Tempe may ditch the inflatable rubber dams that have turned a portion of the otherwise dry Salt River into Tempe Town Lake.
The city needs to replace the aging dams in two years but is looking for another type of structure because the rubber ones lasted only half as long as promised.
Arizona’s sun and dry climate took a toll on the dams, so the city will study other types to see if they will hold up longer.
Nobody at the city is sure whether something else will work better. The options are limited because any dam has to lower when water flows through the river. Otherwise, the dams could block the flow and trigger massive flooding.
“You have to have something that has a gate there that can be moved out of the way when large floods come,” said Don Hawkes, Tempe’s water utilities manager.
Now, Tempe can let air out of the dams to lower them so that water will flow over the structures without backing up.
Some city employees involved with the lake know of other dams that pivot on a hinge or move in some way. But Tempe officials aren’t familiar enough with other designs to know whether they’d be as effective, said Basil Boyd, a city hydrologist. That’s why the city approved $99,000 for a private company to study other dams and their cost.
Some dams might cost more initially but would cost less over decades if they have longer life spans.
“The technology has changed and we want to look and see if anything has come up that looks any better,” Boyd said.
The eight dam sections were supposed to last 25 to 30 years but were guaranteed for 10. The city last year determined the dams needed to be replaced starting in 2009 following the inspection that showed they were cracking.
The cost? About $16 million.
Tempe bought the dams in 1998 from Bridgestone, the same company that makes vehicle tires. The city filled the lake the following year.
The city has already looked at keeping the same type of dam or replacing the rubber dams with smaller ones. The city could raise the height of the dam’s concrete foundation a few feet and use smaller, less costly rubber dams.
The dams don’t need to lower as much as when the lake was built, Boyd said, because Roosevelt Dam has been enlarged upstream on the river. Roosevelt Dam am can store more water during a flood, meaning less water will flow down the Salt River in future floods.
If the city keeps the rubber dams, Boyd said, it will probably use a new version that’s more likely to survive a massive failure. Bridgestone now has dams with inner tubes. The inner layer would keep most water in the lake if the outer layer failed. Now, a failure would result in a massive drop in water.
Tempe can keep the lake filled even when it replaces the dam sections. The piers that separate the dam sections are built to hold a temporary dam that lets workers repair or replace the rubber dams.
“We are not seriously considering draining the lake for any reason,” Boyd said.