Infragravity-Frequency (0.005–0.05 Hz) Motions on the Shelf. Part II: Free Waves
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In Part I, the energy levels of ocean surface waves at infragravity frequencies (nominally 0.005-0.05 Hz) locally forced by swell in 13-m water depth were shown to be predicted accurately by second-order nonlinear wave theory. However, forced infragravity waves were consistently much less energetic than free infragravity waves. Here, in Part II, observations in depths between 8 and 204 m, on Atlantic and Pacific shelves, are used to investigate the sources and variability of free infragravity wave energy. Both free and forced infragravity energy levels generally increase with increasing swell energy and decreasing water depth, but their dependencies are markedly different. Although free waves usually dominate the infragravity frequency band, forced waves contribute a significant fraction of the total infragravity energy with high energy swell and/or in very shallow water. The observed h(-1) variation of free infragravity energy with increasing water depth h is stronger than the h(-1/2) dependence predicted for leaky surface gravity waves propagating approximately perpendicular to local depth contours, but is consistent with a heuristic, geometrical optics-based (WKB) model of the refractive trapping of a directionally broad wave field generated close to shore. Preliminary analysis shows that free infragravity waves are indeed directionally broad and that the propagation directions of infragravity waves and incident swell are related. Free infragravity energy levels also depend on the general geographic surroundings. Comparisons of observations from the same depth and with similar swell conditions, but on different shelves, suggest that more free infragravity wave energy is radiated from wide, sandy beaches than from rocky, cliffed coasts and that less energy is trapped on a narrow shelf than on a wide shelf.