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Oroville Dam Crews Prepare for More Rain as Evacuees Return Home Oroville Dam Crews Prepare for More Rain as Evacuees Return Home
(about 2 hours later)
Crews in Oroville, Calif., worked overnight into Wednesday to reduce the risk of catastrophic flooding at the nation’s tallest dam, racing to stay ahead of rains forecast for Wednesday. As the authorities lowered the level of the reservoir behind the dam, more than 100,000 people who had been evacuated began to return to their homes in the valley below it. 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 thread of rain caused a major flood scare last weekend.
“A lot of the work has been just making sure that folks are returning home safely,” said Capt. Dan Olson of the California Department of Forestry and Fire Protection, known as Cal Fire. “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.
Workers moved 1,200 tons of rock and slurry an hour overnight into the dam’s emergency spillway, shoring up erosion that occurred last weekend. Crews are also using helicopters to fling bags of rock into the spillway. “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.
Engineers continued to release massive amounts of water over the dam’s compromised main spillway — a hole was discovered last week — in anticipation of the rains. 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.
“We want to have more water going out then we do have coming in, and what that will provide is essentially a buffer for the upcoming forecasted rain,” Captain Olson said. “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.
The dam itself, 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. The lake was at about 900 feet over the weekend; the goal is to bring it down to 850 feet. On Wednesday morning, it was at about 880 feet.
The authorities said that the affected area was still under an evacuation warning, and that residents should be prepared to leave again if circumstances at the dam changed.
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, prompting 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,” said Kory Honea, the sheriff of Butte County, where the dam is. People were told to leave immediately. “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.
President Trump is monitoring the situation, Sean Spicer, the White House spokesman, said at a news briefing on Tuesday. The spillway problems, he said, are “a textbook example of why we need to pursue a major infrastructure package in Congress.” 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 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. “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.”
“We need to prepare for all contingencies,” said Bill Croyle, the acting director of the California Department of Water Resources. “If we have to use it, we want to make sure it’s in a condition to take higher flows.” 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.
Yes. The authorities turned the evacuation order into an evacuation warning on Tuesday afternoon, which meant 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.”
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 lifting of the mandatory evacuation order brought at least a temporary end to a crisis that led Gov. Jerry Brown to seek an emergency declaration for Yuba, Butte and Sutter Counties.
“This is about engineering, it’s about construction, it’s about repair,” Mr. Brown said on Monday. “We’ll get a lot of eyes on the problem, now that we have a problem — and the problem is big.” The dam was completed in 1968.
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.”
In 2005, environmental groups requested that the state line the earthen emergency spillway with concrete, warning that erosion could become a problem if it were used. That action was not taken, and Mr. Croyle said he was not familiar with the request. 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.
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. 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.
Mr. Mathews, the meteorologist, said there was not a straightforward answer. “I would say for Northern California, obviously we have flooding, so there is no drought,” he said. 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.”
But a hydrologist looking at groundwater in Southern California, where conditions are drier, might answer differently. And Mr. Brown has not declared an end to the drought. “That’s basically how it works in California,” he added. “It’s either feast or famine in the water.”
“We go from several years of below-normal precipitation, and then we go to several years of above-normal precipitation,” Mr. Mathews said. “That’s basically how it works in California. It’s either feast or famine in the water.”