Response of GPS-tracked drifters to wind and water currents in a tidal estuary

Lagrangian field data in tidal shallow waters are rare, but valuable for the understanding of the spatio-temporal structure of flow and particle transport. The response of drifters to the wind and water flow in tidal shallow water was examined using correlation, spectral and coherence analyses. Under moderate wind conditions (0 – 4 m/s), floating drifter motions in bounded sheltered water are affected by wind through low frequency induced wind current rather than direct wind drag when only a small portion of the drifter is unsubmerged. The field validation of both high resolution and low resolution drifters with surface measured velocity from fixed Acoustic Doppler Current Profiler is good in the streamwise direction. The correlation between the drifter and fixed instrument velocities is low in the cross stream direction due to strong spatial variability of the flow field. The evaluation shows that drifters are applicable to studying the flow dynamics of tidal water bodies in relation to small scale processes.