Investigators probing the ongoing Oroville Dam emergency won’t know for some time whether cracks, cavitation or some other monumental engineering failure caused the collapse of the massive concrete spillway at Oroville Dam, the head of the state Department of Water Resources said Friday.
Asked about a story in this newspaper Friday morning that cited prominent independent dam engineers saying that cavitation — a process in which fast-moving water can create explosive vapor pockets that can shatter concrete — is a leading possibility in the cause of the Oroville crisis, acting DWR Director Bill Croyle said his main priority right now is stabilizing the emergency.
“It’s going to be a long time, in my opinion, before we can get access to that site and better understand the dynamics of what may have caused it,” Croyle said. “We really can’t speak to cavitation, erosion or any of the other potential failure modes that caused this. It’s going to be a while.”
On Monday, federal officials ordered Croyle’s department, which owns and operates Oroville Dam, to assemble a panel of five independent dam engineering experts to review the emergency actions designed to prevent a massive, uncontrolled release of water from Lake Oroville, and to make recommendations on ways to improve safety in the future.
The Federal Energy Regulatory Commission, which licenses Oroville and other hydropower dams around the United States, also required the department to do a “forensic analysis” of why a large crater ripped apart the main concrete spillway at Oroville, the nation’s tallest dam, on Feb. 7.
“The forensic analysis must be performed by a fully independent third party with no previous
involvement in assessing the spillway structure at this project,” wrote David Capka, FERC’s acting director of dam safety and inspections.
Capka said the forensic analysis must be overseen by the five-member review board. The investigation into the cause of the collapse should take a back seat, however, to the immediate efforts to stabilize the reservoir, he wrote.
The collapse of the main concrete spillway — a huge structure that is 3,000 feet long and 178 feet wide, as wide as 15 lanes of freeway — limited the department’s ability to drain water from the 10-mile-long reservoir. Swollen by recent storms, the reservoir level rose 50 feet in five days, and by Sunday water was flowing over the top of the emergency spillway for the first time since dam construction was finished in 1968.
When the hillside eroded violently below the emergency spillway on Sunday, the department said it was in danger of imminent collapse, which could send a wall of water 30 feet high or larger raging down on towns, below killing large numbers of people. Local sheriff’s officials ordered the evacuation of 188,000 people who were allowed to return home Tuesday.
On Friday, Croyle reported that his agency has been successful in continuing to drain water out of the broken main spillway. The lake level has fallen 43 feet — from 901 feet above sea level last Sunday to 858 feet Friday afternoon, and falling — just short of the 850 feet goal Croyle set.
He said it was unlikely that strong storm systems coming in this weekend and Monday will dump so much water into the reservoir that its level will rise over the emergency spillway again.
“We have generated a large volume of flood storage space,” he said. “Mother Nature is in the room, but I do not expect there to be a risk. But things can change.”
Two days ago, dam operators successfully ran water over the broken main spillway at 100,000 cubic feet per second — 748,000 gallons a second — and were relieved it did not erode up to the reservoir’s crest or cause erosion on the hillside toward the face of the 770-foot tall dam.
On Friday, after dropping that flow rate to 80,000 cubic feet per second, Croyle said it would continue to be reduced slowly to remove stress on the broken spillway and to reduce the water level in the channel below so workers can remove debris and try to reopen the dam’s shuttered Hyatt Power Plant.
Also Friday, the Associated Press reported that in 2011, state officials told FERC that in the event of a collapse of the entire Oroville Dam, local emergency officials “do not believe there is enough time to perform evacuations in the communities immediately downstream” and that emergency responders would likely withdraw to safer ground and prepare for victims.
Butte County Sheriff Kory Honea did not dispute that Friday when asked by reporters.
“It’s a very, very daunting challenge,” Honea said. “That is why we are taking steps now to refine our notification plan and our evacuation plan, and potential evacuation routes, in the hopes that we can give people more time to exit the area in the event that should happen. Let’s be clear: The things we are dealing with are very serious.”
Meanwhile roughly 100 construction workers continued Friday to pile huge boulders and concrete into four highly eroded areas in the hillside below the emergency spillway in case water does flow over it again anytime this spring.
“Even through this weather we are having now, we will keep pushing to reinforce that area should we ever have to use that structure this year,” said Croyle.
