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Concrete Collapses at CA Dam Spillway

Monday, February 13, 2017

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The tallest dam in the United States is subject to close monitoring after engineers discovered major concrete erosion in its spillway—just as heavy rains set in.

On Tuesday (Feb. 7), officials stopped flows to California’s Oroville Dam Spillway after a massive hole, estimated at 250 feet long, was discovered. Over the next two days, California Department of Water Resources performed controlled releases into the spillway, bringing about further erosion, though, officials say not as much as they feared.

Oroville spillway damage
Images: Kelly M. Grow/California Department of Water Resources

On Feb.  7, officials stopped flows to California’s Oroville Dam Spillway after a massive hole, estimated at 250 feet long, was discovered.

Oroville Dam, about 70 miles north of Sacramento, is the tallest dam structure in the country, at 770 feet, and Lake Oroville, which the dam created, is the second-largest man-made lake in California. The dam was finished in 1968 after a construction period of more than 15 years.

Cause Unknown

State officials said they do not yet know what caused the crater in the spillway, and could not speculate as to how it would be fixed or how long repairs might take. Their current focus is on preventing damage in the area as the waters of Lake Oroville rise.

Oroville damage

Controlled releases in the days since the damage was found have led to more erosion.

The Edward Hyatt Powerplant, an underground hydroelectric power generating plant, is part of the dam complex. Lake Oroville provides water to much of the San Joaquin Valley and Southern California.

Reservoir Levels Rise

Lake Oroville has a capacity of 3.5 million acre-feet, according to the DWR, and as tests continued on the spillway, heavy rains brought water levels closer and closer to that maximum figure. On Tuesday, when the damage was discovered, the reservoir stood at 2.8 million acre-feet. By Thursday, the DWR estimated the lake held 3.22 million acre-feet, over 90 percent capacity.

Oroville recieved a total of more than 2 inches of  rainfall between Tuesday and Thursday, according to weather records.

Inflow as of Thursday was 118,000 cubic feet per second, the DWR said, while outflow into the Hyatt Powerplant was about 13,000 cubic feet per second. By late Thursday, officials increased flows into the damaged spillway to more than 40,000 cubic feet per second.

Crews were preparing for the possible use of an unlined emergency spillway, clearing trees and debris from a hillside where water was anticipated to flow. The use of the emergency spillway, which has never been used before, raises concerns about damage downstream, including to a Feather River salmon hatchery that is important to commercial and recreational fishing in the state.

Editor's note: This story was one of our most popular of 2017, and appeared in our Readers' Choice issue on Dec. 27. After this article was originally published, the DWR attempted to use the emergency spillway at the dam, leading to a situation in which the concrete weir was in danger of collapse. A two-year reconstruction project ensued, with first-year work having wrapped up in November. The rebuild is expected to cost $500 million. 


Tagged categories: concrete; Erosion; Infrastructure; Locks and dams; potable water

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