Runout transition and clustering instability observed in binary-mixture avalanche deposits
Binary mixtures of dry grains avalanching down a slope are experimentally studied in order to determine the interaction among coarse and fine grains and their effect on the deposit morphology. The distance travelled by the massive front of the avalanche over the horizontal plane of deposition area is measured as a function of mass content of fine particles in the mixture, grain-size ratio, and flume tilt. A sudden transition of the runout is detected at a critical content of fine particles, with a dependence on the grain-size ratio and flume tilt. This transition is explained as two simultaneous avalanches in different flowing regimes (a viscous-like one and an inertial one) competing against each other and provoking a full segregation and a split-off of the deposit into two well-defined, separated deposits. The formation of the distal deposit, in turn, depends on a critical amount of coarse particles. This allows the condensation of the pure coarse deposit around a small, initial seed cluster, which grows rapidly by braking and capturing subsequent colliding coarse particles. For different grain-size ratios and keeping a constant total mass, the change in the amount of fines needed for the transition to occur is found to be always less than 7%. For avalanches with a total mass of 4 kg we find that, most of the time, the runout of a binary avalanche is larger than the runout of monodisperse avalanches of corresponding constituent particles, due to lubrication on the coarse-dominated side or to drag by inertial particles on the fine-dominated side.