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CA Dam Situation Spawns Evacuations

Wednesday, February 15, 2017

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The waters in California’s Lake Oroville continue to threaten nearby homes and infrastructure as the Oroville Dam’s main spillway remains in limited use, and an emergency spillway is also limited, due to erosion that could unleash an uncontrolled and potentially catastrophic release of water.

The situation at Oroville began last week, when operators discovered a huge patch of eroded concrete in the surface of the dam’s main spillway. The spillway’s capacity had to be reduced so that the damage could be addressed, and to prevent further erosion that the state’s Department of Water Resources said could threaten nearby power lines.

Release over Oroville auxiliary spillway
Images: Kelly M. Grow/California Dept. of Water Resources

By Saturday morning (Feb. 11), the lake's water level had reached 901 feet above sea level, the level at which water begins to flow over a concrete weir and into the emergency spillway.

But heavy rains—weather records show Oroville got more than 3 inches of precipitation between Feb. 6 and Feb. 10—meant rising levels in the reservoir, which is California’s second-largest. By Saturday morning (Feb. 11), the water level had reached 901 feet above sea level, the level at which water begins to flow over a concrete weir and into the emergency spillway.

Crews had been working to prepare the auxiliary spillway, a hillside that had not been paved with concrete like the main spillway, by clearing away trees and debris. Water flowing down this emergency spillway goes into the Feather River.

Weir in Jeopardy

But by Sunday, officials began extra releases onto the main spillway in an attempt to relieve pressure, because they found that the emergency spillway was eroding, threatening the stability of the concrete weir that holds back the water. Erosion also damaged an access road that had crossed the path of the emergency spillway.

Oroville auxiliary spillway damage

Erosion damaged an access road that had crossed the path of the emergency spillway.

Fearing a sudden and uncontrolled release of water that could result from damage to the weir, officials ordered an evacuation of homes and businesses downstream of the emergency spillway. Oroville and nearby Thermalito were affected by the evacuations, as were low-lying areas further down the Feather River, including parts of Gridley, Biggs, Yuba City and Loma Rica.

A total of about 188,000 people are reportedly affected by the evacuation. Butte County officials lifted the mandatory evacuation Tuesday afternoon, but kept an evacuation warning in place, advising residents to remain vigilant.

Waters Recede but Rain Threatens

As of Monday afternoon (Feb. 13), the lake had receded to 894 feet, partly because the main spillway was operating at 100,000 cubic feet per second, nearly twice the limited rate it had been operating at. Crews were working to shore up the emergency spillway by filling in with large rocks and gravel, in hopes of enhancing the stability of the weir.

Oroville spillway damage

The cause of the original concrete erosion in the dam is still officially a mystery, but some have questioned whether the phenomenon of cavitation may have played a role.

But the rains continue to threaten Oroville: Beginning Wednesday (Feb. 15) and continuing through the weekend, forecasters predict another 3 inches of rain for the area.

Cavitation to Blame?

The cause of the original concrete erosion in the dam is still officially a mystery. One expert, Blake P. Tullis, of Utah State University, told The New York Times that one possibility is cavitation.

Cavitation was to blame for a similar concrete collapse at Glen Canyon Dam, on the border of Utah and Arizona, in 1983. The phenomenon involves water vapor bubbles that can form in high-velocity columns of water. The bubbles implode with great force—enough to damage concrete.

   

Tagged categories: concrete; Erosion; Infrastructure; Locks and dams; potable water

Comment from Tom Schwerdt, (2/15/2017, 8:49 AM)

It should be noted that when this dam was undergoing relicensing in 2005, several groups filed motions to require concreting the emergency spillway to bring it up to modern standards. California DWR claimed it was not needed. Obviously they were very wrong, but the Feds (FERC) also share blame as they did not require the concreting.


Comment from Jacob Willem Knulst, (2/15/2017, 10:48 AM)

If cavitation would be the cause, shouldn't this more likely occur in the lower regions of the spill way as the severity of cavitation is proportional to the velocity of a liquid. Apparently it would.


Comment from Tom Schwerdt, (2/16/2017, 11:48 AM)

Jacob: Depends on how consistent the condition of the spillway was. If there was a particularly vulnerable section, the damage could happen anywhere on the spillway there is cavitation - not just the areas with the highest amount of cavitation stress.


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