Imaging of avalanches in granular materials

Dry, noncohesive granular materials were very slowly driven to the threshold of instability where they produced avalanches. We have used digital image analysis to record avalanches that occurred on the surface of the incline. Results show large slides every 10–13 min which reset the surface from the maximum angle of stability, ${\mathrm{\theta }}_{\mathit{c}}$, back to the angle of repose, ${\mathrm{\theta }}_{\mathit{r}}$. We also find a power-law distribution of sizes for smaller avalanches occurring between the large sliding events. The number of these small avalanches follows a power-law dependence on the reduced angle, (${\mathrm{\theta }}_{\mathit{c}}$-θ)/(${\mathrm{\theta }}_{\mathit{c}}$-${\mathrm{\theta }}_{\mathit{r}}$), or time, which demonstrates that ${\mathrm{\theta }}_{\mathit{c}}$ behaves like a critical point.