Watch crews make progress at Oroville Dam spillway this week (July 25, 2017)
California officials have ordered owners of 93 dams to reinspect their flood-control spillways following the Oroville Dam crisis, saying the spillways need a closer look following a preliminary review.
The list released by the Department of Water Resources includes some of the largest dams in California, such as the New Exchequer Dam on the Merced River, New Bullards Bar on the Yuba River, and Lake Almanor Dam on the Feather River in Plumas County. Each holds back reservoirs roughly the size of Folsom Lake, which can store about 977,000 acre-feet of water.
Also on the list is New Don Pedro Dam, on the Tuolumne River, which is about twice the size of Folsom and contains the sixth largest reservoir in California.
DWR’s list also features scores of obscure facilities, including two owned along the American River by the Sacramento Municipal Utility District: Ice House and Union Valley dams.
The 93 dams represent less than 10 percent of the 1,250 dams overseen by the Department of Water Resources’ dam safety division.
DWR said the probes were ordered in recognition of the emergency at Oroville, which prompted a mass evacuation, and the fact that California’s dams are 70 years old on average.
Preliminary assessments showed each of the spillways on the list “may have potential geologic, structural or performance issues that could jeopardize its ability to safely pass a flood event,” according to letters the dam-safety division sent to the dam owners. “Therefore, we are requesting that you perform a comprehensive condition assessment of the spillway as soon as possible.”
DWR officials have been notifying the owners of the affected dams since June. The complete list was released Thursday by the agency.
“It will not be known which spillways, if any, will need repairs until the comprehensive assessments are completed and reviewed by (the dam-safety division),” DWR said in a note accompanying the list. “Dam owners of these spillways have been directed to perform any needed maintenance repairs prior to the next flood season. (The division) has already received immediate responses from many dam owners in compliance with the notice.”
Calvin Curtin of Turlock Irrigation District, which co-owns New Don Pedro, said the district already conducted several inspections last winter, before and after the Oroville emergency, and “found no items of concern.” Nonetheless, the district will comply with the state’s order and is preparing for an in-depth look at the geologic conditions beneath New Don Pedro.
“We don’t believe the same conditions exist here that exist at Oroville,” Curtin said.
SMUD engineer Dudley McFadden said the Sacramento utility will conduct intensive inspections at Ice House and Union Valley next month, with a particular emphasis on the “geology underneath the spillway.” He said preliminary inspections in April showed some “areas of concrete are starting to peel off” but in general the two facilities are in good shape. Union Valley is SMUD’s largest dam, guarding a reservoir about one-fourth the size of Folsom Lake.
At least one of the dams on DWR’s list has attracted attention from area residents. Robert Eberhardt, a Chico X-ray technician, grew concerned when he was boating on Lake Almanor on the Feather River earlier this year and examined Almanor’s spillway.
He said the spillway appeared to be crumbling in places. Photos he shot with his cellphone and shared with The Sacramento Bee appear to show cracks, places where cracks had been filled but the fill material was crumbling away, and even a small cedar tree growing out of the concrete. The lake can hold 1.3 million acre-feet of water, or 300,000 acre-feet more than Folsom.
Eberhardt said he feared that if the spillway was ever used, it could wash away and threaten the integrity of the main earthen dam.
“The entire lake would drain into Oroville Lake and it would happen overnight,” Eberhardt said.
A 2016 annual inspection report compiled by the Division of Safety of Dams noted some “freeze-thaw” damage to the concrete floor, but inspectors said the chute walls were stable and “the structure remains in satisfactory condition for continued use.”
The dam is owned by Pacific Gas and Electric Company, which has been monitoring “surface damage on the floor of the spillway,” said utility spokesman Denny Boyles. The damage has been caused by water seepage and “affects only the upper surface of the concrete and has not progressed to the point where repairs are necessary,” he said.
“PG&E has inspected these areas and will continue to monitor them for evidence of further deterioration,” Boyles said in an email. “If and when repairs become necessary, PG&E will repair the damaged areas as part of its regular maintenance program.”
In Thursday’s report, the state identified 11 other PG&E dams besides Almanor as needing further examination. Boyles said the utility is performing those inspections, following “in-depth evaluations we conducted this winter.”
“Our inspections to date have not identified any issues that require urgent repairs,” he said.
When it comes to Almanor, at least one local official isn’t particularly worried. Sherrie Thrall, a Plumas County supervisor, told The Bee that Almanor’s spillway wasn’t needed this year or even during the huge storms of 1997 that flooded wide swaths of the Sacramento Valley. She said if water was ever sent down the spillway, there would be so much, the town of Chester would be underwater. The spillway and dam also survived a nearby earthquake that registered 5.7 on the Richter Scale in 2013.
A 2014 inspection report noted that the dam’s main outlet, a structure separate from the spillway, was inspected after the earthquake and showed no apparent problems.
Federal and state officials have openly talked for months about toughening inspection programs following the near disaster at Oroville.
“One thing we all have in common is that we didn’t predict this happening,” said Frank Blackett, a regional engineer at the Federal Energy Regulatory Commission, in a speech in Sacramento in May.
David Gutierrez, a DWR consultant and former chief of the dam-safety division, said routine annual inspections generally aren’t able to detect the types of problems that were lurking at the Oroville spillway. “We’re trying to find the obvious issues,” he said in an interview in June. “It’s a visual inspection. You’re climbing things. You’re not X-raying.”
Oroville’s crisis began when a giant crater opened in the main spillway Feb. 7. DWR officials throttled back water releases to limit the damage, but a heavy rainstorm filled Lake Oroville and water began spilling over the adjacent emergency spillway for the first time in the lake’s 48-year history. When it appeared the emergency spillway might fail, officials ordered the immediate evacuation of 188,000 downstream residents. DWR quickly ramped up releases over the main spillway, arresting the flow of water over the emergency structure, and lake levels fell. The evacuation order was lifted two days later.
The exact cause of the crater has yet to be determined. An independent forensic team has cited two-dozen possible factors but won’t finish its investigation until this fall.