Mechanics of Sediment Transport of Graded Materials and Fluvial Sorting

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Sediment transport of graded materials often shows non-equilibrium behavior and subsequently it accompanies fluvial sorting process. In order to describe the phenomena it brings about, non-equilibrium transport model characterized by pick-up rate and step length for each grain size.When the sediment transport supplied from the upstream region is suppressed, the bed-surface adjusts its composition (size distribution) by selective movement of sediment according to the size. Particularly, the supplied sediment is zero, the bed is coarsened and the transport rate over it becomes zero increasingly from the upstream end (armor coat formation and propagation). While, the suppressed sediment is not zero, all size-fraction of sediment are still transported (the transport rate becomes equal to the supplied one finally) but the bed surface is obviously coarsened (pavement). These two different types of bed coarsening process could be described by the non-equilibrium transport model proposed in th … More is study.Besides monotonous sorting, alternate sortings are often observed : longitudinally alternate sorting and laterally alternate sorting (longitudinal stripes of sorting). These sortings do not require the forced change of supplied sediment. Non-equilibrium transport for each grain size and the change of bed-surface composition brings about an instability of the bed-surface composition. The formation processes could be described by the non-equilibrium transport model proposed in this study.In a stream with longitudinally alternate sorting, the longitudinal change of bed roughness severely changes the turbulent structure and subsequently the characteristics of suspended sediment transport over it. Based on the turbulence measurements, a model to describe the relaxation process of Reynolds-stress distribution due to the change of the bed shear stress was derived, which was a convolution-integral type with an impulse response. The similarity between the turbulent momentum exchange and the turbulent transport of suspended sediment suggested a description of non-equilibrium concentration distribution of suspended sediment with the relaxation model prepared for the Reynolds-stress distribution. Less