lausanne, Suisse. Courriel : [email protected] Kim Pieracci, Christophe Lambiel, Emmanuel Reynard of slopes can be explained by a chimney effect (Delaloye et lanches and the presence of buried ice patches. In most al., 2003; Morard et al., 2008). A chimney effect has also cases, permafrost distribution may be explained by the com- been described in talus within the discontinuous permafrost bination of two or more factors. belt (Delaloye, 2004; Lambiel, 2006; Lambiel and Pierac- In sum, there is a good concordance between results ci, 2008). obtained by thermal and electrical methods: cold thermal Nine talus slopes were investigated in the Grand Chava- regime and permafrost are present mostly at the foot of lard area, in the Dent de Morcles Massif (Western Switzer- slopes. However, the permafrost distribution is quite hetero- land). Lithology is characterised by an alternation of mas- geneous due to the variability of the different controlling sive limestone and limestone schists. Talus slopes formed by factors. The efficiency of the chimney effect strongly massive limestone blocks have a high macro-porosity, whe- depends on the macro-porosity that is highly related to the reas talus slopes formed by limestone schists are much less grain size distribution. Other investigations should be car- porous. All the investigated talus slopes are situated over ried out (2D electrical investigations, seismic refraction, the lower limit of discontinuous permafrost in the area. The thermal measurements) in order to better characterise the talus slopes have been investigated by combining geoelec- permafrost distribution within the investigated slopes. trical methods (DC resistivity soundings, resistivity map- ping) and thermal methods (Bottom Temperature of the win- Introduction ter Snow cover (BTS) measurements and thermal conti- nuous measurements on the ground surface). Jusqu’à ce jour, la plupart des études portant sur les ébou- The results obtained in three talus slopes are presented in lis de la frange inférieure du pergélisol discontinu dans les this paper. The Creux du Bouis talus is a ENE oriented slope Alpes ont porté sur leur morphologie, leur structure et leur situated between 2230 and 2400 m a.s.l. The macro-poro- mise en place incluant des analyses sédimentaires précises sity is high. One DC resistivity sounding and three resistivi- (par exemple Åkerman, 1984 ; Francou, 1988 ; Hinchliffe ty-mapping lines were carried out. The thermal regime was et al., 1998 ; Jomelli et Francou, 2000 ; Sass, 2006). studied through a BTS measurement campaign conducted in W. Haeberli (1975) est le premier auteur à s’être intéressé à March 2005 and the registration of temperature by three la répartition du pergélisol dans les éboulis. Il met en évi- mini-loggers between September 2004 and July 2005. The dence la présence de pergélisol dans la partie basale de lowest temperatures and the highest resistivities are confi- l’éboulis du col de la Flüela (2 400 m, Grisons, Suisse ned at the foot of the slope. Permafrost is probably present orientale) et son absence dans la partie supérieure ; il l’ex- in that part and certainly absent in the upper part of the plique par le rôle des avalanches qui redistribuent la neige slope. The negative thermal anomaly at the foot of the slope vers la partie inférieure de l’éboulis. Ce processus d’accu- is likely due to a chimney effect as the analysis of the ther- mulation induit une fonte tardive de la neige et, par consé- mal regime recorded by the mini-loggers suggests. quent, une anomalie thermique négative en raison du rôle The NW oriented col du Fenestral talus slope is situated isolant de la neige qui, en bas de pente, protège le sol sous- between 2370 and 2600 m a.s.l. The macro-porosity is low. jacent du réchauffement estival. Sur le même site, M. Lerjen Two vertical electrical soundings were carried out and the et al. (2003) concluent toutefois à une faible relation entre la ground surface temperatures were measured between Sep- persistance tardive de la neige et la présence de pergélisol. Ils tember 2004 and July 2005 in two places. There is no remarquent en revanche une bonne corrélation entre la pré- evident correlation between the electrical resistivities and sence de pergélisol et celle de matériaux grossiers à la sur- the temperature of the ground surface. Permafrost could be face de l’éboulis. En raison du granoclassement longitudinal present at the foot of the slope because of the snow redistri- affectant les éboulis d’amont en aval, ils attribuent la pré- bution by avalanches and/or the graded bedding. sence de pergélisol aux différences de texture de l’éboulis. The third talus slope – Les Charmettes – is oriented to De plus, les études de M. Luetschg et al. (2004) montrent Northeast and situated between 2480 and 2610 m a.s.l. The que, dans la partie amont de cette pente, les températures si- morphology is complex and a rock glacier is present at the mulées à l’aide du modèle SNOWPACK (cf. Lehning et al., foot of the talus slope. One DC resistivity sounding and four 1999) sont plus froides que les températures mesurées et electrical mapping lines were realised during summer 2004 que, au contraire, dans la partie aval, elles sont plus chaudes and a BTS measurement campaign was carried out in March que celles qui sont mesurées. Ainsi, il ressort de ces études 2005. The thermal regime of the slope was investigated by que d’autres facteurs que la répartition de la neige et la gra- using mini-loggers at three places. Permafrost is probably nulométrie des éboulis se combinent probablement pour present in the whole slope and we suggest that the high resis- créer l’inversion thermique observée.