Surf zone diffusivity on a rip-channeled beach

Absolute and relative diffusivity are measured on a rip-channeled beach using 30 position-tracking drifters released in clusters (4–12 drifters) deployed on 7 days with different wave forcing and tidal elevations at Sand City, Monterey Bay, California. Diffusivity and dispersion were found to be larger on days with rip current flow patterns and larger waves. Rip currents cause material to diffuse quickly for t < 90 s in the cross shore (Kxx = 5.4–6.1 m2/s) before decreasing to an asymptotic oscillation (Kxx = 0.9–2.2 m2/s), while alongshore material diffusion is initially (t < 170 s) smaller than cross-shore diffusion and asymptotes at a larger value (Kyy = 2.8–3.8 m2/s). The cross- and alongshore absolute diffusivity modulate at ~300 s corresponding to the average circulation time for a rip current. Two-particle relative dispersion (Dp) grows like Dp2 ~ t4/3 and the relative diffusivity (Kp) is scale dependent, Kp ~ d0.2 (d is particle separation). Cluster relative diffusion (Ke) ranged from 1.0 to 4.5 m2/s and cluster relative dispersions (De) are significantly correlated with two-particle relative dispersions (DS) [R2 > 0.9]. Two independent methods are used to measure the small-scale turbulent diffusion contribution (kxy), which are found significantly correlated (R2 = 0.95) with each other and calculated surf zone wave breaking induced turbulent eddy viscosity. Here kxy is small relative to the total dispersion (Ke/kxy = 3–30), indicating that the shear flow is the primary mechanism responsible for dispersion in a rip current system