Model Studies of the Flocculation of Sand-Mud Mixtures

Results are presented from a series of parametric experiments investigating the time-dependent flocculation characteristics for sand-mud suspensions under stationary, nearly-isotropic, grid-generated turbulence. The experimental runs were conducted in a sophisticated 2.1 m by 0.25 m diameter settling column, incorporating a fixed array of oscillating grids to generate predictable and repeatable zero-mean-shear turbulent flow fields under a range of hydrodynamic forcing conditions. Experimental measurements focused on determining the temporal development of mud flocs for different sand-mud mixtures through in-situ visualization and measurement of their geometric properties under specific grid shear conditions generated within the column. In-situ total suspended sediment (TSS) concentration measurements were also obtained at the mid-height and bottom of the main column using calibrated optical backscatter sensor (OBS) probes. The results indicated that, for mud-only suspensions, the measured peak and significant floc sizes increased non-linearly with measured TSS values, while higher values of grid-generated shear rate tended to reduce floc sizes attained within the column. On addition of the sand fraction, smaller overall floc sizes were also attained compared to the mud-only runs, suggesting that the sand particles impose an additional break-up mechanism on the developing flocs, either through direct collisions between sand particles and mud flocs and/or induced shear caused by differential settling effects. In this regard, it is anticipated that the study findings will provide enhanced understanding of sand-mud interactions within mixed suspensions leading to improved representation in mixed sediment transport models (e.g. through the development of population balance models with interactions between multiple sediment fractions).