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

摘要

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 15 s for 5 h 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 w_s 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 ≈ 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.