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Independent forensic investigators released their final report Friday about the spillway emergency, which was very close to a catastrophe, in February at the Oroville Dam in the Sierra Nevada foothills east of the Sacramento Valley. The report cited a “long-term systemic failure” at the California Department of Water Resources.
The 584-page report about the failure at the nation's tallest dam asserted that the structure was flawed from the beginning in design and construction in the 1960s, a situation made worse in the following decades by inadequate repairs.
“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 report said.
The six independent engineering consultants from around the country who formed the investigative team that wrote the report said there was no single root cause for the incident, which began Feb. 7 as California experienced a particularly wet season.
Instead, the consultants pointed to a “complex interaction of relatively common physical, human, organizational and industry factors” that precipitated the disaster. The giant crater that erupted in the dam's concrete chute resulted in the evacuation of 188,000 residents downstream on Feb. 12.
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The 3,000-foot-long chute had steel rebar and anchors corrosion almost from the get-go. It was also characterized by poor foundation conditions directly beneath the concrete, the engineers wrote.
All that came to a head Feb. 7 when the dam was releasing water down the spillway during a major rainstorm. Water crept into the cracks and joints of the concrete chute, causing the spillway to erupt.
The investigators said the entire dam industry, including federal regulators, needs to learn from what happened 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 report said.
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