The flocculation of fine‐grained sediments in estuarine waters

Abstract

Experimental and theoretical investigations of the effects of salinity, fluid shear, and sediment concentration on the rates of aggregation and disaggregation of a fine‐grained sediment in estuarine waters were made. Experiments were performed in fresh water, in seawater, and in an equal mixture of fresh and sea waters. The applied shears ranged from 100 to 600 s−1, while the sediment concentrations ranged from 10 mg/L to 800 mg/L. From the experimental and theoretical results and for the above range of parameters, the following was demonstrated: (1) the steady state particle size distribution is independent of the manner in which the steady state is approached; (2) the median floc size decreases as the shear stress increases; (3) the median floc size decreases as the suspended sediment concentration increases; (4) particles flocculate faster in seawater than in fresh water; (5) flocs are smaller in seawater than in fresh water; (6) the sizes of flocs in estuarine waters seem to be weighted averages of the sizes of flocs in fresh and sea waters; (7) in order to explain the observed decrease in floc size as the sediment concentration increases, the analysis requires disaggregation due to three‐body collisions; (8) the theory does not require disaggregation due to fluid shear. For the present range of parameters, fluid shear seems to have a negligible direct effect on disaggregation, while collisions between particles (possibly due to shear but also due to differential settling and Brownian motion) are the dominant mechanism for disaggregation; and (9) the theory can accurately reproduce the experimental results of mean diameter versus time and of steady state particle size distribution.