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[[File:Pdf image.png|100px|thumb|right|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Water_quality_benefits_of_GSI.pdf Download pdf]</font size>]]
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[[File:Pdf image.png|100px|thumb|right|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Water_quality_and_hydrology_benefits_of_GSI.pdf Download pdf]</font size>]]
  
This page provides information on the water quantity and hydrology benefits of <span title="Green stormwater infrastructure (GSI) describes practices that use natural systems (or engineered systems that mimic or use natural processes) to capture, clean, and infiltrate stormwater; shade and cool surfaces and buildings; reduce flooding, create wildlife habitat; and provide other services that improve environmental quality and communities’ quality of life. (City of Tucson)"> '''green stormwater infrastructure'''</span> (GSI) practices (<span title="One of many different structural or non–structural methods used to treat runoff"> '''best management practices'''</span>). The water quantity benefit of a practice is defined by its ability to retain runoff or detain and slowly release runoff. These benefits include the following.
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This page provides information on the water quantity and hydrology benefits of <span title="Green stormwater infrastructure (GSI) describes practices that use natural systems (or engineered systems that mimic or use natural processes) to capture, clean, and infiltrate stormwater; shade and cool surfaces and buildings; reduce flooding, create wildlife habitat; and provide other services that improve environmental quality and communities’ quality of life. (City of Tucson)"> '''green stormwater infrastructure'''</span> (GSI) practices (<span title="One of many different structural or non–structural methods used to treat runoff"> '''best management practices or bmps'''</span>). The water quantity benefit of a practice is defined by its ability to retain runoff or detain and slowly release runoff. These benefits include the following.
 
*Decreased downstream flooding as water is either captured or slowly released to reduce peak runoff volumes (rate control). These reductions in downstream runoff volume also reduce impacts to <span title="A stream, river, lake, ocean, or other surface or groundwaters into which treated or untreated wastewater is discharged"> '''receiving waters'''</span> by reducing erosion and protecting habitat.
 
*Decreased downstream flooding as water is either captured or slowly released to reduce peak runoff volumes (rate control). These reductions in downstream runoff volume also reduce impacts to <span title="A stream, river, lake, ocean, or other surface or groundwaters into which treated or untreated wastewater is discharged"> '''receiving waters'''</span> by reducing erosion and protecting habitat.
 
*Increased groundwater recharge, which can lead to improved baseflow and deep aquifer recharge
 
*Increased groundwater recharge, which can lead to improved baseflow and deep aquifer recharge
*Reductions in downstream runoff volumes also provide additional benefits,  
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*Reductions in downstream runoff volumes also provide additional benefits, such as reduced erosion.
  
 
Water quantity benefits vary between each practice, primarily as a result of the mechanism by which stormwater runoff is captured and by its ultimate fate.
 
Water quantity benefits vary between each practice, primarily as a result of the mechanism by which stormwater runoff is captured and by its ultimate fate.
*Constructed ponds (<span title="A stormwater retention basin that includes a combination of permanent pool storage and extended detention storage above the permanent pool to provide additional water quality or rate control"> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_ponds '''wet ponds''']</span>) and wetlands (<span title="Stormwater wetlands are similar in design to stormwater ponds and mainly differ by their variety of water depths and associated vegetative complex."> '''[https://stormwater.pca.state.mn.us/index.php?title=Stormwater_wetlands stormwater wetlands]'''</span>) temporarily capture and store water, releasing it slowly. This decreases peak discharges downstream. There is some water loss through seepage and evapotranspiration, but these losses are considered negligible. See [[Calculating credits for stormwater ponds]]. For more information on sedimentation processes, [https://stormwaterbook.safl.umn.edu/sedimentation-practices link here].
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*Constructed ponds (<span title="A stormwater retention basin that includes a combination of permanent pool storage and extended detention storage above the permanent pool to provide additional water quality or rate control"> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_ponds '''wet ponds''']</span>) and wetlands (<span title="Stormwater wetlands are similar in design to stormwater ponds and mainly differ by their variety of water depths and associated vegetative complex."> '''[https://stormwater.pca.state.mn.us/index.php?title=Stormwater_wetlands stormwater wetlands]'''</span>) temporarily capture and store water, releasing it slowly. This decreases peak discharges downstream. There is some water loss through seepage and e<span title="Loss of water to the atmosphere as a result of the joint processes of evaporation and transpiration through vegetation"> '''evapotranspiration'''</span>, but these losses are considered negligible. See [[Calculating credits for stormwater ponds]]. For more information on sedimentation processes, [https://stormwaterbook.safl.umn.edu/sedimentation-practices link here].
*<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=Stormwater_filtration_Best_Management_Practices '''Filtration''']</span> practices include bmps that have an <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrain'''</span> (<span title="A bioretention practice having an underdrain. All water entering the practice is filtered through engineered media and filtered water is returned to the storm sewer system."> [https://stormwater.pca.state.mn.us/index.php?title=Bioretention '''biofiltration''']</span>, <span title="Permeable pavements allow stormwater runoff to filter through surface voids into an underlying stone reservoir for temporary storage and/or infiltration. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP)."> '''[https://stormwater.pca.state.mn.us/index.php?title=Permeable_pavement permeable pavement]'''</span>, <span title="A tree trench, often known as a "vertical rain garden," is a system that consists of piping for water storage, structural soils and a tree."> '''[https://stormwater.pca.state.mn.us/index.php?title=Trees tree trench]'''</span>, <span title="Are configured as shallow, linear channels. They typically have vegetative cover such as turf or native perennial grasses"> [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) '''swales''']</span>, <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>, and <span title="Filtration of stormwater through a sand filtering material whose purpose is to remove pollution from runoff"> '''[https://stormwater.pca.state.mn.us/index.php?title=Filtration media filters]'''</span>) or bmps that provide some water retention in soil or engineered media (<span title="Vegetated filter strips are designed to provide sedimentation and screening (by vegetation) to treat stormwater runoff prior to entering a structural stormwater BMP. Vegetated filter strips are especially effective at capturing excess sediment in stormwater runoff by settling solids. Vegetated filter strips provide limited (due to size) volume reduction, peak flow reduction, infiltration, and biological treatment. Stormwater management processes not provided in vegetated filter strips include filtration and sorption."> [https://stormwater.pca.state.mn.us/index.php?title=Overview_for_pretreatment_vegetated_filter_strips '''vegetated filter strips''']</span>, swales, green roofs).
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*<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=Stormwater_filtration_Best_Management_Practices '''Filtration''']</span> practices include bmps that have an <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrain'''</span> (<span title="A bioretention practice having an underdrain. All water entering the practice is filtered through engineered media and filtered water is returned to the storm sewer system."> [https://stormwater.pca.state.mn.us/index.php?title=Bioretention '''biofiltration''']</span>, <span title="Permeable pavements allow stormwater runoff to filter through surface voids into an underlying stone reservoir for temporary storage and/or infiltration. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP)."> '''[https://stormwater.pca.state.mn.us/index.php?title=Permeable_pavement permeable pavement]'''</span>, <span title="A tree trench, often known as a "vertical rain garden," is a system that consists of piping for water storage, structural soils and a tree."> '''[https://stormwater.pca.state.mn.us/index.php?title=Trees tree trench]'''</span>, <span title="Are configured as shallow, linear channels. They typically have vegetative cover such as turf or native perennial grasses"> [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) '''swales''']</span>, <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>, and <span title="Filtration of stormwater through a sand filtering material whose purpose is to remove pollution from runoff"> '''[https://stormwater.pca.state.mn.us/index.php?title=Filtration media filters]'''</span>) or bmps that provide some water retention in soil or engineered media (<span title="Vegetated filter strips are designed to provide sedimentation and screening (by vegetation) to treat stormwater runoff prior to entering a structural stormwater BMP. Vegetated filter strips are especially effective at capturing excess sediment in stormwater runoff by settling solids. Vegetated filter strips provide limited (due to size) volume reduction, peak flow reduction, infiltration, and biological treatment. Stormwater management processes not provided in vegetated filter strips include filtration and sorption."> [https://stormwater.pca.state.mn.us/index.php?title=Overview_for_pretreatment_vegetated_filter_strips '''vegetated filter strips''']</span>, swales, green roofs). In practices with underdrains, there is always some infiltration below the underdrain, provided the practice does not have an <span title="Impermeable means not allowing something, such as water, to pass through. Some materials considered impermeable may actually allow water to pass through at very slow rates, such as 10(-8) cm/sec."> '''impermeable'''</span> liner. The annual volume lost depends on the position of the underdrain and the infiltration characteristics of the underlying soil. In vegetated practices with underdrains, there is also water loss through evapotranspiration. Annual losses in these practices are typically in the 5 to 20 percent range. Green roofs are effective at retaining water, while volume reduction in swales and filter strips is generally small.
*<span title="Infiltration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium and into underlying soil, where it may eventually percolate into groundwater. The filtering media is typically coarse-textured and may contain organic material, as in the case of bioinfiltration BMPs."> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_Best_Management_Practices '''Infiltration''']</span> practices remove pollutants by capturing runoff and infiltrating it vertically into underlying soil, the <span title="The vadose zone is the variably saturated zone between the ground surface and the permanent water table of the groundwater."> '''vadose zone'''</span>, and groundwater. Attenuation occurs primarily through adsorption and filtering, though dilution in groundwater may also be a mechanism for reducing pollutant concentrations.
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*<span title="Infiltration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium and into underlying soil, where it may eventually percolate into groundwater. The filtering media is typically coarse-textured and may contain organic material, as in the case of bioinfiltration BMPs."> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_Best_Management_Practices '''Infiltration''']</span> practices retain all water captured by the practice. This captured water is infiltrated through the soil, <span title="The vadose zone is the variably saturated zone between the ground surface and the permanent water table of the groundwater."> '''vadose zone'''</span>, and into groundwater.
  
 
{| class="wikitable" style="float:right; margin-left: 10px; width:500px;"
 
{| class="wikitable" style="float:right; margin-left: 10px; width:500px;"
 
|-
 
|-
! Practice !! Water quality benefit !! Notes
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! Practice !! Water quanity benefit !! Notes
 
|-
 
|-
| Bioretention || <font size=6><center>&#9685;</center></font size> || Infiltration is most effective; potential phosphorus leaching in filtration practices
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| Bioretention || <font size=4><center>&#9685;</center></font size> || Benefit is high for infiltration, low for filtration
 
|-
 
|-
| Tree trench and tree box || <font size=6><center>&#9685;</center></font size> || Infiltration is most effective; potential phosphorus leaching in filtration practices
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| Tree trench and tree box || <font size=4><center>&#9685;</center></font size> || Benefit is high for infiltration, low to moderate for filtration
 
|-
 
|-
| Green roof || <font size=4><center>&#9685;</center></font size> || Potential phosphorus leaching
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| Green roof || <font size=4><center>&#9685;</center></font size> || Benefit is associated with rate control; effectiveness increases with media thickness
 
|-
 
|-
| Vegetated swale || <font size=4><center>&#9684;</center></font size> || Infiltration is most effective; less effective for dissolved pollutants
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| Vegetated swale || <font size=4><center>&#9684;</center></font size> || Benefit is high for infiltration, low for filtration
 
|-
 
|-
| Vegetated filter strip || <font size=4><center>&#9684;</center></font size> || Removes solids; less effective for dissolved pollutants
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| Vegetated filter strip || <font size=4><center>&#9684;</center></font size> ||  
 
|-
 
|-
| Permeable pavement || <font size=4><center>&#9685;</center></font size> || Infiltration is most effective
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| Permeable pavement || <font size=4><center>&#9685;</center></font size> || Benefit is high for infiltration, low for filtration
 
|-
 
|-
| Constructed wetland</td> || <font size=4><center>&#9681;</center></font size> || Removes solids; less effective for dissolved pollutants
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| Constructed wetland</td> || <font size=4><center>&#9681;</center></font size> || Benefit is associated with rate control
 
|-
 
|-
 
|Rainwater harvesting || <font size=4><center>&#9685;</center></font size> || Can be used on low permeability soils
 
|Rainwater harvesting || <font size=4><center>&#9685;</center></font size> || Can be used on low permeability soils
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There is no universal definition of GI or GSI ([https://stormwater.pca.state.mn.us/index.php?title=Green_infrastructure_and_green_stormwater_infrastructure_terminology link here for more information]). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to [[Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services]].
 
There is no universal definition of GI or GSI ([https://stormwater.pca.state.mn.us/index.php?title=Green_infrastructure_and_green_stormwater_infrastructure_terminology link here for more information]). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to [[Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services]].
  
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==Bioretention==
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[[File:Bioretention facility in St Paul MN.PNG|right|thumb|300 px|alt=This is a picture of Bioretention facility in St Paul MN|<font size=3>Bioretention practices can be incorporated into street landscapes. Image Courtesy of Emmons & Olivier Resources, Inc.</font size>]]
  
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Bioretention can be designed as an effective infiltration / recharge practice (<span title="A bioretention practice in which no underdrain is used. All water entering the bioinfiltration practice infiltrates or evapotranspires."> '''bioinfiltration'''</span>) when parent soils have high permeability. For lower permeability soils an <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrain'''</span> is typically used and some infiltration and rate control can be achieved.
  
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Bioinfiltration practices, which are bioretention practices with no underdrain and designed to infiltrate water, are effective at reducing runoff volume and peak rates for small and medium intensity storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on <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 soils, for example, will capture about 90 percent of annual runoff.
  
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Effects of bioinfiltration on groundwater hydrology are poorly understood. It is often assumed increased recharge will result in increased aquifer recharge and/or increased <span title="Baseflow (also called drought flow, groundwater recession flow, low flow, low-water flow, low-water discharge and sustained or fair-weather runoff) is the portion of streamflow delayed shallow subsurface flow".> '''baseflow'''</span> to surface waters, but limited data exists to support these assumptions.
  
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The following design considerations can increase the water quantity/hydrologic benefits of bioretention practices.
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*Maximize infiltration through sizing and identifying the most permeable soils on a site
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*Utilize [http://chesapeakestormwater.net/wp-content/uploads/downloads/2014/03/Internal-Water-Storage-for-Bioretention-2009.pdf internal water storage]
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*Maximize water storage in <span title="Engineered media is a mixture of sand, fines (silt, clay), organic matter, and occasionally other amendments (e.g. iron) 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>
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*Bioinfiltration practices scattered throughout a watershed can reduce runoff volumes and peak runoff
  
Bioinfiltration practices, which are bioretention practices with no underdrain and designed to infiltrate water, are effective at reducing runoff volume and peak rates for relatively small storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on B soils, for example, will capture about 90 percent of annual runoff. On a watershed scale, bioinfiltration practices scattered throughout a watershed can reduce runoff volumes and peak runoff but not to predevelopment levels. Hunt et al (2009; 2012) discuss the importance of determining hydrologic goals prior to designing and constructing bioretention practices. The researchers state "A large cell media volume: drainage area ratio, and adjustments to the drainage configuration appear to improve the performance". In particular, the researchers advocate for deeper basins and thicker media depths.
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See [[Calculating credits for bioretention]] and Hunt et al (2009; 2012).
  
:Effects of bioinfiltration on groundwater hydrology are poorly understood. It is often assumed increased recharge will result in increased aquifer recharge and/or increased baseflow to surface waters, but limited data exists to support these assumptions.  
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==Tree Trench==
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[[File:Marquette avenue 5.jpg|thumb|300px|alt=photo of completed project, Marquette Avenue, Minneapolis|<font size=3>Completed tree system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.</font size>]]
  
:Additional recommend reading:
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Tree trenches are a type of bioretention practice. However, their potential benefits for water quantity and hydrology are greater than for traditional bioretention practices due to their ability to treat runoff from larger areas, hence more runoff volume, because of the enhanced evapotranspiration provided by trees, and due to <span title="Interception refers to precipitation that does not reach the soil, but is instead intercepted by the leaves, branches of plants and the forest floor"> '''interception'''</span> of rainfall by tree canopies. On higher permeability soils they can be designed as infiltration practices, while on lower permeability soils they typically have an underdrain.
:- [http://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2008)13:2(90) Field Performance of Bioretention: Hydrology Impacts]
 
:- [http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0000448 Bioretention Hydrologic Performance in an Urban Stormwater Network]
 
:- [http://ascelibrary.org/doi/abs/10.1061/(ASCE)EE.1943-7870.0000504 Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design]
 
:- [http://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2009)14:4(407) Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland]
 
:- [http://www.sciencedirect.com/science/article/pii/S0022169409000158 Spatio-temporal effects of low impact development practices]
 
  
===Green Roofs===
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Tree trenches with no underdrain are effective at reducing runoff volume and peak rates for small and medium intensity storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on B soils, for example, will capture about 90 percent of annual runoff.
Green roofs have the ability to store significant amounts of water in their growing media. Water can evaporate from the soil and be transpired by the vegetation and plans growing in the media. As a result, runoff from the roof is reduced which decreases flow to storm sewer systems and can ultimately reduce overflow events and flooding events. This can be beneficial especially in highly developed urban areas where stormwater is conveyed to storm sewer systems that can frequently experience combined sewer overflows. In some cases both laboratory and field measurements show a reduction in stormwater runoff volume by 30% to 86%, a reduction in peak flow rate by 22% to 93% and a delay in peal flow by 0 to 30 minutes using green roofs<sup>[https://stormwater.pca.state.mn.us/index.php?title=Green_Infrastructure_References#Water_quantity_and_hydrology_benefits_of_Green_SW_Infrastructure_Page 1]</sup>.  The selection of growth medium, plant species, and roof slope of green roof can have a significant effect on the effectiveness and performance of a green roof for hydrologic purposes. Green roofs can generally be categorized into two categories; intensive and extensive. Intensive roofs have a soil depth of 6 inches or greater, whereas extensive roofs have less than 6 inches of soil depth<sup>[https://stormwater.pca.state.mn.us/index.php?title=Green_Infrastructure_References#Water_quantity_and_hydrology_benefits_of_Green_SW_Infrastructure_Page 2]</sup>. One experimental case study that implemented an extensive roof in Michigan showed a peak discharge reduction of 54% to 99%.
 
  
===Tree Trench===
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Reduction in runoff due to trees is highly variable and depends on many factors such as, but not limited to, tree characteristics, planting configuration, soil conditions, and precipitation event characteristics. For examples, studies show that annual rainfall interception by urban trees and forests can range from 6% to 66%, while urban trees can transpire from 0.2 to 46.7 gallons per tree per day (Center for Watershed Protection, 2005).
Trees can help reduce stormwater runoff by intercepting rainfall, promoting infiltration by increasing the presence of macrospores and the ability of soil to store water, transpiration, and evapotranspiration. Tree canopies also help reduce the impact raindrops have on barren surfaces. Through all of these mechanisms a tree canopy temporarily detain rainfall and gradually releases it, and regulates the flow of stormwater runoff downstream to storm sewer networks or other waterways.  Reduction in runoff due to trees is highly variable and depends on many factors such as, but not limited to, tree characteristics, planting configuration, soil conditions, and precipitation event characteristics. For examples, studies show that annual rainfall interception by urban trees and forests can range from 6% to 66%, while urban trees can transpire from 0.2 to 46.7 gallons per tree per day<sup>[https://stormwater.pca.state.mn.us/index.php?title=Green_Infrastructure_References#Water_quantity_and_hydrology_benefits_of_Green_SW_Infrastructure_Page 3]</sup>. That being said, it is difficult to estimate and quantify runoff reduction, thus results are likely to vary.
 
  
When selecting trees species, species appropriate for the local current and future water conditions should be selected in preference to production species, which typically combine high rates of biomass accumulation with high evapotranspiration. Additionally, fast-growing tree species such as production species are likely to reduce runoff more than slow-growing species, and may be more susceptible to drought and climate.
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The following design considerations may increase the water quantity and hydrology benefits of tree-based practices.
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*Maximize infiltration by designing larger practices, if feasible, and identifying the most permeable soils on a site
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*Utilize [https://epa.ohio.gov/static/Portals/41/storm_workshop/lid/IWS.Dec10.pdf internal water storage]
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*Maximize water storage in media
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*Select species appropriate for the local current and future water conditions, which typically combine high rates of biomass accumulation with high evapotranspiration.
  
:Additional recommended reading
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See [[Calculating credits for tree trenches and tree boxes]]
:-[https://www.sciencedirect.com/science/article/pii/S1433831915000463 Balancing the environmental benefits of reforestation in agricultural regions]
 
  
===Bioretention===
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==Permeable pavement==
Bioretention practices are able to store and infiltrate stormwater, which helps mitigate flood impacts and prevents stormwater from polluting local waterways. A study analyzing 2 bioretention cells subject to over 49 rainfall events showed flow peak reductions of 44% to 63%<sup>[https://stormwater.pca.state.mn.us/index.php?title=Green_Infrastructure_References#Water_quantity_and_hydrology_benefits_of_Green_SW_Infrastructure_Page 4]</sup>. The study summarized that “from a hydrological perspective, the bioretention facilities were successful in minimizing the hydrologic impact of the impervious surface and major reductions can be expected for about 1/3 to 1/2 of the rainfall events. 
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[[File:Picture of porous asphalt 1.jpg|thumb|300 px|alt=This photo illustrates porous asphalt. Porous asphalt is standard hot-mix asphalt that allows water to drain through it.|<font size=3>Photo illustrating porous asphalt. Porous asphalt is standard hot-mix asphalt that allows water to drain through it.</font size>]]
A separate study that analyzed 5 bio-retention cells found that “bioretention cells were able to mitigate peak flows because of their infiltration rales, potential to store water in soil pores, and slow drawdown time.<sup>[https://stormwater.pca.state.mn.us/index.php?title=Green_Infrastructure_References#Water_quantity_and_hydrology_benefits_of_Green_SW_Infrastructure_Page 5]</sup>
 
  
===Permeable Pavement===
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Permeable pavement provides reduction in total water volume movement and retardation of peak flow from rainfall events. When constructed without an underdrain (as an infiltration practice), they provide significant volume reduction if properly maintained, though they are typically small in size and therefore treat runoff from limited areas. When an underdrain is incorporated into the design, they provide rate control by temporarily storing and infiltrating water before the water is captured by the underdrain.
Permeable pavement reduces surface runoff volumes by allowing stormwater to infiltrate into underlying soils as opposed to allowing stormwater to flow into storm drains and out to receiving water as effluent. Additionally, permeable pavement helps reduce peak flow rates and decreases the risk of flooding through this same process by preventing large, fast pulses of precipitation through stormwater collection systems<sup>[https://stormwater.pca.state.mn.us/index.php?title=Green_Infrastructure_References#Water_quantity_and_hydrology_benefits_of_Green_SW_Infrastructure_Page 6]</sup>. A study conducted by the Wisconsin Department of Natural Resources using a Permeable Interlocking Concrete Pavement (PICP) found that a permeable pavement system facilitate a volume reduction of 56%. It should be noted that the deteriorating upland drainage area is thought to have clogged the porous pavement, allowing a greater percentage of surface runoff to bypass the system than originally hypothesized.
 
  
:Additional recommended reading
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Permeable pavement can be used in conjunction with other GSI practices, such as bioretention and harvest and reuse systems.
:-[http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=0ahUKEwiQ5-izxZTZAhVJ74MKHfCpALkQFghYMAc&url=http%3A%2F%2Fwww.mdpi.com%2F2073-4441%2F10%2F1%2F33%2Fpdf&usg=AOvVaw00y3F4iqWqpKN9Sol7G_vd Hydrologic and Water Quality Evaluation of a Permeable Pavement and Biofiltration Device in Series]
 
  
===Water Re-use and Harvesting===
+
The following design consideration may increase the water quantity and hydrology benefits of permeable pavement practices.
Water re-use and harvesting systems help capture rainfall and minimize stormwater runoff volumes and rates to receiving stormwater sewer systems and conveyances. Re-use and Harvesting systems capture water and increase a user’s water supply that can be stored and used on site, immediately or in the future, in lieu of the local water supply. This can be particularly advantageous in times of droughts when water may not be readily available. These systems also help alleviate pressure on the local water supply and allows communities to allocate water to other users. Reusing rainwater for irrigation purposes can also help increase groundwater recharge.
+
*Ensure the subgrade is flat. Since roads are typically sloped, utilize terracing in the subgrade to achieve flat slopes. See [https://files.nc.gov/ncdeq/Energy%20Mineral%20and%20Land%20Resources/Stormwater/BMP%20Manual/C-5%20%20Permeable%20Pavement%2004-06-17.pdf page 5 of the North Carolina design guidance].
 +
*Incorporate signage into the design to ensure maintenance activities do not affect the infiltration properties of the pavement.
 +
*Design to maximize retention time and prevent <span title="A condition that occurs when water flows along a nearly direct pathway from the inlet to the outlet of a tank or basin, often resulting in shorter contact, reaction, or settling times in comparison with the calculated or presumed detention times."> '''short-circuiting'''</span>. Storage may be increased by use of geotextile subgrades. An example is presented by Nnadi et al, (2014).
 +
*Plan for the expected loading on the permeable pavement and ensure capabilities and reduce compaction or clogging
 +
*Use in conjunction with other treatments to establish a treatment train or reuse water on site
  
:Additional recommended reading
+
See [[Calculating credits for permeable pavement]]
:-[https://www3.epa.gov/region9/water/recycling/pdf/brochure.pdf Water Recycling and Re-use: The Environmental Benefits]
+
 
:-[https://www.epa.gov/green-infrastructure/what-green-infrastructure#rainwaterharvesting Rainwater Harvesting ]
+
==Green Roofs==
:-[https://www.water.ca.gov/LegacyFiles/pubs/conservation/water_facts_no._23__water_recycling/waterfact23.pdf Water Facts: Water Recycling]
+
[[File:Target Center Arena Green Roof 2, Minneapolis, MN.jpg|300px|thumb|alt=image of target center green roof, Minneapolis, MN|<font size=3>Vegetation on the Target Center Arena green roof. vegetation consisted of a pregrown Sedum mat supplemented with 22 species of plugs and 16 species of seed native to Minnesota’s bedrock bluff prairies. Image Courtesy of The Kestrel Design Group, Inc.</font size>]]
 +
 
 +
Green roofs have the ability to store significant amounts of water in their growing media. Water can evaporate from the soil and be transpired by the vegetation and plants growing in the media. As a result, runoff from the roof is reduced which decreases flow to storm sewer systems and can ultimately reduce overflow events and flooding events. This can be beneficial especially in highly developed urban areas where stormwater is conveyed to storm sewer systems that can frequently experience combined sewer overflows.
 +
 
 +
The selection of growth medium, thickness of the growth medium, plant species, and roof slope can have a significant effects on the effectiveness and performance of a green roof for hydrologic purposes. Green roofs can generally be categorized as either intensive or extensive. Intensive roofs have a soil depth of 6 inches or greater, whereas extensive roofs have less than 6 inches of soil depth ([https://www.mi-wea.org/docs/Carpenter_-_Hydrologic_Design_of_Vegetated_Roofs.pdf Carpenter, 2016]).
 +
 
 +
The following design considerations may increase the water quantity and hydrologic benefits of green roofs.
 +
*Increase media depth to extent economically feasible
 +
*Maximize the sorptive and retention properties of the media. Increasing the organic fraction can increase water retention but may result in phosphorus export and other concerns if drying occurs. Additives such as perlite and pumice increase water retention. See [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6997945/ Bollman et al., 2019])
 +
*Select plants with high evapotranspiration rates, keeping mind the harshness of green roof microclimates
 +
*Combine green roofs with other treatment practices, such as harvest and reuse systems and roof disconnection
 +
 
 +
See [[Calculating credits for green roofs]]
 +
 
 +
==Water Re-use and Harvesting==
 +
[[File:Woodbury case study 2.jpg|300px|thumb|alt=Photo of harvest and use system Woodbury|<font size=3>Photo of storage pond used for irrigation. Image courtesy Emmons and Olivier Resources.</font size>]]
 +
 
 +
Water re-use and harvesting systems help capture rainfall and minimize stormwater runoff volumes and rates to receiving stormwater sewer systems and conveyances. Harvest and reuse systems capture water and increase a user’s water supply that can be stored and used on site, immediately or in the future, in lieu of the local water supply, including use of potable water. This can be particularly advantageous in times of droughts when water may not be readily available. These systems also help alleviate pressure on the local water supply and allows communities to allocate water to other users. Reusing rainwater for irrigation purposes can also help increase groundwater recharge.
 +
 
 +
The potential benefits delivered by a harvest and reuse system depend on the type of system. Practices utilizing storage tanks typically have limited storage capacity and are used for site applications. Constructed ponds and wetlands have much greater storage capacity and can be used in larger applications. See [[Design considerations for constructed stormwater ponds used for harvest and irrigation use/reuse]].
 +
 
 +
The following design considerations may increase the potential water quantity and hydrology benefits of harvest and reuse systems.
 +
*Consider using harvesting on low permeability soils (<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 and D soils), where captured water can be distributed through irrigation and/or used indoors.
 +
*Size the system to meet the intended uses of the harvest system. This includes ensuring appropriate water supply in response to demand. See [[Determining the appropriate storage size for a stormwater and rainwater harvest and use/reuse system]] and [[Design criteria for stormwater and rainwater harvest and use/reuse]].
 +
*Construct the distribution system to reach all areas of the site that require water when economically feasible
 +
*Determine the sites’ water needs for vegetation and plant systems over a given time period and design the water storage container to meet these needs. Typically, harvest systems are designed and built with a specific use and vegetation scheme in mind, but if feasible, consider adopting vegetation to an intended harvest system (i.e. if the objective is driven by performance goals for the harvest system, vegetation considerations should be built into the design considerations).
 +
*Utilize tandem systems which combine multiple storage units, such as multiple rain barrels in sequence ([https://www.ecolandscaping.org/04/managing-water-in-the-landscape/rain-gardens/rainwater-harvesting-a-simple-approach-to-conservation/ Kwiatkowski, 2012])
 +
*Determine the feasibility of distributed systems, which employs a combination of residential (parcel) harvesting, neighborhood harvesting, and regional harvesting, matching the system to the appropriate storage capacity ([https://iwaponline.com/bgs/article/4/1/58/89139/Multi-scale-stormwater-harvesting-to-enhance-urban Nguyen et al., 2022]).
 +
 
 +
See [[Calculating credits for stormwater and rainwater harvest and use/reuse]]
 +
 
 +
==Constructed ponds and wetlands==
 +
[[File: Photo1 of stormwater wetland.jpg|right|thumb|300 px|alt=This photo shows an example of a stormwater wetland|<font size=3>Example of a stormwater wetland in a suburban area.</font size>]]
 +
 
 +
Constructed ponds and wetlands capture stormwater runoff and slowly release the runoff, thus decreasing peak flows downstream of the practice. Although there may be some seepage through the bottom of the pond or wetland, and some evaporative loss, these losses are negligible. The primary benefit of these practices is therefore on rate control, which can reduce downstream flood impacts. Wet ponds and wetlands can be designed to capture runoff from large areas, thus making these practices potentially effective over large areas.
 +
 
 +
The following design considerations may increase the water quantity and hydrologic benefit of constructed ponds and wetlands.
 +
*Distribute constructed wetlands systemically throughout a watershed to increase potential for delivering networked benefits
 +
*Utilize the practice for reuse purposes, which allows for increased storage
 +
 
 +
==Swale with Check Dam (with and without underdrain)==
 +
[[File:Dry swale.jpg|300 px|thumb|alt=photo of a dry swale|<font size=3>Photo of a dry swale. Courtesy of Limnotech.</font size>]]
 +
[[File:Wet swale.jpg|300 px|thumb|alt=photo of a wet swale|<font size=3>Photo of a wet swale. Courtesy of Limnotech.</font size>]]
 +
[[File:Step pool.jpg|300px|thumb|alt=image of step pool|<font size=3>Stormwater step pool. Courtesy of Limnotech.</font size>]]
 +
 
 +
The water quantity and hydrologic benefits of swales depends on the type of swale and the presence or absence of impermeable <span title="A check dam is a structure installed perpendicular to flow in a natural or manmade conveyance channel to reduce flow velocity. By slowing flow velocities, check dams can serve multiple functions including reduction of channel scour and erosion, enhancement of sediment trapping, and greater treatment of the water quality control volume via enhanced water detention or retention. Typical check dam materials include rock, earth, wood, and concrete. "> '''check dams'''</span>. Swales without check dams convey stormwater and therefore provide limited hydrologic benefit, though on flatter slopes they provide some rate control.
 +
*Impermeable check dams can be employed for dry swales and stormwater step pools on permeable soils. Water stored behind the check dams will infiltrate into the underlying soil.
 +
*On soils with lower permeability, permeable check dams may be employed to slow water movement and provide some rate control
 +
*Some infiltration may occur on permeable soils when check dams are not employed
 +
 
 +
The following design considerations may potentially increase the water quantity and hydrologic benefits of swales.
 +
*If underlying soils are permeable, incorporate impermeable check dams into the design to promote infiltration.
 +
*For wet swales incorporate permeable check dams to slow water movement (rate control)
 +
*Engineered media may be used in some designs to increase water storage. This stored water may infiltrate or be utilized by plants.
 +
 
 +
See [[Calculating credits for dry swale (grass swale)]], [[Calculating credits for high-gradient stormwater step-pool swale]], and [[Calculating credits for wet swale (wetland channel)]]
 +
 
 +
==Additional recommended reading==
 +
*[http://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2008)13:2(90) Field Performance of Bioretention: Hydrology Impacts]
 +
*[http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0000448 Bioretention Hydrologic Performance in an Urban Stormwater Network]
 +
*[http://ascelibrary.org/doi/abs/10.1061/(ASCE)EE.1943-7870.0000504 Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design]
 +
*[http://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2009)14:4(407) Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland]
 +
*[http://www.sciencedirect.com/science/article/pii/S0022169409000158 Spatio-temporal effects of low impact development practices]
 +
*[https://www3.epa.gov/region9/water/recycling/pdf/brochure.pdf Water Recycling and Re-use: The Environmental Benefits]
 +
*[https://www.epa.gov/green-infrastructure/what-green-infrastructure#rainwaterharvesting Rainwater Harvesting ]
 +
*[https://www.researchgate.net/publication/322240640_Hydrologic_and_Water_Quality_Evaluation_of_a_Permeable_Pavement_and_Biofiltration_Device_in_Series Hydrologic and Water Quality Evaluation of a Permeable Pavement and Biofiltration Device in Series]
  
 
===References===
 
===References===
# https://www.ncbi.nlm.nih.gov/pubmed/24569270
+
*Bollman, M.A., G.E. DeSantis, R.M. DuChanois, M. Etten-Bohm, D.M. Olszyk, J.G. Lambrinos, P.M. Mayera. 2019. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6997945/ A framework for optimizing hydrologic performance of green roof media]. Ecol Eng. 140:1–105589. doi: 10.1016/j.ecoleng.2019.105589.
# https://www.michigan.gov/documents/deq/Hydrologic_Performance_of_Vegetated_Roofs_-_Carpenter_457042_7.pdf
+
*Carpenter, D. 2016. [https://www.mi-wea.org/docs/Carpenter_-_Hydrologic_Design_of_Vegetated_Roofs.pdf Hydrologic Design of Vegetated Roofs]. 20 June 2016 MWEA Annual Conference.
# https://www.cwp.org/wp-content/uploads/2017/01/cwp_rr_jan17.7.pdf
+
*Center for Watershed Protection. 2017. [https://www.cwp.org/wp-content/uploads/2017/01/cwp_rr_jan17.7.pdf Runoff Rundown]. ISSUE 66, JANUARY 2017.
# http://chesapeakestormwater.net/wp-content/uploads/downloads/2012/01/JHE20Davios20Bioretention20Paper1.pdf
+
*Center for Watershed Protection. 2005. [https://www.csu.edu/cerc/documents/UrbanWatershedForestryMaual-Part2-ConservingandPlantinTreesatDevelopmentSites.pdf URBAN WATERSHED FORESTRY MANUAL Part 2: Conserving and Planting Trees at Development Sites]
# https://web.sbe.hw.ac.uk/staffprofiles/bdgsa/11th_International_Conference_on_Urban_Drainage_CD/ICUD08/pdfs/211.pdf
+
*Hunt, W.F. and W.G. Lord. 2006. Bioretention Performance, Design, and Construction and Maintenance. North Carolina Cooperative Extension Service.
# https://www.usgs.gov/science/evaluating-potential-benefits-permeable-pavement-quantity-and-quality-stormwater-runoff?qt-science_center_objects=0#qt-science_center_objects
+
*Hunt, W., Davis, A., and Traver, R. 2012. ''Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design''. J. Environ. Eng., 138(6), 698–707. doi: 10.1061/(ASCE)`EE.1943-7870.0000504
 +
*Kwiatkowski, P. 2012. [https://www.ecolandscaping.org/04/managing-water-in-the-landscape/rain-gardens/rainwater-harvesting-a-simple-approach-to-conservation/ Rainwater Harvesting: A Simple Approach to Conservation]. Ecological Landscape Alliance.
 +
*Nnadi, E.O., S.J. Coupe, L.A. Sañudo-Fontaneda, and J. Rodriguez-Hernandez. 2014. ''An evaluation of enhanced geotextile layer in permeable pavement to improve stormwater infiltration and attenuation''. International Journal of Pavement Engineering, Volume 15, - Issue 10.
 +
*Nguyen, T.T., P.M. Bach, and M. Pahlow. 2022. [https://iwaponline.com/bgs/article/4/1/58/89139/Multi-scale-stormwater-harvesting-to-enhance-urban Multi-scale stormwater harvesting to enhance urban resilience to climate change impacts and natural disasters]. Blue-green Systems, Volume 4, Issue 1.
 +
*Selbig, W.R. 2019. [https://www.usgs.gov/science/evaluating-potential-benefits-permeable-pavement-quantity-and-quality-stormwater-runoff?qt-science_center_objects=0#qt-science_center_objects Evaluating the potential benefits of permeable pavement on the quantity and quality of stormwater runoff]. United States Geological Survey.
 +
*Yanling, L., R.W. Babcock Jr. 2014. ''Green roof hydrologic performance and modeling: a review'' Water Sci Technol., 69(4):727-38. doi: 10.2166/wst.2013.770.
 +
 
 +
[[Category:Level 2 - Management/Green infrastructure]]

Latest revision as of 22:10, 16 February 2023

image

This page provides information on the water quantity and hydrology benefits of green stormwater infrastructure (GSI) practices ( best management practices or bmps). The water quantity benefit of a practice is defined by its ability to retain runoff or detain and slowly release runoff. These benefits include the following.

  • Decreased downstream flooding as water is either captured or slowly released to reduce peak runoff volumes (rate control). These reductions in downstream runoff volume also reduce impacts to receiving waters by reducing erosion and protecting habitat.
  • Increased groundwater recharge, which can lead to improved baseflow and deep aquifer recharge
  • Reductions in downstream runoff volumes also provide additional benefits, such as reduced erosion.

Water quantity benefits vary between each practice, primarily as a result of the mechanism by which stormwater runoff is captured and by its ultimate fate.

  • Constructed ponds ( wet ponds) and wetlands ( stormwater wetlands) temporarily capture and store water, releasing it slowly. This decreases peak discharges downstream. There is some water loss through seepage and e evapotranspiration, but these losses are considered negligible. See Calculating credits for stormwater ponds. For more information on sedimentation processes, link here.
  • Filtration practices include bmps that have an underdrain ( biofiltration, permeable pavement, tree trench, swales, green roofs, and media filters) or bmps that provide some water retention in soil or engineered media ( vegetated filter strips, swales, green roofs). In practices with underdrains, there is always some infiltration below the underdrain, provided the practice does not have an impermeable liner. The annual volume lost depends on the position of the underdrain and the infiltration characteristics of the underlying soil. In vegetated practices with underdrains, there is also water loss through evapotranspiration. Annual losses in these practices are typically in the 5 to 20 percent range. Green roofs are effective at retaining water, while volume reduction in swales and filter strips is generally small.
  • Infiltration practices retain all water captured by the practice. This captured water is infiltrated through the soil, vadose zone, and into groundwater.
Practice Water quanity benefit Notes
Bioretention
Benefit is high for infiltration, low for filtration
Tree trench and tree box
Benefit is high for infiltration, low to moderate for filtration
Green roof
Benefit is associated with rate control; effectiveness increases with media thickness
Vegetated swale
Benefit is high for infiltration, low for filtration
Vegetated filter strip
Permeable pavement
Benefit is high for infiltration, low for filtration
Constructed wetland
Benefit is associated with rate control
Rainwater harvesting
Can be used on low permeability soils
Level of benefit: ◯ - none; ; - small; - moderate; - large; - very high

Green infrastructure and multiple benefits

Green infrastructure (GI) encompasses a wide array of practices, including stormwater management. Green stormwater infrastructure (GSI) encompasses a variety of practices primarily designed for managing stormwater runoff but that provide additional benefits such as habitat or aesthetic value.

There is no universal definition of GI or GSI (link here for more information). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services.

Bioretention

This is a picture of Bioretention facility in St Paul MN
Bioretention practices can be incorporated into street landscapes. Image Courtesy of Emmons & Olivier Resources, Inc.

Bioretention can be designed as an effective infiltration / recharge practice ( bioinfiltration) when parent soils have high permeability. For lower permeability soils an underdrain is typically used and some infiltration and rate control can be achieved.

Bioinfiltration practices, which are bioretention practices with no underdrain and designed to infiltrate water, are effective at reducing runoff volume and peak rates for small and medium intensity storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on hydrologic soil group B soils, for example, will capture about 90 percent of annual runoff.

Effects of bioinfiltration on groundwater hydrology are poorly understood. It is often assumed increased recharge will result in increased aquifer recharge and/or increased baseflow to surface waters, but limited data exists to support these assumptions.

The following design considerations can increase the water quantity/hydrologic benefits of bioretention practices.

  • Maximize infiltration through sizing and identifying the most permeable soils on a site
  • Utilize internal water storage
  • Maximize water storage in engineered media
  • Bioinfiltration practices scattered throughout a watershed can reduce runoff volumes and peak runoff

See Calculating credits for bioretention and Hunt et al (2009; 2012).

Tree Trench

photo of completed project, Marquette Avenue, Minneapolis
Completed tree system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.

Tree trenches are a type of bioretention practice. However, their potential benefits for water quantity and hydrology are greater than for traditional bioretention practices due to their ability to treat runoff from larger areas, hence more runoff volume, because of the enhanced evapotranspiration provided by trees, and due to interception of rainfall by tree canopies. On higher permeability soils they can be designed as infiltration practices, while on lower permeability soils they typically have an underdrain.

Tree trenches with no underdrain are effective at reducing runoff volume and peak rates for small and medium intensity storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on B soils, for example, will capture about 90 percent of annual runoff.

Reduction in runoff due to trees is highly variable and depends on many factors such as, but not limited to, tree characteristics, planting configuration, soil conditions, and precipitation event characteristics. For examples, studies show that annual rainfall interception by urban trees and forests can range from 6% to 66%, while urban trees can transpire from 0.2 to 46.7 gallons per tree per day (Center for Watershed Protection, 2005).

The following design considerations may increase the water quantity and hydrology benefits of tree-based practices.

  • Maximize infiltration by designing larger practices, if feasible, and identifying the most permeable soils on a site
  • Utilize internal water storage
  • Maximize water storage in media
  • Select species appropriate for the local current and future water conditions, which typically combine high rates of biomass accumulation with high evapotranspiration.

See Calculating credits for tree trenches and tree boxes

Permeable pavement

This photo illustrates porous asphalt. Porous asphalt is standard hot-mix asphalt that allows water to drain through it.
Photo illustrating porous asphalt. Porous asphalt is standard hot-mix asphalt that allows water to drain through it.

Permeable pavement provides reduction in total water volume movement and retardation of peak flow from rainfall events. When constructed without an underdrain (as an infiltration practice), they provide significant volume reduction if properly maintained, though they are typically small in size and therefore treat runoff from limited areas. When an underdrain is incorporated into the design, they provide rate control by temporarily storing and infiltrating water before the water is captured by the underdrain.

Permeable pavement can be used in conjunction with other GSI practices, such as bioretention and harvest and reuse systems.

The following design consideration may increase the water quantity and hydrology benefits of permeable pavement practices.

  • Ensure the subgrade is flat. Since roads are typically sloped, utilize terracing in the subgrade to achieve flat slopes. See page 5 of the North Carolina design guidance.
  • Incorporate signage into the design to ensure maintenance activities do not affect the infiltration properties of the pavement.
  • Design to maximize retention time and prevent short-circuiting. Storage may be increased by use of geotextile subgrades. An example is presented by Nnadi et al, (2014).
  • Plan for the expected loading on the permeable pavement and ensure capabilities and reduce compaction or clogging
  • Use in conjunction with other treatments to establish a treatment train or reuse water on site

See Calculating credits for permeable pavement

Green Roofs

image of target center green roof, Minneapolis, MN
Vegetation on the Target Center Arena green roof. vegetation consisted of a pregrown Sedum mat supplemented with 22 species of plugs and 16 species of seed native to Minnesota’s bedrock bluff prairies. Image Courtesy of The Kestrel Design Group, Inc.

Green roofs have the ability to store significant amounts of water in their growing media. Water can evaporate from the soil and be transpired by the vegetation and plants growing in the media. As a result, runoff from the roof is reduced which decreases flow to storm sewer systems and can ultimately reduce overflow events and flooding events. This can be beneficial especially in highly developed urban areas where stormwater is conveyed to storm sewer systems that can frequently experience combined sewer overflows.

The selection of growth medium, thickness of the growth medium, plant species, and roof slope can have a significant effects on the effectiveness and performance of a green roof for hydrologic purposes. Green roofs can generally be categorized as either intensive or extensive. Intensive roofs have a soil depth of 6 inches or greater, whereas extensive roofs have less than 6 inches of soil depth (Carpenter, 2016).

The following design considerations may increase the water quantity and hydrologic benefits of green roofs.

  • Increase media depth to extent economically feasible
  • Maximize the sorptive and retention properties of the media. Increasing the organic fraction can increase water retention but may result in phosphorus export and other concerns if drying occurs. Additives such as perlite and pumice increase water retention. See Bollman et al., 2019)
  • Select plants with high evapotranspiration rates, keeping mind the harshness of green roof microclimates
  • Combine green roofs with other treatment practices, such as harvest and reuse systems and roof disconnection

See Calculating credits for green roofs

Water Re-use and Harvesting

Photo of harvest and use system Woodbury
Photo of storage pond used for irrigation. Image courtesy Emmons and Olivier Resources.

Water re-use and harvesting systems help capture rainfall and minimize stormwater runoff volumes and rates to receiving stormwater sewer systems and conveyances. Harvest and reuse systems capture water and increase a user’s water supply that can be stored and used on site, immediately or in the future, in lieu of the local water supply, including use of potable water. This can be particularly advantageous in times of droughts when water may not be readily available. These systems also help alleviate pressure on the local water supply and allows communities to allocate water to other users. Reusing rainwater for irrigation purposes can also help increase groundwater recharge.

The potential benefits delivered by a harvest and reuse system depend on the type of system. Practices utilizing storage tanks typically have limited storage capacity and are used for site applications. Constructed ponds and wetlands have much greater storage capacity and can be used in larger applications. See Design considerations for constructed stormwater ponds used for harvest and irrigation use/reuse.

The following design considerations may increase the potential water quantity and hydrology benefits of harvest and reuse systems.

  • Consider using harvesting on low permeability soils ( hydrologic soil group C and D soils), where captured water can be distributed through irrigation and/or used indoors.
  • Size the system to meet the intended uses of the harvest system. This includes ensuring appropriate water supply in response to demand. See Determining the appropriate storage size for a stormwater and rainwater harvest and use/reuse system and Design criteria for stormwater and rainwater harvest and use/reuse.
  • Construct the distribution system to reach all areas of the site that require water when economically feasible
  • Determine the sites’ water needs for vegetation and plant systems over a given time period and design the water storage container to meet these needs. Typically, harvest systems are designed and built with a specific use and vegetation scheme in mind, but if feasible, consider adopting vegetation to an intended harvest system (i.e. if the objective is driven by performance goals for the harvest system, vegetation considerations should be built into the design considerations).
  • Utilize tandem systems which combine multiple storage units, such as multiple rain barrels in sequence (Kwiatkowski, 2012)
  • Determine the feasibility of distributed systems, which employs a combination of residential (parcel) harvesting, neighborhood harvesting, and regional harvesting, matching the system to the appropriate storage capacity (Nguyen et al., 2022).

See Calculating credits for stormwater and rainwater harvest and use/reuse

Constructed ponds and wetlands

This photo shows an example of a stormwater wetland
Example of a stormwater wetland in a suburban area.

Constructed ponds and wetlands capture stormwater runoff and slowly release the runoff, thus decreasing peak flows downstream of the practice. Although there may be some seepage through the bottom of the pond or wetland, and some evaporative loss, these losses are negligible. The primary benefit of these practices is therefore on rate control, which can reduce downstream flood impacts. Wet ponds and wetlands can be designed to capture runoff from large areas, thus making these practices potentially effective over large areas.

The following design considerations may increase the water quantity and hydrologic benefit of constructed ponds and wetlands.

  • Distribute constructed wetlands systemically throughout a watershed to increase potential for delivering networked benefits
  • Utilize the practice for reuse purposes, which allows for increased storage

Swale with Check Dam (with and without underdrain)

photo of a dry swale
Photo of a dry swale. Courtesy of Limnotech.
photo of a wet swale
Photo of a wet swale. Courtesy of Limnotech.
image of step pool
Stormwater step pool. Courtesy of Limnotech.

The water quantity and hydrologic benefits of swales depends on the type of swale and the presence or absence of impermeable check dams. Swales without check dams convey stormwater and therefore provide limited hydrologic benefit, though on flatter slopes they provide some rate control.

  • Impermeable check dams can be employed for dry swales and stormwater step pools on permeable soils. Water stored behind the check dams will infiltrate into the underlying soil.
  • On soils with lower permeability, permeable check dams may be employed to slow water movement and provide some rate control
  • Some infiltration may occur on permeable soils when check dams are not employed

The following design considerations may potentially increase the water quantity and hydrologic benefits of swales.

  • If underlying soils are permeable, incorporate impermeable check dams into the design to promote infiltration.
  • For wet swales incorporate permeable check dams to slow water movement (rate control)
  • Engineered media may be used in some designs to increase water storage. This stored water may infiltrate or be utilized by plants.

See Calculating credits for dry swale (grass swale), Calculating credits for high-gradient stormwater step-pool swale, and Calculating credits for wet swale (wetland channel)

Additional recommended reading

References

This page was last edited on 16 February 2023, at 22:10.