Cities need permeable pavements for resiliency to flooding: experts

A person bikes across a flooded street due to a water main break in Montreal, Friday, August 16, 2024, causing flooding in several streets of the area. (Graham Hughes, The Canadian Press)

Researchers are developing more permeable pavements to allow water to flow through streets rather than run off the surface and end up in residential basements in an effort to make cities more resilient to flooding caused by climate change.

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

Experts say rainwater needs to infiltrate the ground without passing through the underground networks by making parking lots, pavement and streets permeable, for example.

"In addition to nature-based solutions such as sponge pavement, porous concrete, permeable asphalt and pavement with permeable joints are all pavements that can be used to make cities more resilient to flooding," explained Sophie Duchesne, a professor at INRS and a specialist in urban water management.

However, it is easier to make a parking lot or bike path permeable than a street or boulevard, mainly 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 can store the water and often in streets, under the pavement, you have drinking water pipes, gas pipes, sewer pipes, which complicates water storage," said Duchesne, who works with municipalities to help them manage rainwater.

The Riviere-du-Loup example

Across the province, pilot projects are being developed to make parking lots, bike paths and sections of street more permeable, as in Riviere-Du-Loup, in the Bas-Saint-Laurent region.

Municipal engineer Pascal Gamache told The Canadian Press that the town first carried out two tests with porous asphalt in two parking lots and that "in both cases, the results were satisfactory."

Porous asphalt is obtained by reducing the fine materials in the mix, which leaves space for water to infiltrate.

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

"With this type of infrastructure," Gamache explained, "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 surface, reducing its ability to absorb water. The pavement, therefore, requires special maintenance, including high-pressure water jetting.

The city has also tested another type of pavement that absorbs rainwater by installing permeable pavements along the edges of two streets.

"Permeable pavements are essentially concrete blocks, but rather than bonding them with polymer sand, which would make the pavement watertight, gravel is used to allow water to infiltrate," the engineer said.

He added that the projects have produced "good results" so far.

"What we want in the future is not just to capture the water and bring it from point A to point B, we want it to stay in the ground, we want to avoid the water travelling through the pipes to avoid overloading the network," said Gamache.

The heavy traffic challenge

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

"I live in Washington State, and here, it's a very popular material," she said. "It can be used for pavements, but it can also be very effective for low-traffic streets."

Almeida's research focuses on sustainable materials for pavements.

"However, one of the difficulties encountered when applying it to roads with heavy traffic is that permeable concrete is not as strong as traditional concrete and cannot be reinforced," she said.

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

"But you can't use steel in permeable concrete roads, because the water will cause oxidation," explained Almeida, adding that she was "very interested in the use of steel in concrete roads."

"All kinds of pollution can be found in run-off water, for example, pollution caused by vehicle tyres, and run-off water can carry this pollution into rivers, lakes and even 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 areas, "there are a number of things we can do," explained professor Sophie Duchesne, such as reducing the width of streets.

"If we reduce the width of a street by a third, we will have at least reduced the amount of asphalt by a third, and therefore the amount of impermeable surface by a third, which will generate run-off that we will have to manage," she said.

"Even if the streets are made of the traditional "impermeable asphalt," run-off water will run from the streets to areas that are permeable, for example pavements filled with vegetation and then with drainage material," she added, referring to what is commonly known as a "sponge pavement" or "sponge parks."

Demineralising certain types of soil, i.e. removing unnecessary asphalt to leave natural areas to absorb rainwater, is recognised as an effective measure for adapting to climate change.

This report by The Canadian Press was first published in French on Aug. 18, 2024.