Suspended sediment in most estuaries is found in the form of floes or aggregates. The movement of sediments in estuaries is important in the maintenance of port areas, the transport of adsorbed contaminants and the turbidity of the water column, which affects the penetration of light. Turbidity is not only related to the concentration of sediment, but also to the particle size distribution of the sediments,
as small particles diffract more light than large ones.
Most sediment transport models can not predict changes in turbidity and particle size as they do not have any information about the particles' size distribution and their changes. This is partly due to the fact that the modelling of flocculation is highly computationally demanding, and partly because there is not a solid framework to describe the flocculation of sediments. The particle size distribution of the floes is constantly changing under the influence of the flow conditions through flocculation and breakage. The characteristics of the floes
such as size and porosity determine their settling behaviour and ultimately their fate.
A set of flocculation experiments were performed using sediments from the Brisbane River, Australia. The sediments were flocculated in a Couette device, with particle size measured on-line using laser reflectance, to evaluate the influence of shear and salinity on the flocculation efficiency and breakage rates.
Particle size dynamics were modelled using population balances to provide a measure of collision efficiencies and breakage rates of estuarine sediments. The population balance equations used flexible grids instead of the fixed grid methods common in most flocculation studies. The grid flexibility allowed the use of a reduced set of equations resulting in increased computational efficiency. Flocculation was modelled using the shear mechanism, and breakage was considered to occur by erosion and splitting. The collision efficiency was found to be inversely proportional to the increase in shear and to a minor extent, directly proportional to an increase in salinity. The breakage rates are directly proportional to shear but do not depend on salinity.
The settling velocity of individual floes was measured using a settling column. The use of a weak density gradient was sufficient to eliminate convection currents in the column without interfering with the settling of the floes. Particle size and settling velocity were determined using image analysis and it is shown that settling velocity increases with particle size, but with a large variability for a given size. The results also show that when flocculated in the laboratory, estuarine particles have a smaller settling velocity than before flocculation, probably due to the reduction in flow through the floes.
The population balance model and the settling velocity information were used to solve a model with the advection diffusion equation coupled with flocculation and breakage. The one dimensional model was used to simulate the changes in particle size and concentration in a water column. The results were compared to data collected around a slack tide in the Brisbane River. The model could track the changes in the concentration profile and particle size distribution through most of the water column. It is demonstrated that the results are highly dependent on vertical diffusion and settling velocity description. It is
also shown that the number of equations in the population balance can be reduced to gain computational efficiency while still achieving good results.