BY ANISSA GROUT
On Feb. 7, damages were discovered on the Oroville Dam located in Northern California, causing authorities to use the dam’s emergency spillway. Recent storms caused the adjacent reservoir to swell, they expected to let water flow for at least 32 hours, 58 at the most.
Oroville Dam is an earth fill embankment dam on the Feather River east of the city of Oroville, Calif. At 770 feet high, it is the tallest dam in the U.S. The dam impounds Lake Oroville, the second largest man-made lake in the state of California.
The California Department of Water Resources (DWR) and the incident command team managing Lake Oroville, counties and cities near Lake Oroville and the surrounding area issued evacuation orders for residents. The concern was that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville. The potential flows could exceed the capacity of downstream channels.
By Feb. 14, residents were allowed back into their homes, according to the Washington Post.
DWR stated on Feb. 9 that there is no imminent or expected threat to public safety or the integrity of Oroville Dam. To avert more erosion at the top of the auxiliary spillway, DWR doubled the flow down its main spillway from 55,000 cubic feet per second (cfs) to 100,000 cfs. Flow over the auxiliary spillway weir began Feb. 11 and has slowed considerably. DWR officials expect that flow to stop entirely soon, which will reduce the erosion on the downstream side of the structure. Oroville Dam itself is sound and is a separate structure from the auxiliary spillway.
According to the Los Angeles Times, Bill Croyle, acting director of the state DRW, called the storms “fairly small” and said the public “won’t see a blip in the reservoir levels,” now dropping about eight inches an hour. Croyle said it was not the weather he was concerned about so much as the damage done to the dam’s already compromised main spillway during days of sustained heavy releases of water. “It’s holding up really well,” Croyle said, but continued mass water releases could be causing hidden damage to the rocky subsurface adjacent to the concrete chute.
DRW will continue releasing updates on the damage over the next couple of days.