Under reconstruction: Oroville Dam spillway like you've never seen it
The reconstruction of Oroville Dam’s flood-control spillways is underway, and California officials vow the structures will become stronger and safer than ever.
More than three months after a near disaster forced the emergency evacuation of thousands of downstream residents, California officials announced Wednesday that permanent repairs have begun on the two spillways.
“This week symbolizes a huge, important and positive step,” said Bill Croyle, acting director of the state Department of Water Resources.
Croyle said work officially began Saturday with the start of demolition of the lower portion of the main spillway – the structure that’s taken the heaviest beating since the crisis began in early February. Demolition will continue through mid- to late-June, when reconstruction will start. Work is also proceeding on the adjacent emergency spillway, whose near-failure in February prompted the evacuations.
Speaking to reporters on a conference call, officials said the state’s designers have taken into account criticisms leveled by an outside team of forensics investigators and others. They said the February emergency resulted from design and construction flaws dating from the dam’s construction in the 1960s, including an inadequate drainage system beneath the spillway and critical inconsistencies in the thickness of its concrete slabs.
Those issues and others will be addressed, said Jeanne Kuttel, chief of DWR’s engineering division.
“The concrete will be thicker,” Kuttel said, adding that the spillway will be supported by “state-of-the-art drains” to prevent new problems.
Dam officials “adopted design measures to mitigate any of the challenges that the forensics team identified as possible contributors (to the crisis),” she said.
Kiewit Corp. of Omaha, Neb., which was awarded a $275.4 million contract to fix the dam’s two spillways, has more than 200 employees on the site, a workforce that will balloon to 500 by August. The company and its subcontractors will work 20 hours a day, six days a week, in an effort to get as much work done as possible this year, Kuttel said.
The complete repair is expected to take two years, although DWR officials say the spillways will be functional by November, the start of the upcoming rainy season. This season’s work will focus mainly on replacing the badly damaged lower portion of the main spillway. Next year much of the attention will shift to the upper section of the structure, which wasn’t damaged in the February incident but will be replaced anyway.
The dam’s emergency spillway – the epicenter of February’s crisis – will be fortified, too. Among other things, Kiewit will embed a vertical “cutoff wall” in the hillside beneath the spillway to prevent the type of erosion that nearly toppled the spillway structure three months ago.
The state expects the Federal Emergency Management Agency to reimburse much of the repair cost. The water agencies that store water in Lake Oroville are expected to pick up the rest of the expense.
State officials shut off the battered main spillway for the season last Friday in order to start the repairs. DWR said it believes the outlet from the dam’s hydro plant can release enough water this summer to keep Lake Oroville levels in check, despite a heavy snowpack starting to melt in the Sierra Nevada. If the snowmelt proves heavier than expected, DWR has said it could reopen the spillway one more time.
While state officials sketched the broad outlines of the two-year repair plan, they have kept many of the technical details sealed from public view.
Oroville’s emergency began Feb. 7, when a massive crater was discovered in the main spillway. That prompted dam operators to limit water releases as they sought to contain the damage. Inflow from a heavy storm filled Lake Oroville and caused water to pour over the adjacent emergency spillway – a concrete lip on top of an unlined hillside – for the first time ever Feb. 11.
The flow caused massive erosion on the hill, prompting fears that the emergency structure would crumble and unleash a “wall of water.” Officials ordered the evacuation of 188,000 downstream residents. The immediate crisis ended when state officials dramatically ramped up water releases over the damaged main spillway.
That brought lake levels down and arrested the flow of water over the emergency spillway. But it also increased the devastation to the main spillway, leaving a gaping chasm in the concrete chute. Much of the adjoining hillside also had been carved out. That hillside will be repaired as well, although Kuttel said it isn’t clear if that will occur this year or next.