2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/28025Recent 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.submitted to `Granular Matter'Soft Condensed Mattertext