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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]. | *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]. | ||
− | *<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). | + | *<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 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. |
− | *<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 | + | *<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;" | ||
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! Practice !! Water quality benefit !! Notes | ! Practice !! Water quality benefit !! Notes | ||
|- | |- | ||
− | | Bioretention || <font size=6><center>◕</center></font size> || | + | | Bioretention || <font size=6><center>◕</center></font size> || Benefit is high for infiltration, low for filtration |
|- | |- | ||
− | | Tree trench and tree box || <font size=6><center>◕</center></font size> || | + | | Tree trench and tree box || <font size=6><center>◕</center></font size> || Benefit is high for infiltration, low to moderate for filtration |
|- | |- | ||
− | | Green roof || <font size=4><center>◕</center></font size> || | + | | Green roof || <font size=4><center>◕</center></font size> || Benefit is associated with rate control; effectiveness increases with media thickness |
|- | |- | ||
− | | Vegetated swale || <font size=4><center>◔</center></font size> || | + | | Vegetated swale || <font size=4><center>◔</center></font size> || Benefit is high for infiltration, low for filtration |
|- | |- | ||
− | | Vegetated filter strip || <font size=4><center>◔</center></font size> || | + | | Vegetated filter strip || <font size=4><center>◔</center></font size> || |
|- | |- | ||
− | | Permeable pavement || <font size=4><center>◕</center></font size> || | + | | Permeable pavement || <font size=4><center>◕</center></font size> || Benefit is high for infiltration, low for filtration |
|- | |- | ||
− | | Constructed wetland</td> || <font size=4><center>◑</center></font size> || | + | | Constructed wetland</td> || <font size=4><center>◑</center></font size> || Benefit is associated with rate control |
|- | |- | ||
|Rainwater harvesting || <font size=4><center>◕</center></font size> || Can be used on low permeability soils | |Rainwater harvesting || <font size=4><center>◕</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]]. | ||
− | + | ==Bioretention== | |
This page provides information on the water quantity and hydrology benefits of green stormwater infrastructure (GSI) practices ( best management practices). 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.
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.
Practice | Water quality 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 (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.
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.
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 roofs1. 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 depth2. One experimental case study that implemented an extensive roof in Michigan showed a peak discharge reduction of 54% to 99%.
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 day3. 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.
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%4. 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. 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.”5
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 systems6. 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.
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.