United States Office of Water EPA 832-F-99-023
Environmental Protection Washington, D.C. September 1999
Agency
&EPA Storm Water
Technology Fact Sheet
Porous Pavement
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v>EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-99-023
September 1999
Storm Water
Technology Fact Sheet
Porous Pavement
DESCRIPTION
Porous pavement is a special type of pavement that
allows rain and snowmelt to pass through it, thereby
reducing the itinoff from a site and surrounding areas.
In addition, porous pavement filters some pollutants
from the runoff if maintained.
There are two types of porous pavement: porous
asphalt and pervious concrete. Porous asphalt
pavement consists of an open-graded coarse
aggregate, bonded together by asphalt cement, with
sufficient interconnected voids to make it highly
permeable to water. Pervious concrete consists of
specially formulated mixtures of Portland cement,
uniform, open-graded coarse aggregate, and water.
Pervious concrete has enough void space to allow
rapid percolation of liquids through the pavement.
The porous pavement surface is typically placed over
a highly permeable layer of open-graded gravel and
crushed stone. The void spaces in the aggregate layers
act as a storage reservoir for runoff. A filter fabric is
placed beneath the gravel and stone layers to screen
out fine soil particles. Figure 1 illustrates a common
porous asphalt pavement installation.
Two common modifications made in designing porous
pavement systems are (1) varying the amount of
storage in the stone reservoir beneath the pavement
and (2) adding perforated pipes near the top of the
reservoir to discharge excess storm water after the
reservoir has been filled.
Some municipalities have also added storm water
reservoirs (in addition to stone reservoirs) beneath the
pavement. These reservoirs should be designed to
accommodate runoff from a design storm and should
provide for infiltration through the underlying subsoil.
APPLICABILITY
Porous pavement may substitute for conventional
pavement on parking areas, areas with light traffic, and
the shoulders of airport taxiways a runways, provided
that the grades, subsoils, drainage characteristics, and
groundwater conditions are suitable. Slopes should be
flat or very gentle. Soils should have field-verified
permeability rates of greater than 1.3 centimeters (0.5
inches) per hour, and there should be a 1.2 meter
(4-foot) minimum clearance from the bottom of the
system to bedrock or the water table.
ADVANTAGES AND DISADVANTAGES
The advantages of using porous pavement include:
• Water treatment by pollutant removal.
• Less need for curbing and storm sewers.
• Improved road safety because of better skid
resistance.
• Recharge to local aquifers.
The use of porous pavement may be restricted in cold
regions, arid regions or regions with high wind erosion
rates, and areas of sole-source aquifers. The use of
porous pavement is highly constrained, requiring deep
permeable soils, restricted traffic, and adjacent land
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Berm Keeps Off-site Runoff
and Sediment Out, Provides
Temporary Storage
Asphalt is Vacuum Swept,
Followed by Jet Hosing to
Keep Pores Open
Sign Posted to Prevent
Resurfacing and Use of
Abrasives, and to Restrict
Truck Parking
Overflow
Pipe
Filter Fabric Lines Sides
of Reservoirto Prevent
Sediment Entry
Perforated Pipe Discharges
Only When 2-Year Storage
Volume Exceeded
Stone Reservoir Drains in 48 - 72 Hours
Observation Well
Gravel Course or 6-
Inch Sand Layer
Undisturbed Soils with a Field Capacity > 0.27
Inches/Hour Preferably >0.50 Inches/Hour
Source: Modified from MWCOG, 1987.
FIGURE 1 TYPICAL POROUS PAVEMENT INSTALLATION
uses. Some specific disadvantages of porous
pavement include the following:
• Many pavement engineers and contractors
lack expertise with this technology.
• Porous pavement has a tendency to become
clogged if improperly installed or maintained.
Porous pavement has a high rate of failure.
• There is some risk of contaminating
groundwater, depending on soil conditions and
aquifer susceptibility.
• Fuel may leak from vehicles and toxic
chemicals may leach from asphalt and/or
binder surface. Porous pavement systems are
not designed to treat these pollutants.
• Some building codes may not allow for its
installation.
• Anaerobic conditions may develop in
underlying soils if the soils are unable to dry out
between storm events. This may impede
microbiological decomposition.
As noted above, the use of porous pavement does
create risk of groundwater contamination. Pollutants
that are not easily trapped, adsorbed, or reduced, such
as nitrates and chlorides, may continue to move
through the soil profile and into the groundwater,
possibly contaminating drinking water supplies.
Therefore, until more scientific data is available, it is not
advisable to construct porous pavement near
groundwater drinking supplies.
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In addition to these documented pros and cons of
porous pavements, several questions remain regarding
their use. These include:
Whether porous pavement can maintain its
porosity over a long period of time, particularly
with resurfacing needs and snow removal.
Whether porous pavement remains capable of
removing pollutants after subfreezing weather
and snow removal.
The cost of maintenance and rehabilitation
options for restoration of porosity.
DESIGN CRITERIA
Porous pavement - along with other infiltration
technologies like infiltration basins and trenches - have
demonstrated a short life span. Failures generally have
been attributed to poor design, poor construction
techniques, subsoils with low permeability, and lack of
adequate preventive maintenance. Key design factors
that can increase the performance and reduce the risk
of failure of porous pavements (and other infiltration
technologies) include:
• Site conditions;
Construction materials; and
• Installation methods.
These factors are discussed further in Table 1.
PERFORMANCE
Porous pavement pollutant removal mechanisms
include absorption, straining, and microbiological
decomposition in the soil. An estimate of porous
pavement pollutant removal efficiency is provided by
two long-term monitoring studies conducted in
Rockville, MD, and Prince William, VA. These
studies indicate removal efficiencies of between 82 and
95 percent for sediment, 65 percent for total
phosphorus, and between 80 and 85 percent of total
nitrogen. The Rockville, MD, site also indicated high
removal rates for zinc, lead, and chemical oxygen
demand. Some key factors to increase pollutant
removal include:
• Routine vacuum sweeping and high pressure
washing (with proper disposal of removed
material).
• Drainage time of at least 24 hours.
Highly permeable soils.
• Pretreatment of runoff from site.
Organic matter in subsoils.
• Clean-washed aggregate.
Traditionally, porous pavement sites have had a high
failure rate - approximately 75 percent. Failure has
been attributed to poor design, inadequate construction
techniques, soils with low permeability, heavy vehicular
traffic, and resurfacing with nonporous pavement
materials. Factors enhancing longevity include:
Vacuum sweeping and high-pressure washing.
• Use in low-intensity parking areas.
Restrictions on use by heavy vehicles.
• Limited use of de-icing chemicals and sand.
Resurfacing.
• Inspection and enforcement of specifications
during construction.
• Pretreatment of runoff from offsite.
• Implementation of a stringent sediment control
plan.
OPERATION AND MAINTENANCE
Porous pavements need to be maintained.
Maintenance should include vacuum sweeping at least
four times a year (with proper disposal of
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TABLE 1 DESIGN CRITERIA FOR POROUS PAVEMENTS
Design Criterion
Guidelines
Site Evaluation
Traffic conditions
Design Storm Storage Volume
Drainage Time for Design Storm
Construction
Porous Pavement Placement
Pretreatment
Take soil boring to a depth of at least 1.2 meters (4 feet) below bottom of stone
reservoir to check for soil permeability, porosity, depth of seasonally high water
table, and depth to bedrock.
Not recommended on slopes greater than 5 percent and best with slopes as
flat as possible.
Minimum infiltration rate 0.9 meters (3 feet) below bottom of stone reservoir:
1.3 centimeters (0.5 inches) per hour.
Minimum depth to bedrock and seasonally high watertable: 1.2 meters (4
feet).
Minimum setback from water supply wells: 30 meters (100 feet).
Minimum setback from building foundations: 3 meters (10 feet) downgradient,
30 meters (100 feet) upgradient.
Not recommended in areas where wind erosion supplies significant amounts
of windblown sediment.
Drainage area should be less than 6.1 hectares (15 acres).
Use for low-volume automobile parking areas and lightly used access roads.
Avoid moderate to high traffic areas and significant truck traffic.
Avoid snow removal operations; post with signs to restrict the use of sand,
salt, and other deicing chemicals typically associated with snow cleaning
activities.
Highly variable; depends upon regulatory requirements. Typically design for
storm water runoff volume produced in the tributary watershed by the 6-month,
24-hour duration storm event.
Minimum: 12 hours.
Maximum: 72 hours.
Recommended: 24 hours.
Excavate and grade with light equipment with tracks or oversized tires to
prevent soil compaction.
As needed, divert storm water runoff away from planned pavement area before
and during construction.
Atypical porous pavement cross-section consists of the following layers: 1)
porous asphalt course, 5-10 centimeters (2-4 inches) thick; 2) filter aggregate
course; 3) reservoir course of 4-8 centimeters (1.5-3-inch) diameter stone; and
4) filter fabric.
Paving temperature: 240° - 260° F.
Minimum air temperature: 50° F.
Compact with one or two passes of a 10,000-kilogram (10-ton) roller.
Prevent any vehicular traffic on pavement for at least two days.
Pretreatment recommended to treat runoff from off-site areas. For example,
place a 7.6-meter (25-foot) wide vegetative filter strip around the perimeter of
the porous pavement where drainage flows onto the pavement surface.
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removed material), followed by high-pressure hosing to
free pores in the top layer from clogging. Potholes and
cracks can be filled with patching mixes unless more
than 10 percent of the surface area needs repair.
Spot-clogging may be fixed by drilling 1.3 centimeter
(half-inch) holes through the porous pavement layer
every few feet.
The pavement should be inspected several times during
the first few months following installation and annually
thereafter. Annual inspections should take place after
large storms, when puddles will make any clogging
obvious. The condition of adjacent pretreatment
devices should also be inspected.
COSTS
The costs associated with developing a porous
pavement system are illustrated in Table 2.
Estimated costs for an average annual maintenance
program of a porous pavement parking lot are
approximately $4,942 per hectare per year ($200 per
acre per year). This cost assumes four inspections
each year with appropriate jet hosing and vacuum
sweeping treatments.
REFERENCES
Field, R., et al., 1982. "An Overview of
Porous Pavement Research." Water
Resources Bulletin, Volume 18, No. 2, pp.
265-267.
Metropolitan Washington Council of
Governments, 1987. Controlling Urban
Runoff: A Practical Manual for Planning
and Designing Urban BMPs.
Metropolitan Washington Council of
Governments, 1992. A Current Assessment
of Best Management Practices: Techniques
for Reducing Nonpoint Source Pollution in
a Coastal Zone.
Southeastern Wisconsin Regional Planning
Commission, 1991. Costs of Urban
Nonpoint Source Water Pollution Control
Measures, Technical Report No. 31.
U.S. EPA, 1981. Best Management
Practices Implementation Manual.
TABLE 2 ESTIMATED COSTS FOR A POROUS PAVEMENT SYSTEM
Component
Excavation Costs
Filter Aggregate/Stone Fill
Filter Fabric
Porous Pavement
Overflow Pipes
Observation Well
Grass Buffer
Erosion Control
Subtotal
Contingencies (Engineering,
Unit Cost
740cyX$5.00/cy
740cyX$20.00/cy
760syX$3.00/cy
556syX$13.00/sy
200ftX$12.00/ft
1 at $200 each
822syX$1.50/sy
$1000
25%
Total
$3,700
$14,800
$2,280
$7,228
$2,400
$200
$1,250
$1,000
$32,858
$8,215
Administration, etc.)
Total
$41,073
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6. U.S. EPA, 1992. StormwaterManagement
for Industrial Activities: Developing
Pollution Prevention Plans and Best
Management Practices. EPA 833-R-92-
006.
7. Washington State Department of Ecology,
1992. Stormwater Management Manual
for the Puget Sound Basin.
ADDITIONAL INFORMATION
Andropogon Associates, Ltd.
Yaki Modovnik
374 Shurs Lane
Philadelphia, PA 19128
Cahill Associates
Thomas H. Cahill
104 S. High Street
West Chester, PA 19382
Center for Watershed Protection
Tom Schueler
8391 Main Street
Ellicott City, MD 21043
Fairland Park, Maryland
Ken Pensyl
Nonpoint Source Program
Water Management Administration
Maryland Department of the Environment
2500 Broening Highway
Baltimore, MD 21224
Fort Necessity National Battlefield
National Park Service
1 Washington Parkway
Farmington, PA 15437
Massachusetts Highway Department
Clem Fung
Research and Materials Group
400 D Street
Boston, MA 02210
Morris Arboretum
Robert Anderson
9414 Meadowbrook Avenue
Philadelphia, PA 19118
Washington Department of Ecology
Linda Matlock
Stormwater Unit
P.O. Box 47696
Olympia, WA 98504-7696
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for the use by the U.S. Environmental Protection
Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, DC, 20460
IMTB
MUNICIPAL TECHNOLOGY BRANCH
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