The sampling variability of the Reynolds stress and its relation to boundary shear stress and drag coefficient measurements

Abstract

Large momentum fluxes occurring intermittently in the bottom boundary layer of a tidal current have been found to result in a high sampling variability for the Reynolds stress ( − puw ). The sampling error estimated on the assumption that all data points are independent, has been found, typically, to vary between ±40% and ±10% of the mean in Reynolds stress values of 1–8 dynes cm −2. It is shown that autocorrelation between contributions to the stress, may increase this by a factor of approximately 2–3 and it is suggested that the scatter which is frequently observed in estimates of the drag coefficient, derived from logarithmic velocity profiles, is explained by a similar variability in the bed shear stress.