Florence Magnin

Ice Loss and Slope Stability in High-Mountain Regions

The present time is one significant stage in the adjustment of mountain slopes to climate change, and specifically atmospheric warming. This review examines the state of understanding of the responses of mid-latitude alpine landscapes to recent cryospheric change, and summarizes the variety and complexity of documented landscape responses involving glaciers, moraines, rock and debris slopes, and rock glaciers. These indicate how a common general forcing translates into varied site-specific slope responses according to material structures and properties, thermal and hydrological environments, process rates, and prior slope histories. Warming of permafrost in rock and debris slopes has demonstrably increased instability, manifest as rock glacier acceleration, rock falls, debris flows, and related phenomena. Changes in glacier geometry influence stress fields in rock and debris slopes, and some failures appear to be accelerating toward catastrophic failure. Several sites now require expensive monitoring and modeling to design effective risk-reduction strategies, especially where new lakes as multipliers of hazard potential form, and new activities and infrastructure are developed.

The morphodynamics of the mont blanc massif in a changing cryosphere: a comprehensive review

Abstract One of the most glacierized areas in the European Alps, the Mont Blanc massif, illustrates how fast changes affect the cryosphere and the related morphodynamics in high mountain environments, especially since the termination of the Little Ice Age. Contrasts between the north‐west side, gentle and heavily glaciated, and the south‐east side, steep and rocky, and between local faces with varying slope angle and aspect highlight the suitability of the study site for scientific investigations. Glacier shrinkage is pronounced at low elevation but weaker than in other Alpine massifs, and supraglacial debris covers have developed over most of the glaciers, often starting in the nineteenth century. Lowering of glacier surface also affects areas of the accumulation zone. While modern glaciology has been carried out in the massif for several decades, study of the permafrost has been under development for only a few years, especially in the rock walls. Many hazards are related to glacier dynamics. Outburst flood from englacial pockets, ice avalanche from warm‐based and cold‐based glaciers, and rock slope failure due to debuttressing are generally increasing with the current decrease or even the vanishing of glaciers. Permafrost degradation is likely involved in rockfall and rock avalanche, contributing to the chains of processes resulting from the high relief of the massif. The resulting hazards could increasingly endanger population and activities of the valleys surrounding the Mont Blanc massif.

Determination of warm, sensitive permafrost areas in near‐vertical rockwalls and evaluation of distributed models by electrical resistivity tomography

Alpine rockwalls with warm permafrost (near 0°C) are the most active rockfall detachment zones in the Mont Blanc massif (MBM, French Alps) with more than 380 recent events. Near-vertical rockwall permafrost is spatially controlled by variations in rock fractures, snow cover, and microtopography. A reliable method to validate the distribution of permafrost in critical and unstable areas does not yet exist. We present seven electrical resistivity tomography (ERT) surveys measured on five near-vertical rockwalls in the MBM from 2012 and 2013 that have been calibrated with measurements on a granite sample in the laboratory. ERT shows consistent measurements of remaining sensitive permafrost relating to inferred temperatures from 0 to −1.5°C. ERT results demonstrate evidence of topographic controls on permafrost distribution and resistivity gradients that appear to reflect crest width. ERT results are compared to two permafrost index maps that use topoclimatic factors and combine effects of thin snow and fractures, where index model spatial resolution is crucial for the validation with ERT. In cryospheric environments, index maps seem to overestimate permafrost conditions in glacial environments. As a consequence, the sensitive areas of permafrost may slightly deviate from the results from distributed models that are only constrained by topoclimatic factors and interpreted with consideration of local fracture and snow conditions. This study demonstrates (i) that the sensitive and hazardous areas of permafrost in near-vertical rock faces can be assessed and monitored by the means of temperature-calibrated ERT and (ii) that ERT can be used for distributed model validation.

Assessment of Permafrost Distribution in the Mont Blanc Massif Steep Rock Walls by a Combination of Temperature Measurements, Modelling and Geophysics

Permafrost degradation is thought as an important triggering factor of rockfall affecting the steep rockwalls of the Mont Blanc massif. We investigate permafrost distribution by a combination of rock temperature measurements, statistical modelling and Electrical Resistivity Tomography (ERT). Mean Annual Rock Surface Temperature (MARST) is predicted at regional scale on a 4-m-resolution DEM by implementing a multiple linear regression model with Mean Annual Air Temperature (MAAT) of the 1961–1990 period and Potential Incoming Solar Radiation (PISR) as explanatory variables. Based on recent studies, we assume that fracturation and heterogeneous snow deposit induce a temperature offset between the surface and depth of negligible annual temperature variations that may reach 3 °C. This assumption is supported by temperature measurements in 11-m-deep boreholes at the Aiguille du Midi (AdM, 3842 m a.s.l) and verified with ERT measurements. The underlying hypothesis is that permafrost occurs below MARST ranging up to 3 °C. The preliminary results suggest that permafrost possibly occurs even below warmer MARST than expected.