Geomorphology and kinematics of debris flows with high entrainment rates: A case study in the South French Alps

Debris flows consist of a fully saturated mixture of water, sediment and debris that can travel a distance of several kilometers. The entrainment processes occurring during the run-out phase are little discussed in the literature, especially from the discussion of observed events. In the early evening of 5 August 2003, a large debris flow occurred at the Faucon torrent in the Barcelonnette Basin (South French Alps). This debris flow caused substantial damage to several buildings located on the torrential fan. Some morphological aspects of this debris-flow event are presented through a careful field survey and the characterization of the transported sediment. First, the observations show that the triggering area corresponds to a strong incision in scree slopes; the grain-size distribution of the debris-flow deposits in this area is typical of granular debris flows. Second, the total volume of the debris-flow was estimated at 60,000 m3. Channel scouring is responsible of the large difference between the mobilized volume and the deposited volume; an entrainment rate of 15 m3.min−1 is estimated. Laboratory tests show that the 2003 debris-flow event started as a granular flow and then transformed into a muddy debris flow by incorporating clay-rich marly sediments along the flow track.