Observations of near-bed velocity profiles and seabed roughness in tidal currents flowing over sandy gravels

Abstract

Measurements of near-bed velocity profiles and bottom roughness in tidal currents flowing over sandy gravels in the west Solent, England, have shown that, although the seabed in the immediate vicinity of the study area was generally level and devoid of regular bed forms, the observed velocity profiles were consistently curved concave downwards. This behaviour may be attributed to the presence of an internal boundary layer which was due to form drag on irregular topography upstream of the measurement position. Loosely packed gravels of the type found in the west Solent had a geometric mean roughness length of about 0·3 cm. Comparison of the observed ratio of roughness length to mean particle size with laboratory data, suggests an equivalent two-dimensional roughness element spacing of about three particle diameters. In contrast to tidal flows over rippled sand beds, the roughness lengths for sandy gravels remained relatively constant throughout the tidal cycle. Furthermore, these measurements provided no systematic evidence of the effects of flow acceleration on profilederived estimates of the friction velocity u ∗ and the roughness length z 0.