Rebuilt Oroville Dam spillway could be used next week after storm hits. Is it ready?

Water may cascade down Oroville Dam’s rebuilt spillway next week for the first time since a massive crater formed in its nearly half-mile long surface two years ago — a major milestone in the saga that triggered the evacuation of 188,000 people and a $1.1 billion repair job to the country’s tallest dam.

A storm forecast to hit this week is expected to fill Lake Oroville to the point that state dam operators might need to open the spillway gates to manage lake levels, state officials said Tuesday.

To prepare for the possibility, officials said the Department of Water Resources is removing construction equipment from the spillway chute. Crews also are removing a temporary road below the spillway that has been used during construction.

State officials said they’re expecting no problems following the massive spillway reconstruction effort, which required pouring more than 1.2 million cubic yards of concrete — enough to fill 372 Olympic-sized swimming pools. The main spillway alone has enough concrete to build a sidewalk from Oroville to Amarillo, Texas, state officials said.

“We are confident in the reconstruction work, certainly,” said Erin Mellon, a spokeswoman for the Department of Water Resources, which manages the dam.

Jeff Mount, a geologist and water expert at the Public Policy Institute of California, said it isn’t surprising that the spillway could finally be used, as a winter of heavy rain and snow gives way to the beginning of the spring snow-melt season in the Sierra.

“It was only a matter of time,” Mount said. “They’re going to have to use it.”

Although the reservoir is filling up, water is accumulating slowly enough that Mount said it’s doubtful that DWR’s dam operators will have to release copious amounts next week.

“This is unlikely to be a scary thing,” he said. “It’s a nice, gentle test of the system.”

But residents downstream of the spillway are watching the spillway nervously. They’ve been skeptical about DWR’s insistence that small amounts of water flowing down the spillway in recent weeks wasn’t from cracks or other problems. DWR says it’s normal seepage from the spillway gates.

“I think the residents do not have any trust in the DWR people,” said James Stone, a Sutter County fishing guide who’s been critical of the state’s handling of the near disaster, including the amount of debris left in the Feather River during the crisis.

The main spillway was releasing water at more than 50,000 cubic feet per second in February 2017 when an enormous crater erupted in the middle of the concrete chute. Dam operators throttled back water releases to try to limit the damage, which raised lake levels to the point that water started pouring over the adjacent emergency spillway — a concrete lip resting on a natural hillside — for the first time since the dam was completed in 1968.

A day later, engineers noticed the hillside was eroding badly, prompting fears that the emergency spillway could crumble and unleash a “wall of water” on communities below. The Butte Sheriff’s Department ordered the immediate evacuation of 188,000 downstream residents.

Dam operators ramped up the water releases on the main spillway, which drew down the water levels and essentially ended the immediate crisis.

Last year, an independent forensic team the state hired to come up with causes of the spillway failures heavily criticized California officials, saying DWR did a poor job of designing, building and maintaining the structure and neglected safety while focusing on the “water delivery needs” of the districts that store water in Oroville.

The forensic team described the festering problems at Oroville as a “long-term systemic failure.”

In response, DWR revamped its dam safety programs and ordered 93 dams it oversees to conduct thorough inspections and other ongoing safety upgrades.

Earlier this month, the Federal Emergency Management Agency refused the state’s request for $306 million in repairs, saying federal taxpayers shouldn’t have to pay for problems in the spillway that existed prior to the crisis. The state plans to appeal FEMA’s decision.

FEMA has approved $333 million in funding for the state.

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