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Abstract: Differential Transport of Sand Grains: Ripples and Dunes, a Suspension Criterion
John F. Bourke (1)
Differential transport of sand grains by unidirectional water flow over a cohesionless bed material having superimposed ripples or dunes results in hydraulic sorting. Hydraulic sorting occurs on many scales in natural fluvial systems, e.g., flood plain versus channel deposits, bed versus load in transit, various portions of cross-bed sets or individual laminae, etc. The mechanisms of hydraulic sorting include differential entrainment at the source, differential deposition and differential transport rates which are sensitive to prevailing bed configurations. By comparing transport rates of individual grain sizes calculated from size analysis of bed material and load under ripple and dune conditions, it is shown that the sampled load may be differentiated into two subpopulations of grains moved by different transport mechanisms, namely, those particles confined to transport within bedforms, and those particles carried in intermittent suspension. Due to practical limitations in the flume, this study is limited to the two coarsest subpopulations observed in typical samples of river sediment. Theory and results of flume experiments suggest the following approximate hydraulic criterion for separating the two mechanisms of sediment transport: the point where the relative difference in transport rates between size fractions increase abruptly. This criterion is confirmed by flume experiments for flow conditions spanning the stability fields for ripples and dunes.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Arco Exploration Co., P. O.Box 2819, Dallas, Texas 75521.
Copyright © 1999 by The Gulf Coast Association of Geological Societies