Calculating credits for permeable pavement

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

• providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
• complying with permit requirements, including antidegradation (see [1]; [2]);
• meeting the MIDS performance goal; or
• meeting or complying with water quality objectives, including Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs).

This page provides a discussion of how biofiltration practices can achieve stormwater credits.

Overview

Schematic illustrating dimensions used to calculate storage volume for permeable pavement. The volume equals the reservoir depth (d_p) times the permeable pavement surface area. Design A shows a system with no underdrain in which d_p equals the height of the reservoir layer. Design B shows an elevated underdrain, with d_p equal to the distance from the bottom of the underdrain to the underlying soil. Design C shows an underdrain at the bottom.

Permeable pavements with underdrains are a stormwater quality practice that allows runoff to pass through surface voids into an underlying stone reservoir/subbase for temporary storage and filtration before being discharged to a downstream BMP via an underdrain. Stormwater quantity may be achieved if storage and infiltration are allowed below the underdrain. The most commonly used types of permeable pavement are pervious concrete, porous asphalt, and permeable interlocking concrete pavers.

Pollutant removal mechanisms

Permeable pavements provide stormwater pollutant removal by reducing the volume of runoff from a site and the pollutant mass associated with that volume.

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. Under the Treatment Train approach, stormwater management begins with simple methods that prevent pollution from accumulating on the land surface, followed by methods that minimize the volume of runoff generated and is followed by Best Management Practices that reduce the pollutant concentration and/or volume of stormwater runoff.

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 .

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). Permeable pavement is also effective at reducing concentrations of other pollutants including nitrogen and metals. 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 kerplunk method is assumed in calculating volume and pollutant reductions. This method assumes the water quality volume (WQV) is delivered instantaneously to the BMP. The WQV is stored as water ponded above the filter media and below the overflow point in the BMP. The WQV can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch off new impervious surface. For MIDS, the WQV is 1.1 inches.

In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The kerplunk method therefore may underestimate actual volume and pollutant losses.

Volume Credit Calculations

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. Instantaneous volume reduction, or event based volume reduction, of a BMP can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.

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 below an underdrain, and the area of permeable pavement, and the bottom surface area. The volume credit (V) for the infiltration storage, in cubic feet, is given by

\( V = (A_O + A_B) / 2 * D_M * n \)

where:

• V = Volume reduction capacity of the permeable pavement system via infiltration (cubic feet).
• A_O = Overflow surface area of the permeable pavement system. For installations without an underdrain this will be the top surface area. For installations with underdrains this will be the area of the permeable pavement system at the bottom of the underdrain (square feet).
• A_B = Bottom surface area of the permeable pavement system (square feet).
• D_M = Depth of the reservoir/subbase layer (engineered media). D_M is the distance from the bottom of the permeable pavement material to the underlying soil subgrade (feet).
• n = Porosity of the reservoir/subbase (cubic feet per cubic foot).

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 is complete, called field capacity, is less than 5 percent of total porosity for a permeable pavement system.

The volume reduction credit (V) can be converted to annual volume reduction percentage if the annual volume reduction quantity is desired. This conversion can be generated using the MIDS calculator or other appropriate modeling techniques. The MIDS calculator obtains the percentage annual volume reduction through performance curves developed from multiple modeling scenarios using the volume reduction capacity of the BMP, the infiltration rate of the underlying soils, and the contributing watershed size and imperviousness .

Water quality credit calculations

Water quality credits applied to permeable pavement can be calculated per rain event or based on total annual rainfall. Permeable pavement provides water quality benefits through infiltration of stormwater runoff. Because permeable pavement without underdrains assumes 100 percent infiltration for runoff captured by the BMP, water quality credits are represented entirely by the infiltrated credit and no additional credit is assumed for filtration.

Total Suspended Solids (TSS)

TSS reduction credits correspond directly with volume reduction. The water quality credits available for installation of permeable pavement depend on the design of the storage volume below the pavement. Total removal of Total Suspended Solids by permeable pavement is given by

\( M_{TSS} = M_{TSS_I} \)

where:

• M_TSS =Annual or event TSS removal (pounds per event or pounds per year).
• M_TSSI = Mass Total Suspended Solids removed by infiltration (pounds per event or pounds per year).

Annual pollutant reduction calculations are dependent on the annual volume reduction credit (V_Annual). The Annual TSS credit (MTSS-I) for infiltrated runoff is given by

\( M_{TSS_I} = 2.72 * V_{Annual} * EMC_{TSS} \)

where:

• M_TSSI =Annual TSS removal from infiltrated runoff (lb/yr).
• V_Annual = Annual volume reduction credit calculated above (acre-ft).
• EMCTSS = Event Mean Concentration, concentration of TSS in the runoff. (mg/L). Note: if 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.
• Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.

Event pollutant volume reduction calculations are dependent on the volume reduction capacity (V) of the BMP calculated above. The storm event based TSS credit (MTSS-I) for infiltrated runoff is given by

\( M_(TSS-I)=2.72*V/43,560*〖EMC〗_TSS \)

where:

• MTSS-I =Event TSS removal from infiltrated runoff (lb/event).
• V = Event volume reduction credit calculated above (cf).
• EMCTSS = Event Mean Concentration of TSS in the runoff. (mg/L). Note: if 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.
• Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds. A Factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

Total Phosphorus (TP)

TP reduction credits correspond directly with volume reduction through infiltration. Removal is considered to be 100 percent for storm water that is captured and infiltrated by the BMP and 0 percent for storm water that is not captured by the BMP.

Total removal of Total Phosphorus Solids by permeable pavement is given by

\( M_{TP} = M_{TP_I} \)

where:

• MTP =Annual or event TP removal (lb/yr or lb/event).
• MTP-I =Annual or event TP removal from infiltrated runoff (lb/yr or lb/event).

Annual volume reduction TP credits are dependent on the annual volume reduction (VAnnual), as well as the annual runoff volume calculated above. The annual TP credit (MTP-I) for infiltrated runoff is given by

\( M_{TP_I} = V_{Annual} * EMC_{TP} * 2.72 \)

where:

• MTP_I =Annual TP removal (lb/yr).
• V_Annual = Annual volume reduction credit calculated above (acre-ft).
• EMC_TP = Event Mean Concentration of TP in runoff. (mg/L). Note: if permeable pavement is not the upstream most BMP in the treatment train, *EMC_TP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.
• Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.

Event based volume reduction TP credits are dependent on the volume reduction (V) and the filtration volume (VF) capacities of the BMP calculated above. The storm event based TP credit (MTP_I) for infiltrated runoff is given by

\( M_{TP_I} = 2.72 * V / 43,560 * EMC_{TP} \)

where:

• V = Event volume reduction credit calculated above (cf).
• EMC_TP = Event Mean Concentration of TP in the runoff. (mg/L). Note: if permeable pavement is not the upstream most BMP in the treatment train, EMC_TP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.
• Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds. A Factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

Related articles

• Overview for permeable pavement
• Types of permeable pavement
• Design criteria for permeable pavement
• Construction specifications for permeable pavement
• Assessing the performance of permeable pavement
• Operation and maintenance of permeable pavement
• Calculating credits for permeable pavement
• Additional considerations for permeable pavement
• References for permeable pavement
• Fact sheets for permeable pavement