Oroville Dam earthquake investigation may be needed

A recent federal inspection has concluded that Oroville Dam, the tallest dam in the nation, needs a comprehensive earthquake safety assessment.

The dam on the Feather River is the primary storage facility for the State Water Project, the state-owned plumbing system that provides drinking water to more than 23million people across California. Failure of the dam could inundate not only the city of Oroville but numerous other communities downstream, including Yuba City, Marysville and even West Sacramento.

The inspection was conducted in 2010 by consultants working for the Federal Energy Regulatory Commission, which oversees hydroelectric dams in the United States. It is the most recent inspection of its kind, which are conducted every five years.

No significant flaws were found in the dam itself. Inspectors recommended the earthquake safety assessment based on newer information about earthquake hazards in the vicinity of the dam. A copy of the inspection was reviewed by The Bee following a Public Records Act request.

Officials at the California Department of Water Resources, which manages the dam, say they don’t plan to conduct the recommended review because they don’t think the expense is justified. But they may be compelled to do it by state or federal regulators who are still considering the recommendation.

Oroville Dam, at 742feet high, was completed in 1968 and is the tallest dam in the United States. It stores 3.5million acre-feet of water, or enough to serve 7 million average households for a year. In California, only Shasta Dam on the Sacramento River stores more water.

State water officials say the dam is sufficiently strong. “Even with today’s understanding of seismicity and ground motions, Oroville still would meet the criteria that would be set today,” said David Panec, chief of dam safety at the DWR’s operations and maintenance division. “The dam is essentially overbuilt.”

Independent experts concur. Like Panec, they point to a 5.7-magnitude quake that occurred near the dam in 1975 that caused no damage to the structure. Subsequent investigations showed that the dam performed well and was not weakened by the quake.

“We’ve seen these types of dams perform very well in earthquakes under very strong shaking,” said Ross Boulanger, a civil engineering professor at UC Davis and an expert on earthquake risk involving earthen and rockfill dams, like Oroville. Boulanger said he has done consulting work for the DWR, but not involving Oroville Dam. “And we know their behavior can be relatively insensitive to modest changes in the seismic hazard.”

Not everyone is confident. Ronald Stork, a senior policy advocate at Friends of the River, said the call for further study of Oroville Dam should not be taken lightly. In recent years, after modern studies, numerous other earthen dams in California have been shown to be vulnerable to earthquakes. For instance, Folsom Dam upstream of Sacramento is now undergoing millions of dollars in upgrades to withstand earthquakes. Similar work is planned at Lake Perris in Riverside County and San Luis Reservoir in Merced County, both also part of the State Water Project.

Stork noted Oroville is also considered a “classic case” for something known as reservoir-induced seismicity. In this phenomenon, the reservoir itself can cause earthquakes, because the enormous weight of water stored behind the dam is enough to shift the Earth’s crust. Studies have documented the phenomenon at Oroville.

“A dam with a whole lot of shaking going on … does seem to be something you need to be really careful about,” said Stork, who monitors dam regulation across the state. “You lose a 3.5million acre-foot reservoir – the tallest in the United States – and that could cause a whole mess of downstream trouble.”

As one piece of the State Water Project, Oroville Dam is part of a complex bureaucratic and political system. Water management at Oroville affects habitat in the Sacramento-San Joaquin Delta and water rates for millions of Californians who depend on that water, from Napa to San Diego.

Any repairs or problems at Oroville Dam are paid for by the 29 urban and agricultural water contractors that buy water from the State Water Project, which include the Metropolitan Water District of Southern California, Kern County Water Agency, Santa Clara Valley Water District and others. These contractors, in turn, pass along their costs to ratepayers, including homeowners, business owners or farmers.

Those costs have been mounting at Oroville Dam after two expensive and high-profile accidents.

In 2009, five workers were injured when a steel bulkhead inside the dam blew out when two large river outlet valves were opened during a test. The valves have been inoperable ever since, and millions of dollars in repair costs are still looming. A subsequent investigation found that poor safety practices at the DWR contributed to the accident.

In 2012, a major fire occurred at the Thermalito Pumping-Generating Station, which is part of the hydroelectric system at the dam. The fire significantly reduced electric generating capacity at the dam and complete repair costs are still unknown. The DWR recently opened bids for a major cleanup project, the lowest of which was $11.9million. Additional work is needed to get the station operating again.

Negotiations are underway now between the DWR and its contractors to draft new terms for long-term water supply contracts. Among the key disagreements is how much the contractors will pay into a contingency fund so the department can respond more rapidly to emergencies.

The parties also are wrangling over other contract terms to finance two massive water diversion tunnels in the Delta. The $25billion project, supported by Gov. Jerry Brown, will be the most expensive water project ever undertaken in the state.

Leah Wills, a water consultant to Plumas County involved in the negotiations, said the contractors are reluctant to take on more expenses. Plumas County is a state water contractor, though often a contrarian in such negotiations because it is the only state contractor north of the Delta.

“There is no doubt in my mind this is all a big dance over how these new costs are going to be apportioned,” Wills said. “The contractors are kind of legitimately panicked. Somewhere there’s going to be this huge blank check that lands in their laps.”

The 1975 earthquake at Oroville revealed for the first time an active fault directly south of Lake Oroville. Known as the Cleveland Hills Fault, it was traced at the time to within just 1.4miles of Bidwell Canyon Saddle Dam, an earthen dyke that encloses part of the reservoir. Studies after the 1975 quake found no significant damage to the saddle dam or the main dam.

Federal inspectors in 2010 recommended a new seismic evaluation to consider the potential of two other earthquake sources in the vicinity: the Foothill Fault System and the Prairie Creek Fault. They recommended a “finite element analysis,” which involves extensive computer modeling to determine how the dam will perform in earthquake scenarios.

“That would not be uncommon for the consultant to recommend a very cautious and conservative approach,” said Boulanger, of UC Davis. Such studies can take months and cost several hundred thousand dollars, he said.

The DWR could be required to conduct the study, either by the Federal Energy Regulatory Commission, or by California’s Division of Safety of Dams, a separate regulatory unit within the DWR.

Dave Gutierrez, chief of the state’s dam safety division, said he expects his agency will decide in Januarywhether to order further earthquake studies. Asked generally about the dam, he said, “Oroville is not one that keeps me up at night from a seismic stability standpoint.”