Permeable pavement is a tool that can achieve reductions in stormwater volume and pollutant loading, thereby generating stormwater credits. 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. When sized to capture all rain events with no overflow ever occurring, this design retains 100% of the annual runoff volume and 100% of annual pollutant loading. 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.
A permeable pavement system achieves volume reductions through
The overall credit will be the sum of the storage and infiltration credit.
The storage credit is a function of the design and dimensions of the permeable pavement system, specifically the depth of the subbase below an underdrain, the area of permeable pavement and the porosity of the subbase. The storage credit (Vs) is given by
\(V_s = A_s d_p n\)
where
The storage credit, as defined by the above credit, assumes the entire pore space is available for water storage. In reality, some pores will be taken up by water and use of field capacity may provide a more accurate estimate of the storage credit. However, the storage volume is also based on the kerplunk method, in which all stormwater runoff is assumed to enter the BMP at one time. This results in an underestimate of the actual volume credit.
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.
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 (Vi) while the reservoir is filling is given by
\(V_i = A_s i/2 t_f\)
where
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 tf is 2 hours (0.083 day).
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.
There are several models and calculators that can be used to calculate volume reductions associated with use of permeable pavement.
The MIDS calculator provides a BMP volume credit based on storage within the reservoir layer (subbase) below the permeable pavement. Calculator inputs include
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 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.
The Manual does not provide specific recommendations for which values or models to use when calculating volume credits for permeable pavement. The models discussed above have been peer reviewed and are appropriate for calculating volume credits provided the model assumptions are met and ther permeable pavement is properly designed, constructed and maintained. Below is a summary of a literature review examining methods for calculating volume reduction for permeable pavement.
The following models or calculators can be used to calculate the credit:
The following models or calculators can be used to calculate the credit:
In addition to TSS and phosphorus, permeable pavement can reduce loading of the following pollutants:
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.
NOTE - WHAT ARE WE GOING TO SAY ABOUT INFILTRATION CREDIT Table X.3 specifies how to estimate the volume of reservoir storage required for this performance goal. 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. This design may reduce some outflow from the pavement base. Such designs offer some treatment of pollutants. The volume and pollutant reductions for permeable pavement listed in Table X.1 correspond (MIDS calculator. A project can be recognized for higher pollutant reductions if demonstrated by the project designer. Besides adequate design and construction, maintenance is critical to permeable pavement performance. All three aspects must be demonstrated for each project in order to qualify for the stated credits.
Information in this article is intended to aid in determining the best method for calculating credits and to lead the user to the appropriate resources for calculating credits. While it may be desirable to establish specific values that can be used to calculate credits, this prevents flexibility and does not allow for consideration of the range of factors that affect the volume or pollutant reductions associated with any one BMP.
There are several potential reasons for calculating credits. It is important to identify the reasons for calculating a credit and the information and resources available for calculating credits. In some cases it may be appropriate to use simple spreadsheet calculations, while in other cases more sophisticated modeling may be warranted.
This article provides users with basic equations used in calculating credits, suggests some models that may be used to calculate credits, and presents information on BMP performance that can also be used to calculate credits. The user will ultimately have to choose the most appropriate method.
The amount of credit given for volume reduction is a function of the design and performance (construction and maintenance) of the permeable pavement system.
The credit is given by the following equation
V = As * Do * n
where V is volume of storage (ft3), As is the area of permeable pavement (ft2), Do is the depth from the underdrain outflow pipe to the soil subgrade (ft.; not including surfacing thickness), and n is the porosity of stone per ASTM C29 or AASHTO T-19 (decimal). If there is no underdrain, the equation becomes
V = As * D * n
where D is the depth of base /subbase (ft. not including surfacing thickness). This credit assumes no infiltration of water stored in the permeable pavement system. Infiltration will increase the credit.
There are many models and calculators that can be used to calculate volume reductions associated with use of permeable pavement, including the following:
Assumptions used to calculate credits may 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 each model or calculator are briefly discussed in the previous sub-section. More detailed discussions of assumptions may be found in user's manuals or other documentation for the model or calculator. The following general assumptions apply in calculating the credit for permeable pavement. If any of these assumptions is violated, the credit will be reduced.
Table X summarizes information on volume reductions achieved with permeable pavement. Below is a list of literature sources for this information. The literature articles contain additional information regarding the values cited in Table X. We include a short overview for some of the references.
The following models or calculators can be used to calculate the credit:
The following models or calculators can be used to calculate the credit:
In addition to TSS and phosphorus, permeable pavement can reduce loading of the following pollutants:
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.