Lateral Variation of Tidal Mixing Asymmetry and Its Impact on the Longitudinal Sediment Transport in Turbidity Maximum Zone of Salt Wedge Estuary

Abstract

The lateral bathymetry in the estuary results in different degrees of tidal mixing asymmetry, which has significant impacts on the longitudinal sediment transport by changing the temporal variation of vertical eddy diffusion. This study focus on the lateral variation of tidal mixing asymmetry and longitudinal sediment transport at the landward boundary of turbidity maximum zone in the North Channel of Yangtze estuary, which is a typical time-dependent salt wedge estuary. A transect survey was carried out in December, 2018; five vertical profiles of flow velocity, salinity and suspended sediment concentration were simultaneously measured covering a spring tidal cycle. Analysis of the data revealed that, after the maximum ebb, the stratification in the main and secondary channel was stronger than that on the shoal. In the channel, during ebb tide, the stronger stratification restrained the turbulent mixing induced by vertical shear, vertical mixing during the flood tide was stronger than that during ebb tide and vertical mixing coefficients ranged from 0.06 to 0.12, showing regular tidal mixing asymmetry over a flood–ebb tidal cycle. Therefore, stronger eddy diffusion caused by vertical mixing resulted in higher suspended sediment concentrations during flood tide, the larger landward tidally averaged sediment transport rate was induced by tidal pumping with the transportation of flood tidal current and the net sediment transport over a flood–ebb tidal cycle in the channel was landward. Meanwhile, on the shoal, under the effect of vertical shear, the vertical mixing during flood tide was weaker than that during ebb tide; vertical mixing coefficients ranged from −0.27 to −0.02, showing the reversed tidal mixing asymmetry. Higher suspended sediment concentration was transported seaward by the ebb current, the tidally averaged sediment transport rate by both tidal pumping and advection was seaward and the net sediment transport was seaward. Furthermore, large river discharge increased the seaward advection sediment flux on the surface layer in the main channel, resulting in the seaward tidally averaged sediment flux. Strong resuspension increased the seaward advection sediment flux near the bottom in the main and secondary channel, resulting in the seaward tidally averaged sediment flux.

Impacts of Marine Construction on Tidal and Residual Characteristics: An Example from the Incheon Bridge

“…These increase and decrease in tidal asymmetry also affect the residual flows (Wang, Jeuken, and De Vriend, 1999). The increased vertical salinity gradient near the bridge can enhance the tidal asymmetry (Teng et al, 2022). In Gyeonggi Bay, the horizontal gradient of water temperature is low at less than 5 degrees, whereas the gradient of salinity is high at more than 20 psu.…”

Contribution of coastal seiches to sediment transport in a microtidal semi-enclosed bay

“…For all behaviors of the suspended sediments over the entire period, the horizontal fluxes for those uplifted above the position of suspended sediments assumed to be free of fluctuations by coastal seiches reached 51×10 −3 kg m −1 s −1 (standard SSC: 20 mg l −1 ) at M1 and 83×10 −3 kg m −1 s −1 (standard SSC: 20 mg l −1 ) at M2 (Figure 10). Although the sediment diffusion was suppressed by enhanced s 2 in the neap tides (N I and N II ) (Teng et al, 2022; Zhang…”

Size Distribution and Variation in Surface-Suspended Sediments in the North Passage Estuarine Turbidity Maximum of the Yangtze Estuary

“…Regardless of the year and season, the low slack water seems to be the most important tidal period because there are always large surface-suspended sediment concentrations and variations at that particular period. The SSSC recorded during low slack water could be the remaining SSSC from ebb resulting from the resuspension of the bottom deposited sediment by the stronger currents [52,59]. It can be observed that throughout the tidal periods, high SSSC location is slightly shifted in the upstream direction during ebb at stations C, D, and E in spring tides and in stations B and D during neap tides before coming back to its normal area D, E, and F at the other tidal phases, especially in spring tides (Figure 10b).…”