Two of Calgary's oldest bridges are soon going to be torn down and replaced.

A city report says parts of the 12th Street bridge by the Calgary Zoo are in poor condition, and the Ninth Avenue bridge by Fort Calgary isn't far behind.

If the zoo bridge isn't replaced, it will have to be closed to traffic later this decade, according to the report. After last June's flooding, traffic was restricted and no trucks have been allowed on it. 

Mac Logan, Calgary's manager of transportation and infrastructure, says the zoo bridge — which crosses over the Bow River — will be the first to go with work starting as soon as next year.

“There's been some scouring underneath the bridge based on the flood. Some of the old steel is really getting to the end of its life. Yeah, it's time to go. It's served us for about 100 years, but the time has come.”

Another century-old bridge, the Ninth Avenue bridge by Fort Calgary that passes over the Elbow River, is also nearing the end of its usable life.

The Ninth Avenue bridge is crossed by roughly 21,000 vehicles per weekday, and the 12th Street crossing sees about 8,000 per day, according to city traffic data from 2012.

By comparison, the upper deck of the Centre Street bridge is crossed by 25,000 vehicles per day. 

Flood damage

Logan says inspections after last year's flood found the bridges must be replaced sooner than expected.

“We're certainly looking at options on the zoo bridge where potentially, can you build beside it? Build parallel to it and then realign the roads afterwards? The Ninth Avenue is a tougher situation, you basically have to put the new bridge where the existing bridge is.”

Coun. Gian-Carlo Carra wants the city to explore keeping the old zoo bridge for another purpose.

“Much like the historic bridge that crosses into Bowness, we can retain this historic bridge as a pedestrian bridge.”

Logan says it’s estimated it will cost $30 million to replace both bridges, but design work has not yet been completed.

With files from Scott Dippel/CBC