Bioretention

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

Bioretention cell capturing and treating runoff 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 bioretention see Bioswales.
For planted systems that do not infiltrate any water, see Stormwater planters.

Overview

Bioretention systems may be the most well recognized form of low impact development (LID). They can fit into any style of landscape and encompass all mechanisms of action: infiltration, filtration, attenuation and evapotranspiration.

Bioretention is 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:

• A 'filter bed' containing a filter media
• A storage layer of reservoir aggregate
• planting, and
• a finishing surface layer (e.g. mulch and/or stone)

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

Infiltration

Some form of stormwater landscaping (bioretention) can be fitted into most spaces. Although there are some constraints to infiltrating water, it is preferable to do so where possible. Designing bioretention without an underdrain 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 evapotranspiration. As there is no outflow from this BMP under normal operating conditions, it is particularly useful in areas where nutrient management is a concern to the watershed.

Bioretention 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 BMP can only drain through infiltration. This creates a fluctuating anaerobic/aerobic environment which promotes denitrification. Increasing the period of storage has benefits for promoting infiltration, but also improves water quality for catchments impacted with nitrates. A complimentary technique is to use fresh wood mulch, 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 bioretention facilities should receive runoff 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 catchment's total area for bioretention facility placement.
• Bioretention cells work best when distributed, so that no one facility receives runoff from more than 0.8 Ha.

Although, there is a trade off to be considered regarding distributed collection and treatment against ease of maintenance.

• Bioretention 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.

The principles of bioretention 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 stormwater.

Streetscape

Bioretention 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 bioretention facility 'disappear' into green space surroundings. In some scenarios, a larger bioretention (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 infiltration.

Design

Optimizing bioretention 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 pretreatment	Pretreatment provided as part of treatment train design
Over-sized underdrain	Moderately sized underdrain (or no underdrain)
Filter bed < 0.5 m	Filter bed > 0.75 m
Filter media Phosphorus > 30 ppm	Filter media Phosphorus < 30 ppm
Filter media predominantly sand	Filter media contains fractions of fines and organic material in sand
Surface covered with stone (or uncovered)	Surface covered with mulch and dense vegetation

Sizing and Modelling

Bioretention facilities should be sized to accommodate runoff from approximately 10 to 20 times the footprint area of the facility. i.e. they should have an I/P ratio of 10 to 20. When the drainage area is too large, silt can accumulate very rapidly, overwhelm the pretreatment devices, and lead to clogging of the facility. When the drainage area is relatively small compared to a bioretention facility, it can make the facility appear unreasonably costly.

• Sizing
• Modelling

Inlets and pretreatment options

Options for pretreatment include:

• A gravel diaphragm for sheet flow
• Vegetated filter strips for sheet flow
• A forebay for concentrated overground flow
• An oil and grit separator for concentrated underground flow

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 inline or offline from the drainage system. See figure to the right for an illustration.
• Inline facilities accept all of the flow from a drainage area 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 maximum surface ponding elevation or furthest downgradient end of the facility to limit surface ponding during periods of flow in excess of the facility storage capacity.
• Offline facilities use flow splitters or bypass channels that only allow the design storm runoff storage volume to enter the facility. Higher flows are conveyed to a downstream storm sewer or other BMP by a flow splitting manhole weir or pipe, or when the maximum surface ponding depth has been reached, by by-passing the curb opening and flowing into a downstream catchbasin connected to a storm sewer.

Overflow Elevation

The invert of the overflow should be placed at the maximum water surface elevation of the practice (i.e. the maximum surface ponding level). A good starting point is 150 to 350 mm above the surface of the mulch cover. However, consideration should be given to public safety, whether or not an underdrain is included, the time required for ponded water to drain through the filter bed surface, and if no underdrain is present, into the underlying native soil (must drain within 48 hours). See Bioretention: Sizing and Stormwater planters for more details.

Freeboard

• In swales conveying flowing water a freeboard of 300 mm is generally accepted as a good starting point.
• In bioretention the freeboard is the difference between the invert elevation of the inlet and overflow structure. 150 mm will usually suffice, so long as the inlet will not become inundated during design storm conditions.
• In above grade stormwater planters, the equivalent dimension would be the difference between the invert elevation of the lip of the planter and the overflow structure (150 mm minimum).

Overflow Outlet 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 basin	x
Ditch inlet catch basin	x		x
Curb cut	x	x	x

Gallery

• 

  Flat metal overflow with stone surround to reduce erosion around the cast concrete structure. Mississauga Road, ON

• 

  Domed, metal overflow grate. Being flush with the surface reduces potential infiltration of ponded water. Photo credit: Aaron Volkening

• 

  Overflow inlet for newly constructed stormwater bioretention areas in median of Bradley Road. Village of Brown Deer, Wisconsin. Bradley Road, east of 51st Street. Photo from October 2015. Constructed summer 2015.
  Photo credit: Aaron Volkening

• 

  Flat, metal overflow grate in Emeryville California Stormwater Curb Extension

• 

  Note the greyish colour change indicating UV degradation of this plastic overflow inlet in a bioretention cell; metal is recommended instead. Mississauga ON.

Plant Selection

The nature of bioretention 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
   • Wetland ‘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.

1. 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.

1. 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 stormwater.
• 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.

1. Exposure to roadway or parking lot runoff
   • 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 runoff
   • Practices allow for a greater range of species selection.
   • These receive runoff from rooftops or areas that use no deicing salt and have low pollutant exposure, such as courtyard bioretention.

Other selection factors:

• Most bioretention 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 bioretention 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 bioretention 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 garden.

Tables for identifying ideal species for bioretention are found in the Plant lists. See plant selection and planting design for supporting advice.

See also

• Bioswales
• Rain gardens
• Trees

External links

• Design Guidelines (2014) Healthy Waterways

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

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.

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

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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

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.

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

Reduction of peak flow and increase of the duration of the flow event.

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

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

Penetration of water through the ground surface.

A perforated pipe used to assist the draining of soils.

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.

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.

The 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.

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

Penetration of water through the ground surface.

Refers 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.

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

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Initial 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.

Initial 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.

Stormwater 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.

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.

Mineral 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.

Soil particles with a diameter less than 0.050 mm.

The ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.

The 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.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

Refers 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.

A 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

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

Refers 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.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

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

Ground depression acting as a flow control and water treatment structure, that is normally dry.

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

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