The effect of suspended sediment concentration on the settling velocity of cohesive sediment in quiescent water

A laboratory study was carried to qualitatively investigate the effect of suspended sediment concentration C on the settling velocity w(s) of cohesive sediment in quiescent water. A bay mud sample was mixed with water in a cylindrical container, and three optical back scatterance sensors were then used to measure suspended sediment concentrations of the mud-water mixture at three levels every 15s for 5h while sediments were settling in the quiescent water. Based on the measured sediment concentrations, the settling velocities at different concentrations were derived from the depth-integrated mass balance equation. This study has found that the settling velocity ws is independent of C in the free settling regime of C < 0.3 g/l, and then increases nonlinearly with C in the enhanced settling regime of 0.3 < C < 4.3 g/l, and finally decreases sharply with C in the hindered settling regime of C > 4.3 g/l. The maximum settling velocity occurs at C approximate to 4.3 g/l and is about nine times faster than the settling velocity in the free settling regime. A single empirical formula is also proposed to calculate the settling velocities at different sediment concentrations. (C) 2004 Elsevier Ltd. All rights reserved.