Oroville Dam: Spillway releases dialed back to relieve pressure on downstream levees, repair power plant
Saying the reservoir has receded enough to handle inflows from approaching storms, operators at troubled Oroville Dam said Friday they would continue to dial back releases from its cracked main spillway in hopes of easing pressure on the Feather River and levees downstream.
With Lake Oroville approaching normal flood-control levels for this time of year, dam operators on Friday cut outflows from 80,000 cubic feet per second down to 70,000 cfs. They will re-evaluate Saturday to see if they can dial back even further, to 60,000 cfs. In the heat of the crisis that erupted Sunday, the spillway roared at 100,000 cfs.
Dam operators are prepared to ramp up flows again should the series of storms forecast to drop rain and snow on the Sierra Nevada watershed over the next week come in wetter and more powerful than expected, said Bill Croyle, acting director of the Department of Water Resources, which operates Oroville Dam.
“The spillway has been stable through this entire process for a number of days,” Croyle said. “We can feel confident to use that spillway not only this week but on into the future.”
In the meantime, he said, lowering releases from the dam will ease pressure on downstream levees in the Feather and Sacramento river basins.
The problems at Lake Oroville, California’s second largest reservoir, have stretched more than a week, and involve serious malfunctions in both its main and emergency spillways.
Early last week, in the midst of winter storms, DWR 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 the rainy season. Fearing the spillway would become inoperable, dam operators stopped the flows for a time, then gradually reactivated releases.
With runoff from the stormy 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, the adjacent emergency spillway dumps water in uncontrolled sheets over a 1,700-foot concrete lip onto a steep, wooded hillside.
By Sunday afternoon, a day and a half after the emergency system activated, the hillside just below the spillway lip was showing serious erosion, raising fears the structure would collapse. The concerns prompted mandatory evacuation orders for Butte, Yuba and Sutter counties that sent nearly 200,000 people fleeing for safety. The order was lifted Tuesday, after DWR cranked up releases on the main spillway, despite the damage to its midsection, and managed to lower reservoir levels below the emergency lip.
By Friday afternoon, so much water had been flushed from the lake that the level had dropped to 859 feet, or 42 feet below the emergency spillway.
In addition to relieving pressure on downstream channels, DWR is hoping that dialing back the punishing flows on the damaged main spillway will allow cranes and barges to safely operate in the channel below. The aim is to start digging out a massive pile of concrete, trees and other debris that has accumulated in the channel since the main spillway fractured. The debris has clogged the channel below the dam, raising water levels to the point that its power plant – the dam’s primary release outlet outside of flood season – can’t operate.
Once the debris is cleared and the power plant is restarted, the facility is capable of draining another 14,000 cfs from the lake. DWR doesn’t expect to have the plant running before Monday, when the brunt of a wet, heavy storm known as an atmospheric river is forecast to hit the area. Forecasters say the system could bring as much as 7 to 15 inches of precipitation to the watershed above the dam in the coming week, much of it as snow in the higher elevations.
On Friday, work crews continued to haul rock to the scarred hillside below the emergency spillway, to shore up the eroded sections in the event reservoir levels once again overtop the lip this winter. “We’ll keep working until the heavy equipment can’t run anymore,” said DWR spokesman Chris Orrock.
Meanwhile Friday, Assemblyman Marc Levine, D-Greenbrae, said he is carrying legislation requiring the state to perform annual inspections of auxiliary spillways at DWR-managed dams. He said in a prepared statement that he also wants to require that dam operators update their procedures and manuals. The Oroville Dam manual has not been updated since 1970.
How full are Northern California reservoirs and rivers?
AP Exclusive: If California dam failed, people likely stuck
Communities immediately downstream of California's Lake Oroville dam would not receive adequate warning or time for evacuations if the 770-foot-tall dam itself — rather than its spillways — were to abruptly fail, the state water agency that operates the nation's tallest dam repeatedly advised federal regulators a half-decade ago.
Regulators at the time recommended that state officials implement more public-warning systems, carry out annual public education campaigns and work to improve early detection of any problems at the dam.
Six years later, state and local officials have adopted some of the recommendations, including automated warnings via reverse 911 calls to residents. But local officials say the state hasn't tackled other steps that could improve residents' response, such as providing routine community briefings and improving escape routes.
The catastrophic scenario of a sudden breach at California's second-largest water reservoir, outlined between 2010 and 2012 in online archives of federal dam regulators, is a different and far graver situation than the concern that prompted sudden evacuation orders Sunday for 188,000 downstream residents. Operators of the nearly half-century-old dam in California's Sierra Nevada foothills became worried that the water cascading from the reservoir after a series of winter storms could roar uncontrolled down a rapidly eroding emergency spillway toward towns downstream.
The shortfalls in organization as well as infrastructure to quickly get residents out were on full display in the chaotic hours after the evacuation order. Residents found themselves caught in traffic jams for hours on clogged roads, leading some families to abandon their cars. While many local officials and ordinary people rushed to help direct traffic and staff emergency shelters, evacuees also reported seeing fistfights on gridlocked roads.
In an email Thursday, state water agency spokesman Ed Wilson said that despite the repeated back-and-forth correspondence between state and federal officials about reducing detection and response times in a sudden dam failure, the scenario was "hypothetical" and "not how dams typically fail in real life."
Asked Friday whether residents immediately downstream would have time and warning to get out if the dam itself failed, Sheriff Kory Honea in Butte County, where Lake Oroville is located, answered, "it's a very, very daunting challenge."
"That is why we're taking steps now to refine our notification plan and our evacuation plan, potential evacuation routes, in hope that we can give people more time to exit the area should that happen," Honea said.
Local officials, residents and a Florida-based evacuation expert said the federal-state discussion highlights the steps that the state Department of Water Resources and others still should take to improve warning and escape for people downstream.
"You know what the evacuation plan is? 'Get the hell out of town!'" said Kevin Zeitler, a critic of the state water agency's interactions with communities downstream of the dam.
The state informed federal dam regulators that local emergency officials "do not believe there is enough time to perform evacuations in the communities immediately downstream of the dam during a sudden failure," according to a Feb. 8, 2011, letter reviewed by The Associated Press.
Absent significant advance warning, emergency responders instead would likely withdraw to safer ground and prepare for victims, said the same letter by the Federal Energy Regulatory Commission, which oversees safety of hydroelectric dams, in a summary of the state's conclusions.
The federal government in recent years has made evacuation and emergency-response plans for major dams off-limit information for the public, for fear details could be exploited for terror attacks or hacking. California officials cited that reason this week in declining to release the latest emergency plans for the dam.
Wilson, the state water agency spokesman, said authorities have implemented the reverse-911 automated warnings recommended by federal regulators, and also activated an emergency broadcast system locally. Residents confirmed the reverse-911 system worked Sunday.
With months left in the rainy season, state spokesperson Nancy Vogel said California now has drones, cameras and human lookouts watching the dam and its spillways. Operators have been releasing torrents of water down the damaged main spillway to avoid a repeat of last weekend.
Even with round-the-clock efforts by dam operators, Oroville Mayor Linda Dahlmeier immediately began praying for those closer to the dam when she heard the first drops of rain hit the metal roof of her home Thursday.
"You just start bawling," Dahlmeier said. "This is Mother Nature's hand."
Oroville used to have civil-defense sirens for emergencies, Dahlmeier said, but funds for such public expenses have dwindled in the Sierra Nevada foothill counties. Neither she nor others recalled the annual safety briefings for the public that federal regulators urged of the state water agency.
A May 2013 hazards-assessment report by Butte County estimates 8,735 people live in a so-called inundation zone in Oroville that would likely be under water in the event of a sudden rupture at the dam, which is five miles from the vulnerable area in Oroville.
Since the 1990s, Oroville and other communities in Butte County have asked the state for the $300 million it would take to widen the full route of a key highway out of the county from two lanes to four, said Jon Clark, head of the Butte County Association of Governments.
Unquestionably, that would have helped in the evacuations, Clark and others said. For Butte County's many low-income retirees and others unable to drive, Clark's association got buses on the road Sunday to carry people to safety.
In a disaster as sudden as a major problem with a dam, authorities will have had warning signs telling them to increase their vigilance, even if that is just forecasts of storms coming, said John Renne, an urban-planning professor at Florida Atlantic University.
And the public can almost always be warned, even if it entails greater government investment in public-warning technology. "Minutes can save lives," Renne said.
Updates: Deadly storm slams Southern California
Torrential downpours were inundating Southern California on Friday, flooding streets and prompting water rescues. Officials said at least two people had died in the storm. One was found in a vehicle that was under water.
Here are the latest developments from one of the worst storms in the area in recent memory:
[Breaking news update, posted at 10:15 p.m. ET]
• A motorist in Victorville was found dead when San Bernardino County firefighters searched a submerged car. The department didn't give any details about the victim.
• Eight people were rescued when 10 vehicles got stuck on a road in Sun Valley. No one was injured, the Los Angeles Fire Department said.
• In just four hours, beginning at noon, the fire department responded to 150 calls of downed power lines.
• It was snowing heavily at higher elevations. "Chains required in all mountain areas. Unknown duration," tweeted the California Department of Transportation office for Riverside and San Bernardino counties.
• The National Weather Service in San Diego said a "severe squall line" was moving through Orange County and was approaching San Diego County. One wind gust was 75 mph.
• Duarte, a city northeast of Los Angeles, said residents of 200 homes were under a mandatory evacuation order.
• The Ventura County sheriff's air unit said that it has rescued three people who had been trapped on an island in a river. A helicopter crew was still searching for a missing person, it said on Twitter.
• More than 470 flights that originated at Los Angeles International Airport or were due to land there had been delayed and more than 130 were canceled, according to the flight tracking website FlightAware.com.
• At least one person has died, officials said. The 55-year-old man was electrocuted after a power line fell Friday afternoon in Van Nuys, Los Angeles police Officer Drake Madison said.
• Santa Barbara Airport has one closed runway. "The main airfield is under water and is closed for commercial activity," director Hazel Johns said. Runway 7 has 6 to 9 inches of water on it. Some employees were prevented from getting to work by road closures. The terminal is above the flood, Johns said.
• More than 116,000 customers in Southern California had lost power as of about 6 p.m. PT, officials said.
• A tweet from the Cleveland National Forest account showed an SUV with water almost up to the top of the rear wheel well. "The OC/Trabuco area is already experiencing some flooding. Be careful when driving and stay off the roads, if possible," the agency wrote.
An inch of rain an hour
Flash-flood warnings were in effect for Orange, Riverside and San Bernardino counties, the National Weather Service said. Parts of Los Angeles and Ventura counties were under a similar watch.
The rain was so furious a parking garage in Los Angeles turned into a waterfall.
In southwest California, the National Weather Service warned the area will see the strongest storm to hit this season. "It is likely the strongest within the last six years and possibly even as far back as December 2004 or January 1995," the weather service said.
Rainfall totals by the National Weather Service showed that parts of Santa Barbara County have already seen more than 7 inches of rain in the past two days. Parts of Ventura County have seen totals of more than 6 inches.
While these areas will see the bulk of rain before the end of the weekend, parts of northern California can expect a storm late Monday into Tuesday. The question is how much impact that might have on Oroville Dam's damaged spillway.
"The Oroville Dam and areas around it will see some rain Friday, but the bulk of the rain has shifted south. ... Unfortunately, it looks like another storm will pound the region again on Monday," said CNN senior meteorologist Dave Hennen. "Over the next week, areas around the dam could pick up another foot of rain, which will again likely raise the lake level."
Earlier this week, authorities ordered evacuations over concerns that an emergency spillway at the dam could fail and threaten nearby communities.
On Tuesday, officials downgraded the evacuation order to a warning, allowing 188,000 evacuees from Butte, Sutter and Yuba counties to return home.
Parts of the south-facing foothills and coastal mountain slopes could see up to 10 inches of rain through the weekend, meteorologists said.
"The incessant heavy rains, expected to reach up to 1 inch per hour, will dramatically increase the threat of urban flooding, as well as mud and debris flows from recent burn areas near mountainous terrain," CNN meteorologist Derek Van Dam said. "The highest elevations of Los Angeles and Ventura counties will likely experience 1 to 2 feet of snow, with near hurricane-force winds near the tops of mountain overpasses."
Oroville Dam 'is holding up'
The weather brings more worries for communities south of Oroville Dam. Rainfall over the next seven days could total more than 12 inches.
But on Friday, officials were optimistic the dam and lake could handle the upcoming rain.
Controlled water releases have dropped the lake level to 861 feet, more than 30 feet below the top, Bill Croyle, acting director of the California Department of Water Resources, said at a news conference. The normal lake level is 850 feet, he said. The amount of water released daily is exceeding the amount of water flowing into the lake.
"We have generated a large volume of storage space so we can take on a very big storm," Croyle said.
The threat level has been reduced for residents living near the dam, but Butte County officials on Wednesday advised those returning to their homes to "remain vigilant and prepared."
"The dam is holding up, it's structurally sound," said Jay Smith, a spokesman for the California Department of Forestry and Fire Protection.
But Hennen, the CNN meteorologist, said, "The next week of storms could potentially bring hundreds of billions of gallons of water into Lake Oroville, adding pressure back onto the already compromised structure of the concrete spillway and emergency spillway next to Oroville Dam."
Evacuation warnings remain in place for Oroville and Thermalito as well as "all low-lying areas around the Feather River, which includes Gridley, Biggs, Yuba City, Loma Rica, and anywhere south of Butte County along the River," according to the Butte County Sheriff's Office website.
The Butte County Sheriff's Office said 578 inmates had been transferred to the nearby Alameda County Jail.
With Lake Oroville approaching normal flood-control levels for this time of year, dam operators on Friday cut outflows from 80,000 cubic feet per second down to 70,000 cfs. They will re-evaluate Saturday to see if they can dial back even further, to 60,000 cfs. In the heat of the crisis that erupted Sunday, the spillway roared at 100,000 cfs.
Dam operators are prepared to ramp up flows again should the series of storms forecast to drop rain and snow on the Sierra Nevada watershed over the next week come in wetter and more powerful than expected, said Bill Croyle, acting director of the Department of Water Resources, which operates Oroville Dam.
“The spillway has been stable through this entire process for a number of days,” Croyle said. “We can feel confident to use that spillway not only this week but on into the future.”
In the meantime, he said, lowering releases from the dam will ease pressure on downstream levees in the Feather and Sacramento river basins.
The problems at Lake Oroville, California’s second largest reservoir, have stretched more than a week, and involve serious malfunctions in both its main and emergency spillways.
Early last week, in the midst of winter storms, DWR 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 the rainy season. Fearing the spillway would become inoperable, dam operators stopped the flows for a time, then gradually reactivated releases.
With runoff from the stormy 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, the adjacent emergency spillway dumps water in uncontrolled sheets over a 1,700-foot concrete lip onto a steep, wooded hillside.
By Sunday afternoon, a day and a half after the emergency system activated, the hillside just below the spillway lip was showing serious erosion, raising fears the structure would collapse. The concerns prompted mandatory evacuation orders for Butte, Yuba and Sutter counties that sent nearly 200,000 people fleeing for safety. The order was lifted Tuesday, after DWR cranked up releases on the main spillway, despite the damage to its midsection, and managed to lower reservoir levels below the emergency lip.
By Friday afternoon, so much water had been flushed from the lake that the level had dropped to 859 feet, or 42 feet below the emergency spillway.
In addition to relieving pressure on downstream channels, DWR is hoping that dialing back the punishing flows on the damaged main spillway will allow cranes and barges to safely operate in the channel below. The aim is to start digging out a massive pile of concrete, trees and other debris that has accumulated in the channel since the main spillway fractured. The debris has clogged the channel below the dam, raising water levels to the point that its power plant – the dam’s primary release outlet outside of flood season – can’t operate.
Once the debris is cleared and the power plant is restarted, the facility is capable of draining another 14,000 cfs from the lake. DWR doesn’t expect to have the plant running before Monday, when the brunt of a wet, heavy storm known as an atmospheric river is forecast to hit the area. Forecasters say the system could bring as much as 7 to 15 inches of precipitation to the watershed above the dam in the coming week, much of it as snow in the higher elevations.
On Friday, work crews continued to haul rock to the scarred hillside below the emergency spillway, to shore up the eroded sections in the event reservoir levels once again overtop the lip this winter. “We’ll keep working until the heavy equipment can’t run anymore,” said DWR spokesman Chris Orrock.
Meanwhile Friday, Assemblyman Marc Levine, D-Greenbrae, said he is carrying legislation requiring the state to perform annual inspections of auxiliary spillways at DWR-managed dams. He said in a prepared statement that he also wants to require that dam operators update their procedures and manuals. The Oroville Dam manual has not been updated since 1970.
How full are Northern California reservoirs and rivers?
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AP Exclusive: If California dam failed, people likely stuck
Communities immediately downstream of California's Lake Oroville dam would not receive adequate warning or time for evacuations if the 770-foot-tall dam itself — rather than its spillways — were to abruptly fail, the state water agency that operates the nation's tallest dam repeatedly advised federal regulators a half-decade ago.
Regulators at the time recommended that state officials implement more public-warning systems, carry out annual public education campaigns and work to improve early detection of any problems at the dam.
Six years later, state and local officials have adopted some of the recommendations, including automated warnings via reverse 911 calls to residents. But local officials say the state hasn't tackled other steps that could improve residents' response, such as providing routine community briefings and improving escape routes.
The catastrophic scenario of a sudden breach at California's second-largest water reservoir, outlined between 2010 and 2012 in online archives of federal dam regulators, is a different and far graver situation than the concern that prompted sudden evacuation orders Sunday for 188,000 downstream residents. Operators of the nearly half-century-old dam in California's Sierra Nevada foothills became worried that the water cascading from the reservoir after a series of winter storms could roar uncontrolled down a rapidly eroding emergency spillway toward towns downstream.
The shortfalls in organization as well as infrastructure to quickly get residents out were on full display in the chaotic hours after the evacuation order. Residents found themselves caught in traffic jams for hours on clogged roads, leading some families to abandon their cars. While many local officials and ordinary people rushed to help direct traffic and staff emergency shelters, evacuees also reported seeing fistfights on gridlocked roads.
In an email Thursday, state water agency spokesman Ed Wilson said that despite the repeated back-and-forth correspondence between state and federal officials about reducing detection and response times in a sudden dam failure, the scenario was "hypothetical" and "not how dams typically fail in real life."
Asked Friday whether residents immediately downstream would have time and warning to get out if the dam itself failed, Sheriff Kory Honea in Butte County, where Lake Oroville is located, answered, "it's a very, very daunting challenge."
"That is why we're taking steps now to refine our notification plan and our evacuation plan, potential evacuation routes, in hope that we can give people more time to exit the area should that happen," Honea said.
Local officials, residents and a Florida-based evacuation expert said the federal-state discussion highlights the steps that the state Department of Water Resources and others still should take to improve warning and escape for people downstream.
"You know what the evacuation plan is? 'Get the hell out of town!'" said Kevin Zeitler, a critic of the state water agency's interactions with communities downstream of the dam.
The state informed federal dam regulators that local emergency officials "do not believe there is enough time to perform evacuations in the communities immediately downstream of the dam during a sudden failure," according to a Feb. 8, 2011, letter reviewed by The Associated Press.
Absent significant advance warning, emergency responders instead would likely withdraw to safer ground and prepare for victims, said the same letter by the Federal Energy Regulatory Commission, which oversees safety of hydroelectric dams, in a summary of the state's conclusions.
The federal government in recent years has made evacuation and emergency-response plans for major dams off-limit information for the public, for fear details could be exploited for terror attacks or hacking. California officials cited that reason this week in declining to release the latest emergency plans for the dam.
Wilson, the state water agency spokesman, said authorities have implemented the reverse-911 automated warnings recommended by federal regulators, and also activated an emergency broadcast system locally. Residents confirmed the reverse-911 system worked Sunday.
With months left in the rainy season, state spokesperson Nancy Vogel said California now has drones, cameras and human lookouts watching the dam and its spillways. Operators have been releasing torrents of water down the damaged main spillway to avoid a repeat of last weekend.
Even with round-the-clock efforts by dam operators, Oroville Mayor Linda Dahlmeier immediately began praying for those closer to the dam when she heard the first drops of rain hit the metal roof of her home Thursday.
"You just start bawling," Dahlmeier said. "This is Mother Nature's hand."
Oroville used to have civil-defense sirens for emergencies, Dahlmeier said, but funds for such public expenses have dwindled in the Sierra Nevada foothill counties. Neither she nor others recalled the annual safety briefings for the public that federal regulators urged of the state water agency.
A May 2013 hazards-assessment report by Butte County estimates 8,735 people live in a so-called inundation zone in Oroville that would likely be under water in the event of a sudden rupture at the dam, which is five miles from the vulnerable area in Oroville.
Since the 1990s, Oroville and other communities in Butte County have asked the state for the $300 million it would take to widen the full route of a key highway out of the county from two lanes to four, said Jon Clark, head of the Butte County Association of Governments.
Unquestionably, that would have helped in the evacuations, Clark and others said. For Butte County's many low-income retirees and others unable to drive, Clark's association got buses on the road Sunday to carry people to safety.
In a disaster as sudden as a major problem with a dam, authorities will have had warning signs telling them to increase their vigilance, even if that is just forecasts of storms coming, said John Renne, an urban-planning professor at Florida Atlantic University.
And the public can almost always be warned, even if it entails greater government investment in public-warning technology. "Minutes can save lives," Renne said.
Updates: Deadly storm slams Southern California
Torrential downpours were inundating Southern California on Friday, flooding streets and prompting water rescues. Officials said at least two people had died in the storm. One was found in a vehicle that was under water.
Here are the latest developments from one of the worst storms in the area in recent memory:
[Breaking news update, posted at 10:15 p.m. ET]
• A motorist in Victorville was found dead when San Bernardino County firefighters searched a submerged car. The department didn't give any details about the victim.
• Eight people were rescued when 10 vehicles got stuck on a road in Sun Valley. No one was injured, the Los Angeles Fire Department said.
• In just four hours, beginning at noon, the fire department responded to 150 calls of downed power lines.
• It was snowing heavily at higher elevations. "Chains required in all mountain areas. Unknown duration," tweeted the California Department of Transportation office for Riverside and San Bernardino counties.
• The National Weather Service in San Diego said a "severe squall line" was moving through Orange County and was approaching San Diego County. One wind gust was 75 mph.
• Duarte, a city northeast of Los Angeles, said residents of 200 homes were under a mandatory evacuation order.
• The Ventura County sheriff's air unit said that it has rescued three people who had been trapped on an island in a river. A helicopter crew was still searching for a missing person, it said on Twitter.
• More than 470 flights that originated at Los Angeles International Airport or were due to land there had been delayed and more than 130 were canceled, according to the flight tracking website FlightAware.com.
• At least one person has died, officials said. The 55-year-old man was electrocuted after a power line fell Friday afternoon in Van Nuys, Los Angeles police Officer Drake Madison said.
• Santa Barbara Airport has one closed runway. "The main airfield is under water and is closed for commercial activity," director Hazel Johns said. Runway 7 has 6 to 9 inches of water on it. Some employees were prevented from getting to work by road closures. The terminal is above the flood, Johns said.
• More than 116,000 customers in Southern California had lost power as of about 6 p.m. PT, officials said.
• A tweet from the Cleveland National Forest account showed an SUV with water almost up to the top of the rear wheel well. "The OC/Trabuco area is already experiencing some flooding. Be careful when driving and stay off the roads, if possible," the agency wrote.
An inch of rain an hour
Flash-flood warnings were in effect for Orange, Riverside and San Bernardino counties, the National Weather Service said. Parts of Los Angeles and Ventura counties were under a similar watch.
The rain was so furious a parking garage in Los Angeles turned into a waterfall.
In southwest California, the National Weather Service warned the area will see the strongest storm to hit this season. "It is likely the strongest within the last six years and possibly even as far back as December 2004 or January 1995," the weather service said.
Rainfall totals by the National Weather Service showed that parts of Santa Barbara County have already seen more than 7 inches of rain in the past two days. Parts of Ventura County have seen totals of more than 6 inches.
While these areas will see the bulk of rain before the end of the weekend, parts of northern California can expect a storm late Monday into Tuesday. The question is how much impact that might have on Oroville Dam's damaged spillway.
"The Oroville Dam and areas around it will see some rain Friday, but the bulk of the rain has shifted south. ... Unfortunately, it looks like another storm will pound the region again on Monday," said CNN senior meteorologist Dave Hennen. "Over the next week, areas around the dam could pick up another foot of rain, which will again likely raise the lake level."
Earlier this week, authorities ordered evacuations over concerns that an emergency spillway at the dam could fail and threaten nearby communities.
On Tuesday, officials downgraded the evacuation order to a warning, allowing 188,000 evacuees from Butte, Sutter and Yuba counties to return home.
Parts of the south-facing foothills and coastal mountain slopes could see up to 10 inches of rain through the weekend, meteorologists said.
"The incessant heavy rains, expected to reach up to 1 inch per hour, will dramatically increase the threat of urban flooding, as well as mud and debris flows from recent burn areas near mountainous terrain," CNN meteorologist Derek Van Dam said. "The highest elevations of Los Angeles and Ventura counties will likely experience 1 to 2 feet of snow, with near hurricane-force winds near the tops of mountain overpasses."
Oroville Dam 'is holding up'
The weather brings more worries for communities south of Oroville Dam. Rainfall over the next seven days could total more than 12 inches.
But on Friday, officials were optimistic the dam and lake could handle the upcoming rain.
Controlled water releases have dropped the lake level to 861 feet, more than 30 feet below the top, Bill Croyle, acting director of the California Department of Water Resources, said at a news conference. The normal lake level is 850 feet, he said. The amount of water released daily is exceeding the amount of water flowing into the lake.
"We have generated a large volume of storage space so we can take on a very big storm," Croyle said.
The threat level has been reduced for residents living near the dam, but Butte County officials on Wednesday advised those returning to their homes to "remain vigilant and prepared."
"The dam is holding up, it's structurally sound," said Jay Smith, a spokesman for the California Department of Forestry and Fire Protection.
But Hennen, the CNN meteorologist, said, "The next week of storms could potentially bring hundreds of billions of gallons of water into Lake Oroville, adding pressure back onto the already compromised structure of the concrete spillway and emergency spillway next to Oroville Dam."
Evacuation warnings remain in place for Oroville and Thermalito as well as "all low-lying areas around the Feather River, which includes Gridley, Biggs, Yuba City, Loma Rica, and anywhere south of Butte County along the River," according to the Butte County Sheriff's Office website.
The Butte County Sheriff's Office said 578 inmates had been transferred to the nearby Alameda County Jail.
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