Courtesy California Department of Water Resources.
OROVILLE, Calif. — A move to draw down Lake Oroville this fall to accommodate rebuilding of the dam won’t immediately affect water contractors, officials said.
The state Department of Water Resources plans to take the lake’s surface down to below 700 feet elevation this fall to ensure safety for workers fixing the dam, said Erin Mellon, a project spokeswoman.
That’s lower than the lake’s normal mid-autumn levels, she said, and lower than the lake’s current surface level of 800 feet. Weather permitting, draining the lake will enable crews to work past Nov. 1, which was their self-imposed deadline for getting the dam’s spillways ready for next winter’s rain and runoff, she said.
The state will be able to meet all of its water obligations this year, said Joel Ledesma, the State Water Project’s deputy director. However, next year’s deliveries remain to be seen, he said.
“We’ll be waiting for next winter’s snowpack and weather to come in,” Ledesma said during a conference call with reporters. “Then we can determine what the allocations are.”
While crews are making plans to work into the winter, Mellon insists the project hasn’t fallen behind schedule.
“We are still on schedule to build what we’ve committed to by Nov. 1, to make sure we can handle those flows coming in for the winter season,” she said.
Their remarks came during a progress report on the $275.4 million effort to repair and rebuild the nation’s tallest dam, whose spillways nearly failed in February.
The DWR’s final 2017 construction plan won approval in mid-July from the California Division of Safety and Dams and the Federal Energy Regulatory Commission, meaning this year’s work has gained all of its required approvals from federal, state and independent oversight groups, officials said.
This summer’s first phase includes removing and rebuilding 2,270 feet of the main spillway, temporarily fixing the top portion of the main spillway that connects to the radial gates, and building an underground cutoff wall below the emergency spillway to prevent erosion if it is used again.
Next year’s work is slated to include adding structural concrete to the entire main spillway, resurfacing and hydro-blasting energy dissipators at the base of the spillway and building a roller-compacted concrete wall and splashpad on the emergency spillway to dissipate the force of rushing water, officials said. The work in 2018 will also need federal and state approval.
Lake Oroville is the main reservoir for the State Water Project, which irrigates more than 600,000 acres of Central Valley farmland and serves 20 million urban customers in the San Francisco Bay area and Southern California.
Spillway ruptures in February led to the two-day evacuation of about 188,000 area residents and threatened a large portion of the Eastern Sacramento Valley’s $1.5 billion agriculture industry, including rice and tree crops and several processors along the Highway 99 corridor between Chico and Yuba City.
Emergency and long-term repairs this spring delayed the State Water Project’s setting final allocations to its 29 contracting water agencies. The delay caused some growers to put off planting decisions or rely on guesswork to determine how many acres they’d plant.
The state finally announced in mid-April that SWP contractors north of the Sacramento-San Joaquin River Delta would get their full allocations for the first time since 2006, while agencies south of the Delta would get 85 percent of normal supplies.
Officials began to slowly drain Lake Oroville earlier this summer. The DWR’s releases into the Feather River are now at 6,500 cubic feet per second. As of July 26, the lake was at 62 percent of capacity and 83 percent of average for the date.
With guidance from FERC, the agency has established a projection schedule to draw down the reservoir’s elevation to enable late-season work and to provide some wiggle room if it starts raining.
The schedule calls for the lake to be taken down to 670 feet on Nov. 1 if inflows are low and 700 feet if they are high. By the end of December, the plan calls for a surface elevation of 640 feet to 680 feet.
“We will have a significant amount of flood capacity and storage space if we have a storm later in the year with a lot of rainfall,” Ledesma said.
Since 1968, Lake Oroville’s Oct. 31 surface elevation has averaged about 812 feet, though it has dipped below 700 feet on five occasions during dry years, according to a spreadsheet provided by the DWR. Last year’s Oct. 31 surface elevation was about 737 feet, up from 665 feet a year earlier.