Bioretention

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These bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cells at Edwards Gardens in Toronto receive inflow from hydraulically connected permeable pavingAn 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. parking stalls
BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell capturing and treating runoffThat potion of the water precipitated onto a catchment area, which 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. from an adjacent parking lot at the Kortright Centre, Vaughan.

This article is about planted installations designed to capture and infiltrate some or all of the stormwater received.
For simple systems, without underdrains or storage reservoir (typically found n residential settings), see Rain gardens.
For linear systems, which convey flow, but are otherwise similar to bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. see Bioswales.
For planted systems that do not infiltrate any water, see Stormwater planters.

Overview

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. systems may be the most well recognized form of low impact development (LIDLow 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.). They can fit into any style of landscape and encompass all mechanisms of action: infiltration, filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil., attenuationReduction of peak flow and increase of the duration of the flow event. and evapotranspiration.

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. is an ideal technology for:

  • Fitting functional vegetation into urban landscapes
  • Treating runoffThat potion of the water precipitated onto a catchment area, which 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. collected from nearby imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surfaces

The fundamental components of a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell are:

Additional components may include:

  • An underdrain to redistribute or remove excess water
  • Soil additives intended to enhance nutrient and related water quality pollutant removal

Planning considerations

Note Site Considerations from the BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. Fact Sheet [1] in the 2010 CVCCredit Valley Conservation/TRCAToronto Region Conservation Authority LIDLow 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. Stormwater Management Planning Design are detailed below and within links included

InfiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.

Some form of stormwater landscaping (bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.) can be fitted into most spaces. Although there are some constraints to infiltrating water, it is preferable to do so where possible. Designing bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. without an underdrainA perforated pipe used to assist the draining of soils. is highly desirable wherever the soils permit infiltration at a rate which is great enough to empty the facility between storm events. Volume reduction is achieved primarily through infiltration to the underlying soils, with some evapotranspirationThe 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/dayThe combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.. As there is no outflow from this BMPBest 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. under normal operating conditions, it is particularly useful in areas where nutrient management is a concern to the watershedThe drainage area of a river.An area of land that drains into a river or a lake. The boundary of a watershed is based on the elevation (natural contours) of a landscape..

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. with an underdrain is a popular choice in areas with 'tighter' soils where infiltration rates are ≤ 15 mm/hr. Including an perforated pipe in the reservoir aggregate layer helps to empty the facility between storm events, which is particularly useful in areas with low permeability soils. The drain discharges to a downstream point, which could be an underground infiltration trench or chamber facility. Volume reduction is gained through infiltration and evapotranspiration. By raising the outlet of the discharge pipe the bottom portion of the BMPBest 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. can only drain through infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.. This creates a fluctuating anaerobic/aerobic environmentRefers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer. which promotes denitrification. Increasing the period of storage has benefits for promoting infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., but also improves water quality for catchments impacted with nitrates. A complimentary technique is to use fresh wood mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales., which also fosters denitrifying biological processes.

Where infiltration is entirely impossible, but the design calls for planted landscaping, try a stormwater planter instead.

Space

  • For optimal performance bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facilities should receive runoffThat potion of the water precipitated onto a catchment area, which 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. from between 5 to 20 times their own surface area.
  • In the conceptual design stage it is recommended to set aside approximately 10 - 20 % of a catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.'s total area for bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facility placement.
  • BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cells work best when distributed, so that no one facility receives runoffThat potion of the water precipitated onto a catchment area, which 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. from more than 0.8 Ha.
Although, there is a trade off to be considered regarding distributed collection and treatment against ease of maintenance.
  • BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. can be almost any shape, from very curving, soft edges with variable depth, to angular, hard sided and uniform depth.
For ease of construction and to ensure that the vegetation has adequate space, cells should be no narrower than 0.6 m at any point.
The maximum width of a facility is determined by the reach of the construction machinery, which must not be tracked into the cell.
  • Setback from Buildings: A typical four (4) metre setback is recommended from building foundations. If an impermeable liner is used, no setback is needed.
  • Proximity to Underground Utilities and Overhead wires - consult with local utility companies regards to horizontal and vertical clearance required between storm drains, ditches, and surface water bodies. Further, check whether the future tree canopy height in the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. area will not interfere with existing overhead wires

The principles of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. can be applied in any scenario where planting or vegetation would normally be found.

Private sites

In single family residential sites rain gardens most often take the form of a soft edged, traditional perennial planting bed. As many private industrial, commercial and institutional sites have landscaping around their parking lots, Bioretention: Parking lots is an increasingly popular choice to manage stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events..

Streetscape

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. is a popular choice for making urban green space work harder. Design configurations include extending the cells to accommodate shade trees, and using retrofit opportunities to create complete streets with traffic calming and curb extensions or 'bump outs'. See Bioretention: Streetscapes

Parkland and natural areas

Naturalized landscaping and soft edges can make a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facility 'disappear' into green space surroundings. In some scenarios, a larger bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. (50 - 800 m2) cell may be used as an end-of-pipe facility treating both sheet flow and concentrated flow before it enters an adjacent water course. In these larger installations care must be made in the design to distribute the inflow, preventing erosion and maximizing infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..

Design

Optimizing bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. for water quality
Poor design choice:
Limits outflow water quality
Better design choice:
Improves outflow water quality
Single large cell design Several smaller distributed cells
Single concentrated inflow Forebays or distributed flow
No pretreatmentInitial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms. PretreatmentInitial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms. provided as part of treatment trainStormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.A combination of lot level, conveyance, and end-of-pipe stormwater management practices. design
Over-sized underdrainA perforated pipe used to assist the draining of soils. Moderately sized underdrainA perforated pipe used to assist the draining of soils. (or no underdrainA perforated pipe used to assist the draining of soils.)
Filter bed < 0.5 m Filter bed > 0.75 m
Filter mediaThe 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. Phosphorus > 30 ppm Filter mediaThe 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. Phosphorus < 30 ppm
Filter mediaThe 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. predominantly sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. Filter mediaThe 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. contains fractions of finesSoil particles with a diameter less than 0.050 mm. and organic material in sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.
Surface covered with stone (or uncovered) Surface covered with mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. and dense vegetation

Sizing and Modelling

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facilities should be sized to accommodate runoffThat potion of the water precipitated onto a catchment area, which 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. from approximately 10 to 20 times the footprint area of the facility. i.e. they should have an I/P ratioThe ratio of the catchment (impervious area) to the footprint area of the receiving BMP (pervious area). of 10 to 20. When the drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. is too large, siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) can accumulate very rapidly, overwhelm the pretreatment devices, and lead to clogging of the facility. When the drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. is relatively small compared to a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facility, it can make the facility appear unreasonably costly.

Inlets and pretreatmentInitial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms. options

Options for pretreatment include:

Simple (non-treating) inlets include:

  • Sheet flow from a depressed curb
  • One of more curb cuts
  • Covered drains

Overflow routing

Conceptual diagram of the excess routing alternatives: On the left, excess flow leaves the cell via an overflow; on the right, excess flow is diverted so that only the design volume enters the cell.

Routing

  • Infiltration facilities can be designed to be inlineRefers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet. or offlineRefers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed. from the drainage systemA system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006. See Inlets
  • InlineRefers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet. facilities accept all of the flow from a drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. and convey larger event flows through an overflow outlet. The overflow must be sized to safely convey larger storm events out of the facility.
  • The overflow must be situated at the far end of the facility to prevent any localised ponding to cause bypassing of the infiltration facility.
  • OfflineRefers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed. facilities use flow splitters or bypass channels that only allow the required water quality storage volume to enter the facility.
Higher flows are diverted and do not enter the infiltration practice. A pipe can by used for this, but a weir or curb cut minimizes clogging and reduces the maintenance frequency.

Overflow elevation

The invert of the overflow should be placed at the maximum water surface elevation of the practice. i.e. the maximum ponding depth. A good starting point is around 300 mm over the surface of the practice. However, consideration should be given to public safety and drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level).|time for the ponded water to drain. See Bioretention and Stormwater planters

Freeboard

  • In swales convey flowing water a freeboard of 300 mm is generally accepted as a good starting point.
  • In bioretention the freeboard is being defined as the depth between the invert of the overflow and the the inlet 150 mm would suffice, so long as the inlet will not become inundated during design storm conditions.
  • In above grade stormwater planters above grade, the equivalent dimension would be the depth between the invert of the overflow and the lip of the planter (150 mm minimum)
  • Where the stormwater planterA vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration. is configured more like a lined/non-infiltrating bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. system, the inlet will be the depth to which this is measured, as above (150 mm minimum).

Options

Metal grates are recommended (over plastic) in all situations.

Feature Anti Vandalism/Robust Lower Cost Option Self cleaning
Dome grate x
Flat grate x
Catch basinGround depression acting as a flow control and water treatment structure, that is normally dry. x
DitchA long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line. inlet catch basinGround depression acting as a flow control and water treatment structure, that is normally dry. x x
Curb cut x x x

Gallery

Plant Selection

The nature of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cells is to attenuate stormwater from rainfall events of varying intensities. For this reason, the vegetation used must be suitable for the varying moisture conditions and is often categorized into three zones related to the grading of the feature.

  1. Low Zone -- This area is frequently inundated during storm events, and is well-drained between rainfall events.
    • Mineral Meadow Marsh plant community
    • Grasses, Sedges, rushes, wildflowers, ferns and shrubs that have an ‘Obligate’ to ‘Facultative’ designation
    • WetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. ‘Obligate’ species that are flood tolerant as they will persist in average years and flourish in wetter years.
    • Plants that are likely to occur in wetlands or adjacent to wetlands.
    • Plants with dense root structure and /or vegetative cover are favoured for their ability to act as pollution filters and tendency to slow water velocity
    • Be advised these practices are not constructed wetlands and are designed to fully drain within 48 hours.
  2. Mid Zone -- This zone is inundated less frequently (2 – 100 year storm events) and has periodically high levels of moisture in the soil. The ecology of this zone is a transition from the Mineral Meadow Marsh/Beach-type community to an upland community.
    • Plants able to survive in soils that are seasonally saturated, yet can also tolerate periodic drought.
    • Species include grasses and groundcovers, as well as low shrub species.
  3. High Zone -- The ecology of this zone is terrestrial due to its elevation in relation to the filter bed. The zone most closely resembles a Cultural Meadow or a Cultural Thicket community, depending on the mix of grasses, herbaceous material, shrubs and trees utilized.
    • Plants should have deep roots for structure, be drought-tolerant and capable of withstanding occasional soil saturation.
    • Trees and large shrubs planted in this zone will aid in the infiltration and absorption of stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events..
    • This area can be considered a transition area into other landscape or site areas.
    • A variety (min. five) species should be used to prevent a monoculture.

Exposure to roadway or parking lot runoffThat potion of the water precipitated onto a catchment area, which 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. must be considered.

  1. Exposure to roadway or parking lot runoffThat potion of the water precipitated onto a catchment area, which 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.
    • Select salt tolerant grasses, other herbaceous material and shrubs.
    • These can take on several forms, including parking lot islands, traffic islands, roundabouts, or cul-de-sacs and are often used as snow storage locations.
  2. No exposure to roadway or parking lot runoffThat potion of the water precipitated onto a catchment area, which 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.
    • Practices allow for a greater range of species selection.
    • These receive runoffThat potion of the water precipitated onto a catchment area, which 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. from rooftops or areas that use no deicing salt and have low pollutant exposure, such as courtyard bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation..

Other selection factors:

  • Most bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cells will be situated to receive full sun exposure. The ‘Exposure’ column in the master plant list identifies the sun exposure condition for each species.
  • Facilities with a deeper media bed (greater than 1 m) provide the opportunity for a wider range of plant species (including trees).
  • The inclusion of vegetation with a variety of moisture tolerances ensures that the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell will adapt to a variety of weather conditions.
  • Proper spacing must be provided for above-ground and below-ground utilities, and adjacent infrastructure.
  • Where possible, a combination of native trees, shrubs, and perennial herbaceous materials should be used.
  • A planting mix with evergreen and woody plants will provide appealing textures and colors year round, but they may not be appropriate for snow storage areas.
  • In areas where less maintenance will be provided and where trash accumulation in shrubbery or herbaceous plants is a concern, consider a “turf and trees” landscaping model.
  • If trees are to be used, or the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. is located in a shaded location, then ensure that the chosen herbaceous plants are shade tolerant.
  • Spaces for herbaceous flowering plants can be included. This may be attractive at a community entrance location or in a residential rain gardenA lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts..


Tables for identifying ideal species for bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. are found in the Plant lists. See plant selection and planting design for supporting advice.

See also

External links