Toronto is finally getting a relief line, but it’s not one that’s going to move people.

Instead, a new $400-million overflow tunnel to be built along the Don River will stop combined sewage and stormwater from overflowing into the river and Lake Ontario during storms and serve as a back up for an existing 60-year old sanitary sewer.

It’s the first stage of a 25-year project to stop the combined overflow going into the lake. That overflow contributed to a joint commission between Canada and the U.S. in 1987 naming the Toronto Harbour as one of 43 areas of ecological concern.

Then, “governments on both sides of the border committed themselves to implementing initiatives and making funding available to clean up those areas,” says Frank Quarisa, the city’s acting general manager of Toronto Water. Smaller projects to clean up the harbour began in 1994.

Council approved the tunnel project earlier this year, which Quarisa described as a “big step forward.” Two more phases of the storm water overflow project, the Taylor-Massey Creek and inner harbour tunnels, have yet to be approved.

The ambitious first phase includes the 10.6-kilometre tunnel, which will run from the Ashbridge’s Bay wastewater treatment plant at Coxwell Ave. and Lake Shore Blvd. E., west to Broadview Ave., and then north along the Don River to just north of Pape Ave.

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Twenty-three per cent of the city has combined sewers, which means sewage from those homes and rainfall going into the catchbasin at the side of the road are all travelling down the same pipes.

“We’re getting more extreme storms more frequently, where more rain is falling over a shorter period of time,” Quarisa says. When the system can’t handle the flow, the water goes into one of the city’s 309 outfalls that lead to the Don River, the Humber River or Lake Ontario.

A new pumping station, to handle the millions of litres of combined sewage and stormwater overflow that previously went into the lake, will be built at Ashbridge’s, replacing two smaller, outdated pumping stations — located in the park opposite Ashbridge’s at Eastern and Coxwell Aves. — which push the dirty water into the treatment plant.

Already, about 60 trees have been removed in front of the Ashbridge’s plant to make way for a staging area and shaft, where the tunnel boring machine will be lowered into the ground. The city will replace them with 200 trees in a decade when the tunnel is complete.

Quarisa says a tunnel will be built under Lake Shore Blvd. E. so that the dirt and muck coming from the tunnel boring can be loaded on trucks in an area on the north side of the street where the rugby field is currently located.

Traffic disruptions will be kept to a minimum, he says.

A new treatment plant to clean the overflow will be built south of the existing Ashbridge’s plant on a piece of land that isn’t there yet. It will be created from the rocks removed during the tunnelling project.

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“This is a significant project with a 25-year projected time frame,” Quarisa says. “The next 10 years will be entirely focused on constructing the new infrastructure. And in the following 15 years we will incrementally start to see benefits as all the different projects come online.”

The new tunnel along the Don will also serve as a backup for the Coxwell sanitary trunk, which handles 75 per cent of the city’s sewage.

In 2009, there was a series of cracks in the trunk and workers had to build a mini-bypass. Councillor Glenn De Baeremaeker, who at the time chaired Toronto’s public works committee, told the Star: “The alarm bells have gone off for the City of Toronto. If this pipe collapses, we would have a catastrophic event on our hands because you have more water going through that sewer pipe than is in the Don River.”

The contents of the pipe would then spill into the Don. “The Don River, from bank to bank, would be covered with raw human sewage,” De Baeremaeker told the Star at the time.

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