Close to 200,000 people have been ordered to evacuate from areas surrounding the Lake Oroville Dam in California amid fears of severe flooding.
The National Weather Service urged people living downstream of the tallest dam in the US to flee to higher ground on Sunday, after authorities said that a damaged auxiliary spillway was on the verge of collapse.
“Immediate evacuation from the low levels of Oroville and areas downstream is ordered,” the Butte County sheriff said in a statement posted on social media. “This in NOT A Drill. This in NOT A Drill. This in NOT A Drill.”
Weeks of heavy rain had left Lake Oroville – the second largest reservoir in California – nearly full, leaving rural communities along the Feather river in danger.
However, relief over the end of a devastating four-year drought in California was short-lived after authorities and engineers began carefully releasing water from the dam after noticing that a large patch of concrete had been eroded from the spillway.
Helicopters were seen dropping rocks onto the spillway in an attempt to plug the hole in the concrete that threatens to cause a collapse.
“What we are working with is approximately a 30-foot (10 metre) wall of water coming out of the lake. Not the lake draining but a 30-foot wall of water. That is why we took the appropriate measures that we did and implemented the evacuation process,” California Fire Incident Commander Kevin Lawson told reporters.
Hundreds of cars could be seen along Highway 99 as residents tried to make their way South or West to safety.
When the reservoir reaches capacity, up to 150,000 cubic feet (4,250 cubic metres) of water can be released down the spillway.
Unfortunately, due to erosion, damage to the concrete path for the water run-off put releases on hold.
If lake elevation tops 900 feet (275 metres), water can be released down the emergency spillway on the adjacent hillside.
The fear is that water pressure from the large amount of rainfall could cause a further breach.
Graphics #OrovilleDam #Oroville how water is flowing down emergency spillway. Fear is water pressure may force larger breach
nbcbayarea</a> <a href="https://t.co/YKEJP5qeol">pic.twitter.com/YKEJP5qeol</a></p>— Jeff Ranieri (JeffRanieri) February 13, 2017
Built between 1962 and 1968, the Oroville Dam lies about 150 miles (241 km) northeast of San Francisco.
It stands at 230m high, besting the famous Hoover Dam by over 12m.