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Why does the Don Valley Parkway flood: Geology, hydrology, urban development

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By Scott Sutherland
Meteorologist/Science Writer
Thursday, October 16, 2014, 8:49 PM

EDITOR'S NOTE: This article originally ran on June 26, 2014.

The situation was so 'classic' that it was instantly recognizable. Staring down a line of ominous dark-grey, rain-laden clouds that stretched across the entire Greater Toronto Area, moving slowly down from the north towards the lake shore, one thought crossed the mind: It wouldn't be too long before social media would be lighting up with reports that the south-end of Toronto's Don Valley Parkway was flooded.

How many times has this happened before? A particularly strong band of rain washes past the GTA, or a concentrated heavy downpour drenches the east end of Toronto, and suddenly anyone driving along the Don Valley Parkway not only has to deal with the standard woes that particular traffic artery is known for, but on top of that there's the worry of their car stalling out in the middle of a rapidly-flowing waterway as the Don River spills over its banks and overtakes the entire road.

The answer to the above question is: often. In fact, ever since people settled in this area, 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, which is about 15 metres wide these days, managed to carve out the entire 400-metre-wide Don Valley since that time, meandering back and forth and cutting down through the layers of Earth. Even in the times when the first people settled here, up through the establishment of the colony here and the city of Toronto building up around it, the meanders in the river made it prone to overland flooding whenever heavy rains fell across the area and the river crested its banks. The Don Improvement Plan, in 1886, dug out a long trench that cut through several of the meanders at the south end of the river, and then filled in all the twists and turns, effectively deepening and straightening that section of it. This did help with the issue of overland flooding, but with the city growing up and around the valley, and with developments spreading down into it as well, any benefits weren't going to last. 

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, which is 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, 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, and 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.

RELATED: Why are southern Alberta communities prone to flooding?

This is what it really comes down to for the Don River and the Don Valley Parkway. The water that falls across that section of the city really has nowhere to go but straight into the river flow. If rainfall rates are high enough, there's going to be overland flooding anyway, regardless of where you are. This is because the ground can only absorb so much of that water so fast, depending on what that ground is made of. Artificial surfaces - like concrete and asphalt - don't generally absorb water as well as natural surfaces, or let that water flow through them as well either. With all the urbanization that's grown up on both sides of the Don Valley, storm water ends up flowing over the abundant artificial surfaces, into the city's storm sewers and then straight into the river. In the past, a large percentage of that water would have soaked into the ground and would have slowly make its way past the soil particles towards the rivers and streams. Now, most of it gets dumped into the river flow in a much more sudden and rapid rate, joining all the water that's fallen straight into the river and its tributaries, or run off from the valley floor. The result: all that flow converging on the Lower Don turns the Parkway into an extension of the river flow.

Recently, 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 of this year, both the Lower Don and Brickworks regions of the Don River (both bordering 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 (click here to see a report).

Is this going to get any better? In all likelihood, not anytime too soon. Conservation Authority staff work very hard on flood management of course, but there's only so much that you can do with a river that's been flooding since before the city was even here. However, there is one way that we can help. As the city has grown, many of the streams and rivers that run through it have been preserved, but they run through artificial environments now - metal pipes and concrete sewers which direct the flow but keep the water from being absorbed by natural soils. A new process, called daylighting, returns these streams and rivers to a more natural state, allowing more water to soak into the ground, which reduces the rate of flow in storm sewers and the rate at which that water gets dumped into the remaining open waterways.

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