Peter Moe, Director of Operations, University of Minnesota Landscape Arboretum
Permeable, or porous, paving is a type of paving that allows water to drain through to the ground below. Water can then directly infiltrate on-site rather than being diverted to a drain for off site treatment. These systems protect surface water, recharge aquifers, reduce flooding and reduce the need for traditional storm sewer systems.
There are three basic types of permeable paving:
• Porous asphalt or porous concrete mixes contain little sand or fine materials and rely on the concrete or asphalt binder to hold the stones and crushed rock together, but still leave voids for water to pass through. Both resemble a rice crispy bar.
• Permeable paver systems typically consist of highly drainable layers below a layer of concrete pavers that are designed to allow water to pass through joints between them. They resemble typical modular pavers or bricks but have corners missing or built in spacers that allow for water infiltration.
• Turf reinforcement systems offer support for vehicles and protects the grass from the traffic damage while allowing natural groundwater replenishment. These systems come as plastic grids with cells that are filled with a sandy topsoil mix and planted with grass. When properly installed, the turf grows over the height of the plastic reinforcing panels and is indistinguishable from a non-reinforced lawn.
Permeable paving should be considered on all projects and is a useful tool for stormwater management. It is particularly practical for projects with tight sites and strict stormwater run-off regulations.
Permeable asphalt and concrete and permeable paving blocks are designed for the heaviest use; they are meant for daily, low-speed use such as in parking lots, driveways, residential drives and similar areas. Generally, turf support structures are intended for infrequently used areas, such as event grounds, fire lanes, overflow or event parking, and other spaces where there is little regular traffic.
The use of permeable paving decreases peak flow rates in area lakes, streams and rivers during a rain event because it allows water to infiltrate into the ground on site. This infiltration reduces the amount and speed of water leaving the site during a rain event. Permeable paving also reduces the urban heat island effect and can reduce the need for air conditioning.
Design and Installation
• Soil type will affect infiltration rates and must be considered when designing a permeable paver system. Subsurface drain tile may be required if clay subsoils are present.
• Subbase depth depends on the system selected, existing soils and the type of traffic the site will receive. Additional coarse, crushed rock with no fine materials may be required for temporary water storage.
• Careful installation by trained personnel will prevent settling and result in a system that will continue to function properly and be easier to maintain.
Porous asphalt, porous concrete and permeable pavers should be cleaned 2-3 times each year with a street sweeper to prevent sand, dirt, seeds and other materials from plugging the surface. Sand should not be applied and typically isn't needed since water does not pool on the surface and freeze. Snow plows and snow blowers will not damage a properly installed system.
Reinforced turf may require more watering since the sand and gravel base do not hold much water in between rain showers. Slow release fertilizer will keep the grass vigorous and allow for faster recovery after vehicle or foot traffic.
The cost of permeable paving systems varies by technology and location. The estimates provided here are based on information from several sources including, the Low Impact Development Center, ToolBase.org, and the Seattle Rite of Way Manual.
Paving Type Cost/sf (installed)
Asphalt $0.50 - $1.00
Porous Concrete $2.00 - $6.50
Grass Reinforcing System $1.50 - $5.75
Interlocking Concrete Paving Blocks $5.00 - $10.00
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