Reconstitution de la dynamique de lave torrentielle sur un cône forestier combinant dendrogéomorphologie et analyse diachronique d’orthophotographie (Mont Granier, Massif de la Chartreuse, Préalpes)

Abstract

The past debris flow dynamics of the Ravin du Diable are analyzed by an integrative approach based on dendrogeomorphological methods coupling with diachronic orthophotography analysis. The aim is to reconstruct (i) both frequency and spatial extent of the recent debris flow activity, (ii) key debris flows on the cone dynamic and then (iii) the global torrential dynamics on a depositional forested cone in the French Prealps. Due to one of the most important landslide in Europe occurred in 1248, the Ravin du Diable is part of a very young and steep highly active catchment of 16 ha. Situated on the north face of the Mont Granier, part of Chartreuse Massif, this catchment is now well known for being an active debris flow corridor. Based on a dendrogeomorphological analysis, 315 growth disturbances from 189 trees (68% of resinous trees: Picea abies, Abies alba and 32% of broadleaved trees: Fraxinus excelsior, Alnus Incana) allowed a reconstitution of 14 events since 1968 including 7 unknowns debris flows in the historical archives and an estimation of 21 debris flows since 1915. Coupling with dendrogeomorphology, 13 orthophotographies since 1948 revealed channels activity, channels mobility, and different stages of the forest stand re-colonization. As a result obtained from this integrative approach, the spatial minimum extent of every major debris flows was mapped. Furthermore, 4 spatial morphological changing periods of the depositional cone dynamics were identified, resulting from debris flows intense activity.