Stormwater Management Tool Evaluation: Porous Asphalt

There are several types of green stormwater infrastructure tools to manage urban stormwater runoff.  Tree trenches, sidewalk bump-outs, planters, pervious pavement, green roofs, rain barrels, and rain gardens are common examples. 

The Darby-Cobbs watershed has implemented some porous basketball courts, as mentioned in my last post, to manage stormwater runoff.  Water can soak into the ground slowly through the porous asphalt rather than rush to a stormwater drain and go through the combined sewer system.  This helps replenish groundwater, prevent flash flooding, and protect rivers and streams within a watershed. 

Porous asphalt, pervious concrete, and interlocking concrete pavers are the three main types of pervious pavement.  The following is a review of porous asphalt as a practice to manage stormwater runoff.  

Porous asphalt (Source: National Asphalt Pavement Association)

Function
Porous asphalt used in place of traditional impervious paving materials allows stormwater runoff to filter through voids in the pavement surface into an underlying stone reservoir, where it is temporarily stored and/or infiltrated. This also assists with reducing peak runoff velocity and volume.  It is well suited for parking lot areas, playgrounds, and basketball courts.

A schematic cross section of permeable paving. Image: Cahill Associates, Inc. (Source: Dauphin County, PA Porous Asphalt Fact Sheet).

 

Porous asphalt itself provides for some pretreatment of runoff.  Porous asphalt consists of standard bituminous asphalt in which the fines have been screened and reduced, creating void space to make it highly permeable to water.  The void space of porous asphalt is approximately 16%, as opposed to two to three percent for conventional asphalt (Dauphin County, PA Porous Asphalt Fact Sheet).  Letting oil into a porous pavement’s voids is the whole idea in water-quality improvement.  In the pavement, naturally occurring microorganisms biodegrade hydrocarbons (i.e. oil) before they migrate to the bottom of the pavement.  The constituents go off as carbon dioxide and water vapor, and very little else; the hydrocarbons cease to exist as water-quality pollutants (American Society of Landscape Architects).

It’s also recommended for a stone-edge around the paved surface to act as a back-up method for water to enter the underground reservoir.  This will allow water to reach underground even if the lot is repaved with impervious surface (Cahill et al. 2003).

Cost

Porous asphalt does not usually cost more than conventional asphalt on a yard-by-yard basis.  The underlying stone bed is usually more expensive than a conventional compacted sub-base, but this cost difference is usually offset by the reduced need for stormwater pipes and inlets (Cahill et al. 2003).  When considering cost savings by eliminating the need for a separate detention basin for stormwater retention, the overall cost for porous asphalt makes even more economic sense.

Maintenance

It is recommended that all porous pavement surfaces, including porous asphalt, is vacuum-swept twice a year with an industrial vacuum sweeper to remove sediment from the pores.  Annual maintenance inspections in the spring are recommended to check for signs of clogging and the general condition of the site (Virginia Department of Conservation and Recreation).  With proper maintenance, porous asphalt can have a minimum life span of 20 years (Dauphin County, PA Porous Asphalt Fact Sheet).

Limitations

Porous paving materials are not effective at removing dissolved nutrients from water; therefore, they should be installed at least 100 feet from drinking water sources.  Porous asphalt should also not be located: within four feet above bedrock or a water table’s high point, within 10 feet of a building foundation that is above proposed pavement or 100 feet from a building foundation that is below proposed pavement, within close proximity of sources of contaminants (e.g. gas stations), and on slopes that exceed five percent (Lake Superior Streams).

Porous asphalt should be placed in areas with highly permeable soils, with a preferred rate of 0.5 inches per hour (Lake Superior Streams). 

Recommendations

Proper site selection and construction methods are vital to porous asphalt’s success.  It’s advised not to use porous asphalt, and other pervious pavements, in northern climates due to concern for heavy clay soils and sanding used to prevent icing of roads (Lake Superior Streams), unless these factors can be avoided. 

For more information, check out the National Asphalt Pavement Association’s Design, Construction, and Maintenance Guide.