Wavenumber‐frequency structure of infragravity swash motions

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Abstract
Alongshore‐separated time series of natural swash motions were obtained over a range of environmental conditions using a video technique. Although the frequency spectra and normalized wavenumber spectra for these motions were particularly bland, wavenumber‐frequency spectra of these data showed clear partitions of infragravity band energy levels associated with various wave types. For the frequencies 0.025 < ƒ ≤ 0.05 Hz, 45 ± 13% of the shoreline variance was, on average, associated with high‐mode (n≥ 2) edge waves and/or leaky waves, while approximately half that amount was associated with low‐mode edge waves. Gravity wave motions (comprising both edge and leaky modes) were typically dominant in a lower‐frequency band (0.001 < ƒ ≤ 0.025 Hz). A substantial portion of the variance in this band (21 ± 10%, with a maximum of 38%), however, was identified as a nondispersive waveform with wavenumbers well outside of the wavenumber‐frequency bounds for gravity waves. Surprisingly, this nongravity swash variance showed no significant dependence on mean alongshore current strength or mean alongshore current shear as measured in the surf zone trough separating the shoreline from an offshore bar. In addition, the celerities of these swash zone nondispersive waves were found to differ in magnitude, and in one instance, sign, from celerities of similarly structured waves measured farther offshore in the surf zone. These unexpected observations with respect to low‐frequency, nongravity swash energy imply a strong decorrelation between trough and shoreline fluid motions.

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