GROUP BOUND LONG WAVES AS A SOURCE OF INFRAGRAVITY ENERGY IN THE SURFZONE

Measurements of water surface elevation and cross-shore currents were made under moderate-energy conditions (H-rms = 1.1 m and T-z = 7.5 s) on a low-gradient macrotidal beach in central Queensland, Australia. T he infragravity signal (<0.04 Hz) is separated into incoming and outgo ing components and compared with the wind wave envelope. The variances associated with the incoming and outgoing infragravity signals are co mparable and increase four- to five-fold from a water depth of around 4 m to a depth of 1 m. Cross-correlation and cross-spectral analysis i ndicate the presence of an incoming bound long wave (BLW) that lags ap proximately 5 s behind the wind wave envelope. The BLW decouples from the wind wave groups as the incident waves break. The released BLW the n travels onshore through the surf zone as a free wave and reflects so me distance seaward (5 m) of the shoreline. The superposition of the r eleased BLW and the reflected leaky wave results in standing wave moti on in the inner surf zone. Coherence-spectra suggest that about 40% of the incoming infragravity energy is forced by the wind wave envelope.