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

[1] Eulerian and Lagrangian measurements of nearshore circulation on a longshore uniform beach with perpendicularly incident swell waves are described. Both the Eulerian and Lagrangian data show a variable rotational part of the current field that is distinct from, but coexists with, that due to infragravity waves. A well-defined feature of the flow field includes transient rip currents, which occur in varying locations and are not topographically controlled. Drifter trajectories show that the rips behave like a shallow plane 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 length scale across the rip neck using two-particle statistics are 20 - 30 m. Well-defined discrete vortex features are observed at the initiation point of one of the necks and in one of the rip heads. Estimated dispersion values suggest that mixing is greatly enhanced outside the surf zone by the intermittent presence of the rip heads. The dispersion calculated from both inside and outside the surf zone appears to show scale dependence with a power law close to 4/3 in a similar way to oceanic turbulence at much larger scales.