Coherence of eddies in a rip channeled surfzone
Joseph Geiman, James T. Kirby, Ad J.H.M. Reniers, Jamie MacMahan, Jeff Brown, Jenna Brown, Tim P. Stanton
University of Delaware
(Abstract received 08/14/2009 for session B)
ABSTRACT
Breaking waves on a rip-channeled beach often generate mean two-dimensional vortical structures that contribute to most of the mixing of fluid inside the surfzone. The RCEX experiment (Monterey, CA in 2007) investigated the rip current circulation over a bathymetry consisting of persistent shoals and rip channels using GPS aided drifters. The experiment revealed instances where a well developed rip current circulation pattern suprisingly did not lead to frequent transport of drifters beyond the outer edge of the surfzone. This trend of drifter retention is echoed in recent numerical simulations of the RCEX field experiment using the wave and current resolving model Funwave. Model output reveals dipolar rip structures that oscillate at a time period much longer than the incident wave period, leading to episodic ejections of simulated drifters. Coherence of these rip current eddies is assessed by calculating finite-time invariant manifolds using FTLE fields as well as methods used by Miller et al. (1997). Eddy coherence is also assessed by mapping the retention time of fluid particles inside the surfzone.
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