GENERATION AND PROPAGATION OF INFRAGRAVITY WAVES

The generation and propagation of infragravity waves (frequencies nomi nally 0.004-0.04 Hz) are investigated with data from a 24-element, coh erent array of pressure sensors deployed for 9 months in 13-m depth, 2 km from shore. The high correlation between observed ratios of upcoas t 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 depen dent on incident swell propagation directions. However F-up(IG)/F-down (IG) is usually much closer to 1 (i,e., comparable upcoast and downcoa st fluxes) than is F-up(swell)/F-down(swell), suggesting that upcoast propagating swell drives both upcoast and downcoast propagating infrag ravity waves. These observations agree well with predictions of a spec tral WKB model based on the long-standing hypothesis that infragravity waves, forced by nonlinear interactions of nonbreaking, shoreward pro pagating swell, are released as free waves in the surf zone and subseq uently reflect from the beach. The radiated free infragravity waves ar e predicted to be directionally broad and predominantly refractively t rapped on a gently sloping shelf. The observed ratios F-sea(IG)/F-shor e(IG) of the seaward and shoreward infragravity energy fluxes are inde ed 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, sugg est that high-mode edge waves generated near the shore can be suppress ed by nonlinear dissipation processes (e.g., bottom friction) on the s helf.