Water & Drought

Use of untested emergency spillway yet again a possibility at crippled Oroville Dam

Water continues to rush down Oroville spillway on Friday

The Department of Water Resources said it planned to slow releases from Oroville Dam’s damaged main spillway “to prevent erosion along the north side of the spillway from compromising nearby power line towers.” The lines run to the dam’s power pla
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The Department of Water Resources said it planned to slow releases from Oroville Dam’s damaged main spillway “to prevent erosion along the north side of the spillway from compromising nearby power line towers.” The lines run to the dam’s power pla

Pressing down on the accelerator to flush water out of their swollen reservoir, the operators of crippled Oroville Dam thought they had achieved a breakthrough Friday: So much water was cascading out over the damaged main spillway, it looked as if they could keep the lake from reaching the brim and emptying through the dam’s untested emergency spillway.

But by Friday evening, more unexpected trouble: The Department of Water Resources announced it was dialing back water releases over the battered main spillway by about 15 percent to keep erosion along the side of the spillway from “compromising” the power line towers that fuel the dam’s power plant. That reduced releases to 55,000 cubic feet per second.

With that, the possibility that the reservoir would crest the lip of the emergency spillway was raised anew. The slower releases “may keep the lake level below 901 feet,” the point at which water would start topping the emergency structure, the department announced. However, “there are many variables involved, and the public should not be surprised if some water flows into the emergency spillway.”

That structure, which empties onto a forested hillside, hasn’t been used in the dam’s 48-year history.

The department wanted to avoid using the emergency system because the cascade of water will scrape trees and other debris into the Feather River. If it is activated, officials said, the release should not be enough to cause flooding in nearby Oroville or further downstream.

DWR officials continued to stress that they do not believe the massive earthen dam that holds back Lake Oroville is at risk of failure.

“Whether the emergency spillway is used or not, Oroville Dam itself is sound and there is no imminent threat to the public,” William Croyle, the acting director of the California Department of Water Resources, said in a statement.

Still, the decision to dial back releases was a stark reminder of how quickly the situation at Oroville Dam could change. Tuesday afternoon, engineers shut down the main spillway after discovering a giant pothole had formed in the concrete chute. After a lull of about 24 hours, they began running relatively small amounts of water down the structure, knowing it would cause more erosion to the spillway, but racing to stay ahead of rising reservoir levels as drenching rains pounded the Sierra watershed that feeds the Feather River and its tributaries.

On Thursday, the pothole had expanded to a massive fissure that split the 3,000-foot main spillway in two. Nonetheless, just after midnight Friday – with the reservoir just feet below the dam’s crest and still rising – engineers made the decision to open the spillway’s giant 48-year-old gates and release a whopping 55,000 cubic feet of water per second down the battered chute. Two hours later, they ramped up the releases to 65,000 cfs, nearly double the amount they had been releasing the day before.

By daybreak Friday, the spillway was a boiling, foaming monster depositing huge volumes of water into the Feather River. Some of the water, misdirected by the jagged 300-foot-long gash in the span, sloshed over the east side of the chute and continued to erode a hillside.

Although the Butte County Sheriff’s Office has advised residents to stay alert for evacuation orders, state officials said they do not think the region downstream of the dam is at risk of flooding. Even if the reservoir overtopped the emergency spillway and cascaded over the adjacent hillside, it would come down in volumes that wouldn’t cause a disaster, said department spokesman Eric See.

“We call it a high flow event – it’s not a flood event,” See said. He said the Feather River below Lake Oroville isn’t expected to flow at levels exceeding 75,000 cfs, the level seen during the extreme storms of 2006.

Department of Water Resources engineer Kevin Dossey said, should water in the reservoir reach the emergency spillway, the releases wouldn’t be “catastrophic.” The emergency structure measures 1,700 feet across, and water likely would flow out in a broad sheet about 2 feet deep, he said.

Crews were bringing in concrete to bolster the lip of the emergency spillway “just as a precaution,” he said.

Two independent experts interviewed by The Sacramento Bee agreed with state officials that the situation posed no immediate threat to the integrity of the dam. But they also said erosion is an ongoing concern.

At The Bee’s request, Jeffrey Mount, former chairman of the UC Davis Department of Geology, reviewed video footage taken by helicopter Friday of the canyon that is forming at the bottom of the damaged spillway as a cascade of water rushes down.

“What it’s doing is what rivers do when they flood across bedrock. They tend to cut downward, like a knife,” Mount said. “You’re getting rapid, dramatic incision into the underlying bedrock, making essentially its … own steep canyon.

“Your No. 1 worry,” he said, “will be that it will continue to erode in the upstream direction (toward the top of the spillway) until it destroys ... the spillway gates. Then, you’re close to the abutment of the dam. But you’ve got a long way to go for that.”

He said it’s possible that officials could use the damaged main spillway for the rest of the rainy season, but it would depend on several factors, including how much water is spilling down the structure, and the rate of its degradation and that of the bedrock below.

DWR officials said Friday it wasn’t yet clear what had caused the spillway to splinter. Inspectors at Oroville Dam found “minor” cracks in the dam’s main spillway in 2009, according to a Bee review of state inspection reports. The reports indicate the damage was repaired the following year.

Dossey, the DWR engineer, said additional cracks had appeared in the main spillway as recently as 2013 but also were repaired. With water hitting the concrete chute at an estimated 50 mph, “spillway erosion is a natural thing,” he said.

For most of Friday, the main spillway of Oroville Dam was a stunning sight, a miniature version of Niagara Falls as water cascaded in unruly fashion down the damaged concrete chute. Department of Water Resources officials, sheriff’s deputies and others watching from a quarter mile away were feeling the spray even as patches of blue appeared in the sky directly above the dam.

The water pouring down the chute ran clear, a sign that although the spillway likely continues to erode, soil and dirt aren’t getting kicked up by the raging water to the same extent that happened Thursday. “It’s hitting bedrock, which is a good sign,” said Cal Fire spokesman Scott McLean.

However, large gushes of water were being misdirected by the massive fracture that now splits the spillway in two, and streaming down the hillside just east. The resulting debris formed a kind of natural dam at the bottom of the hill, raising water levels so high in the channel below the spillway that engineers had to shut down the dam’s power plant. The plant could have released up to 15,000 cfs.

Releasing water from Lake Oroville

With the first-ever use of the facility’s emergency spillway still a possibility, crews worked to remove trees and power transmission lines in the path of the potential water flow.
Diagram of Oroville Dam 
Image: Google Earth Pro (2015 aerial image)
The Sacramento Bee

The murky, debris-filled water threatens to suffocate millions of baby salmon at the Feather River Hatchery below Oroville Dam. Hatchery crews Friday continued their rescue operation, trucking more than half the 8 million salmon being reared at the hatchery to clearer water at a small annex nearby. The hope was that, by sunset Friday, some 5 million of the salmon would be in those holding ponds adjacent to Highway 99 and the Thermalito Afterbay, a small reservoir west of Oroville.

The other 3 million baby salmon will remain at the main hatchery. To keep them safe, crews have rigged a system of pumps, pipes and generators to filter water to keep two holding areas operational. The hatchery also holds 1 million steelhead eggs. In an effort to keep them alive, crews devised a filtration system using water from a municipal fire hydrant that is passed through charcoal filters.

“Pretty amazing stuff on the fly,” said Andrew Hughan, a spokesman for the Department of Fish and Wildlife.

In Oroville, just downstream of the dam, residents and tourists gathered on the levees to take stock of the raging, turbid Feather River. While many celebrated that the worst of the crisis appeared to be over, others reflected on the sometimes uneasy relationship between the river and this town of 15,000, a largely agricultural community with roots in the Gold Rush.

“Most of the time the river brings a lot of joy to this town,” said Michelle Gillstrap-Willard, 36, as she stood on a levee with her husband, Jeremy. “But when it goes a little haywire, people start to panic.”

Dale Kasler: 916-321-1066, @dakasler

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