Sediment Sorting in Alluvial Channels
The formation of primary sedimentary structures in and along an alluvial channel occurs as a result of interactions between gravity, the physical characteristics of the sediment and fluid as well as the hydraulic environment. The occurrence of many of these structures in channels results from the presence of ripples, dunes, bars, and antidunes on the bed. However, the actual process by which recognizable structures develop arises from sorting of sediment with respect to size, shape, and density along the bed and within the stream. The settling rates of the particles, turbulent diffusion, gravitational sliding and boundary shear stress contribute to the sorting processes. From laboratory experiments, quantitative descriptions of sorting within the suspended load are presented and compared to the theoretical diffusion model. A description of the sorting processes occurring within dunes and ripples is also presented together with a discussion of some of the interactions between dunes and ripples and hydraulic roughness.