Experimental Study of Longitudinal Sorting of Particles Differing in Size and Density.

摘要

Transport, deposition and erosion of sediment particles differing in size, shape and density may result in particle segregation, which in geosciences applications is generally referred to as sediment sorting. A thorough understanding of sediment sorting processes is important to describe and model a wide variety of natural processes such as the decrease in particle size and/or density in a fluvial system (downstream fining and/or lightening), the formation of economic placers, concentration of heavy minerals, chemical and metal pollutants etc. Sorting of sediment grains associated with sediment transport in the streamwise direction results in the development of longitudinal sorting patterns. Vertical sorting patterns are the results of sediment sorting within the alluvial deposit. Laboratory experiments were conducted at the Hydraulics Laboratory, University of South Carolina to study the physical processes associated with the transport of a mixture of particles differing in size and density, and the resulting longitudinal sorting patterns. Experiments were performed in a sediment feed flume, which is an experimental set up that is traditionally used for these type of studies. Three experiments were performed with sediment mixture differing in both size and density, the sediment feed rate and the flow rate of water were held constant in each experiment. The sediment feed rate was the only parameter that changed from one experiment to the next. In each experiment data were collected to characterize equilibrium and non-equilibrium conditions, with equilibrium referring to a condition in which the characteristics of the flow and the sediment transport can be reasonably considered steady and uniform. The analysis of the experimental data shows that 1) a downstream lightening pattern developed in the experiment with the highest feed rate, which means that the heavy particles were preferentially deposited in the upstream part of the deposit and the light particles traveled further downstream; 2) a downstream fining pattern was observed in the experiments with a comparatively low feed rate, with the coarse particles deposited in the upstream part of the deposit and the finer particles deposited further downstream.