Update, 3:25 p.m. Friday: To start with the numbers: Department of Water Resources data show that despite cutting back releases down Oroville Dam’s shattered spillway and the return of storms to the Feather River basin, Lake Oroville continues to empty.
At 3 p.m. Friday, DWR’s running statistics on the reservoir show that its surface is now a little more than 42 feet below the lip of the dam’s emergency spillway. The lake is falling at a rate of roughly 3 to 4 inches an hour.
The second in a series of winter storms arrived in the region on Friday, dropping moderate amounts of rain and snow on the 3,600-square-mile Feather River watershed. Forty-eight-hour rain totals in the area ranged from 1.36 inches at Oroville Dam to 2.44 inches at the Humbug gauge in the mountains north of the reservoir.
Thursday, DWR cut releases from Lake Oroville from 100,000 cubic feet per second to 80,000 cfs. The reduction was designed to give crews a chance to begin removing the mass of concrete rubble, rock and sediment that tumbled into a channel that issues from the bottom of the dam. The agency said Friday it would cut spillway flows further — down to 70,000 cfs — as part of the effort to clear the channel.
As we’ve reported every day this week, work continues to repair erosion damage to the hillside below the dam’s emergency spillway structure. That erosion, which occurred when the water rose above the weir at the top of the emergency spillway and gouged out huge sections of the slope below as it rushed downhill, prompted last Sunday evening’s mass evacuation from Oroville and communities as far as 35 miles downriver from the dam.
Butte County Sheriff Kory Honea reiterated during a press briefing Friday that those who live downstream from the dam need to be prepared to leave if trouble recurs at the dam.
“The likelihood (of trouble) is low,” Honea said. “But — and I don’t want to sound like a broken record, but that’s my job. My job is to keep people prepared. So they’ve got to pay attention, they’ve got to be vigilant, they’ve got to be prepared, they’ve got to sign up for their emergency warning notification system. And if you’re tired of hearing my say that, I’m sorry, but I’m going to keep saying it until this situation is well past us.”
Honea also addressed again a question that has arisen in the aftermath of last weekend’s evacuation: Whether Oroville or other communities in the evacuation zone had experienced looting after people left town.
The sheriff has said while there had been burglaries and thefts during the roughly 48-hour evacuation, there had been no looting. Friday, he clarified that a little.
“Now that my staff has had a better opportunity to talk with me, we find that a couple of those burglary- or theft-related crimes, we can charge … the individuals responsible with an enhancement of looting,” Honea said. He did not immediately offer specific details of those episodes.
Update, 3:20 p.m. Thursday: The Feather River watershed has gotten its first dose of rain and snow from an expected series of storms, with moderate amounts of precipitation that haven’t yet caused a major increase in flows into the reservoir behind Oroville Dam.
For the 12 hours ending at 9 a.m., precipitation totals ranged from about a half-inch at the dam itself to 1.50 inches near Bucks Lake, in the higher country of the Feather River watershed..
At the same time, the Department of Water Resources announced today it was reducing flows down the dam’s main spillway as crews get ready to remove the large volume of debris that has fallen into the channel below.
Rubble from the main release structure, and rock and sediment eroded from the adjacent slope, have filled the 80-foot channel immediately below the spillway chute.
DWR reduced the spillway flows from 100,000 cubic feet per second to 80,000 cubic feet per second.
The 100,000 cfs rate, which commenced Sunday afternoon as fears mounted that the dam’s emergency spillway system might fail and unleash an uncontrolled surge of water down the Feather River, helped lower the lake’s level 34 feet over the past four days.
The DWR has said the reduced releases will be sufficient to continue lowering the lake and make room for runoff from future storms and snowmelt.
The wettest storm in the series of storms that began Thursday is expected to arrive Monday. One precipitation forecast, from NOAA’s California-Nevada River Forecast Center, says that system could drop as much 6 inches of water — either rain or snow — on the higher elevations of the Feather River watershed.
Update, 4:30 p.m. Wednesday: Officials raced to drain more water from Lake Oroville as new storms began rolling into Northern California on Wednesday.
The three storms were expected to stretch into next week. Forecasters said the first two storms could drop a total of 5 inches of rain in higher elevations.
However, the third storm, starting as early as Monday, could be more powerful.
“There’s a potential for several inches,” National Weather Service forecaster Tom Dang said. “It will be very wet.”
Nonetheless, California Department of Water Resources chief Bill Croyle said water was draining at about four times the rate that it was flowing in and the repairs should hold at the nation’s tallest dam.
About 100,000 cubic feet of water was flowing from the reservoir each second, enough to fill an Olympic-size swimming pool.
Croyle said work crews had made “great progress” cementing thousands of tons of rocks into holes in the spillways.
“We shouldn’t see a bump in the reservoir” from the upcoming storms, he said.
The reservoir has dropped 20 feet since it reached capacity Sunday. Croyle said officials hope it falls 50 feet by this Sunday.
Still, officials warned residents who have returned to their homes that the area downstream of the dam remained under an evacuation warning and they should be prepared to leave if the risk increases.
KQED’s Dan Brekke hosted a live Facebook video below the Oroville Dam spillway earlier Wednesday:
Update, 2:45 p.m. Tuesday: The evacuation order affecting about 180,000 residents along the course of the Feather River below Oroville Dam has been reduced to a warning, allowing residents to return to their homes, Butte County Sheriff Kory Honea said during a press conference.
“Taking into account the current level of risk, the predicted strength of the next round of inclement weather and the capacity of the lake to accommodate increased inflow associated with those storms, we have concluded that it is safe to reduce the immediate evacuation order currently in place to an evacuation warning,” Honea said.
The Department of Water Resources indicated during the conference that the inflow of water to the reservoir continues to drop and that about 100,000 cubic feet of water per second is being released.
“We’re continuing to make significant gains in removing water from the reservoir,” acting DWR chief Bill Croyle said.
DWR officials said the goal is to get the level of the reservoir down to flood control storage, which is about 850 feet.
Update, 8:15 a.m. Tuesday: Large-scale releases of water continue at Oroville Dam, and the level of the giant reservoir there has dropped to about 12 feet below the emergency spillway structure that engineers believed was on the verge of failure on Sunday.
The Department of Water Resources and other agencies are continuing to assess the condition of the slope below the dam, parts of which were scoured down to rock by the force of water rushing over the emergency release structure over the weekend.
The crisis was triggered a week ago, when serious damage to the dam’s main spillway was detected just as runoff began cascading into the nearly full lake after a series of wet, warm storms.
Gov. Jerry Brown has issued an emergency declaration to help speed up state agencies’ response to the Oroville crisis. On Monday, he told reporters at a Sacramento-area media briefing with emergency officials that he’s confident the Trump administration will respond promptly to the state’s requests for aid.
Responding to questions about whether the Department of Water Resources should have done more to reinforce the emergency spillway system — as suggested by environmental groups during a 2005 relicensing process — Brown said:
“Every time you have one of these disasters, people perk up and start looking at analogous situations and things that you might not have paid as much attention to. But we live in a world of risk – the earthquake shook the Bay Bridge, and then we the state and all the different governors had to put up a new bridge.”
Tuesday morning, 180,000 people remain evacuated along the course of the Feather River in the east-central Sacramento Valley. At a media briefing Monday, Butte County Sheriff Kory Honea said the evacuation order, issued hurriedly on Sunday, would be in place until agencies handling the situation at the dam say the danger of a catastrophic emergency spillway failure has passed.
A series of storms expected to begin rolling across Northern California on Wednesday night are expected to trigger a new rise in Lake Oroville — the reason dam managers are continuing to try to lower the lake as fast as the damaged main spillway will allow.
Update, 1:20 p.m. Monday: Here are four big takeaways from the Department of Water Resources (with other local officials’ noontime briefing on the situation at Oroville Dam:
First: The evacuation order that forced 180,000 people from their homes on Sunday will remain in place for now. Butte County Sheriff Kory Honea, whose jurisdiction includes the dam and the communities immediately downstream, said he is depending on the advice of “subject-matter experts” from the DWR and other agencies before people are allowed to return home.
Second: The desperate effort to lower Lake Oroville’s level after an imminent failure of the dam’s emergency spillway continues. With water pounding down the severely damaged main spillway at nearly 100,000 cubic feet per second — that’s about 750,000 gallons, for those of us who don’t measure water in cubic feet — the giant reservoir is falling at about 4 inches per hour and is now about 5 feet from the top of the emergency spillway.
Third: Dam and water managers are preparing for the resumption of winter storms over the Feather River watershed above Lake Oroville. The DWR’s 10-day precipitation forecast, based on analysis of weather models, suggests that the next round of storms will be much colder and drop less than half the precipitation than the very warm weather systems that helped trigger the Oroville crisis.
Fourth: The DWR and other state and federal agencies are going to face very tough questioning about whether something should have been done years ago to shore up the emergency spillway structure and adjacent hillside. Those questions will be prompted by a story by KQED Science Managing Editor and San Jose Mercury News reporter Paul Rogers, who details concerns raised about the soundness of the emergency spillway system back in 2005.
Update, 5:40 p.m. Sunday: Officials say the emergency spillway at Oroville Dam could fail at any time and are ordering evacuations from Oroville to Gridley.
The California Department of Water Resources urged residents of Oroville to head north, toward Chico. Residents elsewhere downstream should follow the orders of their local law enforcement, the department said. Officials have set up an evacuation shelter at the Silver Dollar Fairgrounds in Chico.
The auxiliary spillway is separate from the main dam structure. It’s essentially an ungated 1,700-foot-wide notch in the rim of the reservoir, which began overflowing Saturday morning, days after the dam’s concrete-lined main spillway began to crumble. Below an initial concrete lip, water courses over bare earth all the way to the Feather River below, scouring the incline of earth, rocks and trees. Despite earlier reports that the lake level was dropping on Saturday, erosion on the auxiliary spillway has increased beyond expectations. Oroville Dam contains California’s second-largest reservoir, and is currently holding back more than 3.5 million acre-feet of water.
Update, 9:40 a.m. Sunday: After rising to record high levels, the water level in Lake Oroville appears to be dropping.
Data from the California Department of Water Resources — see real-time Lake Oroville levels here — show the reservoir’s surface crested at 902.59 feet above sea level at 3 a.m. Sunday.
With the volume of runoff into the lake decreasing and about 500,000 gallons of water flowing out of the lake every second down the badly damaged main spillway and the emergency outlet, reservoir levels had dropped to 902.39 feet by 9 a.m. That drop is equivalent to about 2.5 inches.
The lake is considered full at 901 feet, and it’s at that level that it began pouring over an emergency spillway early Saturday. The emergency outlet is being used for the first time since the dam went into operation in 1968.
DWR managers say water should stop flowing over the emergency spillway sometime Monday.
Update, 4:45 p.m. Saturday: The real news of this afternoon came from a media briefing with acting Department of Water Resources chief Bill Croyle, who gave new details about the work ahead to replace Oroville Dam’s shattered spillway.
Before we get to that, though, let’s take a glance once more at Lake Oroville, which has continued to rise and spill over on this sparkling midwinter Saturday. The giant reservoir, California’s second-largest, is now a foot over the dam’s never-before-used emergency spillway.
DWR officials say that with several days of dry weather in store and the volume of runoff dropping, they expect water to continue to flow over the emergency weir until sometime Monday. Video posted Saturday afternoon (see below) showed a muddy, debris-laden torrent pouring into the waterway below the spillway.
At his noon-hour media briefing, Croyle said the damaged main spillway will need to be completely rebuilt. He said he told Gov. Jerry Brown in a discussion on Friday the cost would come to $100 million to $200 million.
“My objective is to get a spillway back in operation before the wet season next year, which is typically Oct. 15 or so,” Croyle said.
Croyle said he can only give “a very rough range” of the eventual cost because of the many unknowns involved in the project, including exactly where the replacement spillway will be built.
“We haven’t gone in and looked at it, we don’t know how much more damage we’re going to do, decisions have to be made on a new one … so the range is huge,” Croyle said. “What we told the governor yesterday afternoon is a hundred to two hundred million. Again, with the caveats we don’t know a lot about the site itself.”
He added that while the agency has the resources it needs to carry out the new spillway project and associated cleanup and repairs, he’s hoping for support from the federal government.
Croyle said dam managers face a long, complex juggling act to deal with the impact of the spillway failure amid a continuing very wet winter.
One of the biggest challenges engineers and work crews face is how to clear the Thermalito Diversion Pool immediately below the wrecked spillway of a large volume of concrete debris and sediment that have dammed the waterway and forced closure of the hydroelectric plant at the base of Oroville Dam.
Muddy water rose and backed up toward the powerhouse as the lower section of the main spillway disintegrated under high flows. To avoid contaminating the power facility, it was shut down early Friday. That had an unfortunate side effect: Outflows through the plant, which can handle a maximum of 12,000 cubic feet per second, were halted. That, in turn, limited the amount of water managers could release from the fast-filling reservoir.
To remove the debris blocking the waterway, Croyle said, flows down the damaged main spillway will probably need to be halted temporarily. With another series of storms forecast to arrive in the region starting Thursday, that’s not something that can be done immediately.
One piece of good news about the forecast, though: The next round of storms is expected to be colder, meaning they are far less likely to unleash the torrents of runoff produced by the last group of extremely warm weather systems.
Update, 11:30 a.m. Saturday: Floodwaters began flowing over Oroville Dam’s emergency spillway early Saturday morning.
It’s the first time since the dam went into operation in 1968 that the emergency outlet from Lake Oroville has been used. The lake filled rapidly this week after severe damage to the main spillway forced dam managers to decrease the volume of water being released at the same time a series of warm storms triggered heavy runoff into the reservoir.
California Department of Water Resources officials said water began moving over the 1,700-foot-long emergency weir just before 8 a.m.
TV helicopter video soon after showed sheets of water cascading over the concrete structure, although heavy flows did not appear to have begun downhill.
And the lake continues to rise. By 11 a.m., the reservoir’s surface was 901.55 feet, 6 inches over the top of the emergency spillway.
DWR spokesman Doug Carlson said the rate of flow over the auxiliary release structure was expected to increase from an estimated 660 cubic feet per second at 9 a.m. to 6,000 to 12,000 cubic feet per second.
He said dam and water managers estimate the flow will continue for 40 to 56 hours — a time frame that runs roughly between midnight Sunday and 4 p.m. Monday.
Update, 12:35 a.m. Saturday: Anyone who’s been watching the numbers associated with Oroville Dam and Lake Oroville this evening — how much water is flowing into the lake, how much is flowing out through the partially destroyed spillway — probably has come to a conclusion similar to this one: At some point during the next few hours, water from the state’s second-largest reservoir is likely to start pouring across the dam’s emergency spillway and start racing down an adjacent slope toward the waterway below.
At midnight Friday, Lake Oroville had risen to within about 18 inches of the lip of the emergency spillway. With water still coming into the lake from the Feather River watershed faster than it can be released down the damaged spillway, the level is rising at about 3 inches per hour. At that rate, simple spectator arithmetic tells you that the lake will overtop the emergency spillway as early as 6 a.m. Saturday.
Dam managers with the Department of Water Resources had calculated releasing 65,000 cubic feet of water per second down the damaged spillway would slow the lake’s rise enough to keep water from reaching the emergency structure. Those hopes dimmed Friday evening when releases were cut to 55,000 CFS to lower the risk of erosion that would threaten the stability of nearby power line towers.
DWR is unable to use another release point in the dam, a hydroelectric generating station that can handle another 12,000 CFS. Debris from the shattered spillway wound up in the channel just downstream from the power plant, causing water to back up and forcing officials to shut it down.
This would mark the first time water has flowed over the emergency facility since the dam began operating in 1968. (The closest call since then: January 1997, when an inrush of floodwaters after a huge New Year’s Day storm brought the lake to within a foot of the emergency spillway.)
Beyond the history, the event brings uncertainty about what happens next. Crews from DWR, Cal Fire and private contractors scurried over the landscape immediately below the emergency weir over the last two days, trying to prepare the way for the cataract that soon might be pouring down the slope. Preparations included clearing trees and brush and cementing boulders into place at the edge of the emergency spillway. (See KCRA-Channel 3’s helicopter footage of the scene Friday afternoon.)
To corral any debris that comes tumbling down the slope as the water comes down, log booms have been placed in the channel below the spillway (a waterway known as the Thermalito Diversion Pool) with crews ready to tow large objects to a nearby cove.
Of the highest immediate concern to people residing downstream is whether the water coming over the emergency spillway will represent a flood threat. The Department of Water Resources says it will not.
Longer term, the deeper interest will be finding out whether DWR did everything it could and should have to ensure the integrity of the spillway, and what it will do to design and build a repaired structure.
And now, just after midnight early Saturday morning, we’ll sign off by saying: We’ll see what happens after day breaks.
Update, 12:45 p.m. Friday: State Department of Water Resources officials now say they believe the volume of water rushing into Lake Oroville is slowing enough — and releases down a badly damaged spillway have increased enough — that the giant reservoir will not flow over an emergency spillway as feared.
Dam managers increased the flow of water down the broken main spillway to 65,000 cubic feet per second — 486,000 gallons — in the early morning hours Friday. While department officials say damage to the structure is continuing, the erosion does not appear to pose a threat to the spillway gates or other critical infrastructure.
At the same time, DWR officials noted at a noon media briefing, runoff into the lake is decreasing. The inflow hit a peak of 190,000 cubic feet per second Thursday evening and had fallen to 130,000 by midnight Friday.
The difference between the inflow and outflow means the lake is still rising — about 4 inches per hour at noon. Lake Oroville’s surface is about 5 feet below the lip of the emergency spillway. But DWR officials say with rains having stopped for the time being, the volume of water coming into the lake should continue to drop and the lake’s rise will stop short of overflowing.
Update, 9:20 a.m. Friday: Two things have changed overnight at Oroville Dam and the giant reservoir behind it.
First: Inflow from the Feather River watershed into Lake Oroville, while still very high, has dropped from its peak levels Thursday.
Second: California Department of Water Resources managers followed through with a plan to ramp up releases down the dam’s wrecked spillway (for their rationale for doing that, see our earlier updates, below).
The rate of rise in the lake — see the DWR’s real-time data for yourself –has decreased from nearly a foot an hour at times Thursday to about 4 or 5 inches an hour Friday morning. The reservoir surface at 9 a.m. was reported to be 895 feet — up 45 feet from Tuesday when the spillway damage was discovered and just 6 feet below the dam’s emergency spillway.
The net result: That rate of increase would mean water from the reservoir would begin cascading over the emergency spillway sometime early Saturday morning. The lake, which has a stated maximum capacity of 3.5 million acre-feet, is now 98 percent full.
Mostly light rain and snow are expected across the Feather River watershed today before clear weather Saturday. Colder weather and a break from heavy rain could help reduce the volume of water flowing into the lake.
Update, 7:15 p.m. Thursday: The situation surrounding the damaged spillway at Oroville Dam has escalated into a crisis, with state water managers hoping they can dump enough water down the badly compromised structure to prevent the state’s second-largest reservoir from pouring over an emergency release point that has never been used before.
Flow rates down the collapsing spillway were increased late Thursday morning to 35,000 cubic feet per second. The result was a spectacle of churning mud and water and further damage to the concrete structure.
Whoa! Look at 35,000 CFS battering the damaged spillway at Lake Oroville: https://t.co/OLSANzgCvH https://t.co/zjgnZ8TxAd
But with storms continuing to pound the northern Sierra and torrents of water quickly filling Lake Oroville, the huge reservoir behind the dam, crews from the Department of Water Resources and Cal Fire are getting ready for what officials previously called “a very last-ditch measure.”
Crews on Thursday began cutting down trees and bulldozing brush on the steep slope below an emergency spillway to try to minimize downstream debris flows should the lake exceed its 3.5 million acre-feet capacity.
“We have crews out there just as a precaution,” said DWR spokesman Eric See during a media briefing at midday Thursday. “We’re still taking every measure we can to not have to use the emergency spillway, but if we do, we’re actually removing that debris right now so it doesn’t get mobilized” into an adjacent waterway.
But the possibility that Lake Oroville would overflow for the first time in its half-century history grew stronger as the day progressed, despite the water being released down the damaged spillway.
Acting DWR chief Bill Croyle said at an evening press conference that it was becoming more and more likely that water would pour uncontrolled over the emergency spillway.
“To be very clear, with the hydraulic conditions we have now, and with the flow that we have coming down out of the spillway chute, unless conditions change, we anticipate there may be a release of water over the emergency spillway,” Croyle said. “Maybe sometime on Saturday.”
That event has become imminent because the volume of water flowing into the lake increased dramatically during the day as heavy rain fell across the Feather River watershed. Some locations in upstream mountains had received 4 to 5 inches of rain in the last 24 hours, with another inch or two expected before clear weather arrives Saturday.
Lake Oroville will overflow the emergency spillway if it reaches an elevation of 901 feet above sea level. On Tuesday, when the spillway damaged was first noted, the lake’s surface was at about 850 feet. With the spillway shut down for most of the last 48 hours, the lake has risen to 887 feet as of 7 p.m. Thursday. (See DWR’s real-time Lake Oroville statistics.)
“The downside of having water go over the emergency spillway is that it would go down the hillside and take out trees and soil and create a big mess in the diversion down below,” the DWR’s See said.
See said the severe erosion seen on and around the spillway structure is being closely monitored by crews on the ground, remote cameras and drones. Engineers believe the heavy flow of water will scour its way down to bedrock before long, See said, but acknowledged there are risks to allowing the erosion to continue.
“Erosion is occurring in multiple ways,” See said. “You can have erosion to the side and erosion going down the hill, and then you can have ‘head cutting,’ which is erosion that can actually work its way back upstream. So that’s the one that’s of most concern.”
If engineers detect that uphill erosion, See said, it would be “a trigger point” that would prompt another shutdown of releases down the spillway.
The erosion has already released massive flows of sediment into the adjacent waterway, a canal called the Thermalito Diversion Pool. The canal carries water from the dam down to and around the city of Oroville. Among the facilities to which it conveys water is the Feather River Hatchery, which raises millions of chinook salmon and steelhead trout.
Heavy sediment in the water can kill juvenile salmonids. With muddy water cascading into the hatchery facility Thursday morning, the Department of Fish and Wildlife began an emergency rescue of salmon and steelhead, trucking the young fish to a satellite hatchery on the Thermalito Afterbay, west of Oroville.
From the Sacramento Bee’s account of the fish rescue:
At the hatchery Thursday, workers waded waist-deep through concrete holding ponds filled with water the color of chocolate milk. They used screens to push baby fish toward tanker trucks that would transport them a few miles southwest to Thermalito.
[Department of Fish and Wildlife spokesman Harry] Morse said that wild steelhead and salmon are spawning in the Feather River, fueling concern that sediment could overwhelm their nests and kill eggs and juvenile fish.
Officials at the media briefing repeated further reassurances that the integrity of Oroville Dam, one of the largest in the United States, has not been affected by the spillway collapse.
Butte County Sheriff Kory Honea said that while local emergency agencies are preparing for evacuations downstream of the dam, he didn’t believe the spillway situation posed an imminent threat.
Update, 11:55 a.m. Thursday: The California Department of Water Resources is fast running out of time and options for dealing with the badly damaged spillway at Oroville Dam.
With Lake Oroville rapidly approaching full, water managers increased flows down the spillway Wednesday afternoon and early Thursday to test the effect on the damaged structure. The result was both unsurprising and sobering.
The department said it expected the test, which involved releasing about 20,000 cubic feet per second down the long concrete spillway chute, would cause further damage to the structure.
But they may not have anticipated the extent of the damage that daylight revealed early Thursday. Photos from the scene showed that the massive cavity in the face of the spillway had grown several times larger and that the adjacent slow had suffered extensive new erosion. Here are a couple of views tweeted out early Thursday:
Looks like a significant increase in the hole in the #oroville dam spillway overnight.
Lake #Oroville #Spillway #Update Thursday. Both sides have crumbled off. Dam road closed
With the spillway mostly out of commission since major releases were curtailed, Lake Oroville has been rising at the rate of about half a foot an hour since midday Tuesday. Its level has increased 30 feet since then, with the reservoir’s surface now 20 feet below an emergency spillway.
The emergency spillway, which would release water down a steep slope adjacent to the spillway, has never been used in the dam’s half-century of operation. DWR officials and others say water flowing down the slope will likely result in a large volume of debris being dumped into the Feather River, which flows through the city of Oroville on its way to the Sacramento Valley.
That’s one reason dam managers are willing to risk the destruction of the concrete spillway, calculating that would be preferable to the unknowns involved in an uncontrolled emergency spillover.
“It’s going to be rocks, trees, mud — liquid concrete — going down that river,” retired DWR engineer Jerry Antonetti told Sacramento’s KCRA as he watched the spillway Wednesday night. “I’d open ‘er up, sacrifice the bottom of that thing — it’s going to go in the river — clean it out next year and build a new spillway.”
Update, 8:45 a.m. Thursday: State water officials say they may be forced to continue using a badly damaged spillway at Oroville Dam to prevent the lake from reaching capacity in the next few days.
Doing that would likely cause further damage to the spillway structure and continue eroding the surrounding area, Department of Water Resources spokesman Doug Carlson said Wednesday afternoon. But that could be preferable to allowing the lake to begin flowing over an emergency spillway on the dam.
Carlson called the alternate spillway — which would send water cascading down a long tree- and brush-covered slope containing roads and power lines, a “very last-ditch measure.”
“It’s an outcome that DWR is committed to not allowing to happen,” Carlson said. Like other DWR officials, he was quick to add that the spillway damage does not pose a threat to the dam itself, one of the largest ever built in the United States.
The department conducted an experiment during the day Wednesday in which it began sending a limited amount of water — about 20,000 cubic feet per second — down the damaged concrete spillway structure. The purpose of the test, Carlson said, was to see how much additional damage was done.
“We may just let the spillway do its job” despite the damage, Carlson said. Then, after the rainy season, “we could shut off the spillway, keep it dry, put construction people in there, whatever has to be done — rocks, fill, concrete mix, whatever — and get it back to 100 percent efficiency.”
The DWR’s spillway test came as Lake Oroville, the state’s second-largest reservoir, is filling rapidly with runoff from recent storms.
In order to maintain enough space in the lake to accommodate in-rushing floodwaters, managers would normally release water down the dam’s massive concrete spillway. That was just what was happening Tuesday when bystanders alerted dam personnel that there appeared to be damage to the structure.
Releases that were being ramped up to about 60,000 cubic feet per second were abruptly halted so that Department of Water Resources crews could assess the situation.
In the meantime, a high volume of runoff into the lake has continued, raising it more than 20 feet since early Tuesday. Late Wednesday afternoon, the reservoir was just 30 feet below an emergency spillway that has never been used in the dam’s half-century of use.
“It’s quite serious,” Carlson said of the dam and reservoir’s status. “The good news is that we think we have it under control.”
Below: DWR photo gallery depicting damage to spillway and erosion to adjacent area.
Update, 12:25 p.m. Wednesday: State water officials say engineers are still in the process of assessing damage to the spillway at Oroville Dam and figuring out what they can do to fix it.
“They’re evaluating the situation intensively this morning,” said Ted Thomas, the chief spokesman for the Department of Water Resources. “They’re looking at what their options are for repair.”
An extensive section of concrete on the spillway, which is used to manage the level of Lake Oroville, has peeled away or collapsed.
At the time the problem was spotted at midday Tuesday, water managers were in the process of ramping up the volume of water being dumped down the spillway into the Feather River. That was necessary to make room for high flows coming into the reservoir, the state’s second largest, from a series of storms that have dumped very heavy rain over the Feather River watershed.
Releases were reduced from about 60,000 cubic feet per second to just 5,000 CFS — the amount being routed through the dam’s hydroelectric generating facility.
The immediate result of curtailing the releases while huge amounts of runoff stream into the reservoir has been a very rapid rise in the lake’s level. In the 20 hours after releases were reduced at midday Tuesday, Lake Oroville has risen 10 feet and added 150,000 acre-feet.
If current release and flow rates persisted — and that’s not a sure thing by any means — the reservoir would reach its 3.5 million acre-foot capacity in the next three or four days.
If that happens, Thomas said, the dam’s emergency spillway — which has not been used since the dam was finished in the late 1960s — would channel floodwaters down a hillside into the river.
Thomas said he expected details on a proposed fix for the spillway damage later Wednesday.
Previous update, 5:35 p.m. Tuesday: California Department of Water Resources crews are assessing a potentially serious problem with Oroville Dam, the giant structure that impounds the Feather River to create the state’s second-largest reservoir.
Tuesday morning, the spillway that managers use to release water from Lake Oroville into the river appeared to suffer a partial collapse. That led to the shutdown of the spillway while engineers assess its condition.
Department officials say the dam itself, perched above the Sacramento Valley about 130 miles northeast of San Francisco, is not in danger.
The timing of the shutdown is critical: A huge amount of runoff is coming into Lake Oroville from the Feather River watershed after recent storms. To maintain room in the reservoir to contain the incoming flows, a high volume of water — about 55,000 cubic feet per second — was being released down the spillway.
With the spillway closed for the time being, there’s no way to release water from the dam except through a hydroelectric powerhouse built into the structure. Only about 5,000 cubic feet per second can be released through the powerhouse.
The net effect is that with releases virtually halted and heavy inflows from a series of very wet winter storms continuing to pour into the reservoir, the lake is rising steadily.
Another view of the damaged spillway at Lake Oroville. (Water releases temporarily halted.) #CaDrought
As of 3 p.m. Tuesday, Lake Oroville was 82 percent full and was 150,000 acre-feet above the storage level prescribed to maintain room for incoming floodwaters.
The Department of Water Resources said in a statement that “sufficient capacity exists within the reservoir to capture projected inflows for at least days, and DWR expects to resume releases from the gated spillway at a rate deemed later today after a thorough inspection is performed.”
Oroville Dam is an earth-fill dam and was dedicated in 1968. At 700 feet high, it’s the highest dam in the United States.
What does the spillway look like under normal conditions? Here’s a video shot Monday, when managers had ramped up releases from 25,000 cubic feet per second top 50,000 CFS (see below for some perspective on the flow numbers):
The flow perspective: One cubic foot of water is 7.48 gallons. So 55,000 cubic feet per second, roughly the volume being released down the spillway before problems were detected Tuesday, comes out to 411,400 gallons a second. That equals 1.26 acre-feet — enough water to flood a football field to a depth of 15 inches.
An acre-foot, in turn, is roughly the amount of water used each year by two “average” California households. So the volume of water pounding down the spillway every second is close to what three households would use in a year.