State officials said Wednesday the damaged Oroville Dam flood-control spillway is ready for the rainy season, and will be able to fully blast water down its half-mile long concrete chute for the first time in nearly two years if lake levels rise.
Work on the adjacent emergency spillway is ongoing.
Both the main and emergency spillways that allow the dam to release water to prevent overflowing were severely damaged by heavy rains in February 2017. A massive crater erupted in the main flood-control spillway, and the never-before-used emergency spillway failed. The crisis at America’s tallest dam triggered the frantic evacuation of 188,000 Sacramento Valley residents as fear mounted that the structure could burst.
The Department of Water Resources said it has largely completed the $1.1 billion reconstruction and recovery project prompted by that near-disaster.
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The last of the new concrete slabs on the main spillway will have to cure over the next month to ensure their long-term strength, but DWR spokeswoman Erin Mellon said it could handle water releases from Lake Oroville if necessary. DWR had set a Nov. 1 target for having all the concrete placed.
“If we needed to use it now, we could,” she said in a conference call with reporters.
Work will continue through early 2019 on the dam’s emergency spillway, which is being reinforced with additional concrete.
As a precaution, DWR plans on keeping lake water levels about 13 feet lower than normal “to prevent the use of the emergency spillway this winter,” said John Leahigh, DWR water operations manager. The emergency spillway is used when lake levels threaten to overtop the dam.
Last winter, officials said the partially-rebuilt main spillway could have withstood water releases at only 100,000 cubic feet per second — less than half the structure’s original capacity. Now, they said, the chute has been rebuilt to its original design capacity of 270,000 CFS.
Since the dam was completed in 1968, the most water ever sent down the chute was in the massive floods of 1997, when more than 160,000 CFS was released down the spillway.
The rebuilt structure is a “new and modern spillway using the best available engineering technology and construction practices, all completed within an incredibly compressed timeline,” said Joel Ledesma, deputy director of DWR.
State officials said they’ve used a massive amount of materials in a short time to get the dam back in order. Workers poured more than 1.2 million cubic yards of concrete — enough to fill 372 Olympic sized swimming pools. The main spillway alone has enough concrete to build a sidewalk from Oroville to Amarillo, Texas, state officials said.
The construction milestone marks the final chapter in a saga that began Feb. 7, 2017 when officials discovered a massive hole in the middle of the main spillway.
DWR’s dam operators dramatically reduced water releases down the chute in an effort to limit the damage. As the reservoir filled, water poured over the emergency spillway, which rests atop a natural hillside that holds back the lake.
The hillside began eroding. Fearing it would collapse and unleash “a wall of water,” law enforcement ordered the immediate evacuation of 188,000 downstream residents. The crisis passed two days later when dam operators dramatically ramped up water releases from the main spillway, lowering lake levels.
The relentless water releases over the next few weeks destroyed much of what was left of the main spillway, necessitating the two-year repair effort led by contractor Kiewit Corp. of Omaha, Neb.
Neither spillway has been used since spring 2017. State officials said the main spillway had been rebuild to the point it could have safely handled up to 100,000 cubic feet per second of water last winter, but a release was not needed.
After the crisis, the federal government ordered the state to convene a forensic team to investigate what went wrong.
Early this year, the team heavily criticized California officials, saying the state did a poor job of designing, building and maintaining the structure and neglected safety while focusing on the “water delivery needs” of the water districts who keep water in Oroville.
The forensic team described the festering problems at Oroville as a “long-term systemic failure.”
In response, the state revamped its dam safety programs and ordered 93 dams it oversees to conduct thorough inspections and other ongoing safety upgrades.