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Pilot projects are being developed across Quebec to make parking lots, bike paths or portions of streets more resilient to climate change.
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To make cities more resilient to flooding caused by climate change, researchers are developing more permeable pavements to allow water to flow through streets instead of running off the surface and ending up in residential basements.
This is happening more and more in Quebec: torrential rains overload underground networks that are not able to deal with such large quantities of water. Then the sewers back up, the streets are transformed into swimming pools and homes are flooded.
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To make cities more resilient to climate change, one idea is to allow rainwater to infiltrate the ground without passing through underground networks by making parking lots, sidewalks and streets more permeable, for example.
Beyond nature-based solutions such as sponge sidewalks, “porous concrete,” or “permeable asphalt” or “pavers with permeable joints” are all types of pavements that can be used to make cities more resilient to flooding, said Sophie Duchesne, a professor at INRS and specialist in urban water management.
However, it’s easier to make a parking lot or bike path permeable than a street or boulevard, because of what lies beneath the pavement.
“When you use a permeable surface, you still have to send the water somewhere, so you need a pavement underneath that will be able to store the water” and often in the streets, under the pavement, you have water pipes, gas pipes, sewer pipes, which complicates water storage, said Duchesne, who works with municipalities to help them manage stormwater.
The Rivière-Du-Loup example
Across the province, pilot projects are being developed to make parking lots, bike paths or portions of streets more permeable, notably in Rivière-Du-Loup, in the Lower St. Lawrence.
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City engineer Pascal Gamache explained the city first carried out two tests with porous asphalt, in two parking lots, and “in both cases, we had satisfactory results.
Porous asphalt is obtained by reducing the fine materials in the mix, leaving space for water to infiltrate.
Tests carried out by the city have shown this type of pavement can absorb a significant amount of rainwater, but the pavement is not flawless, particularly in a northern climate.
With “this type of infrastructure,” Gamache said, “you have to avoid using de-icing salt or abrasives as much as possible.”
The salts used on roads in winter can “clog the pores” of the pavement, reducing its ability to absorb water. This pavement, therefore, requires special maintenance, including high-pressure water cleaning.
The city has also tested another type of pavement that absorbs rainwater by installing “permeable pavers” along the edges of two streets.
These pavers are essentially concrete blocks, but “rather than bonding them with polymer sand,” which would make the pavement watertight, “we use gravel, which allows water to infiltrate”, Gamache said.
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These projects have yielded good results so far, he added.
“What we want in the future is not just to capture water and bring it from point A to point B, we want it to stay in the ground, we want to avoid water running through the pipes to avoid overloading the network”, the engineer explained.
The challenge of heavy traffic
In some places in the U.S., alternatives to traditional road surfaces, such as pervious concrete, are gaining in popularity, according to Nara Almeida, a researcher and assistant professor at the University of Washington.
“I live in Washington State (…) and here it’s a very popular material, you can use it for sidewalks, but it can also be very effective for low-traffic streets,” explained Almeida, whose research focuses on sustainable materials for pavement manufacture.
“However, one of the difficulties in applying it to roads with heavy traffic is permeable concrete is not as strong as traditional concrete and cannot be reinforced,” Almeida said.
Reinforced concrete roads, built for heavy traffic, are made up of steel rods to reinforce the structure.
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“But you can’t use steel in permeable concrete roads, because water will cause oxidation”, Almeida, explained, adding she is currently studying different materials that could make permeable concrete stronger.
According to this U.S. researcher, one of the indirect benefits of this type of pavement is it filters contaminants present in stormwater runoff.
“All kinds of pollution are found in stormwater runoff, for example, pollution caused by vehicle’s tires, and runoff water can carry this pollution into rivers, lakes, even all the way to the ocean”, but permeable concrete captures and filters out some of these pollutants.
Only part of the solution
Pavements such as porous asphalt or permeable concrete can absorb rainwater and make cities more resilient to climate change, but they are only part of the solution.
To mitigate the consequences of flooding in residential neighbourhoods, “there are a number of things we can do,” Duchesne said, such as reducing the width of streets.
“If we reduce the width of a street by a third, we’ll have at least reduced the amount of asphalt by a third, so reduce by a third the amount of impermeable surface that will generate runoff that we’ll have to manage.”
Even if the streets are “traditional impermeable asphalt, we can still send runoff from the streets to areas that are permeable, for example sidewalks that are going to be filled with vegetation, then with draining material,” she added, referring to what is commonly called sponge sidewalk or sponge park.
Demineralizing certain soils, i.e. removing unnecessary asphalt to leave natural spaces to absorb rainwater, is recognized as an effective measure for adapting to climate change.
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