A study of the significance of flocculation for the in situ settling velocities of suspended particles in a tidal channel

Measurements concerning current velocities, mean suspended sediment concentrations and in situ floc settling diameters were carried out in a tidal channel in Lister Tief (The German Wadden Sea). Current velocities and mean suspended sediment concentrations follow a pattern well-known from other parts of the Wadden Sea, with rapidly accelerating current velocities at the beginning of the flood period compared to slowly accelerating currents in the ebb period. The suspended sediment concentration mainly follows the variation in current velocities. At the surface, the median fall diameters were very constant, about 25 mu m, compared to between 22 and 80 mu m at the bottom. Flocculation is influencing the settling velocities throughout the tidal period regardless of concentration. The sediment is flocculated at all concentrations and current velocities. The size of the flocs is influenced by the current velocities, the suspended sediment concentration and distance from the bottom. At the surface, the largest part of the flocs are in the 24 to 32 mu m fraction. At the bottom, the sediment has a large percentage of fecal pellets material giving larger fall diameters. The results are comparable to investigations from other parts of the Northern Wadden Sea area.