The Formation of Turbidity Maximum Zones by Minor Axis Tidal Straining in Regions of Freshwater Influence

Abstract

This study investigates the influence of tidal straining in the generation of turbidity maximum zones (TMZ), which are observed to extend for tens of kilometers along some shallow, open coastal seas. Idealized numerical simulations are conducted to reproduce the cross-shore dynamics and tidal straining in regions of freshwater influence (ROFIs), where elliptical current patterns are generated by the interaction between stratification and a tidal Kelvin wave. Model results show that tidal straining leads to cross-shore sediment convergence and the formation of a nearshore TMZ that is detached from the coastline. The subtidal landward sediment fluxes are created by asymmetries in vertical mixing between the stratifying and destratifying phases of the tidal cycle. This process is similar to the tidal straining mechanism that is observed in estuaries, except that in this case the convergence zone and TMZ are parallel to the shoreline and perpendicular to both the direction of the freshwater flux and the major axis of the tidal flow. We introduce the term minor axis tidal straining (MITS) to describe the tidal straining in these systems and to differentiate it from the tidal straining that occurs when the major axis of the tidal ellipse is aligned with the density gradient. The occurrence of tidal straining and the coastal TMZ is predicted in terms of the Simpson (Si) and Stokes (Stk) numbers, and top–bottom tidal ellipticity difference (Δε). Based on our results, we find that SiStk2 > 3 and Δε > 0.5 provide a limiting condition for the required density gradients and latitudes for the occurrence of MITS and the generation of a TMZ.

Observations of Multiple Internal Wave Packets in a Tidal River Plume

“…The occurrence of large Stokes numbers means that there is strong tidal mixing, in contrast to systems like the Columbia and the Mississippi. The presence of this large mixing intuitively suggests that the presence of the internal waves would be suppressed, however, the mixing is not strong enough to remove the freshwater stratification and it allows for the formation of counter rotating tidal currents, which drives tidal straining and the trapping of the fronts (Rijnsburger, Flores, Pietrzak, Horner‐Devine, et al., 2021; Flores et al., 2020) may be responsible for the maintenance of multiple IWS packets.…”

“…The mechanisms associated with cross‐shore tidal straining and the relationship to are discussed in detail in Flores et al. 2020. Tidal straining is a defining dynamical feature of the Rhine plume.…”

“…Tidal straining is a defining dynamical feature of the Rhine plume. It generates strong shear and mixing, impacts the ecosystem, and influences patterns in coastal turbidity (Flores et al., 2020; Huthnance, 1997; Joordens et al., 2001).…”

“…Model predictions find that turbulence is elevated at the tidal plume front (Fischer et al, 2009), which then influences coastal mixing and transport. Rhine plume fronts have been shown to propagate toward the coast, increase bed stresses and generate a cross-shore exchange flow causing offshore sediment transport (Flores et al, 2017(Flores et al, , 2020Horner-Devine et al, 2017;Rijnsburger et al, 2018).…”

The Formation of Coastal Turbidity Maximum by Tidal Pumping in Well‐Mixed Inner Shelves

“…Estuarine circulation processes have already been related to CTM formation (Burchard et al., 2008; Flores et al., 2020). Burchard et al.…”