Colliery Dams - Extremely Low Probablity of Failure

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Colliery Dams Update

Risks significantly lower than previously believed

The Colliery Dams Technical Committee has finally released another public update and while the remediation options won't be decided upon for another 4-6 weeks it does shed some light on the current state of the dams.

The technical team working on a Colliery Dam solution engaged Golder Associates based on their high level of expertise in this area. The balance of the team is made up of city of Nanaimo staff, CDPPS and SFN and is facilitated by Katherine Gordon.

Some of the most significant findings in this recent report is the fact the Dam Safety Section has decided to reduce the hazard rating, or consequence classification of the dams, from their current 'Extreme' rating to 'High' for the Middle Dam and 'Very High' for the Lower Dam.

The dams appear to be in the same condition as they were in 1980 when remediation work was done on them. Testing shows the concrete walls are in good condition with very limited signs of deterioration.

Extremely low probability of dramatic rupture in an earthquake

The probability of risk to public safety as a result of the dam rupturing in a seismic event is extremely low. It was established the dams are very unlikely to fail during an earthquake in a manner that would result in the loss of life. If the dams were to be damaged by an earthquake, it would most likely be manifested through slow leaking through cracks in the dam rather than a sudden failure of the walls.

The results of their analysis also indicates that risks of serious damage resulting from a severe storm are lower than previously thought. However, spillway capacity must be addressed to reduce the risk further to the level required by dam safety standards.

The committee expects to be able to provide detailed information as to the remediation options within the next 4 - 6 weeks.

In the meantime, the City is reviewing its current short term emergency mitigation plan in light of this new information.