A damaged flood control spillway at the Oroville Dam may have to be used as early as next week as storm runoff and snowmelt continue to fill the massive reservoir on the Feather River, state water officials said.
The spillway has been dry since Feb. 27, when engineers with the Department of Water Resources rapidly reduced the flow of water down the concrete chute from 50,000 cubic feet per second to zero so they could repair the spillway and restart a nearby hydroelectric plant.
But since that shutdown, the water level in the reservoir has climbed 21 feet, even though the power station began operating last week. The reservoir's water level was at 860 feet Friday, 40 feet below capacity and only 5 feet shy of the threshold at which engineers have said they'll want to use the spillway again.
But using the spillway in its current state carries risks, officials said.
Engineers discovered last month that if they allow water to flow down the spillway too slowly, it erodes the earth beneath the structure, further compromising the spillway and possibly, eventually, the dam.
Crews have spent the last week filling the area below the damaged section with rocks and concrete slurry to prevent further erosion, but officials couldn't say Thursday if the spillway could handle reduced flows.
Instead, they'll only use the spillway when water can be released at upward of 40,000 cubic feet per second, Department of Water Resources spokeswoman Lauren Bisnett said. The spillway's limited capabilities mean that in the short term, engineers will continue to operate the reservoir like a yo-yo, allowing it to fill up before opening the spillway gates and releasing massive amounts of water, then closing them and repeating the process when it fills up again, officials said.
On Thursday, Butte County Sheriff Kory Honea revealed a new warning and evacuation plan for the county's residents, many of whom were forced to flee to high ground with only an hour's notice last month when state officials feared a portion of the reservoir's emergency spillway was nearing failure.
"Our goal is to give people 12 to 24 hours' notice" and reduce choke points for fleeing residents, Honea said.
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3:50 p.m.: This article has been updated with more current numbers