The July 2003 Frébouge debris flows (Mont Blanc Massif, Valley of Aosta, Italy): water pocket outburst flood and ice avalanche damming

Abstract

In July 2003, during a dry period, debris flows occurred on the large polygenic fan (0.85 km² ; elevation difference of 500 m) at the foot of the Glacier de Frébouge (Val Ferret). These events deeply incised the western fan sector and deposited ~ 30000 m³ of coarse material in the distal part of the fan. The first debris flow on 17 July may have been triggered by a glacial water pocket outburst flood, a not uncommon process in the Mont Blanc massif. The other ones developed in three steps: (i) ice avalanching of a part of the Glacier de Frébouge front; (ii) damming of the proglacial torrent gorge by the ice deposit; (iii) outburst flood due to dam collapse, with debris flow formation. Increasing ice avalanching at the hanging front of retreating glaciers constitutes an important natural hazard factor in alpine high mountain tourist areas.