In a 584-page dissection of the disaster at America’s tallest dam, the investigative team said Oroville Dam was designed and built with flaws from the beginning, which were exacerbated by inadequate repairs in the years that followed.
“Due to the unrecognized inherent vulnerability of the design and as-constructed conditions and the chute slab deterioration, the spillway chute slab failure, although inevitable, was unexpected,” the panel reported.
The panel also said the Department of Water Resources, which runs the dam, has been “somewhat overconfident and complacent” and gave “inadequate priority for dam safety.” At the same time, the investigators said the entire dam industry, including federal regulators who oversee the facility’s operations, needs to heed the lessons learned at Oroville.
“Although the practice of dam safety has certainly improved since the 1970s, 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 panel wrote. “Challenging current assumptions on what constitutes ‘best practice’ in our industry is overdue.”
Made up of six independent engineering consultants from around the country, the investigative team said there was “no single root cause” for the crisis, which began Feb. 7. Rather, the panel pointed to a “complex interaction of relatively common physical, human, organizational and industry factors.” The giant crater that erupted in the concrete chute set off a slow-motion emergency that culminated five days later with the evacuation of 188,000 downstream residents.
Problems emerged almost immediately after the dam, the linchpin of the State Water Project, opened in 1968. The concrete chute slab cracked above and along underdrain pipes, leading to higher-than-expected flows of water directly beneath the chute, the panel wrote.
“The slab cracking and underdrain flows, although originally thought of as unusual, were quickly deemed to be ‘normal,’ and as simply requiring on-going repairs,” the panel wrote. “However, repeated repairs were ineffective and possibly detrimental.”
The 3,000-foot-long chute deteriorated over the years, with its steel rebar and anchors corroding. The chute was also plagued by “poor foundation conditions” directly beneath the concrete, which were documented in geological reports but weren’t properly addressed in the design and construction of the spillway, the panel wrote.
All these factors came together Feb. 7, as the dam was releasing water down the spillway to lower reservoir levels during a massive rainstorm. Water that crept into the cracks and joints of the concrete chute resulted in “uplift forces beneath the slab,” causing the spillway to erupt.
The panel’s findings are in line with its interim reports, although the final report ventures into new territory by criticizing Department of Water Resources officials for their handling of the first few days of the emergency once the initial fracture appeared in the main spillway.
After the crater formed, dam operators decided to curtail water releases on the chute the next few days in an effort to limit the damage. Water levels at Lake Oroville, the state’s second largest reservoir, rose so high that water started pouring over the nearby emergency spillway, which consists of a concrete lip sitting on an unlined hillside, for the first time since the dam opened.
The forensic team said the DWR dam operators didn’t realize just how risky it was to let water pour over the 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 forensic team wrote.
One day after water started running over the emergency structure, the hillside had eroded so badly that dam officials feared the lip of the emergency spillway would crumble, releasing a “wall of water” on communities below the dam. That necessitated the evacuation. Faced with imminent disaster, dam operators then dramatically ramped up water releases over the main spillway, which lowered lake levels to the point that water stopped flowing over the emergency spillway.
The evacuation ended two days later, although the heavy water releases over the main spillway turned the initial fracture into a massive canyon that would take months to fix.
The panel had other criticisms for DWR, calling it a “somewhat insular organization” that hasn’t tapped “industry knowledge” to improve its “technical expertise.”
“Lake many other large dam owners, DWR has been somewhat overconfident and complacent regarding the integrity of its civil infrastructure and has tended to emphasize shorter-term operational considerations,” the panel added.
The report was a blow for an agency that has built a reputation for diligently policing California’s dams. An audit in 2016 by the Association of State Dam Safety Officials concluded that California operates “the leading dam safety program in the nation.” Yet a Sacramento Bee investigation last fall showed that dam owners are allowed to let flaws and deficiencies go unrepaired for years, even after repeated notifications from DWR’s inspectors.
The independent investigation was ordered by the Federal Energy Regulatory Commission, which licenses Oroville Dam. State officials said they welcomed the investigation and incorporated its preliminary findings into the massive repair project that has been underway since last spring.
DWR Director Grant Davis, in a prepared statement, said “we will carefully assess this report, share it with the entire dam safety community and incorporate the lessons learned going forward to ensure California continues to lead the nation on dam safety.”
The flood control spillway has been largely repaired in time for the rainy season, although portions of the chute have been patched or partially replaced and are awaiting further repairs later in 2018.
The total emergency, including the repairs, is expected to cost around $500 million or more. State officials are expecting the Federal Emergency Management Agency to pick up most of the expense. Any leftover costs are to be covered by the water agencies that store water behind Oroville Dam, such as the Metropolitan Water District of Southern California.
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