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Flood Diversion Tunnel Plan Raises Tough Questions for Calgary

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When flood mitigation experts mused about a tunnel to divert water from the Glenmore Reservoir to the Bow River, it struck even the mayor as something from science fiction.

In the first half of 2014, Calgary will get a Choose Your Own Adventure report on the proposed tunnel.

Should it go under 58th Avenue South or Heritage Drive? Should it protect Elbow River communities from a repeat of 2013’s flood, or a much worse disaster?

Will the benefits be worth the cost of a $200-million tunnel? What if it’s a $300-million tunnel?

And if a flood-sized gusher is siphoned off the Elbow, what damage does it wreak when the water empties into the Bow?

Tricky options and daunting scenarios, in the feasibility study planned for the Great Calgary Flood Tunnel. But the technical study and cost-benefit analysis leave out another vital question: if not the water tunnel, then what will governments do to protect against the next big floods?

“If you’re not going to have a drain, then you’ve got to stop the water in the first place,” said Wolf Keller, head of the city’s flood mitigation advisory panel.

In November, the Redford government announced it was moving forward on three major flood prevention projects that may have seemed like crazy precautions before June, but are now portrayed as reasonable disaster insurance policies.

The province is confident enough to begin environment reviews of projects to run a diversion channel around High River and a dry dam for the Elbow River near Bragg Creek. Both projects would probably cost less than $100 million each, according to a Stantec Consulting report for the province’s flood mitigation panel.

The ballpark estimates for the flood tunnel run $200 million to $300 million. Between that price and the technical challenges of tunnelling between one water body and another — at least 30 metres below the surface — the province decided to proceed more cautiously. It granted the city $250,000 for a tunnel feasibility study.

Much about the project is in question, according to the city’s proposal for would-be contractors.

Provincial officials have routinely called it the 58th Avenue tunnel — skirting by the Chinook Centre and the Calgary Golf and Country Club — but the city is also studying a Heritage Drive alignment, just south of Heritage Park and directly beneath Deerfoot Meadows retail centre.

The province’s flood panel recommended 58th Avenue because it doesn’t have as steep a slope toward the Bow as Heritage does, Keller said. To be sure of the route decision, the feasibility study will consider both, he said.

An engineering firm has already studied rock types up to 60 metres below 58th Avenue to assess the viability of a tunnel there. Neither road would be dug up to install the eight-metre-wide water pipe — several dozen will be dozens of metres underground — a tunnel-boring machine will do the job.

The provincial panel also considered a diversion tunnel from the reservoir to Fish Creek, but discarded the option early because of impacts on the provincial park’s creek, the Stantec report states.

While much of the Redford government’s post-flood projects — and property buyouts — are aimed at protecting against another 1-in-100-year flood, the city’s feasibility study will model impacts of a flood with a 1-in-200 probability.

“A lot of the advice we’re hearing from the insurance industry, climatologists, engineers and other people is that (1-in-100) might be too low, too conservative,” Keller said.

The peak flow into the reservoir in June was 700 cubic metres per second. The proposed tunnel, at eight metres in diameter, would be able to handle 500 cubic meres per second. Preliminary studies say that if another 2013-scaled flood hits with the diversion tunnel in place, the extra flow would be “low enough that protection measures such as sand bagging will not be required for properties,” the Stantec paper says.

The study will also consider the downstream impacts on the Bow River. An Enmax substation and Sue Higgins dog park were badly damaged by the swollen Bow last year, and evacuations were ordered in Riverbend, Deer Run and Douglasdale.

San Antonio, Texas, and Seward, Alaska, currently have water diversion tunnels. A $106-million flood control tunnel in Austin, Texas, is scheduled to be ready in 2015, four years after crews broke ground.

The study will measure the project’s price tag against the potential benefits, although Keller couldn’t say exactly how that equation would work. The damages in Elbow River neighbourhoods such as Roxboro and Erlton and to the Stampede grounds may eclipse the cost of a diversion tunnel, but it’s not a certainty that another flood as severe as 2013 will hit again in the near future.

Keller puts it another way: with more than a million people in Calgary, the cost spread over a decade could be $20 or $30 each.

“What would you pay to prevent damage like this? Would you pay $20? I’m sure a lot of people would say sure,” Keller said.

Both tunnel alignments go through Ward 11. Coun. Brian Pincott said he’s not heard much from residents about the subterranean plan, which goes predominantly under city roads. They’re mostly curious if the tunnel is feasible, but the questions may start coming when the study comes in. It’s due in June.

“Large areas of my ward are ghost towns,” Pincott said. “Nobody’s thinking about cost-benefit analysis.”


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