Wavelength selection and symmetry breaking in orbital wave ripples
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Perron_Wavelength selection.pdf
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Author(s)
Perron, J. Taylor
Kao, Justin C. T.
Myrow, Paul M.
Nienhuis, Jacob Harm
Date Issued
October 2014
Journal
Journal of Geophysical Research: Earth Surface
Publisher
American Geophysical Union (AGU)
Citation
Nienhuis, Jaap H., J. Taylor Perron, Justin C. T. Kao, and Paul M. Myrow. “Wavelength Selection and Symmetry Breaking in Orbital Wave Ripples.” J. Geophys. Res. Earth Surf. 119, no. 10 (October 2014): 2239–2257. © 2014 American Geophysical Union
Version
Final published version
Abstract
Sand ripples formed by waves have a uniform wavelength while at equilibrium and develop defects while adjusting to changes in the flow. These patterns arise from the interaction of the flow with the bed topography, but the specific mechanisms have not been fully explained. We use numerical flow models and laboratory wave tank experiments to explore the origins of these patterns. The wavelength of “orbital” wave ripples (λ) is directly proportional to the oscillating flow's orbital diameter (d), with many experimental and field studies finding λ/d ≈ 0.65. We demonstrate a coupling that selects this ratio: the maximum length of the flow separation zone downstream of a ripple crest equals λ when λ/d ≈ 0.65. We show that this condition maximizes the growth rate of ripples. Ripples adjusting to changed flow conditions develop defects that break the bed's symmetry. When d is shortened sufficiently, two new incipient crests appear in every trough, but only one grows into a full-sized crest. Experiments have shown that the same side (right or left) wins in every trough. We find that this occurs because incipient secondary crests slow the flow and encourage the growth of crests on the next flank. Experiments have also shown that when d is lengthened, ripple crests become increasingly sinuous and eventually break up. We find that this occurs because crests migrate preferentially toward the nearest adjacent crest, amplifying any initial sinuosity. Our results reveal the mechanisms that form common wave ripple patterns and highlight interactions among unsteady flows, sediment transport, and bed topography.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.
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DOI of Published Version
https://doi.org/10.1002/2014JF003158
