Tempe Town Lake Dam: The Best Laid Plans...

If you design something, chances are it will not work exactly as you intended. This can be especially true after exposure to the elements. Tempe Town Lake Dam (Arizona, 2010) | Case Study

The dam failed on July 20, 2010.

Tempe Town Lake Dam consisted of rubber, inflatable dams on both the upstream and downstream sides of the lake. They could be inflated and deflated to capture the spring flooding and create a lake for recreation without flooding the areas upstream of the lake. The image at the right, which is from Wikimedia Commons, is of the dam from 2009, prior to the incident in 2010. Click on it for a larger view and more info.

In Arizona it is quite hot, and they were concerned that the rubber could not stand the heat, so they designed a system that would enable water to flow over the dams to cool them, while capturing that water, so as to lose very little during the dry season.

One important feature of the dam was an innovative water capture and recycling system incorporated into the stilling basin design. In order to protect the rubber bladders from the intense heat of the desert sun (sometimes reaching 150°F at the ground surface), water was allowed to flow over the bladders continuously. This water was then captured in the stilling basin and pumped back into the lake to provide a cooling system with minimal water loss.

There was a problem however; the dams were not perfect.

Unfortunately, this unique system did not function as intended. The rubber bladders sagged at the middle which did not allow for distribution of flow over the entire bladder. As part of the regular dam inspection process, it was found that the rubber bladders were deteriorating more quickly than anticipated as a result of exposure to extreme heat. In 2007, the bladder manufacturer evaluated the condition of the rubber dams and recommended replacing the bladders after 10 years of service.

You can see in the image above that water is only flowing over a portion of the rubber bladder.

They were supposed to be replaced in early 2010. That date got pushed to July of 2010.

At 9:44 PM on July 20, 2010, less than 12 hours before construction crews were scheduled to begin work on replacing the bladders, Bladder No. 2 at the downstream dam burst and immediately deflated. Water poured out into the empty river bottom, and the lake was essentially drained by noon the next day.

There was no loss of life.

The bladders were replaced temporarily with new bladders, but in 2016 they where replaced with a steel hydraulic system.

Ultimately, the rubber dam was replaced with a hydraulically operated steel gate dam located approximately 200 feet downstream of the original rubber dam structure. Construction of the replacement structure was started in 2014 and completed in 2016.

So what are the "lessons learned?"

What are the modes of failure?

Dam designers and inspectors should understand the vulnerabilities of the specific technologies or materials that are being employed. There may be unique potential failure modes that need to be addressed or considered as well as site-specific environmental factors (e.g. extreme heat and repetitive wave motion as was the case here, but also potential for ice loading, rigorous operational conditions, debris loading or clogging, etc.).

What are the non-critical aspects of the system?

In the case of Tempe Town Lake Dam, the dysfunction of the cooling system accelerated or even led to deterioration and failure of the rubber bladder. The dam could operate without this cooling system and did so successfully for several years. However, while the cooling system was not critical to the ability to impound water, it was vital to the long-term maintenance and integrity of the structure.

What are the long-term effect of the elements? (Water always wins.)

The hot Arizona sun and mild rocking action of the bladders worked away for more than a decade prior to failure of the dam. This progression of failure is not uncommon.

The final lesson boils down to "The Best-laid plans of Mice and Men, oft go awry." The problems were known. At least some of them were. Plans were put in place to replace the dam, but that still didn't stop a failure. In this case no one died, but infrastructure failures often do result in death, or at least a whole lot of property damage.