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Oroville Spillway Final Report: 50 Years Of Inadequate Action

CA - DWR / Twitter

CA - DWR / Twitter

The final report on last February's failure of the Oroville Dam spillway and resulting overtopping of the emergency spillway says there is plenty of blame to go around. 

A 584-page report by the Independent Forensic Team says a combination of poor design, complacent management, and insufficient response to the spillway's failure contributed to a scenario in which 180,000 people were ordered to evacuate.

The team's report says the California Department of Water Resources was "somewhat overconfident and complacent regarding the integrity of its civil infrastructure." Investigators also blame regulatory and industry practices.

This was in spite of the condition of the rock at the main spillway site. It was identified as "poor" by geologists before the chute was completed in 1968.

Investigators say the design and construction process failed to make proper modifications to fit the site conditions.

The report says the concrete slab cracked almost immediately above and below the drain pipes after it was built in 1968. The cracks and resulting "underdrain flows" were "quickly deemed to be 'normal.'"

The report says the contractor on the project had never built a spillway before.

Although there were frequent repairs made during the next five decades, they did not address the erosion under the slab that was occurring.

In February 2017, investigators say water found its way through cracks and joints in the main spillway chute slab. The water caused an "unexpected" amount of erosion and put enough pressure from underneath the concrete that a rupture occurred.

Investigators also say the Department of Water Resources then failed to have a "full understanding" of the consequences that would come from halting or limiting water releases from Lake Oroville after the damage to the spillway was identified.

The report says flows were limited to protect the powerhouse at the base of the spillway even though the threat to the powerhouse was "clearly diminishing."

The overtopping of the emergency spillway weir lead to "severe and rapid erosion downstream of the weir and, ultimately, the evacuation order."

The report says geologists' reports in the early 1960's showed part of the area below the emergency spillway was not made of hard rock and was prone to erosion. Somehow, the department came to accept as fact that the area below the emergency spillway was all solid rock, covered by three-to-four feet of earth.

The Department of Water Resources issued a statement that says, in part, "We will carefully assess this report, share it with the entire dam safety community and incorporate the lessons learned going forward." 

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