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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