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 grassed channels, see Enhanced grass swales.

Overview

The fundamental components of a bioswale are:

• A graded channel
• Planting
• Filter media, to permit infiltration into the facility (not necessarily to soils below)

Additional components may include:

• Underdrain with clean out and inspection ports
• Impermeable membrane to prevent infiltration to soils below
• Check dams to facilitate short tern ponding

Design

Bioswales are sized as narrow linear bioretention cells. Drawdown times of bioswales are typically lower than other geometric configurations of bioretention facilities. Infiltration: Sizing and modeling

Materials

Sandy filter media mix.

Filter media being used for an online bioretention swale (also of previous, more sandy specification)

It is recommended that the mixture comprises:

One of these two blend options

	Blend A: Drainage rate priority	Blend B: Water quality treatment priority
Application	Impervious area to pervious area (I:P) ratio of 15:1 or greater	More diverse planting options, Improved metals and phosphorus retention.
Proportions	3 parts sand 1 part organic soil components and additives	3 parts sand 2 parts topsoil 1 part organic soil components and additives
Porosity	This mixture may be assumed to have a porosity of 0.4 unless demonstrated otherwise	This mixture may be assumed to have a porosity of 0.35 unless demonstrated otherwise

Filter media should be obtained premixed from a vendor and meet all municipal, provincial and federal environmental standards. Topsoil used to produce the mix should be passed through a 5 centimetre (2 inch) screen to remove large rocks, roots and other debris, while retaining soil peds. Samples of the filter media should be dried, ground and tested by a certified soil testing laboratory to ensure they meet the following specifications:

Bioretention filter media

Characteristic	Criterion	Recommended test method
Particle-size distribution (PSD)	< 25% silt- and clay-sized particles (smaller than 0.05 mm) combined; 3 to 12% clay-sized particles (0.002 mm or smaller)	ASTM D6913/D6913M, Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis (pebble to sand fraction); and ASTM D7928, Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis (silt and clay fraction).
Organic matter (OM)	3 to 10% by dry weight	ASTM F1647, Standard Test Methods for Organic Matter Content of Athletic Field Rootzone Mixes.
Phosphorus, plant-available or extractable	12 to 40 ppm	As measured by the 'Olsen' method for alkaline and calcareous soils (common in Ontario). Alternatives include 'Mehlich I or III', or 'Bray', which is better suited to acidic to slightly alkaline and non-calcareous soils. NB: Results from different test methods are not directly comparable.
Cationic exchange capacity (CEC)	> 10 meq/100 g	ASTM D7503, Standard Test Methods for Measuring the Exchange Complex and Cation Exchange Capacity of Inorganic Fine-Grained Soils.
Hydraulic conductivity, saturated (Kf)	> 75 mm/h; Blend A > 25 mm/h; Blend B < 300 mm/h; Blend A and B	ASTM D2434, Standard Test Method for Permeability of Granular Soils (Constant Head), when the sample is compacted to 85% of its maximum dry density in accordance with ASTM D698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort.

Note that you may choose not to use particle-size distribution as a criterion for acceptance of a filter media blend, but saturated hydraulic conductivity should be one. While information on particle-size distribution and soil texture are useful in selecting plants, the disconnect between hydraulic conductivity and uniformity of gradation makes it far less important than measuring the saturated hydraulic conductivity directly

Sand

Particle size distribution graph for ASTM C33 sand, as described in table

• Coarse sand for LID construction shall be washed clean and free of toxic materials.
• The pH of the sand shall be ≤ 7.0.
• The coarse sand shall have a fineness modulus index between 2.8 and 3.1 according to ASTM C33/C33M, or otherwise meet the gradation below.

Particle size distribution

Sieve	Percent passing
9.5 mm	100
4.75 mm (No.4)	95 - 100
2.36 mm (No.8)	80 - 100
1.18 mm (No.16)	50 - 85
0.60 mm (No.30)	25 - 50
0.30 mm (No.50)	5 - 30
0.15 mm (No.100)	0 - 10
0.075 mm (No.200)	≤ 3

Topsoil

• Topsoil may be material that was stripped from the project site and stored in stockpiles for re-use, or material imported to the site from a supplier provided the physical and chemical characteristics are within acceptable ranges.
• Topsoil shall be in compliance with Ontario Regulation 153/04 Record of Site Condition standards for soil quality or as amended through Ontario Management of Excess Soil - A Guide for Best Management Practices.
• Soil laboratory reports shall certify the material to be suitable for re-use on residential, parkland, institutional, industrial, commercial, or community landscapes for the germination of seeds and the support of vegetative growth.

The factors to consider in determining if a topsoil is suitable for use as planting soil for a vegetated stormwater practice, or use in producing a bioretention filter media mixture include the following:

• Must be friable and capable of sustaining vigorous plant growth;
• Must be free from toxic material and roots, stones or debris over 50 mm (2") in diameter;
• Should not have been passed through sieves or screens smaller than 50 mm (2”) to avoid eliminating peds;
• Should have a Loamy Sand, Sandy Loam, Sandy Clay Loam, Loam or Silty Loam soil texture;
• For use as planting soil for a vegetated stormwater practice , the topsoil must contain a minimum of 5% organic matter by dry weight or be amended so, through addition of an organic soil conditioner;
• For use in producing bioretention filter media Blend B (water quality treatment priority), the topsoil must contain at least 9%, and not greater than 36% clay-sized particles and at least 2% organic matter by dry weight.
• Must have a pH of between 6.0 and 8.0;
• Must have a sodium absorption ratio less than 15;
• Must have a cationic exchange capacity greater than 10 milliequivalents per 100 grams (meq/100 g).

Specify that 4 litre samples of topsoil, from each source to be drawn upon, be provided to the consultant for visual inspection, along with topsoil quality test results from an accredited soil testing laboratory, or a quality assurance certificate from the supplier.

We recommend a planting soil or filter media depth of 300 mm to support grasses, 600 mm for shrubs and perennials, and 1000 mm for trees.

Organic component

This is the first big opportunity to manage phosphorus export from a bioretention or stormwater planter system. While compost is the most common choice, designers working in nutrient-sensitive receiving waters are encouraged to explore the alternatives listed below. Some of these materials may be combined 50:50 with compost to balance providing the nutrients required by the plants with limiting the potential for leaching of excess nutrients.

Compost

Compost is the most widely used organic component. It's use in bioretention facilities is well established and documented. In Ontario, compost should comply with mandatory Ontario Compost Quality Standards for Category 'AA'. See Compost page for a summary of Category 'AA" compost standards. Compost should also be certified to meet quality parameters recommended under the Compost Council of Canada Compost Quality Alliance (CQA) program. Low available phosphorus composts should always be sought for use in low impact development facilities, including bioretention. Low available phosphorus composts are typically created from feedstocks including yard, leaf, and wood waste, and excluding manures, biosolids, and food scraps.

Organic component alternatives

Even low-phosphorus composts are known to export phosphorus over many years. The use of compost is not recommended in nutrient-sensitive watersheds where phosphorus pollution is a concern, or an additive to enhance nutrient retention of the media should also be included in the blend, or a layer be included above the stone reservoir, or a reactive media vault be included in the treatment train downstream of the bioretention (see Additive below for available options). There are a number of alternative sources of soil organic matter which have undergone field studies which have benefits and potential concerns:

Organic soil components

Material	Benefits	Concerns
Coconut coir	Doesn't leach phosphorus	Must be imported
Wood chips	Doesn't leach phosphorus Promotes nitrogen removal from water
Peat moss	Doesn't leach phosphorus	Must be unsustainably extracted from natural wetlands
Shredded paper (e.g., Pittmoss)	Doesn't leach phosphorus Promotes denitrification

Wood derivatives

The 2017 guidance from New Hampshire specifically rules against the inclusion of compost in their bioretention media. Instead they recommend "Shredded wood, wood chips, ground bark, or wood waste; of uniform texture and free of stones, sticks". The use of wood chip has been common in New Hampshire for some time, in this 2006 thesis 20% wood chips (not characterized) were incorporated into all of the test cases to match current practices at the time.

Shredded paper has been tested as an additional source of carbon and as an electron-donor to promote denitrification in a number of successful laboratory and field studies. <ref>

Additives

Typically these components would make up 5 to 10% by volume of the filter media mixture.

A number of granular amendments have been demonstrated to improve nutrient removal from discharge water in BMPs such as bioretention systems, stormwater planters, absorbent landscapes, sand filters or green roofs.

There are two primary processes involved, chemical precipitation and adsorption. Both mechanisms are ultimately finite, but have been shown in come cases to make significant improvements on the discharged water quality over several years.

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Filter Media Additives

Material	Benefits	Potential concerns
Biochar	Renewable Enhances soil aggregation, water holding capacity and organic carbon content	Currently expensive Energy intensive to produce Some sources say ineffective for phosphorus removal
Bold & GoldTM	Documented total phosphorus removal of up to 71%	Proprietary
Iron filings or Zero valent iron (ZVI)	Proven phosphorus retention Retained phosphorus is stable	May harm plants Removal efficiency declines with increased concentration of incoming phosphorus
Red sand or Iron-enriched sand	Proven phosphorus removal Also removes TSS	Poor orthophosphate removal in hypoxic or anoxic conditions
Smart SpongeTM	Removes phosphorus, as well as TSS, fecal coliform bacteria and heavy metals Non-leaching	1-3 year lifespan, after which the product is removed as solid waste Proprietary
Sorbtive MediaTM	High phosphorus removal efficiency	Proprietary
Water treatment residuals	Waste product reuse	Quality control (capabilities depend on source, treatment methods, storage time)

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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.

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

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

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

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

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).

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.

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.

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

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.

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

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.

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.

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).

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

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

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

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.

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.

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.

Any form of rain or snow.

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

Total suspended solids

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

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A perforated pipe used to assist the draining of soils.

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 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.

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