SETTLING VELOCITIES OF MUD AGGREGATES IN THE OOSTERSCHELDE TIDAL BASIN (THE NETHERLANDS), DETERMINED BY A SUBMERSIBLE VIDEO SYSTEM

A submersible video system was used for direct measurements of the for m, size and settling velocity of mud aggregates in the Oosterschelde t idal basin. Continuous in situ measurements were obtained during one t ide. They showed that even at relatively low concentrations (10 mg l(- 1)) large aggregates are present. The largest aggregates and highest s ettling velocities were determined during high-water slack (for aggreg ates greater than or equal to 50 mu m: median aggregate diameter=300 m u m, median settling velocity=1.74 mm s(-1)). The settling velocity of mud aggregates (W) depends on their diameter (d(a)) according to W si milar to d(a)(n). For the Oosterschelde an n-value of 0.92 was found f or aggregates greater than or equal to 50 mu m. This value is within t he range of 0.4-1 which is usually determined for mud aggregates. Like wise, the reduction of aggregate density as they grow can be related t o the diameter of these aggregates (d(a)) according to Delta rho simil ar to d(a)(m). The value of m for the Oosterschelde aggregates >50 mu m ranged from -0.69 to -1.84.