Engineers Work On Their SUNTANS As They Track Waves And Beaches

In recent years, tanker accidents have ruined fisheries and tourist beaches from Alaska to France. But do oil spills always have to end in catastrophe? Perhaps the most vulnerable beaches and coastal habitats could be identified and protected well in advance, if scientists had some way to predict where a glob of spilled crude was likely to end up.

A team of engineers from Stanford University is trying to accomplish that challenging task by developing a computer code capable of tracking massive internal waves that begin on the ocean floor and gather strength as they rise to the surface.

Internal waves can reach heights of 300 feet and often contain enough energy to move pollutants, debris and even boats long distances. Despite their size, internal waves are difficult to detect because they move invisibly below the surface.

"Tracking internal waves is important to the fishing industry and for understanding ocean pollution," said Margot Gerritsen, an assistant professor of petroleum engineering. "If you accidentally drop some pollutants into a coastal region, you'll want to be able to predict how quickly they mix."

Gerritsen and Stanford colleagues Robert Street and Oliver Fringer are spearheading SUNTANS - a federally funded research project to develop a computer code that can identify internal waves and forecast when they will reach the shore.

"What we're trying to do is simulate a coastal region precisely enough to find internal waves with our computer code and predict where they will break. Currently, there is no code that can do this accurately," Gerritsen said. She and her colleagues were scheduled to discuss the SUNTANS project at the annual fall meeting of the American Geophysical Union in San Francisco on Dec. 9.

"SUNTANS certainly will have an impact on the fishing industry, because with this code, we can predict where there will be a lot of vertical mixing, which tells us a lot about the availability of nutrients in the water," Gerritsen noted. "We'll also be able to use it to track the transport of red tides and other toxic blooms."

SUNTANS also could be used to create better forecasting models of global climate change. "Internal waves are generated by the tides, and 75 percent of all tidal energy gets dissipated in coastal regions," Gerritsen said. "If we have a better sense of why this is and how this works, then we can apply that to a global model. Right now, global climate models don't even take the tides into account most of the time."

Navier-Stokes

SUNTANS is the acronym for the Stanford Unstructured Non-hydrostatic Terrain-following Adaptive Navier-Stokes Simulator - a long name that reflects the enormous complexity of trying to simulate and forecast oceanic wave movement. "Navier-Stokes" refers to a set of 19th-century equations that have become standard tools in the field of fluid dynamics.

"The Navier-Stokes equations provide the means of finding the accelerations of fluid masses caused by the forces acting in the ocean," explained Street, the William Alden and Martha Campbell Professor in the School of Engineering. "They're very hard to solve, in part because things like wind on the surface or varying densities in the fluid itself can produce a complex variety of forces that, together, make the fluid accelerate."

The SUNTANS team is using Navier-Stokes equations and cutting-edge computer algorithms to create a universal code that can be applied to any of the world's oceans. So far, the research effort is focused on two sites in the Pacific: Hawaii's Mamala Bay, which includes Pearl Harbor and Waikiki; and California's Monterey Bay, located about 50 miles southwest of the Stanford campus.

Monterey Bay provides an ideal setting because it includes a near-shore canyon that is deeper than the Grand Canyon.

"When water comes up against the slope of Monterey Canyon, it excites internal waves that propagate throughout the canyon," Street said. "Some of these waves get pretty big, but they're very subtle, sometimes appearing as little ripples on the surface."

When internal waves are funneled up the canyon, they intensify and often turn into breaking waves that pick up sand and other material from the ocean floor.

"Internal waves have even been known to move a ship from its anchorage," Street noted, citing a 1991 Coast Guard report that described how a breaking internal wave in Monterey Canyon may have contributed to a mishap in which the propeller of a tanker became entangled with a buoy chain.

"People in Hawaii are interested in SUNTANS from a tourism point of view," Gerritsen added, "especially in Mamala Bay, where tankers and ships pass very close by the coast."

Better mousetrap

Street predicted that the SUNTANS team "could have a code we're happy with a year from now. With Margot and Oliver's numerical expertise, we can build a better mousetrap - a better numerical code that would be very accurate, very fast and would allow people to predict internal waves and circulation accurately to solve coastal environment problems."

SUNTANS requires thousands of parallel computers operating in tandem - the kind of enormous computing power available at only a handful of national centers.

"If we put this on the fastest parallel computer currently available, it would be fast enough to do realistic cases," Gerritsen said. "Of course, the average researcher does not have this kind of computer technology now, but it should be widely available in a few years when the SUNTANS code is up and running."

The SUNTANS project is supported by the National Science Foundation and the Office of Naval Research.