Tempe's quest for a better dam at its Town Lake involves much more than just finding a way to hold water.
At least as important is making sure the new dam gets out of the way quickly to let flood waters rush through the Salt River.
The amount of water could reach 150,000 cubic feet per second. To picture the volume, assistant city manager Jeff Kulaga thinks of each cubic foot as a basketball.
"Imagine pushing 150,000 basketballs past that plain every second," Kulaga said. "If the water doesn't go over fast enough, then water flows over the levies and you get flooding."
The city must decide what kind of dam to use in November in order to meet a December 2015 replacement deadline. The city will consider a rubber inflatable dam, just like the one that ruptured a year ago this week and emptied the billion-gallon lake. On the anniversary, the city outlined the options and what pros and cons are known so far.
New inflatable dams could be more reliable than what was installed when the lake was formed in 1999. Improved materials have been developed that may extend their lifespan, Kulaga said. A pedestrian bridge that will open in September now sits above the four 240-foot long bladders to shade them. Also, the bridge supports a sprinkler system that will spray and cool the rubber during warm weather.
The sprinklers are being tested now and should begin normal operations next week, said Adam Gordon, project manager with contractor PCL. The sprinklers are spaced every 10 feet and overlap to ensure coverage even if a single head gets clogged. They hang from rubber tubing so the lines can bend if high water or debris flows by.
Hinged metal dam
A solid material reduces the odds of a sudden failure but Tempe doesn't yet know if that configuration would pose other issues. The hydraulic systems would have to be used periodically to ensure they work, which could involve some water loss. The city also needs to know the odds of a section getting stuck whether it's up or down and how quickly it could be fixed.
"We want to make sure the maintenance is fairly manageable and parts are available," Kulaga said.
An adjustable gate from Obermeyer Hydro Inc. is a metal structure that's supported by small rubber bladders at the base. The bladders are protected by the structure to minimize the risk of something puncturing the bladder.
A consultant outlined these options in 2008. Tempe is now having Gannett Fleming undertake a $368,000 study that will provide more details in advance of the November decision. Once the study is out, Tempe will hold one or more meetings to give residents a closer look at the options.
The study will identify the upfront costs, maintenance costs and anticipated lifespan. Even if rubber dams are chosen, the existing ones must be returned by 2015 to the manufacturer, Bridgestone Industrial Products. Tempe negotiated that deal because it had been told the bladders would last 20 or 30 years, but later discovered they had to be replaced shortly after their 10-year warranty expired.
Bladders may still be the best option even if Arizona's heat shortens their life, Kulaga said. "If we know they're going to last 15 years, you budget for that the way you budget to repave a street," he said.
Tempe has budgeted $45 million for the replacement based on a 2008 study. Tempe doesn't know how much costs will have changed by the time the project begins.
Gordon, who installed the new bladders and the pedestrian bridge, said it's hard to predict the expense. Labor costs have dropped with the recession, but the cost of materials is soaring.