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 form, size and settling velocity of mud aggregates in the Oosterschelde tidal basin. Continuous in situ measurements were obtained during one tide. They showed that even at relatively low concentrations (10 mg l(-1)) large aggregates are present. The largest aggregates and highest settling velocities were determined during high-water slack (for aggregates greater than or equal to 50 mu m: median aggregate diameter=300 mu 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 similar to d(a)(n). For the Oosterschelde an n-value of 0.92 was found for aggregates greater than or equal to 50 mu m. This value is within the range of 0.4-1 which is usually determined for mud aggregates. Likewise, the reduction of aggregate density as they grow can be related to the diameter of these aggregates (d(a)) according to Delta rho similar to d(a)(m). The value of m for the Oosterschelde aggregates >50 mu m ranged from -0.69 to -1.84.