Dam failure informs protocol

Forensic team finds physical problems with Oroville spillway, offers advice for future

Workers access preliminary damage to Oroville Dam spillway in February.

Workers access preliminary damage to Oroville Dam spillway in February.

Photocourtesy of dwr

While human error or negligence haven’t been ruled out when it comes to the Oroville Dam spillway failure in February, one thing is for certain: Decades-old technology and infrastructure certainly played a part. More significantly, the widely accepted inspection process was lacking in at least one key area.

That’s according to an interim report issued Tuesday (Sept. 5) by the Oroville Dam Spillway Incident Independent Forensic Team, a panel of engineering and other experts that convened in May to study the failure—independently of the Department of Water Resources, which owns the dam. After submitting the results to DWR Tuesday, the Association of Dam Safety Officials and the U.S. Society on Dams moderated a press briefing with team leader John France.

“We’re at a point where we’ve nearly completed [investigating] the physical factors,” France explained during the briefing. “Regarding human and organization factors, it’s still work in progress.”

Perhaps the most significant conclusion the team has reached thus far was that no review of the original dam design and construction had been conducted since the dam was built nearly 50 years ago. “Physical inspections, while necessary, are not sufficient to identify risks and manage safety,” the report reads.

“Any profession, any practice like this, has ways it goes about doing business,” France said. “Sometimes we learn how to improve the way we do things when things don’t go well.

“Some things you can’t identify by physical inspection alone,” he continued. “We’re always learning how to design and do things better. They may have done what others were doing at the time, but we know better now. We need to look at the difference between what was built in 1960 and what we do today—is it significant?”

Based on this finding, the forensic team is recommending a protocol change across the board when it comes to safety inspections: In addition to taking physical stock of the structure, officials also should conduct thorough, critical reviews of original design and construction documents.

“Such a review would likely have ‘connected the dots’ and informed the [evaluation] process, by identifying the physical factors that led to failure of the service spillway chute …,” the report reads. It goes on to suggest that a review of original designs may have indicated potential problems with drain flows and “subsurface geologic conditions that left portions of the spillway susceptible to uplift and subsequent foundation erosion.”

Uplift, it turns out, is the most likely cause of the spillway failure, according to the report. Even a small portion—how big is unknown because most of the original material has since washed away, France said—of a spillway slab that lifted above the rest could allow water to rush underneath, causing the massive erosion seen in February.

“Water pressure developed underneath a section of the spillway chute,” France explained. In addition to that, the investigation revealed some major flaws in both the main and emergency spillways, including a lack of erosion protection. Whether that was a flaw in the original design or something overlooked during more recent inspections is still under investigation and will be included in the team’s final report, France explained.

The main reason the team released an interim report was because the Federal Energy Regulatory Commission and the California Division of Safety of Dams have required evaluations of all dams following the Oroville disaster. Out of 24 dams in Butte County, all but Oroville Dam—the condition of which is labeled “unsatisfactory”—are in “satisfactory” condition. However, three Butte County dams— Magalia, Paradise and Sly Creek—are among 93 in California that are in the process of in-depth spillway re-evaluations. The team wanted its results to inform those evaluations, France explained.

France said the public can expect the forensic team’s full report on the Oroville Dam spillway failure this fall.