Generation and propagation of infragravity waves

TLDR

The study used a 24‑element pressure‑sensor array deployed for nine months at 13 m depth, 2 km offshore to investigate infragravity wave generation and propagation. The measurements show that infragravity wave directionality is strongly linked to incident swell direction, with upcoast swell driving both upcoast and downcoast infragravity fluxes, and the observed energy‑flux ratios largely agree with a spectral WKB model, while deviations at low and high swell energies suggest remote sources and nonlinear dissipation of high‑mode edge waves.

Abstract

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 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.