US: With snow melting, Oroville managers plan to use mangled spillway - - PressFrom - US
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US With snow melting, Oroville managers plan to use mangled spillway

03:30  12 march  2017
03:30  12 march  2017 Source:   sfgate.com

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The massive snowpack in the mountains above Oroville Dam is beginning to melt as temperatures rise and could soon test the troubled reservoir with its biggest inflows since last month’s crisis.

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Managers of the state-run dam say they’ll be forced to rely on the lake’s damaged main spillway to discharge water down the Feather River as soon as next Friday. The concrete chute, which partially collapsed last month amid heavy outflows, has been offline for repairs for nearly two weeks while reservoir levels have been down.

Officials with the state Department of Water Resources are confident the mangled spillway and the eroded hillside below it can handle the releases as they wait for drier months to make permanent fixes.

But the wild card remains the weather — how much water pours into the lake and how quickly.

“It’s going to start to get a lot of snowmelt coming in,” said Roger Bales, a professor of engineering at UC Merced and director of the Sierra Nevada Research Institute. “Depending on the flow rate that you put through there, there’s the potential for more damage and more deterioration.”

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“Yes, I’m concerned,” he said. “Everyone has concerns.”

Problems at the nation’s tallest dam began Feb. 7 when a gaping hole emerged on the main spillway. Without the chute fully functional, lake water rose and days later poured over an emergency spillway, eroding the hillside beneath it. Fear that the hill would give way, sending water uncontrollably downstream, prompted the temporary evacuation of nearly 200,000 people.

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The worst never came, but questions about why and how both spillways failed remain unresolved.

About 170 state employees alongside as many as 500 contractors have been working 24 hours a day to strengthen the lake’s crippled water outlets. Crews have reinforced the concrete main spillway and bolted it to the ground to prevent further collapse, while pouring concrete and laying heavy boulders beneath the emergency spillway to stabilize the scarred hillside.

Repairs have so far cost $100 million, through February, or about $4.7 million a day, according to the state. A long-term fix could be twice as much.

“Public safety is paramount, and we’ll be managing the reservoir through the spring runoff season to ensure public safety while we work towards (permanent) repairs,” said Department of Water Resources spokeswoman Lauren Bisnett in an email to The Chronicle.

Since the main spillway was taken offline Feb. 27, a move made possible when cool and dry weather limited runoff to the lake, workers have cleared out much of the broken-off concrete that washed from the main chute into the Feather River below — about 803,000 cubic yards of it, officials said.

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Removing the concrete allowed operations to resume at the dam’s hydroelectric plant, which not only produces power but offers another outlet for water releases. The power station was shut down after the debris caused water to pool near the plant and threaten its turbines.

As lake levels begin to rise next week, state officials said, the powerhouse will release as much as 13,500 cubic feet of water per second, while the main spillway will release as much as 40,000 cubic feet of water per second.

While the reservoir can discharge as much as 150,000 cubic feet per second when everything’s working properly, the state anticipates that the lower outflow will be sufficient to accommodate the inflow from the mountains — at least in the short-term.

The lake level is expected to rise to 865 feet above sea level by late next week, but no higher. State officials are trying to keep the lake below 901 feet, the point at which water would begin pouring over the emergency spillway again.

Managing the lake level would be tricky this year even without the hobbled spillways. The mountains that feed the Feather River have the most snow they’ve accumulated in years, if not decades, promising near-record runoff in coming months.

In fact, Mount Lassen, to the north, has the state’s biggest snowpack in terms of water content, records show. The snow, which was 238.5 inches deep in the latest snow survey, contains about 118.5 inches of water.

“Everybody’s hope is that they’ll be able to keep up with the snowmelt without putting high flows over the (main) spillway,” said Bales, at UC Merced. “The big concern would be getting more rain during the snowmelt, because then you would have both coming in.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: [email protected] Twitter: @kurtisalexander

What happened at Lake Oroville?

View The Chronicle’s step-by-step, multimedia look at the crisis at projects.sfchronicle.com/2017/oroville-explainer.

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