Flocculated suspended sediment in a micro-tidal environment

Silt and clay particles are unstable when suspended in saline waters. They coagulate into sediment flocs having settling velocities orders of magnitude higher than the individual particles constituting them. In the Danish Wadden Sea in-situ analyses were carried out to obtain the natural textural composition of the suspended sediment. A settling tube, Braystoke SK 110, was used; and the analysis revealed that the suspended sediment at the investigation sites was composed of an unflocculated population coarser than about 4.5 φ and of a flocculated population finer than this grain size. Median settling velocities from 10 −4 to 10 −3 m s −1 were determined within the concentration range 50–1000 mg 1 −1. It is concluded that suspended sediment concentration is a major determinant for the natural textural composition of flocculated sediment. The variation in concentration can account for about 80% of the variance in median settling velocity. Turbulent mixing within the water column is found to be the limiting factor for the growth of sediment flocs.