ission tower they deemed vital to dam operations.
Meanwhile, with runoff from a wet, warm Sierra storm pouring into the reservoir, lake levels continued to climb. And DWR, in effect, was forced to choose between two bad, untested options:
Speed up releases over the damaged main spillway and possibly sacrifice the transmission tower. Or, let lake levels rise to the point that they crested the lip of an adjacent emergency spillway that had never been used in the dam’s 48-year history. Officials wanted to avoid the emergency spillway. They knew the spills would be uncontrolled and send mud and timber rolling down the unlined hillside beneath the structure into the Feather River below.
On Feb. 11, the water from the swollen reservoir rolled over the emergency spillway lip. Although the flows peaked at just 12,000 cubic feet of water per second – a fraction of the 250,000 cfs the structure was supposed to handle – the effect was far worse than imagined. On the afternoon of Feb. 12, the hillside directly beneath the spillway was eroding fast and on the verge of collapse.
The hillside looked “like flour being blown off by a fire hose,” J. David Rogers, a dam expert with the Missouri University of Science and Technology, recalls being told by an engineer at the scene.
In one area, the erosion came dangerously close to the emergency spillway’s concrete apron. If the water were allowed to eat away those remaining yards, officials feared it would undercut the emergency structure. That would have sent an uncontrolled blast of deadly, fast-moving water down the Feather River, flooding a vast swath of the eastern Sacramento Valley from Oroville to Yuba City.
The crisis was averted when dam operators gambled on the broken main spillway. They sent an explosion of water, at 100,000 cfs, nearly twice as strong as what they had been dumping onto the fractured chute just hours before. The move paid off. The main spillway held. The transmission tower stood. And lake levels dropped below the emergency spillway. The mass evacuations in Oroville and other downstream communities were lifted two days later.
How could the main spillway handle so much water in its dilapidated state? Experts say the bedrock below the remaining concrete chute appears to be nothing like the crumbly rock that eroded in the early days of the crisis. It’s certainly much stronger than the weathered material on the upper portion of the hillside below the emergency spillway.
“They’re like two different geologies,” said Rogers. “One’s severely weathered, and the other one isn’t. The other one is youthful and fresh and hard.”
Mount, with the Public Policy Institute of California, said he initially worried the erosion would eat its way up the spillway, possibly endangering its gates and rendering the structure inoperable. At that point, the spillway is also much closer to the earthen dam that holds back the reservoir. But that upward erosion hasn’t happened.
The whole structure “seems to be surprisingly stable,” Mount said.
Paul Tullis, an engineering consultant in Logan, Utah, who has studied spillway designs, said a kind of self-healing mechanism appears to have kicked in: The crater in the chute became large enough, and filled up with so much water, it’s acted something like a trampoline. The biggest gushes bounce away without beating too much into the chute.
“It has the ability to absorb and dissipate the energy,” Tullis said.
In the meantime, helicopters and dump trucks have hauled in thousands of tons of aggregate and concrete slurry to patch up the hillside below the emergency spillway. With the repairs taking hold, Croyle, acting director of DWR, has said “I would not hesitate to use the emergency spillway” if necessary.
Countryman, a member of the state’s Central Valley Flood Protection Board, agreed that it looks as though the emergency spillway “would be operable” given the bolstering. Nevertheless, he and others say it’s preferable to avoid letting the reservoir fill completely again, sending water down the emergency structure uncontrolled.
Over the past week, dam 