Forecasted storms expected in the Feather River basin this weekend may require using Lake Oroville’s flood control outlet spillway (also known as the main spillway) this week or next.
After last year’s spillway incident, the Department created the 2017/18 Lake Oroville Winter Operations Plan to ensure public safety in the event of major storm events. This plan triggers more aggressive outflow from Hyatt Powerplant and potential use of the main spillway should the reservoir’s elevation reach 830 feet during the month of April. The current forecasts show the potential for inflows to raise the reservoir to near the 830-foot trigger elevation by the middle of next week. Currently, the lake elevation is 794 feet.
In anticipation of the incoming weather, DWR began increasing outflows today, April 3, from Hyatt Powerplant to approximately 10,000 cfs. The total capacity of outflows from Hyatt Powerplant is currently 12,500 cfs. If necessary, DWR also has use of the River Valve Outlet System which has an additional maximum outflow capacity of 4,000 cfs.
DWR’s objective for the year has been to minimize use of the main spillway while it is still under construction. However, because forecasts are uncertain, DWR is taking proactive steps such as early notification to downstream communities, regulatory agencies and construction crews to prepare for possible use of the main spillway next week in the event the lake level reaches an elevation of 830 feet, even after increasing outflows through Hyatt Powerplant.
Phase I of the reconstruction of the main spillway was completed in November 2017 to handle outflows of 100,000 cfs. The design of the partially reconstructed spillway was approved by the Federal Energy Regulatory Commission, Division of Safety of Dams and an Independent Board of Consultants that has been overseeing reconstruction. Phase II of construction on the main spillway will begin in May, depending on weather.
Phase I included repair and replacement of the upper chute and end of the chute with structural concrete, and construction of a temporary roller-compacted concrete (RCC) section in the middle chute. Since the surface finish of the temporary RCC section is not as smooth as the structural concrete sections, the flow may be more turbulent than in the upper chute. Additional wear may also occur to the surface of the temporary RCC. Phase I construction included structural concrete cutoff walls at the connections between the structural sections and RCC section as measures to protect against possible wear of the RCC.
If the main spillway is used, inspectors will be closely monitoring the chute and flows.
DWR has notified the following groups about the increase in outflows and potential use of the spillway:
State and federal regulatory agencies
Local and statewide public safety organizations including the Butte County Sheriff’s office
Partner state agencies
Local, state and federal elected officials
Community members and stakeholders
Because the forecasts are dynamic, they could change in magnitude and timing. DWR will provide regular communication to the community, stakeholders and the media about forecasts and their impact to Oroville operations over the coming days as the storms approach.