TUCSON — During rainy seasons, desert washes grow wider and deeper as fast-moving runoff water gobbles up chunks of ground along unreinforced banks.
Now a retired engineering professor is testing a new way to stop the erosion, help restore riparian areas and get rid of an environmental problem.
How? By building a dam made from discarded tires.
"A typical stream here will drop 50 to 80 feet per mile," said Stuart Hoenig, an electrical and mechanical engineer and professor emeritus with the University of Arizona. "So when they run, they really move."
By comparison, the Mississippi River drops only 550 feet between northern Minnesota and New Orleans, more than 1,400 miles.
The 80-mile Brawley Wash starts near the Mexican border and feeds into the Santa Cruz River near Marana. Flows of up to 50 mph are common, and the flooding routinely spreads sand across Arizona 86 about 20 miles west of Tucson. That closes the only highway from Tucson to Sells and Ajo.
"The Brawley a hundred years ago was a wagon road," Hoenig said. "It is now about 200 feet wide and about 20 feet deep. That just gives you an example of how the arroyos have grown.
"There's been a tremendous loss of land from this thing, and all its tributaries keep growing."
One such feeder arroyo is on King's Anvil Ranch, about 10 miles south of Three Points. It washes out a ranch road with most rains, forcing owner John King to scrape it, and the arroyo "gets deeper and deeper, so that makes it worse," Hoenig said.
King, who has been trying to solve his erosion problem, asked Hoenig, an old friend, for help. Hoenig suggested using old tires to slow water flow, halt erosion and trap the sand.
The 30-foot long, 6-foot high dam will cost about $5,000, which does not include the cost of donated labor and tires. Hoenig estimates that a concrete dam would cost at least $35,000.
The dam will include up to 1,200 tires donated by the county's solid waste management tire recycling program.
A crew of Pima County probationers stacks and fastens the tires together, anchoring them with half-inch-thick plastic rope cut about 10 feet into the arroyo's side walls. The dam's "toe" will drop four feet into the subsoil, similarly anchored, Hoenig said.
The tires also are filled with rocks.
The weekend project is supervised by Hoenig and geological engineer Joshua Minyard and monitored by the Arizona Department of Environmental Quality.
Goodyear Tire Co. and the Phelps Dodge Corp. contributed $5,000 to the project to cover costs such as equipment rental.
Soil should cover the tires within the first few rains.
Getting most of the water to sink into the ground and stop the sand would level the ground upstream and turn it fertile over about three to four acres, Hoenig said.
What Hoenig learns from this dam he hopes to bring to other projects, including the Brawley Wash.
If the dam is a success, there would be little trouble finding enough tires or people to participate in more projects. Just last year, Pima County collected about 650,000 used tires and shipped 612,000 of them for recycling.
The Arizona project is an ideal application for used tires, says Michael Blumenthal, president of the Scrap Tire Manufacturers Council in Washington. "They will not decompose, will not collapse, will not emit any odor, will not leach anything," he said.