Permeable surfaces (also known as porous or pervious surfaces) allow water to percolate into the soil to filter out pollutants and recharge the water table. Impermeable/impervious surfaces are solid surfaces that don’t allow water to penetrate, forcing it to run off.

Permeable Surfaces	Impermeable Surfaces
Asphalt	Planting beds
Concrete	Mulch beds
Traditional stone, brick or concrete	Gravel
pavers	Permeable pavers
	Turf

What is the impact of impermeable surfaces on the environment?

Urban and suburban sites typically contain large expanses of impermeable surface, causing a host of problems:

Pollution of surface water. When stormwater runs off impermeable surfaces, it picks up pollutants as it flows into storm drains. The contaminated water then flows directly into rivers, lakes, wetlands and oceans, generating problems for biodiversity as well as public health.
Flooding of surface water and erosion of stream banks. During periods of heavy rainfall, large amounts of impermeable surfaces generate large amounts of runoff. This sudden influx of runoff into rivers can cause flash flooding and erosion of stream banks.
Water table is not adequately recharged. Because impermeable surfaces send rainwater into storm drains rather than allow it to percolate down to our aquifers, groundwater may be used faster than it is recharged.
Formation of stagnate water puddles. On impermeable surfaces where runoff has no drainage route, stormwater can puddle for long periods of time. Stagnate puddles can become breeding places for undesirable insects such as mosquitoes.
Heat island effect. Due to the heat-absorbing quality of asphalt and other paving materials, sites with high ratios of impermeable surfaces increase ambient air temperatures and require more energy for cooling.

How can I reduce the amount or effects of impermeable surfaces?

Provide maximum permeability of surfaces in your landscape. Replace surfaces in your landscape to promote maximum permeability.

Reduce the environmental impact of impermeable surfaces through on-site management of stormwater, such as:
- Rain gardens. Rain gardens, sometimes called bio-retention areas, are shallow depressions in the landscape that capture stormwater and allow it to gradually percolate into the soil. Planted with moisture-loving plants that help filter out pollutants, rain gardens provide an attractive way to reduce the impact of stormwater on the environment. For more information, consult the fact sheet “Rain Gardens"
- Rain barrels and cisterns. Water tanks stored above and/or below ground can capture rainwater from downspouts for later use. They are available in many styles, materials and sizes to accommodate a variety of needs. For more information, consult the fact sheet “Harvesting Water”
- Green roofs. A green roof is a specially-engineered rooftop that supports plant life and captures rainwater before it runs off. Green roofs have been utilized in Europe for 30 years and are quickly gaining popularity in the United States. For more information, consult the fact sheet “Green Roofs"
- French drains. French drains are ditches filled with gravel or rock used to capture stormwater and direct its flow. They can be utilized on the downslope side of impermeable surfaces to move runoff to an area where it can infiltrate the soil.

Permeable Paving Systems

Permeable pavers are specially fabricated paving units designed to replace asphalt and other impermeable paving materials. Interconnected pore spaces within the material channel water into the underlying soil or into a special storage layer which forces slow percolation during periods of heavy rainfall. Permeable pavers are often laid on a bed of sand or gravel to enhance drainage properties.

Examples of permeable paving systems

Grid of concrete pavers and void space filled with turf, sand or gravel
Aggregate of large stone particles and concrete with interwoven pore spaces
Turf system supported by a grid composed of post-consumer recycled plastic

Bibliography

Collins, Kelly. (2007) Permeable Pavement Research. North Carolina State University. Retrieved November 9, 2008
from http://www.bae.ncsu.edu/info/permeable-pavement/

Booth, Derek B., Jennifer Leavitt, and Kim Peterson. (Unknown date). The University of Washington Permeable Pavement Demonstration Project: Background and First-Year Field Results. Center for Urban Water Resources Management, Department of Civil Engineering, University of Washington. Retrieved November 9, 2008 from http://depts.washington.edu/cuwrm/research/rc3.pdf

Balogh, Anne. (2008). Permeable Surfaces: A Hot New Installation Segment. Landscape Online. February 2008.
Retrieved November 9, 2008 from http://www.landscapeonline.com/research/article/10426

University of Washington. (Unknown Date). Center for Water and Watershed Studies. University of Washington.
Retrieved December 4, 2008 from http://depts.washington.edu/cuwrm/

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