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

Abstract
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 in... Abstract 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 in...