abstract: Recent experiments show that an avalanche initiated from a point source propagates downwards by invading a triangular shaped region. The opening angle of this triangle appears to reach 180 o for a critical inclination of the pile, beyond which avalanches also propage upwards. We propose a simple interpretation of these observations, based on an extension of a phenomenological model for surface flows. Well-controlled avalanche experiments have recently been performed by Rajchenbach in (narrow) rotating drums , and by Daerr and Douady on a thin layer of sand atop a wide inclined plane (of high surface friction) at various angles of inclination . In the latter experiments, an avalanche is triggered locally (e.g. by a probe) and its subsequent evolution is observed. One sees very clearly that below a certain inclination angle, the avalanche rapidly dies out, leaving only a small perturbed region around the initiation point. When the angle exceeds a first critical angle, the avalanche progresses downwards indefinitely; the perturbed region has a triangula
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