Bioretention

This article is about planted installations designed to capture surface runoff through an engineered soil with subterranean infrastructure.
For simpler, residential systems, see Rain gardens.
For linear systems, which convey flow, but are otherwise similar see Bioswales.

Overview

Bioretention cells are one of the most well recognized form of Low Impact Development. They can encompass all mechanisms of action: infiltration, filtration and evapotranspiration.

Bioretention cells are an ideal technology for:

• Fitting functional vegetation into urban landscapes
• Treating runoff collected from nearby impervious surfaces

The fundamental components of a bioretention cell are:

• Biomedia: An engineered soil mix
• Planting
• Storage layer of coarse aggregate

Additional components may include:

• Under-drain to redistribute or remove excess water
• Impermeable membrane to prevent infiltration to soils below

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  These bioretention cells at Edwards Gardens in Toronto receive inflow from hydraulically connected permeable paving.

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Planning considerations

Design for maintenance

Will the BMP be a snow storage facility in winter months?

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  Bioretention cell capturing and treating runoff from adjacent parking lot at the Kortright Centre, Vaughan.

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Design

   Underdrain

This is a collection of three articles with the common theme of being aggregate products for various applications in LID.

Underground construction aggregates

For reservoirs

Note the uniform size and angularity of this clear stone sample. Note also that the fragments all appear to have a film of fine particles adhering; this material would be improved by being washed prior to use.

This article gives recommendations for aggregate to be used to store water for infiltration. This is usually called 'clear stone' at aggregate yards.

To see an analysis of Ontario Standard Specifications for granular materials, see OPSS aggregates.

For advice on decorative surface aggregates see Stone

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Gravel used for underdrains in bioretention, infiltration trenches and chambers, and exfiltration trenches should be 20 or 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a porosity of 0.4.

The clean wash to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the porosity). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles . See jar test for on-site verification testing protocols.

Gravel with structural requirements should also meet the following criteria:

• Minimum durability index of 35
• Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions

Standard specifications for the gradation of aggregates are maintained by ASTM D2940

• 

  The highest porosity is found in uniformly graded aggregate, as there are no smaller particles to occupy the inter-particle pores.

• 

  Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity.

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For choking/choker layers

medium sized granular, free from fines

In bioretention systems a choker layer of ≥ 100 mm depth is the recommended method to prevent migration of finer filter media into the underlying storage reservoir aggregate. These same mid-sized granular materials are recommended for use in Stormwater planter underdrains and may be useful in the fine grading of foundations courses for permeable pavements.

Suitable materials include:

High performance bedding (HPB)

Clean, angular aggregate screened to between 6 and 10 mm. Widely available and designed specifically for drainage applications. Free from fines by definition.

HL 6

Is a clean, angular aggregate screened between 10 and 20 mm. Free from fines by definition.

Pea Gravel

Rounded natural aggregate, screened between 5 and 15 mm, and washed free from fines.

In most scenarios, a geotextile layer is unnecessary and has been associated with rapid decline and clogging in some circumstances.

---

OPS Aggregates

Of the standard granular materials in the standard OPSS.PROV 1010 only Granular O is recommended as a substitute for clear stone in LID construction.

Where Granular O is substituted for clear stone in underground reservoir structures, the porosity used in design calculations shall be 0.3 unless laboratory testing proves otherwise.

Examples of BMPs with underground reservoirs include Underdrains, infiltration trenches, permeable pavements, infiltration chambers, exfiltration trenches.

All other mixes must be avoided for free drainage or storage as they are permitted to contain a higher enough proportion of fines to reduce permeability below 50 mm/hr.

For more information see OPS aggregates

Landscaping aggregates

This rain garden in a school yard uses stone as both decorative edging and for erosion control.

This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

For advice on aggregates used in underdrains, see Reservoir aggregate.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions on the surface of an LID practice.

Stone for erosion control

Aggregates used to line swales or otherwise dissipate energy (e.g. in forebays) should have high angularity to increase the permissible shear stress applied by the flow of water. However, in some surface landscaped applications there may be a desire to use a rounded aggregate such as 'river rock' for aesthetic reasons. Rounded stones should be of sufficient size to resist being moved by the flow of water. Typical stone for this purpose ranges between 50 mm and 250 mm in diameter. The larger the stone, the more energy dissipation.

• Stone beds should be twice as thick as the largest stone's diameter.
• If the stone bed is underlain by a drainage geotextile, annual inspection and possible replacement should be performed as there is a potential for clogging of this layer to occur.

• 

  Stone lining the ponding zone of this rain garden. Image credit California Native Plant Society

• 

  Coarse angular stone laid onto a geogrid and geotextile. Image from wikimedia commons

Stone mulch

Finer inorganic mulch materials can be of value applied in areas with extended ponding times i.e. in the the centre of recessed, bowl shaped bioretention, stormwater planters, trenches or swale practices. Inorganic mulches resist movement from flowing water and do not float. Applying a thin layer of inorganic mulch over the top of wood based mulch has been shown to reduce migration of the underlying layer by around 25% . Inorganic mulches which may be available locally, include:

• Pea gravel
• River rock/beach stone
• Recycled glass
• Crushed mussel shells

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On-site verification

Steps in conducting a jar test to detect fines in construction materials

Specifying that aggregates for the construction of LID practices must be free from fines is important. But checking that the delivered materials meet specification is essential to reduce problems with construction and longer term performance.

When possible, Construction Managers should observe the offloading of materials to watch for dust clouds. Aggregates or sand for LID construction should not give rise to clouds of dust when dumped.

A simple jar test can be used to gauge the proportion of fines in an aggregate product before acceptance.

Apparatus:

• A large wide-mouthed jar - glass or clear plastic are both fine,
• Tap water, and
• The aggregate to be tested.

Method:

1. Collect approximately 5 cm of material in the jar (or at least two complete layers of 50 mm clear stone),
2. Add water to around 3/4 full,
3. Secure cap and shake,
4. Leave for at least 30 minutes and until the water is clear - plan to run the test overnight when possible,
5. Examine the layer of sediment - if > 3 mm has been washed from 5 cm of product, the material should be rejected,

Note that the sediment may collect on top of, or at the bottom of the construction material.

External references

• Heger, S. (2014). Critical Aspects During System Installation and Inspection General equipment considerations. In PMSA conference. Retrieved from http://www.psma.net/pdf/14/conference-presentations/Keys_to_Installation_(Heger).pdf
• Manitoba. (2010). Onsite Wastewater Management Systems: Field Reference Guide - JAR TEST. Retrieved from http://www.gov.mb.ca/sd/envprograms/wastewater/pdf/jar_test_reference_03_2010.pdf

Pipes are available with perforations on just one side, these should be situated on the lower half of the pipe. Pipes with 360° perforations should have a strip of geotextile or membrane placed over the pipe to reduce the migration of fines from overlying media.

Perforated pipes are a common component of underdrains used in bioretention, permeable pavements, infiltration trenches and exfiltration systems.

Pipes should be manufactured in conformity with the latest standards by the Canadian Standards Association (CSA) or ASTM International.

• Perforated pipes should be continuously perforated, smooth interior HDPE or PVC.
  • Wherever possible pipes should be ≥200 mm internal diameter to reduce potential of freezing and to facilitate push camera inspections and cleaning with jet nozzle equipment.
  • Smooth interior facilitates inspection and maintenance activities; internal corrugations can cause cameras or hydrojetting apparatus to become snagged.
  • A perforated pipe with many rectangular slots has better drainage characteristics than a pipe with similar open area provided by fewer circular holes .
• Non-perforated pipes should be used for conveyance of stormwater to and from the facility, including overflow. It is good practice to extend the solid pipe approximately 300 mm within the reservoir or practice to reduce the potential for native soil migration into the pipe.

• 

  Pipe with slotted perforations

• 

  Perforated pipes awaiting installation, note the 45 degree couplings used to facilitate push camera inspection and jet nozzle cleaning.

See also: Flow through perforated pipe

---

Underdrain

This is a collection of three articles with the common theme of being aggregate products for various applications in LID.

Underground construction aggregates

For reservoirs

Note the uniform size and angularity of this clear stone sample. Note also that the fragments all appear to have a film of fine particles adhering; this material would be improved by being washed prior to use.

This article gives recommendations for aggregate to be used to store water for infiltration. This is usually called 'clear stone' at aggregate yards.

To see an analysis of Ontario Standard Specifications for granular materials, see OPSS aggregates.

For advice on decorative surface aggregates see Stone

---

Gravel used for underdrains in bioretention, infiltration trenches and chambers, and exfiltration trenches should be 20 or 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a porosity of 0.4.

The clean wash to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the porosity). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles . See jar test for on-site verification testing protocols.

Gravel with structural requirements should also meet the following criteria:

• Minimum durability index of 35
• Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions

Standard specifications for the gradation of aggregates are maintained by ASTM D2940

• 

  The highest porosity is found in uniformly graded aggregate, as there are no smaller particles to occupy the inter-particle pores.

• 

  Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity.

---

For choking/choker layers

medium sized granular, free from fines

In bioretention systems a choker layer of ≥ 100 mm depth is the recommended method to prevent migration of finer filter media into the underlying storage reservoir aggregate. These same mid-sized granular materials are recommended for use in Stormwater planter underdrains and may be useful in the fine grading of foundations courses for permeable pavements.

Suitable materials include:

High performance bedding (HPB)

Clean, angular aggregate screened to between 6 and 10 mm. Widely available and designed specifically for drainage applications. Free from fines by definition.

HL 6

Is a clean, angular aggregate screened between 10 and 20 mm. Free from fines by definition.

Pea Gravel

Rounded natural aggregate, screened between 5 and 15 mm, and washed free from fines.

In most scenarios, a geotextile layer is unnecessary and has been associated with rapid decline and clogging in some circumstances.

---

OPS Aggregates

Of the standard granular materials in the standard OPSS.PROV 1010 only Granular O is recommended as a substitute for clear stone in LID construction.

Where Granular O is substituted for clear stone in underground reservoir structures, the porosity used in design calculations shall be 0.3 unless laboratory testing proves otherwise.

Examples of BMPs with underground reservoirs include Underdrains, infiltration trenches, permeable pavements, infiltration chambers, exfiltration trenches.

All other mixes must be avoided for free drainage or storage as they are permitted to contain a higher enough proportion of fines to reduce permeability below 50 mm/hr.

For more information see OPS aggregates

Landscaping aggregates

This rain garden in a school yard uses stone as both decorative edging and for erosion control.

This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

For advice on aggregates used in underdrains, see Reservoir aggregate.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions on the surface of an LID practice.

Stone for erosion control

Aggregates used to line swales or otherwise dissipate energy (e.g. in forebays) should have high angularity to increase the permissible shear stress applied by the flow of water. However, in some surface landscaped applications there may be a desire to use a rounded aggregate such as 'river rock' for aesthetic reasons. Rounded stones should be of sufficient size to resist being moved by the flow of water. Typical stone for this purpose ranges between 50 mm and 250 mm in diameter. The larger the stone, the more energy dissipation.

• Stone beds should be twice as thick as the largest stone's diameter.
• If the stone bed is underlain by a drainage geotextile, annual inspection and possible replacement should be performed as there is a potential for clogging of this layer to occur.

• 

  Stone lining the ponding zone of this rain garden. Image credit California Native Plant Society

• 

  Coarse angular stone laid onto a geogrid and geotextile. Image from wikimedia commons

Stone mulch

Finer inorganic mulch materials can be of value applied in areas with extended ponding times i.e. in the the centre of recessed, bowl shaped bioretention, stormwater planters, trenches or swale practices. Inorganic mulches resist movement from flowing water and do not float. Applying a thin layer of inorganic mulch over the top of wood based mulch has been shown to reduce migration of the underlying layer by around 25% . Inorganic mulches which may be available locally, include:

• Pea gravel
• River rock/beach stone
• Recycled glass
• Crushed mussel shells

---

On-site verification

Steps in conducting a jar test to detect fines in construction materials

Specifying that aggregates for the construction of LID practices must be free from fines is important. But checking that the delivered materials meet specification is essential to reduce problems with construction and longer term performance.

When possible, Construction Managers should observe the offloading of materials to watch for dust clouds. Aggregates or sand for LID construction should not give rise to clouds of dust when dumped.

A simple jar test can be used to gauge the proportion of fines in an aggregate product before acceptance.

Apparatus:

• A large wide-mouthed jar - glass or clear plastic are both fine,
• Tap water, and
• The aggregate to be tested.

Method:

1. Collect approximately 5 cm of material in the jar (or at least two complete layers of 50 mm clear stone),
2. Add water to around 3/4 full,
3. Secure cap and shake,
4. Leave for at least 30 minutes and until the water is clear - plan to run the test overnight when possible,
5. Examine the layer of sediment - if > 3 mm has been washed from 5 cm of product, the material should be rejected,

Note that the sediment may collect on top of, or at the bottom of the construction material.

External references

• Heger, S. (2014). Critical Aspects During System Installation and Inspection General equipment considerations. In PMSA conference. Retrieved from http://www.psma.net/pdf/14/conference-presentations/Keys_to_Installation_(Heger).pdf
• Manitoba. (2010). Onsite Wastewater Management Systems: Field Reference Guide - JAR TEST. Retrieved from http://www.gov.mb.ca/sd/envprograms/wastewater/pdf/jar_test_reference_03_2010.pdf

Pipes are available with perforations on just one side, these should be situated on the lower half of the pipe. Pipes with 360° perforations should have a strip of geotextile or membrane placed over the pipe to reduce the migration of fines from overlying media.

Perforated pipes are a common component of underdrains used in bioretention, permeable pavements, infiltration trenches and exfiltration systems.

Pipes should be manufactured in conformity with the latest standards by the Canadian Standards Association (CSA) or ASTM International.

• Perforated pipes should be continuously perforated, smooth interior HDPE or PVC.
  • Wherever possible pipes should be ≥200 mm internal diameter to reduce potential of freezing and to facilitate push camera inspections and cleaning with jet nozzle equipment.
  • Smooth interior facilitates inspection and maintenance activities; internal corrugations can cause cameras or hydrojetting apparatus to become snagged.
  • A perforated pipe with many rectangular slots has better drainage characteristics than a pipe with similar open area provided by fewer circular holes .
• Non-perforated pipes should be used for conveyance of stormwater to and from the facility, including overflow. It is good practice to extend the solid pipe approximately 300 mm within the reservoir or practice to reduce the potential for native soil migration into the pipe.

• 

  Pipe with slotted perforations

• 

  Perforated pipes awaiting installation, note the 45 degree couplings used to facilitate push camera inspection and jet nozzle cleaning.

See also: Flow through perforated pipe

---

Mulch

Pine mulch will help hold moisture in the soil

• Mulch is considered to be an normal finishing touch to many types of formal landscaping. Maintaining mulch application can help increase aesthetic value of LID BMPs.
• As in other landscaping applications, the mulch helps to preserve soil moisture for plant survival, and suppresses weed growth.
• Mulch can also help to maintain the organic matter content of underlying filter media, which provides cation exchange capacity for pollutant removal.
• Regular fresh applications of wood mulch can also promote denitrification, reducing nitrates in impacted surface waters.
• Mulch should be applied on the surface of the BMP in a layer of 75 -100 mm.
• Double-shredded hardwood or softwood mulch is recommended for LID facilities. Its fibrous texture knits together somewhat; providing limited erosion control.
• In areas with particularly high flow (e.g. around inlets and forebays) coarse decorative aggregate or stone is recommended to better dissipate energy and protect it from erosion.
• This advice also holds for stormwater planters, which often experience concentrated flow from a roof downspout or drain.
• All organic mulches have the potential to float and migrate in surface flow, particularly after a previously dry period.

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Vegetation

For a detailed list of recommend plant species, see Plants for bioretention Bioretention plant plan

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• 

  Bioretention cell capturing and treating runoff from adjacent parking lot at the Kortright Centre, Vaughan.

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Performance

Performance Content

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• 

  Bioretention cell capturing and treating runoff from adjacent parking lot at the Kortright Centre, Vaughan.

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Incentives and Credits

In Ontario

City of Mississauga
The City of Mississauga has a stormwater management credit program which includes RWH as one of their recommended site strategies[1].

LEED BD + C v. 4

SITES v.2

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See Also

• Bioswales
• Rain gardens
• Trees

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External Links

	Variations
	Overview
	Design Guidance
	BMP Sizing
	Design Specifications
	Construction Considerations
	Inspection and Maintenance Needs
	Life Cycle Costs
	Case Studies
	Typical Design Drawings
	Supporting Material and External Resources

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The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The technique of removing pollutants from runoff as it infiltrates through the soil.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

A perforated pipe used to assist the draining of soils.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

Soil particles with a diameter less than 0.050 mm.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Includes the protection of soil from dislocation by water, wind or other agents.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

The natural ground material characteristic of or existing by virtue of geographic origin.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Rainwater harvesting.