The December 2008 Crammont rock avalanche, Mont Blanc massif area, Italy
Résumé
Rock avalanching is a very hazardous process in high mountain area. Present climatically induced glacier shrinkage and permafrost degradation could increase its frequency and magnitude. A 0.5 M m3 rock avalanche affected the 400-m-high North face of Mont Crammont (western Italian Alps) on 24/12/ 2008. The main part of the rock mass settled at the rockwall foot, whereas a minor part travelled horizontal and vertical distances of 3,050 and 1,560 m, respectively. The mobility was enhanced by the channelization in two torrent gullies, and the incorporation of snow. Three elements suggest that the rock avalanche could have been triggered by the current degradation of the rockwall permafrost: seepage water in the detachment zone; modelled 'warm' permafrost (T > −2 °C); no other Holocene rock avalanche deposit. Collapsed volume was computed by comparison of pre- and post-event DTMs. Back analysis calculation of the rock avalanche runout suggests a two step event