Bioswales

Bioswale with check dams, to facilitate short-term ponding (vertical scale exaggerated for diagram), graded sloped channel, mulch preserve soil moisture for plant survival to maintain the organic matter content of underlying filter media, filter media and vegetation to promote infiltration into the facility, an underdrain and river rock/stone to reduce flow velocity and erosion potential within the facility. A 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.

• This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
• For underground conveyance which promotes infiltration, see Exfiltration trenches.
• For design recommendations on channels in which surface flow is controlled with check dams, see Enhanced swales.

Overview

Similar to swales, biofilters, enhanced grassed swales, and bioretention, bioswales are long vegetated open channels trapezoidal channels which are designed to treat, convey, infiltrate, and attenuate incoming stormwater runoff. Similar to roadside ditches, they are predominately used to help direct large stormwater flow volume from a large catchment area (i.e., road, parking lot, property) to a connected discharge point (i.e. overflow outlet connected to the municipal storm sewer system, or to another LID BMP). Bioswales with the optional use of check dams can also slow and clean suspended sediments along the practice's sloped channel, while also allowing water to infiltrate through the system to an associated underdrain system below (Capital Regional District (CRD), n.d.)

There are several design variations of the bioswales, which include:

• Dry swales
• Enhanced swales
• Grass swales
• Retention swales

Bioswale vegetation is generally lawn grasses, but there are numerous variations that include: perennials, shrubs, small trees and various plants or [[river rock] that allow the practice to still effectively infiltrate and convey incoming stormwater runoff (Capital Regional District (CRD), n.d.).

Take a look at the downloadable Enhanced Grass Swales Factsheet, which includes details about bioswales below for a .pdf overview of this LID Best Management Practice:

The fundamental components of a bioswale are:

• A graded channel
• Planting
• Underdrain with clean out and inspection ports
• Filter media, to permit infiltration into the facility (not necessarily to soils below)

Additional components may include:

• Impermeable liner to prevent infiltration to soils below
• Check dams to facilitate short tern ponding

Planning considerations

Bioswales are sized as narrow linear bioretention cells. Drainage time of bioswales is typically lower than other geometric configurations of similarly sized bioretention facilities, owing to the higher hydraulic radius of the sides.

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

Design

Bioretention: Sizing

Inlets

Inlets for BMPs in the right of way should be located:

• At all sag points in the gutter grade
• Immediately upgrade of median breaks, crosswalks, and street intersections.

It is recommended to include multiple inlets, sized to distribute inflow along the length of the practice or between multiple facilities, where feasible, rather than concentrating all inflow into a single location. (Offline overflow).

Trench drains

Trench drains are long, covered channels that collect and direct water into the BMP. They are an excellent solution for streets where walking across the entire surface is to be encouraged. They can be designed as detectable edges or part of a detectable edge, and may be used to help define curbless or 'complete streets'.

Trenches may either be shallow (where runoff volume is less of an issue) or deep and covered by a metal grate. Deeper trench drains may gather sediment and require frequent maintenance.

Drains may be configured either perpendicular or parallel to the flow direction of the roadway, collecting runoff and directing to a single inlet in the BMP.

Curb cuts

Curb cuts are breaks along the length of a curb system to allow water to flow into a LID/BMP.

Inlet aprons or depressions increase inflow effectiveness of curb cuts. Steeply angled aprons can be hazardous, especially to people bicycling. Curbside and protected bike lanes along concrete aprons should be at least 1.8 m to give cyclists adequate clear width from the curb and any pavement seams. Aprons can also be marked visually to indicate their perimeter. For aprons into bioretention, the curb may angle into the cell to improve conveyance of gutter flow into the facility. Aprons typically drop 50 mm into the bioretention cell, with another 50 mm drop behind the curb to maintain inflow as debris collects. A depressed concrete apron can be cast in place or retrofitted in by grinding down the existing concrete pavement.

Where the curb alignment along the street is straight, the curb opening may optionally have a bar across the top of the inlet.

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  OPSD 605.040 Asphalt Spillway inlet to biofilter swale and road catch basin overflow outlet. County Court Blvd., Brampton, ON.

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  This curb cut has been sawn into existing concrete as part of a retrofit. Note the temporary (erosion log) and permanent stone erosion control measures in place. Mississauga Road, ON.

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  Curb cut used as a controlled overflow route from permeable pavement to a bioretention facility with monitoring well, Lake Simcoe Region Conservation Authority, Newmarket, ON.

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  Curb cut into a bioretention facility in Hinsdale, IL. Stone in the center of the facility reduces erosion and dissipates power inflow around the inlet area. A monitoring/maintenance well can be seen in the foreground. Photo credit: CNT

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  Curb cut into a bioretention facility in Brown Deer, WI. Stone is used to reduce erosion around the inlet area. Photo credit: Aaron Volkening

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  Curb cut into a bioretention facility in Ajax, ON.

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  Stone lined inlet at IMAX site in Mississauga

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  The grading around this inlet prevents flow in the correct direction. i.e. from the pavement onto the grass. Not too critical in this example, as the surface is permeable pavement.

Inlet sumps

An inlet sump is recommended to settle and separate sediments from runoff where a large amount of debris is expected. Water drains into a catch basin, where debris settles in its sump. After pretreatment, water drains via a pipe or opening into the BMP. The sump can be directly connected to a perforated underdrain pipe to distribute the flow to the bioretention, supported soil cells or underground practices such as trenches or chambers.

Sump inlets should not be sited where pedestrians will have to negotiate with them.

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  This pretreatment device, known as a baffle helps to remove trash, oil, TSS, larger sediments (silts, sands, etc.) and other floatables in roadway catch basins. This picture was taken of a SNOUT BAFL in Myrtle Beach, S.C. (Mullen, 2022)

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  Example of an overland flow sump, capturing stormwater runoff from the adjacent roadway. This example showcases a Rain Guardian^(TM) Turret structure with a metal grate overtop to capture larger floatables, trash and detritus from entering a stormwater facility or in this case a bioretention LID feature.

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  This inlet channels flow from an underground pretreatment unit onto the surface of a large bioswale, Mississauga Road, ON

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  A stormwater tree trench, including a sump inlet design used as part of pretreatment. The feature is located in the City of Cambridge, Massachusetts, US. The design was developed and lead by HDR Inc., and Halvorson, Tighe & Bond Studio (Halvorson, Tighe & Bond Studio, n.d.)

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  Example of a sump inlet being used in conjunction with a large bioretention basin feature, located in park space receiving stormwater off of the adjacent roadway (Source: Philadelphia Water Department, n.d.)

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  An inlet sump leading into a bioretention bump out for traffic calming. Inlet sumps help both settle and separate sediment from stormwater road runoff before depositing excess sediment and silt into an infiltration BMP. These sumps allow stormwater to flow into the feature through an underdrain pipe (Source: NACTO, 2017)

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  Example of a sump inlet allowing sediment to settle out of influent stormwater before entering a large infiltration chamber housed under the parking lot (Source: Philadelphia Water Department. 2020)

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  Example of a proprietary inlet device known as a Rain Guardian Bunker, that was installed at the Morningside Extension Project in Scarborough, ON.

Depressed drains

Runoff in the gutter drops into a grate-covered drain before flowing into the BMP. Drain covers must be compatible with bicycling and walking; grid covers are preferred. Depressed drains are a potential solution for bioretention cells on sloped streets where directing runoff into the cell is a challenge.

This style of inlet can be combined with a curb cut, to maintain capacity in case debris clogs the grate.

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  Combination of trench drain and winter shut off gate: King Street, Kitchener, ON

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  Trench drain that outlets to a bioswale at the LSRCA Office in Newmarket, ON

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  Trench drain in parking lot of Evergreen brickworks, Toronto ON

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  Bioretention system, or rain garden with a decorative trench drain cover, in Portland, US Taken in April 2013. Photo credit:EmilyBlueGreen

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  This trench drain has easily opened, hinged sections for maintenance, Singpore

Conceptual diagram of the excess routing alternatives: On the left, excess flow leaves the bioswale via an overflow; on the right, excess flow is diverted so that only the design volume enters the cell.

Overflow

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 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

Inspection and Maintenance

Maintenance requirements for bioswales is similar to [[swales00 and bioretention. It typically involve a low level of activity after vegetation becomes established. Grass channel maintenance procedures are already in place at many municipal public works and transportation departments. These procedures should be compared to the recommendations provided on the Inspection and Maintenance: Enhanced Swales page to assure that the infiltration and water quality benefits of enhanced grass swales are preserved.

Meanwhile, bioswales also require regular 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.

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 any 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.

Take a look at the Inspection and Maintenance: Bioretention & Bioswales page and theInspection and Maintenance: Enhanced Swales page by clicking below for further details about proper inspection and maintenance practices:

Materials

All forms of bioretention are complex in their structure, so please follow separate links for the materials.

• Plants and Planting design
• Bioretention: Filter media
• Choker layer
• Underdrains

Gallery

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  Note the use of stone in the centre of the channel, with mulch cover on the side slopes. Mississauga Road, ON

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  Bioswale in High Point neighborhood, Seattle, WA
  Photo credit: USEPA

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  Streetside swale in Seattle

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  This feature is a bioswale in that the underlying soil has been replaced with engineered filter media. The turf finish simplifies landscape maintenance. Brampton, ON

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  A bioswale located on Country Court Blvd., located in Brampton, ON> Read more about the performance of this feature and associated rpoject, by reading the following Grey to Green Conference Presentation, 2018. Dean Young - STEP, 2018

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  Bioswale located in Downsview Park, Toronto ON. with Check dams.

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  Grassed bioswale with Vegetated filter strips

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  Bioswale located in a parking lot with a 'beehive-styled' overflow, with Curb cuts Inlets at the back of the feature, located in the Burlington ON.

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  Stone-lined bioswale with rock check dams located to helps slow the flow of water as it enter the system from the nearby roads. [[ STEP, 2017

Planting Design Considerations

• Where possible a combination of native trees, shrubs and perennial herbs should be used in addition to grasses.
• Most bioswales will be situated to receive full sun exposure. The ‘Exposure’ column in the master plant list identifies the sun exposure condition for each species.
• Facilities with a deeper media bed (greater than 1 m) provide the opportunity for a wider range of plant species (including trees).
• For applications along roads and parking lots, where snow may be plowed or stored, non-woody and salt tolerant species should be chosen.
• Proper spacing must be provided for aboveground and below ground utilities, and adjacent infrastructure.

For adoption on residential properties see Rain Gardens or TRCA's A Complete Guide to Maintaining a Rain Garden

Performance

While few field studies of the pollutant removal capacity of bioswales are available from cold climate regions like Ontario, it can be assumed that they would perform similar to bioretention cells. Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, plant uptake, soil adsorption, and microbial processes. It is important to note that there is a relationship between the water balance and water quality functions. If a bioswale infiltrates and evaporates 100% of the flow from a site, then there is essentially no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff will continue to occur as it moves through the native soils.

Volumetric runoff reduction from bioswales

LID Practice	Location	Runoff Reduction*	Reference
Bioswale without underdrain	Washington	98%	Horner et al. (2003)
	Scotland	94%	Jefferies (2005)
Bioswale with Underdrain	Maryland	46 to 54%	Stagge (2006)
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)
	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)
Runoff Reduction Estimate*		85% without underdrain; 45% with underdrain

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.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

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

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 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.

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

Penetration of water through the ground surface.

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).

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.

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

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.

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.

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.

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.

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.

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.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

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.

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.

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).

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

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.

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).

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.

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

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

Total suspended solids

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

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.

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.

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

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.

Lake Simcoe Region Conservation Authority

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.

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

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

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.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

(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).

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).

Toronto and Region Conservation Authority

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.

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

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

A perforated pipe used to assist the draining of soils.

Credit Valley Conservation

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

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.

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.

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

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.

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