After review, Tempe may stick with rubber dams at Town Lake - East Valley Tribune: Tempe

After review, Tempe may stick with rubber dams at Town Lake

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Posted: Tuesday, September 20, 2011 10:56 am | Updated: 3:26 pm, Tue Sep 27, 2011.

After studying roughly 20 alternatives to the problematic rubber dams at Tempe Town Lake, experts are suggesting the best option may just be for the city to stick with what it’s got.

The engineering and construction firm Gannett Fleming included the existing type of dam in its list of the five best options as Tempe plans to replace the features in 2015.

The study noted the inflatable rubber bladders aren’t perfect.

They’re vulnerable to vandalism.

They’re costly because they need frequent replacement.

And the desert climate ages them prematurely — which contributed to one of the bladders bursting in July 2010 and draining the lake.

But some options may involve draining the lake during construction. That would go against the city’s desire to keep the lake intact, said Jeff Kulaga, Tempe’s assistant city manager.

And some other types wouldn’t catch the tail end of flood waters, leaving the lake low for some time after water flows in the normally dry Salt River.

That leaves the rubber dam as the only option that would keep the lake full both during construction and after a flood.

Tempe’s City Council will discuss the $367,000 report this Thursday. The other types of dams include concrete bins that tip and spill when full, metal gates pushed up with small inflatable bladders and two variations of hinged gates.

By next month, the city will have a third party look at the Gannett Fleming study to ensure the firm didn’t overlook anything, Kulaga said.

“We’re looking at national experts, peer review,” Kulaga said. “Check the checkers.”

Any of the five options would cost $25 million to $35 million. Kulaga called the estimates extremely preliminary and said further study is needed.

Tempe is hesitant to empty the lake, touting it as the most popular Arizona destination after the Grand Canyon.

But downtown wouldn’t suffer much from a briefly empty lake, said Nancy Hormann, executive director of the Downtown Tempe Community.

When the lake was empty from late July to mid-October, the spectacle drew more visitors than in a normal summer, she said. But boaters, fishers and rowing teams had to go elsewhere.

“A lot of people were inconvenienced by it, but if they do it and they plan it and we know when it’s going to be down, I think it will have minimal impact,” Hormann said.

The empty lake triggered the cancelation of the Soma Triathlon. That translated to an economic impact loss of $550,000, said Toni Smith, communication manager of the Tempe Convention and Visitors Bureau. Had the Ironman Triathlon been cancelled, the city would have lost nearly $4 million of economic impact. The bureau would prefer a construction approach that ensures water-based events continue, Smith said.

Tempe has a December 2015 deadline to replace the bladders under an agreement with Bridgestone Industrial Products, which made the dams. Before Tempe replaced the bladders last summer, it negotiated for four west-end bladders at virtually no cost to Tempe. Bridgestone told Tempe to expect 20 to 30 years of use when the bladders were installed in 1999, but a 2006 inspection found they were aging faster than anticipated and needed replacement. The bladders had a 10-year warranty.

Tempe ordered the replacement study because of the 2015 deadline. But when considering the bladders would be just 5 years old if replaced then, Kulaga said another approach may deserve consideration.

“One of the options is, Why can’t we just keep these?” he said.

The agreement with Bridgestone encourages their replacement by charging Tempe $300,000 a month if they stay in place after the deadline. Any new bladders would have to come from another manufacturer because Bridgestone stopped making rubber dams.

“Because they’re getting out of the business, they have been consistently telling us, ‘We want them out. We want them removed,’ ” Kulaga said.

The city hasn’t yet weighed whether it’s best to rent the bladders at $3.6 million a year, he said. The city had initially expected replacement would cost about $49 million, but the estimates have been falling as research progresses, Kulaga said. Tempe has $3.6 million to put toward replacement. The city hasn’t identified how it will raise the remainder.

Another factor in choosing a type of dam is which companies manufacture rubber bladders. Gannett Fleming considered Bridgestone’s bladders among the industry’s best, Kulaga said.

The city’s agreement with Bridgestone requires Tempe to select a technology this November. By then, the city will have ranked the options, Kulaga said. Also, the city will hold a public meeting at 6 p.m. Oct. 17 at the Tempe Center for the Arts to get feedback.

A concrete dam isn’t an option. During a flood, the Salt River must handle about 210,000 cubic feet per second of water to stay in its banks. A solid structure would get in the way, so any dam must be able to lower quickly.

Even replacing two of the rubber dams with concrete would cause flooding. Tempe studied this option to cut costs, but a hydrological survey shows water would inundate much of downtown and Rural Road south of the Loop 202.

DAM REPLACEMENT OPTIONS

Gannett Fleming chose five technologies for Tempe to further study as the city prepares to replace its rubber dams in 2015.

Inflatable rubber dam

Four bladders can be lowered or raised quickly by adjusting air pressure.

Pros:

• Relatively low construction cost

• Can catch the end of flood waters to keep the lake full

• Known operation

• Does not require additional piers

• Allows the lake to stay full during construction

Cons:

• Past performance problems

• Long-term cost may be high because of replacement frequency

• Manufacturer support and warranty uncertain

• Vulnerable to vandalism

* * *

Fusegates

A row of concrete buckets fill when the lake rises and wash out when overtopped.

Pros:

• Does not require additional piers

• Simple and reliable

• Allows the lake to stay full during construction

Cons:

• Possible high long-term costs

• Cannot catch last floodwaters to keep the lake full

• Uncertain permitting requirements

* * *

Obermeyer crest gates

Steel gate panels are supported on the downstream side with small inflatable air bladders. The lake level can be adjusted by controlling air pressure in the bladders.

Pros:

• Does not require additional piers

• Simple

• Can catch the end of flood waters to keep the lake full

• Allows the lake to stay full during construction

Cons:

• Construction costs more than rubber bladders

• Bladders have same limitations as rubber dams

* * *

Hinged crest gates, hydraulic operation

Steel gate panels feature hinges at the bottom and are operated with overhead pier-mounted hydraulic cylinders.

Pros:

• Simple and reliable

• Durable

• Can catch the end of flood waters to keep the lake full

• Less vulnerable to vandalism than rubber dams

Cons:

• Would require additional piers, which would slow water flow during a flood

• May require draining the lake during installation

• Complicated operation and maintenance

• Would require periodic opening

• Unattractive overhead hydraulic equipment spans the structure

* * *

Hinged crest gates, electric operation

Steel gate panels are hinged at the bottom and are adjusted with a wire rope on hoist

Pros:

• Simple and reliable

• Durable

• Can catch the end of flood waters to keep the lake full

• Less vulnerable to vandalism than rubber dams

Cons:

• Would require additional piers, which would slow water flow during a flood

• May require draining the lake during installation

• Complicated operation and maintenance

• Would require periodic opening

• Overhead equipment spans the structure

• Contact writer: (480) 898-6548 or ggroff@evtrib.com

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