Environment

After heavy rains, first use of partly rebuilt Oroville Dam spillway now 'unlikely'

Watch Oroville Dam spillway progress continue with rock excavation

Repairs continued on the Oroville Dam spillway in late March 2018 as workers excavated and processed rocks into material for roller-compacted concrete.
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Repairs continued on the Oroville Dam spillway in late March 2018 as workers excavated and processed rocks into material for roller-compacted concrete.

After a spring storm system dumped 5 to 7 inches of rain into the Feather River basin over the weekend, state officials said Sunday they likely won't have to use the partly rebuilt flood control spillway at Oroville Dam after all.

Last week, the California Department of Water Resources said the storm might cause water levels in the Lake Oroville reservoir to rise to the "trigger elevation" of 830 feet. At that point, DWR officials planned to open the spillway gates and release water down the 3,000-foot-long concrete chute..

But the lake level only reached 799.7 feet over the weekend, according to DWR.

In anticipation of the storm, water was released from the dam's hydroelectric plant to keep the reservoir level below 800 feet.

DWR has been keeping Lake Oroville's water levels lower than usual in an effort to avoid using the spillway. The department hasn't used the spillway since May, when it was closed off to begin phase one of the reconstruction.

The main spillway broke in two during a major rainstorm in February 2017. Water levels in the reservoir rose so high that the adjacent emergency spillway had to be used for the first time. When the hillside beneath the emergency structure eroded badly, sparking fears that it would fail and unleash a "wall of water," officials ordered a mass evacuation of 188,000 downstream residents.

Since then, most of the spillway has been rebuilt. Crews have refilled the massive hole that formed in the lower half of the main spillway.

State officials say they're confident of the structure's integrity, but crews will resume construction later this spring to make it safer.

Molly Sullivan: 916-321-1176, @SullivanMollyM

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