Flowing sand - a physical realization of Directed Percolation

We introduce and investigate a simple model to describe recent experiments by Douady and Daerr on flowing sand. The model reproduces experimentally observed compact avalanches, whose opening angle decreases linearly as a threshold is approached. On large scales the model exhibits a crossover from compact directed percolation to directed percolation; we predict similar behavior for the experimental system. We estimate the regime where “true” directed percolation morphology and exponents will be observed, providing the first experimental realization for this class of models.