Surf zone surface retention on a rip-channeled beach

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Author
Reniers, A.J.H.M.
MacMahan, J.H.
Thornton, E.B.
Stanton, Timothy P.
Henriquez, M.
Brown, J.W.
Brown, J.A.
Gallagher, E.
Date
2009Metadata
Show full item recordAbstract
The retention of floating matter within the surf zone on a rip-channeled beach is
examined with a combination of detailed field observations obtained during the Rip
Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic
Doppler current profiler–observed hourly vertical cross-shore velocity structure variability
over a period of 3 days with normally incident swell is well reproduced by the
computations, although the strong vertical attenuation of the subsurface rip current
velocities at the most offshore location outside the surf zone in 4 m water depth is not well
predicted. Corresponding mean alongshore velocities are less well predicted with errors on
the order of 10 cm/s for the most offshore sensors. Model calculations of very low
frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the
observed variability, both inside and outside of the surf zone. The model calculations also
match the mean rip-current surface flow field inferred from GPS-equipped drifter
trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters,
the computed instantaneous surface velocity fields are used to calculate the hourly drifter
trajectories. Collecting the hourly drifter exits, good agreement with the observed surf
zone retention is obtained provided that both Stokes drift and VLF motions are accounted
for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated
number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the
O(20)% of observed exits during the drifter deployments. Conversely, when excluding the
VLF motions instead, the number of calculated drifter exits is less than 5%, thus
significantly underestimating the number of observed exits.
Description
The article of record as published may be located at http://dx.doi.org/10.1029/2008JC005153
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Collections
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