Modeling the Lateral Circulation in Straight, Stratified Estuaries

Abstract

The dynamics of lateral circulation in an idealized, straight estuary under varying stratification conditions is investigated using a three-dimensional, hydrostatic, primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary. For all model runs, lateral circulation is about 4 times as strong during flood tides as during ebbs. This flood?ebb asymmetry is due to a feedback between the lateral circulation and the along-channel tidal currents, as well as to time-varying stratification over a tidal cycle. As the stratification is increased, the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself. When rotation is included, a time-dependent, cross-channel Ekman circulation is driven, and the tidally averaged, bottom lateral circulation is enhanced toward the right bank (when looking toward the ocean in the Northern Hemisphere). This asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport, leading to asymmetric channel profiles in straight estuaries. For the weakly stratified model run, advection due to lateral currents is a dominant term in both the along-channel and cross-channel momentum equations over a tidal cycle and for the tidally averaged momentum equations. In the tidally averaged, along-channel momentum equation, lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along-channel pressure gradient. Therefore, it should not be ignored when estimating momentum budgets in estuaries.