Spectral evolution of nearshore wave energy during a sea-breeze cycle

From in situ measurements taken over several sea-breeze cycles off a beach in southwest (SW) Australia, the evolution of the one-dimensional spectrum of wave energy is observed to have a distinctive spectral shape. During the land-breeze phase of the cycle, lower rates of dissipation of wave energy are seen at high frequencies compared to midrange frequencies above the remnant wind-sea peak. A simulation of waves was performed using the Simulating Waves Nearshore (SWAN) model and produced the same spectral evolution, by generating longshore modes, as seen in the observations. The performance of whitecapping schemes available in SWAN was assessed, and the Alves-Banner scheme was found to best simulate the observed growth and decay of the wave spectra. During the onshore phase of the sea-breeze cycle, local wave growth is duration limited, and during the offshore land-breeze phase, wave growth is fetch limited. From an examination of the modeled two-dimensional spectra it is found that quadruplet interactions play a key role in spreading high-frequency wave energy in frequency and direction space.