Based on information received from the California Department of Water Resources (DWR) and the incident command team managing Lake Oroville, counties and cities near Lake Oroville and the surrounding area issued evacuation orders for residents. The concern is that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville. Those potential flows could exceed the capacity of downstream channels.
To avert more erosion at the top of the auxiliary spillway, DWR doubled the flow down its main spillway from 55,000 cubic feet per second (cfs) to 100,000 cfs. The next several hours will be crucial in determining whether the concrete structure at the head of the auxiliary spillway remains intact and prevents larger, uncontrolled flows.
Current flows are contained with downstream channels.
Flow over the auxiliary spillway weir began Saturday morning and has slowed considerably. DWR officials expect that flow to stop entirely soon, which will reduce the erosion on the downstream side of the structure.
Oroville Dam itself is sound and is a separate structure from the auxiliary spillway.
Evacuation ordered for Oroville as dam spillway expected to fail
From the San Francisco Chronicle:
“Butte County residents near Lake Oroville, including the entire town of Oroville and nearby regions, were ordered to evacuate Sunday evening after the emergency spillway next to the reservoir’s dam suffered a possible structural failure, officials said. “There has been severe erosion of the emergency spillway and a possible structural breach that could send uncontrolled water down the stream,” said Chris Orrock, a spokesman with the California Department of Water Resources. Residents downstream from Lake Oroville to the Sutter County line were under mandatory evacuation order. Counties around the reservoir, the second largest in the state, down to Sacramento were warned about the possibility of flooding. ... ” Read more from the San Francisco Chronicle here: Evacuation ordered for Oroville as dam spillway expected to fail
The latest pictures from Oroville …
The heavy flows from the 3,000-foot main Oroville dam spillway continues to push debris into the turbid Feather River as the concrete span further eroded and the jagged fracture in its midsection sends water flowing over an adjacent hillside.
The volume of water is expected to pose no flood threat downstream and should remain within the capacity of the Feather River and other channels to handle. Oroville Dam in Butte County itself remains safe with no imminent threat to the public.
Florence Low / California Department of Water Resources