City council's top committee is backing recommendations for a diversion tunnel that could help protect the central part of Calgary from flooding.

The tunnel, which would divert excess water to the Bow River from the Glenmore Reservoir, could cost up to $500 million, according to a feasibility study led by the head of the city's flood mitigation panel, Wolf Keller.

The study found spending on major mitigation solutions could save billions of dollars in damages later, said Keller.

"If you take the net present value, spread it out over 100 years, 50 years, it looks like there's a payback if you build one of these large structures," he said.

"Our panel really felt that council should advance that issue — at least to the discussion of how do we get to a decision on it and what are the important factors we want to consider. So let's have a really open public debate about it and let's look at all the factors — the cost, the benefits and all the other aspects — and let's really get a serious discussion on it."

While the decision on whether to build the tunnel below Heritage Drive is ultimately up to the province, Keller said, he believes it's up to city council to keep the pressure on.

Mayor Naheed Nenshi agreed with Keller that data on two water storage sites west of the city need to be compared with the tunnel idea before a final decision can be made.

"I think we have that now on the tunnel," Nenshi said. "I'm looking forward to reading the articles on the upstream ones and now it's a matter for the city and the province to work together to determine the best way forward — and I think it's worth taking the time to get it right."

City council will discuss the flood mitigation study next week.