Failure of a granular step.

We investigate the gravity driven rapid failure of a granular step composed of noncohesive steel beads. The step is initially held together with electromagnets, and released when the current is switched off. We visualize the surface and the motion of the grains during the entire relaxation. The initial failure occurs at the surface and the subsequent flow is also confined to the surface as the step relaxes to its final state. The final shape of the surface is almost linear, depends on the initial angle of the step, and is not sensitive to the size of the grains. The average final slope of the pile is only slightly lower than the angle of repose of a pile formed by slowly pouring particles on to a flat surface. The evolution of the step is compared with a proposed convective-diffusion model of our system. The qualitative features of the relaxation are captured by the model after a flow-dependent dissipation parameter is introduced.