The preliminaries are just about over. Permanent structural repairs are about to begin at Oroville Dam.
Five months after an unprecedented emergency forced a mass evacuation, state officials said Wednesday they’re ready to start replacing the now-demolished lower portions of Oroville’s main flood-control spillway.
Construction contractor Kiewit Corp. plans to install the first slabs of concrete Thursday in the lower 350 feet of the spillway chute. The so-called “leveling concrete” will create a smooth foundation for structural concrete that will be poured on top, said Jeff Petersen, Kiewit’s Oroville project director.
“It’s significant,” he said on a media conference call. “We call it a major milestone of permanent work starting in that portion of the spillway.” Leveling concrete has been placed elsewhere on the chute, but in smaller amounts, he said.
There have been some hiccups, including a temporary halt to demolition on a segment closer to the top of the chute to review quality control issues, said Jeanne Kuttel, incident commander at the state Department of Water Resources. Nonetheless, demolition will resume soon, that phase of the project is nearly complete and the overall reconstruction at Oroville remains on schedule, officials said.
DWR officials have sketched out a two-year timeline for repairs.
Kiewit, which was awarded the $275 million contract, will completely replace the lower portion of the spillway this year. It will also make improvements to the adjacent emergency spillway, including fortifications in the hillside below the structure to limit erosion. DWR officials said they believe those repairs will be enough to get through the upcoming rainy season, which begins in November.
Next year Kiewit will complete the project by replacing the 1,000-foot upper portion of the main spillway. The upper portion escaped damage in the February incident, but DWR officials said they want an entirely new spillway anyway. An outside team of forensic investigators has said longstanding flaws might have caused the February failure of the main spillway, including crucial inconsistencies in the thickness of the concrete slabs and an inadequate drainage system beneath the structure.
State officials have said they expect the Federal Emergency Management Agency to reimburse the state for the bulk of the repairs, and state water contractors to pick up the rest. The contractors are the urban and agricultural agencies, such as the Metropolitan Water District of Southern California, that store water at Lake Oroville.
The Oroville crisis began when a giant crater sprouted in the main spillway Feb. 7. Dam operators limited water releases through the spillway’s gates to contain the damage as a massive storm rolled in. Lake levels rose so high that on Feb. 11, water flowed over the emergency spillway – an ungated concrete lip atop an unlined hillside – for the first time since the dam opened in 1968. When the hillside began eroding badly, jeopardizing the integrity of the emergency spillway, officials ordered the evacuation of 188,000 downstream residents. Officials then dramatically ramped up water releases from the main spillway, reducing lake levels enough so residents could return home.
The repair project got under way May 20, when Kiewit began demolishing what was left of the battered lower portion of the main spillway.