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Effects of bed perturbation and velocity asymmetry on ripple initiation: wave-flume experiments
Sekiguchi, T.; Sunamura, T. (2004). Effects of bed perturbation and velocity asymmetry on ripple initiation: wave-flume experiments. : 231-239

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  • Sekiguchi, T.
  • Sunamura, T.

    Laboratory experiments using a wave flume were designed to examine the threshold condition for ripple formationunder asymmetrical oscillatory flows on an artificially roughened bed. Three types of sand beds were prepared in theexperiments: they were flat, notched, and notch-mounded beds with bed roughness increasing in this order. The beds wereconstructed with three kinds of well-sorted sand with similar density, but different diameters. Data analyses were madeusing the two dimensionless parameters: the mobility number, M, a simplified form of the Shields number, and the Ursellnumber, U, a surrogate for asymmetry of flow field. The result confirmed that the threshold for ripple initiation isdecreased with increasing bed perturbation and that as the bed perturbation increases, the dependency of this threshold onthe flow asymmetry becomes less and finally null for the notch-mounded bed. This relationship is quantified by thefollowing equations: M=17-14.5e-0.03U on the flat bed, M= 5.0-2.5e-0.1U for the notched bed, and M= 2.5 for thenotch-mounded bed. A comparison between the previous field data and the present laboratory findings indicates that thethreshold in the notch-mounded bed experiment, M= 2.5, seems to provide a critical condition for rippling in the naturalenvironment.

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