Overview
Permeable hardscaping utilizes paving materials with spaces that allow stormwater to move through the driving or walking surface. Permeable hardscaping can replace asphalt and concrete and can be used for driveways, parking lots, and walkways. The three broad categories of permeable hardscaping are paving blocks; porous asphalt; and other surfaces including gravel, cobbles, wood, mulch, brick, and natural stone. 1
Paving blocks
Paving blocks are cement or plastic grid systems with gaps between them. Paving blocks make the surface more rigid and gravel or grass planted inside the holes allows for infiltration (see photos at right).
Porous asphalt
Porous asphalt has larger particles in the aggregate than traditional asphalt, allowing stormwater to infiltrate.
Other permeable surfaces
Gravel, cobbles, wood, and mulch also allow varying degrees of infiltration. Brick and natural stone arranged in a loose configuration allow for some infiltration through the gaps. Gravel and cobbles can be used as driveway material and wood and mulch can be used to provide walking trails.
Green Parking
Green parking refers to several techniques applied together to reduce the contribution of parking lots to the total impervious cover in a lot. From a stormwater perspective, application of green parking techniques in the right combination can dramatically reduce impervious cover and consequently, the amount of stormwater runoff. Green parking lot techniques include setting maximums for the number of parking lots created, minimizing the dimensions of parking lot spaces, utilizing alternative pavers in overflow parking areas, using bioretention areas to treat stormwater, encouraging shared parking and providing economic incentives for structured parking. 2
Benefits
Permeable hardscaping allows stormwater to infiltrate the soil thereby reducing the amount of runoff leaving a site. This can help to:
- provide for a more aesthetically pleasing site
- eliminate problems with standing water
- provide for groundwater recharge
- facilitate pollutant removal
- decrease thermal pollution (see bioswale page for more information)
- decrease downstream flooding
- reduce erosion of streambeds and riverbanks
- decrease the frequency of combined sewer overflow (CSO) events
- reduce and even eliminate the requirement for underground sewer pipes and conventional stormwater retention / detention systems 3
Cost Considerations
- Unit costs vary from $5 to $10 per square foot.
- Appropriately designed permeable paving areas may need no detention.
- Permeable paver surfaces typically last twice as long as asphalt and require less rehabilitative maintenance (resurfacing, etc.). 4
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