Off the coast of Maui, an underwater ridge acts like a "giant magnifying lens," bending and enlarging Pacific Ocean storm swells to soup up monster surfing waves as high as 70 feet, according to University of Delaware research linked this month to a National Geographic web site.
The waves result from the unique shape of the underwater ridge--not the effects of submerged cliffs, as some observers had previously assumed, says Robert A. Dalrymple, director of the Center for Applied Coastal Research at UD, where faculty and students investigate waves, rip currents and the "swash" of waves sliding up and down the beach face.
Dalrymple, the University's Edward C. Davis Professor of Civil and Environmental Engineering and a professor of marine studies, solved a mystery that had puzzled National Geographic researchers working on the magazine's November 1998 cover story. The profile of "Jaws," a surfing mecca on Maui's northern shore, near the town of Haiku, was prepared by writer Joel Achenbach, a Washington Post reporter and author of Why Things Are.
Stormy weather in the northern Pacific periodically produces swells big enough to be affected by the presence of the ridge, located about 30 feet beneath the ocean's surface, says Dalrymple. "As soon as a big wave runs into that ridge," he says, "it wraps right around it."
As part of a storm swell passes over the ridge crest, it slows down because water travels slower in shallow water, Dalrymple explains. Other parts of the swell travel faster in deeper water, causing the wave to focus on the ridge-a process called refraction. Achenbach's article puts it this way: "The swells on either side of the reef, moving in deeper water, bend inward, focusing much of their energy on the center of the wave crest." In this way, he writes, "the reef squeezes the wave inward and upward" to form a "peaking wave."
Wave Surfin' With UD TechnologyWhen Achenbach set out to solve the mystery of Maui's monster waves, Dalrymple says, initial reports suggested that the unusually large curls resulted from the reflection of the waves from underwater cliffs. Then the magazine commissioned a survey of the water depth over the ridge. This "bathymetric" survey was completed by Sea Engineering.
With Dalrymple's guidance, undergraduate researcher Katie M. Fearing, a civil engineering major set to graduate in the year 2000, fed the depth data into a wave model developed by UD faculty member James T. Kirby and Dalrymple. Called REF/DIF, the program predicts wave patterns when the movement of water is affected by refraction or other processes such as the diffraction and shoaling of waves. (Meanwhile, Achenbach also sought input from other respected experts such as Rick Grigg, a former champion surfer and professor of oceanography at the University of Hawaii, who is quoted in the National Geographic article.)
At Maui's Jaws site, Achenbach's article explains, the ocean depth changes abruptly from 120 feet to just 30 feet, as waves move from deep sea to the beach. Normal waves "don't feel the ridge," Dalrymple says, "because they don't penetrate the entire water column." But, he adds, storm swells longer than about 1,000 feet in length "really trip over the ridge," produced by an ancient lava flow.
When Jaws roars, hard-core surfers flock to Maui, the second largest island in the Hawaiian chain. Wave runners tow the surfers into the path of the big waves, where they ride for up to 22 seconds. In the book, Jaws Maui (written by Charlie and Leslie Lyon, with photography by Blue Max), the first Japanese waterman to ride Jaws describes the experience as exhilarating. "It's just simply one nice, perfect peak, just very tall," says Hidemi Furuya. "It's a perfect wave, actually. Just real stoke, pure adrenaline."
World-Class Research FacilitiesWhen he isn't investigating Hawaiian storm events, Dalrymple studies wave behavior in one of the nation's largest coastal process facilities, the Ocean Engineering Laboratory, home of UD's Center for Applied Coastal Research (CACR).
Researchers at the Center strive to understand the formation of dangerous rip currents that can occur when incoming waves force water through gaps in sandbars. Currently, researchers also are working on wave propagation models for the U.S. Army and the U.S. Navy. These models should prove useful for coastal planning and for safely moving marines from sea to shore, Dalrymple says.
The Laboratory features a large directional wave basin for studying such real-world events as rip currents; a precision wave tank equipped with a computer-controlled hydraulic wave-maker for investigating wave breaking and the interaction of waves with tidal currents; and a spiral wave basin for simulating sandy beach conditions. The facility also makes use of powerful computing technologies including networked stations, a parallel computer and direct access to the University's award-winning network.
NG web site: http://www.nationalgeographic.com/ngm/9811/
Animated gif file: http://www.coastal.udel.edu/ngs/anim.html
Photo gallery: http://www.JawsMaui.com/gallery.shtml
The above story is based on materials provided by University Of Delaware. Note: Materials may be edited for content and length.
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