Generation and propagation of infragravity waves

The generation and propagation of infragravity waves (frequencies nominally 0.004-0.04 Hz) are investigated with data from a 24-element, coherent array of pressure sensors deployed for 9 months in 13-m depth, 2 km from shore. The high correlation between observed ratios of upcoast to downcoast energy fluxes in the infragravity (F-up(IG)/F-down(IG)) and swell (F-up(swell)/F-down(swell)) frequency bands indicates that the directional properties of up infragravity waves are strongly dependent on incident swell propagation directions. However F-up(IG)/F-down(IG) is usually much closer to 1 (i,e., comparable upcoast and downcoast fluxes) than is F-up(swell)/F-down(swell), suggesting that upcoast propagating swell drives both upcoast and downcoast propagating infragravity waves. These observations agree well with predictions of a spectral WKB model based on the long-standing hypothesis that infragravity waves, forced by nonlinear interactions of nonbreaking, shoreward propagating swell, are released as free waves in the surf zone and subsequently reflect from the beach. The radiated free infragravity waves are predicted to be directionally broad and predominantly refractively trapped on a gently sloping shelf. The observed ratios F-sea(IG)/F-shore(IG) of the seaward and shoreward infragravity energy fluxes are indeed scattered about the theoretical value 1 for trapped waves when the swell energy is moderate, but the ratios deviate significantly from 1 with both low- and high-energy swell. Directionally narrow, shoreward propagating infragravity waves, observed with low-energy swell, likely have a remote (possibly trans-oceanic) energy source. High values (up to 5) of F-sea(IG)/F-shore(IG), observed with high-energy swell, suggest that high-mode edge waves generated near the shore can be suppressed by nonlinear dissipation processes (e.g., bottom friction) on the shelf.