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OUT OF THIS WORLD | Earth, Space And The Stuff In Between - a daily journey through weather, space and science with meteorologist/science writer Scott Sutherland

Don Valley Parkway flooding. Here's why it happens so often

Scott Sutherland
Meteorologist/Science Writer

Tuesday, February 5, 2019, 11:00 AM - To everyone who commutes along Toronto's Don Valley Parkway, beware! A remarkably fast melt of all the latest snow, along with pouring rainfall, and that is resulting in lane closures due to flooding!

It's almost a "classic" Toronto experience these days. Every time we see dark, rain-laden clouds sweep across the GTA, and especially if we combine rainfall with a rapid snow melt, it isn't too long before social media lights up with reports that the south end of the Don Valley Parkway is, once again, flooded out.


In the early hours of Tuesday, February 5, 2019, Don River levels at Dundas Street reached the High-High Alarm level - the point were there is significant flooding of the Don Valley Parkway - and peaked at over 77 m before tapering off.

Toronto and Region Conservation Authority real-time river gauge at the Don Valley Parkway and Dundas St, for overnight, Feb 4-5, 2019, showing the water levels cresting the banks of the Don and flooding the DVP between 11:45 p.m. and 3:15 a.m. ET. Credit: TRCA/Scott Sutherland

Along with this, the Don Valley Parkway Twitter account is reporting that both southbound and northbound lanes were closed due to flooding.


What exactly makes the DVP so prone to flooding? It's a combination of geology, hydrology and urban development.

How many times has this happened? A particularly strong rain storm washes over the GTA, a concentrated heavy downpour drenches the east end of Toronto and suddenly commuting along the Don Valley Parkway goes from simply slow to downright hazardous as the Don River crests its banks and envelopes the traffic artery's south end.

The answer to the above question? Too often.

In fact, it's been happening ever since people settled in this region, and even going back much further than that - all the way to the last time the glaciers retreated from southern Ontario, roughly 12,000 years ago.

The Don River. Credit: Raysonho/Wikimedia Commons

The Don River, which is about 15 metres wide these days, managed to carve out the entire 400-metre-wide Don Valley since the last glacial period, as it meandered back and forth, cutting down through the layers of Earth.

Even when the first people settled here, and throughout the establishment of a colony and build-up of the city of Toronto around that colony, the shape of the river made it extremely likely that the land between the winding curves of the river would at experience least some flooding whenever the river flow increased, due to heavy rains or melting snow.

In 1886, the Don Improvement Plan dug out a long trench, which cut through several of the meanders at the south end of the river, and then filled in all the twists and turns with dirt, effectively deepening and straightening that section of the waterway. This helped with the immediate issue of overland flooding, but with the city growing up and around the valley since, and with developments spreading down into it as well, any benefits from the plan were fairly short-lived.

It was Hurricane Hazel, which blasted across Toronto in 1954, that served as an effective wake-up call for the disaster potential that still existed in the valley. River flows during the storm have been estimated at around 1,700 cubic metres per second - over 400 times greater than the normal average flow the river experiences.

There hasn't been anything quite that bad since, but the government did ban development in the valley (as well as other flood plains) as a result of that disaster.

Still, less than four years later, in 1958, construction crews broke ground on the newly approved Don Valley Parkway, which was going to wind its way right through the middle of the Don River's flood plain. Meanwhile, the city continued to grow around it, laying down even more concrete and asphalt up and down the lands that border both sides of the river valley.

This is what it really comes down to for flooding along the Don River and the Don Valley Parkway today - urban development.

If rainfall rates are high enough, or if you have a significant combination of rainfall and rapid snowmelt, overland flooding is a foregone conclusion, regardless of where you live or what waterway you're dealing with. This is simply a combination of hydrology and geology.

The ground, depending on what it's made of, can only absorb so much water so quickly. The more sandy the top soil is, the more water it can absorb, and the faster it can absorb it, since the individual soil particles are larger and have more space in between them. If there is more clay in the soil, the finer grains provide less space between them, resulting in less water absorption and thus slower removal of surface water. No matter what the soil type, though, it does allow at least some water absorption and movement, thus provided a natural buffer to slow the transfer of rain water from where it falls to where it enters the flow of any rivers and streams in the area.

On the other hand, artificial surfaces - concrete and asphalt - are specifically designed to prevent water from penetrating them, or at least to keep water intrusion to a minimum, to improve their durability and strength. Water falling on these surfaces just quickly flows along the top of the surface, pooling in any low areas where flow stagnates, until it reaches ground that will absorb it or it flows into a drain. Drains, for their part, are effective at moving that water along, but since they lead to concrete storm sewers, they just continue the problem, quickly and efficiently moving that rainwater along towards waterways. That removes the water from being an immediate problem where it fell, but it creates a problem farther along.

Since the local streams and rivers formed under conditions where the natural buffer of the soil was in place, they can easily handle your average rainfall or snowmelt, as the water slowly filters through the soil until it joins the flow in the waterway. With the artificial surfaces and drainage funneling water much more rapidly towards those waterways, they can easily swell, crest their banks and cause localized flooding. Add an ice jam to the lower end of the river, where floating chunks of ice pile up on each other during a January thaw or spring melt, and the river flow can slow to a crawl, turning localized flooding into an extreme situation.

This is exactly what happened as the east end of Toronto became more urbanized. Paving over the surfaces there gradually replaced the local geological buffer for rainwater with a series of artificial channels (both on the surface and underground), that quickly funnel rain and melt water directly into the flow of the Don River. The result: all that flow converges on the Lower Don, "piling up" and turning the Parkway into an expansion of the river.


Unfortunately, in all likelihood, this is not going to get better anytime soon.

Toronto and Region Conservation Authority staff work very hard on flood management, of course. There is only so much that you can do, however, with a river that's been cresting its banks since before Toronto was even here, and there's only so much that can be done to counteract the changes we've caused in our climate.

In recent years, a fairly aggressive flood management plan was put into place for the south end of the Don Valley, sparking off major excavation and construction to try to deal with the problem. However, even as of April 2014, both the Lower Don and Brickworks regions of the Don River - both of which border the last stretch of the Parkway before it merges with the Gardner Expressway - were still identified as 'flood vulnerable areas' on Toronto and Region Conversation Authority maps.

There are a few ways that we can help, though.

One is a process called daylighting. As the city has grown, many of the streams and creeks that run through the area have been preserved, but now run through artificial environments - metal pipes and concrete sewers that keep the water from being absorbed by natural soils. Daylighting returns these streams and rivers to a more natural state, allowing more water to soak into the ground, thus restoring part of that natural buffer for the river.

Also, technology can help as well as hinder, like the development of a new product called Topmix Permeable, by British company Tarmac, which has been nickname "Thirsty Concrete."

Simply by replacing the fine grain materials of the concrete mix with coarse grains of crushed granite, Tarmac has produced a porous version that emulates how sandy soils respond to rainfall. According to the company, this concrete can deal with a flow rate of up to 36,000 millimetres per hour. That's over 700 times better than what sand can handle, and somewhere around 2,000 times better than the permeability of your average soil. For comparison, on July 8, 2013, when 126 mm of rainfall caused extensive flooding throughout the GTA, it fell mostly over a two-hour period. However, that doesn't even come close to what this new concrete can handle.

Perhaps, as the city continually repaves Toronto's roadways, a slow replacement of fine-grain impermeable concretes with coarse-grain permeable ones, along with preservation of green-space and an expansion of the daylighting process, could help enough to eventually bring this problem under control.

One important factor working against these preparations and techniques is climate change.

It's a simple scientific fact that when you increase the concentration of greenhouse gases in the atmosphere (carbon dioxide, methane, etc), this raises the temperature of the air. We've known about this connection since at least the late 1800s.

It's another simple scientific fact that warmer air is capable of holding more water vapour than cooler air, and it will fill that capacity from any source of water available (the ground, lakes, rivers, oceans, etc). With an increased amount of water vapour in the air, this translates into more water available to form clouds and precipitation, and an increased likelihood of the precipitation from any storm coming down at extreme rates. We're already seeing this effect - in storms such as Hurricane Harvey, and how many times have we seen torrential downpours so far this year southern Ontario?

As we continue our dangerous dependency on fossil fuels, and continue to pump billions of tonnes of carbon pollution into the atmosphere every year, the added heat and humidity is fueling weather systems of increasing intensity - turning simple rain storms into torrential downpours.

If we can act to drastically reduce greenhouse gas emissions, by breaking our dependency on fossil fuels, we can limit the rise in temperature, and therefore reduce the risks of these downpours and the associated flooding events.

However, if we do not act, Earth's temperature will continue to tick upwards, increasing the humidity along with it, and the risk of extreme rainfall events will continue to rise, making Don River flooding events far more likely.

EDITOR'S NOTE: This article originally ran on June 26, 2014. It has been updated and expanded to reflect current developments.

Sources: TRCA | TRCA Flood Management | Vanishing Point | Tarmac

Related Video: What to do to stay safe during flooding? Shawna Peddle from University of Waterloo gives crucial tips.

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