Crews working to repair the Oroville Dan spillways worked this week placing roller-compacted concrete in between the upper and lower chutes of the Lake Oroville flood control spillway. Workers also put forms in place to begin pouring concrete for spillway walls soon. California Department of Water Resources
Crews working to repair the Oroville Dan spillways worked this week placing roller-compacted concrete in between the upper and lower chutes of the Lake Oroville flood control spillway. Workers also put forms in place to begin pouring concrete for spillway walls soon. California Department of Water Resources

Water & Drought

Those green, wet spots on Oroville Dam aren’t worrisome, state insists

By Ryan Sabalow

rsabalow@sacbee.com

August 30, 2017 02:47 PM

UPDATED September 05, 2017 04:41 PM

State dam operators have issued a new report that refutes troubling allegations raised by a catastrophic engineering expert who contends Oroville Dam may be dangerously leaking.

On Aug. 30, the Department of Water Resources reiterated what state dam managers have insisted for months: that the public is in no risk from the persistent green wet spots near the top left abutment of the nearly 770-foot-tall earthen dam. The report says they’re nothing more than natural vegetation growth caused by rainfall that becomes “temporarily trapped” inside the dam’s outer-most layer and then seeps out.

The spots are currently brown because of the lack of rain.

The state says the wet spots have been there since before the lake was first filled in the 1960s. They’ve been there so long, the DWR says in its report, that they were noted in 1967 by dam inspectors who declared them not “a problem except for public relations.”

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Seepage has been monitored weekly since the dam was constructed, and DWR maintains an alarm system at the dam that would go off if serious leaks occurred.

Leslie Harder, a consultant working for DWR, said the seepage monitoring system works. “It’s quite sensitive ... so we would be able to pick up a concentrated leak,” he said.

Harder said it’s important to note that all earthen dams leak at least little. Oroville Dam – the nation’s tallest – only spills about 8 gallons a minute, which Harder said is a remarkable testament to how well the dam was designed.

“It’s a very small amount of seepage for any dam of any size,” Harder said. “The fact that the highest dam is only seeping 8 gallons a minute or so is amazing.”

A team of independent consultants brought in to monitor repairs after February’s spillway failure reviewed the state’s findings and concurred with them, the DWR says.

The DWR’s rebuttal is in response to a report released last month by Robert Bea of the Center for Catastrophic Risk Management at UC Berkeley.

Bea’s report contended the dam was “facing a breach danger from a serious and a dangerous form of a slow-motion failure mode” from persistent leaks in the main dam, perhaps caused by internal shifting of dam fill.

Bea is a retired engineer whose credentials include conducting an independent investigation into why the levees around New Orleans failed in 2005 during Hurricane Katrina.

In a follow-up report Bea released Tuesday, he said that that most of the documents and other information the state cited last week are not available for review so it’s impossible for outsiders to determine whether the state is being overly rosy with its assurances.

He urged the state to perform additional testing and analysis and make the findings available for review.

“We conclude that DWR’s August 30th report is a superficial summary ‘public relations’ report that is not consistent with the importance of the seepage related hazards and

uncertainties relative to ... this important part of California’s vital water infrastructure system,” Bea wrote.

The report acknowledges one of the more alarming findings in Bea’s analysis. In Bea’s report, he noted that instruments called piezometers that measure water pressure inside the dam have long since failed.

The report notes the devices were “abandoned” in 2000 because they were no longer working. But Harder said that was to be expected since the devices were installed during construction, primarily to ensure no troublesome pressure changes were occurring while workers were building the dam. He said the more robust seepage monitoring system works just fine.

The report notes that federal inspectors in 2014 recommended a comprehensive review of dam seepage, including an evaluation of the green spots on the face of the dam. Harder said that was routine.

“(Inspectors) say ‘Alright, it’s been 20 years since you last looked at it, you should update your previous evaluations and turn that in,’ ” Harder said.

The DWR says the evaluation is expected be be completed before the next federal inspection in 2019.

Scrutiny of Oroville Dam has increased since the opening of a massive hole in its spillway triggered a chain of events that eventually led to the two-day evacuation of 188,000 people living below the dam.

The state estimates that the February crisis and ensuing repairs will cost more than $500 million. The ongoing spillway repairs are expected to span two years.