Bridge survives flood simulation

University of New Mexico professor Bruce Thomson, standing on platform, watches a demonstration of a scale model of Albuquerque's flood control system. (John Fleck/Albuquerque Journal)

University of New Mexico professor Bruce Thomson, standing on platform, watches a demonstration of a scale model of Albuquerque’s flood control system. (John Fleck/Albuquerque Journal)

Copyright © 2014 Albuquerque Journal

Absent a really giant floating rubber ducky, the Rail Runner bridge across Albuquerque’s main flood control channel appears to be safe.

“The thing about water that people don’t understand is that it is so powerful,” said Adrienne Martinez as she stood over water rushing beneath a scale model of the flood control channel and the Rail Runner bridge. The bridge held, intact, against a simulated flood flow larger than anything nature has yet provided since the Albuquerque system was built in the 1970s.

Until Martinez released the rubber duckies into the channel. Wham! The little toy Rail Runner train was in the drink.

“And that’s what would happen if we had really big ducks,” Martinez said with a smile.

The duck is for entertainment value, but the model is dead serious. Martinez, a University of New Mexico engineering student who also works as an intern for the Albuquerque Metropolitan Arroyo Flood Control Authority, is part of a team that built the realistic scale model to simulate flood flows at the end of the authority’s North Diversion Channel, which dumps metro area storm water into the Rio Grande at the southern boundary of Sandia Pueblo.

For years, flood control workers have known the railroad bridge was a choke point, vulnerable as water slowed and “standing waves” backed up under the bridge during major floods. At the right flow levels, the water had the chance to take out the bridge, explained Jerry Lovato, chief engineer for the flood control authority.

“We’re trying to keep that bridge from washing out,” explained AMAFCA engineer Brad Bingham.

The physics of flowing water are sufficiently complex that you cannot simulate the problem on a computer, Lovato said, so Martinez and her colleagues have been hard at work in the basement of UNM’s Centennial Engineering Center with the same sort of plastic sheets used to build model airplanes.

Beyond the rubber ducks, toy trains and trucks, the resulting model provided a realistic way of experimenting with changes to the shape of the flood control channel to reduce the risk of washing out the bridge.

“You build it, you run water, you find a problem, you rebuild it,” Martinez explained.

What the engineers found was an array of large concrete block-like structures built to slow down water on its way to the Rio Grande had become a bottleneck, creating the standing waves that in a severe flood would threaten the bridge. Following a summer thunderstorm in July, the water ran 10 feet deep, reaching within a few feet of the bridge.

Last month, based on the research done in the basement lab, AMAFCA removed the concrete blocks. Now they’re using the model to study further modifications to the flood channel as it approaches the Rio Grande, hoping to further reduce the risk of water backing up and threatening the bridge.

And the ducks? Not as far-fetched an idea as it might seem, said Bruce Thomson, a University of New Mexico engineering professor and member of the flood control authority’s board of directors.

Trucks and cars, while not as cute as rubber duckies, have occasionally been known to wash down the channel, he said: “The duck is not totally unrealistic.”

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