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Journal of Geophysical Research: Oceans

Observations of wave energy fluxes and swash motions on a low-sloping, dissipative beach

Authors

  • Rafael M. C. Guedes,

    Corresponding author
    1. Department of Earth and Ocean Sciences, University of Waikato, Hamilton, New Zealand
    2. Now at MetOcean Solutions Ltd., Raglan, New Zealand
    • Corresponding author: R. M. C. Guedes, MetOcean Solutions Ltd, 5 Wainui Road, Raglan 3225, New Zealand. (r.guedes@metocean.co.nz)

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  • Karin R. Bryan,

    1. Department of Earth and Ocean Sciences, University of Waikato, Hamilton, New Zealand
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  • Giovanni Coco

    1. National Institute of Water and Atmospheric Research, Hamilton, New Zealand
    2. Now at Environmental Hydraulics Institute, “IH Cantabria,” Universidad de Cantabria, Santander, Spain
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Abstract

[1] Field observations of swash and ocean waves show that runup saturation at infragravity frequencies (<0.05 Hz) can occur under mild offshore energy conditions if the beach slope is sufficiently gentle. Infragravity saturation was observed for higher-frequency (>0.025–0.035 Hz) infragravity waves, where typically less than 5% of the (linear) energy flux was reflected from the beach and where, similar to the sea swell band, the swash energy was independent of offshore wave energy. The infragravity frequency range of saturation was determined by the tide, with saturation extending to lower frequencies at low tide when the local beach face slope over the concave-shaped profile was gentler. Runup was strongly dominated by infragravity frequencies, which accounted on average for 96% of the runup variance, and its energy levels were entirely consistent with strong infragravity wave dissipation observed in the surfzone, particularly when including the nonlinear contributions to the wave energy fluxes. The infragravity wave dissipation was strongly associated with breaking of sea swell waves, which abruptly decreased nonlinear transfers to infragravity frequencies and made infragravity dissipation prevail over forcing within the breaking region. Our observations show evidence of nonlinear interactions involving infragravity and high-frequency, harmonic waves and suggest that these harmonics could play a role in the wave energy balance near the shoreline on low-sloping, dissipative beaches.

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