Turbulence, Sediment‐Induced Stratification, and Mixing Under Macrotidal Estuarine Conditions (Qiantang Estuary, China)

Abstract

Time series of in situ measured velocity and suspended sediment concentration from Qiantang Estuary (China), and estimates of turbulence and sediment stratification parameters are presented. The data span a period of 9 days, and after phase averaged, they are used to explore spring‐neap tidal variations in flow, turbulence, and sediment stratification. A local balance between shear production, sediment‐induced buoyancy flux, and dissipation is found to hold during ebb for both neap and spring tides. During flood elevated turbulence dissipation rates are observed, attributed to nonlocal turbulence, most likely due to horizontal advection. Our results show that the effect of sediment stratification is successfully parameterized by adding the Monin‐Obukhov length scale to the classical logarithmic layer theory. Flood‐ebb asymmetry in both Rig and Rf is observed, with higher values attained during flood due to the higher sediment concentrations and the corresponding weaker velocity shear found at these times. Rf is found to increase with Rig when Rig < 0.25, and it attains a maximum value of ~0.2. Our estimates, consistently with those from previous studies, fall slightly below the commonly used theoretical predictions. Sediment stratification contributes to the decay of turbulence, and weak mixing still exists under high Rig numbers.

Bottom Drag Variations Under Waves and Currents: A Case Study on a Muddy Deposit off the Shandong Peninsula

“…For ebb tides, stronger stratification was observed during the event period. Previous studies have shown that sediment stratification could suppress turbulence and reduce the bottom drag (Adams and Weatherly, 1981;Tu et al, 2019;Wu et al, 2022). However, when comparing data from before and after the event, estimations C D were generally the same during ebb tides (Figure 10).…”

Acoustic Observations of Kelvin‐Helmholtz Billows on an Estuarine Lutocline

“…Vertical gradients in SSC were stronger at ebb than at flood, especially at late ebb. This is due to the flood‐ebb asymmetry in mixing (Lian & Liu, 2015; Simpson et al, 1990; Tu et al, 2019). During the flood tide, sediment tends to be resuspended by strong current and then mixed into the upper water column (e.g., −2 hr relative to second high water [referred to as HW hereafter] in Figure 2b), resulting in relatively homogeneous SSC profiles (Figure 2e).…”

Scaling the Mixing Efficiency of Sediment‐Stratified Turbulence

“…Flood tide begins with the passage of one or more powerful tidal bores whose effects modify the subsequent turbulence. Turbulent dissipation rates are elevated compared to ebb tide (Figure 1b; also Tu et al, 2019). In contrast, the buoyancy flux (Figure 1b) is low during flood tide, resulting in very low (Figure 1c), suggestive of a strongly turbulent boundary layer (e.g., W. D. Smyth, 2020).…”