Velocity profiles of tidal currents over sand waves

Abstract

Velocity profiles were measured over a sand wave in the Southern Bight of the North Sea. The height of the sand wave is about 8 m, its wavelength 300 m. The depth of the trough is 36 m. During one tidal cycle, velocity and depth measurements were taken regularly. The behaviour of the current in the trough was very similar to that over a flat seabed, except that a van Veen fit during the quasi-stationary period around maximum tide had a larger power: U = α z p with p = 0.22 to 0.29, versus p = 0.15 to 0.20 for a flat seabed. Exactly on the crest and just downstream of it, the profiles were significantly different; compared with a van Veen fit, the flow was faster in the lowest 8 m and at 4 m beneath the surface: the highest measuring point. Advection was shown to be important in forming these profiles. Depending on the method of extrapolation, the spatial variation in surface current was calculated to be 5 to 26 cm·s −1, compared with a surface current of ± 65 cm·s −1 in the trough. The ratio of surface current variation was, depending on the extrapolation method, 0.3 to 1.7.