Rapid currents are the big challenge for engineers designing the foundations of the new Tacoma Narrows bridge.
Above water, the main concern is wind.
Wind, after all, is what set the first bridge over the Narrows dancing the fandango and then flying to pieces Nov. 7, 1940. Compared to what the Narrows is capable of, the storm that downed Galloping Gertie was a minor blow, with prevailing winds of just 42 mph.
Gusts twice that strong whip through the Narrows every few years, causing drivers to make sudden, inadvertent lane changes and occasionally lifting 50-foot mobile homes into the air.
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Engineers say the completed structure should be able to withstand sustained winds of 109 mph and gusts of 127 mph, both of which theoretically occur just once every 10,000 years.
Why is the wind so wild at the Narrows?
Weather experts say global weather patterns and local topography make the narrow passage a natural wind tunnel.
Cliff Mass, a meteorologist at the University of Washington, explains it this way: In the winter, when the worst storms occur, low-pressure areas develop over the Pacific and move over the northwestern part of the state.
Air tends to move counterclockwise around these low-pressure areas, a pattern that predictably sends strong winds moving up the Sound from the south and southwest.
"Wind just accelerates up the Sound, basically," he said.
The Cascades and the Olympics create a convenient pathway for the wind, and the Narrows parallels its path, offering a long stretch of open water, or "fetch," over which it tends to accelerate.
When the new bridge is finished, it will have a complete weather station built into it, including an anemometer on top of the Tacoma-side tower to measure wind speed.
(This story was published 7/13/03)