Saying the reservoir has receded enough to handle inflows from approaching storms, operators at troubled Oroville Dam said Friday they would continue to dial back releases from its cracked main spillway in hopes of easing pressure on the Feather River and levees downstream.
With Lake Oroville approaching normal flood-control levels for this time of year, dam operators on Friday cut outflows from 80,000 cubic feet per second down to 70,000 cfs. They will re-evaluate Saturday to see if they can dial back even further, to 60,000 cfs. In the heat of the crisis that erupted Sunday, the spillway roared at 100,000 cfs.
Dam operators are prepared to ramp up flows again should the series of storms forecast to drop rain and snow on the Sierra Nevada watershed over the next week come in wetter and more powerful than expected, said Bill Croyle, acting director of the Department of Water Resources, which operates Oroville Dam.
“The spillway has been stable through this entire process for a number of days,” Croyle said. “We can feel confident to use that spillway not only this week but on into the future.”
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In the meantime, he said, lowering releases from the dam will ease pressure on downstream levees in the Feather and Sacramento river basins.
The problems at Lake Oroville, California’s second largest reservoir, have stretched more than a week, and involve serious malfunctions in both its main and emergency spillways.
Early last week, in the midst of winter storms, DWR engineers discovered a cavernous hole in the lower section of the dam’s main spillway, a 3,000-foot concrete span that acts as the dam’s primary flood-control outlet during the rainy season. Fearing the spillway would become inoperable, dam operators stopped the flows for a time, then gradually reactivated releases.
With runoff from the stormy Sierra Nevada still rushing in, reservoir levels climbed, and early Saturday, water overtopped the dam’s emergency spillway for the first time in its 48-year history. Unlike the main spillway, which is lined in concrete, the adjacent emergency spillway dumps water in uncontrolled sheets over a 1,700-foot concrete lip onto a steep, wooded hillside.
By Sunday afternoon, a day and a half after the emergency system activated, the hillside just below the spillway lip was showing serious erosion, raising fears the structure would collapse. The concerns prompted mandatory evacuation orders for Butte, Yuba and Sutter counties that sent nearly 200,000 people fleeing for safety. The order was lifted Tuesday, after DWR cranked up releases on the main spillway, despite the damage to its midsection, and managed to lower reservoir levels below the emergency lip.
By Friday afternoon, so much water had been flushed from the lake that the level had dropped to 859 feet, or 42 feet below the emergency spillway.
In addition to relieving pressure on downstream channels, DWR is hoping that dialing back the punishing flows on the damaged main spillway will allow cranes and barges to safely operate in the channel below. The aim is to start digging out a massive pile of concrete, trees and other debris that has accumulated in the channel since the main spillway fractured. The debris has clogged the channel below the dam, raising water levels to the point that its power plant – the dam’s primary release outlet outside of flood season – can’t operate.
Once the debris is cleared and the power plant is restarted, the facility is capable of draining another 14,000 cfs from the lake. DWR doesn’t expect to have the plant running before Monday, when the brunt of a wet, heavy storm known as an atmospheric river is forecast to hit the area. Forecasters say the system could bring as much as 7 to 15 inches of precipitation to the watershed above the dam in the coming week, much of it as snow in the higher elevations.
On Friday, work crews continued to haul rock to the scarred hillside below the emergency spillway, to shore up the eroded sections in the event reservoir levels once again overtop the lip this winter. “We’ll keep working until the heavy equipment can’t run anymore,” said DWR spokesman Chris Orrock.
Meanwhile Friday, Assemblyman Marc Levine, D-Greenbrae, said he is carrying legislation requiring the state to perform annual inspections of auxiliary spillways at DWR-managed dams. He said in a prepared statement that he also wants to require that dam operators update their procedures and manuals. The Oroville Dam manual has not been updated since 1970.
Staff writer Christopher Cadelago contributed to this report.