Residual Circulation in and Classification of Shallow, Stratified Estuaries

Abstract

Residual flows in shallow estuaries arise from the interaction of the density field, river flow and tidal currents. Both barotropic and baroclinic mechanisms are important, but to varying degrees in different systems. An application of finite amplitude wave theory by means of a perturbation analysis of the time-dependent equations of mass, salt and momentum conservation in a uniform channel defines three distinct circulation types for shallow estuarine systems (highly stratified, partially mixed and weakly stratified). It indicates that each corresponds to a different dominant vertical exchange mechanism on flood, and allows identification of the nonlinear mechanisms bringing about the transitions between the various states. Correct specification of vertical mass and momentum exchange results in prediction of realistic tidal and residual density and velocity fields for each case. The analysis further suggests an improved classification system for shallow estuaries, the primary subject of this contribution.

The classification system employs two parameters that arise from the perturbation analysis. The first is an internal Froude number that measures the stability of a tidal frequency internal oscillation. The second is the ratio of tidal amplitude to mean depth, a Froude number that is a measure of the barotropic nonlinearity of the tidal flow. The present classification system has an advantage relative to that of Hansen and Rattray (1966) in that classification is based directly on the strength of the barotropic and baroclinic nonlinearities in the time-dependent flow that drive the residual flow.