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Phoenix bridges and a dry riverbed: storm dilemma

By Penny L. HatcherSpecial to The Christian Science Monitor / December 26, 1980



Phoenix, Ariz.

Engineers are keeping a wary eye on the highway bridges here because of the severe blow dealt them by last winter's storms. Will the area face a similar blow in the months just ahead?

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Half of this city's population depends on four state bridges to get to work. When the eight-lane Interstate 10 bridge over the Salt River shut down for two weeks last February, a huge traffic tie-up was the result.

"We went in a boat regularly for sonar readings during the flooding period and found that the foundations of the bridge were being reached, so we closed it ," says Ron Brechler, Arizona's state structures engineer, who has been in the center of a controversy ever since.

"When the water later receded, silt was left that recovered the foundations, so we opened it again," he adds.

"There was no damage to the bridge."

Other communities in the Southwest face a situation that is similar to that here. Not only do the bridges cross dry riverbeds but modern roads, ravines, and canyons that never, or seldom, see water.

A heavy rainfall results in a torrent of water that rushes around the bridges and threatens to damage them.

The newer bridges are built for a storm that might occur only once in 50 years, yet southern Arizona has had a devastating storm in each of the last three years. The conflict comes in trying to balance economics with sound construction.

"Hydraulic engineers study weather and climatic patterns, water flow, elevation etc., to determine statistically the violence and strength of a 50- or 100-year storm," Mr. Brechler says. "But these storms could come every year or every 200 years.

"Statistics are only a tool to judge the needs of a bridge."

Thus, at a time when taxpayers are sensitive to governmental spending and inflation, the failure of a bridge and inadequate means to control ruinous flooding frustrate the public, which looks for solutions that stick.

"We have a unique problem with the Salt River bridges in that the Salt River is a privately owned river," Mr. Brechler reports.

"Because its dry bed contains sand and gravel, it is a source for concrete, and several companies along the river mine the sand and gravel for the concrete. Once a pit is mined and becomes a hole, it is refilled by flooding waters during heavy rains, bringing silt into the pits and lowering the riverbed."

The I-10 bridge into Phoenix, for example, is about 20 years old. Yet in those 20 years the riverbed beneath it has dropped at least 20 feet and is continuing to fall.

"That is our concern," Mr. Brechler declares.

According to US geological surveys of the Colorado River Basin which cover most of New Mexico and Arizona, through 1970 the heaviest flow of the Gila River and its tributaries, of which the Salt is one, measured a maximum of 52,900 cubic feet per second on Feb. 7, 1937, with a height taken on the Salt of 15.18 feet.