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[[File:Stillwater rain garden 1.JPG|thumb|300px|alt=photo showing a rain garden in a residential area|<font size=3>Photo showing a bioretention facility (rain garden) in a residential area. note the curb cut that allows water to enter the facility.</font size>]]
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[[File:Pdf image.png|100px|thumb|alt=pdf image|<font size=3>[https://stormwater.pca.state.mn.us/index.php?title=File:Types_of_bioretention_-_Minnesota_Stormwater_Manual.pdf Download pdf]</font size>]]
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[[File:General information page image.png|right|100px|alt=image]]
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[[File:Stillwater rain garden 1.JPG|thumb|300px|alt=photo showing a rain garden in a residential area|<font size=3>A bioretention facility (rain garden) in a residential area. Note the curb cut that allows water to enter the facility.</font size>]]
  
As bioretention becomes a more common tool in the stormwater management toolbox and as the number of design variants increases, so does the number of names for each of these variants. As an example of the ongoing evolution of bioretention terminology, the terms “rain garden” and “rainwater garden” have recently caught on with the public and are being used interchangeably with bioretention. In most instances, rain garden designs are utilizing the processes of bioretention, but the term rain garden is also being loosely used to describe BMPs that are operating more as stormwater ponds (or as other BMPs) than as bioretention facilities.
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{{alert|The information on this page was updated in 2015 and placed on a new page. We recommend you use the information from the new page, which is called [[Bioretention terminology]]|alert-info}}
  
Further confusion stems from the using the terms “process” and “practice” interchangeably. As mentioned earlier, bioretention is not a “practice” per se, but rather a process or group of processes that can be incorporated into many different practices. This section is provided to clarify the more common bioretention terminology being used in the field of stormwater management today.
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As <span title="Bioretention, also called rain gardens, is a terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Bioretention employs a simplistic, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation. Bioretention areas are suitable stormwater treatment practices for all land uses, as long as the contributing drainage area is appropriate for the size of the facility. Common bioretention opportunities include landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage and street-scapes (i.e., between the curb and sidewalk). Bioretention, when designed with an underdrain and liner, is also a good design option for treating Potential stormwater hotspots. Bioretention is extremely versatile because of its ability to be incorporated into landscaped areas. The versatility of the practice also allows for bioretention areas to be frequently employed as stormwater retrofits."> '''bioretention'''</span> becomes a more common tool in the stormwater management toolbox and as the number of design variants increases, so does the number of names for each of these variants. As an example of the ongoing evolution of bioretention terminology, the terms “rain garden” and “rainwater garden” are used interchangeably with bioretention. In most instances, rain garden designs are utilizing the processes of bioretention, but the term rain garden is also being loosely used to describe <span title="One of many different structural or non–structural methods used to treat runoff"> '''best management practices'''</span> (bmps) that are operating more as stormwater ponds (or as other BMPs) than as bioretention facilities.
  
==Performance Types (adapted from [[References for bioretention|Prince George’s County]], 2002)==
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Further confusion stems from the using the terms “process” and “practice” interchangeably. As mentioned earlier, bioretention is not a “practice” per se, but rather a process or group of processes that can be incorporated into many different practices. This section provides clarity the more common bioretention terminology being used in the field of stormwater management today.
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==General discussion of types of bioretention BMPs==
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This section provides an overview of the general types of bioretention practices.
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===Bioinfiltration with no underdrain===
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[[File:Bioinfiltration.png|thumb|300px|alt=image of bioinfiltration device|<font size=3>Bioinfiltration device</font size>]]
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<span title="A bioretention practice in which no underdrain is used. All water entering the bioinfiltration practice infiltrates or evapotranspires."> '''Bioinfiltration'''</span> is suitable for areas where significant recharge of groundwater is possible and would be beneficial. Because there is no <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrain'''</span> the in-situ soils need to have a high infiltration rate to accommodate the inflow levels. The <span title="The infiltration rate is the velocity or speed at which water enters into the soil"> '''infiltration rate'''</span> of the in-situ soils must be determined through proper [[Design criteria for bioretention|soil testing/diagnostics]]. The ''Recommended'' filter media <span title="Engineered media is a mixture of sand, fines (silt, clay), and organic matter utilized in stormwater practices, most frequently in bioretention practices. The media is typically designed to have a rapid infiltration rate, attenuate pollutants, and allow for plant growth."> [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_bioretention#Materials_specifications_-_filter_media ('''engineered media''')]</span> depth is 2.5 feet or more to allow adequate <span title="Filtration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium, such as sand or an organic material. They are generally used on small drainage areas (5 acres or less) and are primarily designed for pollutant removal. They are effective at removing total suspended solids (TSS), particulate phosphorus, metals, and most organics. They are less effective for soluble pollutants such as dissolved phosphorus, chloride, and nitrate."> [https://stormwater.pca.state.mn.us/index.php?title=Filtration '''filtration''']</span> processes to occur.  Most [[Design criteria for bioretention#Materials specifications - filter media|media mixes]] are suitable because phosphorus is not a significant concern with this practice. The [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_program Construction General Permit] requires that water captured by the BMP be drawn down within 48 hours. <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> A and B soils are commonly suitable for bioinfiltration. Bioinfiltration is suitable for areas and land uses that are expected to generate nutrient runoff (e.g. residential and business campuses) that can be infiltrated and captured by the practice. Fresh mulch rather then aged shredded bark mulch can be used to enhance <span title="The loss or removal of nitrogen or nitrogen compounds specifically: reduction of nitrates or nitrites commonly by bacteria (as in soil) that usually results in the escape of nitrogen into the air.> '''denitrification'''</span> processes if nitrate leaching is a concern. Bioinfiltration is not recommended for <span title="Stormwater Hotspots (PSHs) are activities or practices that have the potential to produce relatively high levels of stormwater pollutants"> '''[https://stormwater.pca.state.mn.us/index.php?title=Potential_stormwater_hotspots potential stormwater hotspots]'''</span>. Other [[Stormwater infiltration and constraints on infiltration|infiltration constraints]] apply to bioinfiltration practices.
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===Biofiltration with underdrain at bottom===
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[[File:Biofiltration with underdrain at bottom.png|thumb|300px|alt=image of biofiltration device with an underdrain at the bottom|<font size=3>Biofiltration device with an underdrain at the bottom</font size>]]
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This bioretention practice is designed with an underdrain at the <span title="Invert level is the base interior level of a pipe, trench or tunnel; it can be considered the "floor" level"> '''invert'''</span> of the planting soil mix to ensure drainage at a desired rate. The practice allows for partial recharge and an impervious liner is not used. The depth is also shallow (2.5 feet) to allow high capacity flows if necessary. Siting is suitable for visually prominent or gateway locations in a community. The practice is suitable for areas and land uses that are expected to generate metals loadings (e.g. residential, business campus, or parking lots).  The practice is suitable for areas with high nutrient loadings provided the media has a low phosphorus concentration or phosphorus-sorbing amendments are used (see [[Design criteria for bioretention#Materials specifications - filter media|section on filter media]]). This type of facility is also recommended for soils where infiltration is limited (C and D soils). Some volume reduction will be seen from <span title="Loss of water to the atmosphere as a result of the joint processes of evaporation and transpiration through vegetation"> '''evapotranspiration'''</span> and partial infiltration below the underdrain.
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===Biofiltration with elevated underdrain===
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[[File:Biofiltration with elevated underdrain.png|thumb|left|300px|alt=image of biofiltration device with elevated underdrain|<font size=3>Biofiltration device with elevated underdrain</font size>]]
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A biofiltration practice with a raised underdrain provides a storage area below the invert of the underdrain discharge pipe. This area provides a recharge zone and quantity control can also be augmented with this storage area. The storage area is equal to the void space of the material used. Since the practice utilizes both infiltration and an underdrain, considerations include those for both bioinfiltration practices and biofiltration with an underdrain at the bottom.  These include an assessment of [[Stormwater infiltration and constraints on infiltration|infiltration constraints]] and [[design criteria for bioretention#Materials specifications - filter media|media]].
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===Biofiltration with a liner (at bottom and or on sides)===
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[[File:Bioretention with liner.png|thumb|300px|alt=image of bioretention device with a liner|<font size=3>Biofiltration device with a liner</font size>]]
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This type of facility is recommended for areas that are known as [[Potential stormwater hotspots|potential stormwater hot spots]] (e.g. gas stations, transfer sites, and transportation depots). An important feature of this type of facility is the impervious liner designed to reduce or eliminate the possibility of groundwater contamination. The facility provides a level of treatment strictly through filtration processes that occur when the runoff moves through the soil material to the underdrain discharge point. In the event of an accidental spill, the underdrain can be blocked and the objectionable materials siphoned through an observation well and safely contained.
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===Biofiltration with internal water storage===
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[[File:Biofiltration with internal water storage.png|thumb|300px|alt=image of biofiltration device with internal water storage|<font size=3>Biofiltration device with internal water storage</font size>]]
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The biofiltration practice with internal water storage is not commonly used in the Midwestern United States but is widely used in some places on the east coast, such as North Carolina.  The use of an upturned elbow in this practice allows water to be retained within the practice, leading to increased pollutant removal, increased infiltration, and increased evapotranspiration.  The practice is particularly effective at removing nitrogen through denitrification. The media should be 3 feet or more thick to allow water to be drawn down below the root zone. Underlying soils should be permeable enough to allow water stored within the practice to infiltrate.
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==Performance types (adapted from Prince George’s County, 2002)==
 
The following facility performance types have been slightly modified for Minnesota to optimize the expected or anticipated pollutant loadings based on the proposed land use. All of these facilities may be used as high-hydraulic-capacity filtration systems. High-hydraulic-capacity filtration systems are defined as systems that are composed of essentially a shallow sandy soil mix, thick layer of mulch and an underdrain/gravel discharge system.
 
The following facility performance types have been slightly modified for Minnesota to optimize the expected or anticipated pollutant loadings based on the proposed land use. All of these facilities may be used as high-hydraulic-capacity filtration systems. High-hydraulic-capacity filtration systems are defined as systems that are composed of essentially a shallow sandy soil mix, thick layer of mulch and an underdrain/gravel discharge system.
  
[[file:Infiltration recharge facility.jpg|thumb|300px|alt=schematic showing an infiltration recharge facility|<font size=3>Schematic showing an infiltration partial recharge facility. Note there is no underdrain or liner and the underlying soil has a high infiltration rate. (Source: [[References for bioretention|Prince George’s County Bioretention Manual]], 2002)</font size>]]
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[[file:Infiltration recharge facility.jpg|thumb|300px|alt=schematic showing an infiltration recharge facility|<font size=3>An infiltration recharge facility. Note there is no underdrain or liner and the underlying soil has a high infiltration rate. (Source: [[References for bioretention|Prince George’s County Bioretention Manual]], 2002)</font size>]]
===Infiltration / Recharge Facility===
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===Infiltration / recharge facility===
 
This type of facility is suitable for areas where significant recharge of groundwater is possible and would be beneficial. Because there is no underdrain, the in-situ soils need to have a high infiltration rate to accommodate the inflow levels. The infiltration rate of the in-situ soils must be determined through proper soil testing/diagnostics. Preferably, facilities of this type should have infiltration rates of 1 inch per hour or greater. Facility filter media depth must be at least 2.5 feet to allow adequate filtration processes to occur. Siting of these facilities should be in areas where visibility is not a concern because hydraulic overload can cause extended periods of standing water conditions, although the [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html CGP] requires that the water quality volume (see [[Unified sizing criteria]]) be drawn down within 48 hours. This facility type is suitable for areas and land uses that are expected to generate nutrient runoff (e.g. residential and business campuses) that can be infiltrated and captured by the facility. Fresh mulch rather then aged shredded bark mulch can be used to enhance [[Glossary#D|denitrification]] processes.
 
This type of facility is suitable for areas where significant recharge of groundwater is possible and would be beneficial. Because there is no underdrain, the in-situ soils need to have a high infiltration rate to accommodate the inflow levels. The infiltration rate of the in-situ soils must be determined through proper soil testing/diagnostics. Preferably, facilities of this type should have infiltration rates of 1 inch per hour or greater. Facility filter media depth must be at least 2.5 feet to allow adequate filtration processes to occur. Siting of these facilities should be in areas where visibility is not a concern because hydraulic overload can cause extended periods of standing water conditions, although the [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html CGP] requires that the water quality volume (see [[Unified sizing criteria]]) be drawn down within 48 hours. This facility type is suitable for areas and land uses that are expected to generate nutrient runoff (e.g. residential and business campuses) that can be infiltrated and captured by the facility. Fresh mulch rather then aged shredded bark mulch can be used to enhance [[Glossary#D|denitrification]] processes.
  
[[file:Filtration partial recharge facility.jpg|thumb|300px|alt=schematic showing a filtration partial recharge facility|<font size=3>Schematic showing a filtration partial recharge facility. Note the underdrain at the bottom of the facility. (Source: [[References for bioretention|Prince George’s County Bioretention Manual]], 2002)</font size>]]
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[[file:Filtration partial recharge facility.jpg|thumb|300px|alt=schematic showing a filtration partial recharge facility|<font size=3>A filtration partial recharge facility. Note the underdrain at the bottom of the facility. (Source: [[References for bioretention|Prince George’s County Bioretention Manual]], 2002)</font size>]]
===Filtration/Partial Recharge Facility===
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===Filtration/partial recharge facility===
 
This type of facility is suitable for areas where high filtration and partial recharge of runoff would be beneficial. This facility is designed with an underdrain at the invert of the planting soil mix to ensure that the facility drains at a desired rate. The facility allows for partial recharge and an impervious liner is not used. The depth is also shallow (2.5 feet) to allow the facility to handle high capacity flows if necessary. Siting of this performance type is suitable for visually prominent or gateway locations in a community. The facility type is suitable for areas and land uses that are expected to generate nutrient and metals loadings (e.g. residential, business campus, or parking lots). Attention to mulch type and amount will ensure the adequate treatment of the anticipated loadings. The facility shown in the schematic on the right incorporates a filter material between the gravel blanket the side walls. The filter fabric may be installed horizontally above the gravel blanket, extending just 1 to 2 feet on either side of the underdrain pipe below. Do not wrap the underdrain with filter fabric. Instead of using a filter fabric, the designer may opt to utilize a pea gravel diaphragm over the underdrain gravel blanket. This type of facility is also recommended for tight impermeable soils where infiltration is limited. Some volume reduction will be seen from evapotranspiration.
 
This type of facility is suitable for areas where high filtration and partial recharge of runoff would be beneficial. This facility is designed with an underdrain at the invert of the planting soil mix to ensure that the facility drains at a desired rate. The facility allows for partial recharge and an impervious liner is not used. The depth is also shallow (2.5 feet) to allow the facility to handle high capacity flows if necessary. Siting of this performance type is suitable for visually prominent or gateway locations in a community. The facility type is suitable for areas and land uses that are expected to generate nutrient and metals loadings (e.g. residential, business campus, or parking lots). Attention to mulch type and amount will ensure the adequate treatment of the anticipated loadings. The facility shown in the schematic on the right incorporates a filter material between the gravel blanket the side walls. The filter fabric may be installed horizontally above the gravel blanket, extending just 1 to 2 feet on either side of the underdrain pipe below. Do not wrap the underdrain with filter fabric. Instead of using a filter fabric, the designer may opt to utilize a pea gravel diaphragm over the underdrain gravel blanket. This type of facility is also recommended for tight impermeable soils where infiltration is limited. Some volume reduction will be seen from evapotranspiration.
  
[[File:Infiltration filtration recharge facility.jpg|thumb|300px|alt=schematic showing an infiltration filtration recharge facility|<font size=3>Schematic showing an infiltration filtration recharge facility. Note the use of a raised underdrain. (Source: [[References for bioretention|Prince George's County Bioretention Manual], 2002)</font size>]]
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[[File:Infiltration filtration recharge facility.jpg|thumb|300px|alt=schematic showing an infiltration filtration recharge facility|<font size=3>An infiltration filtration recharge facility. Note the use of a raised underdrain. (Source: [[References for bioretention|Prince George's County Bioretention Manual]], 2002)</font size>]]
===Infiltration/Filtration/Recharge===
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===Infiltration/filtration/recharge===
 
This type of facility is recommended for areas where higher nutrient loadings, particularly nitrates, are anticipated. The facility is designed to incorporate a fluctuating aerobic/anaerobic zone below the raised underdrain discharge pipe. This fluctuation created by saturation and infiltration into the surrounding soils will achieve denitrification. With a combination of a fresh mulch covering, nitrates will be mitigated through the enhancement of natural denitrification processes. This type of facility would be suitable for areas where nitrate loadings are typically elevated, such as in residential communities. The raised underdrain has the effect of providing a storage area below the invert of the underdrain discharge pipe. This area provides a recharge zone and quantity control can also be augmented with this storage area. The storage area is equal the void space of the material used.
 
This type of facility is recommended for areas where higher nutrient loadings, particularly nitrates, are anticipated. The facility is designed to incorporate a fluctuating aerobic/anaerobic zone below the raised underdrain discharge pipe. This fluctuation created by saturation and infiltration into the surrounding soils will achieve denitrification. With a combination of a fresh mulch covering, nitrates will be mitigated through the enhancement of natural denitrification processes. This type of facility would be suitable for areas where nitrate loadings are typically elevated, such as in residential communities. The raised underdrain has the effect of providing a storage area below the invert of the underdrain discharge pipe. This area provides a recharge zone and quantity control can also be augmented with this storage area. The storage area is equal the void space of the material used.
  
[[File:Filtration only facility.jpg|thumb|300px|alt=schematic showing a filtration only facility|<font size=3>Schematic showing a filtration only facility. Note the underdrain at the bottom of the facility. (Source: Prince George's County Bioretention Manual, 2002)</font size>]]
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[[File:Filtration only facility.jpg|thumb|300px|alt=schematic showing a filtration only facility|<font size=3>A filtration only facility. Note the underdrain at the bottom of the facility. (Source: [[References for bioretention|Prince George's County Bioretention Manual]], 2002)</font size>]]
  
===Filtration Only===
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===Filtration only===
This type of facility is recommended for areas that are known as [[Potential stormwater hotspots|potential stormwater hot spots]] (e.g. gas stations, transfer sites, and transportation depots). An important feature of this type of facility is the impervious liner designed to reduce or eliminate the possibility of groundwater contamination. The facility provides a level of treatment strictly through filtration processes that occur when the runoff moves through the soil material to the underdrain discharge point. In the event of an accidental spill, the underdrain can be blocked and the objectionable materials siphoned through an observation well and safely contained.
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This type of facility is recommended for areas that are known as [[Potential stormwater hotspots|stormwater hot spots]] (e.g. gas stations, transfer sites, and transportation depots). An important feature of this type of facility is the impervious liner designed to reduce or eliminate the possibility of groundwater contamination. The facility provides a level of treatment strictly through filtration processes that occur when the runoff moves through the soil material to the underdrain discharge point. In the event of an accidental spill, the underdrain can be blocked and the objectionable materials siphoned through an observation well and safely contained.
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-->
  
==Design Types for Various Land Uses==
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==Design types for various land uses==
It should be noted that the layout of the bioretention area will vary according to individual sites, and to specific site constraints such as underlying soils, existing vegetation, drainage, location of utilities, sight distances for traffic, and aesthetics. Designers are encouraged to be creative in determining how to integrate bioretention into their respective site designs. With this in mind, the
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It should be noted that the layout of the bioretention area will vary according to individual sites, and to specific site constraints such as underlying soils, existing vegetation, drainage, location of utilities, sight distances for traffic, and aesthetics. Designers are encouraged to be creative in determining how to integrate bioretention into their respective site designs. With this in mind, the following are presented as alternative options.
following are presented as alternative options.
 
  
===On-lot / Rain garden===
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===On-lot / rain garden===
Simple design that incorporates a planting bed in the low portion of the site. On-lot systems are designed to receive flows from gutters, and/or other impervious urfaces.
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Simple design that incorporates a planting bed in the low portion of the site. On-lot systems are designed to receive flows from gutters, and/or other impervious surfaces.
  
===Parking Lot Islands (Curbless)===
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===Parking lot islands (curbless)===
 
In a paved area with no curb, pre-cast car-stops or a “ribbon curb” can be installed along the pavement perimeter to protect the bioretention area. This application of bioretention should only be attempted where shallow grades allow for sheet flow conditions over level entrance areas. Water may be pooled into the parking area where parking spaces are rarely used to achieve an element of stormwater quantity control beyond the confines of the bioretention surface area (Prince George’s County, 2002).
 
In a paved area with no curb, pre-cast car-stops or a “ribbon curb” can be installed along the pavement perimeter to protect the bioretention area. This application of bioretention should only be attempted where shallow grades allow for sheet flow conditions over level entrance areas. Water may be pooled into the parking area where parking spaces are rarely used to achieve an element of stormwater quantity control beyond the confines of the bioretention surface area (Prince George’s County, 2002).
  
[[file:Bioretention parking lot island.jpg|thumb|300px|alt=schematic showing a bioretention parking lot island|<font size=3>Schematic illustrating a bioretention parking lot island. Note the use of other BMPs, including permeable pavement and tree trenches. (Source: [[References for bioretention|Minnehaha Creek Watershed District]]</font size>]]
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[[file:Bioretention parking lot island.jpg|thumb|300px|alt=schematic showing a bioretention parking lot island|<font size=3>A bioretention parking lot island. Note the use of other BMPs, including permeable pavement and tree trenches. (Source: [http://www.minnehahacreek.org/ Minnehaha Creek Watershed District</font size>])]]
  
===Parking Lot Islands (Curb-cut)===
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===Parking lot islands (curb-cut)===
For curb-cut entrance approaches, the water is diverted into the bioretention area through the use of an inlet deflector block, which has ridges that channel the runoff into the bioretention area ([[References for bioretention|Prince George’s County]], 2002). Special attention to erosion control and pre-treatment should be given to the concentrated flow produced by curbcuts.
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For curb-cut entrance approaches, the water is diverted into the bioretention area through the use of an inlet deflector block, which has ridges that channel the runoff into the bioretention area ([https://www.slideshare.net/Sotirakou964/md-prince-georges-county-bioretention-manual Prince George’s County], 2002). Special attention to erosion control and pre-treatment should be given to the concentrated flow produced by curbcuts.
  
===Road Medians / Traffic Islands===
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===Road medians / traffic islands===
 
A multifunctional landscape can be created by utilizing road medians and islands for bioretention. There is no minimum width recommended for traffic islands from street edge to edge. A buffer may be necessary along the outside curb perimeter to minimize the possibility of drainage seeping under the pavement section, and creating “frost heave” during winter months. Alternately, the installation of a geotextile filter fabric “curtain wall” along the perimeter of the bioretention island will accomplish the same effect.
 
A multifunctional landscape can be created by utilizing road medians and islands for bioretention. There is no minimum width recommended for traffic islands from street edge to edge. A buffer may be necessary along the outside curb perimeter to minimize the possibility of drainage seeping under the pavement section, and creating “frost heave” during winter months. Alternately, the installation of a geotextile filter fabric “curtain wall” along the perimeter of the bioretention island will accomplish the same effect.
  
[[File:Tree box filter.jpg|thumb|300px|alt=schematic of a tree box filter|<font size=3>Schematic showing a tree box filter. Note the curb cuts. (Source: [[References for bioretention|Minnehaha Creek Watershed District]])</font size>]]
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===Tree pits / tree box filters===
===Tree Pits / Tree Box Filters===
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[[File:Tree box filter.jpg|thumb|300px|alt=schematic of a tree box filter|<font size=3>A tree box filter. Note the curb cuts. (Source: [http://www.minnehahacreek.org/ Minnehaha Creek Watershed District])</font size>]]
Tree Pits and Tree Box Filters afford many opportunities for bioretention. Designs vary widely from simple “tree pits”, used for local drainage interception to more formal Tree Box Filters, which are a useful tool for highly urbanized streetscapes.  
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Tree pits and tree box filters afford many opportunities for bioretention. Designs vary widely from simple “tree pits”, used for local drainage interception to more formal Tree Box Filters, which are a useful tool for highly urbanized streetscapes.  
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The tree pit technique provides very shallow ponding storage areas in a “dished” mulch area around the tree or shrub. Typically, the mulched area extends to the dripline for the tree and is similar to conventional mulching practices, except that the mulch area is depressed at least 2 to 3 inches rather than mounded around the tree ([http://www.lid-stormwater.net/ Low Impact Design Center], 2005).
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Tree box filters are bioretention areas installed beneath trees that can be very effective at controlling runoff, especially when distributed throughout the site. Runoff is directed to the tree box, where it is cleaned by vegetation and soil before entering a catch basin. The runoff collected in the tree-boxes helps irrigate the trees. The system consists of a container filled with a soil mixture, a mulch layer, under-drain system and a shrub or tree. Stormwater runoff drains directly from impervious surfaces through a filter media. Treated water flows out of the system through an underdrain connected to a storm drainpipe/inlet or into the surrounding soil. Tree box filters can also be used to control runoff volumes/flows by adding storage volume beneath the filter box with an outlet control device.
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<noinclude>
  
The Tree Pit technique provides very shallow ponding storage areas in a “dished” mulch area around the tree or shrub. Typically, the mulched area extends to the dripline for the tree and is similar to conventional mulching practices, except that the mulch area is depressed at least 2 to 3 inches rather than mounded around the tree ([[References for bioretention|Low Impact Development Center]], 2005).
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==Related pages==
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*[[Bioretention terminology]]
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*[[Overview for bioretention]]
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*[[Types of bioretention]]
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*[[Design criteria for bioretention]]
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*[[Construction specifications for bioretention]]
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*[[Operation and maintenance of bioretention]]
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*[[Cost-benefit considerations for bioretention]]
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*[[Soil amendments to enhance phosphorus sorption]]
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*[[Supporting material for bioretention]]
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*[[External resources for bioretention]]
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*[[References for bioretention]]
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*[[Requirements, recommendations and information for using bioretention BMPs in the MIDS calculator]]
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</noinclude>
  
Tree Box Filters are bioretention areas installed beneath trees that can be very effective at controlling runoff, especially when distributed throughout the site. Runoff is directed to the tree box, where it is cleaned by vegetation and soil before entering a catch basin. The runoff collected in the tree-boxes helps irrigate the trees. The system consists of a container filled with a soil mixture, a mulch layer, under-drain system and a shrub or tree. Stormwater runoff drains directly from impervious surfaces through a filter media. Treated water flows out of the system through an underdrain connected to a storm drainpipe/inlet or into the surrounding soil. Tree box filters can also be used to control runoff volumes/flows by adding storage volume beneath the filter box with an outlet control device ([[References for bioretention|Low Impact Development Center]], 2005).
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<noinclude>
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[[Category:Level 3 - Best management practices/Guidance and information/BMP types and terminology]]
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[[Category:Level 3 - Best management practices/Structural practices/Bioretention‏‎]]
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</noinclude>

Latest revision as of 18:52, 15 August 2022

image
photo showing a rain garden in a residential area
A bioretention facility (rain garden) in a residential area. Note the curb cut that allows water to enter the facility.
Information: The information on this page was updated in 2015 and placed on a new page. We recommend you use the information from the new page, which is called Bioretention terminology

As bioretention becomes a more common tool in the stormwater management toolbox and as the number of design variants increases, so does the number of names for each of these variants. As an example of the ongoing evolution of bioretention terminology, the terms “rain garden” and “rainwater garden” are used interchangeably with bioretention. In most instances, rain garden designs are utilizing the processes of bioretention, but the term rain garden is also being loosely used to describe best management practices (bmps) that are operating more as stormwater ponds (or as other BMPs) than as bioretention facilities.

Further confusion stems from the using the terms “process” and “practice” interchangeably. As mentioned earlier, bioretention is not a “practice” per se, but rather a process or group of processes that can be incorporated into many different practices. This section provides clarity the more common bioretention terminology being used in the field of stormwater management today.

General discussion of types of bioretention BMPs

This section provides an overview of the general types of bioretention practices.

Bioinfiltration with no underdrain

image of bioinfiltration device
Bioinfiltration device

Bioinfiltration is suitable for areas where significant recharge of groundwater is possible and would be beneficial. Because there is no underdrain the in-situ soils need to have a high infiltration rate to accommodate the inflow levels. The infiltration rate of the in-situ soils must be determined through proper soil testing/diagnostics. The Recommended filter media (engineered media) depth is 2.5 feet or more to allow adequate filtration processes to occur. Most media mixes are suitable because phosphorus is not a significant concern with this practice. The Construction General Permit requires that water captured by the BMP be drawn down within 48 hours. Hydrologic soil group A and B soils are commonly suitable for bioinfiltration. Bioinfiltration is suitable for areas and land uses that are expected to generate nutrient runoff (e.g. residential and business campuses) that can be infiltrated and captured by the practice. Fresh mulch rather then aged shredded bark mulch can be used to enhance denitrification processes if nitrate leaching is a concern. Bioinfiltration is not recommended for potential stormwater hotspots. Other infiltration constraints apply to bioinfiltration practices.

Biofiltration with underdrain at bottom

image of biofiltration device with an underdrain at the bottom
Biofiltration device with an underdrain at the bottom

This bioretention practice is designed with an underdrain at the invert of the planting soil mix to ensure drainage at a desired rate. The practice allows for partial recharge and an impervious liner is not used. The depth is also shallow (2.5 feet) to allow high capacity flows if necessary. Siting is suitable for visually prominent or gateway locations in a community. The practice is suitable for areas and land uses that are expected to generate metals loadings (e.g. residential, business campus, or parking lots). The practice is suitable for areas with high nutrient loadings provided the media has a low phosphorus concentration or phosphorus-sorbing amendments are used (see section on filter media). This type of facility is also recommended for soils where infiltration is limited (C and D soils). Some volume reduction will be seen from evapotranspiration and partial infiltration below the underdrain.

Biofiltration with elevated underdrain

image of biofiltration device with elevated underdrain
Biofiltration device with elevated underdrain

A biofiltration practice with a raised underdrain provides a storage area below the invert of the underdrain discharge pipe. This area provides a recharge zone and quantity control can also be augmented with this storage area. The storage area is equal to the void space of the material used. Since the practice utilizes both infiltration and an underdrain, considerations include those for both bioinfiltration practices and biofiltration with an underdrain at the bottom. These include an assessment of infiltration constraints and media.

Biofiltration with a liner (at bottom and or on sides)

image of bioretention device with a liner
Biofiltration device with a liner

This type of facility is recommended for areas that are known as potential stormwater hot spots (e.g. gas stations, transfer sites, and transportation depots). An important feature of this type of facility is the impervious liner designed to reduce or eliminate the possibility of groundwater contamination. The facility provides a level of treatment strictly through filtration processes that occur when the runoff moves through the soil material to the underdrain discharge point. In the event of an accidental spill, the underdrain can be blocked and the objectionable materials siphoned through an observation well and safely contained.

Biofiltration with internal water storage

image of biofiltration device with internal water storage
Biofiltration device with internal water storage

The biofiltration practice with internal water storage is not commonly used in the Midwestern United States but is widely used in some places on the east coast, such as North Carolina. The use of an upturned elbow in this practice allows water to be retained within the practice, leading to increased pollutant removal, increased infiltration, and increased evapotranspiration. The practice is particularly effective at removing nitrogen through denitrification. The media should be 3 feet or more thick to allow water to be drawn down below the root zone. Underlying soils should be permeable enough to allow water stored within the practice to infiltrate.


Design types for various land uses

It should be noted that the layout of the bioretention area will vary according to individual sites, and to specific site constraints such as underlying soils, existing vegetation, drainage, location of utilities, sight distances for traffic, and aesthetics. Designers are encouraged to be creative in determining how to integrate bioretention into their respective site designs. With this in mind, the following are presented as alternative options.

On-lot / rain garden

Simple design that incorporates a planting bed in the low portion of the site. On-lot systems are designed to receive flows from gutters, and/or other impervious surfaces.

Parking lot islands (curbless)

In a paved area with no curb, pre-cast car-stops or a “ribbon curb” can be installed along the pavement perimeter to protect the bioretention area. This application of bioretention should only be attempted where shallow grades allow for sheet flow conditions over level entrance areas. Water may be pooled into the parking area where parking spaces are rarely used to achieve an element of stormwater quantity control beyond the confines of the bioretention surface area (Prince George’s County, 2002).

schematic showing a bioretention parking lot island
A bioretention parking lot island. Note the use of other BMPs, including permeable pavement and tree trenches. (Source: Minnehaha Creek Watershed District)

Parking lot islands (curb-cut)

For curb-cut entrance approaches, the water is diverted into the bioretention area through the use of an inlet deflector block, which has ridges that channel the runoff into the bioretention area (Prince George’s County, 2002). Special attention to erosion control and pre-treatment should be given to the concentrated flow produced by curbcuts.

Road medians / traffic islands

A multifunctional landscape can be created by utilizing road medians and islands for bioretention. There is no minimum width recommended for traffic islands from street edge to edge. A buffer may be necessary along the outside curb perimeter to minimize the possibility of drainage seeping under the pavement section, and creating “frost heave” during winter months. Alternately, the installation of a geotextile filter fabric “curtain wall” along the perimeter of the bioretention island will accomplish the same effect.

Tree pits / tree box filters

schematic of a tree box filter
A tree box filter. Note the curb cuts. (Source: Minnehaha Creek Watershed District)

Tree pits and tree box filters afford many opportunities for bioretention. Designs vary widely from simple “tree pits”, used for local drainage interception to more formal Tree Box Filters, which are a useful tool for highly urbanized streetscapes.

The tree pit technique provides very shallow ponding storage areas in a “dished” mulch area around the tree or shrub. Typically, the mulched area extends to the dripline for the tree and is similar to conventional mulching practices, except that the mulch area is depressed at least 2 to 3 inches rather than mounded around the tree (Low Impact Design Center, 2005).

Tree box filters are bioretention areas installed beneath trees that can be very effective at controlling runoff, especially when distributed throughout the site. Runoff is directed to the tree box, where it is cleaned by vegetation and soil before entering a catch basin. The runoff collected in the tree-boxes helps irrigate the trees. The system consists of a container filled with a soil mixture, a mulch layer, under-drain system and a shrub or tree. Stormwater runoff drains directly from impervious surfaces through a filter media. Treated water flows out of the system through an underdrain connected to a storm drainpipe/inlet or into the surrounding soil. Tree box filters can also be used to control runoff volumes/flows by adding storage volume beneath the filter box with an outlet control device.


Related pages

This page was last edited on 15 August 2022, at 18:52.