Sources of Uncertainty in Shear Stress and Roughness Length Estimates Derived from Velocity Profiles*

Shear stress is a fundamental parameter in many sediment transport expressions. It is commonly estimated from information contained in the velocity profile, specifically, shear velocity, u_*, and roughness length, z₀ Under ideal conditions, the functional relationship between elevation above the bed and flow velocity is log-linear, as expressed by the “law of the wall.” Unless the field data conform exactly to this ideal relationship, there is uncertainty in estimates of u_* and z₀. derived from velocity profiles. This uncertainty depends on the degree of scatter or deviation from the assumed log-linear model. Expressions are presented to quantify the error and to correct for it. Guidelines are suggested to minimize potential uncertainty, especially with regard to instrument deployment and velocity profile analysis. Calculation of confidence intervals around estimates of u_* and z₀, as proposed by Wilkinson (1984), is necessary and recommended.