Bioretention: Variations

Types of bioretention by underground design

Rain gardens

These are the simplest construction, often used by residents or community groups. Volume reduction is through infiltration and evapotranspiration.

Infiltrating bioretention

This is the most 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 desirable in areas where nutrient management is a concern to the watershed.

Partially infiltrating bioretention

Including an underdrain in the gravel storage layer ensures that the facility will empty between storm events even over ‘tight soils’. The drain discharges to a downstream point, which may require a Limited volume reduction is gained through infiltration and evapotranspiration.

Bioretention with storage

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.

Bioretention planters (non-infiltrating)

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.

Types of bioretention by underground design

Type	Gravel layer	Underdrain	Liner	Mechanisms	Examples
Rain gardens	-	-	-	These are the simplest construction, often used by residents or community groups. Volume reduction is through infiltration and evapotranspiration.	Image of Raingarden
Infiltrating bioretention	yes	-	-	This is the most 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 desirable in areas where nutrient management is a concern to the watershed.	Diagram of bioretention without underdrain.
Partially infiltrating bioretention	yes	yes	-	Including an underdrain in the gravel storage layer ensures that the facility will empty between storm events even over ‘tight soils’. The drain discharges to a downstream point, which may require a Limited volume reduction is gained through infiltration and evapotranspiration.	Image here
Bioretention with storage	yes	yes	-	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.	Image
Bioretention planters (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.	Image here

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

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

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.

A perforated pipe used to assist the draining of soils.

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

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.

A perforated pipe used to assist the draining of soils.

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.