California's Department of Water Resources (DWR) has posted an update for the Lake Oroville Spillways Emergency Recovery Project construction activities.
The 2018 Lake Oroville Operations Plan hopes to begin construction around May 8 at which time the main spillway gates will be closed. The May 8 date is fluid but the DWR hopes to maximize the 2018 construction window and ensure the main spillway is fully reconstructed before next winter. The plan was submitted this week to the Federal Energy Regulatory Commission (FERC) and the California Division of Safety of Dams (DSOD) for approval.
Before the gates are closed, prep work to resurface, or mill, the top layer of the roller-compacted concrete (RCC) middle chute is targeted to begin on April 25 to create a uniform surface to prepare for placement of steel-reinforced structural concrete slabs. Crews will remove up to five inches of the surface of the RCC section of the chute.
On that same date, DWR hopes to begin removal of the remaining surface layer of the grouted rip rap on the emergency spillway hillside. This is the rip rap that was placed as part of the February 2017 emergency response.
Crews are currently placing a concrete cap, or grade beam, on the recently completed underground secant pile cutoff wall. The cap will reinforce the structural concrete piles and secure the wall to the RCC splashpad. The secant pile wall is 1,450 feet long and located 750 feet downhill of the emergency spillway with concrete piles at depths of 35 to 65 feet.
Crews continue to construct the RCC splashpad, which will cover the hillside between the emergency spillway and secant pile wall. The splashpad, in conjunction with the secant pile wall, will armor the existing hillside to significantly reduce the type of uphill erosion that occurred during the February 2017 incident.
Inflows into Lake Oroville are expected to be low this summer due to below-average snowpack and snow water content in the Northern Sierra, so the updated plan targets a lake level of approximately 830 feet before triggering more aggressive outflows.
As of mid-April, the lake was at 808 feet. The DWR expects lake levels to fluctuate through the year as the agency accommodates various uses, including providing water for the 29 State Water Project contractors and senior water rights holders. Other uses include flood protection, environmental releases, recreation and salinity control and flow requirements in the Sacramento-San Joaquin River Delta, officials explain. The operations plan will enable the agency to “maximize our construction window,” says John Leahigh, a State Water Project principal engineer.
Construction plans for the Main Spillway and Emergency Spillway
- Demolition of the original 730 feet of the upper chute leading to the radial gates and reconstruction with steel-reinforced structural concrete slabs and walls.
- Placement of three-foot, steel-reinforced structural concrete slabs over the RCC middle chute, and placement of a drainage system.
- Removal of the RCC walls in the middle chute and replacement with structural concrete walls, with a permanent drainage system.
- Hydro-blasting and resurfacing of the energy dissipaters at the base of the main spillway.
- Later this year, an RCC buttress will be constructed at the base of the emergency spillway structure to provide further reinforcement.
Over the next four to six weeks, DWR will be conducting routine maintenance on three of the six turbines (turbines four, five, and six) at Hyatt Powerplant to ensure the performance of Hyatt Powerplant throughout the construction season when the main spillway will be unavailable.
DWR had planned to begin this maintenance at the beginning of April but delayed the work to provide full outflow capacity from Hyatt Powerplant to safely manage lake levels during recent storms. Turbine one is undergoing an extensive upgrade and has been offline since 2015. Work on turbine one is targeted for completion by the end of this year.
With two turbines active, Hyatt Powerplant has an outflow capacity of 5,000 cubic-feet per second.
Lake Oroville is the chief reservoir for the State Water Project, whose contractors irrigate about 750,000 acres of Central Valley farmland and serve more than 26 million customers.The dam’s near-failure amid heavy storms in February, 2017 prompted the reconstruction project that is expected to take two years and cost $870 million.