Infragravity-Frequency (0.005-0.05 Hz) Motions on the Shelf

Extensive field observations in depths between 8-204 m are used to investigate the sources and variability of infragravity-frequency (nominally 0.005-0.05 Hz) motions on the shelf. The predicted local forcing of ‘bound’ infragravity motions by difference-frequency interactions of swell and sea (Hasselmann, 1962) is verified with array measurements of sea floor pressure in 13 m depth. Observed forced infragravity energy levels agree well with theoretical predictions, but are consistently much lower than the observed total infragravity energy. Wavenumber estimates show that free waves, obeying the surface gravity wave dispersion relation, are frequently the dominant source of energy in the infragravity band. The observed directional properties and cross-shore decay of free wave energy show that refractive trapping, neglected in many current infragravity wave generation models, is of O(l) importance. Both free and forced wave energy levels generally increase with both increasing swell energy and decreasing water depth, but the observed dependencies of free and forced waves are different. The relative contributions of free and forced waves to infragravity energy on the shelf are therefore highly variable. Comparisons of observations in the same water depth at different sites suggest that free wave radiation is relatively stronger from broad, sandy beaches than from rocky, cliffed coasts.