Christophe Lambiel

Evidence of winter ascending air circulation throughout talus slopes and rock glaciers situated in the lower belt of alpine discontinuous permafrost (Swiss Alps)

The winter ascending circulation of air throughout an accumulation of coarse slope sediments (the so-called chimney effect) facilitates the cooling of the ground and even the occurrence of permafrost in the lower part of a deposit. Simultaneously, any freezing is unlikely to occur in the upper part. To date, the chimney effect has been reported mainly for cold and sometimes perennially frozen scree slopes situated at low elevations, far below the regional limit of the discontinuous mountain permafrost. This article reports observations made recently in the western Swiss Alps in several accumulations of coarse sediments (talus slopes, relict or inactive rock glaciers) located at higher elevations (2200–2800 m a.s.l.) within the belt of discontinuous permafrost or close to its lower limit. These observations show that a chimney effect may also occur in debris accumulations situated at ‘usual’ mountain permafrost elevation. This gives rise to multiple questions, in particular about the impact of the chimney effect on both the thermal regime and the spatial distribution of discontinuous mountain permafrost.

Investigating decadal‐scale geomorphic dynamics in an alpine mountain setting

We know little about the effect of recent climate variability upon landscapes at the timescale of decades because of (1) the complex, nonlinear, and path-dependent nature of the response of a landscape to climate forcing and (2) the difficulty of quantifying spatially distributed impacts at the timescale of decades to centuries, despite this being the timescale over which significant hypotheses have been raised over human impacts upon climate change and hence geomorphic systems. A unique resource to investigate the linkages between climatic variability and geomorphic response is provided by the extensive coverage of aerial imagery commonly available since the 1950s. Here we use archival digital photogrammetry to produce high-precision digital elevation models over large spatial scales, and so to reconstruct the quantitative history of surface downwasting and sediment flux in a high mountain alpine system, over the timescales of decades. Propagation of error methods is used to identify locations of significant landscape response and to compute volumes of significant surface change. Orthorectified aerial images are used in an image correlation framework to detect horizontal and vertical displacements of components of the landscape. Results are coupled to extant climate data and modeled snow cover to show how the landscape responds to climate forcing and to geomorphological maps to understand how this response varies between landscape elements. The results show distinct landscape response to both warming and cooling periods and a tendency for the acceleration of surface displacement under warming conditions. Precipitation and snow cover are critical in controlling glacier dynamics and rock glacier displacement velocities. However, while some landforms might lead to locally high sediment flux, landscape heritage can disconnect zones of high change rates from the valley bottom. Hence, the landscape response to climate forcing is not necessarily reflected in valley system processes or sediment deposits.

Instability of a High Alpine Rock Ridge: the Lower Arête Des Cosmiques, Mont Blanc Massif, France

Abstract Rockfalls are dominant in the rock slopes and rock ridge morphodynamics in high mountain areas and endanger people who pass along or stay there, as well as infrastructure that host them (cable cars, refuges). Risks are probably greater now because of fast permafrost degradation and regression of surface ice, two consequences of the atmospheric warming of the last decades. These two commonly associated factors are involved in the instability of rock slopes by modifying the mechanical behaviour of often ice‐filled rock fractures and the mechanical constraints in the rock masses. This paper examines over 15 years the instability of the lower rête des osmiques on the rench side of the ont lanc massif. Its vulnerability is due to the presence of a high‐capacity refuge on its top (3613 m a.s.l.). In 1998, a part of the refuge was left without support when a collapse of 600 m3 occurred immediately below it. Since this date, reinforcement work has been carried out in this area, but the whole ridge has been affected by around 15 relatively shallow rockfalls. Through a multidisciplinary approach, this article assesses the role of the cryospheric factors in the triggering of these rockfalls.

Regional modelling of present, past and future potential distribution of discontinuous permafrost based on a rock glacier inventory in the Bagnes-Hérémence area (Western Swiss Alps).

A regional model was used to draw the permafrost distribution in the 200 km 2 of the Bagnes-Hérémence area (Western Swiss Alps). The model is based on the fact that permafrost distribution depends mainly on altitude and orientation and that the minimal altitude of active/inactive rock glaciers can be used as an indicator of the lower limit of discontinuous permafrost. The lower limit of relict rock glaciers is also used as an indicator of past distribution of permafrost. An inventory of rock glaciers was therefore made in the study area. The lower limit of permafrost during the Younger Dryas was determined by comparing the position of relict rock glaciers and glacier extension during the Older Dryas. The model was then applied to four periods (Younger Dryas, Little Ice Age, current period and future) in order to show the temporal evolution of permafrost distribution and glacier extension.

Geomorphology of the Hérens valley (Swiss Alps)

This paper presents a geomorphological map of the Hérens valley in the Western Swiss Alps. With an area of 270 km2 and altitudes ranging from 470 to 4357 m a.s.l., this valley is one of the main secondary catchments of the Upper Rhône valley. The high differences in altitudes, combined with a varied geology, create an important geomorphic diversity. The main processes active in mountain areas, that is, glacial, periglacial, gravitational and fluvial processes, are well represented. The map was produced in ArcGIS using a specific legend developed at the University of Lausanne.

Internal Structure and Current Evolution of Very Small Debris-Covered Glacier Systems Located in Alpine Permafrost Environments

This contribution explores the internal structure of very small debris-covered glacier systems located in permafrost environments and their current dynamical responses to short-term climatic variations. Three systems were investigated with electrical resistivity tomography and dGPS monitoring over a 3-year period. Five distinct sectors are highlighted in each system: firn and bare-ice glacier, debris-covered glacier, heavily debris-covered glacier of low activity, rock glacier and ice-free debris. Decimetric to metric movements, related to ice ablation, internal deformation and basal sliding affect the glacial zones, which are mainly active in summer. Conversely, surface lowering is close to zero (-0.04 m yr-1) in the rock glaciers. Here, a constant and slow internal deformation was observed (c. 0.2 m yr-1). Thus, these systems are affected by both direct and high magnitude responses and delayed and attenuated responses to climatic variations. This differential evolution appears mainly controlled by (1) the proportion of ice, debris and the presence of water in the ground, and (2) the thickness of the superficial debris layer.

Data-driven mapping of the potential mountain permafrost distribution

Existing mountain permafrost distribution models generally offer a good overview of the potential extent of this phenomenon at a regional scale. They are however not always able to reproduce the high spatial discontinuity of permafrost at the micro-scale (scale of a specific landform; ten to several hundreds of meters). To overcome this lack, we tested an alternative modelling approach using three classification algorithms belonging to statistics and machine learning: Logistic regression, Support Vector Machines and Random forests. These supervised learning techniques infer a classification function from labelled training data (pixels of permafrost absence and presence) with the aim of predicting the permafrost occurrence where it is unknown. The research was carried out in a 588km2 area of the Western Swiss Alps. Permafrost evidences were mapped from ortho-image interpretation (rock glacier inventorying) and field data (mainly geoelectrical and thermal data). The relationship between selected permafrost evidences and permafrost controlling factors was computed with the mentioned techniques. Classification performances, assessed with AUROC, range between 0.81 for Logistic regression, 0.85 with Support Vector Machines and 0.88 with Random forests. The adopted machine learning algorithms have demonstrated to be efficient for permafrost distribution modelling thanks to consistent results compared to the field reality. The high resolution of the input dataset (10m) allows elaborating maps at the micro-scale with a modelled permafrost spatial distribution less optimistic than classic spatial models. Moreover, the probability output of adopted algorithms offers a more precise overview of the potential distribution of mountain permafrost than proposing simple indexes of the permafrost favorability. These encouraging results also open the way to new possibilities of permafrost data analysis and mapping.

Age constraints of rock glaciers in the Southern Alps/New Zealand – Exploring their palaeoclimatic potential:

Two rock glaciers in the valley head of Irishman Stream in the central Ben Ohau Range, Southern Alps/New Zealand, have been investigated using the electronic Schmidt-hammer (SilverSchmidt). Longitudinal profiles on both features reveal a consistent trend of decreasing R(Rebound)-values and, hence, increasing weathering intensity and surface-exposure age on their numerous transverse surface ridges from rooting zone towards the front. Previously published numerical ages obtained by terrestrial cosmogenic nuclide dating (TCND) allowed the calculation of a local Schmidt-hammer exposure-age dating (SHD) age-calibration curve by serving as the required fixed points. Age estimates for the lowermost rock glacier surface ridges fall within the early Holocene between 12 and 10.5 ka and indicate a fast disappearance of the Late Glacial glacier formerly occupying the valley head, followed by the initiation of rock glacier formation around or shortly after the onset of the Holocene. Although it cannot be judged whether the rock glaciers investigated were active within the entire Holocene or only repeatedly during multiple episodes within, their location and intact morphology exclude any substantial glacial activity at Irishman Stream during the Holocene. This has considerable regional palaeoclimatic implications because it opens for the hypothesis that climatic conditions during early Holocene were possibly comparatively dry and favourable for rock glacier initiation, but less so for glaciers. It would also challenge the view that air temperature is the sole major climate driver of glacier variability in the Southern Alps. More work utilising the palaeoclimatic potential of rock glaciers in the Southern Alps is advised.

Monitoring active rock glaciers in the Western Swiss Alps: Challenges of Differential Sar Interferometry and solutions to estimate annual and seasonal displacement rates

This paper describes a new method to monitor active rock glaciers using DInSAR technique with Terrasar-X data acquired with mode facing slope and 11 days time interval during late summers 2009 to 2011. Where the spatial distribution of the rock glacier surface deformation derived from the DInSAR data using conventional unwrapping processes fails due to the relative small size and the complex movement of some rock glaciers, a specific profile is here defined through the rock glacier and is used to analyze the DInSAR products along it. Firstly, the prerequisites to perform such analysis at local scale are given. Then a quantification and discussion of results are achieved through different examples of active rock glaciers encountered in Western Swiss Alps.

Stability Monitoring of High Alpine Infrastructure by Terrestrial Laser Scanning

Rock mass movements are dominant in the morphodynamics of high Alpine rock slopes and are at the origin of significant risks for people who attend these areas and for infrastructures that are built on (e.g. huts, cable cars). These risks are increasing because of permafrost degradation and glacier retreat as consequences of the global warming. These two factors may affect slope stability by changing mechanical properties of the interstitial ice and modifying the mechanical constraints in these rock slopes. The monitoring of rock slopes is thus an essential element for risk management. Our study focuses on two particularly active areas of the Western Alps: the lower Arete des Cosmiques (3613 m a.s.l., Mont Blanc massif, France) on which is located the very popular Refuge des Cosmiques, and the Col des Gentianes (2894 m a.s.l., Valais, Switzerland) where is located a cable car station. Discussed on the basis of geophysical and glaciological data, the evolutions monitored by terrestrial laser scanning probably result from the combination between permafrost activity/degradation and glacier shrinkage.