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The flocculation of cohesive sediment particles is a function of the collision efficiency of sediment particles, which is mainly influenced by local flow hydrodynamics. A detailed study on local hydrodynamic characteristics in a novel stirred tank was done to measure the flow field of a turbulent flow. Instantaneous flow velocity fields were obtained and processed, and the average flow field was computed. Particle image velocimetry (PIV) results measuring the hydrodynamic characteristics (expressed as turbulent kinetic energy (TKE), local dissipation rate of TKE, velocity gradient or shear rate, turbulent shear stress, etc.) were analyzed and discussed. Particle tracking and automated image processing techniques were utilized to analyze the impact of the hydrodynamic conditions on the floc and settling properties. The differences in floc properties under the two turbulent shear states (i.e., steady-state (SS) and unsteady-state (US)) hydrodynamic conditions were examined. The results show that floc size and floc size distribution (FSD) of pure kaolin clay flocs are significantly influenced by the local dissipation rate of TKE and the turbulent intensity states. The floc size, d f, 84 , at local dissipation rate of TKE of 11 × 10−4, 29 × 10−4, 44 × 10−4, 83 × 10−4, and 142 × 10−4 m2/s3 is 97, 108, 107, 124, and 150 μm, respectively. It was found that by increasing the local dissipation rate of TKE, the FSD was skewed left. The US shear condition generated larger flocs (and in higher proportions) relative to the SS condition. The settling velocity and flux settling velocities of flocs formed under SS conditions are higher than those formed under US conditions. The fractal dimension of SS flocs also is higher than the fractal dimension of US flocs. These results highlight a significant impact of the states of hydrodynamics on floc properties such as floc size and FSD as well as the settling and morphological properties of flocs. [ABSTRACT FROM AUTHOR]

Understanding and quantifying mud suspension and sediment transport processes are of great importance for effective exploitation and sustainable management of estuarine environments. Event-based predictive models are widely used to identify the key interactions and mechanisms that govern the dynamics involved and to provide the essential parameterisation for assessing the long-term morphodynamic evolution of the estuaries. This study develops a one-dimensional-vertical (1DV) Reynolds averaged two-phase model for cohesive sediments resuspension driven by tidal flows. To capture the time-dependent flocculation process more accurately, a new drag force closure which relates empirically to settling velocity of mud flocs with suspended sediment concentration (SSC) is incorporated into the two-phase model. The model is then applied to simulate mud suspension in the Ems/Dollard estuary during two periods (June and August 1996) of tidal forcing. Numerical predictions of bed shear stresses and sediment concentrations at different elevations above the bed are compared with measured variations. The results confirm the importance of including flocculation effects in calculating the settling velocity of mud flocs and demonstrates the sensitivity of prediction with the settling velocity in terms of flocs concentration. Although the two-phase modelling approach can in principle better capture the essential interactions between fluid and sediment phases, its practical advantages over the simpler single phase approach cannot be confirmed for the data periods simulated, partly because the overall suspended sediment concentration measured is rather low and the interaction between the two phases is weak and also because the uncertainties in the relationship between the settling velocity and flocs concentration. [ABSTRACT FROM AUTHOR]

Flocculation is an integral part for cohesive sediment transport processes as it will further influence other processes such as settling, deposition as well as erosion processes. To understand the impacts of flocculation on entrainment process of fluid mud with different salinities, a series of entrainment and sedimentation experiments were conducted. Entrainment experiments were conducted under constant hydrodynamic conditions driven by a shear ring on the top of an annular flume. While the sedimentation experiments were conducted within cylinders with different diameters. Results show that fluid mud samples with different salinities can form different distributions of sediment concentration in the vertical direction, which will significantly influence the entrainment rate of fluid mud. Results also show that entrainment rate of fluid mud with higher salinity (E18) is smaller than that of fluid mud with lower salinity (E4) in the upper layer of fluid mud. Sedimentation tests show that the settling rate of fluid mud sample with higher salinity is enhanced and the gelling concentration is reached at a lower sediment concentration. These two mechanisms cause the different distributions of sediment concentration for fluid mud under different flocculation states. Accordingly, the mud characteristics such as density, viscosity, yield strength, buoyancy effects and eventually the entrainment rate vary with different fluid mud samples (the initial sediment concentrations are the same). An effort has also been made to predict the entrainment rate with existing equation. The good agreement between from equation and experimental results further demonstrates that the flocculation process should be considered when calculating the entrainment rate, as accurate prediction of the entrainment rate of fluid mud requires the details of sediment concentration and rheological properties of fluid mud in the vertical direction. • Entrainment and sedimentation tests were conducted in this study. • The flocculation effects will enhance the settling of fluid mud layer. • Fluid mud with higher salinity reaches gelling point at low concentration. • Flocculation effects make the sediment distributed uniformly in the mud layer. • Vertical structure has to be considered to get realistic entrainment rate. [ABSTRACT FROM AUTHOR]