TIDAL ELLIPSES IN THE NEAR-SHORE ZONE (-3 TO -10 M); MODELLING AND OBSERVATIONS

Abstract

Based on theoretical considerations it is demonstrated that in the near-shore zone the tidal water level fluctuations are mainly caused by cross-shore volume fluxes. Phase differences between the tidal wave propagating on the shelf and in the shallow near-shore zone create cross-shore surface gradients which result in an onshore directed flow during rising water and an offshore directed flow during falling water. Observations of near bed velocities in the near-shore zone confirm the presence of these currents. It results in an anti clockwise rotating current vector at a water depth of about 10m . Comparisons of these observations with results obtained from a 1-DV flow model show that the tidal ellipses are not the result of Coriolis forces but are generated by the alternating crossshore water fluxes due to the tide.