Minnesota Pollution Control Agency Logo

Minnesota Stormwater Manual

Difference between revisions of "Calculating credits for green roofs"

(Created page with "Green roofs achieve reductions in stormwater volume compared to conventional roofs. Green roofs can effectively remove or reduce loads of many pollutants that are discharged f...")
 
m
 
(84 intermediate revisions by the same user not shown)
Line 1: Line 1:
−
Green roofs achieve reductions in stormwater volume compared to conventional roofs. Green roofs can effectively remove or reduce loads of many pollutants that are discharged from conventional roofs. Green roofs act as vegetated filters and provide temporary storage of rainwater or snowmelt.  Water kept in storage may eventually be evapotranspired or "bleed' out of the system to the underlying drainage layer. Water reaching the drainage layer is eventually lost from the green roof system. Volume and pollutant 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). Green roofs will achieve the greatest credit when they are properly designed, constructed and maintained.
+
{| class="wikitable" style="float:right; margin-left: 10px; width:100px;"
 +
|+Recommended pollutant removal efficiencies, in percent, for green roofs.<br>
 +
<font size =1>TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen</font size>
 +
|-
 +
| '''TSS'''
 +
| '''TP'''
 +
| '''PP'''
 +
| '''DP'''
 +
| '''TN'''
 +
| '''Metals'''
 +
| '''Bacteria'''
 +
|'''Hydrocarbons'''
 +
|-
 +
| 85
 +
| 0
 +
| 0
 +
| 0
 +
| insufficient data
 +
| insufficient data
 +
| insufficient data
 +
| insufficient data
 +
|}
  
−
Green roofs are used in the beginning of a treatment train. They may receive discharge from another roof, but otherwise do not receive stormwater runoff. Green roofs are excellent BMPs in ultra-urban settings where it is otherwise difficult to achieve volume and pollutant reductions due to space constraints. Since green roofs release water over a period of time following a precipitation event, they are most effective when discharge from the green roof is to a pervious surface, such as turf or a filter strip. Because green roofs effectively remove sediment, discharge from a green roof can be routed to any BMP (e.g. bioretention, infiltration basin, permeable pavement).
+
[[File:Pdf image.png|100px|thumb|left|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Calculating_credits_for_green_roofs_-_Minnesota_Stormwater_Manual_May_2022.pdf Download pdf]</font size>]]
 +
[[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>]]
 +
[[File:Technical information page image.png|100px|left|alt=image]]
  
−
{{alert|The following general assumptions apply in calculating the credit for green roofs. If any of these assumptions is violated, the credit will be reduced.
+
{{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}}
−
*The green roof is [[Design criteria for green roofs|properly designed]].
+
{{alert|Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention.|alert-success}}
−
*The green roof was [[Construction specifications for green roofs|properly constructed]], consistent with the [[Design criteria for green roofs|design criteria]].
 
−
*The green roof is [[Operation and maintenance of green roofs|properly maintained]]. The performance of the green roof should be regularly [[Assessing the performance of green roofs|assessed]].|alert-warning}}
 
  
−
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. Detailed discussions of assumptions may be found in user's manuals or other documentation for the model or calculator.
+
[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
 +
*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);
 +
*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]);
 +
*meeting the [http://stormwater.pca.state.mn.us/index.php/Performance_goals_for_new_development,_re-development_and_linear_projects MIDS performance goal]; or
 +
*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).
 +
This page provides a discussion of how <span title="Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention."> '''[https://stormwater.pca.state.mn.us/index.php?title=Green_roofs green roofs]'''</span> can achieve stormwater credits. Green roofs achieve reductions in stormwater volume compared to conventional roofs. Green roofs can effectively remove or reduce loads of many pollutants that are discharged from conventional roofs, although they may be less effective than other BMPs in removing phosphorus and nitrogen. Green roofs act as vegetated filters and provide temporary storage of rainwater or snowmelt. Water kept in storage may eventually be <span title="Loss of water to the atmosphere as a result of the joint processes of evaporation and transpiration through vegetation"> '''evapotranspired'''</span> or "bleed' out of the system to the underlying drainage layer. Water reaching the drainage layer is eventually lost from the green roof system.
  
−
This section provides specific information on generating and calculating credits from green roofs for volume, TSS and phosphorus. Green roofs may also be effective at reducing concentrations of other pollutants such as metals. 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.
+
==Overview==
 +
[[File:Typical green roof sections 2.jpg|thumb|300px|alt=schematic showing sections comprising a typical green roof|<font size=3>Typical green roof sections. Images from [http://www.greenroofservice.com Green Roof Technology]</font size>]]
  
−
<!--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.-->
+
Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/detention. Green roofs are excellent BMPs in <span title="Highly urban and ultra-urban settings have a large percentage of impermeable surface and typically have limited space to install surface BMPs. An example would be a downtown area."> '''highly urban and ultra-urban environments'''</span> where it is otherwise difficult to achieve volume and pollutant reductions due to space constraints. Since green roofs release water over a period of time following a precipitation event, they are most effective when discharge from the green roof is to a pervious surface, such as turf or a <span title="an area of permanent vegetation or other material used to reduce sediment, organics, nutrients, pesticides, and other contaminants from runoff and to maintain or improve water quality.> '''[https://stormwater.pca.state.mn.us/index.php?title=Overview_for_pretreatment_vegetated_filter_strips filter strip]'''</span>. Because green roofs effectively remove sediment, discharge from a green roof can be routed to any BMP (e.g. [[Bioretention|bioretention]], [[Infiltration basin|infiltration basin]], [[Permeable pavement|permeable pavement]]).
  
−
==Volume credits==
+
===Pollutant removal mechanisms===
 +
Green roofs provide <span title="Filtration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium, such as sand or an organic material. They are generally used on small drainage areas (5 acres or less) and are primarily designed for pollutant removal. They are effective at removing total suspended solids (TSS), particulate phosphorus, metals, and most organics. They are less effective for soluble pollutants such as dissolved phosphorus, chloride, and nitrate."> [https://stormwater.pca.state.mn.us/index.php?title=Filtration '''filtration''']</span> of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream Best Management Practices (BMPs).
 +
 
 +
===Location in the treatment train===
 +
Green roofs occur at the beginning of stormwater <span title="Multiple BMPs that work together to remove pollutants utilizing combinations of hydraulic, physical, biological, and chemical methods"> [https://stormwater.pca.state.mn.us/index.php?title=Using_the_treatment_train_approach_to_BMP_selection '''treatment trains''']</span>. They may receive discharge from another roof, including conventional roofs, but otherwise do not receive 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.
 +
 
 +
Green roofs generate credits for volume by adsorbing water in the planting media. This water may be taken up by plants, evaporate from the media, or be slowly released from the roof. Released water may be routed to a permeable surface, such as a lawn. Although precipitation typically has few pollutants, green roofs can effectively filter many airborne pollutants, including wind blown sediment and pollutants in precipitation, metals and organics. Because the green roof media must support plant growth it typically has a high organic matter content. Green roofs are therefore not effective at removing phosphorus because phosphorus can leach from the organic matter.
 +
 
 +
===Assumptions and approach===
 +
In developing the credit calculations, it is assumed the green roof 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 [[Green roofs|green roof]] section of the Manual.
 +
 
 +
In the following discussion, the <span title="The volume of water that is treated by a BMP."> [https://stormwater.pca.state.mn.us/index.php?title=Water_quality_criteria '''Water Quality Volume''']</span> (V<sub>WQ</sub>) is delivered as an <span title="The maximum volume of water that can be retained by a stormwater practice (bmp) if the water was instantaneously added to the practice. It equals the depth of the practice times the average area of the practice. For some bmps (e.g. bioretention, infiltration trenches and basins, swales with check dams), the volume is the water stored or retained above the media, while for other practices (e.g. permeable pavement, tree trenches) the volume is the water stored or retained within the media."> '''instantaneous volume'''</span> to the BMP. V<sub>WQ</sub> is stored in the media and varies depending on the stormwater management objective(s). For construction stormwater, V<sub>WQ</sub> is 1 inch off new impervious surface. For MIDS, V<sub>WQ</sub> is 1.1 inches.
 +
 
 +
===Volume credit calculations===
 +
[[File:MIDS calculator screen shot 2.png|300px|thumb|alt=screen shot of MIDS calculator showing inputs used to calculate volume credit for a green roof|<font size=3>Screen shot of MIDS calculator showing inputs used to calculate volume credit for a green roof. Inputs include the media depth and green roof area.</font size>]]
 +
 
 +
{{alert|This section does not provide a detailed discussion of green roof hydrology. The article providing an overview of green roofs contains an extensive discussion of [[Overview for green roofs#Water quantity treatment|green roof hydrology and water retention]].|alert-info}}
 +
 
 +
A green roof system acts similar to a bioretention system with an <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrain'''</span>, although the underlying drainage layer may provide some volume control through storage or by retarding water loss. Volume credits for a green roof system are a function of the dimensions of the system, specifically the depth of the <span title="Engineered media is a mixture of sand, fines (silt, clay), and organic matter utilized in stormwater practices, most frequently in bioretention practices. The media is typically designed to have a rapid infiltration rate, attenuate pollutants, and allow for plant growth."> [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_bioretention#Materials_specifications_-_filter_media '''engineered media''']</span>, the storage and hydraulic characteristics of the media, the area of the green roof, and the amount of run-on from adjacent conventional roofs. The volume credit (V), in cubic feet, is given by
 +
 
 +
<math>V = A_s\ D_p\ (MMWR)</math>
 +
 
 +
where
 +
:''A<sub>s</sub>'' = the surface area of the green roof(square feet);
 +
:''D<sub>p</sub>'' = the depth of the media (feet), equal to the area from the bottom of the media (top of underlying drainage layer) to the top of the media; and
 +
:''MMWR'' = moisture content at the maximum media density (cubic feet/cubic foot).
 +
 
 +
The maximum media density (kilograms per cubic meter) is the sample weight (kg) after draining for 120 minutes divided by the initial sample volume (cubic meters). The maximum media density is used to estimate the maximum dead load for green roof assemblies. A value of 0.33 is used for MMWR in the [[MIDS calculator]].
 +
 
 +
A green roof may 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. Specifications for designing green roof systems, including a discussion of how to calculate the reservoir depth, are provided in the section covering [[Design criteria for green roofs|design criteria for green roofs]]. Note that traditional roofs, which are treated as impervious surface, may be routed to a green roof and will therefore affect the achievement of the performance goal. Recommendations about the maximum <span title="The total drainage area, including pervious and impervious surfaces, contributing to a BMP"> '''[https://stormwater.pca.state.mn.us/index.php?title=Contributing_drainage_area_to_stormwater_BMPs contributing drainage area]'''</span> from conventional roofs varies. The MIDS calculator limits the size of a conventional roof that drains to a green roof to an area no greater than the area of green roof.
 +
 
 +
===Total suspended solids (TSS) calculations===
 +
Since all precipitation falling on a green roof is routed through the media, typical sizing calculations do not apply. Typical roof concentrations of TSS are low, with median concentrations of 10.5 milligrams per liter reported in the [https://bmpdatabase.org/ International Stormwater Best Management Practices (BMP) Database]. The event-based or annual reduction in TSS loading, M<sub>TSS</sub> in pounds, is given by
 +
 
 +
<math> M_{TSS} = 2.72\ V\ EMC_{TSS}\ R_{TSS} </math>
 +
 
 +
where
 +
:V is the event or annual precipitation, in acre-feet,
 +
:EMC<sub>TSS</sub> is the event mean concentration in milligrams per liter,
 +
:R<sub>TSS</sub> is the TSS pollutant removal expressed as a fraction, and
 +
:2.72 is a conversion factor.
 +
 
 +
The recommended removal efficiency is 0.85 (85 percent). Typically, annual precipitation can be used for V when computing the annual TSS reduction. This is because all precipitation passes through the green roof media. It is rare that rainfall intensity is such that water flows across the green roof media.
 +
 
 +
:'''Example'''
 +
Assume a 1 inch rain falls on a 1 acre roof. Assume the EMC is 10 milligrams per liter and the removal efficiency is 85 percent (0.85). The TSS removal is equal to 2.72 * 1/12 * 10 * 0.85 = 1.93 pounds.
 +
 
 +
===Total phosphorus (TP) calculations===
 +
Since all precipitation falling on a green roof is routed through the media, typical sizing calculations do not apply. Typical roof concentrations of TP are low, with median concentrations of 0.09 milligrams per liter reported in the [https://bmpdatabase.org/ International Stormwater Best Management Practices (BMP) Database]. The event-based or annual reduction in TP loading, M<sub>TSS</sub> in pounds, is given by
 +
 
 +
<math> M_{TP} = 2.72\ V\ EMC_{TP}\ R_{TP} </math>
 +
 
 +
where
 +
:V is the event or annual precipitation, in acre-feet,
 +
:EMC<sub>TP</sub> is the event mean concentration in milligrams per liter,
 +
:R<sub>TP</sub> is the TP pollutant removal expressed as a fraction, and
 +
:2.72 is a conversion factor.
 +
 
 +
Green roofs have been shown to leach phosphorus from the media, although over time they may begin to attenuate phosphorus. The recommended removal efficiency is 0.
 +
 
 +
==Methods for calculating credits==
 +
The hydrology of green roofs differs from other BMPs and typical sizing calculations do not apply. There are models and calculators that include green roofs, but generally these models are simplistic and do not simulate green roof hydrology. We recommend using simple models and calculators to calculate credits for green roofs. Examples include the [[MIDS calculator]], [http://www2.epa.gov/water-research/storm-water-management-model-swmm Stormwater Management Model] (SWMM), [http://www2.epa.gov/water-research/system-urban-stormwater-treatment-and-analysis-integration-sustain EPA SUSTAIN], [https://greenvalues.cnt.org/ Green Values National Stormwater Management Calculator], and the [https://swbmp.vwrrc.vt.edu/vrrm/ Virginia Runoff Reduction Method]. Other modeling efforts, specific to green roofs, have been conducted (see, for example, Hilten et al, 2008; Hardin et al, 2012).
 +
 
 +
For additional information from the literature pertaining to water quality impacts associated with green roofs, we recommend reviewing [http://stormwater.pca.state.mn.us/index.php/File:Green_roof_pollutant_removal.docx the technical document prepared for this Manual]. This document provides a summary of literature reviewed in development of pollutant removal information for green roofs.
 +
 
 +
==Other pollutants==
 +
Although several studies have examined the quality of water discharged from green roofs, few studies have compared results with those from traditional roofs. Traditional roofs are known to leach metals and other materials. Because the green roof media has a high organic matter content, pollutants such as metals and most organic compounds will likely be attenuated in the media. [http://www.harvesth2o.com/RWH_good_bad_ugly.shtml Nicholson et al.] (2010) provide an overview of water quantity and water quality effects of green roofs compared to traditional roofs, citing limited information that supports retention of several pollutants, particularly metals, by green roofs.
 +
 
 +
==References==
 +
*Hardin, M. M. Wanielista, and M. Chopra. 2012. [http://www.mdpi.com/2073-4441/4/4/914 A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use]. Water. 4:914-931.
 +
*Hilten, R.N., T. M. Lawrence, and E. W. Tollner. 2008. ''Modeling stormwater runoff from green roofs with HYDRUS-1D''. Journal of Hydrology. 358:3–4:288–293
 +
* Nicholson, N., Clark, S., Long, B., Siu, C., Spicher, J., and Steele, K. 2010. ''Roof Runoff Water Quality—A Comparison of Traditional Roofing Materials''. World Environmental and Water Resources Congress 2010: pp. 3349-3355. doi: 10.1061/41114(371)345
 +
 
 +
<noinclude>
 +
 
 +
==Related pages==
 +
*Green roofs
 +
**[[Overview for green roofs]]
 +
**[[Types of green roofs]]
 +
**[[Design criteria for green roofs]]
 +
**[[Construction specifications for green roofs]]
 +
**[[Assessing the performance of green roofs]]
 +
**[[Operation and maintenance of green roofs]]
 +
**[[Calculating credits for green roofs]]
 +
**[[Cost-benefit considerations for green roofs]]
 +
**[[Plant lists for green roofs]]
 +
**[[Case studies for green roofs]]
 +
**[[Links for green roofs]]
 +
**[[References for green roofs]]
 +
**[[Supporting material for green roofs]]
 +
**[[Green roofs terminology and glossary]]
 +
**[[Green roof fact sheet]]
 +
**[[Requirements, recommendations and information for using green roofs as a BMP in the MIDS calculator]]
 +
*Calculating credits
 +
**[[Calculating credits for bioretention]]
 +
**[[Calculating credits for infiltration basin]]
 +
**[[Calculating credits for infiltration trench]]
 +
**[[Calculating credits for permeable pavement]]
 +
**[[Calculating credits for green roofs]]
 +
**[[Calculating credits for sand filter]]
 +
**[[Calculating credits for stormwater ponds]]
 +
**[[Calculating credits for stormwater wetlands]]
 +
**[[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:Level 3 - Best management practices/Guidance and information/Pollutant removal and credits]]
 +
[[Category:Level 2 - Pollutants/Pollutant removal]]
 +
</noinclude>

Latest revision as of 14:22, 26 July 2022

Recommended pollutant removal efficiencies, in percent, for green roofs.
TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen
TSS TP PP DP TN Metals Bacteria Hydrocarbons
85 0 0 0 insufficient data insufficient data insufficient data insufficient data
pdf image
Download pdf
image
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.
Green Infrastructure: Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention.

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 green roofs can achieve stormwater credits. Green roofs achieve reductions in stormwater volume compared to conventional roofs. Green roofs can effectively remove or reduce loads of many pollutants that are discharged from conventional roofs, although they may be less effective than other BMPs in removing phosphorus and nitrogen. Green roofs act as vegetated filters and provide temporary storage of rainwater or snowmelt. Water kept in storage may eventually be evapotranspired or "bleed' out of the system to the underlying drainage layer. Water reaching the drainage layer is eventually lost from the green roof system.

Overview

schematic showing sections comprising a typical green roof
Typical green roof sections. Images from Green Roof Technology

Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/detention. Green roofs are excellent BMPs in highly urban and ultra-urban environments where it is otherwise difficult to achieve volume and pollutant reductions due to space constraints. Since green roofs release water over a period of time following a precipitation event, they are most effective when discharge from the green roof is to a pervious surface, such as turf or a filter strip. Because green roofs effectively remove sediment, discharge from a green roof can be routed to any BMP (e.g. bioretention, infiltration basin, permeable pavement).

Pollutant removal mechanisms

Green roofs provide filtration of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream Best Management Practices (BMPs).

Location in the treatment train

Green roofs occur at the beginning of stormwater treatment trains. They may receive discharge from another roof, including conventional roofs, but otherwise do not receive 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.

Green roofs generate credits for volume by adsorbing water in the planting media. This water may be taken up by plants, evaporate from the media, or be slowly released from the roof. Released water may be routed to a permeable surface, such as a lawn. Although precipitation typically has few pollutants, green roofs can effectively filter many airborne pollutants, including wind blown sediment and pollutants in precipitation, metals and organics. Because the green roof media must support plant growth it typically has a high organic matter content. Green roofs are therefore not effective at removing phosphorus because phosphorus can leach from the organic matter.

Assumptions and approach

In developing the credit calculations, it is assumed the green roof 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 green roof section of the Manual.

In the following discussion, the Water Quality Volume (VWQ) is delivered as an instantaneous volume to the BMP. VWQ is stored in the media and varies depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch off new impervious surface. For MIDS, VWQ is 1.1 inches.

Volume credit calculations

screen shot of MIDS calculator showing inputs used to calculate volume credit for a green roof
Screen shot of MIDS calculator showing inputs used to calculate volume credit for a green roof. Inputs include the media depth and green roof area.
Information: This section does not provide a detailed discussion of green roof hydrology. The article providing an overview of green roofs contains an extensive discussion of green roof hydrology and water retention.

A green roof system acts similar to a bioretention system with an underdrain, although the underlying drainage layer may provide some volume control through storage or by retarding water loss. Volume credits for a green roof system are a function of the dimensions of the system, specifically the depth of the engineered media, the storage and hydraulic characteristics of the media, the area of the green roof, and the amount of run-on from adjacent conventional roofs. The volume credit (V), in cubic feet, is given by

\(V = A_s\ D_p\ (MMWR)\)

where

As = the surface area of the green roof(square feet);
Dp = the depth of the media (feet), equal to the area from the bottom of the media (top of underlying drainage layer) to the top of the media; and
MMWR = moisture content at the maximum media density (cubic feet/cubic foot).

The maximum media density (kilograms per cubic meter) is the sample weight (kg) after draining for 120 minutes divided by the initial sample volume (cubic meters). The maximum media density is used to estimate the maximum dead load for green roof assemblies. A value of 0.33 is used for MMWR in the MIDS calculator.

A green roof may 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. Specifications for designing green roof systems, including a discussion of how to calculate the reservoir depth, are provided in the section covering design criteria for green roofs. Note that traditional roofs, which are treated as impervious surface, may be routed to a green roof and will therefore affect the achievement of the performance goal. Recommendations about the maximum contributing drainage area from conventional roofs varies. The MIDS calculator limits the size of a conventional roof that drains to a green roof to an area no greater than the area of green roof.

Total suspended solids (TSS) calculations

Since all precipitation falling on a green roof is routed through the media, typical sizing calculations do not apply. Typical roof concentrations of TSS are low, with median concentrations of 10.5 milligrams per liter reported in the International Stormwater Best Management Practices (BMP) Database. The event-based or annual reduction in TSS loading, MTSS in pounds, is given by

\( M_{TSS} = 2.72\ V\ EMC_{TSS}\ R_{TSS} \)

where

V is the event or annual precipitation, in acre-feet,
EMCTSS is the event mean concentration in milligrams per liter,
RTSS is the TSS pollutant removal expressed as a fraction, and
2.72 is a conversion factor.

The recommended removal efficiency is 0.85 (85 percent). Typically, annual precipitation can be used for V when computing the annual TSS reduction. This is because all precipitation passes through the green roof media. It is rare that rainfall intensity is such that water flows across the green roof media.

Example

Assume a 1 inch rain falls on a 1 acre roof. Assume the EMC is 10 milligrams per liter and the removal efficiency is 85 percent (0.85). The TSS removal is equal to 2.72 * 1/12 * 10 * 0.85 = 1.93 pounds.

Total phosphorus (TP) calculations

Since all precipitation falling on a green roof is routed through the media, typical sizing calculations do not apply. Typical roof concentrations of TP are low, with median concentrations of 0.09 milligrams per liter reported in the International Stormwater Best Management Practices (BMP) Database. The event-based or annual reduction in TP loading, MTSS in pounds, is given by

\( M_{TP} = 2.72\ V\ EMC_{TP}\ R_{TP} \)

where

V is the event or annual precipitation, in acre-feet,
EMCTP is the event mean concentration in milligrams per liter,
RTP is the TP pollutant removal expressed as a fraction, and
2.72 is a conversion factor.

Green roofs have been shown to leach phosphorus from the media, although over time they may begin to attenuate phosphorus. The recommended removal efficiency is 0.

Methods for calculating credits

The hydrology of green roofs differs from other BMPs and typical sizing calculations do not apply. There are models and calculators that include green roofs, but generally these models are simplistic and do not simulate green roof hydrology. We recommend using simple models and calculators to calculate credits for green roofs. Examples include the MIDS calculator, Stormwater Management Model (SWMM), EPA SUSTAIN, Green Values National Stormwater Management Calculator, and the Virginia Runoff Reduction Method. Other modeling efforts, specific to green roofs, have been conducted (see, for example, Hilten et al, 2008; Hardin et al, 2012).

For additional information from the literature pertaining to water quality impacts associated with green roofs, we recommend reviewing the technical document prepared for this Manual. This document provides a summary of literature reviewed in development of pollutant removal information for green roofs.

Other pollutants

Although several studies have examined the quality of water discharged from green roofs, few studies have compared results with those from traditional roofs. Traditional roofs are known to leach metals and other materials. Because the green roof media has a high organic matter content, pollutants such as metals and most organic compounds will likely be attenuated in the media. Nicholson et al. (2010) provide an overview of water quantity and water quality effects of green roofs compared to traditional roofs, citing limited information that supports retention of several pollutants, particularly metals, by green roofs.

References

  • Hardin, M. M. Wanielista, and M. Chopra. 2012. A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use. Water. 4:914-931.
  • Hilten, R.N., T. M. Lawrence, and E. W. Tollner. 2008. Modeling stormwater runoff from green roofs with HYDRUS-1D. Journal of Hydrology. 358:3–4:288–293
  • Nicholson, N., Clark, S., Long, B., Siu, C., Spicher, J., and Steele, K. 2010. Roof Runoff Water Quality—A Comparison of Traditional Roofing Materials. World Environmental and Water Resources Congress 2010: pp. 3349-3355. doi: 10.1061/41114(371)345


Related pages

Retrieved from "https://stormwater.pca.state.mn.us/index.php?title=Calculating_credits_for_green_roofs&oldid=59698"

This page was last edited on 26 July 2022, at 14:22.