  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.
