Bottom Stress

Bottom stress, or bed shear stress, is the tangential drag force per unit area that moving water exerts on the sea floor, the property that mobilizes sediment. It scales with the square of the near-bed current speed through a drag coefficient, or equivalently with the square of the friction velocity times water density, derived from the logarithmic velocity profile of the benthic boundary layer. When bottom stress exceeds the critical threshold for a given grain size, sediment begins to move as bed load, then enters suspension at higher stress. Tidal currents and storm-wave orbital motion are the main sources, so bottom stress sets where bedforms grow, scour occurs, and channels stay open.