THE DETERMINATION OF THE SIZES AND SETTLING VELOCITIES OF ESTUARINE FLOCS BY AN UNDERWATER VIDEO SYSTEM

The in situ underwater video system VIS is described. VIS determines sizes and settling velocities of suspended mud flocs without destroying the large fragile aggregates. The settling process of the flocs and aggregates can be observed directly on a screen on board the research vessel. Analysis of the videorecordings was possible up to 600 mg.dm(-3), which is of importance in turbid estuarine systems. Experiments with VIS in the Ems estuary showed macroflocs with settling velocities of 0.5 to 2.0 mm.s(-1) and sizes of 200 to 500 mu m. These flocs were observed at relatively low suspended-sediment concentrations (20 to 50 mg.dm(-3)). During certain phases of the tidal cycle (around mid-tide) even larger aggregates were observed with dimensions in the range of 400 to 1100 mu m and settling velocities of 1 to 7 mm.s(-1), surviving relatively high current velocities. The large aggregates are shown to play an essential role in the phenomenon of rapid settling, as observed in various estuaries.