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What went wrong at Oroville?

Dam experts release a report on the California dam’s spillway vulnerabilities.


After intense rainstorms in February 2017, two spillways at Northern California’s Oroville Dam began to disintegrate under the force of the water rushing down them. Officials, concerned that erosion might undermine the side of the reservoir itself, evacuated more than 180,000 people. The reservoir ultimately held, but the spillways required extensive, ongoing reconstruction.

In early January, an independent group of dam experts tasked with determining what went wrong released its final report. It describes the physical factors that caused the spillways to crumble, problems like unstable rock foundations and cracked concrete that allowed water beneath the main spillway.

The report also highlighted human and organizational errors, including faulty design details, shortsighted inspections and “overconfident and complacent” management by the California Department of Water Resources. The department “take(s) the findings very seriously,” said then-director Grant Davis in a statement. “We will … incorporate the lessons learned going forward.”

So what can Western dam owners and regulators learn from Oroville? One lesson, according to the report, is simply to pay more attention to the dangers of deteriorating spillways, which can be overlooked during dam-centric safety assessments. Dam failures are clearly catastrophic, but other emergencies — like a spillway collapse — can also be dangerous and expensive.

Another lesson is the need for inspections that go beyond mere physical checkups. For example, reviews of original design records are necessary to help inspectors determine whether spillways are too outdated to be safe. And those reviews should be independent, not cut-and-paste insertions from previous reports. That could help prevent errors from propagating, an issue at Oroville that led officials to underestimate the risks of spillway erosion.

Oroville’s spillways failed despite state oversight, regulation by a federal agency and repeated evaluations by outside inspectors. That, the report notes, “is a wake-up call for everyone involved in dam safety.”

Hover on the numbers on the photo below to see some of the report’s conclusions.

Emily Benson is an editorial fellow at High Country News.