Permafrost distribution and characteristisation of the Mont Blanc massif rockwalls : an approach combining monitoring, modelling and geophysics

• Main Author: Magnin, Florence
• Other Authors: Environnements, Dynamiques et Territoires de la Montagne (EDYTEM), Centre National de la Recherche Scientifique (CNRS)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry ), Université Grenoble Alpes, Jean-Jacques Delannoy, Philip Deline
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2015
• Subjects: Permafrost; Rockwalls; Massif du Mont Blanc; Monitoring; Modelling; Geophysics; Modélisation; Parois rocheuses; Géophysique; [SHS.GEO]Humanities and Social Sciences/Geography; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Mont Blanc; permafrost
• Online Access: https://tel.archives-ouvertes.fr/tel-01409950; https://tel.archives-ouvertes.fr/tel-01409950/document; https://tel.archives-ouvertes.fr/tel-01409950/file/MAGNIN_Florence_2015_archivage.pdf

Rockwall permafrost investigation is essential for understanding of its role in the triggering of rock falls. To estimate permafrost distribution in the Mont Blanc massif rockwalls, we developed three axes of research based on three methods. The monitoring of rockwall temperatures at the pilot-site, Aiguille du Midi, at the surface since 2005, and down to 10-m-deep since 2010, reveals local characteristics of permafrost: warm (>-2°C) and cold permafrost areas coexist, active layer ranges from ca. 2 m and ca. 6 m according to exposure, lateral heat fluxes, variables effects from snow and fractures. The statistical modelling of permafrost distribution suggests sporadic occurrences of permafrost from 1900 m asl. on north faces and 2300 m asl. on south faces in locally favourable conditions, its presence is more continuous from 2600 and 3000 m asl., respectively. The model has been implemented on a 4-m-resolution DEM and has been comapred to the same model based on a 30-m-resolution DE%: the metric resolution appeared necessary for a relevant prediction at the local scale. Finally, electrical resistivity tomography has been applied on eight 160-m-long and 25-m-deep profiles on six subvertical rock faces, two of which being repeated in 2012 and 2013. The results allow for (ii) a 2D characterisation of permafrost: identification of warm permafrost areas, of the topographical control on sharp crests, suggestion of snow and fracturing effects; and (ii) evaluation of the 4-m-resolution model  the 30-m-resolution being to coarse. Even though the model seems realistic, it overestimates permafrost occurrence in areas characterized by the presence of glaciers, certainly because of the reflected and diffuse radiations that are intensified by the glacial surface. Our results have been used to analyse the distribution of 523 rockfalls triggered from high alpine rockwalls in 2003 and from 2007 to 2014. Nearly 90% of these rockfalls have been triggered in warm permafrost rockwalls, which corroborates that permafrost ...