Shear Stress (t) is a measure of the force of friction from a fluid acting on a body in the path of that fluid. In the case of open channel flow, it is the force of moving water against the bed of the channel. Shear stress is calculated as:

Where:

t = Shear Stress (N/m2,   )

g = Weight Density of Water (N/m3, lb/ft )

D = Average water depth (m, ft)

Sw = Water Surface slope (m/m, ft/ft)

Note: Bed load movement and sediment transport is a function of Shear Stress. When the drag force of flowing water against a rock is greater than the gravitational force holding it in place it begins to move.

Vertical changes in water velocity produces shear forces that are parallel to the bed. These shear forces acting on the bed of a channel generate shear stress, which initiate bedload movement.  The magnitude of these stresses is a function of water surface slope, channel geometry and flow

The moment where the directive forces (shear forces) overcome restrictive forces (inertia, friction) is known as the moment of incipient motion and is the threshold of particle entrainment. The shear stress at this threshold is known as the critical shear stress (t*).