OROVILLE — A new report from a UC Berkeley group researching what caused the Lake Oroville spillway to fail in February is concerned that a green spot on the nation’s tallest dam might mean it is leaking.

This is not the first time the “green spot” on the southern end of Oroville Dam has been brought up. It has been discussed at community meetings, where state Department of Water Resources officials have said it is caused by rain or is a natural spring.

Robert Bea and his team at UC Berkeley are far from convinced. Bea is a professor emeritus at UC Berkeley and a risk management expert who has been recognized by the U.S. Senate for his review of disaster management following the BP oil spill and Hurricane Katrina.

He said a lot of thought went into deciding when to publish the findings, if at all. He understands the unease residents are likely to feel from reading about the report, but in his opinion, the DWR’s explanation for the spot warrants posing the question publicly.

“The consequences of not getting the right answer are very high,” Bea said. “I don’t think anyone should encourage a casual approach.”

DWR officials have addressed the spot of vegetation before, chalking it up to the rainy season or calling it a natural spring. Bea said those claims are both problematic.

For one, he said, the green area was there for years, including during the drought, not solely wet periods. If caused by rain, it shouldn’t be concentrated in one spot. There should be other green spots all around the dam, the authors of the report noted.

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As to the claim the spot is a natural spring, the vegetation does not follow a normal path, Bea said — streams do not go uphill. It is also expanding in a “near perfect” horizontal direction, the report states.

“So why are reputable DWR engineers and representatives giving answers that are easily refuted?” the report asks. “Worse, these answers infer a demonstration of a lack of engineering competency.”

To show the organization is taking the issue seriously, it should fix the dam’s out-of-commission internal piezometers, which measure the pressure of water, test the wet spot, analyze the results and release the findings, he said.

“I’ve got to find out what the patient is sick from,” Bea said.

Based upon the information available, the concern is that the wet spot indicates it could be caused by something called differential settlement, which would likely lead to a failure of the structure, he said.

Differential settlement can occur when soil under the foundation shifts. The foundation settles unequally and can cause cracks to form.

“And water likes cracks,” Bea said.

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The group is concerned that if uncontrolled seepage is happening in the Oroville Dam, it could be in danger of a breach. DWR did drill the spot in 2016, inspection records show. Bea questioned why the organization was excavating there if it knew the spot was from rainfall.

In response to the report, Erin Mellon with the Natural Resources Agency, which oversees DWR, maintained the vegetation is caused by rain.

“These green spots were first noticed just after construction of the dam when there wasn’t even water in the reservoir,” Mellon said. “The preliminary finding from DWR’s inspections and studies show that this is an area that collects water from rainfall, causing vegetation to grow. However, DWR will continue to work with regulating agencies to completely resolve this issue.”

She added that the organization was working on a preliminary report explaining the green spot, which would be made public.

The Oroville Dam Advisory Group, a 15-member team at UC Berkeley, says it has put approximately 3,000 hours of work into analyzing what caused the Oroville Dam spillways to fail. It has received input from former engineers, operators and managers for DWR, concerned citizens and senior faculty.

“These people willingly volunteered their knowledge, experience, documentation and advice as very important resources that have been integrated into this report,” the report states.

Bea and Tony Johnson, co-authors of the report, work with the university’s Center for Catastrophic Risk Management.

Another independent group, the forensic team, is analyzing the root causes of the spillway crisis. Its final report will be released this fall.

DWR has set Nov. 1 as the deadline for this season’s construction work on the spillway. Once high water in the lake stops work, reconstruction will begin in the spring or summer.