  We investigate the gravity driven rapid failure of a granular step composed
of non-cohesive 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.
