Shear stress is the force per unit area exerted by flowing water parallel to a surface, such as the bed or banks of a channel, soil surface, or structural lining. In stormwater management and open-channel hydraulics, shear stress represents the tractive force that moving water applies to boundary materials, influencing their potential to resist or undergo erosion.
Shear stress is generated by the interaction between flowing water and the surface over which it moves, and it is primarily a function of the weight of the water and the slope of the energy grade line or channel. As flow depth and slope increase, the shear stress acting on the boundary also increases. This force acts tangentially, attempting to dislodge soil particles or move sediment along the surface.
The magnitude of shear stress is a critical factor in determining whether erosion will occur. When the applied shear stress exceeds the resisting forces of the soil or channel lining, such as particle cohesion, friction, or vegetative reinforcement, erosion and sediment transport will begin. Conversely, if the applied shear stress is below the material’s resistance, the surface remains stable.
In stormwater design, shear stress is used to evaluate the stability of channels, swales, and other conveyance systems, as well as to select appropriate lining materials such as vegetation, riprap, or geotextiles. It is directly related to concepts such as erosion potential, sediment transport, and permissible shear stress, which defines the maximum shear a material can withstand without failure. Understanding shear stress is essential for preventing channel degradation, protecting infrastructure, and maintaining the integrity of stormwater systems.