Evaluation of the simple settling theory for predicting sediment deposition by overland flow

Experiments were conducted in the laboratory to evaluate the influence of s ediment concentration, sediment grain-size distribution, bed slope and flow discharge on sediment deposition rates and patterns associated with a redu ction in bed slope. The experimental data clearly indicate that sediment de position by overland flow is a very selective process: fine particles remai n almost entirely in suspension and coarse particles are deposited quickly. Analysis of the data shows that up to a critical unit discharge a simple s ettling equation without a transport term, assuming continuous mixing of th e sediment and water, gives a good prediction of the overall sediment deliv ery ratio and the grain-size distribution of the deposited and the exported sediment. However, there are some discrepancies for the clay, the coarse s ilt and the sand fractions. The assumption of continuous mixing is tested b y investigating the sedimentation patterns of very narrow size classes. The observed decrease of sediment concentration versus distance from the inflo w point for these individual sediment size classes closely agrees with the prediction assuming continuous mixing. When the critical unit discharge is exceeded, hydraulic properties of the overland flow do influence the sedime nt delivery outcomes. At discharges exceeding the threshold value the simpl e settling theory underpredicts the sediment delivery ratio. In these hydra ulic conditions, a transport term needs to be incorporated into the simple settling theory. It is shown that the transport capacity and the re-entrain ment model yield similar expressions for the description of sediment transp ort by overland flow over an area of net deposition. The experimental data indicate that the re-entrainment of previously deposited sediment is non-se lective. Copyright (C) 1999 John Wiley & Sons, Ltd.