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Oroville Dam Crews Prepare for More Rain as Evacuees Return Home Oroville Crews Lower Reservoir, Easing Threat Posed by New Storms
(1 day later)
As crews in Oroville, Calif., scrambled on Wednesday to shore up erosion on a spillway at the nation’s tallest dam, the authorities said they had removed enough water from the reservoir behind it to be able to absorb the new rain that had begun to fall. High water levels and the eroded spillway caused a major flood scare last weekend. Engineers in Oroville, Calif., began to slow the draining of a reservoir that had prompted a major flooding scare last weekend, saying that they had lowered the water level enough to make room for a new round of wet weather.
“We still are working through a very difficult situation at the dam,” Bill Croyle, the acting director of the California Department of Water Resources, on Wednesday. He said engineers were releasing 100,000 cubic feet of water per second from the dam’s main spillway, as they have been for much of the week. Bill Croyle, the acting director of the California Department of Water Resources, said on Thursday that the effort so far had provided “more flood storage so we can take on these initial storms.”
“We want to keep that rate up as we continue to move more water out of the reservoir to be able to absorb some of the wet weather that we’re experiencing — apparently right at this moment — and into the future days and weeks,” Mr. Croyle said. As crews continue to shore up an emergency spillway that was eroded last weekend — prompting fears of a flood from the reservoir and an evacuation order for about 180,000 people — a slight reduction in outflows from the reservoir will allow the dam’s operators to turn some attention to another pile of debris near the dam and a power plant that was taken offline.
The dam, a 770-foot-high embankment along Lake Oroville in the foothills of the Sierra Nevada, is not threatened; rather, the crisis emerged from a combination of high water levels and erosion in its spillways. On Sunday, the authorities urged about 180,000 people to evacuate from homes below the dam, but by Wednesday they said it was safe for people to return — at least for now. The dam, a 770-foot-high embankment along Lake Oroville in the foothills of the Sierra Nevada, is not threatened; rather, the crisis last weekend emerged from a combination of high water levels and erosion in its spillways.
“This is still an emergency situation,” said Kory Honea, the sheriff of Butte County, where the dam is. Mr. Honea said 150 National Guard troops were headed to Oroville in case they are needed. “We’ve got to pay attention and have the resources on hand to deal with it,” he said. Kory Honea, the sheriff of Butte County, which contains the dam, said that he was reviewing the county’s contingency plans and that National Guard troops were in place in case another evacuation was necessary.
“I do not have any information to suggest that the threat has increased,” Mr. Honea said on Thursday. “I watch closely, though, and if that comes to be, I want to make sure that we are ready, and that’s why the National Guard is here.”
It has been a wet winter in Northern California, and the water level at Lake Oroville was high after it received six or seven inches of rain last week, in addition to the runoff from 12 to 20 inches of rain that fell in the basin above it, said Jim Mathews, a meteorologist with the National Weather Service in Sacramento.It has been a wet winter in Northern California, and the water level at Lake Oroville was high after it received six or seven inches of rain last week, in addition to the runoff from 12 to 20 inches of rain that fell in the basin above it, said Jim Mathews, a meteorologist with the National Weather Service in Sacramento.
Last week, crews observed a hole in the dam’s concrete spillway, which drains water into the Feather River. The cause of the hole is not yet known. Water was diverted to an earthen emergency spillway, but major erosion was detected there over the weekend. This prompted fears that the spillway could collapse, sending a 30-foot wall of water into the valley below.Last week, crews observed a hole in the dam’s concrete spillway, which drains water into the Feather River. The cause of the hole is not yet known. Water was diverted to an earthen emergency spillway, but major erosion was detected there over the weekend. This prompted fears that the spillway could collapse, sending a 30-foot wall of water into the valley below.
“We determined that although efforts to mitigate potential failure needed to take place, we couldn’t guarantee that they would be completed in the time frame necessary to ensure that people could be evacuated,” Sheriff Honea said. People were told to leave immediately. The authorities have been releasing 100,000 cubic feet per second of water from the dam for much of the week, to make room for new water from storms and from the mountains above it. On Thursday, they said they would reduce that outflow to 80,000 cubic feet per second.
The authorities believe they are lowering the reservoir enough that the emergency spillway will not be needed during the next round of storms, but they are racing to fix it just in case. Workers have been moving 1,200 tons of rock and slurry an hour into the dam’s emergency spillway. Crews are also using helicopters to fling bags of rock into the spillway. The authorities believe that they are lowering the reservoir enough that the emergency spillway will not be needed during the newest round of storms, but they are racing to fix it just in case. Workers have been moving 1,200 tons of rock and slurry an hour into the dam’s emergency spillway.
“The armoring of that material will mitigate the threat of that erosion in the future,” Mr. Croyle said. “It’s kind of like a war zone for all the right reasons out there.”“The armoring of that material will mitigate the threat of that erosion in the future,” Mr. Croyle said. “It’s kind of like a war zone for all the right reasons out there.”
Yes. The authorities turned the evacuation order into an evacuation warning on Tuesday afternoon, which meant that residents who had huddled in Red Cross shelters, a fairground and a Sikh temple were allowed to return home.
But, Sheriff Honea said, “an evacuation warning considers the possibility that future inclement weather or increased lake levels or problems associated with the existing damage to the spillway could elevate risks in the future and necessitate immediate evacuations.”
The troubles at the Oroville Dam are seen by some experts as a warning sign for other dams in California and around the country, which are aging and may face new stresses from climate change.The troubles at the Oroville Dam are seen by some experts as a warning sign for other dams in California and around the country, which are aging and may face new stresses from climate change.
Experts say that the combination of concrete and earthen spillways is not unusual, and that some erosion can be expected.Experts say that the combination of concrete and earthen spillways is not unusual, and that some erosion can be expected.
“That’s a pretty standard design for many dams around the country,” said Mark Ogden, a project manager with the Association of State Dam Safety Officials. “What makes this one unique is the fact that it’s one of the largest dams in the country.”“That’s a pretty standard design for many dams around the country,” said Mark Ogden, a project manager with the Association of State Dam Safety Officials. “What makes this one unique is the fact that it’s one of the largest dams in the country.”
Nonetheless, the authorities seemed to suggest on Monday that the scale of the crisis had emerged suddenly.Nonetheless, the authorities seemed to suggest on Monday that the scale of the crisis had emerged suddenly.
“I’m not sure anything went wrong” with the auxiliary spillway, said Mr. Croyle, who called the use of the spillway a “new, never-happened-before event.”“I’m not sure anything went wrong” with the auxiliary spillway, said Mr. Croyle, who called the use of the spillway a “new, never-happened-before event.”
Mr. Mathews, the meteorologist, said there was no straightforward answer. “I would say for Northern California, obviously we have flooding, so there is no drought,” he said.Mr. Mathews, the meteorologist, said there was no straightforward answer. “I would say for Northern California, obviously we have flooding, so there is no drought,” he said.
But a hydrologist looking at groundwater in Southern California, where conditions are drier, might answer differently. And Gov. Jerry Brown has not declared an end to the drought.But a hydrologist looking at groundwater in Southern California, where conditions are drier, might answer differently. And Gov. Jerry Brown has not declared an end to the drought.
As Mr. Mathews pointed out, “We go from several years of below-normal precipitation, and then we go to several years of above-normal precipitation.”As Mr. Mathews pointed out, “We go from several years of below-normal precipitation, and then we go to several years of above-normal precipitation.”
“That’s basically how it works in California,” he added. “It’s either feast or famine in the water.”“That’s basically how it works in California,” he added. “It’s either feast or famine in the water.”