| Summary: | The study of rockfall (volume > 100 m3) in high mountain is essential to understand landscape evolution and to evaluate natural hazards. The number of rockfalls is presently rising in the Alps, while vulnerability is increasing at high elevation and in valleys. Due to the lack of systematic observations, frequency and volume of rockfalls, as well as their triggering factors remain poorly understood. Until today, most of the studies on rockfall carried out in high Alpine rockwalls were indeed devoted to individual events, while systematic surveys are needed to clarify the role of regional factors such as permafrost degradation. Here we present the network of observers (guides, hut keepers, mountaineers) which sets aside the documentation of all the rockfall events that occur in the central part of the Mont-Blanc massif. Operational since 2007, this network allowed identifying and documenting 251 rockfalls between 2007 and 2011. Checked and completed each year by extensive field work, data from the network are then analysed through a Geographic Information System to statistically characterise these rockfalls. The results of the first five years of survey indicate that permafrost degradation is the main rockfall triggering factor.
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