Dendrogeomorphic reconstruction of debris-flow activity in Sancy Massif (French Massif Central)

Debris flows (DFs) are one of the main geomorphic processes occurring in mountain regions, where they may repeatedly cause economic losses and even fatalities. Usually, along the same track, DFs may recur with various frequencies and magnitudes. Generally, because of the short collective memory and the lack or uncomplete archival records, little is known about the spatial and temporal frequency of past DFs. Dendrogeomorphic methods may be applied to reconstruct debris-flow activity in mountain areas where historical records of past DF events are uncomplete or lacking. In the Sancy Massif (1885 m a.s.l.), one of the medium-altitude stratovolcanoes of the French Massif Central, DFs are triggered during extreme rainfall events. One of the DF tracks located in the Mont Dore tourism resort starts on the steep slopes of unconsolidated volcaniclastic materials and follows a channel which cross a forested debris cone. It is the aim of this study to reconstruct past DF activity which occurred along the track in the Mont Dore tourism resort by using dendrogeomorphic methods. Based on a detailed geomorphic mapping and the identification of growth anomalies found in 42 disturbed silver fir trees (Abies alba Mill.) sampled, a minimum chronology of 13 events has been reconstructed spanning the period 1900 - 2008. Meteorological data recorded by the nearby weather station have been also analysed, to estimate the possible rainfall thresholds for past DF events.