IN-SITU SIZE DISTRIBUTION OF SUSPENDED MATTER DURING THE TIDAL CYCLE IN THE ELBE ESTUARY

In situ size of suspended particles was measured with an underwater photographic system in the Elbe estuary. The data from the measurements at a stationary position indicate that the formation of large flocs took place at low current velocities and that the size distributions varied with the tidal cycle as a consequence of flocculation, deposition and resuspension processes. Flocculation of suspended particles in the estuary is interpreted as a dynamic process: the size of flocs changes with time and their distribution depends on the rates of aggregation and disaggregation of the particles. This is related to physical factors of which turbulence and differential particle settling play key roles in producing large flocs. Fluid shear is responsible for disaggregation of large flocs. Salinity and the organic content of particles may affect flocculation processes with respect to the particle cohesive interaction, but they have no critical effect on the variation of floc size during the tidal cycle in the estuary.