Bioretention

This article is about planted installations designed to capture and filter surface runoff through an engineered soil (biomedia) to underground infrastructure.
For simple systems (typically without regulatory control), 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 (LID). They can fit into any style of landscape and 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:

• Underdrain 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

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 area to the bioretention facility. Bioretention cell(s) 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.

Infiltration

Some form of stormwater landscaping (bioretention) can be fitted into most spaces. One of the key decisions is whether to infiltrate all of some of the collected water into the native soil. Although there are some constraints to infiltrating water, it is preferable to do so where possible. Partial infiltration facilities are a popular choice over 'tight' soils (infiltration ≤ 15 mm/hr).

Types of bioretention by underground design

Type	Gravel layer	Underdrain	Liner	Mechanisms	Schematic
Rain gardens	-	-	-	These are the simplest construction, often used by residents or community groups. Volume reduction is through infiltration and evapotranspiration.
Infiltrating bioretention	yes	-	-	This is a highly desirable type of bioretention where the soils permit infiltration at a great enough rate to empty the facility between storm events. Volume reduction is primarily through infiltration to the underlying soils, with some evapotranspiration. As there is no outflow from this BMP, it is particularly useful in areas where nutrient management is a concern to the watershed.
Partially infiltrating bioretention	yes	yes	-	Including an underdrain in the gravel storage layer help to empty the facility between storm events, even over ‘tight soils’. The drain discharges to a downstream point, which could be an underground infiltration facility. Limited 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.
Biofilters (non-infiltrating)	yes	yes	yes	This type may be required over contamination hot-spots or in very dense urban areas with a lot of other underground infrastructure. The design includes an impermeable base and sides, so that volume reduction is made only through evapotranspiration. This type of cell can be constructed above grade in any waterproof and structurally sound container, e.g. in cast concrete or a metal tank.

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
• A covered drain

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

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

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

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  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

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  Flat, metal overflow grate in Emeryville California Stormwater Curb Extension

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

Design for maintenance

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Design

Sizing

This article describes recommended design approaches when available space for the practice is constrained.

Before beginning the sizing calculations certain parameters must be known or estimated. See Bioretention: Sizing for parameter descriptions and conceptual diagram illustrating key components of bioretention practices. Note that some of these parameters are limited:

1. The maximum total depth will be limited by construction practices i.e. not usually > 2 m.
2. The maximum total depth may be limited by the conditions underground e.g. the groundwater or underlying geology/infrastructure.
3. The minimum total depth will be limited by the need to support vegetation (e.g not less than 0.6 m to support deep rooting perennials and shrubs).
4. Bioretention has a maximum recommended catchment impervious area to practice permeable (footprint) area ratio, R (or I/P ratio) of 20.

Size a bioretention cell receiving flows directly to the storage reservoir for a constrained depth

If there is a constraint to the depth (d_T) of the practice, calculate the required storage reservoir footprint area (A_r), as: $A_{r}={\frac {i\times D\times A_{i}}{(d_{r}\times n')+(f'\times D)}}$

Where:

• A_r = Area of the infiltration practice storage reservoir (m²)
• A_i = Catchment impervious area (m²)
• D = Duration of design storm (h)
• i = Intensity of design storm (mm/h)
• f' = design infiltration rate (m/h)
• n' = Effective porosity of the fill material in the storage reservoir of the practice
• d_r = Storage reservoir depth, based on depth available between the elevation of the invert of the underdrain perforated pipe and one (1) metre above the seasonally high water table or top of bedrock (m) or other value determined to be suitable through groundwater mounding analysis.

If R is greater than 20, consider decreasing catchment impervious area (A_i) by draining less area to the practice.

Size a bioretention cell where drainage area and practice area are fixed

If the land area is limited, determine the I/P ratio, which is the ratio of catchment impervious area (A_i) to practice pervious footprint area (A_p):

$R={\frac {A_{i}}{A_{p}}}$

Where:

• R = Ratio of catchment impervious area to practice pervious footprint area, also referred to as I/P ratio
• A_p = Practice pervious footprint area in m²
• A_i = Catchment impervious area in m²

Then calculate the required storage reservoir depth (d_r), as: $d_{r}={\frac {D\left[(R\times i)-f'\right]}{n'}}$

Where:

• D = Duration of design storm (h)
• i = Intensity of design storm (m/h)
• f' = Design infiltration rate (m/h)
• n' = Effective porosity of the storage reservoir fill material

These equations assume that infiltration occurs primarily through the base of the facility.

This spreadsheet tool has been set up to perform all of the infiltration practice sizing calculations shown above.
Download the infiltration practice sizing tool

Calculate drawdown time

Two footprint areas of 9 m².
Perimeter = 12 m (left) Perimeter = 20 m (right)

Download the Darcy drainage time calculator tool

In some situations, it may be possible to reduce the size of the bioretention required, by accounting for rapid drainage. Typically, this is only worth exploring over sandy soils with rapid infiltration. Note that narrow, linear bioretention features drain faster than round or blocky footprint geometries.

• Begin the drainage time calculation by dividing the storage reservoir area of the practice (A_r) by the perimeter (x).
• Use the following equation to estimate the time (t) to fully drain the facility:

$t={\frac {nA_{r}}{f'x}}ln\left[{\frac {\left(d_{r}+{\frac {A_{r}}{x}}\right)}{\left({\frac {A_{r}}{x}}\right)}}\right]$

Where:

• n is the porosity of the storage reservoir fill material
• A_r is the storage reservoir footprint area (m²),
• f' is the design infiltration rate of the native soil (mm/h),
• x is the perimeter of the practice (m), and
• d_r is the depth of the storage reservoir (m).

This 3 dimensional equation makes use of the hydraulic radius (A_r/x), where x is the perimeter (m) of the facility.
Maximizing the perimeter of the facility directs designers towards longer, linear shapes such as bioswales.
If additional storage volume is desired beneath an underdrain pipe, this may be sized as an infiltration trench.

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

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Construction

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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

Sustainable sites: Rainwater management (up to 3 points)

• Two points (or 1 point for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 95th percentile of regional or local rainfall events."
  
• Three points (or 2 points for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 98th percentile of regional or local rainfall events."
  

OR

• For zero-lot-line projects only, 3 points (or 2 points for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 85th percentile of regional or local rainfall events."

SITES v.2

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See Also

• Bioswales
• Rain gardens
• Trees

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External Links

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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.

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.

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 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.

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.

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

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

Penetration of water through the ground surface.

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

A perforated pipe used to assist the draining of soils.

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.

A perforated pipe used to assist the draining of soils.

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.

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.

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.

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.

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.

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.

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

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.

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

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)

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.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

The period between the maximum water level and the minimum level (dry weather or antecedent level).

Rainwater harvesting.