Secondary circulations near sand banks and in coastal embayments

Abstract

Measurements of the residual tidal currents in the vicinity of a large sand bank indicate secondary circulation effects which might explain the permanence of such features in the marine environment. Strong veering in the mean circulation is observed and this is shown to be associated with flow convergence and divergence effects in a large eddy centred on the sand bank. This eddy is maintained by vorticity generated in the coastal boundary layer as a result of tidal stream curvature and horizontal shear in the vicinity of a headland. Systematic veering in tidal current directions indicates that in the bay adjacent to the headland, flow curvature leads to secondary currents which are about an order of magnitude lower than the primary tidal flows. The directions of residual sediment movement have been calculated from measured near-bottom currents and it is shown that due to the highly non-linear nature of sediment transport processes these can be in markedly different directions to those of the residual water movements. However, the overall movements are, in general, such as to transport sediment towards the bank.