CHANGES IN SUSPENDED-MATTER FLOC SIZE DURING THE TIDAL CYCLE IN THE DOLLARD ESTUARY

Measurements of in situ particle size of suspended matter in a tidal channel in the Dollard (Dutch Wadden Sea) indicated systematic variations in floc size during the tidal cycle that can be explained by assuming settling during slack tide, resuspension during the early ebb and early flood, flocculation of fine particles into large ones during most of the tide, and deflocculation of large flocs into smaller particles during or after settling to the bottom. There was a characteristic difference in floc-size variation during ebb and during flood. During the ebb maximum floc size coincided with maximum suspended-matter concentration (maximum collision frequency); during the flood maximum floc size continued to increase towards high-water slack tide. The adjacent tidal flats had a marked influence on floc size: where the flats were small, the highest percentages of large flocs (>128 mum) occurred around slack tide when current velocities are low. Where the adjacent flats were broad and extensive, high percentages also occurred during intermediate periods. Flocs of maximum size are probably not in equilibrium with the bulk of the suspended matter: they are mainly formed during periods of high suspended-matter concentrations of short duration.