Oroville Dam's main spillway as it appeared Wednesday morning, Nov. 1, the deadline for preparing the structure to handle potential winter releases from Lake Oroville. (Ken James/California Department of Water Resources)
The California Department of Water Resources announced Wednesday it has completed the first phase of its massive reconstruction of Oroville Dam's shattered main spillway -- just in time for the first significant rainfall of the season.
The agency also announced that in the wake of the spillway's failure last February -- and several reports that found the concrete chute appeared to have been poorly designed, built and maintained -- it will conduct a "comprehensive needs assessment" for the dam and its reservoir.
DWR Director Grant Davis said during a media call Wednesday that the agency intends to build on lessons learned in the spillway incident to forge "a meaningful and lasting change in how dams in California" are built and run.
Among the issues that could be studied over the next two years, DWR officials said, is whether the dam needs a second gated spillway, presumably to replace the unlined, uncontrolled emergency spillway that experienced severe erosion during the February crisis.
That part of DWR's announcement was met with skepticism by the Oroville area's two state lawmakers -- state Sen. Jim Nielsen and Assemblyman James Gallagher -- who said that independent experts, not the water agency, should carry out the needs assessment.
All Concrete in Place
At the direction of federal regulators, DWR had set Nov. 1 as the deadline for completing the initial round of repairs and rebuilding needed to allow the spillway to withstand heavy flows of water during the coming winter.
The department said the last of the concrete needed to fill in a 1,050-foot section of the spillway, blown out when the concrete structure failed in February, was placed Wednesday morning. That marked the last major step, for now, of a massive construction project that got underway in May before the spillway design was even close to complete.
The project involved extensive patching of a 730-foot section of the upper spillway; building two brand-new sections of spillway -- a total of about 1,200 feet -- to modern specifications; and filling in the 1,050-foot midsection, which included a crater more than 200 feet deep, with roller-compacted concrete.
Demolishing and replacing the patched upper section and revamping the rough midsection will be the focus of work beginning next year, after the end of the current rainy season.
DWR says the interim spillway is designed to handle flows up to 100,000 cubic feet per second if releases from Lake Oroville are necessary this winter.
That flow volume, equivalent to the amount of water cascading over Niagara Falls during daytime "tourist hours," is relatively rare. The chute was designed in the 1960s to handle a maximum flow of 277,000 cfs, and its highest release ever was about 160,000 cfs, in January 1997. The spillway was discharging about 50,000 cfs when it began to break apart in early February.
Jeff Petersen, who's heading the $500 million-plus construction project for contractor Kiewit Infrastructure West, said he's "very confident" the spillway will stand up to high flows this winter. He also noted some spillway details remain to be completed.
"Now we just have to finish the concrete," Petersen said. "The dry finish work and the joint sealant work you'll see continue inside the spillway."
Wet Weather on the Way
The work in the spillway is wrapping up for the season just as a series of relatively robust early-season storms head for California. Forecast models suggest that as much as 5 inches of rain could fall in the Feather River watershed that feeds into Lake Oroville.
Crews are also working on a project to shore up the dam's emergency spillway -- an unpaved hillside adjacent to the concrete chute. Crews are building a massive wall consisting of piles driven as much as 65 feet, down to strong rock. That project is expected to be finished by January. Later in the year, crews will build a concrete buttress and "splash pad" on the upper reaches of the emergency channel.
All of that work is designed to prevent the kind of erosion that gouged deep channels into the hillside last February. That erosion threatened to undercut a massive concrete weir at the top of the emergency channel and unleash a wall of water down the Feather River. The threat to the weir triggered the emergency evacuation of more than 180,000 people in downstream communities, including Oroville, Yuba City and Marysville.
The two state lawmakers who represent the evacuated area referred to that history in a statement that pointedly omitted praise for the Department of Water Resources in acknowledging completion of the first season of spillway repair work. .
Nielsen, R-Tehama, and Gallagher, R-Yuba City, thanked Kiewit, its workers and subcontractors for accomplishing "a monumental task" over the last six months.
'Work Is Far From Complete'
But, the legislators said, "The work is far from complete. It's not enough to fix the spillways and move on. The impacts of this crisis have not been mitigated."
Gallagher and Nielsen also said that although they welcome DWR's announcement of a comprehensive assessment of the Oroville facility's needs, they don't believe the agency should conduct it.
"This robust inspection work should be conducted completely independent of DWR and the other regulatory agencies that were responsible for the inspection work leading up to the collapse of the Oroville Dam spillways," the legislators said.
In his remarks Wednesday, DWR Director Davis -- appointed after acting agency chief Bill Croyle came under fire for his handling of the Oroville crisis and abruptly retired on July 1 -- acknowledged that communities downstream from the dam were traumatized by February's near-disaster and hurried evacuation.
But he argued that steps DWR has taken since the crisis, including releasing safety information on the more than 1,200 dams the agency regulates and ordering urgent inspections of spillways at 93 of those facilities, signals change in the agency.
"The night of the evacuation was a very terrifying episode for the entire community of Oroville and the three counties below," Davis said. "The reason we're taking such deliberative action and being so decisive about public safety becoming first is that we don't want to have this happen again."
Stay in touch. Sign up for our daily newsletter.