Mass evacuation as rain strains tallest US dam

Water rushes down a spillway as an emergency measure at the Oroville Dam in Oroville, California on February 13, 2017 (AFP Photo/Josh Edelson)
Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States\’ tallest dam.
Officials said the danger had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of San Francisco, had eased. But people were still being told to stay away.
Several weeks of heavy rain had filled the 770-foot (235-meter) high dam to capacity.
The threat comes not from the dam itself, which the California Department of Water Resources said was not in danger of collapse, but from an emergency spillway that channels excess water.
A giant hole opened in the dam\’s main spillway last week, forcing authorities to activate the emergency overflow channel on Saturday for the first time.
But it began eroding, threatening a rupture that would have sent water surging towards the valley below, media reported.
Authorities released 100,000 cubic feet of water per second from the main spillway, bringing down the level of reservoir Sunday, the Sacramento Bee newspaper said, quoting the Department of Water Resources.
Helicopters readied overnight to drop rocks into eroded areas in the emergency spillway ahead of rain that is forecast for Wednesday and Thursday that could fill the reservoir again.
The California National Guard said on Facebook that it had alerted its 23,000 members to be ready to deploy.
– Military on standby –
"Now that there is no more water going over the emergency spillway, though it brings stability to the situation, there are still a lot of unknowns," Butte County Sheriff Kory Honea told a news conference Sunday.
"We\’re not at the point yet where we can make decisions about whether or not it is safe to repopulate areas."
Pentagon spokesman Jeff Davis told reporters on Monday the federal military would be ready, if needed, to provide air transport, water rescue, medical care and shelter.
"While the state first and foremost has the responsibility for doing that there is a federal element, should they need it, that is ready to respond quickly if asked," he said.
About 188,000 people in downstream communities were told to leave on Sunday afternoon as water was still gushing over the top of the wide auxiliary spillway.
In a Facebook post ordering the "immediate evacuation" of low-lying areas of the city of Oroville and downstream communities, the Butte County Sheriff\’s department warned that a "hazardous situation is developing."
"Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville," it said, adding three times that this was "NOT A Drill."
To the south, Yuba County also ordered residents to move to safety.
"Yes, an evacuation has been ordered," the Yuba County Office of Emergency Services said in a Facebook post.
"All Yuba County on the valley floor. The auxiliary spillway is close to failing… Take only routes to the east, south, or west. DO NOT TRAVEL NORTH TOWARD OROVILLE!!!!!"
Water from the dam flows down the Feather River that runs through Oroville, a city of about 20,000 people.
"I\’ve been in close contact with emergency personnel managing the situation in Oroville throughout the weekend," Governor Jerry Brown said in a statement.
"It\’s clear the circumstances are complex and rapidly changing. The state is directing all necessary personnel and resources to deal with this very serious situation."
The Oroville Dam has been in use since 1968. Less famous than America\’s iconic Hoover Dam near Las Vegas, Oroville is still the tallest.
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