Thousands ordered to flee as spillways give on America’s tallest dam

Around 180,000 people in northern California were told yesterday to evacuate their homes immediately or risk being inundated by water rising behind the tallest dam in the US.

Officials called for the emergency evacuation at 5.45pm local time on Sunday, 12 February, telling residents via Twitter and other media to evacuate because the auxiliary spillway at the 230m-high Oroville Dam was predicted to "fail within the next hour".

This is NOT A Drill. This is NOT A Drill. This is NOT A Drill– Butte County Sheriff’s office

The California Department of Water Resources (DWR) warned people in cities downstream of the large, man-made Lake Oroville that erosion of the auxiliary spillway threatened to undermine its concrete weir, leading to "uncontrolled releases" of water down the Feather River, bursting its channels.

After the heaviest rains in several years DWR officials earlier in the day grew alarmed at the water thundering through the compromised auxiliary spillway (pictured) at a peak rate of 12,600 cubic feet per second, recorded at 1am on Sunday.

Repeating the evacuation order on social media, the Butte County Sheriff’s office said: "This is NOT A Drill. This is NOT A Drill. This is NOT A Drill."

So far the auxiliary spillway has held, and water stopped flowing over it early Monday morning, according to the DWR, although water continued to flow over the hydropower dam’s main spillway at 100,000 cubic feet per second. The main spillway also has suffered "serious erosion", DWR said.

After the evacuation order the DWR said it was drawing up plans to use helicopters to drop rocks to "fill in the gouge" in the auxiliary spillway, though it is not clear whether such an operation occurred.

Water thundering through the compromised auxiliary spillway at Oroville Dam, California (Butte County Sheriff’s office/Facebook)

Churches and fairgrounds in the area opened their doors to evacuees needing emergency shelter.

The main structure of the Oroville Dam, located around 240km northeast of San Francisco and completed in 1968, remains sound but the weaknesses of the spillways caught officials by surprise.

On Saturday, 24 hours before the emergency evacuation order, the DWR said it was preparing for Lake Oroville to rise to its highest-ever level.

Earlier in the day water levels rose to 901 feet above sea level, causing water to start flowing over the auxiliary spillway.

"The flows we’re seeing are extremely low compared to the design of the structure," said DWR Acting Director William Croyle. "Based on our current situation, there is no threat."

California state governor Edmund "Jerry" Brown on Sunday night issued an emergency order to bolster the state’s response to the situation at the Oroville Dam.

Top image: The 230-m-high Oroville Hydroelectric dam in calmer times (California Department of Water Resources)