Permissible shear stress is the maximum shear stress that a soil, channel lining, or other surface material can withstand from flowing water without experiencing erosion, deformation, or structural failure. In hydrology and stormwater management, it is used as a design threshold to ensure that channels, ditches, and other conveyance systems remain stable under expected flow conditions.
Shear stress in this context refers to the tractive force per unit area exerted by flowing water on the boundary of a channel, typically expressed in units such as pounds per square foot or Pascals. As water flows, it applies a dragging force along the bed and banks, and if this force exceeds the resistance of the material, particles can be detached and transported, initiating erosion.
Permissible shear stress represents the resisting capacity of the material, which depends on factors such as particle size, gradation, cohesion, compaction, and vegetative reinforcement. Non-cohesive materials like sand and gravel rely primarily on particle weight and interlocking for resistance, while cohesive soils like clay derive additional strength from electrochemical bonding between particles. Vegetation can significantly increase permissible shear stress by anchoring soil and reducing flow velocity near the surface.
In design practice, engineers compare the applied shear stress from anticipated flow conditions to the permissible shear stress of the channel material. If the applied stress exceeds the permissible value, erosion is likely to occur, and stabilization measures must be implemented. These measures may include increasing channel size to reduce velocity, flattening slopes, adding vegetation, or installing protective linings such as riprap, concrete, or geotextiles.
Permissible shear stress is particularly important in the design of open channels, swales, and outfalls, where controlling erosion is essential to maintaining channel integrity, preventing sediment transport, and protecting downstream water quality.
In summary, permissible shear stress is the maximum boundary shear force that a material can resist without eroding, serving as a critical criterion for evaluating and designing stable stormwater conveyance systems.