Calculating credits for permeable pavement

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Schematic of a permeable pavement system with no underdrain. Water infiltrating through the pavement is stored in the reservoir/subbase and infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.

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 permeable pavement practices can achieve stormwater credits.

Overview

Permeable pavements without underdrains 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 underlying soil via infiltration. The most commonly used types of permeable pavement are pervious concrete, porous asphalt, and permeable interlocking concrete pavers.

Pollutant removal mechanisms

Permeable 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 .

Methodology for calculating credits

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

Assumptions and approach

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

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.

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

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

Volume Credit Calculations

Schematic showing terminology for calculating volume credits for permeable pavement.

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

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

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_O = 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 .

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 (M_TSS-I) for infiltrated runoff is given by

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

where

• V_Annual = Annual volume reduction credit calculated above (acre-ft);
• EMC_TSS = Event Mean Concentration, concentration of TSS in the runoff, in mg/L; and
• Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.

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

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

\( M_{TSS - I} = 2.72 * V / 43,560 * EMC_{TSS} \)

where

• M_TSS-I =Event TSS removal from infiltrated runoff (lb/event);
• V = Event volume reduction credit calculated above (ft³);
• EMC_TSS = Event Mean Concentration of TSS in the runoff. (mg/L); and
• 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) credit calculations

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

• M_TP =Annual or event TP removal (lb/yr or lb/event).
• M_TP-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:

• M_TPI =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 M_TP 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, in cubic feet;
• EMC_TP = Event Mean Concentration of TP in the runoff, in mg/L; and
• 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.

Methods for calculating credits

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

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

Credits based on models

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

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

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal by the BMP.

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.

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