Bound infragravity waves

Abstract

Model predictions of bound (i.e., nonlinearly forced by and coupled to wave groups) infragravity wave energy are compared with about 2 years of observations in 8‐ to 13‐m depths at Imperial Beach, California, and Barbers Point, Hawaii. Frequency‐directional spectra of free waves at sea and swell frequencies, estimated with a small array of four pressure sensors, are used to predict the bound wave spectra below 0.04 Hz. The predicted total bound wave energy is always less than the observed infragravity energy, and the underprediction increases with increasing water depth and especially with decreasing swell energy. At most half, and usually much less, of the observed infragravity energy is bound. Bound wave spectra are also predicted with data from a single wave gage in 183‐m depth at Point Conception, California, and the assumption of unidirectional sea and swell. Even with energetic swell, less than 10% of the total observed infragravity energy in 183‐m depth is bound. Free waves, either leaky or edge waves, are more energetic than bound waves at both the shallow and deep sites. The low level of infragravity energy observed in 183‐m depth compared with 8‐ to 13‐m depths, with similarly moderate sea and swell energy, suggests that leaky (and very high‐mode edge) waves contribute less than 10% of the infragravity energy in 8–13 m. Most of the free infragravity energy in shallow water is refractively trapped and does not reach deep water.