Permeable pavement is a tool that can achieve reductions in stormwater volume and pollutant loading, thereby generating volume or pollutant reduction 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, hydrocarbons, nitrogen, and chloride (by requiring less road salt application). 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.

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.

## Volume credits

The amount of credit given for volume reduction is a function of the design and performance (construction and maintenance) of the permeable pavement system.

### Equations and design criteria

File:Volume credit illustration.png
Schematic: The volume reduction associated with permeable pavement is a function of the depth and porosity of the storage media. When an underdrain exists, storage only occurs in the void space below the bottom of the underdrain (Do). When no underdrain exists, the entire subbase can store water. This model does not consider infiltration into the subgrade, which would increase the volume reduction credit.

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.

### Models and calculators for calculating permeable pavement volume credits

There are many models and calculators that can be used to calculate volume reductions associated with use of permeable pavement, including the following:

• MIDS calculator. The MIDS calculator will deliver the credit as a volume reduction and will provide a comparison with the MIDS performance goal of 1.1 inches of runoff retained.

### Assumptions and factors affecting volume credits for permeable pavement

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.

• The permeable pavement is properly designed. Credits can be adjusted using the equations presented in the design section for permeable pavement. Factors that will influence adjustments to the credit calculation include the depth of runoff from the contributing drainage area (not including the permeable paving surface) for the design storm, the ratio of the contributing drainage area (not including the permeable paving surface) to the permeable pavement surface area, the rainfall depth for the treatment volume, the field-verified infiltration rate for native soils, the time to fill the reservoir layer, and the void ratio for the reservoir layer (0.4).
• The permeable pavement was properly constructed, consistent with the design specifications.
• The permeable pavement is properly maintained. The performance of the permeable pavement should be regularly assessed.

### Literature review on volume reductions for permeable pavement

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.

## TSS credits

### Models and calculators for calculating permeable pavement TSS credits

The following models or calculators can be used to calculate the credit:

## Phosphorus credits

### Models and calculators for calculating permeable pavement Phosphorus credits

The following models or calculators can be used to calculate the credit: