Oroville Dam Aerials 3p. MediaSource: 12679879
200,000 flee as dam threatens wall of water
02:01 - Source: CNN
CNN  — 

The nation’s tallest dam reopened its spillway on Tuesday, more than two years after it failed and forced nearly 200,000 people to evacuate their homes, authorities announced.

Construction crews have been working to rebuild the spillway at the Oroville Dam in Northern California since May 2017, and have restored its “full functionality,” said Joel Ledesma, deputy director of the California Department of Water Resources.

Authorities said they will be closely monitoring the spillway’s operations as it faces its first big test since its collapse.

Why did it fail?

Powerful storms across California in February 2017 caused Lake Oroville’s water level to rise and overflow the dam. The water could not be drained fast enough, and the dam’s main spillway – which catches excess water when Lake Oroville’s water level rises to overflow the dam – had developed a hole the size of a football field and 40 feet deep due to erosion.

For the first time in the nearly 50 years since it was built, water was diverted into the emergency spillway, which was covered with brush, trees and rock. As millions of gallons of water washed over the emergency spillway, it eroded further.

Authorities feared that the spillway would fail and send water downstream.

They ordered evacuations in the nearby town of Oroville, which is about 75 miles north of Sacramento. The 770 feet dam provides flood control for the region.

Why is it reopening now?

Water flows down the Oroville Dam spillway on April 2, 2019. California officials opened the flood-control spillway at the nation's tallest dam for the first time since it crumbled during heavy rains two years ago.

It’s been an unusually wet winter in California, and more storms are likely coming later in the week.

State officials said they need to use the spillway to control the rising water in Lake Oroville – one of the state’s key reservoirs.

Ledesma said authorities are “operating the reservoir to ensure public safety of those downstream.”

The Lake Oroville reservoir is currently 81% full at 854 feet, according to DWR’s estimates.

In February 2017, the reservoir topped 900 feet.

How does it work?

The water began flowing Tuesday when the gates at the top of the dam were opened.

The Department of Water Resources says it expects to initially be releasing 8,300 cubic feet per second. The spillway has been rebuilt to handle water flowing at a rate of 270,000 cubic feet of water per second, significantly more than officials would ever expect to release.

Crews have installed temporary cameras and lights along the spillway, so they can monitor the water around the clock as it flows down the chute.

Why do officials think it won’t collapse again?

The damaged main spillway in February 2017.

Built in the 1960s, the spillway lacked modern enhancements and hadn’t been properly fortified, according to a report by a panel of independent investigators released after the spillway collapsed. The panel blamed the problem on “long-term systemic failure,” including issues with the original design.

And a Sacramento environmental group said they had urged officials to reinforce the spillway in 2005 because it didn’t meet modern safety standards.

Officials have expressed confidence that the repairs – costing more than $1 billion – will avert any potential crisis.

“The Oroville main spillway was designed and constructed using 21st century engineering practices and under the oversight and guidance from state and federal regulators and independent experts,” Ledesma said.

The renovation is considered the biggest and fastest construction project in state history. It required enough concrete to fill more than 370 Olympic-sized swimming pools.

During the two phases of the repair, crews worked to cover the spillway with structural concrete, install cutoff walls on the sides and reinforce the concrete with 12.4 million pounds of steel, officials said.

Ron Stork, policy director of the conservationist nonprofit Friends of the River and a flood management expert, said he doesn’t anticipate immediate problems with the spillway but believes more repairs need to be done.

“They’ve got many hundreds of millions of dollars if not more of work yet to be done,” he said.

The strength of a secondary emergency spillway could be an issue, Stork said, as well as potentially broken and cracked “anchor tendons” that are integral for raising and lowering the spillway gates.

The main spillway is prepared to reopen, water resources officials said, and work on the emergency spillway is expected to be finished later this year.

Correction: This story has been updated to reflect that the spillway reopened Tuesday after failing two years ago.

CNN’s Sarah Moon contributed to this report.