m |
m |
||
Line 9: | Line 9: | ||
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 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]. | *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). 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="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 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. | *<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. | ||
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.
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.
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 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.
See Calculating credits for bioretention and Hunt et al (2009; 2012).
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.
See Calculating credits for tree trenches and tree boxes
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.
See Calculating credits for permeable pavement
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.
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.
See Calculating credits for green roofs
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.
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.
The following design considerations may increase the potential water quantity and hydrology benefits of harvest and reuse systems.
See Calculating credits for stormwater and rainwater harvest and use/reuse
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.
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. When impermeable check dams are employed, they can provide effective rate control if the check dams are permeable, or retention through infiltration if check dams are impermeable.
The following design considerations may potentially increase the water quantity and hydrologic benefits of swales.
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)