Calculating credits for permeable pavement

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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.

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 achieves volume reductions through

• storage of water in a reservoir beneath the permeable pavement surface and
• infiltration into the underlying soil.

The overall credit will be the sum of the storage and infiltration credit.

Storage 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 (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 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.

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.

Infiltration credit

For a design with no underdrain, an infiltration credit can also be given if the reservoir storage area is exceeded during a rain event. This is a complicated calculation since infiltration will occur while the reservoir is filling. An approximation of the volume infiltrated (V_i) is given by

\(V_i = A_s i/2 t_f\)

where

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

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

2 is a safety factor

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

There are several models and calculators that can be used to calculate volume reductions associated with use of permeable pavement.

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 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.

File:MIDS screen shot 2.png

This screen shot provides a summary of output 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 (%)).</font size

Literature review on volume reductions for permeable pavement

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.

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TSS credits

Models and calculators for calculating permeable pavement TSS credits

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

Assumptions and factors affecting TSS credits for permeable pavement

Literature review on TSS reductions for permeable pavement

Recommended values for TSS reductions from permeable pavement

Phosphorus credits

Models and calculators for calculating permeable pavement Phosphorus credits

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

Assumptions and factors affecting phosphorus credits for permeable pavement

Literature review on phosphorus reductions for permeable pavement

Recommended values for phosphorus reductions from permeable pavement

Example applications for calculating permeable pavement credits for volume, TSS and phosphorus

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.

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

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.

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

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.

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Recommended values for volume reductions from permeable pavement

TSS credits

Models and calculators for calculating permeable pavement TSS credits

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

Assumptions and factors affecting TSS credits for permeable pavement

Literature review on TSS reductions for permeable pavement

Recommended values for TSS reductions from permeable pavement

Phosphorus credits

Models and calculators for calculating permeable pavement Phosphorus credits

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

Assumptions and factors affecting phosphorus credits for permeable pavement

Literature review on phosphorus reductions for permeable pavement

Recommended values for phosphorus reductions from permeable pavement

Example applications for calculating permeable pavement credits for volume, TSS and phosphorus

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.

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