Oroville Dam emergency spillway in use for first time in history
California’s second-largest reservoir filled with so much water Saturday, thanks to extraordinary winter storms and damage to a release channel, that officials at Oroville Dam took the unprecedented step of opening the lake’s emergency spillway.
Dam operators said the maneuver posed no risk of flooding or dam failure on the Feather River, about 75 miles north of Sacramento. But the move sent lake water cascading down a muddy hillside where boulders and brush in the untested, unpaved spill route threatened to wash into the Feather River and create hazards for fish and levees downstream.
The lake’s power plant and electrical transmission towers at the foot of the dam, the nation’s tallest at 770 feet, were also being monitored for damage.
Officials said the emergency spillway, activated at 8 a.m. Saturday, would remain in use through at least Sunday night as mountain runoff from recent storms continued to fill the lake.
“The event that we never wanted to happen, and didn’t expect to happen, has happened,” said Doug Carlson, spokesman for the California Department of Water Resources, which owns and operates the dam and reservoir. “But it has performed as we hoped it would, even though it was the first time.”
The California Department of Water Resources and host of collaborating agencies continue to monitor the Lake Oroville spillway flows late Thursday afternoon as 35,000 cubic feet per second (cfs) of water was released over the damaged spillway. More erosion is expected, but the releases will help operators absorb the inflow of the storm waters expected Thursday evening and Friday. DWR first noticed erosion on the spillway Tuesday morning and shut off flows to investigate. There is no imminent or expected threat to public safety or the integrity of Oroville Dam in Butte County. Photo taken 3:10 p.m. PST February 9, 2017. Kelly M. Grow/ California Department of Water Resources
Problems for the reservoir began Tuesday when a section of the lake’s primary spillway — a 180-foot-wide, 1,000-foot-long concrete channel to the Feather River below — collapsed amid high-volume water releases.
The resulting craterlike hole has grown dramatically, prompting officials to ease the amount of water released out the main spillway and ultimately use the emergency channel to keep the lake from flowing over the top of the dam. The emergency spillway, which is nothing more than a grassy hillside that drops toward the river, has not been used since the dam was built in 1968.
Lake Oroville is a key state water-storage facility, second in carrying capacity to only Lake Shasta. It supplies water to Central Valley farms as well as several urban water agencies, including the Metropolitan Water District of Southern California and the Santa Clara Valley Water District.
The reservoir also provides flood control for downstream communities and helps regulate salinity in the Sacramento-San Joaquin River Delta.
State officials had hoped to avoid using the emergency spillway and, as late as Friday afternoon, remained optimistic that necessary releases could be handled by the main spillway. Crews, however, took the precautionary measure of clearing the emergency channel of brush, trees and other debris, which served them well once they realized Saturday morning that more water needed to be liberated from the lake.
Once Lake Oroville reaches 901 feet above sea level, which is 21 feet below the top of the dam, water begins to flow automatically into the emergency corridor.
The spillway was expected to release up to 12,000 cubic feet of water per second, a relatively small amount compared with the roughly 90,000 cubic feet of water per second that was pouring into the reservoir Saturday. But it was still enough to send a steady stream of water into a diversion pool below and ultimately into the Feather River.
California Department of Forestry and Fire Protection teams deployed floating devices downriver to catch debris.
Dam operators continued to release water out of the main spillway, too, despite its damage. About 55,000 cubic feet of water per second was being released Saturday afternoon, well short of the 270,000 cubic feet the channel was built to handle.
The total outflow from the lake, anticipated to be as much as 67,000 cubic feet per second, was not likely to create flooding problems, officials said.
“The rated capacity of Feather River is much bigger than that, much larger,” said Carlson. “So there is no public danger. There is no expected evacuation.”
Repairing the channel is estimated to cost between $100 million and $200 million, a fix that can’t be made until winter rains end and water releases are no longer necessary. An alternative option presented by state officials Saturday is to build a new spillway at another point on the lake.
Continuing to use the impaired spillway not only risks more damage to the structure, but also was posing a threat to the Hyatt Powerplant, officials said. Concrete chunks from the spillway’s tear were piling up beneath the dam, causing water to pool up behind the debris and flow toward the utility station.
“The water in the pool creates a certain amount of back pressure,” said Eric See with the Department of Water Resources. “That can lead to damage. We definitely don’t want to damage our power plant.”
The station was shut down late Friday. As of Saturday afternoon, no damage had been reported.
Two transmission towers along the emergency spillway were also at risk of collapsing as water releases softened the ground and destabilized the soil, officials said.
Water releases earlier this week on the main spillway already have turned the Feather River’s normally clear water brown with silt and debris, a problem for fish.
At the Feather River Fish Hatchery about 4½ miles downstream, where endangered salmon are reared, the cloudiness of the water was running “off the charts,” said a spokesman for the California Department of Fish and Wildlife.
With the turbidity threatening to asphyxiate the salmon, hatchery workers were frantically collecting fish and trucking them to a nearby holding pond. By Saturday afternoon, 10 million salmon had been moved.
The hatchery is very important to California salmon production, said John McManus, executive director of the Golden Gate Salmon Association.
“It provides a lot of the fish that are caught in the ocean,” McManus said. “The loss of those fish would indeed be a blow to the salmon fishery.”
