Abstract

Detailed tidal current information collected in the embayments along the coastline of the central northern Bristol Channel, using both Eulerian and Lagrangian techniques, has demonstrated the existence of tidally induced eddies. These eddies are considered, by the authors, to be the result of flow separation downstream of the headlands, due to the interaction between the coastal irregularities and the offshore rectilinear tidal current system. Supplementary, tidal current observations, adjacent to the coastal discontinuities in the area under investigation, are indicative of the formation of similar eddies. The tidally induced eddies along the northern Bristol Channel coastline are related to the formation of either mud or sand (send ridge) deposits, which occur along the coastal boundary. The eddies lead to the deposition of fine grained material at their center, due to a reduction in the magnitude of the current speed, bed shear stress and turbulence. The mud deposits are without any relief, therefore the eddies are the sole mechanism responsible for their formation and maintenance. In contrast, the presence of appreciable relief in the sand deposits (sand ridges) introduces secondary flows and brings other hydraulic mechanisms into play. The fact that the main axes of the sand ridges are sub-parallel to the tidal flow, suggests that Smith's and Huthnance's mechanisms are also operational and therefore enhancing the maintenance and evolution of the sand ridges. Sand ridges in the English Channel which occur off promontories are also related to tidally induced eddies.

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