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Dramatic new images show scale of damage to Oroville Dam spillway

San Francisco Chronicle logo San Francisco Chronicle 1/03/2017 By Peter Fimrite

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Geologists attempted for the first time Tuesday to figure out what to do about the vast, yawning canyon dug out of the earth after a crater opened up in the Oroville Dam’s concrete spillway and diverted water at high speed into the adjacent hillside.

Monday’s shutoff of water flowing down the main spillway revealed a shocking panorama of damage, forcing experts with the California Department of Water Resources to scramble to figure out how the crippled chute can possibly be fixed by the end of the year — in time for the next rainy season.

Photographs and video released by the department Tuesday showed that an enormous section of the spillway, which is as wide as a 15-lane freeway, had been wiped out. In its place is a giant rocky channel carved out of the earth leading down to the Feather River.

“With drones, helicopters, and a team of geologists and other experts, we’re still gathering information about the extent of the erosion on the main spillway,” said Nancy Vogel, a spokeswoman for the California Natural Resources Agency, which oversees the water resources department. “The damage to the lower portion of the spillway is severe.”

The crisis began unfolding Feb. 7 amid rainstorms that raised Lake Oroville toward capacity. While dumping water downstream, crews discovered a gaping hole in the concrete spillway, forcing them to reduce the flow.

That caused water to flow over the reservoir’s earthen emergency spillway for the first time in the dam’s 48-year history, causing rapid erosion and fear of a collapse of the emergency spillway. The danger prompted the Feb. 12 evacuation of as many as 188,000 people living in downstream communities.

The ravaged main spillway was dry Monday for the first time in about two weeks, allowing workers to clear out debris below in a bid to get the dam’s hydroelectric plant back up and running, officials said. The plant acts as another water release system.

“It’s certainly a big hole,” said Jay Lund, director of the Center for Watershed Sciences at UC Davis, of the damage on and around the main spillway. “They’ll be looking to see how well that gully stabilizes, because later on this season they will need to use that spillway.”

The spillway was expected to be shut down for a week, Vogel said, but she acknowledged that water will again have to flow down through the gully as rains continue and snow melts.

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Gov. Jerry Brown sent a letter to President Trump last week requesting a waiver that would allow the Oroville spillway repair to happen without review under the National Environmental Policy Act.

“We have suspended requirements for state environmental review due to the emergency,” Brown wrote. “I would ask that the White House take similar steps.”

Vogel said the department has been working since Feb. 7 “to frame options for repairs or replacement before next winter.”

Robert Bea, an engineer and professor emeritus at UC Berkeley, said the main spillway problem could have been worse.

“We were really fortunate (lucky) the spillway degradation progression did not proceed farther up-slope,” Bea said in an email, and that the water flowed to the river “without causing further major damage to the dam system.”

He said the department has big challenges ahead as the spring thaw approaches and the likelihood of using the spillway again increases.

The amount of water flowing into the reservoir from the surrounding mountains has decreased to about 22,000 cubic feet per second, down from about 40,000 cubic feet per second on Feb. 13, and it will continue decreasing throughout the week, which is forecast to be mostly dry, officials said.

Lake Oroville’s level dropped Tuesday to 840 feet above sea level — 60 feet below the lip of the emergency spillway. Lake levels aren’t expected to rise more than 15 feet during the shutdown of the main spillway, officials said.

Construction crews are continuing to install rock and concrete below the emergency spillway, in case it is needed in the future.

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