Abstract

Rip currents are seaward flowing jets of water that extend from the shoreline to the outer extent of wave breaking, and episodically exit beyond. Field measurements were obtained from 11 stationary instruments and 30 drifters equipped with a modified low-cost (∼$200), hand-held Global Positioning System deployed on a sandy beach with persistent, quasi-periodic alongshore spaced O(125 m) rip channels (currents) at Sand City, Monterey Bay, CA. The rip currents retained and re-circulated the drifters within the surf zone, increasing the number of repetitive drifter observations. Only ∼10% of drifters that entered a rip current exited the surf zone. Owing to the high drifter density (∼75 drifter measurements/bin), time-averaged synoptic maps of velocity, vorticity, horizontal eddy diffusivity over multiple rip current cells (>400 m alongshore, >200 m cross-shore) were computed in the field for the first time. Four different circulation patterns were observed that differed from traditional views. The cross-shore rip current velocity distribution had a maxima (40-60 cm/s) located inside the surf zone, decaying rapidly seaward of wave breaking. Mean onshore shore-connected shoal velocities were found to be of similar magnitude to offshore rip current velocities. The drifter rip current diffusivities in the cross-shore have a periodic response, which modulates at ∼300 s, the time required for a drifter to complete one revolution around a rip current cell, before decreasing to an asymptotic limit. This suggests that material initially (t<90 s; κ _xx = 4.9 - 6.1 m²/s) diffuses, then re-collects, reaching a lower asymptote (t>200 s; κ_xx = 0.9 - 2.2 m²/s). The alongshore diffusivity is also periodic, but its asymptotic limit is larger (κ_yy = 2.8 - 3.9 m²/s), as the drifters spread to neighboring rip currents, whereas the cross-shore offshore limited by the surf zone width, reducing material transport. Asymptotic rip current diffusivities are similar to other asymptotic diffusivities of surf zones that support nonrip current circulation patterns. Turbulent eddy diffusivity (k_xy') accounts for 3 - 33 % of the total diffusivity (K_e = 0.8 - 2.7 m²/s), indicating that the rip current shear is the primary mechanism of material diffusion.