I just received this update on the Oroville Dam spillway hole from the Department of Water Resources:
SACRAMENTO – While there is still a chance the emergency spillway at Lake Oroville may be used this weekend, reservoir operators say that if current releases from the nearly full reservoir can be maintained, it is less likely the lake will rise to the point where water flows into the emergency spillway channel.
Whether the emergency spillway is used or not, Oroville Dam itself is sound and there is no imminent threat to the public, said California Department of Water Resources (DWR) Acting Director William Croyle.
“We’re ready to use the emergency spillway if needed,” he said. “But we’re trying to avoid it because there will be sediment and debris impacts downstream.”
As storm runoff into Lake Oroville began to wane late Thursday, operators boosted releases from 40,000 cubic feet per second (cfs) to 65,000 cfs. Releases from the nearly full reservoir are hampered by erosion damage to the main spillway, but DWR officials said Friday that if the 65,000 cfs discharge can be maintained, use of the emergency spillway may be avoided.
If the reservoir does rise enough to tip overflow into the emergency spillway channel, that likely would happen Saturday. In that event, DWR will give the public as much notice as possible. DWR does not expect any emergency spillway discharge from the reservoir to exceed the capacity of any channel downstream as the water flows through the Feather River, into the Sacramento River, and into the San Francisco Bay.
The emergency spillway has not been used in Oroville Dam’s 48-year history, but Lake Oroville came within a foot of spilling into it in January 1997.
Reservoir operators were using the main, gated spillway at the dam on Tuesday when the lower portion of the concrete chute began to erode. DWR shut off flows temporarily to investigate but has resumed using the gated spillway channel in order to try to avoid having the lake rise to elevation 901 feet above sea level, the point at which water naturally flows over a concrete weir and down a wooded hillside. That hillside is being rapidly cleared of trees and brush to minimize the amount of debris that would flow into the Feather River in the event of an emergency spill. DWR also is reinforcing the emergency spill corridor to minimize erosion.
Thursday’s storm arrived warmer, wetter and stayed longer than anticipated. To help handle inflow, reservoir operators upped releases down the damaged spillway overnight Thursday from 40,000 cubic feet per second (cfs) to 65,000 cfs. The higher flows further eroded the gated spillway channel, but not to a point that would compromise the integrity of the dam upstream. DWR and many federal and state partners are monitoring the spillway erosion carefully and are prepared to ramp down flows if necessary.
In addition, DWR crews are working to restore the use of the Hyatt power plant at the dam, which would allow operators to release up to an additional 14,000 cfs. Flows out of the power plant were halted Thursday evening because debris downstream of the damaged spillway had caused water to back up in a diversion pool, and the elevated levels affect the ability of DWR to operate the power plant.
Current inflow to the lake is approximately 140,000 cfs and lake elevation is 892 feet, with approximately 139,000 acre-feet of vacant space in the reservoir.
DWR is coordinating closely with state and federal wildlife and dam safety officials as it responds to the gated spillway erosion and manages reservoir operations. Those involved in contingency planning and response include the Federal Energy Regulatory Commission, the U.S. Army Corps of Engineers, Butte County Sheriff’s Office, the Governor’s Office of Emergency Services, the state’s Division of Safety of Dams, CAL FIRE and state and federal wildlife agencies.
Lake conditions, including lake levels, inflows, and outflows can be obtained via a recorded message at 530-534-2307. More information is available here at http://cdec.water.ca.gov/cdecapp/resapp/resDetailOrig.action?resid=ORO.
Ted Thomas, Information Officer – (916) 653-9712