The structure of the deposit produced by sedimentation of polydisperse suspensions

To interpret the deposits from particle-laden flows it is necessary to understand particle settling at their base. In this paper a quantitative model is developed that not only captures how particles settle out of suspension but also the composition of the final deposit in terms of its vertical distribution of grain sizes. The theoretical model is validated by comparison to published experimental data that has been used to interpret the field deposits of submarine sediment-laden flows (Amy et al., 2006). The model explains two intriguing features of the experimental deposits that are also observed in natural deposits. First, deposits commonly have an ungraded, or poorly normally graded, region overlain by a strongly normally graded region. Second, the normalized thickness of the ungraded region increases as the initial concentration of the suspension is increased. In the theoretical model, the poorly normally graded region results from a constant mass flux into the bed that persists until the largest grain size present within the flow has been completely deposited. The effect of increasing the concentration of the initial suspension is to increase the thickness of the poorly graded part of the deposit and to decrease its average grain size. This work suggests that deposits with relatively thick, poorly graded bases can form from relatively high-concentration polydisperse suspensions, when the initial volume fraction of sediment is greater than approximately 20% and indicates that it is important to include these hindered settling effects in models of depositing flows.