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 s izes and settling velocities of suspended mud flocs without destroying the large fragile aggregates. The settling process of the flocs and a ggregates can be observed directly on a screen on board the research v essel. Analysis of the videorecordings was possible up to 600 mg.dm(-3 ), which is of importance in turbid estuarine systems. Experiments wit h 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 ob served at relatively low suspended-sediment concentrations (20 to 50 m g.dm(-3)). During certain phases of the tidal cycle (around mid-tide) even larger aggregates were observed with dimensions in the range of 4 00 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 observe d in various estuaries.