Paving over porous paradise, or any absorbent ground, increases the risk of basement flooding, say researchers using City of Toronto data.

Their study could help GTA homeowners make sense of the sometimes-ridiculed notion that the amount of land people cover with buildings, parking pads, parking lots and more, is linked to the costly rising tide of urban flooding that is expected to worsen in coming years.

“From what we have seen, areas with less green space — mostly developed areas, without any consideration for pervious or green areas — are more likely to have this kind of basement flooding,” says Yekenalem Abebe, a University of British Columbia engineering PhD candidate who co-authored the study with UBC civil engineering professor Solomon Tesfamariam and Golam Kabir, a University of Windsor assistant professor in engineering.

Other factors make basements prone to invasion from “pluvial” flooding — rainwater unable to soak into the ground, as opposed to “fluvial” flooding, which occurs when bodies of water overflow. They include aging, deteriorating sewer pipes and other infrastructure meant to steer rainfall away from homes. In Toronto, some of those pipes are more than a century old and, when overwhelmed by big storms, send untreated sewage from downtown into Lake Ontario.

Images of residential flooding, including Brantford in recent weeks, Toronto Islands last summer and across the U.S., are becoming more common.

The researchers didn’t get all the data they wanted while developing a “flood vulnerability index” that any city can use. But even if they had, Abebe said, “I would still expect pavement and impervious surfaces to be one of the most important factors,” in basement flooding risk. Urbanization is accelerating the loss of absorbent ground, communities are being hit more frequently with intense storms, and costly-to-replace infrastructure is failing to handle the runoff.

The researchers divided Toronto into 760 “grid cells,” each about one square kilometre. Using city data they assigned each grid a probability of risk.

At “very high risk” for basement flooding, according to the study, is an area spreading north from Humber Bay and widening to include neighbourhoods around High Park, Swansea and The Junction, and the downtown core. Safest is the high ground in Scarborough from the Bluffs north.

The findings of the study, published late last year in the Journal of Cleaner Production, come as no surprise to Shawna Peddle, a director of Partners for Action, which promotes flood resiliency and is based at the University of Waterloo.

“We’re definitely seeing an increase in urban flooding everywhere — it’s not just Toronto, it’s everywhere across the country and it’s because we are paving over what would normally soak up the water — the water has nowhere to go,” Peddle said in an interview.

“Even if we’re having just a little more rain than we used to, the water ends up in basements and flooding roadways, flooding parks. The weather is changing, we are seeing more rain events more often. That combined with increased development, infrastructure that’s aging, us paving over areas that used to be able to soak up the water — the result is more (flood) events and bigger losses, too.”

Partners for Action offers homeowners tips, including how to check on the flood risk for their homes, on keeping valuables upstairs and checking with their insurance companies on the kinds of water damage they cover.

Cities are encouraging green roof construction, which sees rooftops covered with vegetation, downspout disconnections, and the use of porous hard surfaces and cisterns to capture and reuse rainwater.

They are also trying to figure out where to find the canyons of cash needed to replace and upgrade storm sewer systems and fund flood-risk-reducing incentive programs. Toronto last year paid $7.1 million just to subsidize homeowner installation of backwater valves and other measures to reduce the chance of residential basement flooding, up from $3.1 million in 2013.

Two years ago, Mississauga added a stormwater charge on Peel region water bills ranging from $50 to $170 per year, depending on the size of the roof and runoff potential. More than $30 million in annual revenues are being pumped into a dedicated fund to pay for stormwater infrastructure maintenance and upgrades.

Torontonians pay for pipe replacement and other parts of a stormwater management plan through water consumption fees. City staff had suggested shifting more of the costs to home- and business-owners with the biggest hard surfaces, including parking lots and roofs.

But last May, Toronto Mayor John Tory’s executive committee shelved indefinitely a staff recommendation to propose options for a stormwater charge.

The idea of such a charge had been ridiculed by councillor Giorgio Mammoliti and former councillor Doug Ford, who is now seeking the leadership of Ontario’s Progressive Conservative party, as a “roof tax,” an attack on suburban homeowners and “a measure to get into people’s pockets.”

For basement damage, the cost is borne by homeowners and their insurers. A massive thunderstorm that hit Toronto in July 2013 became one of Canada’s most expensive insured losses at almost $1 billion, mostly from sewer backup claims.

The head of one of Europe’s largest insurers recently warned the World Economic Forum that, if climate change advances, basements in some cities could be uninsurable, Bloomberg news agency reported.

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But GTA homeowners have more options for flood coverage than ever because the risk is top of mind, according to the Insurance Bureau of Canada.

“Water is the new fire because, in the past, fire damage to someone’s home used to be the predominant peril or event that people wanted to protect their property or homes from,” said Pete Karageorgos, the bureau’s Ontario director of consumer and industry relations. “Now it’s water damage of all sorts, from plumbing fixtures and internal leaks, but now more so from external type of water,” that might require special coverage.

“People are recognizing there are severe weather events such as rainstorms that are occurring that are depositing larger amounts of rain in shorter amounts of time and impacting our communities.”

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