Emergency Spillway for Crippled California Dam is Used for First Time

For the first time since Oroville Dam was completed in 1968, water from its storm-swollen reservoir overtopped the emergency spillway Saturday, sending sheets of water down a forested hillside and adding to the murk and debris churning in the Feather River below.

State officials said they did not expect the flows to cause flooding in Oroville or other communities downstream, but the emergency releases underscored the perilous situation confronting the operators of California’s second-largest reservoir for the rest of this extraordinarily rainy winter.

Unable to release enough water from the dam’s 3,000-foot main spillway, which split open Tuesday and continues to erode, the California Department of Water Resources announced that stormwaters reached the top of the dam at around 8 a.m Saturday and began flowing over the concrete lip of the adjacent emergency spillway onto a wooded ravine below.

The flow began as a steady, smooth spill across the 1,700-foot-wide lip of the emergency structure, and was expected to peak at 6,000 to 12,000 cubic feet of water per second at around midnight Saturday. With dry weather in the near-term forecast for the Sierra and inflows to the reservoir slowing, the lake level should fall below the emergency spillway as of Monday night, said Doug Carlson, a DWR spokesman.

The emergency spillway exists to prevent water from topping the wall of the massive earthen dam that holds back Lake Oroville, a situation that could cause problematic erosion. Department of Water Resources Acting Director Bill Croyle stressed Saturday that the dam itself “is not threatened by these conditions.”

But the crisis at Lake Oroville won’t abate any time soon. Northern California is on pace for its wettest winter ever, and Croyle said an estimated 2.8 million acre-feet of snow blankets the Sierra above the dam. Depending on how quickly that melts, it will put additional strain on Oroville Dam in the months to come.

“Our next 60 to 90 days will be critical, how we route this (snow) runoff through this reservoir,” Croyle said. “There’s a lot of snow up there.”

​On Saturday, working to relieve pressure on the dam, operators continued to blast water down the battered main spillway, which developed a pothole in the past week that has expanded to a cavernous 300-foot fissure. The main spillway was releasing 55,000 cfs, meaning a total of 60,000 to 65,000 cfs was pouring out of the dam.

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SOURCE: DALE KASLER AND PETER HECHT 
Sacramento Bee