The Boundary Layer Character of Tidal Currents in the Eastern Irish Sea

The vertical profile of the tidal currents observed by a ship borne ADCP at a site in the eastern Irish Sea where the water depth is 50 m was logarithmic with height above the bed in the lower 50–60% of the water column except for periods of about 1 h either side of slack water. The maximum height of the boundary layer reached 40 m during the flood tide. The shearing stress, determined by the method of Bowden et al. (1959), showed a tendency to be uniform with height in the lower half of the water column while the eddy viscosity increased linearly away from the bed, scaling on u*z where u* is the friction velocity. Both of these features are characteristic of the logarithmic profile region of a boundary layer flow. Estimates of zo, the roughness length, and CD, the bottom drag coefficient, were around 10 3 m and 2·4 10 3, consistent with the known character of the bed. The scatter in the derived boundary layer parameters was reduced significantly by averaging the data within 30-min intervals which suggests the presence of tidal eddies having periods comparable to the 5-min sampling period of the ADCP. Short period fluctuations in the flow were coherent over the lower 20 m of the water column and propagated vertically away from the bed.