Surfzone drifters: applications and observations

A GPS-tracked drifter, designed for surfzone use and providing real-time current information, has been developed/deployed over the last 5 years. The positively-buoyant, 50 cm-draft drifter is constructed of PVC and ballasted for near-complete submergence. GPS and spread-spectrum radio antennae reside on a 70 cm-tall protruding spar. Real-time DGPS positions (∼3-5 m) are first-differenced for drifter velocity and direction. More precise (sub-meter) position information is obtained by post-processing L1 carrier-phase data. The present design has seen 25 deployments (4-6 hrs each) since July 2000 and the current fleet of 10 drifters has logged ∼1000 hrs sea time in breaking wave heights up to 3 m. Studies have focused on rip currents and surfzone circulation, lagoon outflow, and surfzone mixing/dispersion. The drifter flow field observations are presented in conjunction with high-resolution digital video, bathymetric, and tidal data. These observations reveal novel features of nearshore circulation patterns that are difficult to obtain using alternate methods. The spatial structure of rip current velocity appears to be strongly linked to surfzone width and geometry. Similarly, surfzone eddies, long predicted but rarely witnessed, are observed to be long-lived (>2 hrs) and large (∼50 m diameter) bathymetrically-controlled features. The drifter is thought to be a suitable platform for additional sensor systems and capable of riverine/estuarine applications.