Secondary Circulation Asymmetry in a Meandering, Partially Stratified Estuary

Abstract

Numerical model experiments are used to study the effects of multiple channel bends on estuarine dynamics and, in particular, on secondary flows. These effects are demonstrated by comparing experiments with two different idealized trumpet‐shaped estuaries, one straight and another one with a ∼8 km meandering section in the middle of the estuary. Meanders complicate the flow field by introducing secondary processes. For instance, meanders increase turbulence and associated mixing locally within the water column, as well as outside the meandering portion. Furthermore, meanders transform up to 30% of the along‐channel momentum into secondary circulation. Production of turbulence and secondary currents is different at flood and ebb tidal phases. At flood, meanders lead to unstable stratification and increased turbulence. At ebb, the flow develops a helical pattern and adjusts to the channel curvature with minimal decrease in density stability. The secondary circulation asymmetry is caused by an interplay between the across‐channel baroclinic pressure gradient force and the centrifugal force. During ebb both forces enhance each other, whereas they oppose during flood. As a consequence of this interaction between baroclinic forcing and curving morphology, ebb flows and horizontal buoyancy fluxes increase relative to flood. The enhanced ebb dominance shifts a density front toward the mouth of the estuary, thus reducing salt intrusion.

Reversed Lateral Circulation in a Sharp Estuarine Bend with Weak Stratification

“…Snag Channel (Lacy and Monismith 2001) also shows some stratification influence, but that case is predominantly governed by differential advection arising from the confluence of two streams. Like in the North River, laterally sheared streamwise velocities generating ebb-flood asymmetries in the lateral salinity gradient and secondary flow were found also in the numerical study by Pein et al (2018). However, the meander geometry (W/R, c f 21 H/R, W/H) was very different, with significantly lower relative curvature and relative depth.…”

“…The table shows that the relative curvature W/R and particularly the relative depth c f 21 H/R in the North River bend are significantly greater than in the other cases from the literature, with Elkhorn Slough (Nidzieko et al 2009) having the next highest values. The aspect ratio W/H is the smallest in the North River and the greatest for Pein et al (2018). The salinity gradient ratio for the North River is about the same as for Pein et al (2018), but greater than other cases.…”

“…The aspect ratio W/H is the smallest in the North River and the greatest for Pein et al (2018). The salinity gradient ratio for the North River is about the same as for Pein et al (2018), but greater than other cases.…”

“…Ebb-flood differences in secondary flow may also be induced by differences in lateral salinity gradients (Lacy and Monismith 2001;Winterwerp et al 2006), resulting from lateral differences in streamwise velocity produced by, for example, confluences, harbors, or local geometric features. In a numerical study of an idealized, partially stratified estuary, Pein et al (2018) found persistent secondary circulation during ebb and reversed circulation at the end of flood, explained by an ebb-flood asymmetry in differential advection leading to a lateral baroclinic pressure gradient force acting with the centrifugal force during ebb and against it during flood. Ebb-flood asymmetry could also be induced by varying water levels, affecting the friction, and by asymmetry in the sense of the Coriolis force w.r.t.…”

Lateral Circulation in a Partially Stratified Tidal Inlet

“…Interactions between barotropic pressure gradient and bathymetry can generate convergence of lateral flow, producing flows rotating toward the channel from the shoals [4,5]. In curved estuaries, an alternative driving mechanism for lateral circulation is the centrifugal acceleration [6][7][8] and advection [9]. Winds can enhance or degrade the local-curvature-induced, two-layer flow and can drive three-layer flow [10].…”