Application, modelling and validation of surfzone drifters

Lagrangian data of surfzone flows are scarce, but important for understanding the spatial variability of currents. Surfzone drifters capable of collecting high frequency, accurate, Lagrangian data in the surfzone have been developed. The main issues of a robust design capable of autonomous positioning fixing, the dynamic response of the drifters, position fixing accuracy and field validation are addressed. A simple, low-cost drifter design using non-differential Global Positioning System (GPS) position fixing is described. A drogue consisting of a series of small parachutes prevents the drifter surfing in breaking waves. The methodology and results of simulations of drifter response in idealised surfzone waves are presented and predict that the drifter will perform well in low to medium energy surfzones. An analysis of positioning errors shows that only surfzone motions with frequencies of less than 0.05 Hz can be reliably resolved without differential correction. A field validation of the drifters shows that although there is significant scatter in the data, the best-fit correspondence of the drifter velocity and depth-averaged Acoustic Doppler Current Profiler (ADCP) measurements is good. (C) 2004 Elsevier B.V. All rights reserved.