Experiments on the Transformation of Mud Flocs in Turbulent Suspensions

This dissertation aims to better understand how floc aggregate characteristics and behaviors are modified under different local conditions and how such alterations impact the floc settling velocity, which is one of the most crucial parameters influencing sediment transport modeling.

A series of laboratory experiments were conducted to examine the impact of suspended sediment concentration, mixes of clay and silt, and resuspension process to equilibrium floc size and floc settling velocity. In order to observe floc size evolution, a new floc imaging acquisition was first developed. This new method allows flocs in suspended sediment concentration up to C = 400 mg/L can be imaged non intrusively. This new method was applied in all three individual studies, which are composed of this dissertation.

The first chapter investigates the behaviors of flocs under constant and decay suspended sediment concentrations within a steady turbulent suspension. In the constant-concentration set of experiments, floc size time series were measured for 12 h for each of the concentration C = 15, 25, 50, 100, 200, 300, and 400 mg/L. In the decay-concentration experiments, clear water was introduced to the mixing tank, simultaneously the suspension was drained out of the mixing tank at the same rate to make the suspended sediment concentration reduce while the turbulent shear was remained unchanged. The data shows that the equilibrium floc size is a weak, positive function of concentration. For example, in order to increase 20% of floc size (approximate 22 um) the concentration needs to be increased by 700% (going from 50 to 400 mg/L). The data also illustrates that during the decrease of concentration from C = 400 to 50 mg/L, the floc size responses to the changes of concentration in the order of 10 min or less.

The second chapter examines how silt particles and clay aggregates interact in a turbulent suspension. Floc sizes and settling velocity of three different suspensions, i.e., pure clay, pure silt, and a mixture of clay and silt, were monitored. The floc size data show that the presence of silt particles does not have significant impacts on clay aggregate sizes. Silt particles, however, get bound up within floc aggregates, which in turn increase the settling velocity of the floc by at least 50%.

The third chapter examines whether any changes in floc properties during the deposition and resuspension processes. The floc sizes and shapes in a set of experiments with different consolidation times, concentrations, and shear patterns were measured. The conditions at which the flocs deposited or resuspended were maintained the same. The data reveal that floc size and shape of freshly deposited and after resuspended are unchanged. The erosion rate and concentration is a function of consolidation time and the applied shear stress during the deposition phase. Hence, there is a small reduction in resuspended concentration resulting in a slight decrease in resuspension floc size since floc size is also a function of concentration.