Temporal variation of floc size and settling velocity in the Dollard estuary

Temporal changes in flee size and settling velocity were measured in the Dollard estuary with an under water video camera. The results show that the flocs in the Dollard are very heterogeneous and that larger flocs have much lower effective densities than smaller flocs. Due to this density decrease, flee settling velocities show only a minor increase with increasing flee size. Flee sizes and settling velocities correlate with the suspended sediment concentration (SSC) on a tidal time scale, but not on a seasonal time scale. On a seasonal time scale flee sizes depend on the binding properties of the sediment, while flee settling velocities show hardly any variation, as an increase in floc size is mainly counterbalanced by a decrease in flee density. Tidal variations in settling velocity occur but cannot be modeled solely as a function of SSC, as the relation between flee size/settling velocity and SSC constantly changes in time and space. Settling velocity variations throughout the tide can however be expressed as a function of tidal phase. (C) 2000 Elsevier Science Ltd. All rights reserved.