2001–2002 - California Sea Grant - UC San Diego

Surf-Zone Drifters: A New Tool for Studying Nearshore Circulation
With California Sea Grant funding, coastal oceanographers
have built a satellite-tracked drifter capable of withstanding
the pounding force of breaking waves. The surf-zone
drifter, the first of its kind, represents an important addition to an ever
increasingly sophisticated arsenal of tools for studying the coastal ocean.
Unlike current meters, which measure water velocities at fixed points, the
new drifter moves with ocean currents, tracing out a trajectory that
represents the space-time evolution of a water parcel through the surf
zone. These trajectories make it possible to unravel some of the more
subtle physics of surf-zone circulation, such as the evolution and development
of rip currents.
The new instrument also has many potential applications for coastal
communities interested in maintaining beach water quality. A fleet of the
drifters, for instance, could be used to help estimate the rate at which
water along the coast is exchanged with deeper, cleaner waters outside the
surf zone. This exchange rate in turn could be used to help quantify the
dispersion and fate of coastal pollution from point sources and urban
runoff. The drifters could also conceivably be used to help coastal engineers
evaluate the effects of jetties, groins, artificial reefs or seawalls on
nearshore circulation, sand loss and coastal erosion.
The drifter, designed by Sea Grant trainee Wilford Schmidt and engineers
Brian Woodward and Kimball Millikan, all of Scripps Institution of
Oceanography, stands about a meter tall and looks like a long white can
with an antenna sticking out the top. Its exterior hull is made of white
polyvinyl chloride (PVC) piping. Sealed inside, there is a Global Positioning
System receiver, a data logger and a radio transmitter. There are also
batteries and an internal lead weight for ballasting.
Each drifter receives its position from earth-orbiting satellites and
transmits its position to a shore-based tracking system, making it possible
for researchers to monitor the movements of many drifters on a single plot
in real-time.
18
Technicians wade into the surf to deploy new surf-zone drifters. Photo: Georgia
Ratcliffe, California Sea Grant
By design, the drifter floats in a vertical, upright position, its black cap
skimming just beneath the surface. Only its antenna actually protrudes
above the waterline. A thin PVC disk at the base of the drifter is designed
to help dampen vertical excursions (e.g., cork-like bobbing) of the drifter
in the surf.
Laboratory tests in wave tanks at Scripps have proven the drifter’s
fidelity to its purpose. The drifter essentially moves with water parcels and
only to a much lesser extent is pushed by breaking waves and winds. Wind
tunnel tests indicated a 1-centimeter-per-second slippage for every meterper-second
of wind. Subjected to nonbreaking waves, the drifter moved
only slightly, as predicted by linear wave theory. In breaking waves, the
drifter moved slightly in the horizontal direction but not vertically,
relative to the seafloor. This vertical stability prevents the drifter from
being washed to shore, like a beach ball or lost surfboard.
One of the immediate applications of the new tool has been to study rip
currents, seaward jets that often carry swimmers out beyond the surf
zone. In the summer of 2001, Schmidt, Scripps professor Robert Guza,