Angle of repose
the steepest stable angle of a pile of sediment grains relative to a horizontal plane.
Interparticle friction allows a pile of sediment grains to be stable under a slope to a horizontal plane. When sediment grains are piled up at a steeper angle, the grains will slide down the slope to restore the angle of repose.
The angle of repose of loosely packed medium sand in still water is about 30 degrees. The angle of repose increases with increasing grainsize. For sediment mixtures (clay, silt, sand) the angle of repose increases with increasing consolidation (decreasing water content).