The evidence for self-organized criticality in sandpile dynamics

Self-organized criticality (SOC) is thought to describe avalanche dynamics in sandpiles. Experiments on the dynamics of ''sandpiles'' fall in two broad categories. Sandpiles in rotating drums exhibit periodic large avalanches, which is inconsistent with SOC. Other experiments study sandpiles on a circular support driven by the addition of grains of sand at a low rate from above and centered with respect to the support. The mass of avalanches that drop sand off the support is typically measured by a balance. It has been claimed that the observed avalanche statistics is consistent with SOC. However, I find that experiments described in the literature all have avalanche-size distributions that can be very well described by stretched-exponential distributions. Since the stretched-exponential distribution has a characteristic size, I conclude that the experiments described in the literature are inconsistent with SOC.