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The velocity field and turbulence structure within the surf and swash zones forced by a laboratory-generated plunging breaking wave were investigated using a particle image velocimetry measurement technique. Two-dimensional velocity fields in the vertical plane from 200 consecutive monochromatic waves were measured at four cross-shore locations, shoreward of the breaker line. The phase-averaged mean flow fields indicate that a shear layer occurs when the uprush of the bore front interacts with the downwash flow. The turbulence characteristics were examined via spectral analysis. The larger-scale turbulence structure is closely associated with the breaking-wave- and the bore-generated turbulence in the surf zone; then, the large-scale turbulence energy cascades to smaller scales, as the turbulent kinetic energy (TKE) evolves from the outer surf zone to the swash zone. Smaller-scale energy injection during the latter stage of the downwash phase is associated with the bed-generated turbulence, yielding a −1 slope in the upper inertial range in the spatial spectra. Depth-integrated TKE budget components indicate that a local TKE equilibrium exists during the bore-front phases and the latter stage of the downwash phases in the outer surf zone. The TKE decay resembles the decay of grid turbulence during the latter stage of the uprush and the early stage of the downwash, as the production rate is small because of the absence of strong mean shear during this stage of the wave cycle as well as the relatively short time available for the growth of the bed boundary layer.

The flow structure of oscillatory broken waves within surf and swash zones was investigated using particle image velocimetry (PIV) in the laboratory. With the resolved spatial distribution of the velocity field, vorticity was computed directly. The results show that flow separation at the bed occurs during the interaction between the uprush bore front and the downwash flow. The separation point advances in the onshore direction with the bore front until the bore reaches the lowest point of the moving shoreline. The bore front continues to climb up onshore and collapses at the highest point of the moving shoreline. In the swash zone, flow attachment to the bed occurs during the transition from the uprush to downwash process. An internal flow circulation is generated at the flow reversal phase as the flow near the bed responds to the gravitational force earlier than the flow in the upper water column, where the uprush momentum is sustained later in phase. The magnitude of the downwash velocity in the swash zone is greater than that of the uprush process. The swash zone flow observed in the experiments should be erosive in the lower part of the swash and accretive in the upper part where flow attachment occurs.

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