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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