The flocculation dynamics of cohesive sediments in the St. Lucia and Mfolozi estuaries, South Africa.
Increasing turbidities due to land use changes and poor catchment management can cause negative impacts on estuaries worldwide. High turbidity has an impact on the biological functioning of estuaries which are amongst our most productive ecosystems. This study focuses on the St Lucia estuary on the east coast of South Africa, a UNESCO World Heritage Site and Ramsar wetland of international importance. Increased turbidity due to suspended inorganic sediments has been identified as an important threat to the sustainability of biodiversity in the St Lucia system. In order to determine the influence of increased cohesive sediment loads on the estuarine system it is necessary to understand how flocculation affects the fate and transport of cohesive sediment. Flocculation describes the processes of aggregate formation and breakup. Suspended sediment concentration, salinity and turbulent shear rates have been identified as key drivers of estuarine flocculation. This study investigates flocculation by measuring how the floc size distribution and settling velocities of flocs vary with the key drivers. A laboratory technique was developed where flocculation was simulated in an agitated beaker. Digital imaging techniques were used to measure changes in the size of flocs within the beaker and floc settling velocities in a still settling column. Results show reduced aggregation and floc size with increases in turbulent shear. Floc settling velocities were observed to increase with floc size while the effective density was observed to decrease. The study is concluded by investigating potential applications for the results obtained.