Bioswales

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

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 membrane 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 are typically lower than other geometric configurations of similarly sized bioretention facilities, owing to the higher hydraulic radius of the sides.

Design

• Bioretention: Sizing

Overview

Concentrated flow inlets are associated with LID practices such as Bioretention, Stormwater planters, Infiltration trenches and chambers. Sheet flow alternatives include level spreaders, gravel diaphragms and vegetated filter strips. Practices such as permeable pavements and green roofs receive precipitation directly, whilst exfiltration trenches are connected directly to conventional storm sewers.

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.

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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

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

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  This bioretention facility is sunken from it's surrounding landscape, in part to accommodate the drop the the catchbasin inlet. Elm Drive, Mississauga, ON

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  A clogged inlet at the bioretention facility on Elm Drive in Mississauga in need of maintenance.

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. Depressed drains: Gallery

External links

https://nacto.org/publication/urban-street-stormwater-guide/stormwater-elements/bioretention-design-considerations/inlet-design/

Overflow

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 Inlets
• 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 overflow structure and the inlet. 150 mm will 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 overflow structure and the lip of the planter (150 mm minimum)

Overflow outlet options

Metal grates are recommended (over plastic) in all situations.

Feature	Anti Vandalism/Robust	Lower Cost Option	Self cleaning
Dome grate			x
Flat grate		x
Catch basin	x
Ditch inlet catch basin	x		x
Curb cut	x	x	x

Gallery

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

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

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  Overflow inlet for newly constructed stormwater bioretention areas in median of Bradley Road. Village of Brown Deer, Wisconsin. Bradley Road, east of 51st Street. Photo from October 2015. Constructed summer 2015.
  Photo credit: Aaron Volkening

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  Flat, metal overflow grate in Emeryville California Stormwater Curb Extension

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

Materials

• Bioretention: Filter media

Wonderful plants varying in form, texture and colour, TBG ON

The vegetation is a big opportunity to maximize the co-benefits of biodiversity and amenity. Planting plans can be formalized or naturalized to suit the surrounding style. In addition to aesthetic qualities, plants have specific functions in several LID practices. These include promotion of infiltration, treatment of pollutants and stabilization of soil. When selecting plants for an LID practice, aim for species with high functionality, survivability, suitability and availability. Landscape professionals should use these lists as guides, taking into consideration the appropriate planting zone, the size of the planting area versus size of the plant at maturity, tolerances to drought or periodic inundation, maintenance requirements and adaptability.

• To help you select appropriate plants for your site, we've developed tables to indicate the suitability for use in LID features.
• For resilient and robust planting, native species which can tolerate periods of drought and periodic inundation are recommended.
• Woody and evergreen plants should not be planted in any areas of the bioretention cell to be used as snow storage.
• Dense shrubby plants should be avoided in locations where the accumulation of trash is anticipated as a maintenance problem, or where their growth can hinder maintenance and inspection of inlets or other structures.
• Trees should not be planted directly over underdrains, and may be better sited at the perimeter of bioretention cells.
• While it is not always necessary to use an entirely native planting palette, invasive plants are inappropriate for LID practices.

Plant Selection

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Select trees Select shrubs Select perennials Select climbing plants Select tall grasses Select plants for green roofs Select plants for wetlands

Plant Characteristics

Soil moisture

Plant species are adapted to specific levels of moisture to achieve establishment and sustained growth. Soil moisture has been characterized by three categories: dry (1), moist (2) and wet (3). Some plants can tolerate a wide range of moisture regimes, whereas others perform optimally in a more narrow range of soil moisture conditions. Species ranked with a dash between two numbers can tolerate a range of conditions.

Shade tolerance

Plant species react differently to varying levels of sunlight and shade. Plant adaptations to these parameters are referred to in terms of degree of exposure. Most of the LID practices will be installed in newly developed areas, thereby providing exposure to full sun, meaning at least 6 full hours of direct sunlight for plantings. As trees develop over several years, or if an LID practice is installed in an area where there are existing trees or buildings providing partial shade, plants adapted to 3 to 6 hours of sunlight exposure should be used. Plants tolerant of full shade require less than 3 hours of direct sunlight each day. However, some shade-adapted species come into leaf early in the growing season in order to take advantage of full sunlight before tree leaves emerge and create shade.

Our tables indicate whether the species in question is tolerant of shade at all. For more information, consult the sources listed below.

Drought tolerance

These categories represent broad generalisations regarding drought tolerance.

Salt tolerance

The low, medium and high categories indicate the tolerance of plant species to salt exposure and/or uptake. Plant species with low salt tolerance should not be used in any LID practice receiving runoff from salted roads and parking lots. Species with medium salt tolerance can be utilised in LID practices that will be receiving road runoff but should not be in the line of salt spray or be receiving the bulk of the runoff. Species with high salt tolerance should be planted in LID practices that receive road or parking lot runoff that routinely contains road salt. Few plants are truly halophytic or “salt-loving”. In most cases, elevated salt levels are temporary and precipitation quickly dilutes and removes salt from the soil profile. The plant lists below include recommended species for LID practices likely to receive road or parking lot runoff.

Compaction and pollution tolerance

Development nearly always causes compaction of on-site soil, and bioretention facilities in road-right-of-ways should be pollution tolerant.

STEP stars

These are species which have demonstrated good performance in projects designed, installed and monitored by the Sustainable Technologies Evaluation Program.

Advanced plant selection criteria

Plant characteristic	Potential benefit to LID performance
Plant mass	Higher biomass consumes more nutrients (decreases nutrient discharge from bioretention) and increases transpiration rate.
Growth rate	Higher growth rate consumes more nutrients, particularly in combination with root characteristics as below.
Root lipid content	Higher root lipids have been associated with increased plant uptake of organic contaminants such as polyaromatic hydrocarbons (PAHs)
Root length	Longer roots are associated with plants consuming more nutrients, although roots which reach the bottom to the media may contribute nutrient...
Root mass and thickness	Larger overall root mass and many dense fine roots are associated with increased nutrient uptake by plants. Thicker roots help to preserve hydraulic conductivity of the media.
High-nutrient tolerance	Plants adapted to high nutrient environments are likely to uptake nutrients at a higher rate.

See Also

• Planting design
• Bioswales
• Rain gardens
• Materials
• Bioretention
• Green roofs
• Wetlands

External resources

Organization	Coverage	Types of Material	Website
Watersheds Native Plant Database	Canada / Ontario	Grasses, Ferns, Shrubs, Trees, Vines	https://watersheds.ca/plant-database/
Online Plant Guide	USA	Grasses, Ferns, Herbaceous, Shrubs, Trees, Vines, Ornamental	http://onlineplantguide.com/Index.aspx
North American Native Plant Society	N. America	Grasses, Ferns, Herbaceous, Shrubs, Trees, Vines	http://www.nanps.org/plant/plantlist.aspx
United States Department of Agriculture	N. America	Grasses, Ferns, Herbaceous, Shrubs, Trees, Vines, Ornamental	https://plants.usda.gov/java/

• Leaf and fruit identification for trees and shrubs

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• See also Planting design

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

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.

Design	Location	Runoff reduction
No underdrain	Washington	>98 %
No underdrain	United Kingdom	>94 %
With underdrain	Maryland	46 - 54 %
Runoff reduction estimate		85 %

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.

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.

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.

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

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.

Any form of rain or snow.

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.

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.

Lake Simcoe Region Conservation Authority

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.

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.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

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.

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.

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

A perforated pipe used to assist the draining of soils.

The 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.

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.

Sustainable Technologies Evaluation Program

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

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.

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.