Dynamics of grain avalanches

We study the nucleation and the growth of avalanches in a model experimental system consisting of a bidimensional packing of noncohesive grains positioned in a rotating drum. We show that the avalanche mass increases linearly in time, and that the growth rate is governed by the velocities of the two up and down fronts. The upper front is shown to propagate upwards with a velocity which is equal to the averaged velocity of the flowing grains, whereas the velocity of the downslope propagating front is approximately equal to twice the avalanche velocity. We describe simple mechanisms which quantitatively account for the observed dynamical properties.