Authors: Jian Zhou  Mark T. Stacey  Rusty C. Holleman  Emma Nuss  David B. Senn

The internal, density‐driven channel‐shoal interaction in partially stratified estuaries is numerically investigated. Idealized General Estuarine Transport Model (GETM) simulations with the rigid‐lid assumption are performed in a two‐dimensional cross‐sectional mode forced by a constant longitudinal salinity gradient and an M2 tidal current. Simulation results show that within each tide cycle, the abrupt flattening of bathymetry at the channel‐shoal interface leads to the trapping of a patch of high‐salinity water mass on the shoal. The resulting local salinity maximum near the interface interacts with the differential advection by the gentle slope of shoal, driving complex density‐driven on‐shoal lateral circulations. Next, it is found that the presence of the shoal in turn enhances the longitudinal residual circulation in the channel. As a storage of estuarine water that is partially isolated from the main stream, the shoal traps saltier water that originates from the channel during slack after ebb (thus a less stratified flood), while injects fresher water into the surface of the channel during slack after flood (thus a more stratified ebb). These lateral baroclinic exchanges across the channel‐shoal interface give rise to a new mechanism for generating tidal asymmetry of eddy viscosity and shear in the channel (i.e., tidal straining circulation), which has the same orientation with the classical estuarine circulation. The interfacial salt trapping and the accompanied shoal‐induced modification of channel residual circulation are demonstrated to possess a highly localized nature, with their characteristics largely independent of shoal bathymetry far away from the channel‐shoal interface.

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