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Hurricane Ivan generated monster waves – study

August 4, 2005

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) – Hurricane Ivan, which caused a
swathe of destruction across the Caribbean last September
before crashing into the U.S. Gulf coast, generated ocean waves
more than 90 feet high, researchers said on Thursday.

They may have been the tallest waves ever measured with
modern instruments, suggesting that prior estimates for maximum
hurricane wave heights are too low, William Teague of the Naval
Research Laboratory at the Stennin Space Center in Mississippi
and colleagues reported.

“Our results suggest that waves in excess of 90 feet are
not rogue waves but actually are fairly common during
hurricanes,” Teague said in a telephone interview.

A wave that big would snap a ship in two or dwarf a
10-floor building, Teague said. And the sensors may have missed
the largest waves, which the authors estimate had
crest-to-trough wave heights exceeding 40 meters or 130 feet,
the researchers said.

Such giant waves disintegrated before they ever touched
land, they said.

Luckily, said colleague Doug Mitchell, ships rarely ever
encounter such waves.

“They know better than to be out there in those
conditions,” he said in a telephone interview.

But oil platforms and other equipment cannot get out of the
way.

“So when there were reports of an 80-foot (24-meter) wave
striking an oil platform, they called it a rogue wave. We think
it wasn’t a rogue wave,” Teague said.

Ivan sank seven oil platforms and set five adrift, Teague
said, and his team’s findings might help explain it.

It was pure luck that they did. The instruments the
researchers used were out there to measure currents.

They were recorded by sensors about 75 miles south of
Gulfport, Mississippi on six moorings resting on the ocean
floor, Teague, Mitchell and colleagues report in Friday’s issue
of the journal Science.

“It was very fortuitous that our instruments were there to
measure the wave in the first place and even more fortuitous
that they survived the storm,” Teague said.

The six sensors turned on only once every eight hours, and
sample water flow and pressure for 9 minutes.

“We would see 50 or more waves,” Teague said. “It turned
out that during one of these periods of recording, the biggest
crest-peak wave that we measured was 91 feet.”

There were five taller than 65 feet, he added.

The previous record height recorded by the U.S. National
Oceanic and Atmospheric Agency was 52 feet, also recorded
during Ivan.

What would a 90-foot wave look like? Mitchell said the film
“The Perfect Storm” might provide an idea. In “The Perfect
Storm” a 100-foot (30 meter) wave sinks a fishing vessel,
drowning its crew. The images are computer-generated.

“The waves are about 600 feet long and there are ships out
there that are that long,” Mitchell said. “If the wave is under
the ship, with a crest in front and a crest in back, there is
nothing supporting the middle,” he added.

“The chances of a ship surviving something like that are
not good.”




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