COLLABORATIVE RESEARCH: Experimental Study of the Effect ofSediment Sorting on Bed Forms, Bed-Surface Texture, and Fractional Transport Rates in Coarse Mixed-Size Sediments

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When a unidirectional current passes over a bed of coarse mixed-size sediment, a variety of bed forms are generated and there is a segregation of particle sizes among the load, the bed surface layer and the substrate. There is a strong and complex interdependence between the bed forms, the transported sediment and the substrate. Many studies in well sorted sands, both in the laboratory and in the field, have led to fairly good understanding of the behavior of the familiar dune bed forms and the interaction between bed forms and transport. More recently, features variously described as bed-load sheets, gravel waves, and clast jams have been reported in flows over poorly sorted sand-gravel mixtures. The relationships among these seemingly diverse bed features, and their dependence on size segretation in the bed, is unclear. This is an experimental study of the nature of bed features in unidirectional flows as a function of flow strength and sediment sorting involving several sediment mixtures with the same mean size of about 4mm but with widely varying sorting and making series of runs at several mean flow velocities over each sediment mixture. The objective is a unified picture of the fundamental characteristics and interrelationships of the various kinds of bed features produced in these flows. The work will be done in cooperation with Prof. Hiroshi Ikeda, Prof. Shigemi Takayama, and Dr. Fujiko Iseya of the Environmental Research center at the University of Tsukuba. This proposal covers salary and travel for the PI. A companion proposal with identical scientific text is being submitted by Prof. John B. Southard (MIT). The Japanese group already has funds for building the experimental apparatus and conducting the experiments.