Drifter measurements in a laboratory rip current

This paper investigates circulation in a laboratory rip current system using large numbers of Lagrangian drifters, with supplementary current meter and water level measurements. These represent the most dense measurements to date of laboratory rip circulation over a large spatial area. Overall, circulation is found to be very unsteady, with strong changes in circulation apparent at many length and time scales. Drifter and current meter measurements are compared in the rip channel, and Stokes drift is found to be a significant component of drifter velocities in this region. Estimated generation of circulation is compared with computed bottom friction dissipation, with good agreement. The mean cellular circulation about a dividing streamline is found to be surprisingly small when compared with computed rates of generation of circulation. Finally, the divergence of volume flux computed using drifters is found to be significant, with depth-varying wave-induced mass transport, depth-varying Eulerian velocities, and velocity/drifter concentration effects the likely sources of error.