Hervé Michallet

122. A LARGE-SCALE LABORATORY EXPERIMENT OF RIP CURRENT CIRCULATIONS OVER A MOVEABLE BED: DRIFTER MEASUREMENTS

The present paper presents a laboratory experiment of rip current circulations over a moveable bed. The rip current characteristics over four distinct beach morphologies, exhibiting more or less developed nature-like bar-rip morphology, were investigated. For each video run, the same offshore shore-normal waves were generated by the wavemaker with the same mean water level in order to study the sensitivity of the rip current characteristics as a function of the beach morphology only. In each case, a 1-hour video run was used to track a large number (~30) of drifters released within the surf zone. Image coordinates were then rectified to still water level Cartesian coordinates to compute drifter velocities, mean characteristics and surf zone retention rates. Results show the presence of classic rip current patterns with counter-rotating cells and a relatively narrow offshore-directed jet with, for three of the situations, a reasonably symmetric shape. Non-surprisingly, it was found that rip current intensity increases with increasing relative depth of the rip channel. The wave-driven circulations were strongly unstable. Computed standard deviation in flow intensity and direction provides high resolution information on the spatial variability of the rip current instabilities with, for instance, highly-pulsating and weakly directionally variable offshore-directed flow in the rip channel. Conversely to what was previously hypothesized in the literature, there was hardly trace of vortices being shed offshore and drifters exiting the surf zone compartment were not systematically caught by a pulsating jet. The cause for drifter exiting the semi-enclosed surf zone compartment remains, however, elusive and deserves further investigations. The computed surf zone retention rates (~90%) were of the order of those previously observed in the field, with no clear relationship with the mean rip current velocity or relative depth of the rip channel. Further video-runs will have to be analyzed to explore potential explanations.