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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
Permeable pavements allow stormwater runoff to filter through surface voids into an underlying stone reservoir where it is temporarily stored and/or infiltrated. 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 contributing drainage area.
Pretreatment 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 treatment train if underdrains are utilized within the storage reservoir. The underdrains will typically be routed to a bioretention area.
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 ( credits).
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.
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.
Many of the same design considerations and limitations apply to permeable pavement as to other infiltration practices.
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.
Link to this table
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
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).
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.
The most commonly used permeable pavement 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) (MPCA 2008). This document focuses on pervious concrete, porous asphalt and permeable interlocking concrete pavements. A general comparison of their properties is provided in the table below. 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 stormwater pond 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 below. 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.
Link to this table
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 |
|
|
|
Installed surfacing costc | 3 to $4/square foot | $2/square foot | 3 to $4/square foot |
Minimum batch size | |||
Longevityd | |||
Overflow | |||
Runoff temperature reduction | |||
Surface colors/texture | Range of light colors and textures | Black or dark grey colors | Wide range of colors, textures and patterns |
Load bearing capacitye | |||
Surface cleaningg | Periodic vacuuming; replace jointing stones if completely clogged and uncleanable | ||
Other issues |
|
|
Avoid winter sanding |
Design reference | Report 522-2010 | Hansen 2008NAPA | Smith 2011 ICPI |
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
This section provides information on design considerations, criteria and specifications for permeable pavement. Base/subbase thickness is determined for water storage using hydrologic sizing and/or dynamic modeling over time. Base/subbases thickness for supporting traffic is determined using structural design methods. The thicker of the two resulting designs is employed.
Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include
For more information on design information for individual infiltration and filtration practices, link here.
Permeable pavement is subject to the following design considerations, including benefits and constraints.
For more information on contributing area, see Contributing drainage area to stormwater BMPs.
Base/subbase thickness is determined for support traffic using structural design methods and for water storage using hydrologic sizing and/or dynamic modeling over time. The thicker of the two resulting designs is used.
The structural design process for supporting vehicles varies according to the type of pavement selected. The pervious concrete industry is in the process of developing ASTM test methods for characterizing compressive or flexural strengths of pervious concrete. These tests are needed to model fatigue under loads. As an interim step, fatigue equations published by the American Concrete Pavement Association (ACPA 2010) assume such inputs to be comparable in nature (but not magnitude) to those used for conventional concrete pavements. The ACPA design method should be consulted for further information as well as pervious concrete industry software. General guidelines for pervious concrete surface thickness are published by the National Ready Mix Concrete Association NRMCA and the Portland Cement Association (Leming 2007).
Porous asphalt (Hansen 2008) and permeable interlocking pavements (Smith 2011) use flexible pavement design methods adopted from the 1993 AASHTO Guide for Design of Pavement Structures (AASHTO 1993). In addition, MnDOT design methods, approved mechanistic principles, and manufacturer’s specific recommendations should be consulted.
There has been limited research on full-scale testing of the structural behavior of open-graded bases used under permeable pavements to better characterizing relationships between loads and deformation. Therefore, conservative values (i.e., AASHTO layer coefficients) should be assumed for open-graded base and subbase aggregates in permeable pavement design.
Regardless of type of permeable pavement, structural design methods consider the following in determining surface and base thicknesses to support vehicular traffic:
Soil stability under traffic should be carefully reviewed for each application by a qualified geotechnical or civil engineer and lowest anticipated soil strength or stiffness values under saturated conditions used for design. Structural design for vehicular applications should generally be on soil subgrades with a CBR (96-hour soaked per ASTM D 1883 or AASHTO T 193) of 4 percent, or a minimum R-value = 9 per ASTM D 2844 or AASHTO T-190, or a minimum Mr of 6,500 pounds per square inch (45 Mega Pascals) per AASHTO T-307. Soils with lower strengths typically require thickened permeable bases or those using cement or asphalt stabilized open-graded aggregates per Mn/DOT Pavement Manual, Section 3-3.01.02 Treated Base.
Soil compaction required to achieve these soil strengths will reduce the infiltration rate of the soil. Therefore, the permeability or infiltration rate of soil should be assessed at the density required to achieve one of these values. If soils under vehicular traffic have lower strengths than those noted above, or are expansive when wet, there are several options, including
These options are typically used in combination. Pedestrian applications can be placed on lower strength soils than those noted.
Pedestrian applications can be placed on lower strength soils than those noted.
If the depth of the base/subbase for the full infiltration system is excessive, because, as an example, the design subgrade soil infiltration rate is not adequate to remove the water from the design storm within the designated period of time, then the design should include underdrains. The following procedure is for sizing the base/subbase for partial infiltration designs (i.e. contains underdrains).
The outflow rate from underdrain(s) can be approximated by
<math>q_u = k m</math>
where
This equation is based on Darcy’s Law, which summarizes several properties that groundwater exhibits while flowing in aquifers. Although the hydraulic conductivity (measure of the ease with which water can move through pore spaces of a material) of the aggregate subbase is very high (approximately 17,000 feet per day or 8,500 inches per hour), the discharge rate through underdrains is limited by the cross sectional area of the pipe. As the storage volume above/around the underdrain(s) decreases (i.e., the hydraulic head or water pressure decreases), the base/subbase and in turn the underdrain(s) will drain increasingly slower. To account for this change in flow conditions within the subbase and underdrain(s) over time, a very conservative coefficient of permeability (k) of 100 feet per day per pipe can be used to approximate the average underdrain outflow rate.
Once the outflow rate through each underdrain has been approximated, the depth of the base/subbase needed to store the design storm can be determined by
<math>d_p = ((0.95 * RP) – (i/2)t_f)) / n</math>
where
To estimate the number of underdrain pipes (N), take the dimension of the parking lot in the direction the pipes are to be placed and divide by the desired spacing between pipes – round down to the nearest whole number.
With full infiltration systems, the maximum allowable drain time (td) needs to be calculated to make sure the stored water within the base/subbase does not take too long to infiltrate into the soil subgrade.
<math>t{d} = (d_pn) / (0.5i) </math>
The total storage in the permeable pavement system, Vs, is given by
<math> V_{s} = A_{p} (d_pn + 0.5it_{f}) </math>
where
When calculating the storage volume for compliance with the Construction Stormwater Permit, only the instantaneous storage volume is considered
<math> V_{s} = A_{p} (d_pn) </math>
Permeable pavement can also be designed to augment detention storage needed for channel protection and/or flood control. The designer can model various approaches by factoring in storage within the base/subbase, expected infiltration and any outlet structures used as part of the design.
Once runoff passes through the surface of the permeable pavement system, designers should calculate outflow pathways to handle subsurface flows. Subsurface flows can be regulated using underdrains, the volume of storage in the reservoir layer, the bed slope of the reservoir layer, and/or a control structure at the outlet.
Permeable pavements can be designed to reduce nutrient loadings to the ground or surface waters. The design needs to be specifically designed to capture phosphorus. The permeable pavement system can also be designed to capture nitrogen, although it is important to note that nitrogen and phosphorus each require specific designs to facilitate their removal from stormwater. The following paragraphs describe the design characteristics necessary for the removal of phosphorus and nitrogen.
A study by (Bean, 2007a) showed higher nitrate concentrations in the exfiltrate compared to the infiltrate. Nitrogen reduction capabilities of permeable pavement can be enhanced in partial infiltration designs that detain water in the base/subbase for over 24 hours. This time is required to ensure complete de-nitrification occurs.
PICP can use specially coated aggregates in the joints and bedding and all systems can use them in the base to reduce phosphorous. Coated aggregates (sometimes called “engineered aggregates”) have an effective life of seven to ten years and target the removal of dissolved phosphorous, according to manufacturer’s literature.
A filter layer made of sand or fine aggregate placed under or sandwiched within permeable pavement bases are occasionally used as a means to reduce nutrients. This layer can be enhanced with iron filings for phosphorous reduction (Erickson 2010). Their effectiveness, initial cost, reduction in flow rates, and maintenance costs should be weighed against other design options for nutrient reductions. Sand filters will incur additional construction expense and this can be reduced by placing sand filters under the subbase at the down slope end of a permeable pavement. The disadvantage of sand filters is that they will eventually require removal and restoration if continued phosphorus reduction credit is desired. Concentrating their location in the down slope areas of the site can help reduce future maintenance costs and site disruptions.
A second approach useful for nutrient and TSS reduction can occur on sloping sites by creating intermittent berms in the soil subgrade. These enable settlement of suspended solids and encourage de-nitrification if appropriately designed. A third alternative is using a “treatment train” approach where a permeable pavement initially filters runoff and the remaining water outflows to bioswales or rain gardens adjacent to the pavement for additional processing and nutrient reduction. There may be additional BMPs used to remove nutrients as the water moves through the watershed.
Prior to infiltration testing, soil borings should be taken with an auger to assess the consistency of the soil type and horizons. Guidance for conducting infiltration tests and for determining the number of borings can be found here.
Permeable pavement designs should include methods to convey larger storms (e.g., 2-year, 10-year) to the storm drain system. The following is a list of methods that accomplish this.
The reservoir below the permeable pavement surface should be composed of clean, washed crushed stone aggregate and thickness sized for both the storm event to be stored and the structural requirements of the expected traffic loading. The recommended minimum void ratio should be 40 percent per ASTM C29. Reservoir base layers for pervious concrete are typically washed AASHTO No. 57 stone and those for porous asphalt are AASHTO No. 2, 3, or 5. PICP uses AASHTO No. 2, 3, or 4 stone.
If exposed to vehicular loads, all crushed stone should be Minnesota Department of Transportation (MnDOT) Class A or B coarse aggregate, minimum 80 percent crushed, typically granite, basalt, gneiss, trap rock, diabase, gabbro, or similar material. The maximum Los Angeles Rattler Loss should be 35 percent per AASHTO T-96 and no greater loss than 10 percent per AASHTO T-104 Magnesium Sulfate Soundness Test on the non-igneous portions and as modified by the MnDOT Laboratory Manual (MNDOT 2005). Limestone aggregates not meeting these requirements are not recommended in vehicular applications. Class C and D aggregates may be used in areas subject only to pedestrian traffic.
Underdrains install quickly when placed on or in the soil subgrade, surrounded by stone base materials. The outflow portion at the end is not perforated and is raised to a designed height that allows for some water detention prior to outflow. Placement at this elevation also protects the pipe with aggregate during base compaction. For permeable pavement bases/subbases using 2 or 3 inch maximum size aggregates, underdrain pipes with them should be surrounded with at least 4 inches of ASTM No. 57 (maximum 1 inch size aggregate) to protect the pipes during compaction. An underdrain(s) can also be installed and capped at a downstream structure as an option for future use if maintenance observations indicate a reduction in the soil permeability.
Proper maintenance of permeable pavement is crucial for ensuring its longevity and functionality. Some portions of the maintenance plan require planning during the design stages. These items are noted below.
The following design elements apply to permeable pavement.
Local authorities may require greater separation depths.
It is HIGHLY RECOMMENDED that infiltration practices not be hydraulically connected to structure foundations or pavement, to avoid seepage and frost heave concerns, respectively. If groundwater contamination is a concern, it is RECOMMENDED that groundwater mapping be conducted to determine possible connections to adjacent groundwater wells.
Required and recommended minimum vertical and horizontal separation distances. This represents the minimum distance from the infiltration practice to the structure of concern. If the structure is above-ground, the distance is measured from the edge of the BMP to the structure. If the structure is underground, the vertical separation distance represents the distance from the point of infiltration through the bottom of the system to the structure, while the horizontal separation (often called setback) distance is the shortest distance from the edge of the system to the structure.
Link to this table
Structure | Distance (feet) | Requirement or recommendation | Note(s) | |
---|---|---|---|---|
Vertical | Saturated soil | 3 | Requirement1 | |
Bedrock | 3 | Requirement1 | ||
Horizontal | Public supply well | 100 for sensitive wells; 50 for others3 | Requirement | |
Building/structure/property line2 | 10 | Recommended | ||
Surface water | none unless local requirements exist | If nearby stream is impaired for chloride, see [7] | ||
Septic system | 35 | Recommended | ||
Contaminated soil/groundwater | No specific distance. Infiltration must not mobilize contaminants. | |||
Slope | 200 | Recommended | from toe of slope >= 20% | |
Karst | 1000 up-gradient 100 down-gradient | Requirement1 | active karst |
1 Required under the Construction Stormwater General Permit
2 Minimum with slopes directed away from the building
3If treating an average of 10,000 gallons per day; otherwise separation distance is 300 feet
Karst: It is HIGHLY RECOMMENDED that infiltration practices not be used in active karst formations without adequate geotechnical testing.
Wellhead Protection Areas: It is HIGHLY RECOMMENDED to review the Minnesota Department of Health guidance on stormwater infiltration in Wellhead Protection Areas.
It is HIGHLY RECOMMENDED that the following pretreatment sizing guidelines be followed:
It is HIGHLY RECOMMENDED that pretreatment practices be designed such that exit velocities from the pretreatment systems are non-erosive (less than 3 feet per second) and flows are evenly distributed across the width of the practice (e.g., by using a level spreader).
The depth of an infiltration practice is a function of the maximum drawdown time and the design infiltration rate.
Permeable pavement material specifications vary according to the specific pavement product selected. The following table describes general material specifications for the components installed beneath the permeable pavement. Note that the size of stone materials used in the reservoir and filter layers differ depending whether the system is pervious concrete, porous asphalt or permeable interlocking concrete pavement.
Summary of specifications for materials under the pavement surface. For more information, see the footnote (1). Reference or links to any specific commercial product, process, or service by trade name, trademark, service mark, manufacturer, or otherwise does not constitute or imply endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency.
Link to this table
Bedding/choker layer |
|
Washed free of fines |
Reservoir Layer |
|
Stone layer thickness based on the pavement structural and hydraulic requirements. Stonewashed and free of fines. Recommended minimum void ratio = 0.4. |
Underdrain (optional) | Use 4 to 6 inch diameter perforated PVC (AASHTO M-252) pipe or corrugated polyethylene pipe. Perforated pipe installed for the full length of the permeable pavement cell, and non-perforated pipe, as needed,connected to storm drainage system. | |
Filter Layer (optional) | Sand filter layeris separated from base above and native soils with geotextile. Sand layer typically ASTM C33 gradation, 6 to 12 inches thick. | The Filter Layer is REQUIRED if using the permeable pavement system to meet permit requirements. The sand layer may require a choker layer on surface to provide transition to base layerstone. |
Geotextile(optional) | Comply with AASHTO M-288 Standard Specification for Geotextile Specification for Highway Applications, drainage and separation applications, Class I or II. Porous asphalt industry recommends non-woven geotextile. | |
Impermeable Liner | Use a minimum 30mil PVC liner covered by 12 ounce/square yard non-woven geotextile. EPDM and HDPE liner material is also acceptable. | |
Observation Well | Use a perforated 4 to 6 inch vertical PVC pipe (AASHTO M-252) with a lockable cap, installedflush with the surface (or under pavers). |
1for additional information on materials referenced in this table (e.g. stone dimensions), see the following links:
A general comparison of different permeable pavements is provided in the following table. Designers should consult industry association and manufacturer’s technical specifications for specific criteria and guidance.
This table shows summarizes specifications for permeable pavement.
Link to this table.
Permeable Pavers | Surface open area: typically 5% to 15%; minimum thickness: 3 inches for vehicles; minimum compressive strength: 8,000 psi | Concrete pavers conform to ASTM C936 and clay pavers C1272. Reservoir layer required to support the structural load. |
Pervious Concrete |
|
May not require a reservoir layer to support loads, but a layer is required for storage/infiltration. In no case should plain steel rebar or mesh be used in pervious concrete as this invites corrosion. |
Porous Asphalt |
|
Reservoir layer contributes to structural load support. |
There are additional design considerations for permeable pavement, including use of permeable pavement in karst terrain and winter considerations.
A detailed geotechnical investigation may be required for any kind of stormwater design in karst terrain. Permeable pavements, as with other infiltration practices, are not recommended at sites with known karst features as they can cause the formation of sinkholes and can provide a direct link for stormwater to access groundwater without providing any treatment.
Plowed snow piles should be located in adjacent grassy areas so that sediments and pollutants in snowmelt are partially treated before they reach all permeable pavements. Sand is not recommended for winter traction over permeable pavements. If sand is applied, it must be removed with vacuum cleaning in the spring. Traction can be accomplished on PICP using jointing stone materials, some of which will find its way into the joints by springtime. A significant winter advantage of permeable pavements is that they require less deicing materials than their impervious counterparts. Use of deicing material on permeable pavement is therefore not recommended.
Permeable pavements can be used as opportunities for public education with signs explaining how they work. Infiltration demonstrations also help show how the pavements work. Signs provide a reminder to maintenance crews of their presence and list maintenance do’s and don’ts specific to the permeable pavement type.
Design checklists have not been developed for the Minnesota Stormwater Manual. Below are links to checklists developed by other organizations.
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.
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.
The following is a typical construction sequence to properly install permeable pavement, which may be modified depending on the pavement type.
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 [8], [9], [10]
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.
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:
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.
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
Sediment management
Excavation
Geotextiles
Impermeable liners (if specified; see here)
Drain pipes/observation wells
Aggregates
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 have not been developed for the Minnesota Stormwater Manual. Below are links to checklists developed by other organizations.
Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.
Link to this table
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
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.
Permeable pavement main page
</noinclude>
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.
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.
The following is a typical construction sequence to properly install permeable pavement, which may be modified depending on the pavement type.
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 [12], [13], [14]
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.
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:
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.
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
Sediment management
Excavation
Geotextiles
Impermeable liners (if specified; see here)
Drain pipes/observation wells
Aggregates
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 have not been developed for the Minnesota Stormwater Manual. Below are links to checklists developed by other organizations.
Recommended number of soil borings, pits or permeameter tests for bioretention design. Designers select one of these methods.
Link to this table
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
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.
Permeable pavement main page
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Assessing the performance of permeable pavement 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.
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 activities do not produce numerical performance data that could be used to obtain a stormwater management credit (stormwater 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:
Additional information on designing a monitoring network and performing field monitoring are found at this link.
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:
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.
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 |
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.
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.
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.
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 .
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.
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.
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.
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
<math> V_{inf_b} = D_o\ n\ (A_O + A_B)\ / 2 </math>
where
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 hydrologic soil group 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.
Link to this table
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 volume credit (V) for permeable pavement systems with underdrains, in cubic feet, is given by
<math> V = V_{inf_b} + V_U </math>
The infiltrating volume (Vinfb), in cubic feet, is given by
<math> V_{inf_b} = A_B DDT I_R / 12 </math>
where
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
<math> V_U = D_u\ n\ (A_U + A_B)\ / 2 </math>
where
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 hydrologic soil group C soil with an infiltration rate of 0.2 inches per hour, the maximum depth below the underdrain is 0.8 feet.
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
<math> M_{TSS} = M_{TSS_I}\ + M_{TSS_F} </math>
where
The annual TSS credit (MTSSI) for infiltrated runoff is given by
<math> M_{TSS_I} = 2.72\ V_{_{Annual}}\ F_I\ EMC_{_{TSS}} </math>
where
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
<math> M_{TSS_F} = 2.72\ R_{_{TSS}}\ V_{_{Annual}}\ (F_F)\ EMC_{_{TSS}} </math>
where
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.
Link to this table
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
<math> M_{TSS_I} = 2.72\ V_I\ EMC_{_{TSS}}\ / 43,560 </math>
where
The storm event based TSS credit (MTSS-F) for filtered runoff is given by
<math> M_{TSS_F} = R_{_{TSS}}\ 2.72\ V_F\ EMC_{_{TSS}}\ / 43560 </math>
where
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
<math> M_{TP} = M_{TP_I}\ + M_{TP_F}</math>
where
The total annual TP removal for infiltrated runoff is given by
<math> M_{TP_I} = 2.72\ V_{annual}\ F_I\ EMC_{_{TP}} </math>
where
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
<math> M_{TP_F} = 2.72\ R_{_{TP}}\ V_{_{Annual}}\ F_F\ EMC_{_{TP}} </math>
where
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.
Link to this table
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
<math> M_{TP_I} = 2.72\ V_I\ EMC_{_{TP}}\ / 43,560 </math>
where
The storm event based TP credit (MTP-F) for filtered runoff is given by
<math> M_{TP - F} = R_{_{TP}}\ 2.72\ V_F\ EMC_{_{TP}}\ / 43560 </math>
where
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
<math> 2.72 (2.3446) (1) (54.5) = 347 </math>
Annual TP removal is given by
<math> 2.72 (2.3446) (1) (0.3) = 1.91 </math>
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
<math> (2.72 (2.3446) (0.68) (54.5)) + ((2.72 (2.3446) (0.32) (0.74) (54.5)) = 319 </math>
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
<math> (2.72 (2.3446) (0.68) (0.3)) + ((2.72 (2.3446) (0.32) (0.55) (0.74) (0.3)) = 1.55 </math>
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.
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:
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:
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.
Link to this table
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 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, runoff coefficients 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.
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.
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.
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
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.
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.
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.
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:
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.
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.
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:
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.
Link to this table
Pollutant | Constituent | Treatment capabilities1 |
---|---|---|
Metals2 | Cadmium, Chromium, Copper, Zinc, Lead, Nickel | Medium/High |
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
Cost/benefit considerations for permeable pavement
All permeable pavements are ADA compliant. PICP is compliant if designs are used with joints less than ½ inch wide.
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.
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.
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).
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.