State water official vows new Oroville Dam spillway by winter

California’s top water manager said Monday that the problem-plagued Oroville Reservoir will have a new spillway in place to prevent potentially dangerous outflows of water in time for the next rainy season.

The pledge follows concern that the reservoir’s concrete main spillway, which fractured in February, would require more than the dry summer and fall months to fix because of the extent of damage. A panel of dam experts hired by the state said two years might be needed for the work and warned that without substantial repairs by November the situation would pose “very significant risk.”

The state is looking for a way through the crisis that unfolded nearly two months ago. The main spillway at the nation’s tallest dam partially collapsed amid heavy outflows, prompting dam managers to rely on an emergency spillway that sent water over a bare hillside. As the hillside quickly eroded, fears that unconstrained water would pour out of the lake prompted nearly 200,000 temporary evacuations downstream.

In this Feb. 28, 2017, file photo, construction crews clear rocks away from Oroville Dam's crippled spillway in Oroville, Calif. In this Feb. 28, 2017, file photo, construction crews clear rocks away from Oroville Dam's crippled spillway in Oroville, Calif. Photo: Rich Pedroncelli, Associated Press Photo: Rich Pedroncelli, Associated Press Image 1 of / 30 Caption Close State water official vows new Oroville Dam spillway by winter 1 / 30 Back to Gallery

“If I have anything to say about it, we’ll have a spillway to use by Nov. 1,” said Bill Croyle, acting director of the California Department of Water Resources, addressing reporters Monday morning. “Whether that’s a permanent or temporary structure is still to be determined.”

Croyle said a plan for the rebuild would be unveiled this week or early next week. As for what exactly the new chute will look like, he said all options are on the table.

He acknowledged that the timetable for restoring the roughly 3,000-foot-long spillway so that it can safely release water is tight.

After a design concept is chosen, bids for the work must be submitted, a contractor selected and a final plan agreed upon — all before construction begins.

A memo this month from four engineering consultants working with the state identified, to the greatest extent yet, the problems experienced at the broken spillway.

Although the cause of the fracture is yet to be determined, the panel found that water is seeping up from the ground beneath the chute, that the structure is too thin in many places to support outflows, and that the earth that holds up the spillway is riddled with empty space.

The consultants concluded that the spillway could not likely be replaced this year, though they said enough fixes could be completed to make it safe and functional by winter.

“A very significant risk would be incurred if the gated spillway is not operational by Nov. 1,” the memo reads.

To get through the current wet season, dam managers have been releasing limited amounts of water down the mangled main spillway while hoping it doesn’t further degrade. They’re hoping that snowmelt from the mountains above the lake is gradual so reservoir outflows can remain slow and steady.

State officials do not want to have to rely on the emergency spillway. However, construction crews have armored it with concrete just in case.

On Monday, dam managers were working to halt all flows down the main spillway after 10 days of sustained releases. The reservoir level stood at about 837 feet above sea level, well below the 860-foot mark that the state sees as the maximum at which Lake Oroville can safely absorb upcoming snowmelt.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander