Oroville Dam spillway is ready for rainy season
The Oroville Dam flood control spillway has been fixed.
Eight and a half months after the gravest emergency in the dam’s history forced 188,000 residents to flee, state officials said Wednesday that Oroville’s structures have been largely rebuilt and can withstand a rainy Northern California winter. A second phase of work will be completed next year.
“Lake Oroville’s main spillway is indeed ready to safely handle winter flows if needed,” said Grant Davis, director of the Department of Water Resources, in a conference call with reporters.
Noting that a heavy storm could hit the watershed as early as Thursday, the DWR director spoke directly to the downstream residents who had to evacuate in February and remain mistrustful of the state’s operation of Oroville: “You indeed had a terrifying experience and we are working hard to ensure it never happens again.”
Davis outlined further steps to strengthen Oroville and other dams around the state. California officials will undertake a broad “needs assessment” of Oroville, the tallest dam in America, with an eye toward possibly building a second gated spillway to increase redundancy, along with other potential improvements.
In addition, Davis said “repairs and updates” are already being made at some of the 93 other dams around California where the state ordered intensive inspections in the wake of the Oroville crisis. He had no details on the repairs.
As a practical matter, state officials won’t be able to test the work done at Oroville until lake levels rise and the spillway has to release water. For the time being the reservoir is being kept at a lower level than usual as a safety precaution. Nonetheless, DWR officials said they’re certain the spillway can handle a release of 100,000 cubic feet of water per second – nearly twice as much as what was being released when the spillway cracked in February. The spillway has never released more than 160,000 CFS.
“We’ve done hydraulic modeling and physical modeling,” said Joel Ledesma, deputy director of the State Water Project. “Those have shown it’s capable of those flows and a little bit higher.”
Jeff Petersen, project manager for general contractor Kiewit Corp., said some construction will continue through the winter but the 700-person workforce is starting to dial back its hours. Reconstruction work will resume in earnest sometime next spring.
Kiewit won a $275 million contract to rebuild the main spillway and the adjacent emergency spillway, but DWR officials revealed two weeks ago that Kiewit’s costs are likely to rise to $500 million. On Wednesday, DWR spokeswoman Erin Mellon said the $500 million estimate is “a ballpark figure” and the final tally is likely to rise. Adding to the final cost will be the expense of moving some power transmission lines, dredging the river channel below the dam and other functions.
The costs have risen in large part because the bedrock beneath the main spillway wasn’t nearly as strong at critical points as originally believed. That required Kiewit to excavate deeper into the bedrock and lay hundreds of thousands of additional cubic yards of concrete. A similar problem sprang up at the adjacent emergency spillway.
While reconstruction costs have soared, state officials note that the cost of responding to the original emergency at Oroville came in about $100 million below expectations. The state expects the Federal Emergency Management Agency to reimburse up to 75 percent of its costs; the water agencies that store water behind the dam will pick up the balance.
Oroville Dam’s crisis began when a giant crack appeared in the main spillway Feb. 7, as water was being released during a week of heavy rains. Dam operators curtailed water releases in an effort to limit the damage to the spillway. That allowed Lake Oroville to fill to its highest level ever, and water began pouring over the emergency spillway – a concrete lip crowning an unlined hillside – for the first time since the dam opened in 1968. A day later, when the hillside began eroding dangerously close to the lip, authorities ordered the immediate evacuation of 188,000 downstream residents amid fears that the spillway would crumble and unleash a “wall of water.”
Dam operators then ramped up water releases from the main spillway dramatically. The lake level dropped, water stopped flowing over the emergency spillway, and the erosion on the hillside was arrested. But weeks of high-volume water releases left the main spillway almost a complete wreck, with two enormous “scour holes” along its 3,000-foot-long concrete chute.
The first phase of work, which was declared completed Wednesday, consisted of rebuilding two lengthy stretches of the main spillway with erosion-resistant concrete slabs anchored to the underlying bedrock. A third stretch was filled in with less-durable “roller compacted concrete” while a fourth section, at the top of the spillway, was patched.
Phase 2, to begin next spring, will involve rebuilding the top of the spillway from scratch and installing a layer of finished slabs over the section that so far has been filled in with the roller concrete. Next year Kiewit will also complete a fortification of the emergency spillway, which includes lining the top 700 feet of the hillside with concrete and building an underground vertical wall into the bedrock. The wall is designed to halt erosion.
The cause of the original crack in the main spillway is still being investigated, but a forensic team commissioned by DWR has said it believes the problem was the result of long-standing but undetected cracks in the concrete, uneven thickness in the slabs and a faulty drainage system beneath the chute. The drainage flaws allowed water to collect beneath the spillway and gradually weaken the structure. When water was released down the spillway Feb. 7, the heavy flow apparently exploited weaknesses in the concrete and caused one section to lift up, opening up a crater. The forensic team said it isn’t sure why the spillway failed in February after withstanding much heavier flows of water during its history.
Although the forensic investigation won’t be completed until mid-November, Kiewit officials have said they’ve incorporated suggestions made by the investigators into the reconstruction work. A better drainage system has been installed, and the concrete is stronger, Kiewit said.