DOES REENTRAINMENT OCCUR DURING COHESIVE SEDIMENT SETTLING

Experiments on settling of kaolin-distilled water suspensions are carr ied out in a 5-m diameter rotating annular flume in which flows were g enerated by rotating the flume walls in one direction and a top ring i n the opposite direction. Suspensions with initial concentrations vary ing from 250 to 2,000 ppm are allowed to settle under constant bed she ar stress conditions. After equilibrium is reached, the suspensions ar e withdrawn from the flume while the flume volume is maintained consta nt by adding clear water at an equal rate. Size distributions of the d ispersed samples are determined using a Malvern particle size analyzer and concentrations are determined by filtration and weighing. The cha nges in concentration and size distribution during the initial settlin g and the subsequent dilution periods are used to investigate two oppo sing mechanisms of the settling process and to answer the question of whether there is simultaneous deposition and erosion during cohesive s ediment settling.