Turbulent Times at the Beach

After months of careful analysis and dutiful preparation, all was ready. Yet the long-planned start date of Monday, Oct. 20, came and went. Vigorous wave conditions prevented researchers from attempting to install an array of scientific instruments just north of the pier at Scripps Institution of Oceanography at UC San Diego. Tuesday passed with the same hold-up. But Wednesday turned out to be the day, just as the scientists had forecasted.

This group of leading coastal oceanographers and engineers are in the business of deciphering wave processes and other aspects of the coastline. Their research builds upon a decades-long legacy of coastal oceanography at Scripps (See “The Beach of the Future” explorations May 2008.)

Right on cue, on Wednesday, Oct. 22, conditions were ideal to commence the project, led by Falk Feddersen and Bob Guza, designed to probe the intricate dynamics of the surf zone, where waves break and dissipate their energy.

While other Scripps studies have examined the currents that move water along the coast, this project focused heavily on vertical motion, and in particular the strong turbulent motions that mix water in the surf zone.

“This project is an outgrowth of the kinds of things we’ve been doing the past couple of years,” said Feddersen, an associate research oceanographer at Scripps. “When stuff gets put into the water, whether it’s bacteria from sewage spills or some other substance in surf zone waters, we’d like to know how it moves, mixes, spreads, and dilutes.”

The linchpin of the project, sponsored by the National Science Foundation, was a 400-pound instrument frame outfitted with some $80,000 worth of scientific instruments. Engineered over a year, and installed by the Scripps team of engineers and technicians that built it, the frame included acoustic Doppler velocimeters that measure currents, along with sensors that measure wave height, water temperature, and sand level. Fluorescence meters at four vertical levels determined the concentration of a bright, non-toxic pink dye the researchers released into the water. Tracking the dye helps the scientists get a better picture of how pollution moves and mixes in three dimensions: along the coast, offshore, and vertically.

All of the information was measured and recorded eight times per second, pumping upwards of 40 megabytes of data per hour to the project’s information center just up the beach on the Scripps campus.

The researchers spent a month acquiring the data, with analysis continuing in the months ahead.

“This project is part of an ongoing study to understand the transport and dilution of pollution in the surf zone,” said Bob Guza, a Scripps professor of oceanography. “We’re learning more about how pollution, especially urban runoff, mixes and disperses in the surf zone.”

The project was another step in a multi-year journey that began with an experiment in Huntington Beach, Calif. in 2006.  The next step is a much larger project scheduled for fall 2009 at Imperial Beach, Calif.

—Mario C. Aguilera