Observation of the size, settling velocity and effective density of flocs, and their fractal dimensions

In situ instruments, particularly the instrument INSSEV (in situ settling velocity) have given new information on the sizes, settling velocities and effective densities of individual flocs within the spectrum of distribution. The low-density macroflocs (diameter >∼150 μm) contain a mixture of organic and inorganic constituents that become separated when the flocs are disrupted to form microflocs. Representation of the floc characteristics in terms of fractals reveals a range of fractal dimensions representing the distributions varying between 1 and 3, instead of the ideal value of 2. Measurements in estuarine turbidity maxima and on intertidal mudflats show that the fractal dimension is less than 2 in situations where turbulent shearing causes disruption of the flocs. At the same time increasing suspended sediment concentration tends to increase the fractal dimension. Measurements of size using an in situ Malvern sizer show that the floc size distribution is also affected by both turbulent energy dissipation and by concentration. Complementary laboratory studies suggest that, at a constant concentration, flocculation is enhanced by low shear, but that disruption occurs at higher shear. These experiments confirm the relationship between fractal dimension, shear stress and concentration.