Phoenix bridges and a dry riverbed: storm dilemma
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?
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.
On Feb. 21, 1920, 17 years earlier, a recording was taken on the Verde River of 50,600 cubic feet per second and 19.1 feet in height. In 1941, at Redrock and Virden, N.M., towns along the Gila River, the flow on Sept. 29 was measured at some 40,000 cubic feet per second.
None of these early measurements come anywhere near the heavy storms of the 1970s.
"Bridges are being built to withstand 50-year storms," Mr. Brechler says. However, he continues: "We are beginning to consider 75-bridges over the Salt River are designed for a water flow of 170,000 cubic feet per second. At the high point of last winter's flooding the water was moving at 180,000 cubic feet per second and was 40 feet deep."
Thus, the concern of many communities in the Southwest.
The Salt River did not flow appreciably for some 25 years -- between 1941 and 1966.
Since government money is more often directed toward priority items, other structures and roads were improved in that time. "Twenty-five years is a long time to see no water," Mr. Brechler notes.
The occasional rapid flows of a usually dry riverbed are caused by a combination of weather patterns, high mountain ranges, and the proximity of the Pacific Ocean.
As storms drop their moisture along the coastal ranges in moving east, little moisture is left for the inland valleys. Evaporation and any remaining moisture are released, however, as yet another mountain range is crossed, leaving even dryer air on the eastern side for the Sonoran Desert, which covers a large part of the Southwest.
Summer tropical storms moving north relieve this dryness and cause flash flooding. But the ground, parched from the infrequency of regular moisture, behaves with resistance, letting the first water roll off filled with soil.
After thorough saturation, unless the ground is well rooted and founded on a rock base, mud slides occur. Also, the presence of smooth rock encourages runoff. Crevices made from this running water become arroyos.
Edward Abbey in "Desert Solitaire" describes a flash flood in an arroyo: "It advanced in crescent shape with a sort of forelip about a foot high, streaming in front, making hissing, sucking noises like a giant amoeba, nosing to the right and nosing to the left as if on the spoor of something good to eat."
Occasionally in winter the tail end of a heavy storm moving from the South Pacific area across the Northwest part of the United States drops its moisture. This is the main source of the sudden and sometimes violent winter storms in southern California and Arizona.
As the water immediately responds to gravity, it seeks out the crevices, arroyos, canyons, riverbeds, and anyplace else to go. It fills up but later sinks in and disappears in the thirsty earth.
It acts as though one were introducing water to a pot of fresh, absolutely dry soil.
The Salt River has always been important to the Indians and early settlers for irrigation. With its dams and 1,300 miles of canals, it is still an important source of irrigation water.
But it can cause havoc when rampaging water fills it. Bridge engineers are keeping an eye on the riverbed -- and the sky.