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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]

Xu, C. and Dong, P., 2017. A dynamic model for coastal mud flocs with distributed fractal dimension. Suspended sediments entrained from muddy estuarine and coastal areas usually contain a large amount of mud flocs of various sizes and densities. The size and settling velocity of these mud flocs are unsteady and may vary over a large range. In most theoretical descriptions the mud flocs are treated as self-similar fractal entities with the fractal dimension being considered as either a constant or a simple function of the mean floc size. This deterministic description of fractal dimension has recently been found to be inadequate as for a given size class; fractal dimension of the mud flocs is not a single value but is distributed over a certain range. To address this problem this paper proposes a new flocculation model for the temporal evolution of floc size by considering the fractal dimensions for a given floc size class D to be normally distributed and validates the model with available experimental data. The proposed model is found to perform better in predicting the temporal evolution of floc size than that based on a single fixed floc-size-dependent fractal dimension, especially under high shear conditions and with large equilibrium floc size. [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]