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What happens if the Oroville Dam spillway fails?

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Danger appeared to subside Monday morning, but more rain is on the way

Aerial photo released by the California Department of Water Resources shows the damaged spillway with eroded hillside in Oroville.
Aerial photo released by the California Department of Water Resources shows the damaged spillway with eroded hillside in Oroville.
Photo by William Croyle/California Department of Water Resources via AP.

It was a heart-stopping moment Sunday night as officials predicted the “imminent failure” of part of the Oroville Dam in Butte County, about 70 miles north of Sacramento.

The National Weather Service warned locals:

Operation of the auxiliary spillway has lead to severe erosion that could lead to a failure [of] a portion of the structure. Failure of the auxiliary spillway structure would result in an uncontrolled release of flood waters. [...]

Move to higher ground now. Act quickly to protect your life.

Nearly 200,000 (estimates vary) Californians from nearby Oroville, Yuba City, Marysville, Gridley, Live Oak, and other communities were evacuated from their towns in the dead of night.

By very late Sunday the danger seemed to have abated. But the potential for a disaster that would perhaps affect all of California for years is still very real, particularly with yet more rain forecast this week.

A few critical things to know:

  • What’s the Oroville Dam?

At 770 feet, it’s the tallest dam in the United States. Built between 1961 and 1968, it’s “the centerpiece of the State Water Project and its largest water storage facility,” according to the the non-profit Water Education Foundation.

The dam and nearby Lake Oroville create a reservoir of 3.5 million acre feet of water. The dam itself generates enough electricity to power up to 737,000 homes for a year.

Oroville Dam also prevents regular flooding of the Feather River and the related Sacramento River system. According to the U.S. Society on Dams (a professional group of dam engineers and dam builders), “Were it not for Shasta and Oroville dams in particular, people could not live and farm in the Sacramento Valley.”

A huge mass of water behind the Oroville dam.
Oroville dam and its spillway as it’s normally seen.
Photo courtesy of California Department of Water Resources
  • What’s the problem?

There’s no threat to the dam itself. The problem is the spillway, the emergency channel that’s supposed to contain and reroute excess water when the reservoir is in danger of overflowing (spilling).

The University of Calgary’s energy education site explains that it works essentially like the overflowing drain in a bathtub:

If the reservoir is already full but floodwaters enter, the water level will increase and this could result in the over-topping of the dam. Spillways are built to prevent this, as it allows some water to be drawn from the top of the reservoir to make room.

As soon as any excess water enters the reservoir, water will immediately start flowing out through the spillway.

The reservoir is more than 90 percent full. Last Tuesday, dam workers discovered an enormous hole (45 feet deep, 300 feet wide and 500 feet long) in the spillway, which limited the amount of water the channel can handle.

There’s a backup emergency spillway for water the spillway cannot handle, but it’s a mere dirt slope nearby. By Saturday morning water was “running uncontrolled over the emergency spillway,” which had never happened before in the dam’s history.

Still, the Department of Water Resources insisted that there was no danger to the public.

By late Sunday, though, a nightmare scenario almost became very real. As the San Jose Mercury News explains, the fear was that:

Erosion could undercut the 1,730-foot-long concrete lip along the top of the emergency spillway, allowing billions of gallons of water to pour down the hillside toward Oroville and other towns downstream.

  • What would happen if the emergency spillway actually failed?

Disaster. The most immediate result would be a flash flood, a “three-story wall of water,” as a Cal-Fire official described it, that would effectively destroy nearby Oroville, population 16,000.

Flood water would surge down the Feather River, almost certainly devastating those downstream cities evacuated Sunday. The Feather River joins the Sacramento River just north of the Sacramento County line.

After years of drought, water is released down the spillway at Oroville Dam into the Feather River Oroville, California
After years of drought, water let down the spillway at Oroville Dam into the Feather River.
Photo by Barbara Rich/Getty Images

How far south the damage would spread is hard to predict, since it depends on the flow into Lake Oroville and how quickly the spillway might be repaired. But it would be a disaster unlike California has seen in years.

On Saturday, water flows down Oroville Dam's emergency spillway after being severely eroded.
On Saturday, water flows down Oroville Dam's emergency spillway after being severely eroded.
Photo by AP Photo/Rich Pedroncelli

The economic cost could be equally staggering. The Lake Oroville reservoir is critical to the state’s water system—it provides irrigation water as far away as the coast of Southern California—power system, and agricultural infrastructure, so the blow to the economy could smart for years.

  • What’s happening now?

Very late Sunday, DWR reported that hillside erosion had slowed, thus preventing the immediate disaster predicted hours earlier.

A handful of residents in nearby Chico (which is not in danger) told Curbed SF that helicopters ferrying rocks to the dam site, an effort to help fill in the hole, were routinely buzzing their homes.

Oroville residents described the evacuation as “pure chaos.”

Although the reservoir level has dropped, the LA Times notes that DWR is in a “race against time” to diagnose and hopefully repair damage to both spillways before a new storm system moves in on Wednesday.

Three to eight inches of rain could fall on the immediate area over the next week.