Numerical Investigation of Baroclinic Channel‐Shoal Interaction in Partially Stratified Estuaries

Abstract

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.Plain Language Summary Drowned-river estuaries are frequently characterized by a deep channel laterally bounded by shallow shoals with a gentle slope. Many such estuaries are experiencing impairment from nutrient over-enrichment such as low dissolved oxygen level, massive phytoplankton blooms and water quality degradation. Previous field and modeling studies have indicated that the shallow shoals with higher light availability allow for faster phytoplankton growth and that its exchange with the channel can influence the occurrence, timing and extent of large-scale phytoplankton blooms. We perform idealized numerical simulations of straight estuaries with distinct channel-shoal morphology to explore the hydrodynamic processes driven by differences in fluid density inside the water body. We focus on how the salt is exchanged between channel and shoal and how the water on the shoal moves perpendicular to the main axis of the estuary. We also highlight that the presence of the shoal tends to enhance the net along-estuary transport of water in the central channel when the oscillating tide is filtered out. The insights gained are expected to expand our knowledge of the physics of channel-shoal estuaries at various timescales, which can aid us in making management decisions regarding estuarine ecosystems.

Exchange Flow and Material Transport Along the Salinity Gradient of a Long Estuary

“…Besides the steady gravitational circulation, the near‐bed landward residual currents can be enhanced by the tidal asymmetry of eddy viscosity, which is caused by the axial strain‐induced periodic stratification (SIPS, Jay & Musiak, 1994; Simpson et al., 1990), or lateral circulations. Lateral advection can also directly influence the along‐estuary momentum balance (e.g., Geyer et al., 2020; Lacy et al., 2003; Lerczak & Rockwell Geyer, 2004; Li & Li, 2012; Scully et al., 2009; Zhou et al., 2020). On a monthly to annual timescale, studies show that freshwater discharge and wind forcing are two dominant drivers for the seasonal and interannual variations of exchange flow, while exchange flow responds non‐monotonically to the river discharge (Du et al., 2018; Xiong et al., 2021).…”

Residual Sediment Transport in Tidally Energetic Estuarine Channels With Lateral Bathymetric Variation

“…GETM is implemented in a two‐dimensional mode covering the cross‐sectional plane of the estuary. Such lateral‐vertical GETM scenarios have been employed in several previous studies where longitudinal uniformity can be reasonably assumed (Burchard et al., 2011; Burchard & Schuttelaars, 2012; Lange & Burchard, 2019; Schulz et al., 2015; Zhou et al., 2020). All along‐channel gradients vanish except for the longitudinal barotropic and baroclinic pressure gradients.…”

“…Besides the steady gravitational circulation (Hansen & Rattray, 1965), the near‐bed landward currents can be enhanced by tidal asymmetry in eddy viscosity (Jay & Musiak, 1994), which is caused by the along‐channel (axial; longitudinal) strain‐induced periodic stratification (SIPS): the stratifying (destratifying) effect of the vertically sheared ebb (flood) current under the presence of an unidirectional longitudinal salinity gradient (Simpson et al., 1990). Lateral circulations have also been found to enhance the near‐bed landward residual currents by the lateral straining of salinity, which leads to tidal asymmetry in stratification and mixing (Burchard et al., 2011; Geyer & MacCready, 2014; Lacy et al., 2003; Schulz et al., 2015; Zhou et al., 2020). In addition, lateral circulations also directly redistribute longitudinal momentum and generate residual currents in support of the classical estuarine circulation (Lerczak & Geyer, 2004).…”