Transient rip currents and nearshore circulation on a swell-dominated beach

Eulerian and Lagrangian measurements of nearshore circulation on a longshoreuniform beach with perpendicularly incident swell waves are described. Both the Eulerianand Lagrangian data show a variable rotational part of the current field that is distinctfrom, but coexists with, that due to infragravity waves. A well-defined feature of the flowfield includes transient rip currents, which occur in varying locations and are nottopographically controlled. Drifter trajectories show that the rips behave like a shallowplane jet, consisting of a constrained neck region that flows into a spreading head region.There is continuous modulation of the flow within the rip neck. Estimates of the lengthscale across the rip neck using two-particle statistics are 20–30 m. Well-defined discretevortex features are observed at the initiation point of one of the necks and in one of the ripheads. Estimated dispersion values suggest that mixing is greatly enhanced outside thesurf zone by the intermittent presence of the rip heads. The dispersion calculated fromboth inside and outside the surf zone appears to show scale dependence with a power lawclose to 4/3 in a similar way to oceanic turbulence at much larger scales.