INFRAGRAVITY-FREQUENCY (0.005-0.05 HZ) MOTIONS ON THE SHELF .1. FORCED WAVES

This is Part 1 of a two-part study of infragravity-frequency (nominall y 0.005-0.05 Hz) motions on the continental shelf Data from a large ap erture (250 m X 250 m) array of 24 bottom-mounted pressure transducers deployed in 13 m depth is used to investigate the local forcing of in fragravity motions by nonlinear difference-frequency interactions of s urface gravity waves. Second-order nonlinear theory (Hasselmann) and o bserved swell-sea frequency-directional spectra are used to predict th e energy levels of forced infragravity waves. For a wide range of wave conditions, the predicted forced wave levels are lower than the obser ved energy levels, suggesting that the infragravity band contains a mi x of free and forced waves. Bispectral analysis is used to estimate th e relative amounts of free and forced infragravity energy. Good agreem ent between bispectrum-based estimates and theoretical predictions of forced wave energy confirms that second-order nonlinear theory accurat ely 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 a nd free infragravity waves is used in Part 2 to investigate, with data from single-point pressure gauges, the shelfwide variability of free infragravity energy.