State engineers gingerly began releasing water again through the damaged Oroville Dam spillway Wednesday in a controlled test to see how much water the scarred facility could handle, as reservoir levels continued to climb behind the critical flood-control structure.
With dozens of engineers monitoring from strategic locations around the dam’s exterior, a sheet of white water began cascading down the 3,000-foot-long spillway a little after 3 p.m. Within seconds of hitting the damaged area, a crater about 250 feet long, a portion of the water turned into a long ribbon of mud as sediment that had been trapped inside the hole ran down the concrete spillway.
Misdirected by the jagged chunks of concrete lying in the spillway, some of the water left the chute and ran down an adjacent hill.
California Department of Water Resources officials said they planned to open the spillway for more than two hours Wednesday, ramping up the water release to about 20,000 cubic feet per second. That’s considered a “light spill,” engineers said, and was less than half as much water as the spillway was releasing when the giant scar was discovered Tuesday and releases were shut down.
Officials said the spillway would be shut down again later Wednesday so engineers could perform a thorough inspection. They expected the test spill would cause additional erosion, but the question was whether the additional damage would be at acceptable levels.
“We’ll see if anything has changed or anything has moved as far as concrete or materials that were under the spillway,” said department spokesman Eric See.
The concrete spillway was built on top of rock and various soils. It sits beside the main earthen dam that holds back Lake Oroville, the second-largest reservoir in California.
Agency officials said they didn’t expect to release an assessment of the test spill until Thursday morning.
Officials with the Department of Water Resources, which operates the dam, stressed again Wednesday that they believe the dam itself is safe and doesn’t pose a threat to downstream populations, a view echoed by outside experts consulted by The Sacramento Bee.
“We do not believe there’s an imminent danger to the dam, or the flood control … gates that we operate, or the public,” said Acting Director Bill Croyle.
Depending on the results of the spillway test, the agency could resume water releases from the spillway, as a short-term remedy, even if it means creating further erosion in the chute. The alternative, which is considered less preferable, is to let Lake Oroville continue rising until water begins cascading in an uncontrolled fashion over the emergency spillway at the north end of the dam. The emergency spillway – which has never been used – is unlined, so water would flow uncontrolled down a hillside and onto unpopulated land filled with trees and other material that could wind up in the Feather River below the dam and pose problems downstream.
“That is a last resort,” said department spokesman Chris Orrock.
Fixing the massive hole will be expensive, though it is too early to say how much it will cost, Croyle said. “Pushing water down that chute is going to continue to erode that chute,” he said. “We wouldn’t be surprised if by the end of this wet season, much of the lower portion of the spillway has eroded away.”
With the main spillway shut off, the reservoir had taken on a whopping 174,000 acre-feet of water in the 24 hours ending Wednesday afternoon. On an average February day in recent decades, Lake Oroville would normally take in about 7,000 acre-feet, state records show.
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