| Summary: | Poster Rockfall is a major natural hazard in high mountain regions and its frequency is growing, especially since two decades. Given the lack of systematic data on rockfalls, the relationship between permafrost degradation and rockfall has however remained difficult to assess. Data on rockfall were acquired for 2003 and the period 2007-2012 thanks to a satellite image of the Mont Blanc massif and a network of observers (mountain guides, hut keepers and mountaineers) in its central part (57 % of massif area), respectively. To ensure a higher completeness of the inventory, fieldwork is conducted every fall, whereas Summer 2003 rockfalls in the whole massif were identified from their supraglacial deposits. Rockfall parameters were calculated in a GIS. 182 and more than 330 rockfalls (maximal volume: 43 000 m3) occurred during the Summer 2003 heatwave and the period 2007-2012 respectively. Most of the rockfalls occur during the hottest months of the year. Modellingsuggests the presence of permafrost in nearly all of the affected rockwalls, and massive ice was observed in at least 45 scars during the period 2007- 2012. The very high frequency of the Summer 2003 rockfalls can only be explained by permafrost degradation. Several other elements support permafrost degradation as main triggering factor of rockfall: mean elevation of 2003 and 2007-2012 scars (3335 m a.s.l.) is much higher than the mean rockwall elevation (2880 m a.s.l.) while very few detachments occur below 3000 m a.s.l., which suggests that the main triggering factor is not ubiquitous; most affected altitudinal belt is 3200-3600 m a.s.l., with modeled warm permafrost (> -2°C); the hotter the summer, the higher the scar elevation; sharp contrast in scar elevation between north and south faces; rockfall especially affects topography prone to permafrost degradation such as pillars, spurs and ridges.These results suggest that the permafrost warming is the major rockfall triggering factor at high elevation.
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