Hydrodynamics and sediment dynamics of North Sea sand waves and sand banks

Abstract

Seabed drag coefficients have been measured at a site within the Norfolk Banks and at a site within the sand wave field in the southern North Sea, using pressure sensors and moored current meters. At the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used. The results are generally in agreement with values used in numerical models. At the sand banks site, the drag coefficient increases with wind conditions, but at the sand waves site there is a reduction during the highest wave conditions, attributed to the drag reduction caused by sand resuspension from the bed. This result suggests that sediment effects must be considered if wave/current interaction is included in numerical models of the region. The drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face, suggesting a small form drag component. Studies of sand movement used bedform mapping, fluorescent sand tracing and photography of migrating ripples. There is evidence for the early stages of formation of a new bank between Broken Bank and Well Bank. At the sand waves site, ripple migration is found to be a useful estimator of bedload transport under conditions of negligible suspension. Over the sand banks, a definite correlation of surface water properties, notably the turbidity, with the topography of the banks was observed. The mechanism for this surface effect is not known but it is consistent with the observation of features, seen in satellite visible light images, which outline the shapes of the banks.