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Short-term dam mitigation might destroy spillway
Physically changing Nanaimo’s Colliery dams over the short term could carry a $2-million price tag and jeopardize a historic spillway, prompting city officials to now ask if it’s worth it.
Nanaimo city staff is working with the Colliery Dam Preservation Society and Snuneymuxw First Nation on short-term solutions to mitigate flood safety risks posed by the century-old middle and lower Colliery dams this winter.
City officials said top picks included lowering the dams’ spillways to allow them to take on more water, but a recent re-assessment report by engineering firm Klohn Crippen Berger has them second-guessing whether it makes sense to do temporary work on the dams at all.
The report re-assessed the short-term and long-range options for the Colliery dams, which have been flagged by the province as being in extreme risk of failing during a catastrophic flood or quake. According to KCB, work to the spillways’ is the best short-term solution, anticipated to drop water levels by three meters and deepen spillways enough to eliminate flood hazards. But it would also destroy part of Nanaimo’s industrial past – the lower dam spillway – and reduce water levels, which could make the lake warmer in the summer, putting some organisms at risk.
City officials say the report shouldn’t be construed as direction and they are unsure how it will play into ongoing discussions. But Susan Clift, the city’s director of engineering and public works, said it will spark questions about what is practical and whether physically changing the dams is worth the money.
“Perhaps [we] are left with more monitoring and sensing of water levels and dams upstream … adequate early warning systems and emergency measures as the focus,” Clift said.
KCB was hired to take a second look at options for Colliery Dam Park during a 30-day consultation process with the Snuneymuxw First Nation in July. The firm weighed the drawbacks and benefits to temporary fixes as well as long-term rehabilitation and dam construction. A rehabilitation design for the extreme risk structures that would likely crack in a major quake would cost $17.8 million, for example, compared to $23.6 million to ensure there wouldn’t be any damage at all.
A report on short-term options is expected at a city council meeting on Sept. 9.