Abstract:
Particularly in streams, rivers and estuaries, flocculation, the process in which sediment particles join together, is a significant control on the dynamic behaviour (settling, suspension, dispersal etc) of fine sediments as this process alters the sizes and settling velocities of the fines relative to the primary particles. To improve our understanding of the flocculation process, a combined approach of field measurements and modelling of the process was adopted in this thesis. In order to investigate flocculation under field conditions, concurrent measurements of vertical profiles of in-situ particle size distribution _(PSD), primary PSD, salinity, turbulence and suspended sediment concentration (SSC) were made in a tidal channel in the Waitetuna arm of Raglan (Whaingaroa) Harbour, New Zealand. Guidance regarding measurements of PSDs using various means were developed within the present study. The mathematical basis for the flocculation model developed in this thesis is the population balance equation (PBE). PBEs are used extensively in the fields of chemical engineering, wastewater treatment and atmospheric aerosols in order to model temporal changes in PSDs. The implemented PBE model FLOCSIM included the use of advanced modelling kernels to accurately represent the respective influences on flocculation of turbulence and SSC. In addition to the use of advanced modelling kernels, a fractal description of floes was implemented, with a new formulation for the floe factor (representing the degree of flocculation) based solely on particle-size information. The major finding of the field tests is that the degree of flocculation is in phase with both SSC and the rate of turbulence energy dissipation (E) for the measured conditions. Using FLOCSIM as an explorative tool, simulations showed that respective changes in E and SSC typically have opposite effects on flocculation. The field observations and simulations would then suggest that SSC is the primary driver of flocculation at the measurement site as any breakage associated with increasing E is insufficient to overcome the enhanced aggregation driven by the associated increase in SSC. The field measurements also provide some evidence in support of saltwater flocculation. The successful simulations of the field measurements using the FLOCSIM model highlights that the model is at least capable of reproducing general trends in flocculation. This encouraging result, demands further development and testing of the model, potentially in conjunction with numeral modelling of transport processes and floc settling and resuspension processes.