Secret Life of Waves

Duke Researchers Are Finding That the Waves We Never See May Play a Big Role in Shaping the Planet p.3

"I come from outside this particular community," Murray said. "I studied patterns on the earth's surface looking for simple explanations for these complex patterns. We try to throw out all the detail and find out what are the possible simple interactions that would cause this or that to happen." Many researchers in the coastal science don't believe you can learn much without those details, he said. But Murray saw his approach to modeling as a new way of solving an old puzzle. It also lends itself particularly well to shoreline change. Instead of studying change in an isolated surf zone for a short period of time, Murray's model was designed to consider long-range, large-scale change.

Working as a team that included graduate student Andrew Ashton and then-visiting scientist Olivier Arnoult of the Ecole Normal Superieure, the work began not on the beach, but as a physical insight in Murray's head. Laying in bed on vacation, Murray said he realized that there must be a fundamental instability in alongshore sediment transport. From that, he deduced that waves in deep water approaching the shoreline from high angles (between the wave crests and the shoreline) would make bumps grow. These are the same waves that break on the beach, but "at different stages of their journey," Murray said. They have not yet hit shallow water, where they refract, or bend when they "feel the bottom." Murray said.

"If the wave crests are parallel to shore, there can't be any transport, and if wave crests in deep water are perpendicular to shore, they don't even move toward the shore, so approximately nothing happens," he said. "Moving away from either extreme, the transport increases, until you reach a maximum somewhere in the middle."

Drawing a line from sediment transport patterns to the angle between the wave and the shore allowed the team to create a computer model that would explore shoreline change of over hundreds of miles and thousands of years.

"It turns out that rich, complex, fascinating behaviors come from that relationship." He said. "If you start with a shoreline that is more or less straight and has small bumps on it, everyone assumes that these bumps would be smoothed. But that's not true when the waves approach shore at high angle."

His simulations found that, eventually, these high angle waves can create "flying spits" and various other landforms, including capes similar to those spanning most of the coast of the Carolinas. He also found that sandy coastlines continually reshape themselves, and that changes on one beach can impact distant beaches.

"The details of how they interact and what you end up with probably depend on the wave climate," he said.

From that, an immediate, practical application emerged from the work. Wave angles may very well help explain why some "hot spots" along the beach erode so readily, he said.

These simulations are not designed to describe what a specific stretch of beach looks like. Instead, the computer work takes "a much longer term and abstract view of things," Murray said. Still, it turned out the team's hypothetical scenarios strongly resemble actual coastal morphology and patterns of change. That suggests their findings are relevant to shoreline behavior in nature and adds credence to their modeling approach. The team published their findings in the journal Nature last year. Murray may work in the realm of the abstract, but he doesn't spend all his time in the lab. "I get out to the beach as much as I can," he said. "In general you don't discover anything new when you're sitting at your computer. You're not likely to come up with any new ideas unless you are looking at the natural system."

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