Lateral Circulation Generates Flood Tide Stratification and Estuarine Exchange Flow in a Curved Tidal Inlet

Abstract

Cross-channel transect measurements of microstructure and velocity in a well-mixed and curved tidal inlet in the German Wadden Sea show the occurrence of significant late flood stratification. This stratification is found to be a result of lateral straining. This study observes a strong single-cell lateral circulation, which is strongly pronounced at late flood and absent during most of ebb. This tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient. During flood a positive feedback between the terms generates strong lateral circulation, whereas during ebb a negative feedback leads to a suppression of the cross-channel exchange. A theoretical framework based on vorticity is developed, which allows lateral and longitudinal circulation to be studied in a consistent way. With this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal estuarine circulation, here identified with the tidally averaged across-channel vorticity component.

Three-Dimensional Salt Dynamics in Well-Mixed Estuaries: Influence of Estuarine Convergence, Coriolis, and Bathymetry

“…Significant transverse circulations are observed in well-mixed, partially mixed, and stratified estuaries (Nunes and Simpson 1985;Becherer et al 2015;Valle-Levinson et al 2000;Lacy and Monismith 2001). These circulations are usually associated with lateral variations in bathymetry (Nunes and Simpson 1985;Valle-Levinson et al 2000), Coriolis force (Lerczak and Geyer 2004;Valle-Levinson 2010), lateral density gradients (Huijts et al 2006(Huijts et al , 2011, and curvatureinduced centrifugal forces (Lacy and Monismith 2001;Buijsman and Ridderinkhof 2008).…”

“…These circulations are usually associated with lateral variations in bathymetry (Nunes and Simpson 1985;Valle-Levinson et al 2000), Coriolis force (Lerczak and Geyer 2004;Valle-Levinson 2010), lateral density gradients (Huijts et al 2006(Huijts et al , 2011, and curvatureinduced centrifugal forces (Lacy and Monismith 2001;Buijsman and Ridderinkhof 2008). Transverse circulations are also affected by the estuarine width-to-depth ratio (Schulz et al 2015) and temporal/lateral variations of mixing (tidal and lateral straining; see Burchard et al 2011;Becherer et al 2015).…”

Simultaneous measurements of tidal straining and advection at two parallel transects far downstream in the Rhine ROFI

“…This study, in addition to Becherer et al (2015), shows that a simplified ϕ equation such as Eq. 4 is a useful tool to capture the influence of processes such as straining, depth mean advection, nonlinear processes and stirring.…”

“…Turbulent quantities are necessary, which are difficult to measure in the field. One way of determining this term is using the eddy viscosity principle (Becherer et al 2015) . However, the collected dataset does not contain enough information to use this method.…”

“…Becherer et al (2015) used a similar approach in a curved tidal inlet in the German Wadden Sea. Their study demonstrated the importance of lateral circulation, in addition to classic estuarine circulation.…”