Hairline cracks have been detected in sections of the newly reconstructed flood-control spillway at Oroville Dam.
State and federal officials said they’re confident the cracks don’t pose a safety problem and don’t need to be repaired. However, the state Department of Water Resources, which operates the dam, said it will monitor the concrete surface of the spillway during the rainy season and beyond.
“The integrity of the newly reconstructed Oroville spillway is not in question,” said Erin Mellon, a DWR spokeswoman. She said the concrete will have no problem handling powerful flows should the spillway gates need to be opened this rainy season at the state’s second largest reservoir.
DWR hired Omaha-based contractor Kiewit Corp. to rebuild the spillway after it cracked in two last February, triggering an emergency that led to the evacuation of 188,000 downstream residents. Phase 1 of the project, in which much of the spillway was rebuilt from scratch, was completed Nov. 1, with DWR and Kiewit declaring the structure was ready for the rainy season. Next year Kiewit will complete the $500 million job by rebuilding the upper portion of the 3,000-foot-long spillway.
The cracks were first detected more than a month ago, when the first phase was nearing completion. In a letter Oct. 2 to state officials, engineers at the Federal Energy Regulatory Commission told DWR to investigate “cracking of the erosion resistant concrete” on the new spillway. FERC regulates the dam.
The state responded Nov. 7, telling FERC that “the presence of hairline cracks was anticipated and is not expected to affect the integrity of the slabs.”
FERC, in a Nov. 21 reply, said it agrees “with your assessment that the current condition of the hairline cracking does not warrant repair at this time. We understand that you will continue to monitor and evaluate the hairline cracks throughout the 2017 flood season and the 2018 construction season. We also understand that you will be evaluating a number of construction measures this winter that might be implemented next construction season that might reduce the likelihood of hairline cracking.”
Jeff Petersen, Kiewit’s project manager, told The Sacramento Bee that anchoring the slabs to the foundation beneath the spillway, a key design element intended to strengthen the overall structure, contributed to the cracking but doesn’t harm the integrity of the chute.
“The surface of the concrete is in excellent shape,” he said. “All concrete cures and shrinks and has cracks.”
State Assemblyman James Gallagher, whose district includes Oroville, said it’s too soon to determine whether the cracks are a significant problem. But he added, “Any kind of deficiencies should be taken seriously.”