Temporal observations of rip current circulation on a macro-tidal beach

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Author
Austin, Martin
Scott, Tim
Brown, Jeff
Brown, Jenna
MacMahan, Jamie
Masselink, Gerd
Russell, Paul
Date
2010Metadata
Show full item recordAbstract
A field experiment was conducted on a high energy macro-tidal beach (Perranporth, UK) to examine rip
current dynamics over a low-tide transverse bar/rip system in response to changing tide and wave
conditions. Hydrodynamic data were collected using an array of in situ acoustic doppler current meters
and pressure transducers, as well as 12 GPS-tracked Lagrangian surf zone drifters. Inter-tidal and subtidal
morphology were measured through RTK-GPS and echo-sounder surveys. Data were collected for
eight consecutive days (15 tides) over a spring-neap tidal cycle with tidal ranges of 4–6.5m and
offshore significant wave heights of 1–2m and peak periods of 5–12 s.
The hypothesis that rip current dynamics in a macro-tidal setting are controlled by the combination
of variations in wave dissipation and morphological flow constriction, modulated by changes in tidal
elevation was tested. During the measurement period, rip circulation was characterised by a large
rotational surf zone eddy O(200 m) extending offshore from the inner-surf zone to the seaward face of
the inter-tidal transverse bar. During high- and mid-tide, water depth over the bars was too deep to
allow wave breaking, and a strong longshore current dominated the surf zone. As the water depth
decreased towards low-tide, wave breaking was concentrated over the bar crests initiating the
rotational rip current eddy. Peak rip flow speeds of 1.3ms!1 were recorded around low-tide when the
joint effects of dissipation and morphological constriction were maximised. At low tide, dissipation over
the bar crests was reduced by partial bar-emergence and observations suggested that rip flows were
maintained by morphological constriction and the side-drainage of water from the transverse bars.
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