Infragravity-Frequency (0.005–0.05 Hz) Motions on the Shelf. Part I: Forced Waves

This is Part 1 of a two-part study of infragravity-frequency (nominally 0.005–0.05 Hz) motions on the continental shelf. Data from a large aperture (250 m × 250 m) array of 24 bottom-mounted pressure transducers deployed in 13 m depth is used to investigate the local forcing of infragravity motions by nonlinear difference-frequency interactions of surface gravity waves. Second-order nonlinear theory (Hasselmann) and observed swell-sea frequency-directional spectra are used to predict the energy levels of forced infragravity waves. For a wide range of wave conditions, the predicted forced wave levels are lower than the observed energy levels, suggesting that the infragravity band contains a mix of free and forced waves. Bispectral analysis is used to estimate the relative amounts of free and forced infragravity energy. Good agreement between bispectrum-based estimates and theoretical predictions of forced wave energy confirms that second-order nonlinear theory accurately predicts locally forced infragravity motions. The contribution of forced waves to the total infragravity energy, ranging from less than 0.1% to about 30%, is largest when the infragravity energy is maximum, consistent with previously noted trends in similar water depths. The bispectral technique developed here to estimate the energy of forced and free infragravity waves is used in Part 2 to investigate, with data from single-point pressure gauges, the shelfwide variability of free infragravity energy.