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

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 Ri(g) and R-f is observed, with higher values attained during flood due to the higher sediment concentrations and the corresponding weaker velocity shear found at these times. R-f is found to increase with Ri(g) when Ri(g) < 0.25, and it attains a maximum value of similar to 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 Ri(g) numbers.