Applied shear stress is the force per unit area exerted by flowing water parallel to a boundary surface, such as the bed or banks of a channel, pipe, or stormwater conveyance system. It represents the hydraulic force that moving water applies to soil particles, sediments, or structural materials, tending to cause them to move, deform, or erode.
In stormwater management, applied shear stress is a critical parameter for evaluating the stability of channels, swales, culverts, and other open-channel systems. It is generated by the interaction between flowing water and the boundary surface, and its magnitude depends on factors such as flow depth, channel slope, and the density of water. Higher velocities and steeper slopes generally produce greater applied shear stress.
Applied shear stress is commonly estimated using the relationship between hydraulic radius, channel slope, and fluid weight, reflecting how gravitational forces driving flow are transferred into boundary forces. When the applied shear stress exceeds the resistance of the boundary material, erosion occurs. This resistance is typically expressed as permissible shear stress, which defines the maximum shear stress that a given soil, vegetation cover, or lining material can withstand without significant erosion.
Understanding applied shear stress is essential for designing stable stormwater infrastructure. Engineers use it to size channels, select appropriate linings such as riprap or vegetation, and ensure that flow conditions will not cause excessive erosion or sediment transport. It is also used in sediment transport analysis to predict when particles will become mobilized and contribute to downstream sediment loading.
Simply put, applied shear stress quantifies the erosive force of flowing water acting along a surface, and it serves as a key design and analysis parameter for maintaining the integrity and performance of stormwater conveyance systems.