The phenomenon of flocculation has an important role in sediment transport processes such as settling, deposition and erosion. In this study, laboratory experiments were performed to investigate the effect of key hydrodynamic parameters such as suspended sediment concentration and salinity on floc size and settling velocity. Results were compared with previous laboratory and field studies at different estuaries. Materials and methodsĮxperimental tests were conducted in a 1-L glass beaker of 11-cm diameter using suspended sediment samples from the Severn Estuary.

A particle image velocimetry system and image processing routine were used to measure the floc size distribution and settling velocity. The settling velocity was found to range from 0.2 to 1.2 mm s −1. Settling velocity changed in the case of increasing suspended sediment concentration and was controlled by the salinity. The faster settling velocity occurred when sediment concentration is higher or the salinity is lower than 2.5.

On the other hand, at salinities higher than 20, in addition to increasing SSC, it was found that the situation was reversed, i.e. the lower the sediment concentration, the faster the settling velocity. Sediment flocculation is enhanced with increasing sediment concentration but not with increasing salinity.Ĭohesive sediments are regarded as one of the most important features of estuaries around the world. The size of cohesive sediment particles normally ranges from 0.98 to 63 μm (Hjulström 1935). Under certain conditions, these sediments come together (flocculate) to form large aggregates, namely flocs, which are larger but less dense than individual particles. This flocculation phenomenon has a strong influence on the sediment transport processes of deposition, erosion and settling (Fennessy et al. Improving understanding of the significance of flocculation processes is highly desirable because they may exert an impact on the pollutants such as nutrient and heavy metals by which more floc can remove nutrient from the water system (Manning et al. Moreover, the formation of flocs near the surface decreases the penetration of sunlight through the water column, which constrains the production of plankton. These processes have a direct effect on water quality. The accurate prediction of the settling velocity of cohesive sediment in Severn estuary is required for better understanding of (a) estuarine bathymetry changes, (b) where the deposition and resuspension occur and (c) estuarine contaminant transport processes (Kirby et al. In general, flocs are classified into two types, namely, microflocs and macroflocs cohesive sediment flocculates to form small microflocs first, and then macroflocs by combining the microflocs (Eisma 1986 Manning 2001). Microflocs can be classified as those aggregates which do not exceed a spherically equivalent diameter of 100 μm and have a settling velocity of less than 1 mm s −1 (Lafite 2001). The state of microflocs continually changes in response to the hydrodynamic parameters, the physico-chemical and the environmental conditions. These microflocs can develop into larger flocs called macroflocs which behave very differently. Macroflocs have a diameter larger than 100 μm and a settling velocity between 1 and 15 mm s −1(Fennessy et al. 1994 Manning and Dyer 1999 Whitehouse et al. 2000 Lafite 2001 Manning 2001 Manning 2004a Manning and Dyer 2007 Manning et al. 2010b Manning and Schoellhamer 2013 Manning et al.

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