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.

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

“…Although the water bodies are well‐mixed in the nearshore region, a slight salinity stratification exists within waters over the deep offshore (Figures 3 and 4), where tidal straining may have an important effect on sediment convergence (Burchard et al., 2008; Flores et al., 2020). Note that sediment‐induced vertical stratification is also responsible for sediment trapping (van Maren et al., 2020; Zhu et al., 2021).…”

“…The more general choice of simulating coastal sediment transport would be a numerical model (e.g., Flores et al., 2020; van Maren et al., 2020). Setting up a 3D numerical model can help include some processes that are excluded in the analytical solution, and the numerical results can be compared to the observations and analytical calculations.…”

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

“…Although the water bodies are well‐mixed in the nearshore region, a slight salinity stratification exists within waters over the deep offshore (Figures 3 and 4), where tidal straining may have an important effect on sediment convergence (Burchard et al., 2008; Flores et al., 2020). Note that sediment‐induced vertical stratification is also responsible for sediment trapping (van Maren et al., 2020; Zhu et al., 2021).…”

“…The more general choice of simulating coastal sediment transport would be a numerical model (e.g., Flores et al., 2020; van Maren et al., 2020). Setting up a 3D numerical model can help include some processes that are excluded in the analytical solution, and the numerical results can be compared to the observations and analytical calculations.…”

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.…”

Incorporating Non-Equlibrium Ripple Dynamics into Bed Stress Estimates Under Combined Wave and Current Forcing

“…Tides along the Dutch coast are semidiurnal (M2-dominated) with amplitudes typically ranging between 1 and 2 m (Figure 2b). In this region of the North Sea, the tide behaves as a progressive Kelvin wave such that tidal elevation is in phase with the tidal currents; maximum flood (northeast) currents occur at high water, whereas maximum ebb currents (southwest) occur at local low water [53][54][55]. Maximum near-bottom currents at 0.25 mab (meters above the bed) were typically on the order of 0.25-0.3 ms -1 , although peaks of approximately 0.5 ms -1 were frequently observed during spring tides (Figure 2c).…”

“…Numerical models, such as ROMS or COAWST, usually require M and τ cr to be specified by the user for sediment transport calculations (e.g., refs. [55,60]). Field estimates of M and τ cr have been obtained by setting E = αw ′ c ′ , where α is a proportionality constant that is a function of the distance above the bed [48].…”

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

“…Although the water bodies are well‐mixed in the nearshore region, a slight salinity stratification exists within waters over the deep offshore (Figures 3 and 4), where tidal straining may have an important effect on sediment convergence (Burchard et al., 2008; Flores et al., 2020). Note that sediment‐induced vertical stratification is also responsible for sediment trapping (van Maren et al., 2020; Zhu et al., 2021).…”

“…The more general choice of simulating coastal sediment transport would be a numerical model (e.g., Flores et al., 2020; van Maren et al., 2020). Setting up a 3D numerical model can help include some processes that are excluded in the analytical solution, and the numerical results can be compared to the observations and analytical calculations.…”

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.…”

Incorporating Non-Equlibrium Ripple Dynamics into Bed Stress Estimates Under Combined Wave and Current Forcing

“…Tides along the Dutch coast are semidiurnal (M2-dominated) with amplitudes typically ranging between 1 and 2 m (Figure 2b). In this region of the North Sea, the tide behaves as a progressive Kelvin wave such that tidal elevation is in phase with the tidal currents; maximum flood (northeast) currents occur at high water, whereas maximum ebb currents (southwest) occur at local low water [53][54][55]. Maximum near-bottom currents at 0.25 mab (meters above the bed) were typically on the order of 0.25-0.3 ms -1 , although peaks of approximately 0.5 ms -1 were frequently observed during spring tides (Figure 2c).…”

“…Numerical models, such as ROMS or COAWST, usually require M and τ cr to be specified by the user for sediment transport calculations (e.g., refs. [55,60]). Field estimates of M and τ cr have been obtained by setting E = αw ′ c ′ , where α is a proportionality constant that is a function of the distance above the bed [48].…”

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

“…Although the water bodies are well‐mixed in the nearshore region, a slight salinity stratification exists within waters over the deep offshore (Figures 3 and 4), where tidal straining may have an important effect on sediment convergence (Burchard et al., 2008; Flores et al., 2020). Note that sediment‐induced vertical stratification is also responsible for sediment trapping (van Maren et al., 2020; Zhu et al., 2021).…”

“…The more general choice of simulating coastal sediment transport would be a numerical model (e.g., Flores et al., 2020; van Maren et al., 2020). Setting up a 3D numerical model can help include some processes that are excluded in the analytical solution, and the numerical results can be compared to the observations and analytical calculations.…”

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.…”

Incorporating Non-Equlibrium Ripple Dynamics into Bed Stress Estimates Under Combined Wave and Current Forcing

“…Tides along the Dutch coast are semidiurnal (M2-dominated) with amplitudes typically ranging between 1 and 2 m (Figure 2b). In this region of the North Sea, the tide behaves as a progressive Kelvin wave such that tidal elevation is in phase with the tidal currents; maximum flood (northeast) currents occur at high water, whereas maximum ebb currents (southwest) occur at local low water [53][54][55]. Maximum near-bottom currents at 0.25 mab (meters above the bed) were typically on the order of 0.25-0.3 ms -1 , although peaks of approximately 0.5 ms -1 were frequently observed during spring tides (Figure 2c).…”

“…Numerical models, such as ROMS or COAWST, usually require M and τ cr to be specified by the user for sediment transport calculations (e.g., refs. [55,60]). Field estimates of M and τ cr have been obtained by setting E = αw ′ c ′ , where α is a proportionality constant that is a function of the distance above the bed [48].…”