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The critical shear stress and also the critical Shields number (and ) describe the conditions when the sediment starts moving. Note that the shear stress is a property of the current, while the critical shear stress is a property of the sediment.
Excess dimensionless shear stress is a nondimensional measure of bed shear stress about the threshold for motion. ( τ b ∗ − τ c ∗ ) {\displaystyle (\tau _{b}^{*}-\tau _{c}^{*})} , Bed load transport rates may also be given by a ratio of bed shear stress to critical shear stress, which is equivalent in both the dimensional and ...
The shear stress velocity has the dimension of a velocity (m/s), but is actually a representation of the shear stress. So the shear stress velocity can never be measured with a velocity meter. By using the shear stress velocity, the Shields parameter can also be written as:
Shear velocity also helps in thinking about the rate of shear and dispersion in a flow. Shear velocity scales well to rates of dispersion and bedload sediment transport. A general rule is that the shear velocity is between 5% and 10% of the mean flow velocity. For river base case, the shear velocity can be calculated by Manning's equation.
Stream power is closely related to other criteria such as stream competency and shear stress. Stream power is a valuable measurement for hydrologists and geomorphologists tackling sediment transport issues as well as for civil engineers, who use it in the planning and construction of roads, bridges, dams, and culverts.
As discharge increases, shear stress increases above a threshold and starts the process of sediment transport. A comparison of the flow strength available during a given discharge to the critical shear strength needed to mobilize the sediment on the bed of the channel helps us predict whether or not sediment transport is likely to occur, and to ...
The formula to calculate average shear stress τ or force per unit area is: [1] =, where F is the force applied and A is the cross-sectional area.. The area involved corresponds to the material face parallel to the applied force vector, i.e., with surface normal vector perpendicular to the force.
If the cohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. In addition, if the river carries significant quantities of sediment, this material can act as tools to enhance wear of the bed .