CHAPTER 2: LITERATURE REVIEW
2.7. Flocculation in an estuarine context
Flocculation in estuaries is sensitive to hydrodynamic conditions. Estuaries are characterized by varying salinity, shear rate and suspended sediment concentration. Variations in salinity occur as a result of tidal flow. Saline water is driven up into the estuary during flood tide and driven out during ebb tide. Periods of low velocity occur during the changing of tides. Shear rates are low during this time.
Shear rates and bed shear stresses vary during tidal cycles (Verney, 2006; Van Leussen, 1994;
Verney et al., 2010; Verney et al, 2009). Shear rates are typically lower during slack tides (Verney et al., 2010; Verney et al., 2009). Flocculation and deposition are favoured during slack tides, particularly low slack. Turbulent shear is generated by tidal flow, waves, wind-induced waves, and boat traffic (Verney et al., 2006). During tidal flow, turbulent shear is generated in boundary layers. Most turbulent kinetic energy is generated and dissipated in the region near the bed. This is a region of high shear stress where flocs are subject to breakup (Van Leussen, 1994). This region makes up 10-20% of the water depth (Van Leussen, 1994). Regions farther from the bed are characterized by lower shear stresses. Plate 2-7 illustrates the typical velocity profile and layers of turbulent flow. The natural range of shear rates observed in situ is 1-50s-
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1(Kumar et al., 2010). Values of the kolmogorov microscale observed in situ ranges from 300μm to 1000μm (Verney et al., 2009).
Plate 2-7: Typical velocity profile and layers of turbulent flow (adapted from Van Leussen, 1994)
SPM concentration varies on tidal and seasonal scales. Suspended cohesive sediment is the result of soil erosion in river catchments. Marine sediment also enters and leaves estuaries.
This is influenced by littoral processes and wave action. Sediment yield in catchments varies seasonally. During periods of high rainfall, higher sediment runoff results in more turbid fluvial systems the concentration of suspended sediment entering an estuary may vary significantly between seasons. This is particularly the case for rivers with highly variable runoff (e.g. Grenfell
& Ellery, 2009). Concentrations vary over tidal cycles due to deposition, erosion, and dilution in the marine zones of estuaries due to mixing with intruding saline water.
Suspended sediment concentrations of estuaries in literature vary considerably between estuaries. Concentrations may vary between a few mg/L to 10g/L in highly turbid estuaries (Wolanski et al., 1993; Grenfell & Ellery, 2009). The composition of SPM may also vary seasonally (Verney et al., 2009).
30 Erosion and Deposition
Flocs are eroded or deposited depending on the hydrodynamic conditions. Flocs formed during the process of aggregation settle towards the bed. Flocs are deposited on the bed when the bed shear stress is less than the critical shear stress for deposition, τd (Bureau of Reclamation, 1996). If the shear stresses near the bed are too high, flocs are broken up and re-circulated in the water column. In flume experiments Krone (1962, in Bureau of Reclamation, 2006) found τd
to range between 0.06 and 0.078 N/m2.
Erosion occurs when the shear stress at the bed exceeds the critical shear stress for erosion, τc
(Bureau of Reclamation, 2006; Verney, 2006). The critical shear stress is the resistance of the bed to erosion, it is a function of the bed consolidation, proportion of silt, surface roughness, and biological activity in the benthic layer (Verney, 2006).
Continuous erosion and deposition cycles occur in the estuarine turbidity maximum, where the highest suspended sediment concentrations are observed (Van Leussen, 1994). Many estuaries are characterized by erosion-deposition cycles (Fugate & Friedrichs, 2002; Wolanski & Gibbs, 1995). Plate 2- 8illustrates the flocculation process in the water column.
Mixing and Stratification
It is common for stratification to occur in estuaries; whereby two distinct layers form in the water column, one of freshwater and another of saline water. Stratification inhibits mixing between layers, thus influencing flocculation. Other estuaries are well mixed with no stratification .In most estuaries there is a region of high turbidity which occurs due to mixing, this is known as the estuarine turbidity maximum.
Plate 2-8
:
The flocculation process in the water column with erosion and deposition processes. (Adapted from Maggi, 2006)31
2.8. Summary
Flocculation is the dynamic process which controls the behaviour of suspended cohesive sediment. The process is characterized by the formation and breakup of sediment flocs.
Flocculation influences the transport of cohesive sediment. It is stimulated and controlled by a set of drivers. There are three primary drivers of the process – (1) turbulence, a physical driver;
(2) SPM concentration, a physical and statistical driver; and (3) salinity, a chemical driver.
There are numerous other drivers of lesser importance which influence flocculation such as organic matter content, clay mineral content, pH and temperature. The primary drivers have been extensively studied while fewer studies have focussed on the influence of the secondary drivers. Studies on flocculation have been conducted both in situ and in laboratories.
Quantitative results from such studies are mostly empirical and express the settling velocity or floc size as functions of one or more of the primary drivers. The application of quantitative empirical results is limited to particular estuaries or silt types. It is thus necessary to formulate a controlled method to study the flocculation characteristics of a particular estuary or silt type.
Such a method should minimize disruption to flocs given their fragile nature. Qualitative observations in literature of the influence of each driver on flocculation provide an indication of the likely outcome of future studies on flocculation.
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