  A simple model for flowing sand on an inclined plane is introduced. The model
is related to recent experiments by Douady and Daerr [Nature 399, 241 (1999)]
and reproduces some of the experimentally observed features. Avalanches of
intermediate size appear to be compact, placing the critical behavior of the
model into the universality class of compact directed percolation. On very
large scales, however, the avalanches break up into several branches leading to
a crossover from compact to ordinary directed percolation. Thus, systems of
flowing granular matter on an inclined plane could serve as a first physical
realization of directed percolation.
