Group bound long waves as a source of infragravity energy in the surf zone

Measurements of water surface elevation and cross-shore currents were made under moderate-energy conditions (Hrms = 1.1 m and Tz = 7.5 s) on a low-gradient macrotidal beach in central Queensland, Australia. The infragravity signal (<0.04 Hz) is separated into incoming and outgoing components and compared with the wind wave envelope. The variances associated with the incoming and outgoing infragravity signals are comparable 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 indicate the presence of an incoming bound long wave (BLW) that lags approximately 5 s behind the wind wave envelope. The BLW decouples from the wind wave groups as the incident waves break. The released BLW then travels onshore through the surf zone as a free wave and reflects some distance seaward (5 m) of the shoreline. The superposition of the released BLW and the reflected leaky wave results in standing wave motion in the inner surf zone. Coherence-spectra suggest that about 40% of the incoming infragravity energy is forced by the wind wave envelope.