Atmosphere And Weather

Comparing the 2013 Toronto Flash Floods to Hurricane Hazel

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"Comparing the 2013 Toronto Flash Floods to Hurricane Hazel"
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Mention Hurricane Hazel to any Canadian of a certain age, and you will find that it still brings out strong memories. The only hurricane ever to hit Toronto left behind a trail of flooding and carnage that leaves a haunting legacy even to this day. However, the rainfall record set by Hurricane Hazel on October 15, 1954, was beaten at Toronto’s Pearson International Airport on July 9, 2013. This time, all of that rainfall was caused by your standard Great Lakes variety thunderstorms, only they’re not as standard as they used to be.

Hurricane Hazel

Like Superstorm Sandy, the devastation of Hurricane Hazel was caused by the collision of a hurricane with a cold front. That’s what made everything so much worse.

On its own, the cold front would have resulted in possibly severe thunderstorms near Lakes Erie and Ontario, but they would have been isolated and brief. That’s very common for Toronto.

On its own, Hazel would have drifted to the northeast and disappeared, leaving behind a bit of wind damage and about 50 millimetres of widespread rain, which comes out to about two inches. That’s typical for the tag-ends of hurricanes which reach Southwestern Ontario.

Each system on its own could have caused some minor flooding. Together, they started pouring heavy rain over all of Southwestern Ontario, along with a lot of central Ontario, for twenty-two hours straight.

By the time it was over, Pearson International Airport had measured 121 mms of rain, and other places had measured even more. That kind of rainfall’s almost unheard of in the central part of Canada, far away from any ocean coastline.

The flooding of June 9, 2013

Just as during Hurricane Hazel, two storms collided and stalled over the Toronto area. However, this time, both storms were products of the same weather system, a highly unstable hot air mass which was producing wave upon wave of thunderstorms.

One of those waves entered the Southwestern Ontario peninsula near Goderich, which still bears the scars of the F3 tornado which struck it in 2011. That series of thunderstorms beelined straight for Toronto.

The other wave of thunderstorms came in from the south and entered Southwestern Ontario near Lake St. Clair. It was channelled north by the shoreline of Lake Erie, which sent it straight into Toronto.

Any one of these heavy thunderstorms could have caused some minor flooding. When the two waves smashed together and stalled, they started pouring heavy rain over all of south and southwest Toronto like it was painted with a bullseye.

Most of the rain (90 mm) fell in just two hours, right at the beginning of Toronto’s rush hour. That’s nearly four inches of rain! Altogether, the heavy rain continued for seven hours straight. By the time it was over, Pearson International Airport had recorded 126 mm of rain, and set a new record.

So how do they compare?

In the 2013 floods, the major part of the rain only hit in south and southwest Toronto. It also hit neighbouring Mississauga, where Pearson International Airport is located, but that's on high land. The rainfall from Hurricane Hazel was much more widespread. All of that water got into the watersheds and came downriver straight to Toronto. That’s what caused the real problems then, mostly after the actual storm was over. To prevent that kind of thing from ever happening again, a lot of flood-control dams were built and a lot of flood plain areas were cleared. They weren’t ever supposed to be built on again, but sometimes people forget.

There’s no question that the flooding was serious in both cases. There’d already been heavy rain all the previous month both times. The two weeks just before Hurricane Hazel had been among the wettest known. The water table was already as full as it could get, before the dumping by Hazel. Even tree roots can’t drink water that fast! There was nowhere for all that water to go but straight into the rivers.

So tiny creeks suddenly became raging rivers. Many water levels suddenly rose up by eight metres. Some of them grew up to five times wider!

With Hazel, the real problems began after all that rain ended and the watershed flooding rushed downstream, washing away cars and bridges and tearing houses from their foundations. Downstream dams collapsed. Both the 400 and the 401, the two major highways in the area at the time, were underwater. Fred Turnbull, who headed the airport weather office at the time, described it “like dumping a lake the size of Lake Simcoe on the Humber River drainage area and having it all trying to get out of the way of the river at once.”

Just like with Hazel, the soil was also already saturated in the 2013 Toronto flash flood, but that wasn’t the real problem. This time, there was simply nowhere for most of the water to go, because a lot of what used to be soil and streams is now concrete and asphalt. The soil didn’t have a lot of space left for the water, but the roadways had none! So after the rain sewers were overwhelmed, the rain did the only thing it could do. It turned all low-lying underpasses into ponds and all low-lying roadways into rivers.

The 400 and 401 didn’t flood this time around. Most of the rivers flow away from them, and this time, the water didn’t come from far upstream like it did during Hurricane Hazel. The 427 and Don Valley Parkway did flood. Neither of them had been built yet when Hurricane Hazel hit. The DVP’s one of the two major highways which goes straight from the 401 to downtown Toronto, so it really should be considered part of the most heavily used highway system in North America.

Sometimes the water tried to run under the roadways, and that’s when subways flooded and sinkholes appeared. It doesn’t help at all that all the rail and subway systems start off close to the shore of Lake Ontario. Large parts of the Toronto Transit System and the GO system of commuter trains were completely shut down. The daily weekday ridership of the TTC subway alone is over a million people!

There was a subway when Hurricane Hazel hit, barely. The TTC Yonge line had just opened earlier that same year! When Hazel hit, the Yonge Street streetcar tracks had just been torn up, and the grand reopening ceremony after all that deconstruction was finally done was turned into a fundraising event to help the families who had been victimized by Hazel.

But there wasn’t as much rain in Toronto during Hazel as there was in 2013. Besides, at that time, the subway line only ran up the hill from Union Station to Eglinton, so the flooding rivers never touched it.

The really embarrassing part is that it’s not the first time the DVP’s flooded. It’s not even the first time in 2013! It’s one of those roads that’s in a perfectly logical place when it comes to traffic, but it’s also in a perfectly logical place when it comes to the Don River floodplain. It’s kind of ironic that even though a major highway was badly needed in the area, construction was originally banned in the Don Valley after the Hurricane Hazel flooding. Lots of previous development was turned into city parks. However, the city council approved the DVP a couple of years later and turned over some of those parks to Metro, and construction began in 1958.

Put that kind of thinking together with the aging infrastructure that's common to a lot of cities, and it's a sure bet that this kind of flooding's sure to happen again.

The final picture

By the time Hazel finally left Ontario, it had killed 81 people and caused over $135 million in damage. That’s the equivalent of over $1 billion damage in 2009 dollars. Roads, rail, and bridges were washed out as far north as Barrie.

Even though the flooding came on a lot faster in 2013, Toronto’s recent flash floods didn’t kill anyone. Everyone’s happy about that. A couple of people had to be saved from where they were clinging onto trees, but if they hadn’t decided to leave the flooded commuter train and swim for it, they wouldn’t have gotten into trouble in the first place.

The damage is another thing again. The earliest estimates of damage are already over $600 million. That number’s only going to go up from there. It’s probably going to crack a billion before all is said and done. That would put the 2013 Toronto floods among the top ten Canadian weather disasters ever.

More about this author: Cameron Scott

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