Bioretention

Partial infiltration bioretention cell draining a parking lot. Designs for this type of LID practice vary, but typically include a water storage reservoir, optional choker layer, filter media layer, mulch and vegetation. Other design variations include full infiltration and no infiltration configurations which can be explored with their own image maps. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Bioretention refers to vegetated stormwater practices that temporarily store roof and pavement runoff in depressed planting beds or vertical-walled structures. Depending on native soil infiltration rate and physical constraints, the facility may be designed without an underdrain for full infiltration, with an underdrain for partial infiltration, or with an impermeable liner and underdrain for no infiltration, "filtration only" (i.e., a stormwater planter or biofilter) design. Bioretention can be adapted to fit into many different development contexts and provides a convenient area for snow storage and treatment.

This article is about full and partial infiltration design variations that capture and infiltrate some or all of the stormwater received.
For simple systems, without underdrains or storage reservoirs (typically found in residential settings), see Rain gardens.
For linear systems that have a gradually sloping filter media bed and can convey surface flow, but are otherwise similar to bioretention, see Bioswales.
For planted systems that do not infiltrate 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 utilize all of the stormwater treatment mechanisms: sedimentation, infiltration, filtration, attenuation and evapotranspiration.

Bioretention is an ideal technology for:

• Fitting multi-functional vegetation into urban landscapes
• Treating runoff collected from nearby impervious surfaces

Take a look at the downloadable Bioretention Factsheet below for a .pdf overview of this LID Best Management Practice:

The fundamental components of a bioretention cell are:

• Inlets which may be curb openings (e.g. modified curbs, spillways), pipes, road or side inlet catchbasins, trench drains or other pre-fabricated inlet structures;
• A surface ponding area defined by landscaped side slopes or hardscape structures and the invert elevation of the overflow outlet structure;
• A filter bed containing filter media;
• A filter bed surface cover layer (e.g. mulch and stone);
• Plants, and;
• An overflow outlet to limit surface ponding and safely convey excess flow to a downstream storm sewer or the next BMP in the treatment train.

Additional components may include:

• An underdrain to redistribute or remove excess water and access structures or standpipes for periodic inspection and flushing;
• An internal water storage reservoir composed of a reservoir aggregate layer, and may include embedded void-forming structures to minimize depth and conserve aggregate, and organic material derived from untreated wood (aids in dissolved nitrogen removal);
• Monitoring wells installed to the base and screened in the underdrain aggregate to verify and track drainage time; and
• Filter media additives intended to enhance retention of nutrients, metals, petroleum hydrocarbons and/or bacteria.

Planning considerations

Infiltration

Some form of stormwater landscaping (bioretention) can be integrated 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 a 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, creating an internal water storage reservoir. 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 include fresh wood mulch in the storage reservoir aggregate, which fosters denitrifying biological processes.

For more information about constraints to infiltration practices, and approaches and tools for identifying and designing within them see Infiltration.

Where infiltration is infeasible or not permitted, but the design calls for planted landscaping, try a stormwater planter instead.

Native Soil

Bioretention can be constructed over any soil type, but hydrologic soil group A and B are best for achieving water balance objectives. Facilities designed to infiltrate water should be located on portions of the site with the highest infiltration rates. For infiltration rates <15 mm/h an underdrain is recommended. Native soil infiltration rate at the proposed location and depth should be confirmed through in-situ measurements of hydraulic conductivity under field saturated conditions. See Design infiltration rate for further guidance.

Wellhead Protection

Facilities receiving road or parking lot runoff should not be located within the 2 year time-of-travel Wellhead Protection Area for municipal drinking water supply wells (refer to applicable drinking water source protection plan for details).

Available Space

• The principles of bioretention can be applied in any scenario where planting or vegetation would normally be found.
• In the conceptual design stage it is recommended to reserve approximately 10 to 20% of the contributing drainage area for the bioretention facility footprint.
• For optimal performance recommended ratios of impervious drainage area to pervious facility footprint area (I:P area ratio) range from 5:1 on low permeability soils (HSG C and D) to 20:1 on high permeability soils (HSG A and B).
• Minimum bioretention facility footprint area (i.e., filter bed area) is based on the design storm runoff volume and effective surface ponding depth.
• 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 versus ease of maintenance.
• Bioretention can be almost any shape, from having very curvilinear, soft edges with variable depth, to angular, hard-edged and uniform depth.
• Linear trench or swale geometries will drain faster than round or square shapes because they have larger perimeters.
• 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 excavator, which should not be tracked into the cell.

Site Topography

Contributing slopes should be 1 to 5%. The filter bed surface should be flat to allow flow to spread out. A stepped, multi-cell design can also be used.

Available Head

If an underdrain is used, then an elevation difference of 1 to 1.5 m is needed between the inverts of the inlet and the downstream storm sewer.

Water Table

Maintaining a separation of 1 m between the elevations of the bottom of the practice and the seasonally high water table, or top of bedrock, is recommended. Lesser or greater values may be considered based on groundwater mounding analysis.

Pollution Hot Spot Runoff

To prevent groundwater contamination, runoff from pollution hot spots should not be treated by bioretention facilities designed for full or partial infiltration. No infiltration, "filtration only" facilities (with an impermeable liner) can be used.

Proximity to Underground Utilities

Designers should consult local utility design guidance for the horizontal and vertical clearances required between storm drains, ditches, and surface water bodies.

Karst

Infiltration designs are unsuitable in areas of known or implied karst topography and geologic formations.

Overhead Wires

Check whether the future tree canopy height in the bioretention area will interfere with existing overhead wires.

Setbacks From Buildings

If an impermeable liner is used, no setback is needed, although designing to maintain drainage away from the building is crucial. Where no liner is used, a setback of 4 m from the building foundations should be applied.

Context

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 is an increasingly popular choice to manage stormwater in these contexts.

Streetscapes

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.

For a table summarizing information on planning considerations and site constraints see Site considerations.

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 or connected cells
Single concentrated inflow	Forebays or distributed flow
No pretreatment	Pretreatment provided as part of inlet design
Over-sized underdrain	Moderately sized underdrain (or no underdrain)
Filter bed < 0.5 m	Filter bed > 0.75 m
Filter media Plant-Available Phosphorus > 40 ppm	Filter media Plant-Available Phosphorus < 40 ppm
Filter media is predominantly sand	Filter media is a mixture of sand, topsoil and organic material
Surface covered with turf grass and stone	Surface covered with mulch and dense, deeply rooting vegetation

Design Variations

The figures below illustrate three design variations for a parking lot bioretention cell that differ according to whether or not drainage via infiltration into underlying native soils is to be promoted (i.e., lined or unlined) and their outlet configuration (i.e., with or without an underdrain and location of the perforated pipe in the cross-section). Full infiltration design does not include an underdrain and is suitable on highly permeable soils (>15 mm/hr infiltration rate) and residential rain gardens. No infiltration or "filtration only" designs feature an impermeable liner and underdrain and are suitable on all soil types. Partial infiltration design, that includes an underdrain and internal water storage reservoir is recommended where native soil infiltration rate is less than 15 mm/hr and is the most common. All the figures are image map drawings from the following pages to compare side-by-side the differences between varying configurations.

Full infiltration bioretention cell draining a parking lot. This design variation includes a surface overflow pipe/structure to allow excess water to leave the practice. A monitoring well is included so drainage performance can be evaluated over its operating lifespan.Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Partial infiltration bioretention cell draining a parking lot. This design variation includes an underdrain and surface overflow pipes that allow excess water to leave the practice. A monitoring well is included so drainage performance can be evaluated over its operating lifespan. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Stormwater planter / No infiltration bioretention cell draining a parking lot. This design variation includes an impermeable liner, an underdrain and surface overflow pipes to allow excess water to leave the practice. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Sizing and Modelling

Bioretention facilities should be sized to accommodate runoff from approximately 5 to 20 times the footprint area of the facility. i.e. they should have an I/P ratio of 5 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 the bioretention facility, it can make the facility unreasonably costly.

• Sizing
• Modelling

Inlets

• Bioretention may receive runoff via overland sheet flow from a pavement edge or flush curb, concentrated overland flow from a gutter, curb opening or spillway, or as concentrated underground flow from an inlet pipe.
• Distribute concentrated inflows between multiple inlets or facilities and include flow spreading features to dissipate energy and avoid erosion of the filter media bed.
• For concentrated overland flow from roads, inlets should be located at all sag points in the gutter grade and immediately upgrade of median breaks, crosswalks and street intersections. Inlet types include curb openings, side inlet catch basins, trench drains and pre-fabricated inlet structures. Spillways aid in turning flow 30, 45 or 90 degrees into the practice. Incorporate concrete aprons at curb opening or spillway locations to increase inflow effectiveness. Provide a 50 to 100 mm drop in elevation between inlet invert and mulch or concrete apron surface.
• For concentrated underground flow, inlets may include pipes from roof drains, catch basins, storm sewers, oil and grit separators (OGS) or outlet leader from an upstream best management practice.

See Inlets for further guidance.

Pretreatment

Pretreatment prevents clogging by capturing sediment before it reaches the filter bed. It is typically necessary unless runoff sediment loads are very low (e.g. roof drainage). Pretreatment options include:

• Level spreader: A shallow trench structure (with concrete, metal or wood lip), graded to be level and installed parallel to the pavement edge or flush curb. Recommended sizing: (i) 1.4 m of length for every 0.01 m³/s of inflow during the design storm event, (ii) width of 300 mm or 3 times inflow pipe diameter, (iii) depth of 200 mm or half the inflow pipe diameter. Used with any overland flow inlets.
• Gravel diaphragm: A shallow, geotextile-lined depression, filled with clean aggregate, graded level and installed parallel to pavement edge (for sheet inflows) or perpendicular to concentrated inflows. Elevation change of 75 to 100 mm from pavement to top of diaphragm. Typically 600 mm wide by 300 mm deep. Stone: 5 to 20 mm (sheet flow inlets) or rip rap for concentrated inflows. Used with overland flow inlets.
• Vegetated filter strip: Receive runoff as overland sheet flow via pavement edge or flush curb. Use where drainage area flow path length is ≤ 25 m and slopes are <3%. Min. flow path length of 3 m, with 5 m preferred, and slope between 1 and 3%. Must be graded to provide a 75 to 150 mm elevation drop between the pavement and the filter strip surface.
• Catch basin, manhole, or other inlet structure sump: Consider using shield, baffle, trap or filter insert device to increase removal of sediment and debris. Used with overland or underground concentrated inflows.
• Forebay: Constructed with 2:1 length to width ratio and sized to accommodate ponding volume of 25% of the surface ponding storage requirement. Used with concentrated overland flow inlets.
• Oil and grit (hydrodynamic) separator. Used with concentrated underground flow inlets (inlet pipes).

See Pretreatment for further guidance.

Overflow Configuration

Bioretention can be designed to be inline (accepts all flow from the drainage area and conveys large event flows through an overflow outlet) or offline (allows only the design storm runoff storage volume to enter the facility). Overflow structures must be sized to safely convey large event flows out of the facility.

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.

Filter Media Bed

Recommended filter media bed depth is 0.5 to 1.0 m, but in constrained applications pollutant removal benefits may be achieved by beds as shallow as 0.3 m. Plant-specific depths recommended are 0.3 m to support grasses and perennials, 0.6 m to support shrubs and 1.0 metre to support trees. Filter bed must be installed level, with maximum side slopes of 1:3 or 33%. Maintaining a 75 to 100 mm layer of double shredded wood mulch is recommended on portions of the filter bed surface not covered by plantings. Benefits include soil moisture preservation, suppression of weed growth and enhanced water treatment. Erosion-prone inlets may require stone instead of mulch. Filter media should come pre-mixed from an approved vendor, be free of stones or debris greater than 50 mm diameter.

• Blend A - Use when drainage rate is the priority (when I:P ratio ≥15:1). Consists of 3 parts sand to 1 part organic material/additives.
• Blend B – Use when water quality treatment (phosphorus, metals) is the priority and more diverse planting options are desired. Consists of 3 parts sand to 2 parts topsoil to 1 part organic material/additives.

See Filter media for detailed guidance on different blends, required performance criteria, and specifications and laboratory tests for blend components and the filter media itself.

Geotextiles

To avoid clogging, the use of geotextile on the bottom of bioretention facilities or between filter media and underdrain aggregate materials should be avoided. Including a choker layer of clear stone aggregate to maintain separation between filter media and underlying aggregate is recommended instead. Geotextile socks should be installed on underdrain pipes with 360 degree perforations. Alternatively a strip of geotextile may be placed over the pipe to limit migration of fines from the overlying filter media into the pipe. Geotextile may be installed on the sides of the bioretention cell to maintain separation between backfill materials during construction. Geotextile used for weed control (i.e. landscape fabric) within the filter media bed or under mulch should be avoided. Geotextile that is used should have material specifications that conform to Ontario Provincial Standard Specification (OPSS) 1860 for Class II geotextile fabrics. Should be woven monofilament or non-woven needle punched fabrics. See Geotextiles for further guidance.

Underdrain

• Recommended where native soil infiltration rate is less than 15 mm/h (hydraulic conductivity < 1x10-6 cm/s) and needed for non-infiltrating, filtration only designs.
• Typically comprised of a length of perforated pipe embedded near the top of the storage reservoir, with an overlying choker layer of medium-sized aggregate, and structures to provide inspection and maintenance access.
• Alternatively the perforated pipe could be installed on the reservoir bottom and connected to an upturned pipe assembly or riser.
• May include an optional flow restrictor (e.g., orifice cap or valve) on the underdrain outlet pipe or outlet storm sewer, to provide erosion control and optimize infiltration while meeting the required drainage time.
• Continuously perforated, smooth interior HDPE or PVC pipe with diameter ≥ 200 mm to reduce freezing risk and facilitate access by camera and cleaning equip. Solid pipe leaders from inlet structures should extend 300 mm into the storage reservoir before transitioning to perforated pipe. Transitions back to solid pipe to connect to outlet storm drain system.
• See Underdrains for further guidance.

Choker Layer

A minimum 100 mm deep layer of clean 5 to 20 mm diameter angular or rounded stone (e.g. High Performance Bedding, HL-6, pea gravel) should be placed on top of the coarser storage reservoir aggregate to prevent downward migration of filter media into the reservoir and pipe. See Aggregates for further details.

Storage Reservoir

Should be a minimum of 300 mm deep, and filled with granular material of washed 25 or 50 mm diameter crushed angular stone (max. wash loss of 0.5%). See Aggregates for further details. Include organic material derived from untreated wood (e.g. chips, mulch or shavings) to enhance nitrogen removal. Also see Bioretention: Internal water storage. See Bioretention: Sizing for reservoir sizing guidance, including depth and area of surface ponding and reservoir volume needed to capture and store the design storm.

Monitoring wells and perforated pipe access structures

• Monitoring well should be a vertical standpipe consisting of an anchored 100 to 150 mm diameter pipe with perforations along the length within the reservoir, installed to the bottom of the facility, with a lockable cap. The well allows monitoring of post-event drainage times. See Monitoring wells for further guidance.
• Access structures, used for inspection and maintenance of the underdrain perforated pipes may be a maintenance hole or vertical standpipe connected to the pipe. Couplings used for standpipe connections should be 45° to facilitate pipe access by push camera or cleaning equipment. See Underdrains for further guidance.

Plants

Plantings on the filter bed should be dense to help maintain surface infiltration and improve sediment settling and retention of dissolved contaminants. It is recommended that perennial forbs and grasses are planted 0.5 m on center, and no more than 0.6 m on center. Planting plans should feature a mixture of deeply rooting perennials adapted to both wet and dry conditions and local climate. Road salt tolerance should be considered if facility will receive pavement runoff. Native plants are often the most hearty, adaptable, salt tolerant and deeply rooted options for bioretention planting. Invasive plants should not be planted in bioretention areas.

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.

A simplified schematic depicting the three primary zones associated with soil moisture levels and the appropriate plant species selected for each zone's specific soil characteristics, best adapted to the area's growing conditions. (Source: CVC, 2022)

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.
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 stormwater.
   • This area can be considered a transition area into other landscape or site areas.
   • A variety (min. five) species should be used to avoid monocultures.

Exposure to roadway or parking lot runoff must be considered.

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 plant lists identifies the sun exposure condition for each species.
• Facilities with a deeper filter media bed (e.g., 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 are not appropriate for areas where snow will be stored/piled during winter.
• 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.

Construction

Where possible, bioretention areas should be kept offline until construction is complete, the drainage area stabilized, and vehicle mud tracking has stopped. Diverting flows around the facility offers several benefits, including:

• less risk of erosion, clogging, and compaction;
• greater opportunity for establishment of seeded/planted areas; and
• easier access for construction, repairs or maintenance of the area.

Where diverting flows around the facility is not possible, and its location requires that it be used as a temporary runoff detention basin, protection measures can prevent fine sediment from migrating into the subgrade. During construction, the area should only be excavated to 75 cm above the final post-construction base of the facility. This layer of native soil will retain fine particles so that they will not migrate down into the subsoils. To prevent compaction during bioretention construction, heavy equipment should not enter the footprint of the bioretention area. For more detailed guidance, see the LID Construction Guide and the Erosion and Sediment Control Guide for Urban Construction .

Inspection and Maintenance

Bioretention requires regular, routine inspection and maintenance of the landscaping as well as periodic inspection of other parts of the facility. Routine maintenance should include weeding, pruning, and mulching, similar to other landscaped areas, as well as the removal of trash, debris and sediment accumulated in pretreatment areas, inlets and outlets. Watering may be needed until plant establishment (first 2 years).
Inspections should occur twice annually (spring and late fall) and after major storm events. Inspect for vegetation density (≥ 80% coverage), damage by foot or vehicle traffic, erosion, debris and sediment accumulation, and damage to pretreatment devices.
Cleanouts and access points should be provided to allow clean-out of the underdrain and overflow pipe. Camera inspection of these pipes should be conducted every 5 years to ensure pipes are free of roots, sediment and debris. Hydraulic flushing or root removal may be needed to clear debris or obstructions.

Performance

Ability of Bioretention to Meet Stormwater Management Objectives

BMP	Water Balance	Water Quality	Erosion Control
Bioretention with no underdrain	Yes	Yes-size for water quality storage requirement	Partial-based on available storage volume and native soil infiltration rate
Bioretention with underdrain	Partial-based on available storage, native soil infiltration rate and if a flow restrictor is used	Yes-size for water quality storage requirement	Partial-based on available storage, native soil infiltration rate and if a flow restrictor is used
Bioretention with underdrain and liner	Partial-some volume reduction occurs through evapotranspiration	Yes-size for water quality storage requirement	Partial-some volume reduction occurs through evapotranspiration

Water Balance

Bioretention practices have been shown to reduce runoff volume through both means of evapotranspiration and infiltration. The primary body of research is separated into bioretention practices either with underdrains and those without (therefore, relying solely on full infiltration into underlying soils). Volumetric performance improves when:

• Native soils have high infiltration capacity and facility is designed for full infiltration, without an underdrain;
• Size of the impervious drainage area relative to the facility permeable footprint area (i.e., I:P area ratio) is kept within recommended range of 5:1 (HSG C and D soils) to 20:1 (HSG A and B soils).
• Perforated pipe or outlet connection is elevated above the bottom of the practice in the underdrain cross-section;
• A flow restrictor (e.g., orifice, valve) is installed on the underdrain or storm sewer outlet pipe.

Volumetric runoff reduction from bioretention

LID Practice	Location	Runoff Reduction*	Reference
Bioretention without underdrain	China	85 to 100%*	Gao, et al. (2018)
	Connecticut	99%	Dietz and Clausen (2005)
	Pennsylvania	80%	Ermilio (2005)
	Pennsylvania	70%	Emerson and Traver (2004)
Bioretention with underdrain
	Ontario	64%	CVC (2020)
	Ontario	66%	STEP (2019)
	Texas	82%*	Mahmoud, et al. (2019)
	China	35 to 75%*	Gao, et al. (2018)
	Ohio	36 to 59%	Winston et al. (2016)
	Ontario	90%	STEP (2015)
	Ontario	91 to 96%	TRCA (2014)
	Virginia	97 to 99%	DeBusk and Wynn (2011)
	Maryland and North Carolina	20 to 50%	Li et al. (2009)
	North Carolina	40 to 60%	Smith and Hunt (2007)
	North Carolina	33 to 50%	Hunt and Lord (2006)
Bioretention with underdrain & liner
	Ontario	15 to 34%	STEP (2019)
	Queensland, Australia	33 to 84%	Lucke and Nichols (2015)
	Victoria, Australia	15 to 83%	Hatt et al. (2009)
	Maryland	49 to 58%	Davis (2008)
Runoff Reduction Estimate*		85% without underdrain; 45% with underdrain | 30% with underdrain and liner

Water Quality

Performance results from both laboratory and field studies indicate that bioretention systems have the potential to be one of the most effective BMPs for pollutant removal (TRCA, 2009). Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, soil adsorption, microbial processes and plant uptake. It is also important to note that there is a relationship between the water balance and water quality functions. If a bioretention cell infiltrates and evaporates 85 to 100% of the runoff from the drainage area during the design storm event, then there is little to no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff continues to occur as it moves through the native soil.

A comparative performance assessment of bioretention in Ontario was conducted comparing 9 different bioretention facilities in the GTA. The results showed total suspended solids (TSS) load reductions between 88 to 99%, and total phosphorus load reductions between 68 and 92% for unlined facilities. Results for a lined bioretention swale for TSS and Total Phosphorus load reduction were 73 to 79% and -18 to -21% respectively.(STEP, 2019). Negative TP load reduction values were observed because effluent concentrations were higher than influent concentrations, and volume reduction through evapotranspiration was not sufficient to offset the increase in phosphorus concentration in biofilter effluent. (STEP, 2018). Other STEP studies in the Greater Toronto Area have displayed similar results, with 90% reduction in TSS load when compared to nearby asphalt runoff samples having median TSS concentrations near the provincial 30 mg/L standard (median = ~19 mg/L) STEP, 2014 .

Another group of studies of bioretention facilities examines nutrient removal of these LID installation, with mixed results. Some facilities have been observed to increase total phosphorus in infiltrated water (Dietz and Clausen, 2005; Hunt et al., 2006 ; TRCA, 2008). These findings have been attributed to leaching from filter media soil mixtures which contained high phosphorus content. To avoid phosphorus export, the plant-available (extractable) phosphorus content of the filter media soil mixture should be examined prior to installation and kept between 12 to 40 ppm (see Filter media; Hunt and Lord, 2006). A design option to increase phosphorus removal performance of bioretention is to incorporate additives into the filter media bed, either blended into the media or as a layer, such as iron filings (i.e., zero valent iron), fly ash , iron (ferric) or aluminum hydroxide-based water treatment residuals (by-product from drinking water treatment) , biochar , proprietary filter media additives or blends, or by using iron-rich sand in the filter media blend. Read about a field evaluation comparing the phosphorus retention performance of parking lot bioretention cells featuring iron-rich sand and proprietary reactive media additive (Sorptive P^TM) in the STEP technical brief. While moderate reductions in total nitrogen and ammonia nitrogen have been observed in laboratory studies (Davis et al., 2001) and field studies (Dietz and Clausen, 2005), nitrate nitrogen removal has consistently been observed to be low. Design innovations to enhance nitrate-nitrogen removal performance of bioretention is an area of active research. Promising results have been observed from laboratory column and field-scale evaluations of underdrained practices featuring internal water storage reservoirs containing mixtures of clear stone aggregate and shredded newspaper or wood chips, which creates low oxygen or anoxic conditions and promotes conversion of nitrate-nitrogen to nitrogen gas via denitrification .

The mechanisms involved in, and ability of bioretention to reduce bacteria and other microbial pathogen concentrations is also an area of active research. Preliminary laboratory and field study results report good but variable removal rates for fecal coliform bacteria from biofilters and bioretention cells (Rusciano and Obropta, 2005; Hunt et al., 2006; TRCA, 2008). In a recent review, Clary et al. report bioretention E.coli removal efficiency of 42.5% and fecal coliform removal efficiency of 99.4% based on median inlet and outlet concentrations from 12 and 8 studies, respectively . In a recent article, Peng et al. (2016) review factors influencing microbial removal and effects of design choices on treatment performance. They found that approaches for improving the removal of microorganisms by biofilters could involve altering the grain size range and surface properties of the filter media. This could involve the use of filter media with smaller average grain sizes, the inclusion of additives (e.g., activated carbon, zeolite, or biochar) to improve filtration rates, or chemical modifications of filter media grain surfaces (e.g., with biocides) to promote microbial die-off. Including an internal water storage reservoir was also found to improve microbial removal rates .

Recent research into the role of plants in bioretention confirms they play an important roles in hydraulic and nitrogen removal performance. In a recent review of scientific literature, Dagenais et al. (2018) found that planted facilities are more effective than unplanted ones, as the presence of plants increases filter bed permeability and nitrogen removal. Plant species selection can considerably affect hydraulic and nitrogen removal performance, with root traits (e.g., thickness and depth) identified as playing important roles. They identified further research needed to test the hypothesis that native or diversely-planted facilities perform better than ones planted with exotic or fewer species.

Stream Channel Erosion

The feasibility of storing the channel erosion control volume within bioretention areas will be dependent on the size of the drainage area and available space. Meeting the channel erosion control requirement through bioretention is most feasible in the regions of the Greater Toronto Area with Hydrologic Soil Group A and B soils. In these situations, the reduction in runoff volume through infiltration and evapotranspiration may be sufficient. For facilities with constrained footprints, include a flow restrictor on the underdrain perforated pipe or outlet storm sewer to control the release rate, and size the internal water storage reservoir to retain the necessary volume of water. Where acceptable, consider increasing the maximum ponding depth beyond the recommended 350 mm. It is important to note that the bioretention practice will infiltrate runoff into the filter media bed and underlying native soil over the duration of the design storm event, which can be factored into surface ponding and internal water storage reservoir sizing to optimize the design. See Sizing for further guidance.

Other Benefits

The benefits of bioretention reach beyond the specific stormwater management goals to other social and environmental benefits, including:

• Reduced thermal aquatic impacts: Bioretention and other filtration and infiltration practices benefit aquatic life by reducing thermal impacts on receiving waters from urban runoff (Jones and Hunt, 2009). Unlike detention ponds, bioretention does not raise water temperature and can help maintain baseflows through infiltration.

• Snow Storage: Bioretention areas can be used for snow storage and snow melt treatment from the contributing drainage area during winter, especially those located adjacent to parking lots and roadways. To function as snow storage, bioretention must include an overflow for snow melt in excess of the designed ponding depth. Additionally, the plant material must be salt-tolerant, perennial and tolerant of periodic inundation.

• Reduced Urban Heat Island: Bioretention is able to reduce the local urban heat island by introducing soils and vegetation into urban areas, such as parking lots. Vegetation absorbs less solar radiation than hard urban surfaces. Also, the water vapor emitted by plant material also cools ambient temperatures.

See also

• Rain gardens
• Bioswales
• Stormwater planters
• Stormwater tree trenches
• Bioretention: Streetscapes

External links

• City of Toronto Green Streets Technical Guidelines - Version 1.0 (2017) Schollen & Company Inc., Urban Forest Innovators, TMIG, DMG
• CSA W200-18 Design of Bioretention Systems (2018) CSA Group
• CSA W201-18 Construction of Bioretention Systems (2018) CSA Group
• Bioretention Design Guidelines (2014) Healthy Waterways (Australia)

Gallery

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  Parking lot bioretention with surface ponding well in foreground, Kortright Centre, Vaughan, ON. Read about the performance of this practice in the technical brief.

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  Parking lot bioretention sharing underground reservoir with adjacent permeable paving, Edwards Gardens, Toronto, ON. Read a case study about it here.

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  Parking lot bioretention with decorative stone for erosion control, and perforated pipe access standpipe in foreground, Earth Rangers Centre, Vaughan, ON. Read about the performance of this practice in the technical brief.

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  The sunken curb holds the edge of the asphalt pavement and lets water freely flow to the bioretention cell beside the 7sigma parking lot in Minneapolis, MN (USA)
  Photo credit:Brian Ash

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  Bioretention cells on Elm Drive, Mississauga, ON, with vertical sides and hardscape perimeter receive road runoff via leader pipes from road catch basin inlets. Read about the stormwater system at this site in the performance assessment report.

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  The grade and lack of curb allow for sheet-flow runoff into this bioretention facility. Lakeside Park, Mississauga, ON

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  Bioretention cell receiving road and walkway runoff in a high density development setting.

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  Bioretention bump out in a dense urban setting receiving runoff via side inlet catch basin and curb openings on the walkway side of the installation.

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  Fairford Parkette featuring a bioretention cell retrofitted into a roadway in downtown Toronto, ON. Read a case study about it here.

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  Bioretention cell located in Waterfront Park in Ajax, ON.

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  Parking lot bioretention cell at IMAX corporate headquarters in Mississauga, ON. Read about the IMAX stormwater system in the performance assessment report

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  A curb extension bioretention cell in action during a major rain event.

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  Bioretention cell receiving road runoff in the Wychwood neighbourhood of Brampton, ON. The nearby catch basin conveys overflows during major rain events. Read about the stormwater system in this community in the performance assessment report.

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  Kortright Centre parking lot bioretention cell in action during a rain event, Vaughan, ON.

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  Curb extension in Victoria. Image credit Dylan Passmore

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  'Offline' streetscape bioretention with trees, King Street, Kitchener

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  Bioretention on residential street, with covered drain inlet (grate in the gutter) and iron grate covered overflow in the centre of the cell, Hinsdale, IL Photo credit: CNT

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  Curb extension in downtown Indianapolis, IN

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  Curb extension on Queen Lane, in Philidelphia
  Photo credit: PWD

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  Curb extension with Urban Tree in a tree grate, in city showcased by the U.S Department of Transportation's, Federal Highway Administration.
  Photo credit: Jennifer Rosales

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  Boulevard with Urban Trees along the boulevard in tree grates, in Orlando, FL.
  Photo credit: Design by: GAI Consultants/Kevin Aust & Contractor: UCC Group.

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  Schematic of Urban Trees in tree grates in a proposed traffic-calming bump-out as part of the on Columbus Av. New York City, NY. The red arrow depicts the flow of water to the stormwater tree trench system.
  Photo credit: Jonathan Toner

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  Curb extension with planted tree in open tree pit, part of a finished soil cell system - a tree grate for safety reasons and increased aesthetic should be added. Located on Main St. Markham, ON.
  Photo credit: GreenBlue Urban Inc.

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  Rainwater harvesting cistern, which discharges to tree pits during dry conditions. Image credit Mississippi WMO

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  Extended tree pit planting in USA
  Photo credit: USEPA

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  A right of way bioswale installed by the New York City Department of Environmental Protection on Dean Street, Brooklyn

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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.

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 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.

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

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.

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

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

A perforated pipe used to assist the draining of soils.

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.

A vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

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

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

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

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.

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.

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.

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.

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.

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

Penetration of water through the ground surface.

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.

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.

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.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

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

A parameter that describes the capability of a medium to transmit water.

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.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

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 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.

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.

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.

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

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

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

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

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

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

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

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

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.

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

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Soil particles with a diameter less than 0.050 mm.

Includes the protection of soil from dislocation by water, wind or other agents.

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

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Defined as those plants (including grasses, herbaceous species, shrubs, vines and trees) that have historically existed within a particular area. Native plants have co-evolved with the local ecosystems and natural processes.

Typically non-native plants that out-compete native species. These species lack natural predators, grow aggressively and reproduce rapidly and can be problematic in the natural environment.

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

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

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

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Cleanouts are "a fitting access in a drainage system or venting system that is installed to provide access for cleaning and inspection and that is provided with a readily replaceable air tight cover" (O.Reg 332/12: Building Code, 2022).

Cleanouts are "a fitting access in a drainage system or venting system that is installed to provide access for cleaning and inspection and that is provided with a readily replaceable air tight cover" (O.Reg 332/12: Building Code, 2022).

Credit Valley Conservation

Sustainable Technologies Evaluation Program

Toronto and Region Conservation Authority

The attachment of gas, vapour or dissolved matter onto the surface of solid materials.

Total suspended solids

Total phosphorus

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

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.

Tank used to store rainwater (typically roof runoff) for later use.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

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 total mass of a pollutant entering a waterbody over a defined time period.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

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

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

The period of time between consecutive storms or rainfall events.

A perforated pipe used to assist the draining of soils.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

The total amount of particulate matter that is suspended in the water column.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

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.