Author | Lucía Burbano

Water resources are becoming increasingly precious and scarce, necessitating the implementation of sustainable management solutions in urban areas. Permeable paving is gaining attention for its ability to harness rainwater, enabling cities to align sustainable practices with the natural water cycle and implement the sponge city concept.

To underscore the urgency of addressing rainwater management, it is worth noting that in 2023, global rainfall levels reached their lowest point since records began. Rainfall levels were 46.2 millimeters below the average recorded between 1901 and 2000, marking the third consecutive year of negative anomalies.

Non-permeable materials, such as concrete, cause rainwater to accumulate on impermeable surfaces. Furthermore, it collects pollutants such as oils, metals, and chemical products, which ultimately flow into the city’s rainwater systems, contributing to flooding, erosion, and degraded water quality in rivers and lakes.

Concrete also contributes to the heat island effect in built environments, trapping heat and causing urban areas to experience higher temperatures compared to surrounding rural regions.

How does permeable paving work?

The principle of permeable paving or surfaces is to allow water to pass through a subsurface base layer, enabling it to slowly filter into the surrounding soil or urban drainage networks.

There are several types of materials that can be selected based on the intended use of the paving.

For example, open-joint bricks with gaps between them or cobblestone paving combined with grass are suitable only for parking areas and low-traffic access routes. Gravel paving and conglomerates are less durable and are therefore better suited for bike paths, lanes, or private properties.

Semi-permeable paving materials, such as sawdust, seashells, gravel, or stone aggregates, allow rainwater to filter through but are not designed to support heavy loads.

The structure of these types of paving is divided into four layers, each separated by a permeable geotextile membrane.

Surface layer

This is the top layer, made of permeable material, that directly interacts with the surface.

Sand and gravel layer

This layer consists of washed sand and gravel with a minimum thickness of 75 mm, compacted to a modified density of 95%.

Retention trench

This is a weight-bearing substrate made up largely of crushed stone (basalt) and a small amount of filler soil mix at a typical ratio of 5:1 of volume that allows for the controlled passage of oxygen, water, and tree roots deep beneath concreted pavements and roads.

Subsurface layer

This is the most compact layer, designed to adhere to road construction standards.

Advantages and disadvantages

Permeable paving provides numerous advantages over conventional paving systems. For example:

• It reduces the risk of flooding by decreasing rainwater discharge peaks in paved areas.
• It enhances soil health by maintaining higher levels of moisture. This supports healthier trees and green spaces, making them more resilient to drought.
• It reduces the reliance on large-scale rainwater management infrastructures.
• It helps create a cooler urban environment during the summer.

However, this type of paving also has some disadvantages:

• One disadvantage of permeable paving is its high initial cost, as it requires specialized materials and installation techniques.
• Regular cleaning is necessary to maintain their effectiveness, as dirt, debris, and sediments can clog the gaps. Repairs can also be more complicated and expensive.
• Permeable paving is not suitable for all locations. Areas with high sediment loads or risks of groundwater pollution are not suitable for this type of paving. Furthermore, it has a limited load-bearing capacity.
• In colder climates, freeze-thaw cycles can pose challenges for permeable paving, potentially causing cracks and other damage that compromise its durability.

Cities that have implemented permeable paving

Melbourne

The Australian city has implemented permeable asphalt in car parks located at Eades Place and Harris St. In the former, it involves a residential street in West Melbourne, where thermal imaging revealed it to be one of the hottest streets in the city.

Following years of drought, many elm trees were in poor condition and needed replacement. The city used this opportunity to re-urbanize the street, aiming to harness rainwater to support the health of the newly planted trees.

Among its benefits, the transition to permeable paving has created an ideal environment for the growth of ten new trees, providing shade and helping to keep the street cooler. Permeability has increased to 482 m², enabling the capture of up to 790,000 liters of water annually.

New York

In 2023, the city announced plans to expand the use of permeable paving on streets and public spaces to manage rainwater overflow and promote environmental sustainability. Specifically, an infrastructure project in Queens utilized porous paving to absorb approximately forty-nine million liters of rainwater into the soil rather than diverting it to the sewage system.

This expansion continued into the summer of 2024 in Brooklyn, with the installation of seven miles of porous paving along local roads to enhance rainwater management, reduce flooding, and prevent sewer system overflows. This marks the largest implementation of porous paving in the city’s history.

Madrid

The parking area of the Wanda Metropolitano Stadium, home to Atlético de Madrid football club, is constructed with porous paving. This design filters rainwater on-site, preventing it from traveling long distances and reducing the risk of contamination. This paving also captures runoff water from nearby impermeable surfaces.

To complement this installation, underground storage tanks are located beneath the parking spaces to store the recovered water.

Shoreview, Minnesota

In 2007, concerns about the water quality of Lake Owasso in Shoreview prompted the implementation of the largest section of permeable paving in North America in 2009. The city adopted this type of paving to test its potential in colder climates to prevent frozen roads. This paving absorbs water, reducing the likelihood of it freezing over large sections.

After installing the first section, the city distributed leaflets to inform residents about their permeable streets and how they could help maintain their functionality. Suggestions included avoiding spreading soil on the roads during wintry weather. Since then, the city has incorporated an additional five sections of permeable paving through various road reconstruction projects.

Photographs | Unsplash/Harald Arlander, Unsplash/Bruno Martins, Unsplash/Collab media