Fly over the Oroville Dam spillway as repair works continues into September
There was no shortage of red flags at Oroville Dam. It was a matter of knowing where to look.
A team of independent experts charged Tuesday that the state and federal officials who inspected Oroville Dam relied too heavily on visual inspections, ignoring blueprints, construction records and other documented clues that could have warned them about the dam’s troubled flood-control spillway long before it fractured in February. The fracture led to near-catastrophe and the evacuation of thousands of residents.
The team of forensic investigators, commissioned by the state Department of Water Resources to study the cause of the February crisis, said similar problems could be lurking at other dams in California and around the country because of an over-reliance on visual inspections.
In a report released Tuesday, the team said the spillway failure at Oroville was likely caused by long-standing problems with cracks in the concrete and a faulty drainage system underneath the concrete chute that was too thin in places. Visual inspections alone, conducted annually by DWR and once every five years by the federal government, wouldn’t allow regulators to pull all the clues together and point to the likelihood of failure.
“Physical inspections, while necessary, are not sufficient to identify risks and manage safety,” the team said in a seven-page report to DWR. “At Oroville Dam, more frequent physical inspections would not likely have uncovered the issues which led to the spillway incident.”
The panel called on regulators to supplement visual checks with painstaking reviews of original design and construction specifications, as well as maintenance records, with an eye toward finding “design shortcomings” that contrast with current state-of-the-art practices. The reviews should go beyond spillways and take in the entire dam structure, it said.
John France, the leader of the forensic panel, told reporters Tuesday that hints of Oroville Dam’s problems were embedded in records dating to the late 1960s. These include reports of cracks in the concrete right after the dam opened in 1969, documents showing uneven thickness in the concrete slabs and signs that the drains were handling more water than they should have. All were clues that could have foretold the spillway’s failure, France said.
“We believe they were all there,” France said on a conference call. “They were all there in the files.” He added that it didn’t appear that DWR had thoroughly reviewed the blueprints since the dam was completed in 1968.
He added that Oroville was a wakeup call for dam inspectors everywhere, and urged them to go beyond the visual inspections that are typical for the industry.
DWR spokeswoman Erin Mellon said the agency agrees it needs to improve its oversight of dams, because “visual inspections would not have caught the February failure.” She said this recognition influenced DWR’s decision in July to order the owners of 93 dams across the state to conduct thorough inspections of their spillways. The list includes some of the largest dams in California, such as New Exchequer Dam on the Merced River and New Bullards Bar on the North Yuba River.
While the forensic team’s work isn’t complete, France said the panel released interim findings Tuesday in an effort to influence the inspections the state ordered.
Federal officials have also said they’re re-evaluating dam-safety inspections in the aftermath of the Oroville crisis. Frank Blackett, an engineer with the Federal Energy Regulatory Commission, said in May that FERC is ordering operators to see “if they have a potential Oroville waiting to happen.”
FERC regulates Oroville and oversees an inspection every five years. The most recent inspection, conducted in late 2014, concluded that the a failure of the flood-control spillway at Oroville was so unlikely that there was no need to plan for such an emergency.
The forensic team investigating Oroville released a preliminary list in May of 24 possible causes of the spillway crisis.
Tuesday’s report, while not the last word, narrows the focus to the spillway’s drainage system.
The group said the flawed system allowed water to collect beneath the spillway and gradually weaken the structure. This created “uplift pressure” on the concrete slabs that left them vulnerable to heavy water flows.
The problem came to a head Feb. 7, when dam operators released a gusher of water down the spillway to reduce water levels at Lake Oroville. The heavy flows coming down the spillway apparently exploited weak points in the concrete, and one section of the concrete chute was lifted up, creating a giant crater underneath.
It still wasn’t clear, the group said, why the chute had survived heavier water releases than what occurred Feb. 7. The possibilities include inadequate maintenance and corrosion of the rebar inside the concrete.
The group will release a final report later this fall.
The spillway failure led to one of the biggest flood crises in recent California history. Over the days following the initial failure, dam operators limited water releases to minimize damage to the spillway. Water then flowed over the emergency spillway for the first time ever, causing unexpected erosion on the hillside below. Fearing the emergency structure would also fail, officials ordered the evacuation of 188,000 downstream residents.
The forensic team’s report brought a swift response from Assemblyman James Gallagher, R-Yuba City, whose constituents were among those evacuated. “This memo highlights that reviews should be conducted by independent experts who will go deeper into the integrity of structures and not simply provide visual reviews and regurgitate findings of past inspections,” Gallagher said in a prepared statement.
Both spillways are being repaired under a $275 million contract to Kiewit Corp. of Omaha, Neb. The project is expected to take two years, although state officials say the main spillway will be operational in time for the rainy season that begins this November.
Mellon said the repairs “will bring the spillway design and construction up to today’s standards to ensure we address the physical causes that led to the February failure.”