Structure and modeling of surf zone turbulence due to wave breaking

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‐ε) 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‐ε 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.