Floods | Permeable pavements for more resilient cities

To make cities more resilient to flooding caused by climate change, researchers are developing more permeable road surfaces that allow water to flow through streets rather than running off the surface and into basements.

More and more in Quebec, the phenomenon is repeating itself: torrential rains overload underground networks that are not adapted to swallow such quantities of water, sewers back up, streets turn into swimming pools and houses are flooded.

To make cities more resilient to climate change, we must therefore allow rainwater to infiltrate into the ground without going through underground networks, for example by making parking lots, sidewalks and streets permeable.

Beyond nature-based solutions such as sponge sidewalks, “porous concrete”, “permeable asphalt” or even “paving stones with permeable joints” are all coatings that can be used to make cities more resilient to flooding, explained Sophie Duchesne, professor at INRS and specialist in urban water management.

However, it is easier to make a parking lot or a cycle path permeable than a street or a boulevard, in particular because of what is under the roadway.

“When you use a permeable surface, you still have to send the water somewhere, so you need a roadway underneath that can store the water” and “often in the streets, under the roadway, we have drinking water pipes, gas pipes, sewers,” which complicates water storage, explained Professor Duchesne, who works with municipalities to help them manage rainwater.

The example of Rivière-Du-Loup

Across the province, pilot projects are being developed to make parking lots, bike paths or sections of streets more permeable, for example in Rivière-Du-Loup, in Bas-Saint-Laurent.

Municipal engineer Pascal Gamache explained to The Canadian Press that 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 made by reducing the fine materials in the mix, which leaves space for water to seep through.

Tests carried out by the City have shown that this type of roadway can absorb a significant amount of rainwater, but the surface is not flawless, particularly in a northern climate.

With “this type of infrastructure,” explained Pascal Gamache, “we must avoid using de-icing salt or abrasives as much as possible.”

The salts used on roads in winter can “clog the pores” of the coating and therefore reduce its ability to absorb water. This coating therefore requires special maintenance with high-pressure water jets, among other things.

The city also tested another type of roadway that absorbs rainwater, installing “permeable paving stones” along two streets.

“Permeable paving stones” are essentially concrete blocks, but “rather than binding them with polymer sand,” which would make the roadway waterproof, “we use gravel which allows water to infiltrate,” the engineer said.

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 the water traveling through the pipes to avoid overloading the network,” the engineer summarized.

The challenge of heavy traffic

In some places in the United States, alternatives to traditional road surfaces, such as pervious concrete, are gaining popularity, according to researcher and assistant professor at the University of Washington Nara Almeida.

“I live in Washington State […] and here it is a very popular material, we can use it for sidewalks, but it can also be very effective for streets with low traffic,” explained the woman whose research focuses on sustainable materials for making pavements.

“However, one of the difficulties when applying it to roads with heavy traffic is that permeable concrete is not as strong as traditional concrete and cannot be reinforced,” the researcher explained.

Reinforced concrete roads, built for heavy traffic, are made of steel rods to strengthen the structure.

“But you can’t use steel in permeable concrete roads because the water will cause oxidation,” explained Nara Almeida, adding that she is currently studying different materials that could make permeable concrete “stronger.”

According to this American researcher, one of the indirect advantages of this type of coating is that it filters contaminants present in rainwater runoff.

“All kinds of pollution end up in runoff, for example, pollution from vehicle tires, and runoff can carry that pollution into rivers, lakes, all the way to the ocean,” but permeable concrete captures and “filters some of those pollutants.”

Only part of the solution

Surfaces like 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 areas, “there are several things we can do,” explained Professor Sophie Duchesne, such as, for example, reducing the width of streets.

“If we reduce the width of a street by a third, we will have reduced the quantity of asphalt by at least a third, and therefore by a third the quantity of impermeable surface which will generate runoff water that we will have to manage.”

Even though the streets are made of “traditional impermeable asphalt, we can still send the runoff from the streets to areas that are permeable, for example sidewalks that will be filled with vegetation, then with drainage material,” she added, referring to what is commonly called a 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.