On the dense bubble clouds and near bottom turbulence in the surf zone

Measurements of high‐frequency (100 kHz) acoustical backscatter in the surf zone have identified the presence of a wall formed by dense assemblages of bubbles generated by breaking gravity waves and persisting for periods up to 30 min. These bubbles are advected by alongshore and cross‐shore rip currents and are controlled by the local wave field, turbulence, dissolution, and buoyancy. During the summer of 2000 the location of the bubble wall south of Scripps Pier, California, was tracked for a 6‐week period. Distinct categories of bubble wall defined by the backscatter intensity data are observed, distinguished by the presence or absence of rip current activity. The results suggest that presence or absence of long‐period waves (10–20 s) may play a role in controlling the bubble wall type. During periods without rip currents the location of the bubble wall is controlled by the incoming wave field. Simultaneous turbulence dissipation measurements 0.4 m above the seafloor show clear dependence on the local water depth beneath incoming waves. The acoustical backscatter data are interpreted using in situ bubble and turbulence measurements in conjunction with a simple model based on bubble sorting by buoyancy, dissolution, and turbulence.