2001–2002 - California Sea Grant - UC San Diego

A technician radios scientists to tell them he has recovered a
drifter. Photo: Georgia Ratcliffe, California Sea Grant
engineers and technicians successfully deployed a fleet of 10 drifters at the
base of a rip current located about a quarter-mile south of the Scripps pier.
The drifters’ paths revealed a never-before-measured feature of the surf
zone, large eddies (circular currents) at either side of a rip. Swirling waters
have been reported anecdotally, predicted by ocean theoreticians and
suggested in some numerical simulations, but this was the first time
researchers had actually measured them.
By overlaying drifter trajectories on maps of bathymetry, the scientists
have also been able to discern a connection between the shape of the
seabed and the location of rip currents.
The width of the surf zone seems to exert some control over rip currents,
as the scientists reported the drifters accelerated as they traveled
seaward. They
reached a maximum
speed at the edge of
the surf zone—some
two to three times
faster than average.
Width may also play a
role in determining
the seaward extent of
a rip current.
A second deployment
of the drifters
was repeated in the
summer of 2002
along the same
stretch of beach as the
year prior. The
scientists have not yet
analyzed the data, but
it will be a key part of
Schmidt’s doctoral
thesis, “Spatial and
Temporal Variability
of a Rip Current.”
By some accounts, rip currents are the cause of 80
percent of all distressed swimmer rescues. Often
erroneously called undertows, rip currents do not pull
swimmers down, but carry them offshore through the
surf zone. If caught in a “rip,” the swimmer should
not panic, but swim calmly parallel to the shore. Rip
currents are narrow and can be escaped easily in
this manner. Illustration: North Carolina Sea Grant
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