42 areas in GTA at risk from serious flood, experts warn

Hurricane Hazel killed 81 people in southern Ontario in October, 1954. It still ranks as Canada's most lethal natural disaster. GLOBE AND MAIL

Originally published in 2013, this story was updated on August 8, 2018 to reflect a recent storm.

Tuesday’s sudden, violent Toronto rainstorm left widespread flooding, thousands without power, and subjected two men trapped in an elevator to a nightmarish ordeal as it filled with water.

But it’s also an unsettling reminder of the danger that uncontrollable flooding has held for the Greater Toronto Area in the past – and, experts warn, will hold in the future.

In a much bigger storm – which planners call the ‘regulatory storm,’ and popular culture calls the ‘100-year storm,’ dozens of areas in the GTA would be at risk of serious flooding.

For some cities, like New York and New Orleans, the danger from a major hurricane comes in from the sea, lashing buildings with wild winds and pushing ocean water inland.

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Toronto’s nemesis, in a worst-case scenario, works the other way around: unmanageable sheets of rain, running off fields and parking lots, swelling creeks and the city’s rivers with a roaring, lethal flood as it heads downhill towards Lake Ontario.

It has happened before: Hurricane Hazel, a catastrophic 1954 storm, killed 81 people in Ontario. Dozens of houses in Etobicoke, built in a flood plain, were obliterated when the Humber River rose quickly at night. Hazel still stands as Canada’s worst natural disaster in terms of loss of life.

Since the 1950s, Ontario’s emergency plans have assumed a storm like Hazel would happen again, and have worked from there, banning development in river valleys and building systems to channel or deflect large amounts of water.

Still, officials warn, hundreds of buildings, home to thousands of people, remain at some level of risk in a Hazel-scale storm. The GTA has 42 “flood-vulnerable” areas in or near river valleys. Some are residential.

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MAPPING THE GTA’S FLOOD-VULNERABLE AREASToronto’s complex ravine system has the potential for dangerous, locally intense floods. The Toronto Region Conservation Authority map linked below shows places that experts say are in danger of some level of flooding in times of extreme rain.

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“Those areas would have different depths of flooding” in a Hazel-scale storm, explains Laurian Farrell, manager of flood risk management at the Toronto and Region Conservation Authority. “Some would be severe, and some would be very minor, within a few inches or less than a foot, which people can pretty much deal with, but when you get into three feet to six feet of flooding it’s very difficult to deal with.”

Many old village cores in Peel and York Region are defined as flood-vulnerable: Malton, Brampton, Woodbridge, Bolton, Maple, Thornhill, Markham, Unionville, Stouffville and Pickering. Some of these communities, like Bolton, were originally built around a mill, which needed access to a river.

“We have a lot of structures that were built before policies that limited the amount of development within the floodplain,” Farrell explains. “Those areas have a historic significance, and we are not going to start expropriating land or start demolishing houses, but we need to manage them more closely.”

In Toronto itself, officials would be concerned about residential areas in the Weston Road and Black Creek area, and Hoggs Hollow.

Evacuations might be necessary in specific areas, including several hundred from downtown Brampton:

“They’re not massive evacuations where we’re talking about thousands of people. Some areas it’s less than 50 people – it really just depends.”

Whatever the intensity of the storm, Farrell explains, a Hazel-scale loss of life is unlikely today, in part because communities would have several days to prepare:

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“For Hazel, there were warning signs, but people didn’t understand the severity of the situation for our region. They knew the storm was coming, the meteorologists did put out warnings, but the general awareness of what that actually meant wasn’t there. The majority of the issues started overnight, when people were sleeping in their homes, which is why we saw such devastation. Today, that wouldn’t happen.”

Concerned about flood danger? TRCA flood experts can be reached at floodmessage -at- or 416-661-6514.

But a hurricane is only one source of danger, she warns: “Storms are changing. It may not be the Hazel-type storm that causes the problem in the future – it may be one of the summer flash thunderstorms, which would be highly localized flooding.”

(In 2005, a violent summer storm destroyed part of Finch Ave. W. over Black Creek, when storm water overwhelmed a culvert.)

A major flood would also affect the mouth of the Don River, as intense rainfall from a huge area stretching as far north as Richmond Hill would be forced through a large area.

A recently built earth berm protects areas to the west of the river mouth.

“The catastrophic concern has been addressed, because the West Don Lands earthworks that is the floodproofing is floodproofing for whole downtown to the financial district,” says local councillor Pam McConnell, whose ward lies to the west of the river. “So if we had had another Hurricane Hazel hit us, then the Don River would (have flown) over and gone to the financial district. It cannot now.”

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However, areas to the east, around the south end of Broadview Ave., would still be flooded to some extent in an extreme storm.

“There are major concerns still with the lower Don,” warns councillor Glenn De Baeremaeker, who sits on the TRCA’s board. “That area, most of it will be flooded if there’s a big storm. It would be under a foot or two of water if there was a flood.”

At the foot of Carruthers Creek in Ajax, 16 houses are in the way of a potential flood during a Hazel-scale storm, engineers warned in 2011. (That number was revised down from 59.)

One problem, explains Colleen Jordan, a Durham regional councillor who sits on the TRCA’s board, is that houses which were on the safe side of a flood line when they were built can become endangered by later development upstream, as the land becomes paved and less able to absorb rainfall.

“As development on green fields occurs upstream, it places lower-lying communities at risk for flooding, because of the impermeable pavement, so the land isn’t absorbing the water as well. Also, with climate change, the frequency and severity of extreme weather events will be increasing.”

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