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FAQs: Evacuations and the damaged Lake Oroville dam in Northern California

Water continues to run down the main spillway at Lake Oroville on Monday, Feb. 13, 2017, in Oroville, Calif. The water level dropped Monday behind the nation's tallest dam, reducing the risk of a catastrophic spillway collapse and easing fears that prompted the evacuation of nearly 200,000 people downstream. Sunday afternoon's evacuation order came after engineers spotted a hole on the concrete lip of the secondary spillway for the 770-foot-tall Oroville Dam and told authorities that it could fail within the hour. (Randy Pench/The Sacramento Bee via AP)

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SAN FRANCISCO (AP) — Tens of thousands of people below the tallest U.S. dam evacuated after its damaged emergency spillway threatened to give way and cause massive flooding.

Here are answers to key questions about problems at Lake Oroville dam in Northern California:

WHAT'S AT RISK?

Failure of the spillway could send uncontrolled torrents of water flooding downstream from Lake Oroville, which filled to capacity after California and other parts of the West saw some of the heaviest rain and snow in decades this winter.

Authorities say 188,000 people in the Sierra Nevada foothills would be in danger from a 30-foot-high wall of water and ordered evacuations. The Gold Rush-era towns of Oroville, with 16,000 people, and Marysville, with 12,000, are among those that could be inundated.

Specifically, water managers fear uncontrolled flows would overwhelm the Feather River downstream, which winds through downtown Oroville, and possibly trigger a series of subsequent levee failures.

WHAT ARE SPILLWAYS?

When reservoirs get too full, their operators release extra water down long channels, or spillways, designed to carry it downstream in a safe, controlled way.

Lake Oroville dam, which holds back California's second-largest reservoir, has a main concrete spillway that normally is used to release floodwaters into the Feather River downstream.

A second spillway mainly made of earth serves as an emergency backup. It also was supposed to be able to handle high flows from the dam, but it had never been used before Saturday.

The force of floodwater from the dam has damaged both spillways.

WHAT CAUSED THE THREAT?

After five years of drought, a wet winter has strained the system at Lake Oroville, which is receiving runoff from melting snow in the Sierra Nevada as well as from the latest in a series of heavy storms.

Dam operators noticed chunks of concrete in the main spillway last Tuesday. When workers stopped releasing water to investigate, they found that concrete patches the size of football fields had fallen out of the channel. With the reservoir nearing the top of the 770-foot-high dam, dam operators were forced to keep using the main spillway despite increasing damage to it from the rushing water.

The dam reached capacity Saturday, and dam operators had to use the emergency spillway, too. Operators on Sunday noticed water was gouging a hole in the earthen emergency spillway as well. Fearing that the emergency spillway could fail and send torrents of water rushing downstream uncontrolled, authorities ordered the evacuation Sunday evening.

WHAT DOES THIS MEAN FOR CALIFORNIA'S WATER SUPPLY?

Lake Oroville is the main reservoir of California's State Water Project, which supplies water for more than half the state's 39 million residents and for millions of acres of farmland in the leading agricultural state. It's not clear how damage to the two spillways will affect long-term water releases from the dam.

WHEN CAN EVACUEES GO HOME?

Evacuees might not go home until dam spillway is repaired, Don Thompson with The Associated Press reports:

Nearly 200,000 people who were ordered to leave their homes out of fear that a spillway could collapse may not be able to return until the barrier at the nation's tallest dam is repaired, a sheriff said Monday.

The sheriff of California's Butte County, Kory Honea, did not say how long the fixes could take and offered no timetable for lifting the evacuation order. Officials from the state Department of Water Resources were considering using helicopters to drop loads of rock on the eroded spillway at Lake Oroville, about 150 miles northeast of San Francisco.

Meanwhile, the water level behind the dam dropped, easing slightly the fears of a catastrophic spillway collapse. But with more rain expected later in the week, time was running short to fix the damage ahead of the storms.

A day earlier, authorities ordered mass evacuations for everyone living below the lake out of concern that the spillway could fail and send a 30-foot wall of water roaring downstream.

Nancy Borsdorf described a scene of chaos on her way out, including drivers abandoning cars as they ran out of gas.

"People were just panicking," said Borsdorf, who was at a shelter Monday in Chico.

"We've always loved and trusted our dam," she said, having lived in Oroville for 13 years. "I'm really hopeful Oroville wasn't flooded."

Asked if the spillway was supposed to handle far more water, the acting head of California's water agency said he was "not sure anything went wrong" on the damaged spillway.

Bill Croyle said sometimes low-flow water can be high energy and cause more damage than expected. His comments came after officials assured residents for days that the damage was nothing to be concerned, then ordered everyone to get out in an hour.

Department of Water Resources engineer and spokesman Kevin Dossey told the Sacramento Bee the emergency spillway was rated to handle 250,000 cubic feet per second, but it began to show weakness Sunday after flows peaked at 12,600 cubic feet per second.

The water level in the lake rose significantly in recent weeks after storms dumped rain and snow across California, particularly in northern parts of the state. The high water forced the use of the dam's emergency spillway, or overflow, for the first time in the dam's nearly 50-year history on Saturday.

The threat appeared to ease somewhat Monday as the water level fell. Officials said water was flowing out of the lake at nearly twice the rate as water flowing into it.

Documents show environmentalists raised concerns years ago about the stability of the emergency spillway, but state officials dismissed them and insisted the structure was safe.

In 2005, three advocacy groups complained to the Federal Energy Regulatory Commission that using Lake Oroville's earthen spillway would cause significant erosion because it was not armored with concrete.

They said soil, rocks and debris could be swept into the Feather River, potentially damaging bridges and power plants. The groups warned of a failure of the dam itself, threatening lives and property.

Nearly three years later, state officials said no "significant concerns" about the spillway's integrity had been raised in any government or independent review.

Croyle said Monday that he was not familiar with the 2005 warnings.

Sunday afternoon's evacuation order came after engineers spotted a hole in the earthen secondary spillway for the 770-foot-tall Oroville Dam and told authorities that it could fail within the hour.

With more rain expected Wednesday and Thursday, officials were rushing to try to fix the damage and hoping to reduce the dam's water level by 50 feet ahead of the storms.

The sudden evacuation panicked residents, who scrambled to get their belongings into cars and then grew angry as they sat in bumper-to-bumper traffic hours after the order was given.

A Red Cross spokeswoman said more than 500 people showed up at an evacuation center in Chico, California.

The shelter ran out of blankets and cots, and a tractor-trailer with 1,000 more cots was stuck in the gridlock of traffic fleeing the potential flooding Sunday night, Red Cross shelter manager Pam Deditch said.

At least 250 California law enforcement officers were posted near the dam and along evacuation routes to manage the exodus and ensure evacuated towns do not become targets for looting or other criminal activity.

In all, about 188,000 residents of Yuba, Sutter and Butte counties were ordered to evacuate.

The erosion at the head of the emergency spillway threatened to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville. Those flows could overwhelm the Feather River and other downstream waterways and levees and flood towns in three counties.

The California National Guard notified all its 23,000 soldiers and airmen to be ready to deploy, the first time an alert for the entire California National Guard had been issued since the 1992 riots in Los Angeles after a jury acquitted four police officers in the beating of Rodney King.

Unexpected erosion chewed through the main spillway during heavy rain earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing.

The lake is a central piece of California's government-run water delivery network, supplying water for the state's Central Valley agricultural heartland and homes and businesses in Southern California.

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Associated Press writers Jonathan J. Cooper and Kristin J. Bender contributed to this report from San Francisco.

Copyright 2017 The Associated Press

Copyright 2017 The Associated Press

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