Implications of turbulence shear by non-cohesive sediments on the break-up of kaolin flocs

Abstract

The present study investigated the role of non-cohesive fraction of sediments on the flocculation of kaolin suspension. The floc characterization was achieved through micro-scale investigations using an image capturing system followed by an image processing technique. Initially, the variations in the characteristics of flocs of kaolin with salinity and turbulence were examined, followed by characterization of flocs under the addition of sand fraction at different proportions (10%, 20%, 40% and 50%). The presence of sand imparts additional shear causing the break-up of macro-flocs of size >100 µm. The variation of floc volume fraction of macro-flocs with turbulence follows a power law, which was used to quantify the additional turbulence shear created by different % of sand. The breakage coefficient of macro-flocs was determined from the experimental results and a relationship for it in terms of turbulence shear is proposed. The developed relationship was further applied to predict the breakage coefficient of macro-flocs due to the addition of sand. The sensitivity analysis conducted between predicted and observed breakage coefficient of macro-flocs due to the addition of sand yielded a correlation coefficient of 0.99. The study proves that the binary breakage model, which propose the break-up of a floc into two equal sized flocs, can be applied to predict breakage coefficient only at intermediate turbulence shear in the range of 20–40 s−1 ; further it fails to predict the breakage coefficient of macro-flocs in mixed sediment suspensions. The study highlights the role of sand in floc break-up and suggests that the developed model can very well predict the breakage coefficient of macro-flocs in estuaries with mixed sediments as well as at all ranges of turbulence shear encountered in estuaries.