With Oroville Spillway Damage Spreading, Officials Prepare for Reservoir to Overflow

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 the further damage to the concrete structure.

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.

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.”

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.

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.

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Emergency release of water from Oroville Dam escalates from possible to likely, DWR says

With stormwater and snowmelt pouring into the reservoir faster than expected, the operator of the crippled Oroville Dam said it was likely water would have to be released from the facility’s emergency spillway as soon as Saturday – a last-ditch alternative that officials had been hoping to avoid.

William Croyle, director of the state Department of Water Resources, told reporters Thursday evening that water levels in Lake Oroville could reach the brim sometime Saturday, forcing activation of the emergency spillway. The emergency system, which has never been used, would dump water onto an exposed hillside, dislodging trees and earthen debris into the Feather River and potentially affect communities downstream.

If the emergency spillway has to be used, “you’re going to get a lot of debris and erosion,” Croyle said.

The cavernous fracture in the dam’s main spillway continued to grow Thursday, splitting the massive flood-control structure in two and sending a powerful rush of sediment and debris into the Feather River that threatened the lives of millions of fish at a principal downstream hatchery.

The dam was releasing about 40,000 cubic feet of water per second Thursday afternoon, Croyle said, including about 35,000 cfs from the damaged main spillway. But that was not enough to compensate for the 190,000 cfs pouring into the reservoir from continued storms in the vast Sierra Nevada watershed that feeds the Feather River and its tributaries.

“This storm came in a little warmer, a little wetter,” he said.

DWR spokesman Eric See said late Thursday that engineers had ramped up releases at the main spillway to 42,000 cubic feet of water per second, an increase of about 7,000 cfs. That put total releases, including water pouring out of the dam's power plant, to nearly 50,000 cfs.

In addition, See said officials were working on a plan to increase releases from the power plant, which has been running below its maximum.

Use of the emergency spillway could be avoided if the rain stops, or officials are able to ramp up water releases from the damaged main spillway to bring down lake levels, he said.

Following two controlled tests, the Department of Water Resources began releasing water at a continuous flow over the main spillway shortly before 11 a.m. Thursday. Water, along with chunks of mud, gushed down the concrete chute, with some of it spilling out of the channel and onto an adjacent hillside.

With engineers unable to release normal outflows through the damaged spillway, water levels behind the dam continued to rise Thursday. The reservoir added about 205,000 acre-feet of water in the 24 hours prior to 2 p.m. Thursday. By the evening, the lake was at 887 feet, or 14 feet below the brim.

Emergency officials said they were preparing evacuation contingency plans in the event conditions continue to deteriorate and create a flood risk downstream. “If we need them to evacuate, we have the mechanism,” said Butte County Sheriff Kory Honea. He urged residents to sign up for alerts at the Sheriff Department’s website or to call 530-538-7826 for updates.

Honea added that he didn’t believe evacuations would become necessary. Threats to human life “are remote at this point,” he said.

The damaged spillway sits beside the main earthen dam that holds back Lake Oroville, the state’s second largest reservoir and a central piece of California’s government-run water delivery network. The dam can store 3.5 million acre-feet of water, which is divvied out through the year for farming and drinking water needs across great stretches of California. Much of Southern California’s drinking water is stored in the reservoir.

The dam also is a critical piece of the state’s flood-control infrastructure, protecting downstream communities including Oroville, Marysville and Sacramento. The spillway is generally operational only in the rainy season, as a flood-control outlet. Routine water deliveries flow through a power plant at the dam.

DWR officials acknowledged that continued use of the cracked main spillway would cause additional erosion, and might well wipe out the entire bottom half of the structure. As it was, the two test runs held Wednesday roughly doubled the size of the crater that was discovered in the spillway Tuesday, to the point that it severed the concrete, side to side, into two halves.

Kevin Dossey, a DWR engineer, said the department thinks the upper portion of the spillway sits on a layer of solid bedrock. That means officials should be able to continue using the chute without causing serious erosion in the upper reaches, where the spillway gates could be compromised.

“It’s going to continue to chew down,” he said, but “they are very confident it won’t continue to erode to a point of danger.”

As the Feather River below the spillway turned brown with silt, staff with the California Department of Fish and Wildlife raced to transport by truck 4 million baby salmon from a downstream hatchery, fearing they would die in the thick, muddy waters.

“They have turbidity in the river like they’ve never seen before,” said Harry Morse, a spokesman for the department. Turbidity refers to how cloudy the water is. Morse said the hatchery has no method of filtering water as brown as it was Thursday.

Morse said hatchery officials were concerned by the looming possibility of more earth and debris washing into the Feather River should Lake Oroville fill to the point that water would rush uncontrolled over the emergency spillway on the north side of the dam. Work crews from Cal Fire and other agencies were busy chopping down trees in the ravine below the emergency spillway to reduce the amount of debris that would flow into the river if it is triggered.

“Our goal is to get that ravine cleared out,” said Russ Fowler, a Cal Fire battalion chief.

At the fish hatchery just below the dam – one of a handful the state counts on to sustain its $1.4 billion commercial and recreational fishing industries – 4 million salmon were being trucked to holding ponds adjacent to the nearby Thermalito complex, a system of downstream reservoirs. Those ponds would be safe from the cloudy water conditions, Morse said.

That represents just half the baby fish at the hatchery. Morse said the Thermalito facility can’t accommodate all the fish at risk, so more than 4 million will remain in the hatchery while filtration experts try to devise a solution.

Each year the Feather River Hatchery releases 7 million baby salmon into the Central Valley’s waterways. Last March, state officials estimated that fish raised in the Feather River accounted for 63 percent and 76 percent of the state’s recreational and commercial ocean catches, respectively.

“The loss of hatchery-produced salmon from Feather River Hatchery would be a major blow to salmon fishermen in California,” said John McManus, executive director of the Golden Gate Salmon Association.

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.

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.

While problematic for spawning fish, the sediment and debris shouldn’t significantly raise the prospects of flooding in communities downstream of the dam, said Ben Tustison, an engineer who contracts for the Central Valley Flood Control Association.

“I don’t think there’s enough material to make a significant contribution to sediment downstream (that) would really jeopardize the capacity of the system,” he said.

As of Thursday, state officials said they didn’t know what had caused the spillway breach, nor when they might be able to begin repairs. Eric See, a spokesman for the Department of Water Resources, said fixing the spillway could be months off, given that the top priority is using the chute to keep the reservoir from overtopping.

“We need to keep using the spillway to evacuate water from the lake,” he said. “If the weather changes and dries out, we could potentially do a repair.”

With heavy snowpack in the Northern Sierra, it’s possible the state will have to rely on the damaged spillway well into May, said Joe Countryman, a member of the Central Valley Flood Protection Board and a former engineer with the U.S. Army Corps of Engineers.

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