The logarithmic layer in a tidal channel

Twenty-minute averaged velocity profiles taken with a bottom-mounted ADCP in a 30-m deep tidal channel have been fitted to a logarithmic form with 1 % accuracy. The height of the log-layer varies tidally and reaches 20 m above the bottom during peak flows of 1 m s−1. The height is well predicted by 0.04 u*/ω, where u* is the friction velocity and ω is the angular frequency of the dominant tidal constituent. The mean non-dimensional shear,(∂U/∂z)(u*/κz), is within 1 % of unity at the 95% level of confidence inside the log-layer. The friction velocity varies tidally and reachesO(0.05) m s−1 during peak current flow. The bottom drag coefficient referring to the depth-mean flow is 4 × 10−3. The observed log-layer is not connected to the skin friction, but possibly to the form drag. Deviations of the measured velocity from the logarithmic profiles above the log-layer can be explained by the zero-stress boundary condition at surface and by the entrainment of shallow water at mid-depth. The deviations are inconsistent with the effects of acceleration/deceleration and stratification.