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1 April 2013 Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows
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Abstract

Calantoni, J., Landry, B.J. and Penko, A.M., 2013. Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows

The heterogeneity of sand beds has been suggested to significantly impact the resulting sand ripple morphodynamics. However, the majority of previous experiments for sand ripple morphodynamics were conducted using only unimodal grain size distributions. Here we performed a series of ripple growth and transition experiments in a small oscillatory flow tunnel in the Sediment Dynamics Laboratory at the U.S. Naval Research Laboratory. Sand beds were constructed from mixtures of two unimodal sands median grain sizes of 0.30 mm (blue) and 0.70 mm (white), respectively. Experiments were performed with compositions of bimodal mixtures with percent by mass of 10/90, 25/75, 50/50, 75/25, 90/10. Additionally, similar experiments were performed for each of the unimodal cases (i.e., 100/0, 0/100). For each experiment, starting from a planar bed, three different flow forcing conditions were applied in sequential blocks (with minimum of one-hour duration) until the ripples appeared to be uniform and in equilibrium. We analyzed ripple characteristics such as migration rate, wavelength, height, and steepness as a function of the mobility number. Over a range of nearly identical mobility numbers, we observed opposing trends with migration rates increasing in one block forcing and decreasing in another, where the two blocks were comprised of different combinations of the semiexcursion amplitude and oscillatory frequency. The results suggested that the commonly used mobility number might not be appropriate to characterize ripple migration rates, especially for sediment beds composed of bimodal size mixtures. Overall, wavelength, height, and steepness are consistent with empirical ripple predictors. However, observed subtleties existed among the different forcing blocks across the same range of grain size distributions.

Joseph Calantoni, Blake J. Landry, and Allison M. Penko "Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows," Journal of Coastal Research 65(sp2), 1497-1502, (1 April 2013). https://doi.org/10.2112/SI65-253.1
Received: 7 December 2012; Accepted: 6 March 2013; Published: 1 April 2013
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