Crews worked into the night Wednesday to shore up the emergency spillway at troubled Oroville Dam, racing to fortify the structure before the next series of storms, the first of which was forecast to hit before midnight.
Three storm systems will move into Northern California during the next six days, according to the National Weather Service. The first system will drop about an inch of rain in the Oroville area between 10 p.m. Wednesday and 4 p.m. Thursday. Greater amounts of precipitation will fall in the mountains northeast of the reservoir.
Forecasters are confident that the first two storm systems will not cause huge inflows into Lake Oroville. They are less confident about the third system, which is due sometime Tuesday. That storm could be bigger and warmer, meaning more rain and snowmelt streaming into the swollen reservoir.
“The third wave is looking like our problem child,” said Michelle Mead, a warning-coordination meteorologist at the National Weather Service in Sacramento.
Oroville Dam, about 65 miles north of Sacramento in Butte County, holds the state’s second-largest reservoir and serves as a crucial flood-control structure for the eastern Sacramento Valley. Wednesday’s all-out effort to bolster the dam before a new round of storms capped an anxious week that has seen serious malfunctions in both its main and emergency spillways, hobbling the dam’s ability to release water in the midst of an unusually wet winter.
Last week, as winter storms pummeled Northern California, Department of Water Resources engineers discovered a jagged crater in the lower section of the main spillway, a 3,000-foot span that acts as the dam’s primary flood-control outlet. Fearing the spillway would further erode and become inoperable, dam operators stopped the flows for a time, then gradually reactivated releases.
With runoff still rushing in from the snow-packed mountains above the dam, lake levels climbed, and early Saturday, water overtopped the emergency spillway for the first time in the dam’s 48-year history. Unlike the main spillway, which is lined in concrete and controlled via release gates, the emergency spillway dumps water in uncontrolled sheets over a 1,700-foot concrete lip onto a steep, wooded hillside.
Just more than 24 hours later, another problem emerged: The hillside below the emergency spillway’s lip was showing serious erosion, raising fears the structure would collapse and release a crush of water, inundating hundreds of thousands of homes and businesses along the Feather River basin in Butte, Yuba and Sutter counties.
That triggered emergency evacuation orders that sent 180,000 residents fleeing for safety Sunday evening.
With no good options, dam operators raised the gates on the main spillway Sunday evening, and nearly doubled its outflow to 100,000 cubic feet per second despite the crippling fracture in its midsection. The gambit lowered the lake level enough to stop water from flowing over the emergency spillway, and on Tuesday, the mandatory evacuation order was reduced to a warning.
On Wednesday, in an effort to create more space in the reservoir, operators continued to send water down the main spillway at 100,000 cfs. The main spillway is holding under the strain, said Chris Orrock, a DWR spokesman. The water flowing from the spillway is clear, indicating the midsection isn’t further eroding.
Engineers want to keep lake levels well below 901 feet, the depth at which water begins to flow over the emergency spillway. DWR normally keeps lake levels at 850 feet this time of year to mitigate flood risk.
The lake’s level stood at 876 feet at 3 p.m. Wednesday and was falling by a foot every 2 1/2 hours. Dam operators are on pace to lower the level to 850 feet by Saturday night or Sunday.
About 100 workers toiled on the emergency spillway Wednesday, preparing for the possibility that water would flow over it again. A Blackhawk helicopter began dropping large white bags of gravel onto eroded areas around 8 a.m. and continued its runs throughout the day, stopping only briefly to refuel at a busy staging ground overlooking Lake Oroville.
At the same time, trucks barreled through the city of Oroville through the day and into the night, carrying loads of boulders from a nearby quarry up a narrow canyon road and onto a road atop the massive dam itself. There, crews filled the gaping crevices in the hillside below the emergency spillway. Officials said roughly 1,200 tons of rocks an hour were being placed on the spillway.
The plan is to cover much of the reinforced hillside with slurry, further reducing the risk of erosion.
“We continue to armor up the hillside,” Orrock said. “That is our main concern.”
However, once the rain starts, those efforts will have to slow. Wet, muddy conditions would threaten the safety of workers on the ground. Windy conditions would threaten the safety of helicopter pilots in the air.
Asked if officials feel confident about the effort to bolster the emergency spillway, Orrock said, “We are making progress.” Acting DWR director Bill Croyle concurred, describing efforts to bolster the emergency spillway as “kind of like a war zone, for all the right reasons.”
Dam operators are expecting the coming storms to deliver inflows of 40,000 to 50,000 cubic feet per second; outflows over the main spillway will be kept at 100,000 cfs, assuming the structure holds.
By comparison, during last week’s storm – which came in unusually warm and wet – inflow peaked at 197,000 cfs. The storm predicted for Thursday and Friday is expected to be colder, which means more mountain snow and less rain flowing into the reservoir.
It is unlikely that the coming storm will raise reservoir levels enough to activate the emergency spillway over the next couple days, Croyle said. At some point, he said, DWR will look to reduce the massive releases coursing down the dam’s damaged main spillway, but no timeline for those reductions has been set.
“We don’t want to tear up our flood-control structure anymore than it already has been,” he said. “We are feeling very good about the reservoir conditions.”
Wednesday afternoon, Butte County filed an emergency petition with the Federal Energy Regulatory Commission, the agency that issued the license for the dam, demanding it require California officials to act immediately to establish a public safety program that addresses the deficiencies in the dam.
“Butte County and others have warned of the potentially catastrophic consequences of DWR’s failure to adequately address dam safety issues at the Oroville Project, and we are now facing the consequences of DWR’s short-sighted approach,” Butte County Counsel Bruce Alpert said in a letter to federal regulators.
Butte County, along with other local governments and environmental groups, had raised concerns about the emergency spillway’s design during the federal relicensing process in 2005. From 2003 through 2005, three environmental groups – Friends of the River, the South Yuba Citizens League and the Sierra Club – urged the federal government to require the lining of the emergency spillway as part of the relicensing.
Earlier this week, the Federal Energy Regulatory Commission ordered DWR to convene an independent panel of five experts to assess the reasons for the damage and issue recommendations on emergency repairs.