State engineers may spill some water from Oroville Dam’s damaged spillway Wednesday afternoon to test how much water can rush past the massive hole that formed in the concrete structure.
“They’re going to start spilling the water to see what happens,” said Doug Carlson, a spokesman for the Department of Water Resources, which operates the dam.
Engineers conducted their first inspections Wednesday morning of the damaged spillway, about 24 hours after a 250-foot-long pothole was discovered in the massive structure, forcing a halt in water releases.
Officials with the Department of Water Resources said 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.
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A likely short-term remedy is to resume water releases from the spillway, even if it means creating further erosion in the chute, the agency said. “These are things we can repair,” said Eric See, the agency’s environmental program manager, in a briefing with reporters late Tuesday.
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. That would create significant land erosion problems, officials said, although the water would flow into an unpopulated area.
With the main spillway shut off, the reservoir –the second largest in California – had taken on about 150,000 acre-feet of water in about 12 hours. The lake level was about 60 feet below the lip of the dam. The reservoir remained about 15 percent empty.
Engineers halted releases from the main spillway after a hole was found in the bottom half of the concrete chute. Pictures showed a jagged chunk eroded out of the towering 3,000-foot concrete structure. The cause of the problem wasn’t yet determined.
“It’s not a public safety risk,” See said. “Dam failure is not in any way a potential threat.”
Jay Lund, director of the UC Davis Center for Watershed Sciences said, “I wouldn’t evacuate yet. I think they’re a ways from that.”
But Lund and others said the problem is going to make it more difficult to manage high flows from a critical piece of the state’s flood-control network, with two months left in California’s rainy season.
The lake, which is part of the State Water Project, feeds into the Feather River. Lake Oroville, in Butte County, is the state’s second-largest reservoir. Completed in 1968, the 770-foot dam is the tallest in the United States. It can store 3.5 million acre-feet of water.
The last major flood in Northern California, in January 1997, did most of its damage on the Feather River. Since then, significant upgrades have been made to the area’s levees, and Marysville Mayor Ricky Samayoa said he has confidence in the levees that ring the city.
“That last storm showed us how strong our levees are,” he said.
On Tuesday, after the problem was discovered, Department of Water Resources engineers gradually reduced the flows before shutting off the releases altogether.
At that point, Carlson said engineers started releasing water through a power plant at the dam. But the plant released only about 5,000 cubic feet of water per second Tuesday. While that’s expected to be increased to 15,000 cfs on Wednesday, that would still be a fraction of the volume of water flowing into the lake – about 128,000 cfs at midday Tuesday. Until the spillway was damaged, the lake was releasing more than 40,000 cfs, according to state data.
At current rates, the agency said the lake has enough room to absorb three days of inflow.
The agency said it expected to resume releases from the spillway “at a rate deemed safe,” after a more thorough inspection was performed. While resuming releases would worsen the damage to the eroded area, Carlson, the department spokesman, said that’s preferable to letting the water continue to fill the reservoir.
David Gutierrez, a retired Department of Water Resources dam-safety expert, said water would pour out of an emergency spillway if the lake were allowed to fill past the brim of the dam.
The emergency spillway, which has never been used, is designed for the scenario of the “biggest flood that any overgenerous engineer could ever dream of coming through that system with a full reservoir,” said Lund of UC Davis. The flows would be unregulated, meaning the state wouldn’t have any control over how much water pours down the emergency spillway, Lund said.
What’s more, while the top of that secondary spillway is concrete, the main structure is unlined and releasing water could cause erosion, Gutierrez said.
Joe Countryman, a retired engineer at the U.S. Army Corps of Engineers, agreed that the overall structure of the dam doesn’t appear to be at risk. But releasing more water down the cracked spillway could cause serious harm and create “major dollar damage,” said Countryman, a member of the Central Valley Flood Protection Board.
The department is also increasing releases from Thermalito Afterbay, a small downstream reservoir, in order to avoid a “drastic reduction” of flows into the Feather River.
That’s important, Lund said, because levees below the dam could fail if flows are suddenly shut off.
“You could cause some slumping and failure in the levees in the next few days,” Lund said. “And then you have a bigger (storm) event coming in the next few days from now … so not only have you got this problem with the spillway, but you’ve also weakened the channel conveyance downstream.”