Evaluation of Cross-Shore Drifter Performance

Surf zone drifters equipped with GPS units allow for mapping of nearshore circulation patterns, but previous field experiments demonstrate inaccuracies in estimating cross-shore currents. Different field based drifter designs were evaluated in a wave-current flume with a current opposing intermediate, shallow water and breaking waves. Time-averaged drifter velocities are estimated from video images using edge detection techniques. For a quasi-steady current, drifter velocities are correlated to ADV velocities within one standard deviation of the current fluctuation (±5% of the mean current). For wave-current conditions, drifter velocities underestimate ADV velocities by 40-55% for cases using intermediate and non-breaking shallow water waves. Drifter velocities represent Lagrangian observations and ADV velocities represent Eularian observations. To correct for the difference in frame of reference, Stokes drift must be included such that UEularian=ULagrangian+UStokes (Kennedy and Thomas 2004). This correction reduces the error to within the resolution of the ADV. Drifter performance under breaking wave conditions will be discussed further. Kennedy, A. B., and D. Thomas (2004), Drifter measurements in a laboratory rip current, J. Geophys. Res., 109, C08005, doi:10.1029/2003JC001927.