By Deborah M. Todd
OROVILLE, Calif. (Reuters) - Californians who were ordered to evacuate due to a threat from the tallest dam in the United States can now return home after state crews working around the clock reinforced a drainage channel that was weakened by heavy rain.
Officials had ordered 188,000 people living down river from the Oroville Dam to evacuate on Sunday and reduced that to an evacuation warning on Tuesday, Butte County Sheriff Kory Honea said.
That means people can move back to their homes and businesses can reopen, but they should be prepared to evacuate again if necessary, Honea told a news conference.
Both the primary and backup drainage channels of the dam, known as spillways, were damaged by a buildup of water that resulted from an extraordinarily wet winter in Northern California that followed years of severe drought.
The greater danger was posed by the emergency spillway, which was subject to urgent repairs in recent days. Though damaged, the primary spillway was still useable, officials said.
More rain was forecast for as early as Wednesday and through Sunday, according to the National Weather Service, but the state Department of Water Resources said the upcoming storms were unlikely to threaten the emergency spillway.
Evacuees received more good news from President Donald Trump, who declared an emergency in the state, authorizing the Federal Emergency Management Agency and the Department of Homeland Security to coordinate disaster relief efforts.
The lifting of the mandatory evacuation improved the mood among evacuees at Silver Dollar Fairgrounds in Chico, where families packed cars and sifted through piles of donated clothing.
Philip Haar, 37, of Oroville, prepared to take his five dogs back home. He also would be able to feed the rabbit he left behind.
"I'm confident with the warning, at least we'll know the next time something happens to be prepared more than this time," Haar said.
But Richard and Anna Lawson of Oroville said they were not rushing home. Officials last week expressed calm, then abruptly ordered the evacuation on Sunday.
"They kept contradicting themselves. Every time they said something they turned around and said something different," said Richard, 25.
"We're waiting until tomorrow to hear something. We're going to wait until the storm comes through," said Anna, 21.
The sheriff credited swift action by the Department of Water Resources to shore up the emergency spillway and use the main spillway to relieve pressure on the dam, averting the immediate danger of a dam failure, Honea said.
A failure could have unleashed a wall of water three stories tall on towns below.
State officials used 40 trucks carrying 30 tons of rock per hour to reinforce the eroded area around the emergency spillway while two helicopters dropped rock and other materials into the breach.
"We're aggressively attacking the erosion concerns that have been identified," said William Croyle, acting director of the Department of Water Resources.
Water authorities had been relieving pressure on the dam through the concrete-lined primary spillway last week, but lake levels rose as storm water surged in and engineers moderated its use. Then the rising water topped over the earthen backup spillway, which has a concrete top, for the first time in the dam's 50-year history over the weekend.
When the emergency spillway showed signs of erosion, engineers feared a 30-foot-high section could fail, leading to the evacuation order on Sunday. Both spillways are next to the dam, which itself is sound, engineers say.
(Additional reporting by Gina Cherelus and Sharon Bernstein; Writing by Daniel Trotta; Editing by Peter Henderson, James Dalgleish and Lisa Shumaker)