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

<panelSuccess>

• 

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

</panelSuccess>

Types of bioretention cell

Form	Characteristics	Examples
Infiltrating cells	Used in developments with large landscaping areas, parks, parking lot islands, or any areas without tight space constraints. They have side slopes ≥2:1. Often, they receive sheet flow, but in some cases they are surrounded by curbs and will have inlets. The distinction between these options will determine the recommended types of pre-treatment.	Bioretention cell capturing and treating runoff from adjacent parking lot at the Kortright Centre, Vaughan.
Rain gardens	Often found on residential sites or on land managed by community organisations . This simple variation may be constructed by the property owner and usually excludes the storage layer.	See main article on rain gardens
Bioretention planters (stormwater planters)	Typically used in ultra-urban areas adjacent to buildings and in plazas. They appear similar to traditional landscaped beds, but differ by receiving runoff from nearby surfaces.	Image here
Extended tree pits (parallel bioretention)	Located within the right-of-way, occupying the space between sidewalk and street. The inlets can be positioned on either or both sides, and are designed to prevent the system from filling beyond a fixed capacity. When ponding occurs, stormwater bypasses the inlets, making this a 'parallel' system rather than a flow-though or online design.	Extended tree pit on Kings Street, Kitchener.
Curb extensions (bump outs)	Installed in road-right-of-way, these function as a stormwater facility and a traffic calming measure. Inlets are integrated into the raised concrete curb and receive flow from the street side.	Image here

---

Planning Considerations

Planning Content

<panelSuccess>

• 

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

</panelSuccess>

---

Design

Vegetation

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

• The vegetation is a big opportunity to maximize the co-benefits of biodiversity and amenity. Planting plans can be formalized or naturalized to suit the surrounding style.
• For resilient and robust planting, native species which can tolerate periods of drought and periodic inundation are recommended.
• Woody and evergreen plants should be avoided in any areas of the bioretention cell to be used as snow storage.
• Dense shrubby plants should be avoided in locations where the accumultion of trash is anticipated as a maintenance problem.

<panelSuccess>

• 

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

</panelSuccess>

---

Performance

Performance Content

<panelSuccess>

• 

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

</panelSuccess>

---

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

---

See Also

• Bioswales
• Rain gardens
• Trees

---

External Links

	Planning
	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

---

SEND US YOUR QUESTIONS & FEEDBACK ABOUT THIS PAGE

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.

Rainwater harvesting.