Permeable paving

  Permeable paving, also called pervious paving or porous pavement, is a term used to describe paving methods for roads, parking lots and walkways that allow the movement of water and air through the paving material. Although some porous paving materials appear nearly indistinguishable from nonporous materials, their environmental effects are qualitatively different. Whether porous asphalt, concrete, paving stones or bricks, all these pervious materials allow precipitation to percolate through areas that would traditionally be impervious and instead infiltrates the stormwater through to the soil below. The infiltration capacity of the native soil is a key design consideration for determining the depth of base rock for stormwater storage or for whether an underdrain system is needed.

In new suburban growth, porous pavements protect pristine watersheds. In areas already built out, old town centers, redevelopment and reconstruction are opportunities for environmental rehabilitation

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Advantages of permeable paving

  Permeable paving surfaces are highly desirable because of the problems associated with water runoff from paved surfaces. Part of the problem is creating an unnatural volume of runoff from precipitation, which causes serious erosion and siltation in streams and other bodies of waters. Part of the problem is also the washing off of vehicular pollutants into water bodies.

Permeable paving surfaces keep the pollutants in place in the soil or other material underlying the roadway, and allow water seepage to recharge groundwater while preventing the stream erosion problems. They capture the heavy metals that fall on them, preventing them from washing downstream and accumulating inadvertently in the environment. In the void spaces, naturally occurring micro-organisms digest car oils, leaving little but carbon dioxide and water; the oil ceases to exist as a pollutant. Rainwater infiltration through the pavement into the underlying soil reduces stormwater volume and restores natural subsurface flow paths. The cost of porous pavement, with its built-in stormwater management, is usually less than that of an impervious pavement with a separate stormwater management facility somewhere downstream.

Porous pavements give urban trees the rooting space they need to grow to full size. A “structural-soil” pavement base combines structural aggregate with soil; a porous surface admits vital air and water to the rooting zone. This integrates healthy ecology and thriving cities, with the living tree canopy above, the city’s traffic on the ground, and living tree roots below.

However, permeable pavements require frequent maintenance because grit or gravel can block the open pores. This is commonly done by industrial vacuums that suck up all the sediment. If maintenance is not carried out on a regular basis, the porous pavements can begin to hold large amounts of water and cause flooding. With more advanced paving systems the levels of maintenance needed can been greatly decreased, concrete block permeable paving requires no more maintenance than regular concrete paving as the grit between the blocks enhances the filtering properties of the pavement. Also, in a large storm event, the water table below the porous pavement can rise to a higher level preventing the precipitation from being absorbed into the ground.

Porous paving has a disadvantage in places subject to freezing temperatures in winter: as water enter the holes, it may solidify here, and expand in the process, thus fracturing the pavement.

Types of permeable paving surfaces

Installation of porous pavements is not more difficult than that of dense pavements, but it is different, and its different specifications and procedures must be strictly adhered to. Nine different families of porous paving materials present distinctive advantages and disadvantages for specific applications. Here are examples:

"Porous asphalt" is mixed at conventional asphalt plants, but fine (small) aggregate is omitted from the mixture. The remaining large, single-sized aggregate particles leave open voids that give the material its porosity and permeability. Under the porous asphalt surface is a base course of further single-sized aggregate. Porous asphalt surfaces are being used on highways to improve driving safety by removing water from the surface.

"Porous concrete", like porous asphalt, can bear frequent traffic, and is universally accessible. It depends for its quality on a qualified installer.

"Single-sized aggregate" without any binder is the most permeable paving material in existence — and the least expensive. Although it can be used only in very low-traffic settings such as seldom-used parking stalls, its potential cumulative area is great.

"Porous turf", if properly constructed, can be used for occasional parking like that at churches and stadiums. Living turf transpires water, actively counteracting the “heat island” with what appears to be a green open lawn.

"Open-jointed blocks" are concrete or stone units with open, permeable spaces between the units. They give an architectural appearance, and can bear surprisingly heavy traffic, particularly interlocking paving blocks.

See also

• Bioswale

References

• Ferguson, Bruce K. (2005). Porous Pavements. Boca Raton: CRC Press. Ferguson-2005. ISBN 978-0849326707. 
• National Conference on Sustainable Drainage (UK)
• NOVATECH - International Conference On Sustainable Techniques And Strategies In Urban Water Management
• ICUD - International Conference on Urban Drainage