[1] The measurement of mean water levels, roller geometries, and phase ensembleaveraged velocity and turbulence intensity fields under spilling and plunging waves breaking in a two-dimensional laboratory surf zone is presented. The velocities were measured using digital correlation image velocimetry, while water levels and roller geometries were determined through gray scale filtering of video images. The phase ensemble-averaged horizontal and vertical components of velocity and turbulence intensities are measured throughout the entire flow domain, including the wave roller area, by utilizing the aerated areas as part of the flow structure. The time-averaged horizontal velocities (undertow), turbulence intensities, and turbulent kinetic energies are determined by averaging across the wave phase. Turbulence magnitudes are found to compare favorably with existing laser Doppler anemometry measurements below the wave trough level, where such measurements have generally been confined because of aeration contamination effects. The significantly higher velocity and turbulence intensity magnitudes measured above the trough level in the present experiments highlight the novel nature of the present investigation for describing flow regimes in the surf zone. INDEX

[1] The measurement of turbulence dissipation rates and length scales associated with three-dimensional isotropic structures under spilling waves breaking in a laboratory surf zone is presented. Dissipation rates were estimated from the spectral characteristics of the turbulence velocities in the inertial subrange, and length scales were estimated using measurements of the turbulent kinetic energy and dissipation rate. The spatial velocity flow fields for the above analysis were measured using digital correlation image velocimetry. A unique set of measurements that spans the entire water column, including the aerated portion near the crest of the wave, is presented. Dissipation rates were found to reach a maximum above the effective trough level, with over 80% of the depth integrated dissipation occurring in this upper zone. The total depth integrated turbulence dissipation rate is found to be up to an order of magnitude smaller than the local rate of wave energy dissipation due to breaking, the primary turbulence production source. The length scale is found to increase in magnitude below the surface, consistent with the idea that turbulence production occurs above the trough level in the vicinity of the wave roller and is transported downward toward the bed.INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4558 Oceanography: Physical: Sediment transport; KEYWORDS: digital correlation image velocimetry, dissipation rates, length scales, turbulent kinetic energy, wave breaking, surf zone Citation: Govender, K., G. P. Mocke, and M. J. Alport (2004), Dissipation of isotropic turbulence and length-scale measurements through the wave roller in laboratory spilling waves,

Abstract. The turbulence structure under breaking waves in the surf zone is investigated through reference to experimental measurements and turbulence closure modeling. This includes a unique set of recent turbulence measurements that have extended into the roller area through the application of digital particle image velocimetry. Measurements highlight the surface roller as the primary agent for turbulence production, from where it is transported downward principally because of turbulent diffusion. Reflecting the breaker transition process, turbulence intensities are largest some distance shoreward of the breaker point where the roller is fully developed. The temporal variation of turbulence at a fixed point below the wave trough level is generally found to be minimal, supporting the present use of a time-independent turbulence closure model. Production of turbulent kinetic energy in the two-equation (k-e) model is presumed to occur in the vicinity. of the surface roller, from where it is transported downward through the water column by diffusion. This production, which is estimated from the rate of energy dissipation in the wave roller, is the principal input parameter for the k-e model. Froude scaled turbulence measurements from laboratory and field experiments are found to be satisfactorily predicted within the limits of the developed surf zone, with improved methods of incorporating wave breaker transition zone effects leading to further model refinement. The turbulence closure model is further shown to be effective in predicting time mean suspended sediment concentration and undertow velocity profiles in the surf zone.

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