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The West Don Lands along the banks of the Don River are seen in Toronto, Friday June 28, 2013.Kevin Van Paassen/The Globe and Mail

When Waterfront Toronto opens Corktown Common this weekend, visitors to the hilly park overlooking the mouth of the Don River will be able to stroll through a wetland and climb up to a pavilion that looks back over the Pan American Games village.

But the real story of the city's newest public space lies buried under the paths and prairie grasses. The $135-million project doubles as a 8.5-metre-high berm designed to protect the eastern downtown from a major flood. The clay, rock and soil structure can withstand a so-called 500-year storm while shunting raging waters south toward the lake.

The problem of urban flooding has become an increasingly high-profile issue, and not just in Calgary, which saw its downtown inundated, or New York, which just released plans to fortify itself against future storm surges like Hurricane Sandy.

Friday's rainfall in Toronto forced road closures and temporarily turned Lake Shore Boulevard into a roiling river – the second time in a month that Toronto has seen streets temporarily flooded and inaccessible. The new Corktown berm is the sort of seldom-used infrastructure that protects neighbourhoods during extreme weather events.

According to Alberta officials, the Bow River's peak flow during the storm was 1,740 cubic metres per second (cms); the Corktown berm is built to withstand flood waters coursing down the Don River at 1,680 cms, equivalent to the intensity of Hurricane Hazel, the worst storm to have hit the region. (Normal flow is 10 cms, with the Don River rising to 100 cms during the spring runoff.)

Of course, Friday's 40 millimetre downpour was nothing like Alberta's spate of wet weather, which has inflicted billions of dollars in damage. But Waterfront Toronto CEO John Campbell says that without the new berm, a comparable storm could cause the Don River to flood much of the financial district, inundating the subways and the PATH system. "It's far better to pay for an ounce of prevention than a pound of cure," Mr. Campbell said.

When Hurricane Hazel walloped the Toronto region in October, 1954, catastrophic flooding from Etobicoke to the Holland Marsh left 81 people dead, swept away low-lying communities and caused over $1-billion (2010) in property damage. The province responded with strict floodplain protection legislation. Those laws, in turn, blocked the redevelopment of the Lower Don Lands, including the fallow, contaminated industrial land east of Cherry Street. Over time, however, the flood risks have grown. Climate change-related storms have become increasingly frequent and intense, while the proportion of paved land in the Don River watershed has expanded, according to Waterfront Toronto hydrology studies.

In 1990, 70 per cent of the watershed was considered "impervious," meaning rainwater can't seep into the ground. By 2005, that proportion exceeded 85 per cent. "Rainfall can't go anywhere but straight to an overloaded channel," said Tim Dekker, a Michigan-based river hydrologist who modelled floodwater flows for the agency.

WT engineers have spent several years building a "flood protection landform" along the western bank of the Don River, from just north of Keating Channel to Queen Street. The first step, says project manager Brenda Webster, involved piling huge mounds of dirt on the site to force the moisture out of the peaty soil that is a remnant of the wetland that once existed at the mouth of the river. The agency's contractors then constructed the 200,000-cubic-metre clay core, which is 1.5 to four metres deep, and covered it with 180,000 cubic metres of soil, armour stone and prairie grasses. The riverside of the berm has no trees, which can be uprooted in a flood and destabilize the structure.This $135-million structure, Mr. Campbell notes, has paved the way for more than $2-billion in public- and private-sector investment in the vicinity of the West Don Lands.

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