Infragravity energy in the surf zone

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Abstract
Field measurements of onshore and longshore velocities in the surf zone have been obtained on Martinique Beach, Nova Scotia, for the purpose of investigating the dynamics of the infragravity band (0.003–0.03 Hz) of the spectra. A total of 35 data runs were obtained during a 1‐week period. Of particular interest is the response of the infragravity energy to the changing incident waves, which increased considerably in size during the latter half of the week due to an approaching hurricane. It is shown theoretically, using equilibrium arguments, that the infragravity amplitude should vary approximately linearly with incident wave amplitude. This is supported from the field data if significant wave height is used as a measure of incident amplitude. The incident band of spectra observed by instruments in the surf zone is limited by breaking. Thus the infragravity band appears to dominate these spectra during storms. The analysis is carried out in terms of a spectral transformation, the spectrum which would be observed at an offshore instrument if the shoreline amplitude spectrum were white with unit spectral energy density. For onshore velocity the transformation predicts the observed spectral structure in the infragravity band, showing that the structure did not represent any true frequency selection. The match of theory and data also implies that the onshore motions are free waves, forced near resonance. The longshore spectra are red and show no structure which would be associated with free waves. This is consistent with the theoretical prediction that many edge wave modes, including high modes, would be forced given the broad directional spread of the storm waves. It is noted that further field experiments would be simpler on the Pacific coast where the typical, narrow‐band, incident swell should force only a few, low, edge wave modes, a more accommodating situation to observe and analyze.

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