Drifter Experiment to Study Dispersion in Surf Zone
December 11, 2004
Contact: Marsha Gear, Communications Director, mgear@ucsd.edu, 858-534-0581
Kent Smith, a marine technician, carries a drifter from the surf. The drifter was released on a day when waves were often over head-high. A strong alongshore current that day swept this drifter southward. Credit: Christina S. Johnson
In early November, a group of physical oceanographers, technicians and graduate student volunteers released and recaptured a set of drifters just north of the famed Black's Beach surfing spot in La Jolla. The fieldwork was part of an ongoing effort to understand the complex process of dispersion in the surf zone.
In simple terms, dispersion is a measure of dilution and mixing. Imagine pouring cream in a mug of black coffee and stirring. The blob of cream stretches, folds and twists, slowing losing any recognizable form. Eventually, all semblance of the blob disappears, as the black coffee turns uniformly lighter in color as the cream mixes.
If scientists understood dispersion in the mug, and knew how vigorously the coffee was stirred, they could model the spread of cream through the cup. In a sense, this is what Sea Grant researchers Falk Feddersen and Bob Guza of Scripps Institution of Oceanography are trying to do, but for the surf zone. They seek to understand how blobs of water, sewage, runoff pollution, planktonic larvae or fish eggs, for example, are dispersed in the surf. To gather the data that will make this possible, they released pairs of drifters at set locations in the surf zone. The degree to which these pairs separated as they floated along is a measure of dispersion. Drifters that moved together like two
Drifters deployed for the experiment. Sea Grant funding helped in the design and testing of these surf-zone drifters. The drifters' long antennas receive signals from GPS satellites. A computer inside logs raw GPS data. Each drifter also broadcasts its position to a radio receiver located on the Scripps Institution of Oceanography pier. Credit: Christina S. Johnson
ballroom dancers indicate areas where dispersion was weak. In areas where the pairs took markedly different paths, dispersion was strong.
Currently, Feddersen and colleagues are quality controlling the raw data they gathered during the two-day field experiment. As the data is cleaned up, they will produce maps showing the drifters' paths through the surf.
They then plan to compare these maps to ones generated by numerical models of surf-zone dynamics. As the science progresses, it is hoped that simulations of surf-zone dynamics can be integrated into larger-scale, coastal ocean circulation models. This would enable coastal managers, fisheries biologists and others involved in marine science to begin to see linkages between the California current system and surf zone.
Graduate students, post-doctoral fellows and one of the technicians who helped with the experiments. Volunteers swam drifters out to set locations in the surf zone marked by floats. Cued by the blast of a horn, they then released drifters. After twenty minutes of float time, or once a float moved past a certain point on the coast, the drifters were recaptured and brought back to shore. Credit: Dennis Darnell
One of the volunteers floats passively beside a drifter he released. Credit: Dennis Darnell
In bigger surf, drifters were sometimes recovered with the help of a jet ski. Marine technician Bill Boyd is driving the jet ski. Marissa Yates, a physical oceanography graduate student, is holding the recovered drifter. Credit: Dennis Darnell
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