A Laboratory Study of Dilute Suspension Mud Floc Characteristics in an Oscillatory Diffusive Turbulent Flow

GRATIOT, N. and MANNING, A. J., 2007. A Laboratory Study of Dilute Suspension Mud Floc Characteristics in an Oscillatory Diffusive Turbulent Flow, SI 50 (Proceedings of the 9th International Coastal Symposium), 1142 – 1146. Gold Coast, Australia, ISSN 0749.0208 To further the understanding of mud flocculation the COSINUS project funded a series of laboratory experiments whereby dilute mud suspensions were sheared (between 3.7 s -1 - about 20 s -1 ) with nominal concentrations ranging from 200-600 mg l -1 , within a Plexiglas tank, using an oscillating grid. The floc properties were then examined using the LabSFLOC instrument. Results showed that for low concentrations of natural Tamar estuary (UK) mud exposed to high shear, the largest flocs were slightly less than the Kolmogorov eddy size of about 220 μm. The high shear resulted in all flocs having settling velocities of only 0.6 mm s -1 . Lowering the shear for the same mud improved flocculation, and raised the macrofloc settling velocity to 1.8 mm s -1 . This translated into macroflocs constituting 64% of the floc mass, and 80% of the mass settling flux. In contrast, low concentrations of natural Gironde mud displayed significantly faster settling macroflocs at each shear increment than Tamar mud, but the largest flocs were of the same order as the Tamar mud flocs. The suspended matter distribution showed the macroflocs only constituted 20-30% of the particulate mass. However, their faster settling rate transformed the low macrofloc mass into 62% of the settling flux. The removal of organic matter from the Gironde mud resulted in few flocs exceeding 125 μm in diameter.