This document combines several documents related to permeable pavement. Individual documents can be viewed by clicking on the appropriate link below. Fact sheets are not included in this combined document.

Porous pavement articles

# Overview

Example of a new retrofit permeable parking lot at the University of Minnesota
Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

allow stormwater runoff to filter through surface voids into an underlying stone reservoir where it is temporarily stored and/or . The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP). Permeable pavements have been used for areas with light traffic at commercial and residential sites to replace traditionally impervious surfaces such as low-speed roads, parking lots, driveways, sidewalks, plazas, and patios. While permeable pavements can withstand truck loads, permeable pavement has not been proven in areas exposed to high repetitions of trucks or in high speed areas because its’ structural performance and surface stability have not yet been consistently demonstrated in such applications.

While the designs vary, all permeable pavements have a similar structure, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer, optional underdrains, and geotextile over uncompacted soil subgrade. From a hydrologic perspective, permeable pavement is typically designed to manage rainfall landing directly on the permeable pavement surface area. Permeable pavement surface areas may accept runoff contributed by adjacent impervious areas such as driving lanes or rooftops. Runoff from adjacent vegetated areas must be stabilized and not generating sediment as its transport accelerates permeable pavement surface clogging. Additionally, the capacity of the underlying reservoir layer limits the .

Schematic showing the process of infiltration into permeable pavement during and after a rain event. Note how infiltrating water includes precipitation falling directly on the pavement and runoff from the adjacent street directed onto the pavement. Caution should be used when runoff is diverted from impervious surfaces to permeable pavement.

## Benefits and limitations

• Benefits: Permeable pavements allow conversion and/or design of typical impervious areas (i.e. parking lots) to pervious areas that infiltrate stormwater runoff. When compared to typical impervious areas, properly designed and maintained permeable pavements can reduce the runoff quantity, reduce total suspended solids (TSS) and total phosphorus (TP) loads into receiving water bodies, and reduce runoff temperatures. In addition, permeable pavements can reduce nitrogen, metals and process oils. Permeable pavements are well suited for high density urban areas with limited space ( highly urban and ultra-urban environments) for other BMPs such as , or bioretention systems.
• Limitations: As with all BMP’s, permeable pavement has limitations that need to be considered before design and construction. Limitations are discussed in detail in the permeable pavement design section.

## Pretreatment considerations

that removes sediment from runoff draining onto permeable pavement from impervious surfaces is desirable since sediment can clog permeable pavements. For that reason, pretreatment areas should emit practically no sediment onto the permeable pavement surface. Locating such areas next to impervious surfaces upslope from the permeable pavement may not be possible on some sites. Permeable pavement itself can be considered a pretreatment device and included in a stormwater if underdrains are utilized within the storage reservoir. The underdrains will typically be routed to a bioretention area.

## Permit applicability

Permeable pavements can be utilized to assist in meeting stormwater requirements for volume, total suspended solids, and total phosphorus. The section on credits provides guidance on the implementation of permeable pavements that may be utilized to meet various runoff volume and pollutant runoff reduction goals ().

## Retrofit suitability

In most cases, existing impervious surfaces can be replaced with permeable pavements to achieve improved runoff conditions. Retrofit requires the removal of the old pavement and subgrade and the installation of the underlying reservoir layer and the permeable pavement. For the greatest water quality credits, avoid compaction of subgrade soils. If this is not possible, compacted subgrade soils should be removed or loosened to achieve the maximum infiltration rate possible.

## Cold climate suitability

Favorable permeable pavement performance has been documented in cold climates. Air in the aggregate base acts as an insulating layer and the higher latent heat associated with higher soil moisture delays the formation of a frost layer while maintaining permeability and this condition also reduces frost depths when frozen. Winter sanding should be avoided when possible and if used, removed by vacuuming the following spring. Permeable pavements require significantly less use of, or in some cases, no deicing chemicals and sand to maintain a safe walking or driving surface. Other climate considerations include high wind erosion (California 2003). Dramatic reductions in life span of the infiltration properties of the pavement may occur in these areas due to particulate clogging and this may require additional surface vacuum cleaning.

## Special receiving waters suitability

Many of the same design considerations and limitations apply to permeable pavement as to other infiltration practices.

• Infiltration of runoff from (e.g., gas stations, chemical storage areas, etc.) should be carefully considered and in many cases avoided.
• Special consideration also needs to be taken near and basement foundations.
• Some designs may require consideration of storms in excess of the infiltration capabilities of the pavement. For these situations the design should ensure the excess runoff does not negatively impact (e.g.,trout streams) through the implementation of additional BMPs.

The table below provides guidance regarding the use of permeable pavement practices in areas upstream of special receiving waters. Note that the suitability of a practice depends on whether the practice has an underdrain (i.e. filtration vs. infiltration practice).

Infiltration and filtration bmp1 design restrictions for special waters and watersheds. See also Sensitive waters and other receiving waters.

BMP Group receiving water
A Lakes B Trout Waters C Drinking Water2 D Wetlands E Impaired Waters
Infiltration RECOMMENDED RECOMMENDED NOT RECOMMENDED if potential stormwater pollution sources evident RECOMMENDED RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride
Filtration Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments RECOMMENDED RECOMMENDED ACCEPTABLE RECOMMENDED for non-nutrient impairments

1Filtration practices include green roofs, bmps with an underdrain, or other practices that do not infiltrate water and rely primarily on filtration for treatment.
2 Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies

## Water quality

In general, permeable pavement provides removal of TSS and other pollutants through processes similar to other filtration and infiltration BMPs. However, permeable pavements are not suggested for areas that may receive high loading rates of TSS due to their propensity for surface clogging. The expected annual volume and pollutant reductions for designs without an underdrain are a function of the underlying reservoir storage volume. The greater the storage volume, the greater the annual volume and pollutant reductions.

For designs with underdrains, reductions are typically lower depending on the drain outflow location that determines the volume of water removed by the underdrains before infiltration. Of the water intercepted and draining through the underdrain, 45 percent (with upper and lower 90 percent confidence bounds of 65 percent and 24 percent, respectively) of the total phosphorus and 74 percent (with upper and lower 90 percent confidence bounds of 93 percent and 33 percent, respectively) of total suspended solids removal can be expected. These event mean averages and ranges are derived from a literature review on research on permeable pavements. The literature includes 19 studies on pollutant reductions and 10 studies on volume reductions. (See the section on credits for more information on pollutant reduction credits and their relationship to the MIDS credit calculator).

## Water quantity

The primary advantage of permeable pavements is providing volume reduction by reducing runoff from a site and/or providing attenuation from outflows. The volume of water that will be reduced during a given rainfall event will be equivalent to the volume available for storage below the pavement or underdrain (if an underdrain is present). More discussion on this item is available in the section on credits.

# Types of permeable pavement

Photo illustrating pervious concrete. Pervious concrete is a special type of concrete with a high porosity that allows water from precipitation and other sources to pass directly through.
Photo illustrating porous asphalt. Porous asphalt is standard hot-mix asphalt that allows water to drain through it.
Photo illustrating permeable interlocking concrete pavement. Permeable interlocking pavers consist of concrete or stone units with open, permeable spaces between the units.
Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

The most commonly used surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers. Other options include plastic and concrete grids, as well as amended soils (artificial media added to soil to maintain soil structure and prevent compaction). This document focuses on pervious concrete, porous asphalt and permeable interlocking concrete pavements. A general comparison of their properties is provided in the table. Additional requirements specific to each system should be obtained by designers from suppliers and from the local review authority.

For each of the above pavement surfaces, there are many variants depending on the design goals. For instance, permeable pavement can be installed with a deep underlying reservoir consisting of open-graded, crushed rock. This design provides water quality and quantity control by storing runoff and infiltrating it into the subgrade soils over an extended period of time. A second design variation includes a deep underlying reservoir consisting of open-graded, crushed rock above an impermeable layer of soil or a liner and an underdrain. The underdrain typically discharges to a or storm sewer system. This design provides some runoff flow attenuation, filtering, but no volume reduction. These two options provide different levels of treatment.

To assist with selection of a permeable pavement type, a general comparison of the properties of the three major permeable pavement types is provided in the table. Designers should check with product vendors and the local review authority to determine specific requirements and capabilities of each system. Schematic cross sections of each system are illustrated in the design section for permeable pavement.

Summary of properties of permeable pavements.

Properties Pervious concrete Porous asphalt PICP
Typical pavement surface thicknessa 5 to 8 inches 3 to 4 inches (thicker for high wheel load applications) 3 inchesa
Bedding layera,f None 1 in. AASHTO No. 57 stone 2 inches of AASHTO No. 8 stone (MnDOT 3127 FA-3)
Reservoir layerb,f AASHTO No. 57 stone or per hydraulic design AASHTO No. 2, 3, or 5 stone 4 inches of AASHTO No. 57 stone over No. 2, 3 or 4 stone
Construction properties
• Cast in place
• Seven day cure
• Must be continuously covered
• Cast in place
• 24 hour cure
• No cure period
• Manual or mechanical installation of pre-manufactured units
Installed surfacing costc 3 to $4/square foot$2/square foot 3 to \$4/square foot
Minimum batch size
None
Longevityd
20 to 30 years
Overflow
Catch basin, overflow edge, elevated underdrain
Runoff temperature reduction
Cooling at the reservoir layer
Surface colors/texture Range of light colors and textures Black or dark grey colors Wide range of colors, textures and patterns
Handles all vehicle loads with appropriate surface and base/subbase layer material and thickness design
Surface cleaningg
Periodic vacuuming; replace if completely clogged and uncleanable
Periodic vacuuming; replace jointing stones if completely clogged and uncleanable
Other issues
• Avoid concentrated deicers
• Avoid winter sanding
• Avoid seal coating
• Avoid winter sanding
Avoid winter sanding
Design references [1] [2] [3]

aThickness may vary depending on site and traffic conditions
bReservoir storage may be augmented by corrugated metal pipes, plastic arch pipe or plastic lattice crates
cSupply and install minimum surface thickness only; minimum 30,000 sf with Minnesota 2012 prevailing labor wages. Does not include base reservoir, drainage appurtenances, engineering, or inspection
dBased on pavement being properly maintained. Resurfacing or rehabilitation may be needed after the indicated period
eDepends primarily on on-site geotechnical considerations and structural design computations
f ASTM D448 Standard Classification for Sizes of Aggregate for Road and Bridge Construction or ASASHTO M-43
gPeriodic vacuuming frequency determined from inspection, intensity of use, and other potential sediment sources

# Construction specifications for permeable pavement

Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

Proper construction of permeable pavement is critical to its long term performance as a stormwater BMP. Improper or inadequate erosion and sediment control during construction and immediately following construction can cause immediate plugging of the pavement. The construction sequence is also critical to the long term success of the performance of the pavement and is described below. The materials and installation techniques of the three different pavements are very specific and require special attention to detail. Failure to follow the recommendations will likely cause premature structural failure of the pavement or result is pavement without the desired infiltration capacity.

## Essential erosion and sediment controls

All permeable pavement areas should be fully protected from sediment intrusion by silt fence or construction fencing, particularly if they are intended to infiltrate runoff. They should remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. Permeable pavement areas should be clearly marked on all construction documents and grading plans. To prevent soil compaction, heavy vehicular traffic should be kept out of permeable pavement areas during and immediately after construction.

During construction, care should be taken to avoid tracking sediments onto any permeable pavement to avoid surface clogging. Any area of the site intended ultimately to be a permeable pavement area should generally not be used as the site of a temporary sediment basin. Where locating a sediment basin on an area intended for permeable pavement is unavoidable, the invert of the sediment basin must be a minimum of one foot above the final design elevation of the bottom of the aggregate reservoir course. All sediment deposits in the excavated area should be carefully removed prior to installing the subbase, base, and surface materials.

## Permeable pavement construction sequence

The following is a typical construction sequence to properly install permeable pavement, which may be modified depending on the pavement type.

Step 1. Construction of the permeable pavement begins after the entire contributing drainage area has been stabilized. The proposed site should be checked for existing utilities prior to any excavation.
Caution: Do not install pervious concrete or porous asphalt in rain or snow, and do not install frozen aggregate materials under any of the surfaces
Step 2. Temporary erosion and sediment controls are needed during installation to divert stormwater away from the permeable pavement area until it is constructed and contributing drainage areas have been stabilized by a uniform perennial vegetative cover with a density of at least 70 percent over the entire pervious surface area, or other equivalent means. Special protection measures such as erosion control fabrics may be needed to protect vulnerable side slopes from erosion during and after the excavation process. The proposed permeable pavement area must be kept free from sediment during the entire construction process.
Caution: Construction materials contaminated by sediments must be removed and replaced with clean materials
Step 3. Where possible, excavation should work from the sides and outside the footprint of the permeable pavement area (to avoid soil compaction). Contractors can utilize a “cell” construction approach, whereby the proposed permeable pavement area is divided into 500 to 1000 square feet temporary cells with 10 to 15 feet wide earthen bridges between them so that the cells can be excavated from the side. Then the earthen bridges are removed. Excavated material should be placed away from the open excavation to maintain stability of the side walls.
Step 4. The native soils along the bottom of the permeable pavement system can be scarified or tilled to a depth of 3 to 4 inches and graded prior to the placement of the aggregate.
Step 5. Geotextile should be installed on the sides of the reservoir layer applications that do not use concrete curbs extending the full base depth. The design engineer may elect to use geotextile over the soil subgrade as well. Overlap of each sheet should follow recommendations in AASHTO M-288.
Step 6. Provide a minimum of 2 inches of aggregate around underdrain pipes. The underdrains should slope down towards the outlet at a grade of 0.5 percent or steeper. The up-gradient end of underdrains in the reservoir layer should be capped. Where an underdrain pipe is connected to a structure, there should be no perforations within at least one foot of the structure. Ensure that there are no perforations in clean-outs within at least one foot from the surface.

NOTE: Step 7 (below) previously specified minimum 8 inch lifts. A review of literature suggests this should be maximum lifts of 8 inches, with 4 to 6 inch lifts being preferred. See [4], [5], [6]

Step 7. Spread maximum 8 inch lifts (6 inch preferred) of the reservoir base/subbase or base stone. Moistening the aggregate during spreading will facilitate better compaction. Compact reservoir layers (layer with larger than No. 57 stone) with a 10 ton roller with two passes in static mode or until there is no visible movement of the aggregate. For No. 57 or similar sized stone layers, make two passes in vibratory mode and two passes in static mode or until there is no visible movement of the aggregate. Do not crush the aggregate with the roller. Corners and other areas where rollers cannot reach are compacted with a vibratory plate compactor capable of least 13,500 pound force (lbf) and equipped with a compaction indicator. PICP bases require a 4 inch base layer and this is compacted separately from the subbase layer with two passes in vibratory then two in static mode.
Step 8. Install the desired depth of the bedding or choker layer, depending on the type of pavement, as follows.
• Pervious Concrete: No bedding/choker layer is used.
• Porous Asphalt: The choker layer for porous asphalt pavement consists of 1 inch of washed No. 57 stone.
• PICP: The bedding layer for open-jointed pavement blocks should consist of 2 inches of washed No.8 stone. This layer is compacted after pavers are placed on it and their joints are filled with aggregate.
Step 9. Paving materials should be installed according to manufacturer or industry specifications for the particular type of pavement. Installation highlights are provided below. After the installation is complete, the permeable pavement surface should be tested for acceptance using a minimum infiltration rate of 100 inch/hr using ASTM C1701 Standard Test Method for Infiltration Rate of In Place Pervious Concrete. This test method can be used on porous asphalt and PICP.

## Porous asphalt installation

The following has been excerpted from the Minnesota Asphalt Pavement Association (MAPA 2012) and from the National Asphalt Pavement Association (Hansen 2008). These documents should be reviewed for detailed specifications.

• Use PG 58-28 or PG 64-22 asphalt binder.
• Install porous asphalt pavement at according to temperatures recommended in the aforementioned references with a minimum air temperature of 50oF to ensure that the surface does not stiffen before compaction.
• Complete compaction of the surface course when the surface is cool enough to resist a 10-ton roller. One or two passes of the roller are required for proper compaction. More rolling could cause a reduction in the porosity of the pavement.
• The mixing plant must provide certification of the aggregate mix, abrasion loss factor, and asphalt content in the mix.
• Transport the mix to the site in a clean truck with smooth dump beds sprayed with a non-petroleum release agent. The mix should be covered during transportation to control cooling.

## Pervious concrete installation

The basic installation sequence for pervious concrete is outlined by the American Concrete Institute in ACI Specification 522.1 (ACI 2010) and can be purchased from the American Concrete Institute. Guide specifications for Minnesota applications should be obtained from the Aggregate and Ready Mix Association of Minnesota. Concrete installers should successfully complete a recognized pervious concrete installers training program, the Pervious Concrete Contractor Certification Program offered by the National Ready Mix Concrete Association. The basic installation procedure follows:

• Water the underlying aggregate (reservoir layer) before the concrete is placed, so that the aggregate does not draw moisture from the freshly laid pervious concrete.
• After the concrete is placed, approximately 3/8 to 1/2 inch is struck off, using a vibratory screed. This is to allow for compaction of the concrete pavement.
• Compact the pavement with a steel pipe roller. Care should be taken so that over-compaction does not occur.
• Cut joints for the concrete to a depth of ¼ inch.
• Curing: Cover the pavement with plastic sheeting within 20 minutes of the strike-off, and keep it covered for at least seven days. Do not allow traffic on the pavement during this time period.

## Installation of interlocking pavers

The basic installation process is described in greater detail by Smith (Smith 2011). Permeable paver job foremen should successfully complete the PICP Installer Technician Course training program offered by the Interlocking Concrete Pavement Institute (ICPI). The ICPI provides a variety of technical courses.

The following installation method also applies to clay paving units. Contact manufacturers of composite units for installation specifications. Guide construction specifications are available from the Interlocking Concrete Pavement Institute.

• Moisten, place and level the AASHTO No. 2 stone subbase and compact it in minimum 12 inch thick lifts with four passes of a 10-ton steel drum static roller until there is no visible movement. The first two passes are in vibratory mode with the final two passes in static mode. The filter aggregate should be moist to facilitate movement into the reservoir course.
• Place edge restraints before the base layer, bedding and pavers are installed. Permeable interlocking pavement systems require edge restraints to prevent vehicle tires from moving the pavers. Edge restraints may be standard concrete curbs or curb and gutters.
• Moisten, place and level the AASHTO No. 57 base stone in a single lift (4 inches thick). Compact it into the reservoir course beneath with at least four (4) passes of a 10-ton steel drum static roller until there is no visible movement. The first two passes are in vibratory mode, with the final two passes in static mode.
• Place and screed the bedding course material (typically AASHTO No. 8 stone (MnDOT 3127-FA-3), 2 inches thick).
• Pavers may be placed by hand or with mechanical installation equipment.
• Fill gaps at the edge of the paved areas with cut pavers or edge units. When cut pavers are needed, cut the pavers with a paver splitter or masonry saw. Cut pavers no smaller than one-third (1/3) of the full unit size if subject to tires.
• Fill the joints and openings with stone. Joint openings must be filled with AASHTO No. 8 (MnDOT 3127-FA-3), 89 or 9 (MnDOT 3127 FA-2)stone per the paver manufacturer’s recommendation. Sweep and remove excess stones from the paver surface.
• Compact and seat the pavers into the bedding course with a minimum low-amplitude 5,000 lbf, 75- to 95 Hz plate compactor. Do not compact within 6 feet of the unrestrained edges of the pavers.
• Thoroughly sweep the surface after construction to remove all excess aggregate.
• Inspect the area for settlement. Any paving units that settle must be reset and inspected.
• The contractor should return to the site within 6 months to top up the paver joints with stones.

## Construction inspection

Inspections before, during and after construction are needed to ensure that permeable pavement is built in accordance with these specifications. Use a detailed inspection checklist that requires sign-offs by qualified individuals at critical stages of construction and to ensure that the contractor’s interpretation of the plan is consistent with the designer’s intent. The following checklist provides an example.

Pre-construction meeting

• Walk through site with builder/contractor/subcontractor to review erosion and sediment control plan/stormwater pollution prevention plan (SWPPP)
• Determine when permeable pavement is built in project construction sequence; before or after building construction and determine measures for protection and surface cleaning
• Aggregate material locations identified (hard surface or on geotextile)

Sediment management

• Access routes for delivery and construction vehicles identified
• Vehicle tire/track washing station location/maintenance (if specified in the erosion and sediment control plan (SWPPP))
• Ensure that the contributing drainage areas are stabilized and are not eroding

Excavation

• Utilities should be located and marked by local service provider
• The excavated area should be marked with paint and/or stakes
• The excavation size and location should conform to the plan
• Excavation hole as sediment trap: The hole cleaned should be cleaned immediately before subbase stone placement and runoff sources with sediment diverted away from the pavement or all runoff diverted away from the excavated area.
• Temporary soil stockpiles should be protected from run-on, run-off from adjacent areas and from erosion by wind.
• Ensure linear sediment barriers (if used) are properly installed, free of accumulated litter, and built up sediment less than 1/3 the height of the barrier.
• No runoff should enter the pavement until soils are stabilized in the area draining to the pavement
• Foundation walls should be waterproofed
• Soil subgrade: rocks and roots removed, voids should be refilled with base aggregate
• Soil should be compacted to specifications (if required) and field tested with density measurements per specifications
• No groundwater seepage or standing water. If groundwater seepage is present, dewatering and possibly a dewatering permit may be required.

Geotextiles

• Must meet the design specifications
• Sides of excavation should be covered with geotextile prior to placing aggregate base/subbase
• Placement and down slope overlap (minimum of 2 feet) should conform to specifications and drawings
• No tears or holes should be present
• No wrinkles should be present and the fabric should be pulled taught and staked

Impermeable liners (if specified; see here)

• Must meet the specifications
• Placement, field welding, and seals at pipe penetrations should be completed per the design specifications

Drain pipes/observation wells

• Size, perforations, locations, slope, and outfalls must meet specifications and drawings
• Verify the elevation of overflow pipes
• Underdrains should be capped at upslope ends

Aggregates

• Test results should conform to specifications
• Aggregates should be spread (not dumped) with a front-end loader to avoid aggregate segregation
• Storage on hard surface or on geotextile to keep sediment-free
• Thickness, placement, compaction and surface tolerances should meet specifications and drawings

Once the final construction inspection has been completed, log the GPS coordinates for each facility and submit them for entry into the local BMP maintenance tracking database.

## Construction inspection checklists

Construction inspection checklists have not been developed for the Minnesota Stormwater Manual. Below are links to checklists developed by other organizations.

## Minnesota Department of Transportation example construction protocols

### Preliminary analysis and selection

Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.

Surface area of stormwater control measure (BMP)(ft2) Borings Pits Permeameter tests
< 1000 1 1 5
1000 to 5000 2 2 10
5000 to 10000 3 3 15
>10000 41 41 202

1an additional soil boring or pit should be completed for each additional 2,500 ft2 above 12,500 ft2
2an additional five permeameter tests should be completed for each additional 5,000 ft2 above 15,000 ft2

### Field verification testing prior to pond construction

• Soil hydraulic group represent what is stated in SWPPP (Stormwater Pollution Prevention Plan)
• Seasonally high water table not discovered within 3 feet of the excavated pond base within a test pit
• Commonly will test bottom of proposed pond for soil compaction (subsequent subsoil ripping) prior to media placement
• Commonly will test bottom of proposed pond for insitu infiltration rate by test pit or water filled barrel placed on pond base surface

### Filter media and material testing

• Existing soil (option 1 below) or Washed sand (option 2 below), and compost certification
• Washed course aggregate choker certification
• Other treatment material certification of iron filings, activated charcoal, pH buffers, minerals, etc.
• Geotextile separation fabric certification
• Drain-tile certification (if filtration is specified)
• Seed source certification
• Barrel test verification of infiltration rate using 2.5 feet of imported 3877 Type G media

### Field verification testing/inspection/verification during construction

• Water drains away in 48 hours
• Infiltration drainage rate does not exceed 8.3 inches per hour
• No tracking/equipment in pond bottom
• No sediment deposits from ongoing construction activity, media perimeter controls kept functional
• Forebay is trapping settleable solids, floating materials, and oil/grease
• Area staked off

### Notice of Termination (NOT) verification

• Option 1. Amending existing HSG soils with compost or other treatment material. Test the infiltration rate of each infiltration basin using a double ring infiltrometer prior to completion of the basin. Conduct the test at the finished grade of the basin bottom, prior to blending the compost with the in-situ soils or sand. Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests.
• Option 2. Importing 3877 Type G Filter Topsoil Borrow (may be amended with other treatment material). Ensure infiltration rates meet or exceed greater of two times the designed infiltration rate or 2 inches per hour, or rate specified in the plan. Conduct a minimum of five tests per representative acre of basin area and a minimum of five tests per basin. Conduct double ring infiltrometer tests in accordance with ASTM standards. Thoroughly wet test areas prior to conducting infiltrometer tests. Amend soils with additional washed sand if rates less than specified in the contract, or compost if rates exceed 8.3 inches per hour.

The permanent stormwater management system must meet all requirements in sections 15, 16, and 17 of the CSW permit and must operate as designed. Temporary or permanent sedimentation basins that are to be used as permanent water quality management basins have been cleaned of any accumulated sediment. All sediment has been removed from conveyance systems and ditches are stabilized with permanent cover.

## Related articles

Permeable pavement main page </noinclude>

# Operation and maintenance of permeable pavement

Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

## Maintenance and maintenance agreements

The Chesapeake Stormwater Network has developed two videos that illustrate inspection and maintenance of LID BMP practices, including permeable pavement. NOTE: These videos provide useful tips but should not be used for compliance with Minnesota permits.

In addition to the design items previously mentioned, some key actions help ensure the long-term performance of permeable pavement during its operation life. The most frequently cited maintenance problem is surface clogging caused by organic matter and sediment, which can be reduced by the following measures:

• Periodic Vacuuming – The pavement surface is the first line of defense in trapping and eliminating sediment that may otherwise enter the stone base and soil subgrade. The rate of sediment deposition should be monitored and vacuuming done at least two times per year. A typical vacuum cleaning schedule may include the end of winter (April) and after autumn leaf-fall (November). Maintenance records should be maintained by the owner. The vacuuming frequency should be adjusted according to the intensity of use and deposition rate on the permeable pavement surface. At least one pass should occur at the end of winter. Regenerative air vacuum sweepers are the suggested means for regular surface cleaning. For neglected surfaces (i.e., those with no surface cleaning over several years) true vacuum sweepers have the most efficient removal of debris and fine particulates when compared with regenerative air or mechanical sweepers. However, areas on steep slopes or near curbs may limit vacuum sweeper performance (Brown 2013). If a true vacuum sweeper is used on PICP the removed aggregate in the joints should be replaced with the same material.
• Ongoing – Minimizing salt use or sand for de-icing and traction in the winter, keeping the landscaping areas well maintained and preventing soil from being washed onto the pavement helps increase its life. Less salt will be needed. However, such water should not be directed to irrigation uses.
• Maintenance Agreements - Maintenance agreements should note which conventional parking lot maintenance tasks must be avoided (e.g., sanding, re-sealing, re-surfacing, power-washing). Signs should be posted on parking lots to indicate their stormwater function and special maintenance requirements. When permeable pavements are installed on private residential or commercial property, owners must understand routine maintenance requirements. These requirements can be enforced via a deed restriction, drainage easement, maintenance agreement, performance bond, letter of credit or other mechanism enforceable by the local authority to help ensure that the permeable pavement is maintained and continues functioning. The local authority should use this MIDS guideline to establish measurable performance criteria for enforcing maintenance procedures. The mechanism should, if possible, grant authority for local agencies to enter the property for inspection or corrective action.

## Maintenance inspection and maintenance checklists

Inspection and maintenance checklists have not been developed for the Minnesota Stormwater manual. We anticipate developing these in 2018. Below are several links to checklists developed by other organizations.

# Assessing the performance of permeable pavement

Permeable pavement maintenance (Image: United States Geological Survey)
Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

Assessing the performance of includes a review of its condition and performance. A spring maintenance inspection is recommended and cleanup conducted as needed. The following are recommended annual maintenance inspection points for permeable pavements.

• The drawdown rate should be measured at the observation well for three (3) days following a storm event in excess of 1/2 inch in depth. If standing water is still observed in the well after two days, this is a clear sign the system is not performing as desired and subgrade soil clogging is a problem.
• Inspect the surface for evidence of sediment deposition, organic debris, water staining, or ponding that may indicate surface clogging. If any signs of clogging are noted, schedule a vacuum sweeper to remove deposited material. Then test sections using ASTM C1701 to ensure that the surface attains an infiltration rate of at least 10 inches per hour.
• Check inlets, cells and any flow diversion structures for sediment buildup and structural damage. Remove the sediment.
• Inspect any for any controllable sources of sediment or erosion
• Inspect the condition of the observation well and make sure it is capped.
• Inspect the structural integrity of the pavement surface, looking for signs of surface deterioration, such as slumping, cracking, spalling, or broken pavers. Replacement to rectify a damaged surface or one removed for access to utility repairs should be done per industry recommendations. After base compaction, small areas of pervious concrete and porous asphalt can be replaced with conventional (impervious) asphalt or concrete up to 10 percent of the total permeable pavement area. For PICP, paving units are removed for reinstatement later. The remaining undisturbed pavers at the perimeter of the opening are typically held in place with temporary wood or metal braces. Once the base is replaced and compacted, new bedding material and pavers are reinstated and compacted over the base.

An online manual for assessing BMP treatment performance was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory. The manual advises on a four-level process to assess the performance of a Best Management Practice.

• Level 1: Visual Inspection. This includes assessments for infiltration practices and for filtration practices. The website includes links to a downloadable checklist.
• Level 2: Capacity Testing. Level 2 testing can be applied to both infiltration and filtration practices.
• Level 3: Synthetic Runoff Testing for infiltration and filtration practices. Synthetic runoff test results can be used to develop an accurate characterization of pollutant retention or removal, but can be limited by the need for an available water volume and discharge.
• Level 4: Monitoring for infiltration or filtration practices

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management . BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Additional information on designing a monitoring network and performing field monitoring are found at this link.

# Calculating credits for permeable pavement

 Recommended pollutant removal efficiencies, in percent, for permeable pavement with an underdrain. Sources. NOTE: removal efficiencies are 100 percent of captured water for systems with no underdrain. TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen TSS TP PP DP TN Metals Bacteria Hydrocarbons 74 41 74 0 insufficient data insufficient data insufficient data insufficient data
Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.
Schematic of a permeable pavement system with no underdrain. Water infiltrating through the pavement is stored in the reservoir/subbase and infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.
Schematic of a permeable pavement system with an underdrain. Water infiltrating through the pavement is either captured by the underdrain or stored below the underdrain in the reservoir/subbase, where it infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.
Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual best management practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

This page provides a discussion of how permeable pavement practices can achieve stormwater credits. Permeable pavement systems with and without underdrains are both discussed, with separate sections for each type of system as appropriate.

## Overview

Permeable pavements are a stormwater quality practice that allows runoff to pass through surface voids into an underlying stone reservoir/subbase for temporary storage before being discharged to an underdrain and/or underlying soil via infiltration. The most commonly used types of permeable pavement are pervious concrete, porous asphalt, and permeable interlocking concrete pavers.

### Pollutant removal mechanisms

Permeable pavement systems with no underdrains provide stormwater pollutant removal by reducing the volume of runoff from a site and the pollutant mass associated with that volume when infiltration is allowed (Water Environment Federation, 2012). In systems with underdrains most of the water is captured by the underdrain after passing through the subbase. If the underdrain is raised above the underlying soil subgrade, water stored in the reservoir/subbase below the underdrain will infiltrate into the underlying soil. If the underdrain is at the bottom of the reservoir/subbase, a small amount of infiltration may occur. Thus, pollutant removal in a permeable pavement system with an underdrain occurs through filtering of water captured by the underdrain and infiltration for water infiltrating into the underlying soil subgrade.

### Location in the treatment train

Stormwater Treatment Trains are comprised of multiple Best Management Practices that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Permeable pavements are installed near the start of the treatment train as a method that directs the stormwater runoff to a subgrade storage area in order to minimize the volume and pollutant mass of stormwater runoff .

## Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article. Permeable pavement is also effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

### Assumptions and approach

In developing the credit calculations, it is assumed the permeable pavement practice is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the permeable pavement section of the Manual.

Warning: Pre-treatment is required for all filtration and infiltration practices

In the following discussion, the water quality volume (VWQ) is assumed to be delivered instantaneously to the BMP. The VWQ is stored within the reservoir/subbase below the bottom of the pavement and above the soil subgrade. The VWQ can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch times the new impervious surface area. For MIDS, the VWQ is 1.1 inches times the impervious surface area. In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The instantaneous method therefore may underestimate actual volume and pollutant losses.

The approach in the following sections is based on the following general design considerations.

• Credit calculations presented in this article are for both event and annual volume and pollutant load removals.
• Stormwater volume credit for permeable pavements equates to the volume of runoff that is fully contained within the stone reservoir/subbase that will ultimately be infiltrated into the soil subgrade.
• TSS and TP credits for permeable pavements are achieved for the volume of runoff that is filtered and captured by an underdrain and the volume of water that is ultimately infiltrated.

### Volume credit calculations - no underdrain

Schematic showing terminology for calculating volume credits for permeable pavement. AO is the area at the bottom of the pavement, AB the area at the reservoir/soil subgrade interface, and DO the depth or thickness of the reservoir.

Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff via infiltration into the underlying soil from the existing stormwater collection system. These credits are assumed to be instantaneous values entirely based on the capacity of the BMP to capture, store, and transmit water in any storm event.

Volume credits for a permeable pavement system are based on the porosity of the subbase and system dimensions, specifically the depth of the reservoir/ subbase, the area of permeable pavement, and the bottom surface area. The volume credit (Vinfb) for infiltration through the bottom of the BMP into the underlying soil, in cubic feet, is given by

$V_{inf_b} = D_o\ n\ (A_O + A_B)\ / 2$

where

AO is the overflow surface area of the permeable pavement system, in square feet;
AB is the depth at the bottom of the permeable pavement system, in square feet;
DO is the depth of the reservoir/subbase layer (engineered media), equal to the distance from the bottom of the permeable pavement material to the underlying soil subgrade, in feet; and
n is the porosity (f) of the reservoir/subbase, in cubic feet per cubic foot.

The subbase depth should be limited to the drawdown time. The construction stormwater general permit requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). For example, using a B (SM) soil with an infiltration rate of 0.45 inches per hour, the maximum depth is 1.8 feet.

Note that that entire porosity of the subbase layer is used to calculate the volume credit. This slightly overestimates the actual volume infiltrated since some water is held by the media after the runoff infiltrates. The water content after gravity drainage, called field capacity, is less than 5 percent of total porosity for a permeable pavement system.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.

### Volume calculations - underdrain

Schematic of a permeable pavement system with an underdrain. Water infiltrating through the pavement is either captured by the underdrain or stored below the underdrain in the reservoir/subbase, where it infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.

The volume credit (V) for permeable pavement systems with underdrains, in cubic feet, is given by

$V = V_{inf_b} + V_U$

The infiltrating volume (Vinfb), in cubic feet, is given by

$V_{inf_b} = A_B DDT I_R / 12$

where

AB is the surface area at the bottom of the underdrain, in square feet;
DDT is the drawdown time for water stored below the underdrain, in hours; and
IR is the of underlying soil, in inches per hour.
Information: The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to IR. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.

The construction stormwater general permit requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). Note the MIDS calculator does not provide a volume credit for a permeable pavement system with an underdrain at the bottom.

If the underdrain is raised above the bottom of the BMP (i.e. above the interface between the reservoir/subbase and underlying soil subgrade), water stored below the underdrain will infiltrate. The infiltrating volume (VU), in cubic feet, is given by

$V_U = D_u\ n\ (A_U + A_B)\ / 2$

where

Du is the depth of the reservoir layer below the underdrain, in feet;
AB is the surface area at the bottom of the underdrain, in square feet;
AU is the surface area at the bottom of the reservoir layer/subbase, in square feet; and
n is the porosity of the reservoir/subbase layer, in cubic feet per cubic foot.

The depth below the underdrain should be limited to the drawdown time. The construction stormwater general permit requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). For example, using a C soil with an infiltration rate of 0.2 inches per hour, the maximum depth below the underdrain is 0.8 feet.

### Total Suspended Solids (TSS)

The TSS credits available for installation of permeable pavement depend on the design of the storage volume below the pavement and whether the runoff is filtered (through an underdrain), infiltrated, or both. Designs that filter runoff with an underdrain at the bottom of the storage layer are less effective in removing pollutants than infiltration designs. Runoff is filtered while flowing through the permeable pavement and the storage layer and out the underdrain. TSS removal credit of 100 percent is assumed for the infiltrated water. The recommended removal rate for filtered water is 74 percent, based on review of literature.

Removal of TSS by permeable pavement (MTSS), in pounds per event or pounds per year, is given by

$M_{TSS} = M_{TSS_I}\ + M_{TSS_F}$

where

MTSSI = mass of TSS removed by infiltration (pounds per event or pounds per year); and
MTSSF = mass of TSS removed by filtration (pounds per event or pounds per year).

The annual TSS credit (MTSSI) for infiltrated runoff is given by

$M_{TSS_I} = 2.72\ V_{_{Annual}}\ F_I\ EMC_{_{TSS}}$

where

VAnnual is the annual volume treated by the BMP, in acre-feet;
FI is the fraction of the total annual volume treated by the BMP that is infiltrated;
EMCTSS = event mean concentration of TSS in the runoff, in mg/L; and
Factor of 2.72 used for conversion to pounds.

In a permeable pavement system with an underdrain, some of the water captured by the BMP will enter the underdrain while some will infiltrate below the underdrain. The amount infiltrating depends on several factors, including whether the underdrain is raised above the soil subgrade, the infiltration rate of the underlying soil, and size and spacing of the underdrains. Pollutants in water that enters the underdrain are filtered. The Annual TSS credit for filtered runoff (MTSSF) is given by

$M_{TSS_F} = 2.72\ R_{_{TSS}}\ V_{_{Annual}}\ (F_F)\ EMC_{_{TSS}}$

where

FF is the fraction of the total volume treated by the BMP that is filtered; and
RTSS is the pollutant removal fraction for filtered water. A value of 0.74 is recommended.

If the permeable pavement is not the upstream most BMP in the treatment train, EMCTSS should be dependent on the MTSS effluent (mg/L) from the next upstream tributary BMP.

The annual volume treated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils. If an underdrain is in the system, this volume will have to be divided into the portion that infiltrates and the portion that is captured by the underdrain. The MIDS calculator can be used to determine these values.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.

The event (storm) based TSS credit (MTSSI) for infiltrated runoff is given by

$M_{TSS_I} = 2.72\ V_I\ EMC_{_{TSS}}\ / 43,560$

where

VI is the event-based volume infiltrated by the BMP, in cubic feet, and
a factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

The storm event based TSS credit (MTSS-F) for filtered runoff is given by

$M_{TSS_F} = R_{_{TSS}}\ 2.72\ V_F\ EMC_{_{TSS}}\ / 43560$

where

VF is the event-based volume filtrated by the BMP, in cubic feet.

### Total phosphorus (TP) credit calculations

Similar to TSS, TP reduction credits correspond directly with volume reduction through infiltration and filtration of captured stormwater. The water quality credits available for a permeable pavement system depend on the design of the storage volume below the pavement and whether or not the runoff is filtered (through underdrain) or infiltrated. TP credit is divided into particulate phosphorus (PP) and dissolved phosphorus (DP) removal, respectively making up 55 percent and 45 percent of the total TP credit. Because the volume of infiltrated water (calculated above) is completely removed from the existing system, 100 percent TP credit is assumed for all infiltrated stormwater. Filtered stormwater only receives credit for PP credit, and no credit is given for DP. This approach is consistent with the approach used in the MIDS calculator.

Removal of TP by permeable pavement is given by

$M_{TP} = M_{TP_I}\ + M_{TP_F}$

where

MTP is the annual or event TP removal (lb/yr or lb/event);
MTPI is the annual or event TP removal from infiltrated runoff (lb/yr or lb/event); and
MTPF is the annual or event TP removal from filtered water (lb/year or lb/event).

The total annual TP removal for infiltrated runoff is given by

$M_{TP_I} = 2.72\ V_{annual}\ F_I\ EMC_{_{TP}}$

where

Vannual is the annual volume treated by the BMP, in acre-feet,
FI is the fraction of the total annual volume treated by the BMP that is infiltrated,
EMCTP = event mean concentration of TP in the runoff, in mg/L, and
a factor of 2.72 used for conversion to pounds.

In a permeable pavement system with an underdrain, some of the water captured by the BMP will enter the underdrain while some will infiltrate below the underdrain. The amount infiltrating depends on several factors, including whether the underdrain is raised above the soil subgrade, the infiltration rate of the underlying soil, and size and spacing of the underdrains. Pollutants in water that enters the underdrain are filtered. The Annual TP credit for filtered runoff (MTPF) is given by

$M_{TP_F} = 2.72\ R_{_{TP}}\ V_{_{Annual}}\ F_F\ EMC_{_{TP}}$

where

FF is the fraction of the total volume treated by the BMP that is filtered; and
RTP is the pollutant removal fraction for filtered water.

The pollutant removal fraction applies only to particulate phosphorus (PP), which is assumed to be 55 percent of total phosphorus (TP). The recommended removal efficiency for PP is 74 percent. Thus, the recommended value for RTP is 0.55 * 0.74 or 0.41. The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.

If the permeable pavement is not the upstream most BMP in the treatment train, EMCTP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.

The annual volume treated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils. If an underdrain is in the system, this volume will have to be divided into the portion that infiltrates and the portion that is captured by the underdrain. The MIDS calculator can be used to determine these values.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.

The event (storm) event based TP credit (MTPI) for infiltrated runoff is given by

$M_{TP_I} = 2.72\ V_I\ EMC_{_{TP}}\ / 43,560$

where

VI is the event-based volume infiltrated by the BMP, in cubic feet; and
a factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

The storm event based TP credit (MTP-F) for filtered runoff is given by

$M_{TP - F} = R_{_{TP}}\ 2.72\ V_F\ EMC_{_{TP}}\ / 43560$

where

VF is the event-based volume filtered by the BMP, in cubic feet

### Example calculations for TSS and TP

NOTE: The MIDS calculator was used for the following examples. The performance goal was changed from the MIDS default of 1.1 inches to 1 inch.

Assume a permeable pavement system is designed to capture and treat 1 inch of runoff from a 1 acre impervious area. Note that in these calculations, the permeable pavement is considered part of the impermeable surface.

For this example, assume a 9000 square foot surface area at the top of the reservoir/subbase, a 9000 square foot area at the reservoir/soil subgrade, an underlying B soil with an infiltration rate of 0.45 inches per hour, a porosity of 0.4 cubic feet per cubic foot, a depth below the underdrain of 1 foot, a TSS EMC of 54.5 milligrams per liter, and a TP EMC of 0.3 milligrams per liter. With this depth below the underdrain, all the water can be infiltrated (3600 cubic feet per event; 2.3446 acre-feet per year). Annual TSS removal, in pounds, is given by

$2.72 (2.3446) (1) (54.5) = 347$

Annual TP removal is given by

$2.72 (2.3446) (1) (0.3) = 1.91$

If the depth below the underdrain was 0.5 feet instead of 1 foot, only half of the 1 inch performance goal is infiltrated, corresponding to an annual infiltration volume of 1.60 acre-feet. Note that the relationship between infiltration performance goal and annual volume infiltrated is not linear. The first step is to calculate the infiltration and filtered fractions of total volume captured by the BMP. The infiltrated fraction is 1.60/2.3446 or 0.68, leaving a filtered fraction of 0.32.

Annual TSS removal, in pounds, is given by

$(2.72 (2.3446) (0.68) (54.5)) + ((2.72 (2.3446) (0.32) (0.74) (54.5)) = 319$

The first term in parentheses corresponds with the infiltrated portion and equals about 236.3 pounds. The second term in parentheses corresponds with the filtered portion, having a removal efficiency of 0.74 (74 percent), for a total removal of about 82.3 pounds.

Annual TP removal, in pounds, is given by

$(2.72 (2.3446) (0.68) (0.3)) + ((2.72 (2.3446) (0.32) (0.55) (0.74) (0.3)) = 1.55$

The first term in parentheses corresponds with the infiltrated portion and equals about 1.30 pounds. The second term in parentheses corresponds with the filtered portion, having a particulate P fraction of 0.55, and a removal efficiency of 0.74 (74 percent) for the particulate fraction, for a total removal of about 0.25 pounds.

## Methods for calculating credits

This section provides specific information on generating and calculating credits from permeable pavement for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions ("credits”) may be calculated using one of the following methods:

1. Quantifying volume and pollution reductions based on accepted hydrologic models
2. The Simple Method and MPCA Estimator
3. MIDS Calculator
4. Quantifying volume and pollution reductions based on values reported in literature
5. Quantifying volume and pollution reductions based on field monitoring

### Credits based on models

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

1. Model name and version
2. Date of analysis
3. Person or organization conducting analysis
4. Detailed summary of input data
5. Calibration and verification information
6. Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal by the BMP. In using this table to identify models appropriate for permeable pavement, use the sort arrow on the table to select Infiltrator BMPs or Filter BMPs, depending on the type of permeable pavement BMP and the terminology used in the model.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse Manu-
factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X X No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X X X No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X X X No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X Yes Yes Yes
i-Tree-Hydro X No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets No No Yes Computes volume reduction for trees, only.
LSPC X X X Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator X Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet X No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X X X X Yes Yes Yes
PCSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X [ No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA X No No Yes
SHSAM X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X X X Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X Yes Yes Yes
XPSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.

### The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of event mean concentration and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (EPA STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Caution: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

### MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance and examples have been developed for all BMPs in the calculator, including permeable pavement. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

### Credits based on reported literature values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the permeable pavement practice. Concentration reductions resulting from treatment can be converted to mass reductions if the volume of stormwater treated is known.

Designers may use the pollutant reduction values reported in this manual or may research values from other databases and published literature. Designers who opt for this approach should

• select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
• select a pollutant removal reduction from literature that studied a permeable pavement practice with site characteristics and climate similar to the device being considered for credits;
• review the article to determine that the design principles of the studied permeable pavement are close to the design recommendations for Minnesota, as described in this manual and/or by a local permitting agency; and
• give preference to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed permeable pavement practice, considering such conditions as watershed characteristics, permeable pavement practice sizing, soil infiltration rates, and climate factors.

### Credits based on field monitoring

Field monitoring may be made in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

1. Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
1. Which pollutants will be measured?
2. Will the monitoring study the performance of a single BMP or multiple BMPs?
3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
4. Will the results be compared to other BMP performance studies?
5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
2. Plan the field activities. Field considerations include
1. equipment selection and placement;
2. sampling protocols including selection, storage, and delivery to the laboratory;
3. laboratory services;
4. health and Safety plans for field personnel;
5. record keeping protocols and forms; and
6. quality control and quality assurance protocols
3. Execute the field monitoring
4. Analyze the results

This manual contains the following guidance for monitoring.

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

• Chapter 2: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
• Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
• Chapter 4: Methods and equipment for water quality monitoring
• Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
• Chapter 7: BMP Performance Analysis
• Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).
Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.

• Chapter 4: Stormwater Characterization
• 4.2: General Characteristics and Pollutant Sources
• 4.3: Sources of Stormwater Quality data
• Chapter 8: Performance Evaluation
• 8.1: Methodology Options
• 8.5: Evaluation of Quality Performance for Individual BMPs
• 8.6: Overall Hydrologic and Water Quality Performance Evaluation
• Chapter 10: Hydrologic Evaluation
• 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices
• In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).
Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include.

• Chapter 4: Monitoring Methods and Equipment
• Chapter 5: Analytical Methods and Laboratory Selection
• Chapter 6: Monitoring Site Selection
• Chapter 8: Equipment Installation and Maintenance
• Chapter 10: Pre-Storm Preparation
• Chapter 11: Sample Collection and Handling
• Chapter 12: Quality Assurance / Quality Control
• Chapter 13: Laboratory Reports and Data Review
• Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice.

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

## Other pollutants

Permeable pavements provide removal of sediment (TSS), nutrients (phosphorus and nitrogen), and metals through filtration, infiltration, and soil adsorption. Temperature control occurs in the stone reservoir/subbase and soil subgrade. Phosphorus, metals, and hydrocarbons are adsorbed onto soils within the subgrade. In addition, nutrients such as phosphorus and nitrogen may be biologically degraded.

According to the International Stormwater Database, studies have shown that permeable pavements are effective at reducing concentration of pollutants including solids, bacteria, metals, and nutrients. A compilation of the pollutant removal capabilities from a review of literature of permeable pavement studies are summarized in the table below.

Relative pollutant reduction from permeable pavement systems for metals, nitrogen, bacteria, and organics.

Pollutant Constituent Treatment capabilities1
Nitrogen Total nitrogen, Total Kjeldahl nitrogen Medium/High
Bacteria Fecal coliform, e. coli Insufficient data
Organics Medium

1 Low: < 30%; Medium: 30 to 65%; High: >65%
2 Results are for total metals only

• Brown, Chris; Angus Chu; Bert van Duin; Caterina Valeo. 2009. Characteristics of Sediment Removal in Two Types of Permeable Pavement. Water Qual. Res. J. Can. Volume 44, No. 1, 59-70.
• Geosyntec and Wright Water Engineers. 2012. International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals. Prepared under Support from WERF, FHWA, EWRI/ASCE and EPA. July 2012.
• New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B.
• New Jersey Department of Environmental Protection. 2004. New Jersey Stormwater BMP Manual. Standards for Pervious Paving Systems. Chapter 9.7.
• North Carolina Department of Environment and Natural Resources. Water Quality Division. 2012. Stormwater BMP Manual & BMP Forms. Chapter 18. Permeable Pavement.
• Tennis, Paul D.; Michael L. Leming; David J. Akers. 2004. Pervious Concrete Pavements. EB302.02, Portland Cement Association and National Ready Mixed Concrete Association.
• Tota-Maharaj, K. and Scholz, M. 2010. Efficiency of permeable pavement systems for the removal of urban runoff pollutants under varying environmental conditions. Environ. Prog. Sustainable Energy, 29: 358–369. doi: 10.1002/ep.10418
• USEPA. Stormwater Menu of BMPs. Permeable Pavements. 2009.

# Additional considerations for permeable pavement

Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

## Compliance with the Americans with Disabilities Act (ADA)

All permeable pavements are ADA compliant. PICP is compliant if designs are used with joints less than ½ inch wide.

## Groundwater protection and underground injection control permits

The Safe Drinking Water Act regulates the infiltration of stormwater in certain situations pursuant to the Underground Injection Control (UIC) Program, which is administered either by the US EPA or a delegated state groundwater protection agency. The US EPA (USEPA 2008) determined that permeable pavement installations are not classified as Class V injection wells since they are always wider than they are deep. There may be an exception in karst terrain if the discharge from permeable pavement is directed to an improved sinkhole, although this would be uncommon.

## Air and runoff temperature

Permeable pavement appears to have some value in reducing summer runoff temperatures which can be important in watersheds with sensitive cold-water fish populations (Hunt 2011). The temperature reduction effect is greatest when runoff is infiltrated into reservoir layer when underdrains are used. All permeable pavements exhibit cooler summer temperatures than their impervious counterparts. For example, a recent study showed that porous asphalt showed lower nighttime temperatures when compared with materials that have a similar or higher albedo. This was attributed to the insulating properties of porous asphalt due to its high air void content (Stempihar 2011). Pervious concrete and PICP can meet the solar reflectance index (SRI) of 29.

## Sustainable rating systems

All permeable pavements support sustainable rating systems such as LEED and others plus sustainable transportation rating systems such as those published by the Institute for Sustainable Infrastructure (Envision), Federal Highway Administration (INVEST), and the University of Washington (Greenroads).

## Certification

Warning: The MPCA does not endorse any specific product or service, including trainings, workshops, or other similar practices or events

As previously noted, the pervious concrete and PICP industry associations offer education and certification of permeable pavement contractors, i.e., the National Ready Mix Concrete Association (NRMCA) and the Interlocking Concrete Pavement Institute (ICPI). Porous asphalt does not require unique materials and can be installed by most paving equipment. In addition, all plants producing hot-mix asphalt are required to be certified by MnDOT. Industry-trained and experienced supervisory personnel should be required on all jobsites and requirements written into project specifications. A specifications requirement can be contractor submittals demonstrating experience with previous projects.

For design professionals, industry and professional associations offer in-person and online continuing education programs on design, construction and maintenance of permeable pavements. Many of these programs are registered with continuing education programs offered for civil engineering professional development hours, the American Institute of Architects and the American Society of Landscape Architecture continuing education systems, and the Green Building Certificate Institute Credential Maintenance Program for LEED® accredited professionals. Designers are encouraged to participate in these programs.

Industry associations provide literature and design software for design professionals. The National Asphalt Pavement Association offers “Porous Asphalt Pavements for Stormwater Management, Design, Construction, and Maintenance Guide” (Hansen 2008). The Minnesota Asphalt Pavement Association has guidance on their website. The American Concrete Pavement Institute has design software called PerviousPave for design of pervious concrete pavement. The software can be downloaded from their website. Specifications for the design of pervious concrete are provided by the American Concrete Institute (ACI) in ACI 522.1-08 “Specification for Pervious Concrete Pavement”. A report titled ACI 522R-10 “Report on Pervious Concrete” is also available. ICPI offers a course called “Permeable Interlocking Concrete Pavements” covering design, specifications, construction, and maintenance. ICPI also offers Permeable Design Pro software for PICP structural and hydrologic design.