Two-phase Flow Modelling of Mud Flocs Sedimentation in Coastal Waters

  • Chunyang Xu

Student thesis: Doctoral ThesisDoctor of Philosophy


Cohesive sediments can be found widely such as on continental shelves, at estuaries and in coastal waters. Various engineering problems such as siltation of navigation channels and polluting coastal environment can be caused by the cohesive sediments. To solve these problems, field/lab experiments and numerical models are widely used to study the cohesive sediment transport processes.

The flocculation process is usually neglected in the conventional cohesive sediment transport models, despite the fact that flocculation process can significantly influence the dynamics and fate of cohesive sediment. In the last decades, a large number of studies had attempted to describe flocculation effects by relating the settling velocity of mud flocs with mud flocs properties (size, shape and effective density) empirically based on various flocculation models for dilute suspension. A few investigations have also been reported on the cohesive sediment transport processes with high concentration, but the interactions of fluid and high-concentrated suspended sediment under the action of combined waves and currents remain poorly understood. This thesis investigates the cohesive sediment transport process in coastal waters numerically, especially focusing on the flocculation process. The investigation covers a number of aspects of sedimentation processes of cohesive sediments and the insight gained and models developed represent a major advance in understanding the cohesive sediment transport in coastal estuarine waters.

Firstly, 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 in most previous theoretical descriptions. This deterministic description of fractal dimension has recently been found to be inadequate, because for a given size class, fractal dimension of the mud flocs is not a single value but distributed over a certain range. To address this problem, a new flocculation model is proposed in the thesis in which the fractal dimensions for a given floc size class D are taken to be normally distributed rather than a constant. The model is fully validated with available experimental data on the temporal evolution of floc size.

Secondly, a two-phase model for prediction of cohesive sediment suspension is developed and validated using lab experiments. The flocculation process is taken into consideration by incorporating a new drag force closure into the two-phase flow model. This new drag force closure is related to the settling velocity of mud flocs affected by suspended sediment concentration (SSC). The new two-phase model is applied to the simulation of sediment suspension at EMS/Dollard estuary for two measuring period (in June and August 1996). Numerical results are compared with the measured variations of bed shear stresses and sediment concentrations at different elevations above the sea bed where the flocculation process is known to influence the vertical profile of settling velocities and thus the distribution of SSC throughout the water column.

Finally, the two-phase flow model is applied to predict sedimentation processes under both wave and current conditions. The momentum transfer between the two phases is represented by a drag term and the mixing length model is modified to take into account the buoyancy effects due to the gradient of suspended sediment concentration near the seabed. Quantitative comparisons for intra-tide variations of flow properties and mud concentration between the model and the measurements are presented. An interpretation on how the existence of a fluid mud layer may affect the calculated concentration profile and aspects for further improvement of the model are discussed.
Date of Award2016
Original languageEnglish
SupervisorPing Dong (Supervisor)

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