TABLE 1 DESIGN CRITERIA FOR POROUS PAVEMENTS

tn Criterion Guidelines

Site Evaluation

Traffic conditions

o 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 water table: 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.

A typical 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.

eo Field, 1982.


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


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

1. Field, R., et al., 1982. “An Overview of
Porous Pavement Research.” Water
Resources Bulletin, Volume 18, No. 2, pp.
265-267.

2. Metropolitan Washington Council of
Governments, 1987. Controlling Urban
Runoff: A Practical Manual for Planning
and Designing Urban BMPs.

3. Metropolitan Washington Council of
Governments, 1992. A Current Assessment
of Best Management Practices: Techniques
for Reducing Nonpoint Source Pollution in
a Coastal Zone.

4. Southeastern Wisconsin Regional Planning
Commission, 1991. Costs of Urban
Nonpoint Source Water Pollution Control
Measures, Technical Report No. 31.

5. U.S. EPA, 1981. Best Management
Practices Implementation Manual.

6. US. EPA, 1992. Stormwater Management
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 Miodovnik

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


United States Office of Water EPA 832-F-99-023
Environmental Protection Washington, D.C. September 1999

Agency

SEPA Storm Water
Technology Fact Sheet
Porous Pavement


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 USS.
Environmental Protection Agency. :

For more information contact:

Municipal Technology Branch
U.S. EPA

Mail Code 4204

401 M St. S.W.

Washington, DC, 20460

ce)
Excellence in compliance through optimal technical solutions:
MUNICIPAL TECHNOLOGY BRANT ROSS


United States
Environmental Protection
Agency

EPA 832-F-99-023
September 1999

Office of Water
Washington, D.C.

SEPA

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 runoff 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 uses. Some _ specific


Resurfacing and Use of
Berm Keeps Off-site Runoff Asphalt is Vacuum Swept, Abrasives, and to Restrict
and Sediment Out, Provides Followed by Jet Hosing to Truck Parking
‘Temporary Storage Keep Pores Open Posted
“
pS Porous Asphalt aed
Spe x = > = S
Perforated Pipe Discharges
‘Only When 2-Year Storage .
Volume Exceeded Observation Well
Filter Fabric Lines Sides
of Reservoir to Prevent ——>
‘Sediment Entry Stone Reservoir Drains in 48 - 72 Hours

Sign Posted to Prevent

L

Undisturbed Soils with a Field Capacity > 0.27
Inches/Hour Preferably 20.50 Inches/Hour

Source: Modified from MWCOG, 1987.

FIGURE 1 TYPICAL POROUS PAVEMENT INSTALLATION

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.