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Debris Cleared as Oroville Spillway Dries Up

Wednesday, March 1, 2017

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As the waters of its reservoir receded, officials stopped the flow over the Oroville Dam’s main spillway Monday (Feb. 27), revealing the full extent of the damage sustained by the concrete structure and the surrounding hillside.

Concrete erosion was first spotted on the spillway on Feb. 7; the flow of water from the dam was halted briefly, but had to be restarted to stave off a potential disaster, as heavy rains came faster than the dam’s auxiliary spillway could safely handle. The crater in the main spillway is reported to be much larger now than it was when first discovered weeks ago.

Damage to Oroville main spillway
Images: Kelly M. Grow / California Department of Water Resources

Damage to the Oroville spillway and the nearby hillside was revealed as officials ordered the flow over the main spillway slowed to a halt Monday.

Water flow was reduced from about 50,000 cubic feet per second down to zero over a six-hour period. Photos from the site as the water flow was being ramped down show that water was redirected by the crumbling area, onto the surrounding hillside, where it caused major erosion on its way down to the Feather River.

Powering Back Up

The Hyatt Powerplant, a hydroelectric plant that’s part of the Oroville complex, was shut down shortly after the crisis began, due to debris that caused a water backup on the downstream side of the plant. With the flow of water down the spillway and into the diversion pool halted, efforts are underway to get the debris cleared, and restart the plant.

The Powerplant discharges about 14,000 cubic feet per second, which the California Department of Water Resources says will help to manage the reservoir levels, once it’s operational again. Officials expect they will have to use the main spillway again this winter and spring, but hope not to have to use the emergency spillway again.

Oroville spillway damage

Photos from the site as the water flow was being ramped down show that water was redirected by the crumbling area, onto the surrounding hillside, where it caused major erosion on its way down to the Feather River.

The emergency spillway, an unpaved hillside, was used to alleviate rising waters when the main spillway was limited by the concrete damage, but after the emergency spillway showed signs of serious erosion, the DWR had to increase flows onto the main spillway again. For a time, nearly 200,000 people were evacuated, as officials feared the weir above the emergency spillway could give way, causing a rapid and uncontrolled release.

Cause Speculation

While an investigation is underway, and no official cause of the concrete damage has been released, the Los Angeles Times spoke with engineers who speculated that unsophisticated construction techniques and erosion on the nearby hillside, exacerbated by tree roots, could be to blame.

One engineer who worked on the dam during its design in the 1960s told the paper that it never took into account the possibility of cavitation, a phenomenon involving water vapor bubbles that can form in high-velocity columns of water. The bubbles implode with great force—enough to damage concrete.

“It was an accident waiting to happen from day one,” the engineer, Don Colson, told the Times. “This was a mistake that went back to the very beginning.”

Another expert told the paper that gaps caused by the spillway’s tendency to slide downhill may not have been subject to the regular maintenance and repair work required.

“You have to grout these things in perpetuity, and that is what we are doing a poor job on as [a] nation and a state,” J. David Rogers, of the Missouri University of Science and Technology, said.

What’s Next

The DWR has not released a specific plan as to what will be done about the damage to the spillway yet. The agency says on its website that “work is being done around the clock and will continue to be expedited through the summer to ensure the spillways are safe by the next flood season.”

More than 500 people have been involved in the efforts to control the waters at the dam since the crisis began Feb. 7, the agency notes.


Tagged categories: concrete; Federal Energy Regulatory Commission (FERC); Government; Infrastructure; Locks and dams

Comment from Tony Rangus, (3/1/2017, 10:29 AM)

Did this "one engineer" who worked on the dam in the 1960's voice any concern then? Cavitation issues have been known for a VERY long time. Can I assume this engineer was remiss in his or her responsibilities? Or is someone blowing 20-20 hindsight smoke?

Comment from M. Halliwell, (3/1/2017, 11:10 AM)

I wouldn't assume that, Tony...unless Mr. Colson was the principal design engineer, he can raise all the concerns he wants and they may be ignored / disregarded. I've raised concerns on projects in the past only to be told "the book says this"...and that my concern didn't matter because 4" wasn't important on 10' (at least until it became very important later). Perhaps a concern was brought forward in the 60's, perhaps not....but hindsight is always 20/20.

Comment from peter gibson, (3/1/2017, 2:00 PM)

In reality nobody will be bothered with maintaining a spillway. The last thing on peoples minds.Has not been used in decades so was not on their minds. Who would have thunk.Also the spillway was poorly designed at the outset and exposed to the sun/elements for decades.

Comment from Jeff Laikind, (3/1/2017, 4:11 PM)

From what I've found, cavitation was first brought up as an issue in 1983, when underground spillways on the Glen Canyon Dam was severely damaged. The main Oroville spillway was used in March 2016, but at only 10,000 cubic feet per second. It was fine at a much lower rate than the 100,000 cfs releases this year.

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