A quantitative lab examination of floc fractal property considering influences of turbulence, salinity and sediment concentration

Abstract

Flocculation is an important dynamic process for fine sediment transport. A number of physical and biochemical factors affecting flocculation have been identified, but quantitative examination of floc structure and fractal property under varying physical conditions is still needed. In this contribution, we conducted lab experiments using an annular flume and natural sediment to evaluate changes in floc size distributions and fractal dimensions under varying turbulent shear G, salinity and suspended sediment concentration (SSC). The low-intrusive optical instrument of LabSFLOC-2 was deployed to capture images of flocs which enables calculation of floc sizes and settling velocities. Observations showed that the mean floc size in freshwater first increased with increasing G below a threshold of G = 40 s−1, followed by a decrease with further increases in shear. The shear threshold was smaller for saline water (salinity of 7 PSU), i.e., G = 20–30 s−1, than that in freshwater. The fractal dimension of flocs decreased with an increase in size, suggesting more fragile structure for macro-flocs. Turbulent shear appeared to play a dominant role in controlling floc structure. Flocs formed under large shear force were more compacted, and had larger fractal dimensions. The floc fractal dimension was 28–37% greater in freshwater than in saline water. An improved method considering both floc size and variable fractal dimension is proposed for better simulation of floc effective density. The improved understanding of flocculation dynamics and floc structure in this work have implications for fine sediment and adherent nutrients management in varying natural water environments.