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 v elocity) 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 >similar to 150 mu m) contain a mixt ure of organic and inorganic constituents that become separated when the fl ocs are disrupted to form microflocs. Representation of the flee characteri stics in terms of fractals reveals a range of fractal dimensions representi ng the distributions varying between 1 and 3, instead of the ideal value of 2. Measurements in estuarine turbidity maxima and on intertidal mudflats s how that the fractal dimension is less than 2 in situations where turbulent shearing causes disruption of the flocs. At the same time increasing suspe nded sediment concentration tends to increase the fractal dimension. Measur ements of size using an in situ Malvern sizer show that the hoc size distri bution is also affected by both turbulent energy dissipation and by concent ration. Complementary laboratory studies suggest that, at a constant concen tration, 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. (C) 1999 Elsevier Science B.V. All rights reserved.