Binary mixtures of dry grains avalanching down a slope are experimentally studied to determine the interaction among coarse and fine grains and their effect on the morphology of the deposit. The distance traveled by the massive front of the avalanche on 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 in terms of the depth-averaged segregation models that describe how large particles are transported preferentially towards the avalanche front and accumulate there. Segregation by sizes during the avalanching and deposition stages produces distinct morphologies of the final deposit as the coarse-particle content is increased until full segregation and a split-off of the deposit into two well-defined separated deposits occur for certain size ratios. The formation of a separated distal deposit, in turn, depends on a critical number of coarse particles.
A large number of dispersed coarse particles 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, keeping the total mass constant, the change in the amount of fine particles 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 large particles on the fine-dominated side.
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