Permeable paving: a sustainable solution for stormwater runoff

by Madelein
Permeable paving: a sustainable solution for stormwater runoff

Permeable paving is an innovative pavement technique that allows stormwater to pass through the surface, into the subbase and percolate into the ground or find its way to a drainage system. This sustainable technique has been shown to reduce stormwater runoff, keep solids and pollutants from flowing into drainage facilities, naturally filter and recharge stormwater to ground water, and reduce the damage caused to pavement over time by puddles.

While permeable paving is not a new exterior flooring material, it is being used increasingly in projects. Specialist engineer from JC Paving Consulting, John Cairns, says that while projects in Gauteng are using permeable paving because of necessity, it has been legislated in Cape Town, leading to a larger uptake in that region.

“Permeable paving helps with noise reduction, improvement in the urban heat island effect, and it condenses the number or area of drainage facilities needed to sustain a newly paved area. In Johannesburg, developers are often required to install permeable paving because the municipality’s roads can’t always handle the runoff from stormwater,” says John.

The fact that stormwater can enter soil and recharge groundwater is especially helpful during peak rainfall intensities, because porous pavement can decrease the demand on stormwater systems by allowing stormwater to infiltrate the pavement and underlying soil instead of causing stormwater drainage systems to reach capacity or overflow. In addition to reducing the volume of runoff, pervious pavement can filter nutrients and pollutants from the initial flush of stormwater runoff. Studies have seen permeable asphalts, concretes and pavers remove up to 94 percent of toxins from the infiltrated stormwater, preventing those toxins from entering the groundwater or stormwater system.

“With the projects that I have been involved in in the Gauteng area, developers have opted for permeable paving because the municipality insists that the projects control their stormwater. It hasn’t been an environmental consideration, even though there are numerous sustainability benefits,” says John.

Flagship examples
A flagship example of a permeable paving project is the two car parks that the University of the Witwatersrand built near Empire Road. “The municipality told the developers that the Empire Road bridge wouldn’t be able to accommodate a cascade of water from the car parks. The project team solved this challenge by installing permeable paving to control the stormwater from the car parks,” says John.

Permeable paving being installed at the University of the Witwatersrand in May 2008

Although permeable pavement was initially developed for foot and bicycle traffic because of the sustainability benefits it provides, there is growing interest in advancing the use of permeable pavement in highway storm water management.

“Permeable paving is no longer new, but I suspect that it will need to be legislated before we see more installations in Gauteng,” says John.

The permeable paving at the University of the Witwatersrand still looking good in 2009.

What type of installation is required?
The installation of permeable pavement is often more challenging and sensitive than the process of laying asphalt or pouring concrete. For the design of permeable pavement systems to be maximised, there should be no grade within the space.

The materials needed during installation are also very different to those needed during a conventional paving installation. While no reinforcement is required, contractors will need to install mats of geosynthetic material to keep the bedding materials in place.

“Normally, contractors would use river sand for the bedding layer beneath the pavers and plaster sand to fill the joints. These materials won’t work for a permeable paving installation because they do not filter the water fast enough. Small stones should be used instead of sand. Using the right materials will ensure that a certain percentage of voids can be met so that the water filters smoothly,” says John.

Maintenance considerations
Porous pavement requires maintenance, typically annually. Landscaping materials such as mulch or sand cannot be near pervious pavement.

“A certain amount of clogging can be expected because the small stones act as a filter. A permeable paving system is designed for ten years, but test studies in Australia showed that installations still performed well after 15 years. Besides minor maintenance to reduce clogging, your permeable paving system should be maintenance-free for 15 years as long as it was installed correctly,” says John

Load considerations
Permeable pavements cannot withstand as heavy loads as traditional pavements. Permeable pavements function best within parking lots, multi-use paths, sidewalks, trails, shoulders and alleys. The high void ratio results in lower load bearing capacity. Trucks and high-speed vehicles are not ideal users of permeable pavement designs.

During the design phase, two separate designs are done, namely a structural design to ensure that the base is suitable for loading, and a hydraulic design to ensure the base can handle the volume of water. The design used will be based on the design that delivers the thickest base,” says John.

Specialised tip: In Johannesburg, developers are often required to install permeable paving because the municipality’s roads can’t always handle the runoff from stormwater.

Full thanks and acknowledgement are given to John Cairns from JC Paving Consulting and www.il-asphalt.org, for some of the information contained in this article.

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