This research is related to the "debris flow " phenomenon with the aim of identifying necessary conditions for initiation and studying ils mechanisms, in several torrential catchments in the French Alps. . Debris flow occurrence results from a conjunction of several predisposition factors related to climatology, geology, geomorphology and hydrogeology. The triggering factor is most often water provided by violent rainstorms. The study of numerous debris flow deposits, sampled in different catchments, allow to determine two types of debris flows, granular debris flows and cohesive debris flows. Their distinction is based upon several granulometric criteria defined ori the fraction of soil inferior to 20 mm. The granulometric and geotechnical characteristics of initiation zone samples are similar to those of debris flow deposits, which implies that the granulometric sorting between source areas and deposit areas is very low. The initiation mechanisms have been identified and described considering the environmental context. They are numerous, and we established their typology. The knowledge of the initiation mechanisms and the identification of geotechnical criteria for initiation zone materials has been used to elaborate maps of " aptitude for debris flow initiation " . Regarding the initiation mechanisms and the volumes of available materials in the potential initiation zones, we have defined different debris flow scenarios, taking into account the potential damage caused on torrential fans.
