 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 operators have throttled back flows from the main spillway nearly in half, to 50,000 cfs. “We haven’t seen any further damage up and down the spillway,” said Chris Orrock, a DWR spokesman. “We have seen erosion ... carve a new path down the hillside, but we haven’t seen any additional erosion up and down the cement structure.”
Eventually DWR wants to reduce outflows from the main spillway to zero to do a more thorough inspection of the damage and figure out repair options. “We have a lot to learn on the geology,” Croyle said.
Cutting back the releases also will enable crews to start dredging and hauling away the 150,000 cubic yards of rocky debris piled up in the Feather River channel beneath the concrete chute. The piles have raised channel levels to the point that dam operators can’t run Oroville Dam’s hydroelectric power plant, normally its main source of water releases.
“We are going to come in with barges, with heavy equipment on them and just create a channel that will allow the normal flow of water to return,” Orrock said. “We’re days away from that.”
Resuming operations at the hydro plant would play a small but pivotal role in the dam’s overall recovery.
The power plant can only release about 14,000 cfs, a fraction of what the main spillway can discharge. But that could handle a major portion of the Sierra snowmelt that will flow into the reservoir this spring and summer, Countryman said.
The hope, eventually, is that DWR can shut down the main spillway and start a permanent fix. Croyle has said he wants the spillway repaired – or maybe rebuilt in another location – in time for the next rainy season, which begins in October.
Croyle has declined to speculate on what caused the initial fracture in the main spillway. The Federal Energy Regulatory Commission, which licenses the dam, has ordered the state to open an investigation into the cause “as soon as feasible,” in part because the findings could influence how the repairs proceed.
Some outside experts already have weighed in with theories as to why the ch