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Megaripple migration in a natural surf zone

Nature volume 394, pages 165–168 (1998)Cite this article

Abstract

Migrating megaripples are bedforms that appear in the surf zone of sandy coasts1. With heights of 0.1–0.5 m and wavelengths of 1–5 m, they are similar in size and shape to small dunes, large ripples, or sand waves. Such sedimentary bedforms have been studied in subaerial2, steady-flow3 and intertidal4 environments, as well as in laboratory flume experiments5. They affect overlying currents by introducing hydraulic roughness4,6, and may provide a mechanism for sediment transport7,8 as well as forming sedimentary structures in preserved facies9,10. The formation, orientation and migration of such bedforms is not understood well11,12. Dunes, for example, can be aligned with their crests perpendicular to steady unidirectional winds13, but in more complex wind fields their orientation becomes difficult to predict14,15,16,17. Similarly, it is not known how sea-floor megaripples become aligned and migrate in the complex flows of the surf zone. Here we present observations in the surf zone of a natural beach which indicate that megaripples do not migrate in the direction of the vector sum of the currents, but are aligned so that the sediment transport normal to the bedform crest is maximized17. This may need to be taken into account in modelling morphology change and interpreting existing and fossil morphologic patterns.

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Figure 1: Location of the altimeter array in the surf zone.
Figure 2: Distance to the sea floor (every 32 s (ref. 24 for 48 h) from three altimeters (circled asterisks in Fig. 1b) versus time.
Figure 3: Direction of fluid flow versus megaripple migration.
Figure 4: Transport vectors and bedform orientation17.

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Acknowledgements

We thank T. Drake for introducing us to ref. 17 and for sharing his insight into, and enthusiasm for, bedforms; R. T. Guza and B. T. Werner for suggestions and advice; D. Rubin for critically reading the manuscript; P. Fields for comments; and R. T. Guza, T. H. C. Herbers and B.Raubenheimer for help with acquiring field data. The instruments were fabricated, deployed and maintained by staff from the Center for Coastal Studies, Scripps Institution of Oceanography; the US Army Corps of Engineers Field Research Facility provided logistical support. Funding was from the Office of Naval Research, the NSF, and the National Research Council.

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Authors and Affiliations

  1. Oceanography Department, Naval Postgraduate School, Monterey, 93943, California, USA

    Edith L. Gallagher & Edward B. Thornton

  2. School of Electrical Engineering and Computer Science, Washington State University, Pullman, 99164, Washington, USA

    Steve Elgar

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Gallagher, E., Elgar, S. & Thornton, E. Megaripple migration in a natural surf zone. Nature 394, 165–168 (1998). https://doi.org/10.1038/28139

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