SACRAMENTO, Calif. (CN) – Decades of human error, and design and maintenance mistakes on the nation’s tallest dam culminated in the failure of its spillways during California’s history-making rains in 2017, according to a report released Friday by investigators.
The panel of independent forensic investigators found that while the crisis at Oroville Dam couldn’t “reasonably be blamed” on a specific agency or decision, the near-calamity – which forced nearly 190,000 to flee their homes last February – was caused by “long-term systemic failure” within the California Department of Water Resources.
“There was no single root cause of the Oroville Dam spillway incident, nor was there a simple chain of events that led to the failure of the service spillway chute slab, the subsequent overtopping of the emergency spillway crest structure and the necessity of the evacuation order,” the 584-page report concludes.
For decades both state and federal inspectors performed flawed visual inspections and missed indicators that water was seeping below the spillway surface and eroding concrete, the investigators said.
The panel found the state’s original design and construction plans were flawed and that dam’s main spillway, first used in 1969, was ripe for deterioration. Furthermore, the hulking 770-foot dam's spillway was built on poor foundation which eroded during previous spillway repairs.
“The seriousness of the weak as-constructed conditions and lack of repair durability was not recognized during numerous inspections and review processes over the almost 50-year history of the project,” the reports states.
The mounting erosion came to the surface on Feb. 7, 2017, after heightened discharges punctured a hole 200 feet long and 30 feet deep in the spillway. But as another series of heavy storms approached from the Pacific Ocean, the department was forced to continue using the battered spillway.
Burdened by a deficient and obliterated main spillway that couldn’t keep up with the reservoir’s rapidly rising water, officials sent water over the concrete lip of the emergency spillway for the first time in the dam’s history. State and county officials worried that the untested emergency spillway could give out and send a 30-foot wall of water crashing below the dam.
“Officials are anticipating a failure of the auxiliary spillway at Oroville Dam within the next 60 minutes,” the department said in an evacuation order. “Residents of Oroville should evacuate in a northward direction.”
In the end, the department decided to continue using the broken main spillway throughout the rainy season and evacuation orders were lifted.
The final report by six investigators with Members of the Association of State Dam Safety Officials and the U.S. Society on Dams was highly critical of the state’s Department of Water Resources. It calls the department “somewhat insular” and questions its decision to slow releases on the main spillway during the incident and use the untested emergency spillway.
“The decisions were made with the best of intentions, but against the advice of civil engineering and geological personnel, who had by then recognized the poor bedrock conditions and the potential for unsatisfactory performance of the previously untested emergency spillway,” the panel wrote.
The department said a new evaluation of the dam is underway and that it will incorporate the report’s findings.
“During the incident, our sole focus was protecting public safety,” State Water Project deputy director Joel Ledesma said in a statement. “[The department] supported this independent assessment- so we can learn from the past and continue to improve now and into the future.”
The state spent an estimated $500 million this past summer to repair the main spillway, which is now fully operational. Oroville is 69 miles north of Sacramento and 23 miles southeast of Chico.
Republican State Sen. Jim Nielsen, who represents Oroville, said the report “confirmed his suspicions” about the department. He has asked federal regulators to delay issuing a new operating permit for the dam.
“We have a sustained neglect of addressing problems that should have been anticipated such as the deficiency of the geological composition on which the spillway was originally laid,” Nielsen said in a statement. “The department must do better for our community.”
The investigation was commissioned by the Federal Energy Regulatory Commission in wake of the spillway incidents, and preliminary findings were released in May and September 2017. The panel interviewed 75 people involved with dam operation in various capacities.
Investigators warned other dam owners and operators should learn from the prior mismanagement of Oroville Dam.
“The fact that this incident happened to the owner of the tallest dam in the United States, under regulation of a federal agency, with repeated evaluation by reputable outside consultants, in a state with a leading dam safety regulatory program, is a wake-up call for everyone involved in dam safety,” the report states.
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