Energetics of breaking waves within the surf zone

Abstract

Surface elevations and velocities were measured for a variety of breaking wave conditions including collapsing, plunging, and spilling breakers. Turbulent and wave-induced velocity components are separated by associating the wave-induced velocities with contribution coherent with the surface. Most of the measurements were made in the lower half of the water column and are indicative of conditions in this region. The average velocity intensity for all experiments was 85% wave induced, indicating that the kinetic energy is primarily wave induced. Remarkably little difference was found between collapsing, plunging, and spilling breakers in terms of percent wave-induced velocity intensity. Breaking waves can be characterized as highly nonlinear with strong wave-wave interactions resulting in energy being transferred away from the primary wave frequency, resulting in a saturation region at higher frequencies. Similarity arguments assuming kinematic instability with phase speed the only relevant parameter suggest a -3 slope for the log-log frequency spectrum of both the surface and horizontal velocities in shallow water. The surface elevation spectra of collapsing and plunging breakers have a slope more closely approximated by a -7/3 slope, indicating the possible importance of surface tension in the breaking processes: the velocity spectra show a -3 slope. The saturation region for spilling breakers surfaces elevation spectra varied from -3 at lower frequencies to -5 at highest frequencies.