Impervious Surfaces Lose Appeal, Depaving Movement Gains Momentum

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Urban pavements, which help to maintain clean and secure streets, generate large amounts of runoff that aggravate flooding and degrade the quality of surface water. In the context of the ecological and climatic crisis, they also contribute to the creation of heat islands in cities, less water infiltration, less CO2 storage by plants and the soil, and a loss of biodiversity.

Experts agree that the greening of cities must be encouraged through depaving.

Ground surface temperatures of pavement can be significantly warmer than grass — 45 degrees Fahrenheit or higher under direct sunlight. Pavement is a hard, impermeable surface that is ubiquitous in urban areas. From parking lots and sidewalks to streets and highways, pavement covers vast amounts of land, contributing to a host of environmental problems.

Cities need to be designed like giant sponges that allow water to drain away safely. In its simplest iteration, a spongy city has planned green areas and permeable surfaces that absorb water during times of rainfall. The water seeps into an aquifer for later water use or evaporates and cools the city.

Depaving helps a city space to enhance its sponginess. Depaving is the process of removing unnecessary pavement and replacing it with plants or trees, permeable pavers, or permeable asphalt. Depaving allows stormwater to soak into the ground where it falls instead of picking up and carrying pollutants into creeks and waterways. A large network of depaving groups exists globally: Portland, OR, Chicago, Tennessee, Ontario, the United Kingdom, and more.

What are some approaches to mainstream depaving practices in the urban regeneration processes and planning instruments? Which type do you choose?

Water-permeables: With water-permeable ground materials, the water passes through the material because it is porous — the water runs along the material through the joints, perforations in the material, or between the material. Examples of this are grass tiles, shells, grass pebbles, gravel, or bare soil. Depaving and/or use of loose permeable ground covers should take place as much as possible but in ways that allow car circulation and minimize dust formation, especially during wind gusts and dry storm conditions.

Introducing vegetation: Trees, shrubs, and ground covers can be planted in the depaved strips along the sidewalks. Trees and higher shrubs should be planted interspersed to prevent the trapping of longwave radiation, of exhaustion fumes, and the blocking of wind (more effective when the prevailing wind direction is parallel to the street), according to authors in the journal Sustainable Development. Ground covers can be combined with trees and shrubs to reduce their vulnerability to damage by stepping or abusive car parking. These green pockets are alternated with parking spots not to compromise mobility and functionality, such as deliveries to shops. Climbing plants can cover in façades, directly planted on the ground, or in raised planters.

Adapting urban materials: Façade and ground surface dimensions are increased by applying light-colored paint or by re-coating them with cool surface materials. It can be a reflective coating spread over asphalt: a mixture of asphalt, water, mineral fillers, polymers, and recycled material. There should be no harmful chemicals that will run off the surface onto green space or waterways. This isn’t an adequate solution for high vehicle traffic areas, but, if it is a road surface where people walk their dogs, stroll with their children, or play a pickup game of basketball, then it might be a good start.

Depaving Case Study #1: Somerville, Massachusetts

Green & Open has been holding depaving parties in Somerville, MA for a decade. Ripping up asphalt from a yard reveals access to healthy green space. Planted with native plants, the newly exposed earth can soak up rainwater, which would otherwise skip across formerly impermeable surfaces. In heavily paved cities like Somerville, depaving captures excess stormwater runoff, additions of plants and greenery cool down surface temperatures, and carbon dioxide is removed from the atmosphere. Green & Grass has:

• fought against the installation of artificial turf on grass athletic fields;
• lobbied for significantly stronger sustainability language in city zoning;
• advocated for a minimum native plant ordinance — the first in the country;
• brought awareness to the need to halt the deteriorating tree canopy; and ,
• organized community events to plant native species gardens, weed invasive plants, and install planter gardens in highly paved areas.

As described by a Boston Globe article, a volunteer wedges a long metal bar underneath the asphalt where it meet a green space. The first volunteer pushes the bar down with their weight while a second volunteer swings a sledgehammer over their shoulder and head, then smashes it to the ground. After a few heavy swings, the asphalt cracks.

Their advocacy group’s aim is to identify the most critical areas within the urban context that need to be depaved to accommodate green canopies, trees, green roofs, and other water permeable solutions.

Case Study #2: Flanders, Belgium

Flanders is one of the most paved areas in Europe, as 16% of the surface is paved. Projections anticipate more than 20% will be paved by 2050 unless a significant change in urban construction occurs.

Instead of settling for the inevitable, the city is physically removing paved space, such as roads, parking lots, buildings, terraces, and driveways, so that the soil becomes permeable again and allows various other natural ecosystem functions. These actions invite more space for nature and food production, less flooding, a cooler city, greater biodiversity, better soil quality, healthier air, and a more pleasant living environment.

Advocacy works by facilitating, supporting, and supervising depaving projects, as well as by generating and disseminating knowledge and learning lessons related to the depaving that is taking place in Flanders.

In one project, water will be used as efficiently as possible: the collectible rainwater will be disconnected from the sewerage system and buffered on site. Rainwater will be reused and wadis and additional infiltration zones will be created.

Attention is also paid to biodiversity by providing native, regional plants, taking into account the flowering month.

The Flanders project acknowledges, that, due to safety, accessibility, or type of surface, a degree of hard surface is sometimes necessary. In such a case, water-permeable/water-passing paving offers an alternative, as these ensure that more rainwater can infiltrate, which replenishes the groundwater, reduces pressure on the sewers, and limits drought stress.