Effects of Bathymetry, Friction, and Rotation on Estuary–Ocean Exchange

Abstract

An analytical model that includes pressure gradient, friction, and the earth’s rotation in both components of the flow is used to study the transverse structure of estuarine exchange flows and the nature of transverse circulation in estuaries of arbitrary bathymetry. Analytical results are obtained for generic bathymetry and also over real depth distributions and are compared with observations. This study extends previous efforts on the topic of transverse structure of density-induced exchange flows in three main aspects: 1) the analytical model explores any arbitrary bathymetry; 2) the results reflect transverse asymmetries, relative to a midchannel centerline, associated with the effects of the earth’s rotation; and 3) the transverse circulation produced by the analytical model is examined in detail. Analytical results over generic bathymetry show, in addition to the already reported dependence of exchange flow structure on the Ekman number, two new features. First, the transverse structure of along-estuary flows shows the earth’s rotation effects, even in relatively narrow systems, thus producing transverse asymmetries in these flows. The asymmetries disappear under strongly frictional (high Ekman number) conditions, thus illustrating the previously documented pattern of inflow in channels and outflows over shoals for typical estuaries. Second, transverse flows resemble a ‘‘sideways gravitational circulation’’ when frictional effects are apparent (Ekman number greater than ;0.1) responding to a transverse balance between pressure gradient and friction. These transverse flows reverse direction under very weak friction and reflect Coriolis deflection of along-estuary flows, that is, geostrophic dynamics. All examples of observed flows are satisfactorily explained by the dynamics included in the analytical model.