Dynamical equilibrium of avalanches on a rough plane

We present experimental results on avalanches which are triggered in a metastable static layer on a rough inclined plane. We observe that despite the continuous increase in mass of a perturbation while it runs down, the flow reaches a dynamical equilibrium where the fronts evolve at constant speed and the mass is spread. The head front is found to be a shock wave with a crest, and its transient acceleration phase is studied. In contrast, the rear front grows in a self-similar manner from the beginning. An instability is observed which breaks thin flowing layers down into solitary waves.