CHAPTER 5: SUMMARY AND CONCLUSIONS
5.1. Summation
A laboratory technique was developed to investigate flocculation of fine sediments from the Mfolozi and St Lucia estuaries. The tests were performed on suspended sediments sampled from the Mfolozi and St Lucia estuaries. The sediments were silt-dominant with a small clay fraction. Sediment behaviour corresponded to that of cohesive sediment. The technique involved the use of an agitated beaker, still settling column and digital imaging to monitor sediment behaviour. Flocculation was simulated in the agitated beaker. Flocculation drivers were varied and their influence on the formation or destruction of aggregates was measured by digital imaging. The settling velocities of aggregates were measured by analysing images of settling flocs in the still settling column. The laboratory technique was able to fulfil the objectives of the investigations. Different aspects of sediment behaviour were investigated:
aggregation, deflocculation, settling velocity, effective density and particle shape. The results of these are summarized in the subsections which follow.
5.1.1. Aggregation behaviour
The development of aggregates in the agitated beaker over 70minutes was measured. The suspended sediment concentration, shear rate, and salinity of the sediment solution were varied in different tests. The flocculation timescale of both sediments tested was observed to be
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70 minutes Aggregate growth was observed to occur at low shear (10s-1) in the presence of salt. Aggregate growth was most visible in the 90th percentile of the floc size distribution.
Aggregate populations were dominated by microflocs (fine aggregates generally smaller than 100μm). By contrast macroflocs formed a small fraction of the population. The volume-based floc size distribution of aggregates did not follow the population size distribution. It instead showed that the sediment mass was concentrated over a particular size range. This size range depended on the drivers of flocculation. Fine microflocs could not be detected. It is uncertain as to the percentage of total mass these particles constituted. Crude investigations suggested that a bimodal volume-based floc size distribution was present.
Aggregate growth occurred in the presence of salinity. Salinity increased the flocculation potential of the cohesive sediments. Aggregates which formed in saline conditions were larger than those which formed in fresh conditions.
Aggregate growth increased when the suspended sediment concentration increased. The frequency of effective collisions between particles increased with the concentration. This was difficult to observe at high concentrations due to floc overlap in images. Aggregate growth was inhibited at high shear rates. Any flocs which formed were broken up by the high shear stress.
5.1.2. Influence of turbulence
The influence of turbulence on aggregate breakup was investigated in deflocculation tests. This was done by incrementally increasing the shear rate in the agitated beaker. Aggregate size decreased when the shear rate increased. The flocs were broken up due to shear stresses.
The size of aggregates was limited by the Kolmogorov microscale. The largest aggregates observed were smaller than the size of the smallest eddies present.
Turbulence is necessary for aggregation to occur as it induces mixing. At low shear rates, turbulence stimulates aggregation while at higher shear rates it inhibits flocculation by breaking up aggregates. The interaction between turbulence and suspended sediment concentration drives flocculation. It is the combination of these drivers which generates collisions between particles. The probability of aggregate formation as a result of collisions is influenced by the chemical properties of the sediment and solution. This is largely controlled by salinity. The strength of aggregates formed is dependent on this chemical property. Turbulence interacts with the chemical properties to control the rate of aggregate break-up. The points made in this summary have been substantiated by observations in the aggregation and deflocculation tests.
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The settling velocities of flocs which formed during the aggregation tests were measured in a still settling column. Flocs were sampled using a pipette with an opening of sufficient width to prevent floc breakup. Floc size, effective density, and shape parameters were also measured.
Floc settling velocity increased with floc size. Settling velocities ranged from 0.05mm/s to 2mm/s. Fine particles settled at the lower end of this range while macroflocs settled at the upper end. The range of settling velocities observed corresponded to those discussed in chapter 2. The settling velocity of fine microflocs could not be detected. This was however investigated in quiescent settling tests. The settling velocity of these particles was shown to be significantly lower than macroflocs and large microflocs. Bimodal settling behaviour was observed where larger particles settled rapidly during the initial part of the test, thereafter fine particles settled very slowly.
The effective density of flocs decreased as floc size increased. The densities of macroflocs ranged from 1050 to 1100kg/m3 while those of finer particles were typically higher, often matching those of quartz particles. The observed densities suggest that macroflocs are loosely bonded-porous particles while microflocs are denser and more compact particles.
The settling velocities, densities and size range of flocs formed in fresh water were lower than those which formed in saline water. Flocs were typically ellipsoidal in shape. Floc shape did not vary with the drivers of flocculation. Floc shape also did not vary with floc size.
Floc settling velocity did not vary directly with the drivers of flocculation. It varied with floc size.
The floc size distribution varied with the drivers of flocculation. It is therefore necessary to view settling velocities together with the results of aggregation tests in order to understand settling behaviour.
5.1.4. Effectiveness of laboratory technique
The techniques employed were able to demonstrate the behaviour of cohesive sediment as discussed in chapter 2. Aggregate formation, breakup, settlement, and other parameters were all measured. Digital imaging techniques in conjunction with simple laboratory tests may be effectively used to study flocculation. There were however some limitations to the test. The resolution was insufficient to allow the observation of particles finer than 20μm in size. This hindered the analysis of fine and unflocculated solutions. Images could not be analysed at high concentrations due to floc overlap and poor light transmission through the beaker. There are a limited range of conditions in which the technique may be effectively employed.
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In situ sediment behaviour may differ to laboratory behaviour. The laboratory experiment involved isolating and precisely controlling various drivers and conditions (shear rate, salinity, concentration). This does not occur in the field where drivers vary temporally and spatially.
Field conditions are more complex than the simple, controlled environment of an agitated beaker. Sediment will likely respond similarly to drivers in the field. However it is unlikely that the lab conditions will occur in the field for a period of time corresponding to the laboratory timescale. These comments reflect the shortcomings common to most laboratory investigations, which limit their ability to accurately simulate estuarine processes.
5.1.5. General conclusions
Cohesive sediments are present in solution as aggregates. Aggregates are predominantly microflocs, which are fine, robust, densely-compacted particles formed from a combination of primary particles (silt and clay particles). Large flocs known as macroflocs form under certain conditions favour aggregate formation. This typically occurs at low shear rates in the presence of salt. Macroflocs are porous, loosely-bound, low density aggregates which are easily broken up by turbulence. Macroflocs have higher settling velocities than microflocs. When formed, macroflocs dominate the vertical settling flux a suspension. The settling velocities of cohesive sediments are significantly lower than those quartz particles. Flocculation influences the size distribution and settling velocities of cohesive sediments. Flocculation enhances the rate at which settlement occurs. The results of this investigation have implications when considered in the context of the estuarine environment.
Given the observed flocculation timescale of 70minutes, aggregates grow in situ under various conditions. There is potential for aggregation in the saline regions of estuaries, particularly at the freshwater-saline water interface. This is likely to occur in regions of low turbulence.
Flocculation may potentially enhance in situ settling rates. Furthermore large settled aggregates may be resuspended and broken up to form part of a flocculation cycle which occurs with the tidal cycle. These comments require investigation in the field. Laboratory results also require validation. From this it is recommended that a field investigation be undertaken to validate laboratory results and to provide insight into the field processes which occur in the St Lucia and Mfolozi estuaries.