Does Reentrainment Occur during Cohesive Sediment Settling

Experiments on settling of kaolin-distilled water suspensions are carried out in a 5-m diameter rotating annular flume in which flows were generated by rotating the flume walls in one direction and a top ring in the opposite direction. Suspensions with initial concentrations varying from 250 to 2,000 ppm are allowed to settle under constant bed shear stress conditions. After equilibrium is reached, the suspensions are withdrawn from the flume while the flume volume is maintained constant by adding clear water at an equal rate. Size distributions of the dispersed samples are determined using a Malvern particle size analyzer and concentrations are determined by filtration and weighing. The changes in concentration and size distribution during the initial settling and the subsequent dilution periods are used to investigate two opposing mechanisms of the settling process and to answer the question of whether there is simultaneous deposition and erosion during cohesive sediment settling.