Observations of the Surf-Zone Turbulent Dissipation Rate

Abstract

Abstract The contributions of surface (breaking wave) boundary layer (SBL) and bottom (velocity shear) boundary layer (BBL) processes to surf-zone turbulence is studied here. The turbulent dissipation rate ε, estimated on a 160-m-long cross-shore instrumented array, was an order of magnitude larger within the surf zone relative to seaward of the surf zone. The observed ε covaried across the array with changing incident wave height, tide level, and alongshore current. The cross-shore-integrated depth times ε was correlated with, but was only 1% of, the incident wave energy flux, indicating that surf-zone water-column turbulence is driven directly (turbulence injected by wave breaking) or indirectly (by forcing alongshore currents) by waves and that the bulk of ε occurs in the upper water column. This small fraction is consistent with laboratory studies. The surf-zone-scaled (or Froude-scaled) ε is similar to previous field observations, albeit somewhat smaller than laboratory observations. A breaking-wave ε scaling is applicable in the midwater column at certain locations, indicating a vertical diffusion of turbulence and ε balance. However, observations at different cross-shore locations do not collapse, which is consistent with a cross-shore lag between wave energy gradients and the surface turbulence flux. With strong alongshore currents, a BBL-scaled ε indicates that shear production is a significant turbulence source within the surf zone, particularly in the lower water column. Similarly for large currents at one location, the dissipation to shear production ratio approaches one. Both dissipation scalings depend upon wave energy flux gradients. The ratio of BBL to SBL ε has complex dependencies but is larger for a deeper part of the surf zone and more obliquely incident waves.