OROVILLE – Crews are laying the last layer of concrete on the Oroville Dam spillway with one day until the state Department of Water Resources’ deadline to have the structure ready to pass flows of 100,000 cubic-feet per second, or cfs.

Over the last week, Kiewit Corp. construction workers connected the lower and upper chutes and erected the walls. Now all that is left is to place a coating of roller-compacted concrete in the middle chute “that will provide a stronger wearing surface,” said Erin Mellon, the department’s assistant director of public affairs.

The cost for reconstruction rang in at $500 million, nearly double the initial estimate.

Mellon said after reaching the Nov. 1 milestone to beat the rainy season, the focus will be on concrete seams, drain pipes, drain sealing and site clean-up. Work will also continue on the emergency spillway and the cutoff wall below it. DWR projects the underground wall will be completed in January. Its purpose is to prevent erosion, should the emergency spillway be utilized again.

When the emergency spillway was used for the first time in history on Feb. 12, intense erosion and headcutting led officials to order an evacuation for over 180,000 people, out of fear the structure would be compromised. The emergency spillway consists of a concrete weir above an unlined hillside, which water automatically flows down if the lake surpasses its capacity.

In 2018, DWR plans to lay structural concrete on the middle spillway chute and its walls and also fully reinforce the uppermost 730 feet. The walls in the central area now filled with roller-compacted concrete standing at 16 feet tall will be raised to 22 feet.

In its final form, the spillway should be able to handle 270,000 cfs flows.

DWR does not intend to use the spillway this year, instead relying on Hyatt Powerplant and ample storage space. The Federal Energy Regulatory Commission had DWR bring the lake down to 700 feet by Nov. 1. As of Monday evening, the water sat at 696 feet of its 901-feet capacity.

The target for releases from the Hyatt Powerplant this winter is 10,000 cfs, while one penstock and three turbines undergo maintenance. One of those turbines has not been operational since 2015.

Contact reporter Risa Johnson at 896-7763.

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