Oroville, California: Nearly 200,000 people who live below the tallest dam in the United States were ordered to urgently evacuate on Sunday after a spillway appeared to be in danger of imminent collapse.
The abrupt evacuation orders came as authorities near Oroville in northern California said that a crumbling auxiliary spillway on the Lake Oroville Dam could give way, unleashing floodwaters onto rural communities along the Feather River.
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US dam risk triggers urgent evacuation
Residents below the tallest dam in the United States, near Oroville in Northern California, were urgently ordered to evacuate on Sunday after a spillway appeared for a time to be in danger of imminent collapse.
"Immediate evacuation from the low levels of Oroville and areas downstream is ordered," the Butte County sheriff said. "This is NOT A Drill."
Major highways leading south out of the area were jammed as residents fled the flood zone, in which about 188,000 people live. The town of Oroville, which is home to about 16,000 people, was largely deserted by nightfall.
The Butte County Sheriff's Department and the state Department of Water Resources said the failure of the auxiliary spillway was caused by "severe erosion".
"This is not a drill." One of the largest dams in the US, north of San Francisco, expected to fail. https://t.co/IXcHuQFff4 pic.twitter.com/vq1m8muCEY
— Zack Whittaker (@zackwhittaker) February 13, 2017
The water department said on Twitter about 4.30pm (11.30am AEST) that the spillway next to the dam was "predicted to fail within the next hour".
Several hours later, however, the situation appeared less dire as the spillway remained standing and the water department said crews using helicopters would drop rocks to fill a gouge in the spillway.
Authorities were also releasing water to lower the lake's level after weeks of heavy rains in the drought-plagued state.
By 10pm, oficials said the immediate danger had passed with water no longer flowing over the eroded spillway. But they cautioned that the situation remained unpredictable. The evacuation orders would be re-evaluated at dawn.
"Once you have damage to a structure like that it's catastrophic," acting water department director Bill Croyle said. But he stressed "the integrity of the dam is not impacted" by the damaged spillway.
Asked about the evacuation order, Mr Croyle said: "It was a tough call to make." He added: "It was the right call to make."
Butte County Sheriff Korey Honea said he was told by experts earlier on Sunday that the hole in the spillway could compromise the structure. Rather than risk thousands of lives, the sheriff said, a decision was made to order the evacuations.
Officials said they feared the damaged spillway could unleash a nine-metre wall of water on Oroville, north of the state capital Sacramento.
The Yuba County Office of Emergency Services urged evacuees to travel only to the east, south or west. "Do not travel north toward Oroville," the department said on Twitter.
Evacuation centres were set up at a fairgrounds in Chico, California, about 30 kilometres north-west of Oroville.
The evacuations marked a dramatic turn of events at the nation's tallest dam, which stands at 230 metres and was built between 1962 and 1968. For several days, officials have been trying to figure out how to get water out of Lake Oroville after the main spillway was damaged.
The emergency spillway had never been used before - and until the last few hours it seemed to be working well.
Hours earlier at a noon news conference, water department officials said Oroville Lake was draining without incident and that the amount of water it was releasing was beginning to taper off, thanks in part to several days of sunny weather and less runoff flowing into the reservoir.
"We think by this time tomorrow ... the flow over the auxiliary spillway will have ended," Eric See, a spokesman for the water department, said.
The Oroville dam is nearly full after a wave of winter storms brought relief to the state after some four years of devastating drought. Water levels were less than two metres from the top of the dam on Friday.
State authorities and engineers began carefully releasing water from the dam, north of Sacramento, on Thursday after noticing that large chunks of concrete were missing from a spillway.
California Governor Jerry Brown asked the Federal Emergency Management Agency on Friday to declare a major disaster due to flooding and mudslides brought on by the storms.
Diminished by years of drought conditions, the Oroville dam had become a symbol of the state's worsening water crisis. But an unusually wet winter took the lake from nearly full to overflowing in less than a week.
At the same time, the nearly kilometre-long concrete spillway that the dam's managers rely on to release excess water began to crumble, with erosion worsening as millions of litres of water poured over it.
It continued to rain. Realising the lake might rise to a level that would trigger the use of an emergency spillway, state workers began clearing the area of trees and brush that could be sent hurtling downstream.
On Saturday morning, water began washing over the dam's emergency spillway for the first time since it was completed in 1968.
Photographs showed a torrent of water rushing downhill to join the Feather River.
On Sunday, officials said that although they expected the uncontrolled spill to end, they plan to continue using the concrete spillway to create more storage in the reservoir in anticipation of rainfall later in the week.
"We're going to continue to flow water down the spillway and lower the lake," Mr See said. "You're going to see the lake dropping over the next several days."
Officials emphasised that while erosion had carved a massive hole in the main spillway, the dam itself is structurally sound.
They have estimated it could cost $US100 million to $US200 million ($130 million to $260 million) to repair the damage to the spillway and other features.
Reuters, AAP, TNS