Seismic risk rises at some B.C. dams
Six-year BC Hydro study gives better understanding of earthquake impact.
BC Hydro will draw down the water level at an Interior dam and offer to buy 11 homes in the path of a Vancouver Island dam in response to a $10-million study that has better identified major earthquake hazards.
The study, which took six years and involved 25 international experts, used the latest scientific information on earthquakes and applied methods created by the U.S. nuclear industry to determine seismic risks.
The 3,500-page, four-volume report, Probabilistic Seismic Hazard Analysis, determined the hazard from a major earthquake was the same or lower than previously understood at power dams on the Peace River system in northern B.C. and the Columbia River system in the Southern Interior.
But it also found an increase in seismic hazard risks on the Campbell River system and the Jordan River system near Sooke, both on Vancouver Island. The seismic hazard has also increased on the Bridge River system near Lillooet in the Interior.
BC Hydro officials stressed the province’s hydro dams are safe given the new information, but it is important to prepare for the possibility of a major earthquake, the kind that could happen once in 1,000 years.
Officials also said the study will help them better prioritize spending of $1.9 billion earmarked for seismic and safety upgrades in the next decade, $700 million of that targeted for Vancouver Island.
The new information does not affect Hydro’s existing plan for electricity rates to homeowners, businesses and industrial users.
BC Hydro also plans to make a major effort to reach out to the public and First Nations about how to prepare for the consequence of a dam break in the event of a major earthquake, which includes having a plan to get out of the path of the ensuing flood or to high ground. “It’s pretty much a landmark study … It’s excellent, worldleading work. It’s been all scientifically peer-reviewed, poked at and prodded, so we are sure we have the best possible science to support decision making going forward,” said Chris O’Riley, BC Hydro’s executive vice-president of generation.
“We see this as an opportunity to have a larger conversation with the public around emergency preparedness, particularly on Vancouver Island. The seismic risk is real. It’s something we live with every day,” he noted.
The study found the Jordan River system on Vancouver Island near Sooke has the highest seismic hazard in B.C., possibly in Canada.
BC Hydro has concluded it is not feasible to reduce the reservoir level because the water is needed to produce power at peak times for the Victoria area. Nor was it deemed practical to rebuild the dam system, which would cost hundreds of millions of dollars.
That’s why Hydro decided it would offer to buy the 11 homes in the flood path of the dam, to reduce the consequence of a failure, said O’Riley.
The Crown corporation’s plan also includes lowering the maximum reservoir level by 16 metres at the LaJoie dam on the Bridge River system near Lillooet in the Interior. That will take the load off the dam and diminish seismic risks to an acceptable level, said O’Riley.
The lower reservoir level will affect Hydro’s generation capacity, but not significantly.
BC Hydro already has an upgrade plan underway on the Campbell River system, where an increase in seismic hazard was found.
The first stage, which broke ground this fall, is the $1-billion replacement of the John Hart generating station. That project will take three to four years, but will allow the reservoir level to be lowered and more detailed seismic upgrades to take place on the system in the next 10 to 20 years.
The new study allowed BC Hydro officials to decrease the uncertainty of their modelling results and better understand the level of ground motion that would take place at dam sites throughout the province when a major earthquake takes place.
A key finding of the study was that a major earthquake triggered at the Cascadia subduction zone off B.C’s coast – where the ocean plate is pushing underneath North America’s continental plate – will cause longer ground shaking than understood previously.
A 2013 study by Royal Roads University, the Geological Survey of Canada, the University of B.C. and the University of California concluded the Pacific coast has experienced 22 major earthquakes over the past 11,000 years and is due for another.
Researchers said there is evidence the last major earthquake from this zone took place around 1700.
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