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Dam Memo: More than Visual Inspections Needed

Wednesday, September 13, 2017

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An interim memorandum from the independent forensic team looking into February’s incident at California’s Oroville Dam indicates that corrosion and erosion played a role in the concrete slab’s failure, and that the California Department of Water Resources likely relied too heavily on physical inspections of the spillway, where other methods would have “connected the dots” and helped to predict the failure.

The memo, released Sept. 5, explains the team’s current view of what led to the failure of the dam’s main spillway on Feb. 7: Excessive uplift pressure that may have been caused by water injecting itself into open joints, unsealed cracks over herringbone drains and spalled concrete. The memo notes that areas of concrete damage and repairs existed in the affected area prior to the failure.

Oroville Dam damage
Kelly M. Grow / California Department of Water Resources

More visual inspections wouldn't have been enough to reveal the risk for failure at Oroville Dam's main spillway, the new memo from the independent forensic team looking into the incident says.

New damage, deteriorated repairs, long-term erosion under the slab, corrosion of rebar in the slab, and erosion and corrosion on and around the slab’s anchors all may have played a role in the spillway’s eventual failure, the memo says.

Inspections Not Enough

The chief “lesson” the team notes as a takeaway for both the DWR and owners of dams nationwide is that physical inspections can’t be the only method used to identify risks. “More frequent physical inspections would not likely have uncovered the issues which led to the spillway incident,” the team explains in the memo.

Likewise, compliance with regulatory requirements isn’t sufficient: Regulators generally focus on risks for uncontrolled releases, but other types of incidents should be taken into consideration as well.

The team says that comprehensive reviews of the original design, “taking into account comparison with the current state of the practice,” are crucial for dams. Such a review was never done at Oroville, the group says, but if it had been, it would likely have “connected the dots,” helping to identify the risk that existed at the dam.

The original design had shortcomings in comparison with what’s known now, the team says, and procedures and decisions made during construction exacerbated the dam’s shortcomings. The dam was experiencing drain flows higher than what it was designed for, the memo notes, and repairs that were made to the spillway were not always sufficient to withstand flows and other forces.

The DWR said in a statement responding to the memorandum, which comes ahead of the forensic team’s final report, due this fall, that it agrees with the team that “dam owners need to reassess current procedures.” The forensic team’s “research will push the entire dam safety community forward to make lasting changes and improvements internationally,” the department added.

The reconstruction of the dam's main spillway, which failed, and auxiliary spillway, which eroded when used for the first time ever during the February crisis, is underway and is expected to take two years.


Tagged categories: concrete; Corrosion; Locks and dams; Rebar; Risk management

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