Permeable vs. Impermeable Surfaces in the Landscape
What is the difference between permeable and impermeable surfaces?
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. Impervious surfaces are solid surfaces that don’t allow water to penetrate, forcing it to run off.
CHOOSE THIS
Permeable Surfaces
Mulched soil
Gravel & decomposed granite
Permeable pavers
Permeable concrete/asphalt
Decking
NOT THIS
Impermeable Surfaces
Asphalt
Concrete
Traditional stone, brick or concrete
Pavers, traditional
Plastics as weed covers
How do impermeable surfaces affect the environment?
Cities and suburbs typically contain large expanses of non pervious surface, causing a host of problems:
ý Pollution When stormwater runs off impermeable surfaces, it picks up landscape and other pollutants such as fertilizers, herbicides, pesticides, pet feces, etc 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.
ý Flooding and Erosion During periods of heavy rainfall, large amounts of impermeable surfaces generate large amounts of fast moving runoff. This sudden influx of runoff into rivers causes flooding and erosion of stream banks.
ý Water table decimation 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.
ý Undesirable pests On impermeable surfaces where runoff has no
drainage route, stormwater can puddle for long periods of time. Stagnant puddles can
become breeding places for undesirable insects such as mosquitoes.
ý 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 summer cooling.
How can I reduce the amount or effects of impermeable surfaces?
S Replace non porous surfaces in your landscape to promote maximum permeability.
S Slow the flows to reduce the environmental impact of impermeable surfaces through on-site management of stormwater, such as:
S 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, as well as a viable frontyard option to thirsty turf grass. See RiverFriendly.org
S Rain barrels, rain chains and cisterns Water tanks stored above and/or below ground can capture rainwater from downspouts for later use. Rain chains slow the flow from the roof.
S Mulch Keep soils covered with mulch and plant cover which hold moisture in soil during drought and slow rainwater flows during heavy storms.
ACTION
Want to learn more?
S To see permeable landscape options in action, visit the award winning Elk Grove Rain Garden Plaza, the region's largest rain garden. This unique one acre educational watershed park includes a gorgeous low water garden and educational exhibits.
9385 Laguna Springs Drive, Elk Grove CA 95758
S Watch the locally made 29 minute documentary Slow the Flow-- Make Your Landscape Act More Like a Sponge. http://www.waterboards.ca.gov/stormfilm/
S Read Brad Lancaster's books Rainwater Harvesting for Drylands and Beyond Volumes 1-3