A tunnel nine metres in diameter could have prevented the 2013 flooding on the Elbow River downstream from the Glenmore Dam, according to a report from the city's flood-mitigation panel.

The report, issued Thursday, has a number of conclusions, including those flowing from a feasibility study paid for by the province on a possible diversion tunnel to carry excess water from the Glenmore Reservoir to the Bow River.

The report estimates that tunnel would cost $457 million to build, and says the optimal route is under Heritage Drive.

There had been discussion of a tunnel under the 58th Avenue corridor, but the report concludes the intake design for a Heritage Drive tunnel would be simpler and the bedrock conditions there are more consistent for tunnel construction.

As for whether it should be built, the report says the City of Calgary and the province need to compare the tunnel with two water storage proposals the provincial government is looking at for sites west of Calgary, including a dry dam at McLean Creek and an off-stream storage site near Springbank Road.

Because it would be so expensive, Calgary Mayor Naheed Nenshi says there needs to be a thorough policy debate before any commitment is made to building the diversion tunnel.

“Five hundred million dollars would build us a fair bit of LRT. Five hundred million dollars would go a long way towards solving the congestion problems on Crowchild Trail,” he said.

But Emma May, president of the Calgary River Communities Action Group, says it’s time to get on with a major project.

“You know, we need this kind of protection and we need it now,” she said. "Between the tunnel, the diversion and the Dam at McLean Creek, we need communities along the Elbow and the core offered full protection.”

Premier Dave Hancock is more cautious.

"We need to look at the report to determine what it says. We also need to have discussions with people down river. You know, we have to look at projects in the context of the effect on everybody who will be affected by it."

The mitigation report concludes it may be more practical to move some development away from the city's waterways, and that while that may be a painful process, it needs to be considered as part of an overall flood mitigation plan.

The report will be discussed by city council's priorities and finance committee next Tuesday.

A news conference on the report has been scheduled for Friday afternoon.

With files from the CBC's Scott Dippel