Lateral Circulation in Well-Mixed and Stratified Estuarine Flows with Curvature

Abstract

A field experiment was conducted to examine stratified and unstratified curvature-generated lateral circulation and momentum balances in an estuarine tidal channel. Conductivity, temperature, depth, and current profiler data were collected vertically and laterally across the channel at a sharp bend over a fortnightly period to measure the terms of the lateral momentum budget. Well-mixed conditions allow the development of classic two-layer helical flow around a bend. Stratification strengthens curvature-induced lateral circulation, but the development of a lateral baroclinic pressure gradient opposes the resultant motions. The spatial and temporal response of this baroclinic pressure gradient is different than centrifugal acceleration, producing a three-layer profile. As the baroclinic term becomes stronger (or as centrifugal acceleration disappears as the flow exits the bend), two-layer flow with the opposite direction from curvature occurs. In both stratified and well-mixed conditions, downstream adjustment of lateral circulation (nonlinear advective acceleration) is of leading order in the lateral momentum budget; the depth-averaged term adjusts the streamline direction, while vertical deviations from the depth average account for changes in lateral circulation. The asymmetry of forcing mechanisms on flood and ebb, because of variations in stratification and strength of tidal flow, can strongly affect net lateral transport and generation of residual currents in regions of curvature.

Coriolis forces influence the secondary circulation of gravity currents flowing in large‐scale sinuous submarine channel systems

“…We note that a complete description of the flow dynamics around a bend would require a full momentum budget including cross-stream frictional influences and non-local adjustment terms as discussed by Nidzieko et al [2009]. However, we focus on (1) to (3) and point to future work that has to incorporate these features in a more thorough mathematical model.…”

A general model for the helical structure of geophysical flows in channel bends

“…Rivers, estuaries and saline underflows display a helical flow structure when passing through a bend, which can be broken into down-stream and cross-stream components (Rozovskii, 1957;Nidzieko et al, 2009;Parsons et al, 2010;Sumner et al, 2014). The helical structure results from competing forces that drive the flow around a bend.…”

“…Because the densest fluid in a stratified flow is near the bed, this inwardly directed pressure gradient can cause dense fluid to accumulate at the inner bend resulting in lateral stratification of the flow. Lateral stratification within the flow causes lateral pressure gradients; in particular if dense fluid collects near the inner bend, then this will produce a near-bed pressure gradient that pushes fluid back towards the outer bend (Nidzieko et al, 2009;Sumner et al, 2014). The magnitude and rotation direction of helical flow cells is determined by the relative strengths of the above forces, which depend on the specific velocity and density structure of a flow and how this structure evolves around the bend.…”

“…Flow around bends in well-mixed estuaries show a river-like basal helical circulation, while stratified estuaries and saline flows are river-reversed (Nidzieko et al, 2009;Wei et al, 2013). In stratified flows, across-flow variation in stratification (i.e.…”

“…In stratified flows, across-flow variation in stratification (i.e. flow density) sets up an additional lateral pressure gradient that is thought to play a key role in determining the direction of the flow rotation (Nidzieko et al, 2009;Sumner et al, 2014). Such stratificationtriggered pressure gradients have been suggested to be important for turbidity currents, which are density-stratified because of vertical variations in sediment concentration (Sumner et al, 2014;Peakall and Sumner, 2015).…”