Calculating credits for permeable pavement

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Permeable pavement is a tool that can achieve reductions in stormwater volume and pollutant loading. These reductions constitute stormwater credits that can be used to meet various goals (e.g. Total maximum Daily loads (TMDLs), Minimal Impact Design Standards (MIDS) performance goals). Permeable pavement will achieve the greatest credit when it is properly designed, constructed and maintained.

This section provides specific information on generating and calculating credits from permeable pavement for volume, TSS and phosphorus. Permeable pavement may also be effective at reducing concentrations of other pollutants such as metals and nitrogen. This article does not provide information on calculating credits for pollutants other than TSS and phosphorus, but references are provided that may be useful for calculating credits for these other pollutants.

In high-infiltration rate soil subgrades, permeable pavement can be designed without an underdrain. Permeable pavements will typically be built to meet other performance goals. For example, when designing for the MIDS performance goal for new development in sites without restrictions, the pavement must infiltrate the first 1.1 inches of rainfall from impervious surfaces.

File:Design schematic 2.png

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.

Volume credits

A permeable pavement system without underdrains or with a raised underdrain achieves volume credits by allowing water below the underdrain to infiltrate into the underlying soil. Some infiltration will occur even in systems

Basis for volume credits

Volume credits for a permeable pavement system are a function of the dimensions of the system, specifically the depth of the subbase below an underdrain, the area of permeable pavement and the porosity of the subbase. The volume credit (V_s) is given by

\(V_s = A_s d_p n\)

where

A_s = the surface area of the permeable pavement system (square feet);

d_p = the depth of the reservoir layer, equal to the area from the bottom of the underdrain to the underlying soil (do not include the surfacing thickness)(feet); and

n = porosity of the subbase(cubic feet/cubic feet). Note that the term V_r, which represents the void ratio of a material, is analogous to and may be used instead of porosity.

The volume credit assumes the entire pore space is available for water storage. Field capacity represents the amount of water remaining just as all gravitation water has drained thorugh a system. Although use of field capacity rather than the full pore space procides a more accurate estimate of the water stored in a permeable pavement system, field capacity in these systems is assumed to be very low, likely 5 percent or less (see [1]).

Often, permeable pavement will be designed to meet a specific performance goal, such as the 1.1 inch Minimal Impact Design Standards (MIDS) goal for new development sites with no restrictions or a TMDL goal for phosphorus or TSS reductions. Specifications for designing permeable pavement systems, including a discussion of how to calculate the reservoir depth, are provided in the section covering design specifications for permeable pavement. When an underdrain exists at the bottom of the permeable pavement system, the reservoir depth equals zero and there is no storage credit.

File:Design schematic 3.png

Schematic showing storage and infiltration below an elevated underdrain. Infiltration must occur with 48 hours. If the underdrain was placed at the bottom of the design, there would be no storage credit but an infiltration credit can be received.

File:Design schematic 4.png

Schematic showing the sequence of events involved in calculating an infiltration credit. Rainfall begins at t₁. Water infiltrates rapidly and infiltration into the underlying soil begins. At t₂ the reservoir layer is full and infiltration continues into the underlying soil. At t_3a and t_3b, water continues to be delivered to the permeable pavement. In 3a the rate of delivery to the permeable pavement is greater than the rate of infiltration into the underlying soil and infiltration is controlled by the soil infiltration rate. In 3b the rate of water delivery is less than the soil infiltration rate and additional credit for infiltration is controlled by the rate of water delivery. At t_f the reservoir continues to drain but there is no new water delivered to the system, thus resulting in no infiltration credit. The total infiltration credit equals the sum of soil infiltration rate times the time to fill the reservoir plus the the amount of water infiltrated and/or delivered after the reservoir is filled until the time when water is no longer delivered to the system

Infiltration credit

Initial infiltration rates for permeable pavement are initially on the order of hundreds of inches per hour, which is much larger than the intensity that can be produced by a rain event. Infiltration rates usually exceed one inch per hour even when the pavement is substantially clogged (Smith and Hunt 2010). Sites that receive run-on from poorly maintained or disturbed areas had the lowest infiltration rate in a study by Bean et al. 2007. However, the infiltration rates at these sites were still high relative to rainfall intensities.

For a design with no underdrain, an infiltration credit can also be given if the reservoir storage area is exceeded during a rain event. Note that an infiltration credit is not dependent on the reservoir becoming filled since infiltration into the underlying soil begins before the reservoir fills. To avoid double counting, it is necessary to subtract the reservoir volume from the volume that infiltrated through the permeable pavement surface. An approximation of the volume infiltrated (V_i) while the reservoir is filling is given by

\(V_i = A_s i/2 t_f\)

where

i = the soil infiltration rate (feet/day); and

t_f = the time it takes to fill the reservoir layer (day)

Once the reservoir fills and assuming water continues to be delivered to the permeable pavement surface at a rate greater than the underlying soil, water loss will be controlled by infiltration through the underlying soil. If the reservoir fills and water continues to be delivered to the permeable pavement surface but at a rate lower than the infiltration rate into the underlying soil, the rate of water delivery determines the additional infiltration credit. In low-infiltration soils where the design will most likely include an underdrain, some infiltration of water into the subgrade occurs. The volume of water infiltrated depends on the volume of storage available below the underdrain outflow invert. The remaining filtered runoff is collected in the underdrain and exits to the storm drainage system, typically a stream or storm sewer. Equations provided for design specifications for permeable pavement can be used to calculate outflow volumes through underdrains. It is recommended that i be field verified. A typical value for t_f is 2 hours (0.083 day).

Assumptions and factors affecting volume credits for permeable pavement

Warning: The following general assumptions apply in calculating the credit for permeable pavement. If any of these assumptions is violated, the credit will be reduced.

• The permeable pavement is properly designed.
• The permeable pavement was properly constructed, consistent with the design criteria.
• The permeable pavement is properly maintained. The performance of the permeable pavement should be regularly assessed.

Assumptions used to calculate credits may also vary with each calculator or model. To calculate credits it is important to ensure that your calculation is consistent with the assumptions made in the model or calculator you are using. Assumptions for some models or calculators are briefly discussed below. More detailed discussions of assumptions may be found in user's manuals or other documentation for the model or calculator.

Models and calculators for calculating permeable pavement volume credits

The Manual does not provide specific recommendations for which values or models to use when calculating volume credits for permeable pavement. The models discussed below are widely utilized within the stormwater community and are therefore appropriate for calculating volume credits provided the model assumptions are met and the permeable pavement is properly designed, constructed and maintained.

File:MIDS screen shot 1.png

This screen shot of the MIDS calculator provides a schematic showing model inputs and the input portion of the calculator. Boxes in blue are required inputs. Note the "underlying soil" field has a drop-down box that is open. Green and grey fields are calculations made within the worksheet. Note there is a flag, shown in red text, indicating the "Outflow depth" provided by the user does not meet the drawdown time requirement. The calculator therefore calculates a new outflow depth.

Minimal Impact Design Standards (MIDS) calculator

The MIDS calculator provides a BMP volume credit based on storage within the reservoir layer (subbase) below the permeable pavement. Calculator inputs include

• the area of the permeable pavement surface (square feet);
• the reservoir depth (feet)(also called outflow depth). If the user enters a reservoir depth that will not allow drainage within 24 or 48 hours, the model calculates a depth based on the subbase porosity and the underlying soil;
• porosity of the subbase;
• the underlying soil, which the user selects from a dropdown box (14 options plus an option for a user-defined infiltration rate); and
• the required drawdown time (hours), which the user selects from a drop down box (choices are 24 or 48 hours).

The user can specify an impervious area that contributes to the permeable pavement. The user can also route water through downstream BMPs.

Calculator output includes

• the BMP volume capacity (cubic feet);
• the required retention volume for the specified performance goal (cubic feet);
• the BMP volume credit (cubic feet);
• the untreated volume (cubic feet); and
• the percent water removed relative to the performance goal.

The MIDS calculator does not provide credit for infiltration. Thus there must either be no underdrain or the underdrain must be suspended above the botton of the reservoir layer to receive a volume credit.

File:MIDS screen shot 2.png

This screen shot provides a summary of output for volume from the MIDS calculator. Green and grey cells have calculated values. The BMP volume credit (cubic feet) is denoted with a star. The calculator also shows the untreated remaining volume and the percent of the performance goal achieved (Runoff Volume Removed (%)).

TSS credits

The water quality credits available for installation of permeable pavement depend largely on the design of the storage volume below the pavement and whether or not the runoff is filtered (through underdrain) or infiltrated. Infiltration reduces the volume of runoff and results in the most credit. The credit for pollutant reduction corresponds directly with annual volume reduction. The infiltration rate of the subgrade soil affects the annual volume reduction (the higher the infiltration rate, the higher the volume reduction). For example, if a system is designed to store and infiltrate the MIDS performance goal of 1.1 inches off impervious surfaces, it would result in an annual volume reduction of 91% for a site with HSG C subgrade soils, which corresponds to a 91% reduction in TSS. Designs that filter runoff with an underdrain at the bottom of the storage layer (on top of the subgrade) are less effective than infiltration designs. Runoff is filtered while flowing through the permeable pavement and the storage layer and out the underdrain.

Assumptions and factors affecting TSS credits for permeable pavement

Warning: The following general assumptions apply in calculating the credit for permeable pavement. If any of these assumptions is violated, the credit will be reduced.

• The permeable pavement is properly designed.
• The permeable pavement was properly constructed, consistent with the design criteria.
• The permeable pavement is properly maintained. The performance of the permeable pavement should be regularly assessed.

Assumptions used to calculate credits may also vary with each calculator or model. To calculate credits it is important to ensure that your calculation is consistent with the assumptions made in the model or calculator you are using. Assumptions for some models or calculators are briefly discussed below. More detailed discussions of assumptions may be found in user's manuals or other documentation for the model or calculator.

Models and calculators for calculating permeable pavement TSS credits

The Manual does not provide specific recommendations for which values or models to use when calculating TSS credits for permeable pavement. The models discussed below are widely utilized within the stormwater community and are therefore appropriate for calculating TSS credits provided the model assumptions are met and the permeable pavement is properly designed, constructed and maintained.

Minimal Impact Design Standards calculator

If a system is designed to infiltrate the MIDS performance standard of 1.1 inches of runoff from the tributary impervious surfaces, it would result in a 91% annual runoff volume reduction from a site with hydrologic soil group (HSG) C soils (infiltration rate of 0.2 inches/hr). Annual pollutant load reductions for this example are approximately equal to the volume reduction. A site with HSG A soils (infiltration rate of 1.6 inches/hr) would result in higher annual reductions.

For designs with underdrains, the reductions are less because a portion of the water is captured by the underdrains before it can be infiltrated. Of the water intercepted and draining through the underdrain, 74% (with upper and lower 90% confidence bounds of 93% and 33%, respectively) of total suspended solids removal can be expected.

Since TSS credits are a function of the volume infiltrated, the design dimensions control TSS removal. Calculator inputs include

• the area of the permeable pavement surface (square feet);
• the reservoir depth (feet)(also called outflow depth). If the user enters a reservoir depth that will not allow drainage within 24 or 48 hours, the model calculates a depth based on the subbase porosity and the underlying soil;
• porosity of the subbase;
• the underlying soil, which the user selects from a dropdown box (14 options plus an option for a user-defined infiltration rate); and
• the required drawdown time (hours), which the user selects from a drop down box (choices are 24 or 48 hours).

The user can specify an impervious area that contributes to the permeable pavement. The user can also route water through downstream BMPs.

File:MIDS screen shot 3.png

This screen shot shows output from the MIDS calculator. The BMP scenario consisted of a 3000 square foot permeable pavement surface with an underdrain 0.2 feet above the underlying soil, a drawdown time of 48 hours, HSG C, and no impervious surface contributing to the pavement. Click on this picture for more detailed discussion of the output.

In calculating the TSS credit, the incoming TSS load is first reduced by 74%. This accounts for TSS removal in the case where an underdrain is at the bottom of the permeable pavement system. If an underdrain is suspended above the bottom of the design, then additional reduction in TSS loading will occur as a result of infiltration below the underdrain.

Calculator output includes

• annual post development TSS load in pounds, calculated by multiplying an event mean concentration (EMC) of 55.40 mg/L by the area contributing to the pavement. This area equals impervious surfaces discharging to the permeable pavement plus the direct precipitation onto the pavement;
• TSS removed via non-volume reduction treatment, equal to 74% of the incoming TSS load;
• Total TSS removed via volume reduction and other treatment, given as a percent of incoming TSS;
• Annual TSS reduction in pounds; and
• remaining TSS load, in pounds.

For an example with discussion, see the screen shot on the right.

Literature review on TSS reductions for permeable pavement

This section provides links to research and data on removal of TSS for permeable pavement. Users should be aware of assumptions and limitations associated with data presented in these reports or databases.

• New Jersey Stormwater BMP Manual. 2004. Standards for pervious paving systems. Chapter 9.7.
• Chris Brown, Angus Chu, Bert van Duin,and Caterina Valeo. 2009. Characteristics of Sediment Removal in Two Types of Permeable Pavement. Water Qual. Res. J. Can. 2009 · Volume 44, No. 1, 59-70
• North Carolina Department of Environment and Natural Resources. Water Quality Division. 2012. DWQ Stormwater BMP Manual & BMP Forms. Chapter 18.
• New Hampshire Department of Environmental Services. 2008. New Hampsire Stormwater Manual. Appendix B.

Phosphorus credits

Models and calculators for calculating permeable pavement TSS credits

The Manual does not provide specific recommendations for which values or models to use when calculating TSS credits for permeable pavement. The models discussed below are widely utilized within the stormwater community and are therefore appropriate for calculating TSS credits provided the model assumptions are met and the permeable pavement is properly designed, constructed and maintained.

Pollutants other than TSS and phosphorus

In addition to TSS and phosphorus, permeable pavement can reduce loading of the following pollutants:

• Metals such as copper and zinc
• Nitrogen
• Hydrocarbons
• Chloride (indirectly by reducing the amount of road salt applied)
• Oxygen demand

Specific credits and methods for calculating credits are not provided in this section. Information on removal of these pollutant by permeable pavement systems can be found at the following links.

• [2] - information on zinc, copper, lead, nitrate, Kjeldahl nitrogen, and total nitrogen
• [3] - information on total nitrogen, heavy metals, and hydrocarbons.