Flocculation under the control of shear, concentration and stratification during tidal cycles


Tide-dominated estuaries are often characterized by a high variability of turbulent shear, suspended particulate matter (SPM) concentration and salinity, which poses challenges for a comprehensive understanding of its mass transport including cohesive sediment dynamics. Here, a combined in situ and numerical study was undertaken to investigate the mechanism of flocculation during tidal cycles, with the aim to disentangle the impacts of turbulent shear, SPM concentration and salinity on flocs. Results show that microflocs (20–200 μm) dominate in the Pearl River Estuary and floc size variation is caused primarily by exchange between flocculi (4–20 μm) and microflocs. We also identified a critical shear rate (G* ≈ 5/s) below which floc exchange occurs slowly. Above the threshold, the particle size distribution is left-skewed and clustered below 60 μm. Evolutions of flocs with different initial sizes synchronize gradually to adapt to the local hydrological environment. The trends of floc size evolution and absolute net flocculation rates are similar among diverse tidal shear cycles. The reason can be attributed to the turbulent shear which enhances both aggregation and breakup processes, thereby limiting the floc size in a certain range. The higher the concentration, the larger both the particle size and the range of variation. In addition, results of numerical modelling reveal that the flocculation time for primary particles is inversely proportional to shear and concentration. A critical concentration (C* ≈ 50 mg/L), below which the impact of concentration on the equilibrium diameter of flocs is more than twice as strong as shear, whilst above which the equilibrium diameter is inversely proportional to the Kolmogorov microscale and weakly correlated to concentration, was also identified. Furthermore, halocline was found to increase vertical variation of flocs size, suggesting co-existence of different flocculation mechanisms across this layer.
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