California Sea Grant College Program

Currents in the region of actively breaking waves (i.e., the surf zone) present hazards to recreation, transport sediment, and advect nearshore pollutants. While extensively studied with both Langrangian and Eulerian techniques during the last 60 years, many aspects of surfzone currents are still poorly understood, especially over irregular bathymetry. This work describes the design, development, and deployment of a new device for Lagrangian surfzone observations, a GPS-tracked surfzone drifter. The drifters are designed to observe the mean flow in presence of breaking waves, and to withstand harsh surfzone conditions. GPS positions are logged internally and transmitted to shore every 10 s, allowing real-time tracking and partial data backup. Tracking range is approximately 10 km, and battery life is 24 hr. Observations of flows in a frequently-occuring rip current and adjacent waters, made in July 2000, 2001, and 2002, suggest that the location of rip current cross-shore velocity maxima and extent are correlated to the surfzone width. Eddies located entirely withing the surfzone were observed to be bathymetrically-controlled products of the mean flow. Inter-annual flow variations were attributed to subtle variations in the bathymetry, and intra-day variations of two particular days appeared to result from tidal variations of 0.5 m. Numerical solutions of the fully nonlinear 2D Navier-Stokes equations successfully charaterized the flows. Simulated drifter releases also compared favorably to observations, although velocities differ by a factor of 2. Momentum balances near a rip current are discussed.