Impact of the Depth-to-Width Ratio of Periodically Stratified Tidal Channels on the Estuarine Circulation

Abstract

The dependency of the estuarine circulation on the depth-to-width ratio of a periodically, weakly stratified tidal estuary is systematically investigated here for the first time. Currents, salinity, and other properties are simulated by means of the General Estuarine Transport Model (GETM) in cross-sectional slice mode, applying a symmetric Gaussian-shaped depth profile. The width is varied over four orders of magnitude. The individual along-channel circulation contributions from tidal straining, gravitation, advection, etc., are calculated and the impact of the depth-to-width ratio on their intensity is presented and elucidated. It is found that the estuarine circulation exhibits a distinct maximum in medium-wide channels (intermediate depth-towidth ratio depending on various parameters), which is caused by a maximum of the tidal straining contribution. This maximum is related to a strong tidal asymmetry of eddy viscosity and shear created by secondary strain-induced periodic stratification (2SIPS): in medium channels, transverse circulation generated by lateral density gradients due to laterally differential longitudinal advection induces stable stratification at the end of the flood phase, which is further increased during ebb by longitudinal straining (SIPS). Thus, eddy viscosity is low and shear is strong in the entire ebb phase. During flood, SIPS decreases the stratification so that eddy viscosity is high and shear is weak. The circulation resulting from this viscosity-shear correlation, the tidal straining circulation, is oriented like the classical, gravitational circulation, with riverine outflow at the surface and oceanic inflow close to the bottom. In medium channels, it is about 5 times as strong as in wide (quasi onedimensional) channels, in which 2SIPS is negligible.

Flow Reorganization in an Anthropogenically Modified Tidal Channel Network: An Example From the Southwestern Ganges‐Brahmaputra‐Meghna Delta

“…which is the best fit relationship for published non-mouth tidal channel width:depth ratios (compiled from Dury, 1971;Hale et al, 2019;Langbein, 1963;Myrick & Leopold, 1963;Schulz et al, 2015;Wright et al, 1973;Zeff, 1988Zeff, , 1999, and summarized in supporting information Figure S6). Given the tendency for tidal channels to shoal as they approach open water, equation (1) produced implausibly large depths within the funnel-shaped channel mouths.…”

A numerical model for the entire Wadden Sea: Skill assessment and analysis of hydrodynamics

“…There are various explanations for such deviations from theory: For the case of the Sylt-Rømø bight, Purkiani et al [2015] could show for example that a combination of channel curvature and convergence can lead to a complex combination of lateral and vertical residual shear which is not directly connected to longitudinal density gradients. By means of idealized simulations, Schulz et al [2015] demonstrated that in tidally energetic channel flow due to lateral circulation vertical shear and stratification may be stronger during flood than during ebb, other than assumed for classical tidal flow. However, in several inlets a clear correlation of tidal straining with wind stress, longitudinal buoyancy gradient or both is visible.…”

“…By means of idealized simulations, Schulz et al . [] demonstrated that in tidally energetic channel flow due to lateral circulation vertical shear and stratification may be stronger during flood than during ebb, other than assumed for classical tidal flow. However, in several inlets a clear correlation of tidal straining with wind stress, longitudinal buoyancy gradient or both is visible.…”

Suspended Sediment Dynamics in the Macrotidal Seine Estuary (France): 2. Numerical Modeling of Sediment Fluxes and Budgets Under Typical Hydrological and Meteorological Conditions

“…It thus depends on various parameters. For example, a higher river discharge, which implies a larger along‐estuary salinity gradient, increases the gravitational (Festa & Hansen, ; Hansen & Rattray, ), the advectively driven, and the tidal straining circulation (Schulz et al, ), i.e., it increases and thus the mean‐flow sediment flux, .…”