A Numerical Study of the Interaction of Tidal Oscillations and Non-linearities in an Estuary

The interaction of tidal oscillations and buoyancy strongly affects the velocity shear and therefore the dispersion of contaminant in an estuary. A time-dependent quasi-three-dimensional numerical model incorporating a Richardson number based parametrization of vertical and lateral mixing was used to investigate such interactions by exploiting the fact that estuaries are generally much longer than they are wide or deep, by restricting attention to flows where the tidal range to depth ratio is small and, finally, by assuming the estuary to be locally prismatic. The results of the study, applied to the Conwy Estuary in North Wales, show that the lateral circulation strongly affects the longitudinal momentum balance and therefore the longitudinal dispersion. Longitudinal dispersion is further strongly affected by a storage effect near the shallow banks which emphasizes the need to obtain sufficient data in shallow areas if a reliable estimate of the dispersion coefficient is to be obtained. This interaction of tidal oscillations and buoyancy leads to a strong asymmetry in the intra-tidal variation of the dispersion coefficient