THE EFFECT OF STRATIFICATION ON TIDAL CURRENT PROFILES IN A REGION OFFRESH-WATER INFLUENCE

Observations of the semi-diurnal tidal current ellipses within the Rhi ne ROFI are reported. The vertical structure of these tidal ellipses a re observed to be significantly different for stratified and well mixe d conditions. in particular, during well mixed conditions, tidal curre nts are essentially rectilinear and directed parallel to the coast. Wi th the onset of stratification, significant cross-shore components, re aching 40% of the magnitude of the along-shore components appear. Thes e components are 180 degrees out of phase from near surface to near-bo ttom with surface currents rotating anticyclonically, and bottom curre nts cyclonically. It is proposed that this phenomenon may be explained by a decoupling of the upper and lower portions of the water column d uring stratified periods due to reduced viscosity within the pycnoclin e. A two layer model is used to examine this hypothesis and appears to be successful in reproducing many of the features of the observed sem i-diurnal tidal ellipse variability.