The settling of mud flocs in the Dollard estuary, the Netherlands

Abstract

Temporal changes in floc size and settling velocity were measured in the Dollard estuary with an under water video camera, a laser particle sizer and an in situ settling tube. The results showed a strong tidal variation in floc sizes and settling velocities in the Dollard estuary. In the upper part of the tidal channel average floc settling velocities varied between 2 and 3.5 mm/s through the tide.

Floc size variation above the tidal flats differed from floc size variation in the tidal channel. Above the flat the largest flocs occurred during maximum current velocities. Presumably flocs above the tidal flat were broken in the near bed layer of high turbulence during ebb and flood. In the tidal channel, the frequency by which flocs approach the bed is much lower. Therefore breakup processes did not dominate floc sizes in the tidal channel

The measurements on a seasonal time scale showed a strong influence of a spring phytoplankton bloom on the floc sizes in the Dollard estuary. At the end of this bloom there was a large increase of maximum floc sizes, from 400 mu-m ip to 1 millimeter. These floc size increases are thought to be caused by sticky biopolymers excreted by phytoplankton and bacteria that bind the flocs together. The floc size increases due to the plankton bloom did not lead to a large increase in floc settling velocity, since the density of these large flocs was very low. This decreased density of larger flocs diminished the effect of the floc size on the settling velocity.

On a tidal time scale, floc sizes and settling velocities were related with the suspended sediment concentration (SSC), but on a seasonal time scale such a relation was not observed. This suggests that floc size/settling velocity depend more on sedimentation and resuspension of flocs throughout the tide than on the collision frequency (dependent on the SSC) of the flocs in the water column. Therefore tidal variations in settling velocity could not be modeled solely as a function of SSC, since the (indirect) relation between floc size/settling velocity and SSC constantly changes in time and space. The settling velocity variations throughout the tide can better be expressed as a function of tidal phase than as a function of SSC.