There’s another storm bearing down on troubled Oroville Dam, set to begin late Wednesday. But state officials say they believe the precipitation will be mild enough – and the reservoir empty enough – to handle this latest challenge.
The crisis at Oroville Dam continued to ease Tuesday. The dam’s heavily damaged main spillway was still able to expel water at 100,000 cubic feet per second, as it has around the clock since Sunday afternoon, with no apparent signs of significant new erosion. For a second day, crews worked to pack the crevice that formed last weekend in the hillside beneath the dam’s crippled emergency spillway, using helicopters and dump trucks to spread a mixture of boulders and concrete over the eroded section.
“We’re continuing to make significant gains in removing water from the reservoir, which drops the water surface elevation (and is) further reducing the risk to our situation here,” said Bill Croyle, acting director of the state Department of Water Resources, in an afternoon news briefing.
Enough progress has been made that Butte, Yuba and Sutter counties rescinded the mandatory evacuation orders issued Sunday afternoon, allowing an estimated 180,000 downstream residents to return home.
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“The risks that we faced when we initiated those evacuations have been significantly reduced,” Butte County Sheriff Kory Honea said.
The problems at Lake Oroville, California’s second largest reservoir, have stretched over a frenzied week and involve serious malfunctions in both its main and emergency spillways.
Last Tuesday, in the midst of winter storms, Department of Water Resources engineers discovered a cavernous hole in the lower section of the dam’s main spillway, a 3,000-foot concrete span that acts as the dam’s primary flood-control outlet during California’s rainy season. Fearing the spillway would further erode and become inoperable, dam operators stopped the flows for a time, then gradually reactivated releases.
With runoff from the snow-packed Sierra Nevada still rushing in, reservoir levels climbed, and early Saturday, water overtopped the dam’s emergency spillway for the first time in its 48-year history. Unlike the main spillway, which is lined in concrete and controlled via release gates, the adjacent emergency spillway dumps water in uncontrolled sheets over a 1,700-foot concrete lip onto a steep, wooded hillside.
By Sunday afternoon, just more than a day after the emergency system activated, the water flows carved a gash out of the hillside just below the spillway’s lip, raising fears the structure would collapse and send a crush of water cascading out of the reservoir, inundating Oroville, Marysville and other communities along the Feather River downstream.
That triggered the emergency evacuation orders that sent 180,000 residents fleeing for safety.
Sunday evening, dam operators raised the gates on the main spillway and nearly doubled its outflow to 100,000 cfs despite the crippling damage the concrete structure had suffered Feb. 7. The gambit was successful in lowering the lake level enough that water stopped flowing over the suspect emergency spillway.
Honea said Tuesday that a followup inspection of the damaged hillside by federal and state officials showed “there was no piping or other erosion that compromised the overall integrity of the emergency spillway.”
Still, he defended the evacuation orders as a prudent step.
Chris Orrock, a DWR spokesman, said there’s no telling how much additional erosion could have occurred beneath the emergency structure if dam operators hadn’t jammed on the accelerator to push more water out of the main spillway. “That helped to remediate the possibility of the erosion continuing,” Orrock said.
With a few days of dry weather and outflows continuing at 100,000 cfs, dam operators have been able to lower reservoir levels by 8 to 9 feet per day. By Tuesday evening, the lake sat at about 885 feet, or 16 feet below the top. The lake was holding 3.3 million acre-feet, or about 200,000 below capacity.
Croyle said the lake should continue falling. The crippling damage done to the main spillway hasn’t worsened despite the relentless pounding it’s taken since Sunday evening.
“It has been stable for four days,” Croyle said. “With the stability of the spillway, we’re able to maintain those high flows.”
At the current rate of decline, DWR hydrologist Maury Roos said the lake should be down to 3.15 million acre-feet when the next storm hits late Wednesday, leaving about 380,000 acre-feet of empty space. A Sacramento Bee analysis showed the same: The lake should be around 25 feet below the top of the reservoir when the rains begin falling.
“That would be adequate for the size of the storm that’s forecast,” Roos said. “We’d like to have more.”
Ideally, flood-control manuals say the lake should be down to 2.79 million acre-feet for this time of year, but that’s in anticipation of “a very large flood,” Roos said. “There’s nothing in the (weather) outlook that suggests we’re going to get that.”
Alan Haynes, with the federal government’s California Nevada River Forecast, agreed that inflows to the lake should be manageable compared to the volumes that rushed in during last week’s storm. “They’re going to have inflows that are orders of magnitude less,” Haynes said.
Last week’s storm came in wet and warm, with more rain and less snow than expected. Water flowing into the lake peaked at 191,000 cfs last Thursday.
This week’s weather, a series of three storm systems continuing through next Tuesday, should be different. The mountains above Lake Oroville should get only half as much precipitation, and more of it will fall as snow, said Michelle Mead, warning coordination meteorologist at the National Weather Service in Sacramento. Snow doesn’t run off into the reservoir as quickly as rain.
Last week, “we saw inflows peaking at 190,000 cubic feet per second,” Mead said. “With this system you could expect half of that, if not lower.”
Croyle, the DWR acting director, said the new storms will bring a “small increase in water surface elevation later this week.” Nonetheless, he said he thinks DWR can bring the lake down to 850 feet as early as Saturday or Sunday. That would translate into about 750,000 acre-feet of empty space, in line with U.S. Army Corps of Engineers flood-control standards for this time of year. The rain could hamper those efforts.
Croyle said DWR expects to draw down the lake even further, as well as press on with efforts to repair the hillside below the emergency spillway in case the lake fills up again and it has to be used. He said trucks and helicopters were bringing in 30 tons an hour worth of concrete rubble.
“This is an aggressive, proactive attack to address the erosion concern,” he said. “There’s a lot of people, a lot of equipment, a lot of materials moving around, from the ground and from the air.”
Tuesday evening, federal emergency officials approved Gov. Jerry Brown’s request to provide financial assistance to help California respond to the emergency at Oroville Dam, as well as winter-storm damage elsewhere in the state. Repairs at the dam are expected to run in the hundreds of millions.
With the snowpack 45 percent above normal in the northern Sierra, and a heavy spring runoff coming, Croyle said he’s certain the emergency spillway could handle overflows from the lake if it has to.
“I would not hesitate to use the emergency spillway,” he said.