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Colliery dam risk lowered

The technical committee investigating remediation options for the Colliery dams says the amount of work to bring the structures up to an acceptable risk level is much less than previously thought.

The Colliery Dam Technical Committee, made up of city officials and members of the community, released an update this week on its work to investigate remediation options for the century-old dams.

The committee asked engineering firm Golder Associates to evaluate the stability of the dams, which indicated that “there is an extremely low risk of sudden or rapid failure of the dams in an earthquake,” according to the report.

Because of the new information, the Dam Safety Branch lowered the Colliery dam risk from “extreme consequence” to “high” for the middle dam and “very high” for the lower dam.

It means a much lower level of remediation is necessary to maintain safety during an earthquake or high rainfall event, according to Katherine Gordon, the committee’s spokeswoman.

“There is remediation required,” she said. “The spillway capacity must be addressed.

“It’s definitely less than if the dams stayed at an extreme consequence rating.”

Golder’s assessment concluded that the concrete walls of the dams are in good condition, with limited signs of deterioration, with extremely low probability of an extreme rupture in an earthquake that would put public safety at risk. Damage would likely be limited to slow leaking through cracks rather than a sudden failure.

The results are similar for a severe storm with heavy rainfall, but the report notes that spillway capacity must be increased to bring the dams up to the safety level required.

The committee can suggest remediation options that will satisfy public safety based on these lowered classifications.

“It was a huge opportunity for the committee to use that classification,” Gordon said.

She said the technical committee will release a full report on its recommendations for remediation as well as a timeline for the work to be completed in the next four to six weeks. The timing of the release of reports from Golder Associates will also be made then.

The committee’s latest update can be found online at www.nanaimo.ca.

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