Mobile users: see an interactive timeline here.

Oroville >> Repairs to the Oroville Dam spillway are on track for the Nov. 1 deadline, state Department of Water Resources representatives say, but work will be far from over then.

The November deadline was set in the hopes of beating the start of the area’s typical rainy season. The spillway will be functional by then, able to pass flows of 100,000 cubic-feet per second, or cfs, according to DWR’s plans, but the structure will have a higher capacity when the redesign is complete.

Erin Mellon, DWR’s assistant director of public affairs, said in an email Friday there are two critical components that need to be finished in order to allow for 100,000 cfs flows, including the upper and middle portions of the chute. Specifically, 750 feet in the upper region, beginning 730 feet down from the very top of the structure, needs to be lined with structural concrete. Another 1050 feet in the middle, where there was vast erosion, needs to be filled with roller-compacted concrete, or RCC.

About 100,000 cubic yards of RCC has gone into the void, making it about one-third full or complete. Smoothing out the remaining erosion surrounding the structure is not a priority for this year, Mellon said.

“There will be some backfill work on the hillside on either side of the spillway chute but it’s not necessary by Nov. 1 and will continue throughout this year and next,” she wrote. “It’s not necessary for safe operation of the chute.”

Under DWR’s direction, the contractor Kiewit has left in the original concrete of the top 730 feet of the spillway intact, save for some smaller repairs. The bottom 350 feet of the structure was blown up and replaced with structural concrete.

In May, department representatives expressed three main goals to reach by Nov. 1: replacing the lower and middle portions of the spillway – which appears to be on schedule – and also installing a cutoff wall at the bottom of the emergency spillway. In a press release in August, officials announced they would be unable to complete the last goal by the deadline and were now aiming for December or January.

The purpose of the cutoff wall is to keep the dam safe should the department have to use the emergency spillway again. What caused Butte County Sheriff Kory Honea to make the evacuation order in February was fear that rapid erosion toward the concrete weir at the top could cause it to fail and release a torrent of water.

The emergency spillway – an unlined hillside that had never been used before – eroded far worse than DWR had expected.

More repairs at the dam are planned for next year, including adding a “splashpad,” lining the emergency spillway hillside with concrete.

The department will also demolish the top 730 feet of the main spillway, replace it with structural concrete and add structural concrete to the middle portion of the spillway, where RCC is going this year.

Reach reporter Risa Johnson at 896-7763.