Bioswales

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This article is about installations designed to capture and convey surface runoffThat potion of the water precipitated onto a catchment area, which 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. along a vegetated channel, whilst also promoting infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..
For underground conveyance which promotes infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., 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 bioswaleLinear 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. 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:

Planning considerations

BioswalesLinear 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. are sized as narrow linear bioretention cells. Drainage time of bioswalesLinear 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. is typically lower than other geometric configurations of similarly sized bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facilities, owing to the higher hydraulic radius of the sides.

Design

Inlets

Overview

Concentrated flow inlets are associated with LIDLow 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. 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 precipitationAny form of rain or snow. 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 BMPBest 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.. 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 runoffThat potion of the water precipitated onto a catchment area, which 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. volume is less of an issue) or deep and covered by a metal grate. Deeper trench drains may gather sedimentSoil, 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. and require frequent maintenance.

Drains may be configured either perpendicular or parallel to the flow direction of the roadway, collecting runoffThat potion of the water precipitated onto a catchment area, which 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. and directing to a single inlet in the BMPBest 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..

Curb cuts

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

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

Inlet sumps

An inlet sump is recommended to settle and separate sedimentsSoil, 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. from runoffThat potion of the water precipitated onto a catchment area, which 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. where a large amount of debris is expected. Water drains into a catch basinGround depression acting as a flow control and water treatment structure, that is normally dry., where debris settles in its sump. After pretreatment, water drains via a pipe or opening into the BMPBest management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.. The 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.

Depressed drains

RunoffThat potion of the water precipitated onto a catchment area, which 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. in the gutter drops into a grate-covered drain before flowing into the BMPBest 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.. Drain covers must be compatible with bicycling and walking; grid covers are preferred. Depressed drains are a potential solution for bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cells on sloped streets where directing runoffThat potion of the water precipitated onto a catchment area, which 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. 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 inlineRefers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet. or offlineRefers 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. from the drainage systemA 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. See Inlets
  • InlineRefers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet. facilities accept all of the flow from a drainage areaThe 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. 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.
  • OfflineRefers 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. facilities use flow splitters or bypass channels that only allow the design storm runoffThat potion of the water precipitated onto a catchment area, which 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. storage volume to enter the facility. Higher flows are conveyed to a downstream storm sewer or other BMPBest 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. 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 catchbasinBox like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement. 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 mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. cover. However, consideration should be given to public safety, whether or not an underdrainA perforated pipe used to assist the draining of soils. is included, the time required for ponded water to drain through the filter bed surface, and if no underdrainA perforated pipe used to assist the draining of soils. is present, into the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. (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 basinGround depression acting as a flow control and water treatment structure, that is normally dry. x
DitchA long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line. inlet catch basinGround depression acting as a flow control and water treatment structure, that is normally dry. x x
Curb cut x x x

Gallery

Materials

All forms of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. are complex in their structure, so please follow separate links for the materials.

Gallery

Planting Design Considerations

  • Where possible a combination of native trees, shrubs and perennial herbs should be used in addition to grasses.
  • Most bioswalesLinear 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. 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.

Performance

While few field studies of the pollutant removal capacity of bioswalesLinear 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. are available from cold climate regions like Ontario, it can be assumed that they would perform similar to bioretention cells. BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. provides effective removal for many pollutants as a result of sedimentationDeposition 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., filtering, plant uptake, soil adsorptionThe attachment of gas, vapour or dissolved matter onto the surface of solid materials., and microbial processes. It is important to note that there is a relationship between the water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. and water quality functions. If a bioswaleLinear 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. infiltrates and evaporates 100% of the flow from a site, then there is essentially no pollution leaving the site in surface runoffThat potion of the water precipitated onto a catchment area, which 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.. Furthermore, treatment of infiltrated runoffThat potion of the water precipitated onto a catchment area, which 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. will continue to occur as it moves through the native soils.

Design Location RunoffThat potion of the water precipitated onto a catchment area, which 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. reduction
No underdrainA perforated pipe used to assist the draining of soils. Washington[1] >98 %
No underdrainA perforated pipe used to assist the draining of soils. United Kingdom >94 %
With underdrainA perforated pipe used to assist the draining of soils. Maryland[2] 46 - 54 %
RunoffThat potion of the water precipitated onto a catchment area, which 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. reduction estimate 85 %
  1. Horner RR, Lim H, Burges SJ. HYDROLOGIC MONITORING OF THE SEATTLE ULTRA-URBAN STORMWATER MANAGEMENT PROJECTS: SUMMARY OF THE 2000-2003 WATER YEARS. Seattle; 2004. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.365.8665&rep=rep1&type=pdf. Accessed August 11, 2017.
  2. https://www.pca.state.mn.us/sites/default/files/p-gen3-14g.pdf