March 17, 2017 8:48 AM
Oroville Dam’s cracked spillway reopens as state works to lower reservoir levels
In a fresh test of Oroville Dam’s battered infrastructure, water was released from the fractured main spillway Friday for the first time in nearly three weeks in a renewed effort to lower water levels at the troubled reservoir.
As a horn blared and engineers in flourescent vests watched from strategic vantage points, water poured down the concrete chute, reaching flows of 50,000 cubic feet per second within a few hours. The releases are expected to last about a week before another temporary shutdown. The state Department of Water Resources, which operates the dam, expects to use the damaged spillway one or two more times this spring to handle runoff from the heavy snowpack blanketing the Sierra Nevada watershed that feeds the reservoir, California’s second largest.
While DWR’s crews have shored up the spillway with quick-setting concrete and other materials, the water releases might further damage the 3,000-foot chute. But DWR Acting Director Bill Croyle said it was important to push water out of Lake Oroville as water levels continue to rise.
“We’re going to be watching to see what happens, and we believe the measures we’ve taken have really been proactive to try to mitigate the concerns of losing additional concrete off the spillway,” Croyle told reporters. “We may see some of that (concrete) move, but at the moment, I need to get some water out of this reservoir. So, as long as you don’t see catastrophic loss of a lot of concrete, then we’re going to roll through this.”
The flows careened into an explosion of spray Friday as they hit the giant crater that opened in the spillway Feb. 7, the first day of a crisis that ultimately prompted a two-day evacuation of 188,000 downstream residents. The violent froth was a testament to how badly damaged the spillway remains, despite three weeks of repairs. Experts said resuming the water releases was a decision not taken lightly.
“It shouldn’t be as white-knuckle as it was, but I’m sure they’ll be a little worried about it,” said Jay Lund, head of the UC Davis Center for Watershed Sciences.
With the main spillway blasting water in the river channel below, engineers shut down the dam’s hydroelectric plant as a temporary precaution. The power plant serves as the dam’s primary release valve outside of flood season, but can’t function if the river gets too high.
“We want to protect that powerhouse,” Croyle said.
The main spillway was shut off Feb. 27 so crews could dredge out the concrete and debris that eroded into the river channel after the spillway fractured. The debris clogged the channel and made it impossible to operate the power plant. Croyle said DWR’s crews have removed nearly 1.25 million cubic yards of eroded material from the river.
Although the power plant has been releasing water the last two weeks, its outflow capacity is a fraction of the main spillway’s, and lake levels rose 24 feet in the three weeks that the spillway was shut down.
Oroville’s crisis peaked Feb. 12. Five days after the initial crack opened in the main spillway, lake levels rose so high that water topped the adjacent emergency spillway for the first time in dam history. That structure, too, proved faulty. The flows caused severe erosion on the hillside below the emergency spillway, sparking fears of a catastrophic collapse that would bring a “wall of water” down on Oroville and other communities. The evacuation orders followed. The crisis stabilized when DWR officials cranked up releases from the main spillway to 100,000 cubic feet per second, draining lake levels enough to arrest the flow of water over the emergency spillway.
Keeping lake levels low this spring could be a challenge with the heavy snowmelt expected from the Sierra. Another storm is expected in the coming days as well. But Michelle Mead of the National Weather Service said the reservoir should be able to handle the expected inflows. “We’re winding down the storm season,” Mead said.
As long as lake levels stay relatively low, DWR can turn its attention to permanent repairs. Croyle said the agency hopes to announce its plan in two weeks. Meanwhile, he said, crews are shoring up access roads, drilling test borings into bedrock and starting work on an on-site concrete batch plant in anticipation of the repair work.
He said DWR is working with a team of independent consultants to examine why the main spillway fractured, but the cause could be difficult to pin down.
“A lot of what we’d want to know has actually washed down into the river,” he said.