Tempe is turning to an engineering consultant to determine what kind of dam is best to replace the current inflatable rubber bladders on Tempe Town Lake.
The city anticipates getting a recommendation by November from Gannett Fleming, which is expected to be awarded a roughly $368,000 contract on Thursday. The City Council needs to select an option by November to ensure the new dam is completed by December 2015.
Tempe had a smaller-scale study performed in 2008 and needs to learn whether any new technologies have been devised since then, said Jeff Kulaga, the city’s assistant city manager.
“This current phase will dig in a little deeper, do a more exhaustive alternatives analysis to validate the technology that eventually will be recommended to the council in November,” Kulaga said.
The city has been weighing dam options since 2007, when an inspection revealed the bladders were cracking and aging faster than anticipated because of Arizona’s blistering sun.
Tempe installed the bladders on the then-new lake in 1999 with an understanding they’d last 20 or 30 years based on their lifespan in other places across the globe. The study will examine whether it’s best to stay with the rubber dams or use various metal hinged dams.
The other options became more attractive after one of four west-end bladders burst last July, causing the 220-acre lake to empty within hours. The city isn’t ruling out the rubber bladders.
“At this point we’re keeping all alternatives and ideas on the table,” Kulaga said.
Bridgestone Industrial Products manufactured the original bladders and is only letting the city keep the replacement ones through 2015. The bladders were the last made by Bridgestone, which exited the rubber dam business.
Tempe wants to weigh the functions and safety of any replacement against the cost. Also, Gannett Fleming will have to determine whether a different type of dam will fit in the existing four, 240-foot sections of the concrete base. If not, the consultant will need to identify what’s involved with retrofitting the base or building a new one.
The city can’t use a traditional concrete structure. Any dam has to be adjustable to keep the lake full and then lowered so water can pass after storms or the sometimes massive releases from Roosevelt Dam.
“The key here is water in the lake cannot exceed the height of the levies around the lake. That’s an important constraint to design for,” Kulaga said. “The tough part is 90 percent of the time or more, it’s not an issue. It’s those few times a year that we get heavy flooding, you’d better have designed and engineered properly for it because community safety comes into play and that is the pinnacle issue here.”
Tempe’s plans need approval from the Arizona Department of Water Resources, Army Corps of Engineers, Arizona Department of Environmental Quality and Maricopa Flood Control District.
The installation will take 12 to 18 months, Kulaga said.
Gannett Fleming also will study the eastern bladders because they’re cracking as well. They’ve aged much better because upstream water has pooled behind them for most of their lifespan and protected them from sun. The city wants to know whether they need repair or replacement, Kulaga said.
Tempe has estimated a $45 million replacement cost for the downstream dams. City officials acknowledge that’s a guess at best because it isn’t based on any specific design. Also, prices of many materials and services have fluctuated since the last study.
“Honestly, I don’t know where the pricing will fall,” Kulaga said.
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