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A group of people take pictures of the flooding from torrential rain which caused the closure of the Don Valley Parkway in Toronto on July 08, 2013.Deborah Baic/The Globe and Mail

The flood protection landform on the west bank of the Don River is an engineering marvel, an intricately designed, eight-hectare mound of clay and dirt designed to protect the new West Don Lands neighbourhood and the financial district beyond from a catastrophic flood on the scale of Hurricane Hazel in 1954. It is a behemoth, one that took about 40,000 dump trucks' worth of soil to build.

It is also a parable – testament to how complex and costly it can be to try to safeguard the city against extreme weather.

Before it got under way, a City of Toronto backgrounder estimated the landform – it is considered too massive to be called a mere dike or berm – would cost $8-million. By 2007, when federal environment minister John Baird, provincial infrastructure minister David Caplan and mayor David Miller gathered to mark the start of construction, the estimated cost had risen to $25-million.

Today, Infrastructure Ontario puts the final cost at $120-million to $130-million. That includes the cost of rebuilding and realigning two streets, Bayview and River. The bill for the park built on top of the landform, Corktown Common, grew to $26-million from $15-million.

The landform was supposed to be finished in 2008 and the park opened in 2009. After many complications and delays, it was finally completed only last November and the park opened just a couple of weeks ago.

The growing cost of the project shows what Toronto is up against as it rebuilds its infrastructure to protect against storms, such as the one that knocked the city to its knees on Monday. New sewers, flood-proofed highways, a renaturalized Don River mouth – these things don't come cheap.

Groups such as the Toronto Environmental Alliance say that, with climate change threatening, the city has no option but to make the investment. "As Monday showed, our water, electricity, and transportation infrastructures are not ready," the group's executive director, Franz Hartmann, said in a release. "We need new revenues for this work and we need our political leaders to say this is a priority for the city."

Councillor Denzil Minnan-Wong, the city's public works chair, counters that, though much work needs to be done, the city has to be realistic about its means. "If we were Saudi Arabia, where we had the crown princes' money and we could just write a blank cheque, then the money is not an object. But we're not Saudi Arabia."

Officials embarked on the landform project because provincial rules that evolved after Hurricane Hazel prohibited them from building on the West Don Lands site without comprehensive flood protection.

They realized from the start that this was going to be more complicated than building a pile of dirt to block the water. To begin with, the site rests on an underground layer of soggy peat. Anything built on top could settle, shift and become unstable. So, engineers came up with a plan to pile tons of earth on top and squeeze the moisture out of the peat like water from a sponge, a process known as surcharging. It meant bringing in endless truckloads of dirt, dumping it in huge piles and then carting much of it away again. The water was drawn out through 27,000 holes equipped with straw-like wicks.

To add to the engineers' headaches, the main sewer line from downtown to the Ashbridges Bay wastewater treatment plant runs under the site. With all that earth piled on top, the century-old line could easily collapse. They had to build a whole new, double-reinforced sewer pipe and drive pilings down to bedrock to support it.

Then there was the issue of contaminated soil on the site, which was once a garbage dump. Provincial regulations demand that you cart it away for treatment. Waterfront Toronto says environmental costs alone were about $20-million.

All that for a four-metre-high bump by the river? Was it worth it? The long-time project manager, Serge Chukseev of Infrastructure Ontario, gives an instant "yes."

The West Don Lands development would not have been possible without it. On what used to be a rundown quarter of the city, a whole new district is rising.

He says that, without the landform, "the potential damage that could occur as a result of flooding is in the billions of dollars, not millions."

That is what the cost could be if a massive flood spread into the downtown core. The Don's maximum flood zone goes all the way to Bay Street.

"I don't want to sound romantic about it," Mr. Chukseev says, "but it's one of those once-in-a-lifetime projects."

It is also one that might not prove its usefulness for a lifetime. It is designed for a "300-year" storm, one of the kind that happens on average once every three centuries. Monday's storm was a piker by comparison. Although it flooded the Don Valley Parkway on the east side of the Don River, it did not touch the landform. The water didn't rise that high. This multimillion-dollar mound is made for bigger things.