Geology & Geophysics Research Spotlight

Wave Ripples Spaced by Flow Downstream of Ripple Peaks

The well-known ratio between sand ripple spacing and wave-driven flow oscillations may be dictated by flow dynamics downstream of ripple crests.

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The mesmerizing ripple patterns formed by waves and preserved in sedimentary rocks provide valuable paleoenvironmental information. That’s because ripples develop in predictable ways. At equilibrium, they achieve a consistent ratio between the distance between ripple peaks (the wavelength) and the amplitude of the oscillating flow at the bed, which in turn depends on the wave size and water depth.

In a new study, Nienhuis et al. provide the first complete mechanistic explanation for this ideal ratio. Using wave tank experiments and flow modeling, the researchers found that the spacing between ripples depended on the size of the separation zone—the area downstream of a peak where flow is decoupled from the main current and forms a vortex. Their results show that the preferred ratio occurred when the separation zone reached just to the next peak, maximizing scouring in the trough and deposition on the crests and thus reinforcing the existing ripple pattern.

The researchers also investigated what happens to ripple patterns when environmental conditions change. Consistent with previous work, they found that secondary crests formed on both flanks of existing ripples when waves got smaller, but that only one set—left or right—survived the transition. They show that this occurs because the favored crest acts as a speed bump, slowing flow over the main ripple in a way that preferentially scours the competing crest and preserves the corresponding crest on the next ripple.

For increasing wave sizes, the researchers’ results help explain observed increases in ripple sinuosity. They propose that stronger flows can accelerate in the space between widely spaced peaks, scouring them and pushing them farther apart. Conversely, scouring is suppressed between closely spaced peaks, and they migrate closer together. This new insight will improve interpretations of ripples, including those without modern analogs and those on other planets. (Journal of Geophysical Research: Earth’s Surface, doi:10.1002/2014JF003158, 2014)

—Julia Rosen, Freelance Writer

Citation: Rosen, J. (2015), Wave ripples spaced by flow downstream of ripple peaks, Eos, 96, doi:10.2029/2015EO023911. Published on 12 February 2015.

© 2015. The authors. CC BY-NC 3.0