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Difference between revisions of "Calculating credits for permeable pavement"

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{{alert|The anticipated construction period for this page is January through March, 2013|alert-under-construction}}
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{| class="wikitable" style="float:right; margin-left: 10px; width:100px;"
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|-
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| colspan="8" style="text-align: center;" |'''Recommended pollutant removal efficiencies, in percent, for permeable pavement with an underdrain. [http://stormwater.pca.state.mn.us/index.php/Information_on_pollutant_removal_by_BMPs#References Sources]. NOTE: removal efficiencies are 100 percent of captured water for systems with no underdrain'''.<br>
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<font size =1>TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen</font size>
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|-
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| '''TSS'''
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| '''TP'''
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| '''PP'''
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| '''DP'''
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| '''TN'''
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| '''Metals'''
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| '''Bacteria'''
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|'''Hydrocarbons'''
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|-
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| 74
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| 41
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| 74
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| 0
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| insufficient data
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| insufficient data
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| insufficient data
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| insufficient data
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|}
  
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{{alert|The anticipated review period for this page is January through March, 2013|alert-under-review}}
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[[File:Pdf image.png|100px|left|thumb|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Calculating_credits_for_permeable_pavement_-_Minnesota_Stormwater_Manual.pdf Download pdf]</font size>]]
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[[File:Summary image.jpg|100px|left|thumb|alt=image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Credit_page_descriptions.mp4 Page video summary]</font size>]]
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[[File:Technical information page image.png|100px|left|alt=image]]
  
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{{alert|Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.|alert-danger}}
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[[File:Permeable pavement  credits no underdrain 2.jpg|300px|thumb|alt=schematic of permeable pavement no underdrain|<font size=3>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.</font size>]]
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[[File:Permeable pavement with underdrain.png|300px|thumb|alt=schematic of permeable pavement with underdrain|<font size=3>Schematic of a permeable pavement system with an underdrain.  Water infiltrating through the pavement is either captured by the underdrain or stored below the underdrain in the reservoir/subbase, where it infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.</font size>]]
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{{alert|Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.|alert-success}}
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[http://stormwater.pca.state.mn.us/index.php/Overview_of_stormwater_credits Credit] refers to the quantity of stormwater or pollutant reduction achieved either by an individual <span title="One of many different structural or non–structural methods used to treat runoff"> '''best management practice'''</span> (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in
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*providing incentives to site developers to encourage the [[Credits for Better Site design|preservation of natural areas and the reduction of the volume of stormwater]] runoff being conveyed to a best management practice (BMP);
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*complying with permit requirements, including antidegradation (see [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program Construction permit]; [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_Program_for_Municipal_Separate_Storm_Sewer_Systems_(MS4) Municipal (MS4) permit]);
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*meeting the [http://stormwater.pca.state.mn.us/index.php/Performance_goals_for_new_development,_re-development_and_linear_projects MIDS performance goal]; or
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*meeting or complying with water quality objectives, including <span title="The amount of a pollutant from both point and nonpoint sources that a waterbody can receive and still meet water quality standards"> [https://stormwater.pca.state.mn.us/index.php?title=Total_Maximum_Daily_Loads_(TMDLs) '''total maximum daily load''']</span> (TMDL) <span title="The portion of a receiving water's assimilative capacity that is allocated to one of its existing or future point sources of pollution"> '''wasteload allocations'''</span> (WLAs).
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This page provides a discussion of how permeable pavement practices can achieve stormwater credits. Permeable pavement systems with and without [[Glossary#U|underdrains]] are both discussed, with separate sections for each type of system as appropriate.
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==Overview==
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<!--
 
[[File:Design schematic 2.png|thumb|400px|alt=schematic illustrating dimensions used to calculate storage volume for permeable pavement. The volume equals the reservoir depth (d<sub>p</sub>) times the permeable pavement surface area. Design A shows a system with no underdrain in which d<sub>p</sub> equals the height of the reservoir layer. Design B shows an elevated underdrain, with d<sub>p</sub> equal to the distance from the bottom of the underdrain to the underlying soil. Design C shows an underdrain at the bottom.|<font size=3>Schematic illustrating dimensions used to calculate storage volume for permeable pavement. The volume equals the reservoir depth (d<sub>p</sub>) times the permeable pavement surface area. Design A shows a system with no underdrain in which d<sub>p</sub> equals the height of the reservoir layer. Design B shows an elevated underdrain, with d<sub>p</sub> equal to the distance from the bottom of the underdrain to the underlying soil. Design C shows an underdrain at the bottom.</font size>]]
 
[[File:Design schematic 2.png|thumb|400px|alt=schematic illustrating dimensions used to calculate storage volume for permeable pavement. The volume equals the reservoir depth (d<sub>p</sub>) times the permeable pavement surface area. Design A shows a system with no underdrain in which d<sub>p</sub> equals the height of the reservoir layer. Design B shows an elevated underdrain, with d<sub>p</sub> equal to the distance from the bottom of the underdrain to the underlying soil. Design C shows an underdrain at the bottom.|<font size=3>Schematic illustrating dimensions used to calculate storage volume for permeable pavement. The volume equals the reservoir depth (d<sub>p</sub>) times the permeable pavement surface area. Design A shows a system with no underdrain in which d<sub>p</sub> equals the height of the reservoir layer. Design B shows an elevated underdrain, with d<sub>p</sub> equal to the distance from the bottom of the underdrain to the underlying soil. Design C shows an underdrain at the bottom.</font size>]]
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-->
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[[Permeable pavement|Permeable pavements]] 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 an [[Glossary#U|underdrain]] and/or underlying soil via infiltration. The most commonly used [[Types of permeable pavement|types of permeable pavement]] are pervious concrete, porous asphalt, and permeable interlocking concrete pavers.
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===Pollutant removal mechanisms===
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Permeable pavement systems with no underdrains provide stormwater pollutant removal by reducing the volume of runoff from a site and the pollutant mass associated with that volume when [[Glossary#I|infiltration]] is allowed (Water Environment Federation, 2012). In systems with underdrains most of the water is captured by the underdrain after passing through the subbase. If the underdrain is raised above the underlying soil subgrade, water stored in the reservoir/subbase below the underdrain will infiltrate into the underlying soil. If the underdrain is at the bottom of the reservoir/subbase, a small amount of infiltration may occur. Thus, pollutant removal in a permeable pavement system with an underdrain occurs through filtering of water captured by the underdrain and infiltration for water infiltrating into the underlying soil subgrade.
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===Location in the treatment train===
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[[Using the treatment train approach to BMP selection|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. 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 .
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==Methodology for calculating credits==
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This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific [[Calculating credits for permeable pavement#Methods for calculating credits|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 permeable pavement#Other pollutants|calculating credits for other pollutants]].
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===Assumptions and approach===
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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|permeable pavement]] section of the Manual.
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{{alert|Pre-treatment is required for all filtration and infiltration practices|alert-danger}}
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In the following discussion, the water quality volume (V<sub>WQ</sub>) is assumed to be delivered instantaneously to the BMP. The V<sub>WQ</sub> is stored within the reservoir/subbase below the bottom of the pavement and above the soil subgrade. The V<sub>WQ</sub> can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch times the new impervious surface area. For MIDS, the V<sub>WQ</sub> is 1.1 inches times the impervious surface area. In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The instantaneous method therefore may underestimate actual volume and pollutant losses.
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The approach in the following sections is based on the following general design considerations.
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*Credit calculations presented in this article are for both event and annual volume and pollutant load removals.
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*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.
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*TSS and TP credits for permeable pavements are achieved for the volume of runoff that is filtered and captured by an underdrain and the volume of water that is ultimately infiltrated.
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<!--
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===Volume credit calculations===
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Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater from the existing stormwater collection system. These credits are assumed to be instantaneous values entirely based on the capacity of the BMP 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.
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Volume credits for a permeable pavement system are based on the porosity of the subbase and system dimensions, specifically the depth of the subbase below an underdrain and the area of permeable pavement. The volume credit (V) for the infiltration storage is given by
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<math> V = D_o\  n\  (A_O + A_B)\  /  2 </math>
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where
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*A<sub>O</sub> is the overflow surface area of the permeable pavement system, in square feet;
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*A<sub>B</sub> is the bottom surface area of the permeable pavement system, in square feet;
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*D<sub>o</sub> is the depth of the reservoir/subbase layer from the bottom of the permeable pavement material to the top of the underlying soil subgrade, in feet; and
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*n is the porosity of the reservoir/subbase, in cubic feet per cubic foot.
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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. For a permeable pavement system, this is less than 5 percent of total porosity.
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The volume reduction credit (V) can be converted to annual volume reduction percentage (VA%) 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.
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===Total suspended solids (TSS) calculations===
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TSS removal for infiltrated water, M<sub>TSS<sub>i</sub></sub> is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by
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<math> M_{TSS_i} = 0.0000624\ V_{inf_b}\ EMC_{TSS} </math>
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where
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*V<sub>inf<sub>b</sub></sub> is the infiltration through the bottom of the reservoir/subbase and into the underlying soil, in cubic feet;
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*EMC<sub>TSS</sub> is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter); and
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*0.0000624 is a conversion factor.
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The EMC<sub>TSS</sub> entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.
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The above calculations may be applied on an event or annual basis using the appropriate units. The annual TSS credit, in pounds per year, is given by
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<math> M_{TSS_i} = 2.72\ V_{annual}\ EMC_{TSS} </math>
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where
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*V<sub>annual</sub> is the annual volume infiltrated, in acre-feet.
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===Total phosphorus (TP) credit calculations===
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TP removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, M<sub>TP<sub>i</sub></sub> in pounds, is given by
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<math> M_{TP_i} = 0.0000624\  V_{inf_b}\  EMC_{TP} </math>
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where
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*V<sub>inf<sub>b</sub></sub> is the infiltration through the bottom of the reservoir/subbase and into the underlying soil, in cubic feet;
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*EMC<sub>TP</sub> is the event mean TP concentration in runoff water entering the BMP (milligrams per liter); and
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*0.0000624 is a conversion factor.
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The EMC<sub>TP</sub> entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.
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The above calculations may be applied on an event or annual basis using the appropriate units. The annual TP credit, in pounds per year, is given by
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<math> M_{TP_i} = 2.72\ V_{annual}\ EMC_{TP} </math>
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where
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*V<sub>annual</sub> is the annual volume infiltrated, in acre-feet.
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-->
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===Volume credit calculations - no underdrain===
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[[file:Permeable pavement volume credit 1.png|300px|thumb|alt=schematic of permeable pavement system no underdrain|<font size=3>Schematic showing terminology for calculating volume credits for permeable pavement. A<sub>O</sub> is the area at the bottom of the pavement, A<sub>B</sub> the area at the reservoir/soil subgrade interface, and D<sub>O</sub> the depth or thickness of the reservoir.</font size>]]
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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.
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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, the area of permeable pavement, and the bottom surface area. The volume credit (V<sub>inf<sub>b</sub></sub>) for infiltration through the bottom of the BMP into the underlying soil, in cubic feet, is given by
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<math> V_{inf_b} =  D_o\  n\ (A_O + A_B)\ / 2  </math>
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where
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:A<sub>O</sub> is the overflow surface area of the permeable pavement system, in square feet;
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:A<sub>B</sub> is the depth at the bottom of the permeable pavement system, in square feet;
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:D<sub>O</sub> is the depth of the reservoir/subbase layer (engineered media), equal to the distance from the bottom of the permeable pavement material to the underlying soil subgrade, in feet; and
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:n is the <span title="Porosity or void fraction is a measure of the void (i.e. empty) spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%."> '''porosity (f)'''</span> of the reservoir/subbase, in cubic feet per cubic foot.
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The subbase depth should be limited to the <span title="The length of time, usually expressed in hours, for ponded water in a stormwater practice to drain. For stormwater practices where water is stored in media, there is no clear definition of drawdown, but an acceptable assumption is the time for water to drain to field capacity"> '''drawdown time'''</span>. The [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program construction stormwater general permit] requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). For example, using a <span title="A soil classification system (Natural Resource Conservation System) based on runoff potential. Groups include A soils (coarse textured with very low runoff potential), B soils (medium coarse textured with low runoff potential), C soils (fine to moderate textured with moderate runoff potential), and D soils (fine textured with high runoff potential)."> '''[https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rates hydrologic soil group]'''</span> B (SM) soil with an infiltration rate of 0.45 inches per hour, the maximum depth is 1.8 feet.
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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, called <span title="Field capacity is the amount of soil moisture or water content held in soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within 2–3 days after a rain or irrigation in pervious soils of uniform structure and texture."> '''field capacity'''</span>, is less than 5 percent of total porosity for a permeable pavement system.
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The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the [[MIDS calculator]]. Example values are shown below for a scenario using the MIDS calculator.  For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.
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{{:Annual volume treated as a function of soil and water quality volume}}
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===Volume calculations - underdrain===
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[[File:Permeable pavement with underdrain.png|300px|thumb|alt=schematic of permeable pavement with underdrain|<font size=3>Schematic of a permeable pavement system with an underdrain.  Water infiltrating through the pavement is either captured by the underdrain or stored below the underdrain in the reservoir/subbase, where it infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.</font size>]]
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The volume credit (V) for permeable pavement systems with <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrains'''</span>, in cubic feet, is given by
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<math> V = V_{inf_b} + V_U </math>
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The infiltrating volume (V<sub>inf<sub>b</sub></sub>), in cubic feet, is given by
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<math> V_{inf_b} =  A_B  DDT  I_R / 12 </math>
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where
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:A<sub>B</sub> is the surface area at the bottom of the underdrain, in square feet;
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:DDT is the drawdown time for water stored below the underdrain, in hours; and
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:I<sub>R</sub> is the <span title="The assumed infiltration rate into soil or engineered media when determining the dimensions (depth, surface area) of a stormwater practice."> '''[https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rate_as_a_function_of_soil_texture_for_bioretention_in_Minnesota design infiltration rate]'''</span> of underlying soil, in inches per hour.
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{{alert|The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to I<sub>R</sub>. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.|alert-info}}
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The [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program construction stormwater general permit] requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). Note the [http://stormwater.pca.state.mn.us/index.php/MIDS_calculator MIDS calculator] does not provide a volume credit for a permeable pavement system with an underdrain at the bottom.
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If the underdrain is raised above the bottom of the BMP (i.e. above the interface between the reservoir/subbase and underlying soil subgrade), water stored below the underdrain will infiltrate. The infiltrating volume (V<sub>U</sub>), in cubic feet, is given by
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<math> V_U =  D_u\  n\ (A_U + A_B)\ / 2  </math>
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where
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:D<sub>u</sub> is the depth of the reservoir layer below the underdrain, in feet;
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:A<sub>B</sub> is the surface area at the bottom of the underdrain, in square feet;
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:A<sub>U</sub> is the surface area at the bottom of the reservoir layer/subbase, in square feet; and
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:n is the porosity of the reservoir/subbase layer, in cubic feet per cubic foot.
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The depth below the underdrain should be limited to the <span title="The length of time, usually expressed in hours, for ponded water in a stormwater practice to drain. For stormwater practices where water is stored in media, there is no clear definition of drawdown, but an acceptable assumption is the time for water to drain to field capacity"> '''drawdown time'''</span>. The [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program construction stormwater general permit] requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). For example, using a <span title="A soil classification system (Natural Resource Conservation System) based on runoff potential. Groups include A soils (coarse textured with very low runoff potential), B soils (medium coarse textured with low runoff potential), C soils (fine to moderate textured with moderate runoff potential), and D soils (fine textured with high runoff potential)."> '''[https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rates hydrologic soil group]'''</span> C soil with an infiltration rate of 0.2 inches per hour, the maximum depth below the underdrain is 0.8 feet.
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===Total Suspended Solids (TSS)===
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The TSS credits available for installation of permeable pavement depend on the design of the storage volume below the pavement and whether the runoff is filtered (through an underdrain), infiltrated, or both. Designs that filter runoff with an underdrain at the bottom of the storage layer are less effective in removing pollutants than infiltration designs. Runoff is filtered while flowing through the permeable pavement and the storage layer and out the underdrain. TSS removal credit of 100 percent is assumed for the infiltrated water. The recommended removal rate for filtered water is 74 percent, based on review of literature.
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Removal of TSS by permeable pavement (M<sub>TSS</sub>), in pounds per event or pounds per year, is given by
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<math> M_{TSS} = M_{TSS_I}\ + M_{TSS_F}  </math>
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where
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:M<sub>TSS<sub>I</sub></sub> = mass of TSS removed by infiltration (pounds per event or pounds per year); and
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:M<sub>TSS<sub>F</sub></sub> = mass of TSS removed by filtration (pounds per event or pounds per year).
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The annual TSS credit (M<sub>TSS<sub>I</sub></sub>) for infiltrated runoff is given by
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<math> M_{TSS_I} = 2.72\  V_{_{Annual}}\ F_I\ EMC_{_{TSS}} </math>
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where
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:V<sub>Annual</sub> is the annual volume treated by the BMP, in acre-feet;
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:F<sub>I</sub> is the fraction of the total annual volume treated by the BMP that is infiltrated;
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:EMC<sub>TSS</sub> = <span title="The average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentration'''</span> of TSS in the runoff, in mg/L; and
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:Factor of 2.72 used for conversion to pounds.
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In a permeable pavement system with an underdrain, some of the water captured by the BMP will enter the underdrain while some will infiltrate below the underdrain.  The amount infiltrating depends on several factors, including whether the underdrain is raised above the soil subgrade, the infiltration rate of the underlying soil, and size and spacing of the underdrains. Pollutants in water that enters the underdrain are filtered. The Annual TSS credit for filtered runoff (M<sub>TSS<sub>F</sub></sub>) is given by
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<math> M_{TSS_F} = 2.72\ R_{_{TSS}}\ V_{_{Annual}}\ (F_F)\ EMC_{_{TSS}} </math>
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where
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:F<sub>F</sub> is the fraction of the total volume treated by the BMP that is filtered; and
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:R<sub>TSS</sub> is the pollutant removal fraction for filtered water. A value of 0.74 is recommended.
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If the permeable pavement is not the upstream most BMP in the treatment train, EMC<sub>TSS</sub> should be dependent on the M<sub>TSS</sub> effluent (mg/L) from the next upstream tributary BMP.
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The annual volume treated by the BMP can be determined with appropriate modeling tools, including the [[MIDS calculator]]. Example values are shown below for a scenario using the MIDS calculator.  For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils. If an underdrain is in the system, this volume will have to be divided into the portion that infiltrates and the portion that is captured by the underdrain. The MIDS calculator can be used to determine these values.
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{{:Annual volume treated as a function of soil and water quality volume}}
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The event (storm) based TSS credit (M<sub>TSS<sub>I</sub></sub>) for infiltrated runoff is given by
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<math> M_{TSS_I} = 2.72\ V_I\  EMC_{_{TSS}}\ / 43,560 </math>
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where
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:V<sub>I</sub> is the event-based volume infiltrated by the BMP, in cubic feet, and
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:a factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.
  
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Permeable pavement is a tool that can achieve reductions in stormwater volume and pollutant loading. These reductions constitute stormwater [[Overview of stormwater credits|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.
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The storm event based TSS credit (M<sub>TSS-F</sub>) for filtered runoff is given by
  
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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 other pollutants.
+
<math> M_{TSS_F} = R_{_{TSS}}\ 2.72\ V_F\  EMC_{_{TSS}}\ / 43560 </math>
  
−
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.
+
where
 +
:V<sub>F</sub> is the event-based volume filtrated by the BMP, in cubic feet.
  
−
==Volume credits==
+
===Total phosphorus (TP) credit calculations===
−
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
+
Similar to TSS, TP reduction credits correspond directly with volume reduction through infiltration and filtration of captured stormwater. The water quality credits available for a permeable pavement system depend on the design of the storage volume below the pavement and whether or not the runoff is filtered (through underdrain) or infiltrated. TP credit is divided into particulate phosphorus (PP) and dissolved phosphorus (DP) removal, respectively making up 55 percent and 45 percent of the total TP credit. Because the volume of infiltrated water (calculated above) is completely removed from the existing system, 100 percent TP credit is assumed for all infiltrated stormwater. Filtered stormwater only receives credit for PP credit, and no credit is given for DP.  This approach is consistent with the approach used in the [[MIDS calculator]].
  
−
:'''Basis for volume credits'''
+
Removal of TP by permeable pavement is given by
−
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. Assuming all pore space is available for infiltrating stormwater, the volume credit (''V''<sub>''s''</sub>) is given by
 
  
−
<math>V_s = A_s d_p n</math>
+
<math> M_{TP} = M_{TP_I}\ + M_{TP_F}</math>
  
 
where
 
where
−
:''A<sub>s</sub>'' = the surface area of the permeable pavement system (square feet);
+
:M<sub>TP</sub> is the annual or event TP removal (lb/yr or lb/event);
−
:''d<sub>p</sub>'' = 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
+
:M<sub>TP<sub>I</sub></sub> is the annual or event TP removal from infiltrated runoff (lb/yr or lb/event); and
−
:''n'' = porosity of the subbase(cubic feet/cubic feet). Note that the term ''V<sub>r</sub>'', which represents the void ratio of a material, is analogous to and may be used instead of porosity.
+
:M<sub>TP<sub>F</sub></sub> is the annual or event TP removal from filtered water (lb/year or lb/event).
 +
 
 +
The total annual TP removal for infiltrated runoff is given by
 +
 
 +
<math> M_{TP_I} = 2.72\ V_{annual}\ F_I\ EMC_{_{TP}} </math>
 +
 
 +
where
 +
:V<sub>annual</sub> is the annual volume treated by the BMP, in acre-feet,
 +
:F<sub>I</sub> is the fraction of the total annual volume treated by the BMP that is infiltrated,
 +
:EMC<sub>TP</sub> = event mean concentration of TP in the runoff, in mg/L, and
 +
:a factor of 2.72 used for conversion to pounds.
 +
 
 +
In a permeable pavement system with an underdrain, some of the water captured by the BMP will enter the underdrain while some will infiltrate below the underdrain.  The amount infiltrating depends on several factors, including whether the underdrain is raised above the soil subgrade, the infiltration rate of the underlying soil, and size and spacing of the underdrains. Pollutants in water that enters the underdrain are filtered. The Annual TP credit for filtered runoff (M<sub>TP<sub>F</sub></sub>) is given by
 +
 
 +
<math> M_{TP_F} = 2.72\ R_{_{TP}}\ V_{_{Annual}}\ F_F\ EMC_{_{TP}} </math>
 +
 
 +
where
 +
:F<sub>F</sub> is the fraction of the total volume treated by the BMP that is filtered; and
 +
:R<sub>TP</sub> is the pollutant removal fraction for filtered water.
 +
 
 +
The pollutant removal fraction applies only to particulate phosphorus (PP), which is assumed to be 55 percent of total phosphorus (TP). The recommended removal efficiency for PP is 74 percent. Thus, the recommended value for R<sub>TP</sub> is 0.55 * 0.74 or 0.41. The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_and_dissolved_phosphorus_in_stormwater_runoff#Ratios_of_particulate_to_dissolved_phosphorus link here].
 +
 
 +
If the permeable pavement is not the upstream most BMP in the treatment train, EMC<sub>TP</sub> should be dependent on the M<sub>TP</sub> effluent (mg/L) from the next upstream tributary BMP.
 +
 
 +
The annual volume treated by the BMP can be determined with appropriate modeling tools, including the [[MIDS calculator]]. Example values are shown below for a scenario using the MIDS calculator.  For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils. If an underdrain is in the system, this volume will have to be divided into the portion that infiltrates and the portion that is captured by the underdrain. The MIDS calculator can be used to determine these values.
 +
 
 +
{{:Annual volume treated as a function of soil and water quality volume}}
 +
 
 +
The event (storm) event based TP credit (M<sub>TP<sub>I</sub></sub>) for infiltrated runoff is given by
 +
 
 +
<math> M_{TP_I} = 2.72\ V_I\  EMC_{_{TP}}\ / 43,560 </math>
 +
 
 +
where
 +
:V<sub>I</sub> is the event-based volume infiltrated by the BMP, in cubic feet; and
 +
:a factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.
 +
 
 +
The storm event based TP credit (M<sub>TP-F</sub>) for filtered runoff is given by
 +
 
 +
<math> M_{TP - F} = R_{_{TP}}\ 2.72\ V_F\  EMC_{_{TP}}\ / 43560 </math>
 +
 
 +
where
 +
::V<sub>F</sub> is the event-based volume filtered by the BMP, in cubic feet
 +
 
 +
===Example calculations for TSS and TP===
 +
NOTE: The [[MIDS calculator]] was used for the following examples. The performance goal was changed from the MIDS default of 1.1 inches to 1 inch.
 +
 
 +
Assume a permeable pavement system is designed to capture and treat 1 inch of runoff from a 1 acre impervious area. Note that in these calculations, the permeable pavement is considered part of the impermeable surface.
 +
 
 +
For this example, assume a 9000 square foot surface area at the top of the reservoir/subbase, a 9000 square foot area at the reservoir/soil subgrade, an underlying B soil with an infiltration rate of 0.45 inches per hour, a porosity of 0.4 cubic feet per cubic foot, a depth below the underdrain of 1 foot, a TSS EMC of 54.5 milligrams per liter, and a TP EMC of 0.3 milligrams per liter. With this depth below the underdrain, all the water can be infiltrated (3600 cubic feet per event; 2.3446 acre-feet per year). Annual TSS removal, in pounds, is given by
 +
 
 +
<math> 2.72 (2.3446) (1) (54.5) = 347 </math>
 +
 
 +
Annual TP removal is given by
 +
 
 +
<math> 2.72 (2.3446) (1) (0.3) = 1.91 </math>
 +
 
 +
If the depth below the underdrain was 0.5 feet instead of 1 foot, only half of the 1 inch performance goal is infiltrated, corresponding to an annual infiltration volume of 1.60 acre-feet. Note that the relationship between infiltration performance goal and annual volume infiltrated is not linear. The first step is to calculate the infiltration and filtered fractions of total volume captured by the BMP.  The infiltrated fraction is 1.60/2.3446 or 0.68, leaving a filtered fraction of 0.32.
 +
 
 +
Annual TSS removal, in pounds, is given by
 +
 
 +
<math> (2.72 (2.3446) (0.68) (54.5)) + ((2.72 (2.3446) (0.32) (0.74) (54.5)) = 319 </math>
 +
 
 +
The  first term in parentheses corresponds with the infiltrated portion and equals about 236.3 pounds. The second term in parentheses corresponds with the filtered portion, having a removal efficiency of 0.74 (74 percent), for a total removal of about 82.3 pounds.
 +
 
 +
Annual TP removal, in pounds, is given by
 +
 
 +
<math> (2.72 (2.3446) (0.68) (0.3)) + ((2.72 (2.3446) (0.32) (0.55) (0.74) (0.3)) = 1.55 </math>
 +
 
 +
The  first term in parentheses corresponds with the infiltrated portion and equals about 1.30 pounds. The second term in parentheses corresponds with the filtered portion, having a particulate P fraction of 0.55, and a removal efficiency of 0.74 (74 percent) for the particulate fraction, for a total removal of about 0.25 pounds.
 +
 
 +
==Methods for calculating credits==
 +
This section provides specific information on generating and calculating credits from permeable pavement 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:
 +
#Quantifying volume and pollution reductions based on accepted hydrologic models
 +
#The Simple Method and MPCA Estimator
 +
#MIDS Calculator
 +
#Quantifying volume and pollution reductions based on values reported in literature
 +
#Quantifying volume and pollution reductions based on field monitoring
 +
 
 +
===Credits based on models===
 +
{{alert|The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.|alert-danger}}
 +
 
 +
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. Furthermore, many of the models listed below cannot be used to determine compliance with the [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program Construction Stormwater General permit] since the permit requires the water quality volume to be calculated as an instantaneous volume.
 +
 
 +
Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:
 +
#Model name and version
 +
#Date of analysis
 +
#Person or organization conducting analysis
 +
#Detailed summary of input data
 +
#Calibration and verification information
 +
#Detailed summary of output data
  
−
[[File:Design schematic 3.png|thumb|400px|alt=schematic showing storage and infiltration below an elevated underdrain.|<font size=3>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.</font size>]]
+
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. In using this table to identify models appropriate for permeable pavement, use the sort arrow on the table to select Infiltrator BMPs or Filter BMPs, depending on the type of permeable pavement BMP and the terminology used in the model.
−
[[File:Design schematic 4.png|thumb|400px|alt=schematic showing the sequence of events involved in calculating an infiltration credit|<font size=3>Schematic showing the sequence of events involved in calculating an infiltration credit. Rainfall begins at ''t<sub>1</sub>''. Water infiltrates rapidly and infiltration into the underlying soil begins. At ''t<sub>2</sub>'' the reservoir layer is full and infiltration continues into the underlying soil. At ''t<sub>3a</sub>'' and ''t<sub>3b</sub>'', 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<sub>f</sub>'' 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</font size>]]
 
  
−
The volume credit shown above assumes the entire pore space is available for water storage. Although [[Glossary#F|field capacity]] provides 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 [http://web.ead.anl.gov/resrad/datacoll/porosity.htm]).
+
{{:Stormwater model and calculator comparisons}}
  
−
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 [[Acronyms|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 criteria for permeable pavement|design specifications for permeable pavement]].
+
===The Simple Method and MPCA Estimator===
 +
The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of <span title="The average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentration'''</span> and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. [http://www.stormwatercenter.net/Library/Practice/13.pdf Ohrel] (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method.  For more information on the Simple Method, see [http://www.stormwatercenter.net/monitoring%20and%20assessment/simple%20meth/simple.htm The Simple method to Calculate Urban Stormwater Loads] or [[The Simple Method for estimating phosphorus export]].
  
−
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 ([http://content.asce.org/files/pdf/Smith.pdf Smith and Hunt], 2010). Sites that receive run-on from poorly maintained or disturbed areas had the lowest infiltration rate in a study by [http://cedb.asce.org/cgi/WWWdisplay.cgi?158325 Bean] et al. 2007. However, the infiltration rates at these sites were still high relative to rainfall intensities.
+
Some simple stormwater calculators utilize the Simple Method ([https://www.epa.gov/nps/spreadsheet-tool-estimating-pollutant-loads-stepl EPA STEPL], [https://www.stormwatercenter.net/monitoring%20and%20assessment/watershed_treatment_model.htm Watershed Treatment Model]). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the [http://stormwater.pca.state.mn.us/index.php/Guidance_and_examples_for_using_the_MPCA_Estimator '''MPCA Estimator'''], this tool was developed specifically for complying with the [https://stormwater.pca.state.mn.us/index.php?title=2020_MS4_General_Permit_Section_22_Discharges_to_Impaired_Waters_with_a_USEPA-Approved_TMDL_that_includes_an_Applicable_WLA MS4 General Permit TMDL annual reporting requirement].  The MPCA Estimator provides default values for pollutant concentration, <span title="The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land)."> [https://stormwater.pca.state.mn.us/index.php?title=Runoff_coefficients_for_5_to_10_year_storms '''runoff coefficients''']</span> for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available.  Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.
  
−
If a particular storm event exceeds the design storm event, the volume of water infiltrated will exceed the design volume of the permeable pavement system. This is the result of water infiltrating into the underlying soil as precipitation or runoff water continues to be delivered to the system after the design volume has been exceeded. For example, if the performance goal is 1.1 inches and the rain event is 2.0 inches, some of the 0.9 inches will be captured by the permeable pavement system. This additional volume infiltrated will be greater for long duration, lower intensity storms compared to a short duration high intensity event. On an annual basis, this additional volume is relatively small.
+
{{alert|The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.|alert-warning}}
  
−
In a system with an underdrain at the bottom, water will infiltrate at a rate dependent on the underlying soil. The volume infiltrated is equal to the infiltration rate multiplied by the time that infiltration occurs. The length of time for a system to drain is controlled by the spacing of underdrains.
+
Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.
  
−
===Assumptions and factors affecting volume credits for permeable pavement===
+
'''[https://stormwater.pca.state.mn.us/index.php?title=File:MPCA_simple_estimator_version_3.0_March_5_2021.xlsx Download MPCA Estimator here]'''
−
{{alert|The following general assumptions apply in calculating the credit for permeable pavement. If any of these assumptions is violated, the credit will be reduced.|alert-danger}}
 
−
*The permeable pavement is [[Design criteria for permeable pavement|properly designed]].
 
−
*The permeable pavement was [[Construction specifications for permeable pavement|properly constructed]], consistent with the [[Design criteria for permeable pavement|design criteria]].
 
−
*The permeable pavement is [[Operation and maintenance of permeable pavement|properly maintained]]. The performance of the permeable pavement should be regularly [[Assessing the performance of permeable pavement|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.
+
===MIDS Calculator===
 +
[[File:mids logo.jpg|thumb|300 px|alt=mids logo|<font size=3>Download the [[Calculator|MIDS Calculator]]</font size>]]
  
−
===Models and calculators for calculating permeable pavement volume credits===
+
The [[MIDS calculator|Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator]] is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.
−
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|thumb|500px|alt=screen shot of the MIDS calculator showing inputs for permeable pavement|<font size=3>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.</font size>]]
+
The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.
  
−
====[[MIDS calculator|Minimal Impact Design Standards (MIDS) calculator]]====
+
Detailed [[Links to Manual pages that address the MIDS calculator|guidance and examples]] have been developed for all BMPs in the calculator, including [[Requirements, recommendations and information for using permeable pavement BMPs in the MIDS calculator|permeable pavement]]. An overview of individual input parameters and workflows is presented in the [http://stormwater.pca.state.mn.us/index.php/User%E2%80%99s_Guide MIDS Calculator User Documentation].
−
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.
+
===Credits based on reported literature values===
 +
A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the permeable pavement practice. Concentration reductions resulting from treatment can be converted to mass reductions if the volume of stormwater treated is known.
  
−
Calculator output includes
+
Designers may use the pollutant reduction values [http://stormwater.pca.state.mn.us/index.php/Information_on_pollutant_removal_by_BMPs reported in this manual] or may research values from other databases and published literature.  Designers who opt for this approach should
−
*the BMP volume capacity (cubic feet);
+
*select the median value from pollutant reduction databases that report a range of reductions, such as from the [https://bmpdatabase.org/ International BMP Database];
−
*the required retention volume for the specified performance goal (cubic feet);
+
*select a pollutant removal reduction from literature that studied a permeable pavement practice with site characteristics and climate similar to the device being considered for credits;
−
*the BMP volume credit (cubic feet);
+
*review the article to determine that the design principles of the studied permeable pavement are close to the design recommendations for Minnesota, as described in [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_permeable_pavement this manual] and/or by a local permitting agency; and
−
*the untreated volume (cubic feet); and
+
*give preference to literature that has been published in a peer-reviewed publication.
−
*the percent water removed relative to the performance goal.
 
  
−
[[File:MIDS screen shot 2.png|thumb|500px|alt=screen shot showing output from the MIDS calculator|<font size=3>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 (%)).</font size>]]
+
The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed permeable pavement practice, considering such conditions as watershed characteristics, permeable pavement practice sizing, soil infiltration rates, and climate factors.
 +
*[https://bmpdatabase.org/ International Stormwater Best Management Practices (BMP) Database]
 +
**Compilation of BMP performance studies
 +
**Provides values for TSS, Bacteria, Nutrients, and Metals
 +
**Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels
 +
*[http://lshs.tamu.edu/docs/lshs/end-notes/updated%20bmp%20removal%20efficiencies%20from%20the%20national%20pollutant%20re-2854375963/updated%20bmp%20removal%20efficiencies%20from%20the%20national%20pollutant%20removal%20database.pdf Updated BMP Removal Efficiencies from the National Pollutant Removal Database (2007) & Acceptable BMP Table for Virginia]
 +
**Provides data for several structural and non-structural BMP performance evaluations
 +
*[http://www.epa.state.il.us/green-infrastructure/docs/draft-final-report.pdf The Illinois Green Infrastructure Study]
 +
**Figure ES-1 summarizes BMP effectiveness
 +
**Provides values for TN, TSS, peak flows / runoff volumes
 +
**Applicable to permeable pavements, constructed wetlands, infiltration, detention, filtration, and green roofs
 +
*[https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/2020-01/wd-08-20b.pdf New Hampshire Stormwater Manual]
 +
**Volume 2, Appendix B summarizes BMP effectiveness
 +
**Provides values for TSS, TN, and TP removal
 +
**Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
 +
*[https://www.wri.wisc.edu/wp-content/uploads/FinalWR03R001.pdf Design Guidelines for Stormwater Bioretention Facilities].  University of Wisconsin, Madison
 +
**Table 2-1 summarizes typical removal rates
 +
**Provides values for TSS, metals, TP, TKN, ammonium, organics, and bacteria
 +
**Applicable for bioretention
 +
*[https://www3.epa.gov/region1/npdes/stormwater/tools/BMP-Performance-Analysis-Report.pdf BMP Performance Analysis].  Prepared for US EPA Region 1, Boston MA.
 +
**Appendix B provides pollutant removal performance curves
 +
**Provides values for TP, TSS, and zinc
 +
**Pollutant removal broken down according to land use
 +
**Applicable to infiltration trench, infiltration basin, bioretention, grass swale, wet pond, and porous pavement
 +
*Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. [http://www.lrrb.org/media/reports/200523.pdf The Cost and Effectiveness of Stormwater Management Practices: Final Report]  
 +
**Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
 +
**Applicable to wet basins, stormwater wetlands, bioretention filter, sand filter, infiltration trench, and filter strips/grass swales
  
−
==[[Acronyms#T|TSS]] credits==
+
===Credits based on field monitoring===
−
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. 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.
+
Field monitoring may be made in lieu of desktop calculations or models/calculators as described.  Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.
  
−
===Assumptions and factors affecting TSS credits for permeable pavement===
+
#Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
−
{{alert|The following general assumptions apply in calculating the credit for permeable pavement. If any of these assumptions is violated, the credit will be reduced.|alert-danger}}
+
##Which pollutants will be measured?
−
*The permeable pavement is [[Design criteria for permeable pavement|properly designed]].
+
##Will the monitoring study the performance of a single BMP or multiple BMPs?
−
*The permeable pavement was [[Construction specifications for permeable pavement|properly constructed]], consistent with the [[Design criteria for permeable pavement|design criteria]].
+
##Are there any variables that will affect the BMP performance?  Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
−
*The permeable pavement is [[Operation and maintenance of permeable pavement|properly maintained]]. The performance of the permeable pavement should be regularly [[Assessing the performance of permeable pavement|assessed]].
+
##Will the results be compared to other BMP performance studies?
 +
##What should be the duration of the monitoring period?  Is there a need to look at the annual performance vs the performance during a single rain event?  Is there a need to assess the seasonal variation of BMP performance?
 +
#Plan the field activities. Field considerations include
 +
##equipment selection and placement;
 +
##sampling protocols including selection, storage, and delivery to the laboratory;
 +
##laboratory services;
 +
##health and Safety plans for field personnel;
 +
##record keeping protocols and forms; and
 +
##quality control and quality assurance protocols
 +
#Execute the field monitoring
 +
#Analyze the results
  
−
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.
+
This manual contains the following guidance for monitoring.
 +
*[[Recommendations and guidance for utilizing monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing lake monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing stream monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing major stormwater outfall monitoring to meet TMDL permit requirements]]
 +
*[[Recommendations and guidance for utilizing stormwater best management practice monitoring to meet TMDL permit requirements]]
  
−
===Models and calculators for calculating permeable pavement TSS credits===
+
The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.
−
The Manual does not provide specific recommendations for which values or models to use when calculating TSS credits for permeable pavement. The calculators and 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====
+
:[https://www3.epa.gov/npdes/pubs/montcomplete.pdf '''Urban Stormwater BMP Performance Monitoring''']
−
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.
+
Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:
 +
*Chapter 2: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
 +
*Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
 +
*Chapter 4: Methods and equipment for water quality monitoring
 +
*Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
 +
*Chapter 7: BMP Performance Analysis
 +
*Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).
  
−
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.
+
:[http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_565.pdf '''Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)''']
 +
AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center.  The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff.  The document includes chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.
 +
*Chapter 4: Stormwater Characterization
 +
**4.2: General Characteristics and Pollutant Sources
 +
**4.3: Sources of Stormwater Quality data
 +
*Chapter 8: Performance Evaluation
 +
**8.1: Methodology Options
 +
**8.5: Evaluation of Quality Performance for Individual BMPs
 +
**8.6: Overall Hydrologic and Water Quality Performance Evaluation
 +
*Chapter 10: Hydrologic Evaluation
 +
**10.5: Performance Verification and Design Optimization
  
−
Since TSS credits are a function of the volume infiltrated, the design dimensions control TSS removal. Calculator inputs include
+
:[https://www.wef.org/globalassets/assets-wef/3---resources/topics/o-z/stormwater/stormwater-institute/wef-stepp-white-paper_final_02-06-14.pdf '''Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices''']
−
*the area of the permeable pavement surface (square feet);
+
*In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).
−
*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.
+
:'''Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)'''
  
−
[[File:MIDS screen shot 3.png|thumb|400px|alt=screen shot showing output on TSS loads from the MIDS calculator|<font size=3>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.</font size>]]
+
The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program.  However, this manual is among the most thorough for field activities. Relevant chapters include.
 +
*Chapter 4: Monitoring Methods and Equipment
 +
*Chapter 5: Analytical Methods and Laboratory Selection
 +
*Chapter 6: Monitoring Site Selection
 +
*Chapter 8: Equipment Installation and Maintenance
 +
*Chapter 10: Pre-Storm Preparation
 +
*Chapter 11: Sample Collection and Handling
 +
*Chapter 12: Quality Assurance / Quality Control
 +
*Chapter 13: Laboratory Reports and Data Review
 +
*Chapter 15: Gross Solids Monitoring
  
−
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.
+
:[http://stormwaterbook.safl.umn.edu/ '''Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance''']
  
−
Calculator output includes
+
This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency.  The manual advises on a four-level process to assess the performance of a Best Management Practice.
−
*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;
+
*Level 1: [https://stormwaterbook.safl.umn.edu/assessment-programs/visual-inspection Visual Inspection]
−
*TSS removed via non-volume reduction treatment, equal to 74% of the incoming TSS load;
+
*Level 2: [https://stormwaterbook.safl.umn.edu/assessment-programs/capacity-testing Capacity Testing]
−
*Total TSS removed via volume reduction and other treatment, given as a percent of incoming TSS;
+
*Level 3: [http://stormwaterbook.safl.umn.edu/assessment-programs/synthetic-runoff-testing Synthetic Runoff Testing]
−
*Annual TSS reduction in pounds; and
+
*Level 4: [https://stormwaterbook.safl.umn.edu/assessment-programs/monitoring Monitoring]
−
*remaining TSS load, in pounds.
 
  
−
For an example with discussion, see the screen shot on the right.
+
Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit.  BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations.  Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.
  
−
===Literature review on TSS reductions for permeable pavement===
+
Use these links to obtain detailed information on the following topics related to BMP performance monitoring:
−
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.
+
*[https://stormwaterbook.safl.umn.edu/water-budget-measurement Water Budget Measurement]
 +
*[https://stormwaterbook.safl.umn.edu/sampling-methods Sampling Methods]
 +
*[https://stormwaterbook.safl.umn.edu/analysis-water-and-soils Analysis of Water and Soils]
 +
*[https://stormwaterbook.safl.umn.edu/data-analysis Data Analysis for Monitoring]
  
−
*[http://www.njstormwater.org/bmp_manual/NJ_SWBMP_9.7.pdf New Jersey Stormwater BMP Manual]. 2004. ''Standards for pervious paving systems''. Chapter 9.7.
+
==Other pollutants==
−
*[http://www.uni-groupusa.org/PDF/Characteristics%20of%20Sediment%20Removal%20in%20Two%20Types%20of%20Permeable%20Pavement.pdf 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
+
Permeable pavements provide removal of sediment (TSS), nutrients (phosphorus and nitrogen), and metals through filtration, infiltration, and soil adsorption. Temperature control occurs in the stone reservoir/subbase and soil subgrade. Phosphorus, metals, and hydrocarbons are adsorbed onto soils within the subgrade. In addition, nutrients such as phosphorus and nitrogen may be biologically degraded.
−
*[http://portal.ncdenr.org/c/document_library/get_file?uuid=6a5633d6-127e-4fe7-b26c-2a3f53ce7c0a&groupId=38364 North Carolina Department of Environment and Natural Resources]. Water Quality Division. 2012. ''DWQ Stormwater BMP Manual & BMP Forms''. Chapter 18.
 
−
*[http://des.nh.gov/organization/divisions/water/stormwater/manual.htm New Hampshire Department of Environmental Services]. 2008. ''New Hampsire Stormwater Manual''. Appendix B.
 
  
−
==Phosphorus credits==
+
According to the [https://bmpdatabase.org/ International Stormwater Database], studies have shown that permeable pavements are effective at reducing concentration of pollutants including solids, bacteria, metals, and nutrients. A compilation of the pollutant removal capabilities from a review of literature of permeable pavement studies are summarized in the table below.
  
−
===Assumptions and factors affecting phosphorus credits for permeable pavement===
+
{{:Relative pollutant reduction for permeable pavement}}
  
−
===Models and calculators for calculating permeable pavement TSS credits===
+
==References and suggested reading==
−
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.
+
*Brown, Chris; Angus Chu; Bert van Duin; Caterina Valeo. 2009. [http://www.uni-groupusa.org/PDF/Characteristics%20of%20Sediment%20Removal%20in%20Two%20Types%20of%20Permeable%20Pavement.pdf Characteristics of Sediment Removal in Two Types of Permeable Pavement]. Water Qual. Res. J. Can. Volume 44, No. 1, 59-70.
 +
*Geosyntec and Wright Water Engineers. 2012. International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals. Prepared under Support from WERF, FHWA, EWRI/ASCE and EPA. July 2012.
 +
*New Hampshire Department of Environmental Services. 2008. [http://des.nh.gov/organization/divisions/water/stormwater/documents/wd-08-20a_apxe.pdf New Hampshire Stormwater Manual]. Volume 2 Appendix B.
 +
*New Jersey Department of Environmental Protection. 2004. [http://www.nj.gov/dep/stormwater/bmp_manual/NJ_SWBMP_4%20print.pdf New Jersey Stormwater BMP Manual]. Standards for Pervious Paving Systems. Chapter 9.7.
 +
*North Carolina Department of Environment and Natural Resources. Water Quality Division. 2012. [http://www.ncsu.edu/ehs/environ/DWQ_StormwaterBMPmanual_001%5B1%5D.pdf Stormwater BMP Manual & BMP Forms]. Chapter 18. Permeable Pavement.
 +
*Tennis, Paul D.; Michael L. Leming; David J. Akers. 2004. [http://myscmap.sc.gov/marine/NERR/pdf/PerviousConcrete_pavements.pdf Pervious Concrete Pavements]. EB302.02, Portland Cement Association and National Ready Mixed Concrete Association.
 +
*Tota-Maharaj, K. and Scholz, M. 2010. Efficiency of permeable pavement systems for the removal of urban runoff pollutants under varying environmental conditions. Environ. Prog. Sustainable Energy, 29: 358–369. doi: 10.1002/ep.10418
 +
*USEPA. Stormwater Menu of BMPs. [http://water.epa.gov/polwaste/npdes/swbmp/Pervious-Concrete-Pavement.cfm Permeable Pavements]. 2009.
  
−
====Minimal Impact Design Standards calculator====
+
<noinclude>
  
−
===Literature review on phosphorus reductions for permeable pavement===
+
==Related articles==
 +
*[[Overview for permeable pavement]]
 +
*[[Types of permeable pavement]]
 +
*[[Design criteria for permeable pavement]]
 +
*[[Construction specifications for permeable pavement]]
 +
<!--[[Construction observations for permeable pavement]]-->
 +
*[[Assessing the performance of permeable pavement]]
 +
*[[Operation and maintenance of permeable pavement]]
 +
*[[Calculating credits for permeable pavement]]
 +
<!--[[Cost-benefit considerations for permeable pavement]]-->
 +
*[[Case studies for permeable pavement]]
 +
*[[Green Infrastructure benefits of permeable pavement]]
 +
*[[Summary of permit requirements for infiltration]]
 +
*[[Permeable pavement photo gallery]]
 +
*[[Additional considerations for permeable pavement]]
 +
*[[Links for permeable pavement]]
 +
<!--*[[External resources for permeable pavement]]-->
 +
*[[References for permeable pavement]]
 +
<!--*[[Supporting material for permeable pavement]]-->
 +
*[[Fact sheets for permeable pavement]]
 +
*[[Requirements, recommendations and information for using permeable pavement BMPs in the MIDS calculator]]
 +
<!--#[[Permeable pavement credits]]-->
  
−
==Example applications for calculating permeable pavement credits for volume, TSS and phosphorus==
+
[https://stormwater.pca.state.mn.us/index.php?title=Permeable_pavement Permeable pavement main page]
  
−
==Pollutants other than TSS and phosphorus==
+
*Calculating credits
−
In addition to TSS and phosphorus, permeable pavement can reduce loading of the following pollutants:
+
**[[Calculating credits for bioretention]]
−
*Metals such as copper and zinc
+
**[[Calculating credits for infiltration basin]]
−
*Nitrogen
+
**[[Calculating credits for infiltration trench]]
−
*Hydrocarbons
+
**[[Calculating credits for permeable pavement]]
−
*Chloride (indirectly by reducing the amount of road salt applied)
+
**[[Calculating credits for green roofs]]
−
*Oxygen demand
+
**[[Calculating credits for sand filter]]
−
<p>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.
+
**[[Calculating credits for stormwater ponds]]
−
*[http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=browse&Rbutton=detail&bmp=137&minmeasure=5] - information on zinc, copper, lead, nitrate, Kjeldahl nitrogen, and total nitrogen
+
**[[Calculating credits for stormwater wetlands]]
−
*[http://www.invisiblestructures.com/pollutant.html] - information on total nitrogen, heavy metals, and hydrocarbons.
+
**[[Calculating credits for iron enhanced sand filter]]
 +
**[[Calculating credits for swale]]
 +
**[[Calculating credits for tree trenches and tree boxes]]
 +
**[[Calculating credits for stormwater and rainwater harvest and use/reuse]]
  
−
[[category:BMP]]
+
[[Category:Level 3 - Best management practices/Guidance and information/Pollutant removal and credits]]
 +
[[Category:Level 3 - Best management practices/Structural practices/Permeable pavement]]
 +
[[Category:Level 2 - Pollutants/Pollutant removal]]
 +
</noinclude>

Latest revision as of 02:30, 15 December 2022

Recommended pollutant removal efficiencies, in percent, for permeable pavement with an underdrain. Sources. NOTE: removal efficiencies are 100 percent of captured water for systems with no underdrain.

TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

TSS TP PP DP TN Metals Bacteria Hydrocarbons
74 41 74 0 insufficient data insufficient data insufficient data insufficient data
pdf image
Download pdf
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Page video summary
image
Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.
schematic of permeable pavement no underdrain
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.
schematic of permeable pavement with underdrain
Schematic of a permeable pavement system with an underdrain. Water infiltrating through the pavement is either captured by the underdrain or stored below the underdrain in the reservoir/subbase, where it infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.
Green Infrastructure: Permeable pavement can be an important tool for retention and detention of stormwater runoff. Permeable pavement may provide additional benefits, including reducing the need for de-icing chemicals, and providing a durable and aesthetically pleasing surface.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual best management practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

This page provides a discussion of how permeable pavement practices can achieve stormwater credits. Permeable pavement systems with and without underdrains are both discussed, with separate sections for each type of system as appropriate.

Overview

Permeable pavements 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 an underdrain and/or 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 pavement systems with no underdrains provide stormwater pollutant removal by reducing the volume of runoff from a site and the pollutant mass associated with that volume when infiltration is allowed (Water Environment Federation, 2012). In systems with underdrains most of the water is captured by the underdrain after passing through the subbase. If the underdrain is raised above the underlying soil subgrade, water stored in the reservoir/subbase below the underdrain will infiltrate into the underlying soil. If the underdrain is at the bottom of the reservoir/subbase, a small amount of infiltration may occur. Thus, pollutant removal in a permeable pavement system with an underdrain occurs through filtering of water captured by the underdrain and infiltration for water infiltrating into the underlying soil subgrade.

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. 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.

Warning: Pre-treatment is required for all filtration and infiltration practices

In the following discussion, the water quality volume (VWQ) is assumed to be delivered instantaneously to the BMP. The VWQ is stored within the reservoir/subbase below the bottom of the pavement and above the soil subgrade. The VWQ can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch times the new impervious surface area. For MIDS, the VWQ is 1.1 inches times the impervious surface area. In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The instantaneous 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 are achieved for the volume of runoff that is filtered and captured by an underdrain and the volume of water that is ultimately infiltrated.


Volume credit calculations - no underdrain

schematic of permeable pavement system no underdrain
Schematic showing terminology for calculating volume credits for permeable pavement. AO is the area at the bottom of the pavement, AB the area at the reservoir/soil subgrade interface, and DO the depth or thickness of the reservoir.

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.

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, the area of permeable pavement, and the bottom surface area. The volume credit (Vinfb) for infiltration through the bottom of the BMP into the underlying soil, in cubic feet, is given by

\( V_{inf_b} = D_o\ n\ (A_O + A_B)\ / 2 \)

where

AO is the overflow surface area of the permeable pavement system, in square feet;
AB is the depth at the bottom of the permeable pavement system, in square feet;
DO is the depth of the reservoir/subbase layer (engineered media), equal to the distance from the bottom of the permeable pavement material to the underlying soil subgrade, in feet; and
n is the porosity (f) of the reservoir/subbase, in cubic feet per cubic foot.

The subbase depth should be limited to the drawdown time. The construction stormwater general permit requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). For example, using a hydrologic soil group B (SM) soil with an infiltration rate of 0.45 inches per hour, the maximum depth is 1.8 feet.

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, called field capacity, is less than 5 percent of total porosity for a permeable pavement system.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


Volume calculations - underdrain

schematic of permeable pavement with underdrain
Schematic of a permeable pavement system with an underdrain. Water infiltrating through the pavement is either captured by the underdrain or stored below the underdrain in the reservoir/subbase, where it infiltrates into the underlying soil subgrade within a specified drawdown time, usually 48 hours.

The volume credit (V) for permeable pavement systems with underdrains, in cubic feet, is given by

\( V = V_{inf_b} + V_U \)

The infiltrating volume (Vinfb), in cubic feet, is given by

\( V_{inf_b} = A_B DDT I_R / 12 \)

where

AB is the surface area at the bottom of the underdrain, in square feet;
DDT is the drawdown time for water stored below the underdrain, in hours; and
IR is the design infiltration rate of underlying soil, in inches per hour.
Information: The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to IR. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.

The construction stormwater general permit requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). Note the MIDS calculator does not provide a volume credit for a permeable pavement system with an underdrain at the bottom.

If the underdrain is raised above the bottom of the BMP (i.e. above the interface between the reservoir/subbase and underlying soil subgrade), water stored below the underdrain will infiltrate. The infiltrating volume (VU), in cubic feet, is given by

\( V_U = D_u\ n\ (A_U + A_B)\ / 2 \)

where

Du is the depth of the reservoir layer below the underdrain, in feet;
AB is the surface area at the bottom of the underdrain, in square feet;
AU is the surface area at the bottom of the reservoir layer/subbase, in square feet; and
n is the porosity of the reservoir/subbase layer, in cubic feet per cubic foot.

The depth below the underdrain should be limited to the drawdown time. The construction stormwater general permit requires a maximum 48 hour drawdown time (24 hours is recommended for discharges to trout streams). For example, using a hydrologic soil group C soil with an infiltration rate of 0.2 inches per hour, the maximum depth below the underdrain is 0.8 feet.

Total Suspended Solids (TSS)

The TSS credits available for installation of permeable pavement depend on the design of the storage volume below the pavement and whether the runoff is filtered (through an underdrain), infiltrated, or both. Designs that filter runoff with an underdrain at the bottom of the storage layer are less effective in removing pollutants than infiltration designs. Runoff is filtered while flowing through the permeable pavement and the storage layer and out the underdrain. TSS removal credit of 100 percent is assumed for the infiltrated water. The recommended removal rate for filtered water is 74 percent, based on review of literature.

Removal of TSS by permeable pavement (MTSS), in pounds per event or pounds per year, is given by

\( M_{TSS} = M_{TSS_I}\ + M_{TSS_F} \)

where

MTSSI = mass of TSS removed by infiltration (pounds per event or pounds per year); and
MTSSF = mass of TSS removed by filtration (pounds per event or pounds per year).

The annual TSS credit (MTSSI) for infiltrated runoff is given by

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

where

VAnnual is the annual volume treated by the BMP, in acre-feet;
FI is the fraction of the total annual volume treated by the BMP that is infiltrated;
EMCTSS = event mean concentration of TSS in the runoff, in mg/L; and
Factor of 2.72 used for conversion to pounds.

In a permeable pavement system with an underdrain, some of the water captured by the BMP will enter the underdrain while some will infiltrate below the underdrain. The amount infiltrating depends on several factors, including whether the underdrain is raised above the soil subgrade, the infiltration rate of the underlying soil, and size and spacing of the underdrains. Pollutants in water that enters the underdrain are filtered. The Annual TSS credit for filtered runoff (MTSSF) is given by

\( M_{TSS_F} = 2.72\ R_{_{TSS}}\ V_{_{Annual}}\ (F_F)\ EMC_{_{TSS}} \)

where

FF is the fraction of the total volume treated by the BMP that is filtered; and
RTSS is the pollutant removal fraction for filtered water. A value of 0.74 is recommended.

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

The annual volume treated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils. If an underdrain is in the system, this volume will have to be divided into the portion that infiltrates and the portion that is captured by the underdrain. The MIDS calculator can be used to determine these values.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


The event (storm) based TSS credit (MTSSI) for infiltrated runoff is given by

\( M_{TSS_I} = 2.72\ V_I\ EMC_{_{TSS}}\ / 43,560 \)

where

VI is the event-based volume infiltrated by the BMP, in cubic feet, and
a factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

The storm event based TSS credit (MTSS-F) for filtered runoff is given by

\( M_{TSS_F} = R_{_{TSS}}\ 2.72\ V_F\ EMC_{_{TSS}}\ / 43560 \)

where

VF is the event-based volume filtrated by the BMP, in cubic feet.

Total phosphorus (TP) credit calculations

Similar to TSS, TP reduction credits correspond directly with volume reduction through infiltration and filtration of captured stormwater. The water quality credits available for a permeable pavement system depend on the design of the storage volume below the pavement and whether or not the runoff is filtered (through underdrain) or infiltrated. TP credit is divided into particulate phosphorus (PP) and dissolved phosphorus (DP) removal, respectively making up 55 percent and 45 percent of the total TP credit. Because the volume of infiltrated water (calculated above) is completely removed from the existing system, 100 percent TP credit is assumed for all infiltrated stormwater. Filtered stormwater only receives credit for PP credit, and no credit is given for DP. This approach is consistent with the approach used in the MIDS calculator.

Removal of TP by permeable pavement is given by

\( M_{TP} = M_{TP_I}\ + M_{TP_F}\)

where

MTP is the annual or event TP removal (lb/yr or lb/event);
MTPI is the annual or event TP removal from infiltrated runoff (lb/yr or lb/event); and
MTPF is the annual or event TP removal from filtered water (lb/year or lb/event).

The total annual TP removal for infiltrated runoff is given by

\( M_{TP_I} = 2.72\ V_{annual}\ F_I\ EMC_{_{TP}} \)

where

Vannual is the annual volume treated by the BMP, in acre-feet,
FI is the fraction of the total annual volume treated by the BMP that is infiltrated,
EMCTP = event mean concentration of TP in the runoff, in mg/L, and
a factor of 2.72 used for conversion to pounds.

In a permeable pavement system with an underdrain, some of the water captured by the BMP will enter the underdrain while some will infiltrate below the underdrain. The amount infiltrating depends on several factors, including whether the underdrain is raised above the soil subgrade, the infiltration rate of the underlying soil, and size and spacing of the underdrains. Pollutants in water that enters the underdrain are filtered. The Annual TP credit for filtered runoff (MTPF) is given by

\( M_{TP_F} = 2.72\ R_{_{TP}}\ V_{_{Annual}}\ F_F\ EMC_{_{TP}} \)

where

FF is the fraction of the total volume treated by the BMP that is filtered; and
RTP is the pollutant removal fraction for filtered water.

The pollutant removal fraction applies only to particulate phosphorus (PP), which is assumed to be 55 percent of total phosphorus (TP). The recommended removal efficiency for PP is 74 percent. Thus, the recommended value for RTP is 0.55 * 0.74 or 0.41. The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.

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

The annual volume treated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils. If an underdrain is in the system, this volume will have to be divided into the portion that infiltrates and the portion that is captured by the underdrain. The MIDS calculator can be used to determine these values.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


The event (storm) event based TP credit (MTPI) for infiltrated runoff is given by

\( M_{TP_I} = 2.72\ V_I\ EMC_{_{TP}}\ / 43,560 \)

where

VI is the event-based volume infiltrated by the BMP, in cubic feet; and
a factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.

The storm event based TP credit (MTP-F) for filtered runoff is given by

\( M_{TP - F} = R_{_{TP}}\ 2.72\ V_F\ EMC_{_{TP}}\ / 43560 \)

where

VF is the event-based volume filtered by the BMP, in cubic feet

Example calculations for TSS and TP

NOTE: The MIDS calculator was used for the following examples. The performance goal was changed from the MIDS default of 1.1 inches to 1 inch.

Assume a permeable pavement system is designed to capture and treat 1 inch of runoff from a 1 acre impervious area. Note that in these calculations, the permeable pavement is considered part of the impermeable surface.

For this example, assume a 9000 square foot surface area at the top of the reservoir/subbase, a 9000 square foot area at the reservoir/soil subgrade, an underlying B soil with an infiltration rate of 0.45 inches per hour, a porosity of 0.4 cubic feet per cubic foot, a depth below the underdrain of 1 foot, a TSS EMC of 54.5 milligrams per liter, and a TP EMC of 0.3 milligrams per liter. With this depth below the underdrain, all the water can be infiltrated (3600 cubic feet per event; 2.3446 acre-feet per year). Annual TSS removal, in pounds, is given by

\( 2.72 (2.3446) (1) (54.5) = 347 \)

Annual TP removal is given by

\( 2.72 (2.3446) (1) (0.3) = 1.91 \)

If the depth below the underdrain was 0.5 feet instead of 1 foot, only half of the 1 inch performance goal is infiltrated, corresponding to an annual infiltration volume of 1.60 acre-feet. Note that the relationship between infiltration performance goal and annual volume infiltrated is not linear. The first step is to calculate the infiltration and filtered fractions of total volume captured by the BMP. The infiltrated fraction is 1.60/2.3446 or 0.68, leaving a filtered fraction of 0.32.

Annual TSS removal, in pounds, is given by

\( (2.72 (2.3446) (0.68) (54.5)) + ((2.72 (2.3446) (0.32) (0.74) (54.5)) = 319 \)

The first term in parentheses corresponds with the infiltrated portion and equals about 236.3 pounds. The second term in parentheses corresponds with the filtered portion, having a removal efficiency of 0.74 (74 percent), for a total removal of about 82.3 pounds.

Annual TP removal, in pounds, is given by

\( (2.72 (2.3446) (0.68) (0.3)) + ((2.72 (2.3446) (0.32) (0.55) (0.74) (0.3)) = 1.55 \)

The first term in parentheses corresponds with the infiltrated portion and equals about 1.30 pounds. The second term in parentheses corresponds with the filtered portion, having a particulate P fraction of 0.55, and a removal efficiency of 0.74 (74 percent) for the particulate fraction, for a total removal of about 0.25 pounds.

Methods for calculating credits

This section provides specific information on generating and calculating credits from permeable pavement 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

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

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. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume.

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. In using this table to identify models appropriate for permeable pavement, use the sort arrow on the table to select Infiltrator BMPs or Filter BMPs, depending on the type of permeable pavement BMP and the terminology used in the model.

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.


The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of event mean concentration and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (EPA STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Caution: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

Download MPCA Estimator here

MIDS Calculator

mids logo
Download the MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance and examples have been developed for all BMPs in the calculator, including permeable pavement. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

Credits based on reported literature values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the permeable pavement practice. Concentration reductions resulting from treatment can be converted to mass reductions if the volume of stormwater treated is known.

Designers may use the pollutant reduction values reported in this manual or may research values from other databases and published literature. Designers who opt for this approach should

  • select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
  • select a pollutant removal reduction from literature that studied a permeable pavement practice with site characteristics and climate similar to the device being considered for credits;
  • review the article to determine that the design principles of the studied permeable pavement are close to the design recommendations for Minnesota, as described in this manual and/or by a local permitting agency; and
  • give preference to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed permeable pavement practice, considering such conditions as watershed characteristics, permeable pavement practice sizing, soil infiltration rates, and climate factors.

Credits based on field monitoring

Field monitoring may be made in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

  1. Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
    1. Which pollutants will be measured?
    2. Will the monitoring study the performance of a single BMP or multiple BMPs?
    3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
    4. Will the results be compared to other BMP performance studies?
    5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
  2. Plan the field activities. Field considerations include
    1. equipment selection and placement;
    2. sampling protocols including selection, storage, and delivery to the laboratory;
    3. laboratory services;
    4. health and Safety plans for field personnel;
    5. record keeping protocols and forms; and
    6. quality control and quality assurance protocols
  3. Execute the field monitoring
  4. Analyze the results

This manual contains the following guidance for monitoring.

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

  • Chapter 2: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
  • Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
  • Chapter 4: Methods and equipment for water quality monitoring
  • Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
  • Chapter 7: BMP Performance Analysis
  • Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).
Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.

  • Chapter 4: Stormwater Characterization
    • 4.2: General Characteristics and Pollutant Sources
    • 4.3: Sources of Stormwater Quality data
  • Chapter 8: Performance Evaluation
    • 8.1: Methodology Options
    • 8.5: Evaluation of Quality Performance for Individual BMPs
    • 8.6: Overall Hydrologic and Water Quality Performance Evaluation
  • Chapter 10: Hydrologic Evaluation
    • 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices
  • In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).
Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include.

  • Chapter 4: Monitoring Methods and Equipment
  • Chapter 5: Analytical Methods and Laboratory Selection
  • Chapter 6: Monitoring Site Selection
  • Chapter 8: Equipment Installation and Maintenance
  • Chapter 10: Pre-Storm Preparation
  • Chapter 11: Sample Collection and Handling
  • Chapter 12: Quality Assurance / Quality Control
  • Chapter 13: Laboratory Reports and Data Review
  • Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice.

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Other pollutants

Permeable pavements provide removal of sediment (TSS), nutrients (phosphorus and nitrogen), and metals through filtration, infiltration, and soil adsorption. Temperature control occurs in the stone reservoir/subbase and soil subgrade. Phosphorus, metals, and hydrocarbons are adsorbed onto soils within the subgrade. In addition, nutrients such as phosphorus and nitrogen may be biologically degraded.

According to the International Stormwater Database, studies have shown that permeable pavements are effective at reducing concentration of pollutants including solids, bacteria, metals, and nutrients. A compilation of the pollutant removal capabilities from a review of literature of permeable pavement studies are summarized in the table below.

Relative pollutant reduction from permeable pavement systems for metals, nitrogen, bacteria, and organics.
Link to this table

Pollutant Constituent Treatment capabilities1
Metals2 Cadmium, Chromium, Copper, Zinc, Lead, Nickel Medium/High
Nitrogen Total nitrogen, Total Kjeldahl nitrogen Medium/High
Bacteria Fecal coliform, e. coli Insufficient data
Organics Medium

1 Low: < 30%; Medium: 30 to 65%; High: >65%
2 Results are for total metals only


References and suggested reading

  • Brown, Chris; Angus Chu; Bert van Duin; Caterina Valeo. 2009. Characteristics of Sediment Removal in Two Types of Permeable Pavement. Water Qual. Res. J. Can. Volume 44, No. 1, 59-70.
  • Geosyntec and Wright Water Engineers. 2012. International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals. Prepared under Support from WERF, FHWA, EWRI/ASCE and EPA. July 2012.
  • New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B.
  • New Jersey Department of Environmental Protection. 2004. New Jersey Stormwater BMP Manual. Standards for Pervious Paving Systems. Chapter 9.7.
  • North Carolina Department of Environment and Natural Resources. Water Quality Division. 2012. Stormwater BMP Manual & BMP Forms. Chapter 18. Permeable Pavement.
  • Tennis, Paul D.; Michael L. Leming; David J. Akers. 2004. Pervious Concrete Pavements. EB302.02, Portland Cement Association and National Ready Mixed Concrete Association.
  • Tota-Maharaj, K. and Scholz, M. 2010. Efficiency of permeable pavement systems for the removal of urban runoff pollutants under varying environmental conditions. Environ. Prog. Sustainable Energy, 29: 358–369. doi: 10.1002/ep.10418
  • USEPA. Stormwater Menu of BMPs. Permeable Pavements. 2009.


Related articles

Permeable pavement main page

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