Less than nine months after two massive holes formed in Lake Oroville’s main spillway, construction crews wrapped up their first phase of rebuilding it.
Some local residents have expressed concerns that the quick turnover could result in faults or design flaws, but an official with the Department of Water Resources said if any crew can accomplish the feat, it would be Kiewit Infrastructure West Co.
“Kiewit Infrastructure is one of the leaders in dam construction,” said Erin Mellon, assistant director of public affairs for DWR. “The company recently finished the new auxiliary spillway at Folsom Dam before being awarded the contract for the Lake Oroville spillways.”
DWR awarded the contract to Kiewit in April with only 30 percent of the design completed. Mellon said that was necessary to begin work as quickly as possible before the rains came back later in the year.
Construction officially began in May, and the final design plans were completed and signed off on by federal regulators in July.
“In 165 days, DWR and Kiewit successfully met the Nov. 1 milestone of repairing and rebuilding Lake Oroville’s main spillway to handle flows of 100,000 (cubic feet of water per second),” Mellon said.
But with more than a month between the start of construction and the final design being approved, some Yuba-Sutter residents have questioned whether DWR and Kiewit covered all their bases in time.
Mellon said DWR engineers have addressed all of the factors laid out by an independent forensic team listing potential causes to the damage done to both the main spillway and emergency spillway, which is a total of 28 factors.
She said DWR is already working on a comprehensive needs assessment of the entire Lake Oroville facility, which should be completed near the end of 2019. The assessment will look at additional measures that can be taken at the reservoir to help prevent another event like the one experienced last February.
“DWR has stated publicly that all options are on the table for long-term additions at the Oroville Dam facilities,” Mellon said. “This includes a fully lined emergency spillway to the Thermalito Diversion Pool, a lower elevation outflow facility, or another auxiliary spillway.”
History of flood protection:
Though the main spillway and emergency spillway nearly failed last February, when Lake Oroville’s facilities function correctly the reservoir and dam provide downstream communities with flood protection.
“Theoretically, Oroville Dam has reduced flood risk to downstream communities during every wet year since 1968,” Mellon said. “Massive flood damage from years with high precipitation – 1986, 1997, 2017 – was significantly reduced because of the Oroville Dam. In 1964, a partially completed Oroville Dam saved downstream communities from one of the worst floods on record on the Feather River.”
Last February wasn’t the only time the reservoir’s facilities have experienced problems. In 2009, one of the Hyatt Powerplant river valves malfunctioned during testing, and in 2012, a fire damaged several parts of the Thermalito Pumping-Generating Plant located downstream, Mellon said.
It also wasn’t the first time the emergency spillway gave DWR and public safety officials a scare.
“In 1997, a non-mandatory evacuation was ordered for the city of Oroville on New Year’s Day as water came close to engaging the emergency spillway,” she said. “Fortunately, the emergency spillway was not used.”
However, the emergency spillway was used last February – for the first time in its history – and was the reason officials declared the mandatory evacuation, as the hillside under the spillway eroded.
Work still needed:
Once the first phase of construction was completed on Nov. 1, the second phase officially began. While major construction is on hold until the rainy season ends, Mellon said finishing work has already begun on drains, sealing joints and anchors on the main spillway, as well as some erosion mitigation at the job site.
Once heavy construction begins – most likely this spring – crews will begin repairing the uppermost portion of the chute, and will go back over the middle portion that received more of a temporary fix in 2017.
“In 2018, the existing 730 feet of the upper main spillway connected to the radial gates will be removed and rebuilt with structural concrete to final design,” Mellon said. “The roller-compacted concrete middle chute will be completed to final design with a 2.5-foot structural concrete overlay. The energy dissipaters at the bottoms of the main spillway will be hydro-blasted and resurfaced.”
Though the first phase of major construction on the main spillway finished up in November, work on the emergency spillway – primarily with the secant pile wall, or cut-off wall – has continued. DWR is planning to complete the wall, which is intended to help prevent uphill erosion from occurring, by this spring.
Also, crews removed bedrock between the secant pile wall and the top of the emergency spillway in late 2017 in preparation of the placement of a roller-compacted concrete buttress and splashpad, which is expected to be constructed in 2018.
“The splashpad will extend from the buttress about 750 feet down slope to the beginning of the underground secant pile wall,” Mellon said. “The buttress and splashpad will help to further prevent uphill erosion and bolster the emergency spillway.”
The entire reconstruction process of both spillways is expected to be completed by early 2019.