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Corps narrows options for Isabella dam fixes

June 6, 2011

By Mr Tyler Stalker (USACE)

SACRAMENTO, Calif. - The U.S. Army Corps of Engineers’ two dams at Isabella Lake in California count among the nation’s riskiest. Now the Corps’ Sacramento District is closer than ever to finalizing plans to fix them.

After five years of intensive study, the Sacramento District has identified several different options to consider as part of the Isabella Lake Dam Safety Modification Study.

Widening both sides of the auxiliary dam would reduce the threat of earthquakes from the underlying Kern Canyon Fault. Adding seepage filters and drains, which neither dam currently has, would reduce the risk of leakage. Lastly, relocating the Borel Canal, which runs through the auxiliary dam and provides power to much of southern California, would further improve dam stability by removing a potential path for leakage.

Those options were presented during three public meetings May 17-19 in the communities surrounding Isabella Lake. Public comments from the meetings will inform the study’s draft environmental impact statement, an assessment of the possible environmental consequences of the solutions. As the district completes the EIS, it will also evaluate the relative costs and benefits of each option, and could propose a final solution as early as November.

“The dam has done its job so far,” Corps project manager Veronica Petrovsky said. “Now, we’re looking to find a solution that will ensure it will continue to do its job.”

Data shows that, without the dams, the Kern River would’ve have flooded in as many 16 years since they were completed in 1953.

The district has studied both the Isabella main and auxiliary dams since 2006, when the Corps inventoried and classified all of its dams based on their risk. While the 57-year-old Isabella Dam showed a need for modernization, the more than 300,000 residents of Bakersfield downstream of the dam and the potential for larger storms than were previously thought likely made it one of the most at-risk dam facilities in the nation.

According to David Serafini, the project’s technical lead, there are three main problems with the dams. The main dam’s spillway is too small, and couldn’t pass all the water it would need to if big storms filled the reservoir filled too quickly; the auxiliary dam’s foundation is leaking, which could threaten its stability; and the auxiliary dam sits on The Kern Canyon Fault, which U.S. Geological Survey and Corps studies have recently shown to be active.

Together, they mean the dams couldn’t stand up to the kind of worst-case flood scenario engineers now envision for the area.

To reduce the risk to the dam until it can determine and build a fix, the district has kept the reservoir from filling to its designed capacity - reducing pressure on the dams and minimizing the potential consequences if they were to fail - and continues to conduct a heightened inspection regime to detect potential stability problems as early as possible. In 2010, the district also shored up a low spot on the auxiliary dam’s crest, reducing the risk of overtopping.

“Some of the measures, such as restoring the left abutment of the auxiliary dam, will continue to help the facility after the project is completed,” Serafini said. “Others, including the temporary pool restriction and increased monitoring, will likely go away once a fix has been completed.”

Public meetings throughout the study have given the public a voice in determining the best solution.

“We are looking for local citizens and agencies to provide input on proposed solutions, mitigation needs, environmental consideration, construction resources, such as staging areas, borrow sites and utilities or just any general comments about the project,” Petrovsky said.

Comments will be addressed in the draft EIS to help the district better evaluate all the potential impacts and consequences as a result of the dam safety project, Petrovsky said.

When presented with the consequences of dam failure - large waves racing 40 miles down the narrow canyon and inundating Bakersfield - many at the meetings asked if the district currently has an emergency action plan in place.

The district has worked with local agencies, including the Kern County Office of Emergency Services, to develop these plans should the unthinkable occur, project managers explained.

Closer to the dam, recreation-dependent Kernville and Lake Isabella residents said they hope the Corps will consider the potential economic impacts on the small California towns.

Hiring of non-local workers or reducing the reservoir’s water level could hurt local cities, residents said.

“When implementing any fixes we will be looking to minimize the impacts wherever possible,” Petrovsky said.

The project team will now refine the potential alternative plans to analyze their costs, benefits and any potential environmental impacts before returning for more public meetings in August 2011.

“If all goes well, we anticipate being able to begin construction in 2014,” Petrovsky said.

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