Observations of nearshore circulation: Rip currents

A sector-scanning Doppler sonar was deployed at the end of the Scripps Institution of Oceanography pier, aimed in toward the surf zone. Estimates of acoustic backscatter intensity and radial velocity were obtained seaward of the break point, over a wedge with an arc of about 45° and a radius of 200 to 400 m. The acoustic intensity is proportional to some combination of the bubble density and the suspended sediment in the water. Well-defined jets of water extending seaward from the surf zone or "rip currents" were observed. These rip currents occurred episodically, recurred aperiodically one to four times per hour or so, and lasted several minutes each. The water transported offshore within the rips was rich in suspended sediment and bubbles, producing distinctive bright patterns that can be easily recognized in successive images of backscatter intensity from the sonar. Fluid velocities were estimated from the Doppler shift of the sonar signal, approaching 0.7 m/s in some of the events. Simultaneously, a transect of temperature, density and turbidity within the rip current was obtained with conductivity-temperature-depth (CTD) and optical backscatter strength (OBS) sensors. These data describe the ejection of warm turbid water from the surf zone and provide a comparison between the acoustic intensity and the optical turbidity of the fluid. The horizontal extent of the sonar measurements and the vertical resolution of the CTD/OBS measurements complement each other in the study of the dynamics of the nearshore environment. Exchange rates are not fully resolved here, since only one seaward jet is contained within the field of view; however, these observations suggest that rip currents could cause significant exchange between the nearshore and offshore waters.