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 chute, rated to handle 250,000 cfs, broke open Feb. 7 with a relatively modest 55,000 cfs pouring down.
Some, including Tullis, point to a phenomenon known as “cavitation,” in which the blast of tiny water bubbles gushing down the chute at 50 mph effectively jack-hammered holes in weakened sections of concrete. Others, such as Robb Moss, a professor of geotechnical engineering at Cal Poly San Luis Obispo, speculate that roots from trees growing along the chute expanded weaknesses in the concrete.
Rogers, the dam failure expert from Missouri, said the spillway may have had cracks that weren’t properly patched. He also theorized the failure may have been tied to California’s five-year drought: The aging spillway could have weakened as it underwent contractions due to the sudden heavy soaking following years of dry weather.
Whatever the cause, the experts say, problems at Oroville should serve as a wake-up call on the state of America’s aging dams. At 48 years old, Oroville is actually slightly younger than the average American dam. According to the American Society of Civil Engineers, the average age of the 84,000 dams in the country is 52 years.
“The aging issues are without precedent,” Rogers said. “You’re going to see failure modes that you haven’t commonly seen before.”