New storms could imperil California’s massive earthen dam in Oroville where 200,000 were evacuated, now back

UPDATE: The game plan is to get water behind the Oroville Dam below what its engineering designs call “flood control storage,” and keep it there. At that depth, the dam would have a buffer capacity of half a million acre-feet of water. Cold storm and snow could help avert disaster at the Dam, Los Angeles Times reports.

(BE2C2 Report) — More than a decade ago, officials rejected concerns that the massive earthen spillway at Oroville Dam in California could erode during heavy winter rains. It is now at risk of collapse, the Mercury News and USA Today reported on Monday.

Unexpected erosion chewed through the main spillway of the dam earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers don’t know what caused the cave-in, AP report said.

Ironically, the state’s five years of drought caused Lake Oroville’s water levels to plunge to a low of 33 percent of capacity, according to the Los Angeles Times. The lake became a poster child for the drought. In a dramatic shift, Northern California witnessed an extraordinarily rainy winter this year that caused waters to rise to their highest levels in decades, The Washington Post report said.

According to media reports, about 200,000 residents were ordered to evacuate Sunday after a hole in the emergency spillway of the Dam threatened to flood the surrounding area. By early Monday, Lake Oroville’s water level had dropped to a point at which water was no longer spilling over, and the crisis appeared to be stabilizing. However, officials warned that the damaged infrastructure could create further dangers as storms approach in the week ahead.

It also remains unclear when residents will be allowed back into their homes.

Lake Oroville is one of California’s largest man-made lakes, with 3.5 million acre-feet of water and 167 miles of shoreline. And the 770-foot-tall Oroville Dam is the nation’s tallest, about 44 feet higher than the Hoover Dam on the Colorado River. The lake is the linchpin of California’s government-run water delivery system.

There was never any danger of the dam collapsing. The problem was with the spillways, which are safety valves designed to release water in a controlled fashion, preventing water from topping over the wall of the colossal dam that retains Lake Oroville.

Earlier this month, unexpected erosion crumbled through the main spillway, sending chunks of concrete flying and creating a large hole. Then sheets of water began spilling over the dam’s emergency spillway for the first time in its nearly 50-year history.

Water from rain and snow rapidly flowed into the lake, causing it to rise to perilous levels, and sending water down the wooded hillside’s emergency spillway, carrying murky debris into the Feather River below.

Officials also will have to determine whether the damaged primary spillway will be able to handle high levels of water through the rest of the rainy season, Jay Lund, a civil engineering professor at the University of California at Davis, told the Sacramento Bee.

“Once we have damage to a structure like that, it’s catastrophic,” Bill Croyle, acting director of the state’s Department of Water Resources, said at a news conference late Sunday, in reference to the erosion of the main spillway. “We determined we could not fix the hole. You don’t just throw a little bit of rock in it.”

Anticipating a possible catastrophe for the Lake Oroville area, located about 75 miles north of Sacramento and about 25 miles southeast of Chico, the Butte County Sheriff’s Office ordered evacuations, adding in a news release that it was “NOT a drill.”

The dam itself remained structurally sound, the state Department of Water Resources said, and officials said helicopters would be deployed to drop bags of rocks into the crevice and prevent any further erosion.

The California National Guard will provide eight helicopters to assist with emergency spillway repair, Adjutant General David S. Baldwin said. All 23,000 soldiers and airmen statewide received an alert to be “ready to go if needed,” Baldwin said. The last time such an alert was sent out to the entire California National Guard was the 1992 Los Angeles riots, which erupted after a trial jury acquitted four officers of the Los Angeles Police Department of the use of excessive force in the videotaped arrest and beating of Rodney King.

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