Secret Life of Waves

Duke Researchers Are Finding That the Waves We Never See May Play a Big Role in Shaping the Planet p.2

That unanswered question has always intrigued Dr. David Cacchione, a consultant and former U. S. Geological Survey scientist. Cacchione met Pratson when the two worked on the STRATAFORM (Strata Formation on Margins) project. This Office of Naval Research collaboration brought together scientists in the mid-1990s to study the evolution of sedimentary deposits in the continental margin. While not the main focus of his career research, the impact of internal waves on the continental slope has always intrigued Cacchione, who was the lead author on a paper the researchers published last year in the journal Science.

"I've done sort of back of the envelope calculations on this for years," he said. "I've always had this idea that the slope is somehow in equilibrium with the shear and the energy in the internal tides."

The STRATAFORM project brought the two scientists together, but it also made the research possible in another way. Data collected for STRATAFORM allowed them to further explore the impact of tide-driven internal waves on the slope. What they found was that, under the right conditions, internal waves have the capacity to keep sediment from settling on the slopes at a steeper angle.

"When the slope is very low - less than a couple of degrees - the energy of internal waves basically gets reflected up," Pratson said. "If the slope is very steep, then the internal wave energy is deflected back out in to the ocean. But when the continental slope is at an order of 2 to 4 degrees, the internal waves break and create a bore that moves up the slope surface."

The relationship between wave angle and slope, "could be a coincidence," Pratson said. But, at the very least, it suggests that this is a mechanism that should be considered as a factor in the formations of the slopes.

While Pratson did much of his work on the computer, the third co-author of the paper, Andrea Ogston of the University of Washington in Seattle, was the one who got her feet wet. She collected data on internal wave motions from buoy-bound instruments that reached to the bottom of their California study site. Her observations confirmed the researcher's theoretical work.

In a sense, new data allowed Pratson's team to do its work. For Brad Murray, it was a new approach to modeling that led him to his cutting-edge research. Murray's work focuses on the applications of chaos and complex systems theory to geology. The approach is based on the tendency of complicated systems with seemingly irregular behavior to develop from simple interactions. Complex systems theory applies to a range of scientific endeavors, as evidenced by the faculty who staff the Duke Center for Nonlinear and Complex Systems. They come from math, physics, engineering and neurology, to name a few.

Prior to turning his attention to the sea, Murray used complex systems theory to study river and landscape pattern formations. So he was not part of the shoreline change crowd when he turned his attention to the beach. But when it comes to systems, you can't find many that get more nonlinear or complex than the processes that drive shoreline change.

Traditionally, coastal scientists and engineers use numerical simulation-style modeling to analyze shoreline change. With that approach, they try to create formulas that mimic natural systems as accurately as possible, including all the details of the processes and inputs that might impact the system, Murray said.

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