Stretched exponentials and power laws in granular avalanching

We introduce a model for granular surface flow which exhibits both stretche d exponential and power law avalanching over its parameter range. Two modes of transport are incorporated, a rolling layer consisting of individual pa rticles and the overdamped, sliding motion of particle clusters. The crosso ver in behaviour observed in experiments on piles of rice is attributed to a change in the dominant mode of transport. We predict that power law avala nching will be observed whenever surface Row is dominated by clustered moti on.