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LID SWM Planning and Design Guide β


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.



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:

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.



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 paving 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 good practice to have several inlets sized to split higher flow between a number of smaller BMPs or along the length of a linear pratice (Offline overflow).

Trench drains Curb cuts Inlet sumps Depressed drains
  • A long, covered channel that collects directs water into the BMP.
  • 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.
  • 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.
  • An inlet sump is recommended to settle and separate sediment 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 a trenches or chambers .
  • Sump inlets should not be sited where pedestrians will have to negotiate with them.
  • 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


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.


  • 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 far end of the facility to prevent any localised ponding to cause bypassing of the infiltration facility.
  • Offline facilities use flow splitters or bypass channels that only allow the required water quality storage volume to enter the facility.
Higher flows are diverted and do not enter the infiltration practice. A pipe can by used for this, but a weir or curb cut minimizes clogging and reduces the maintenance frequency.

Overflow elevation

The invert of the overflow should be placed at the maximum water surface elevation of the practice. i.e. the maximum ponding depth. A good starting point is around 300 mm over the surface of the practice. However, consideration should be given to public safety and drainage time|time for the ponded water to drain. See Bioretention and Stormwater planters


  • In swales convey flowing water a freeboard of 300 mm is generally accepted as a good starting point.
  • In bioretention the freeboard is being defined as the depth between the invert of the overflow and the the inlet 150 mm would suffice, so long as the inlet will not become inundated during design storm conditions.
  • In above grade stormwater planters above grade, the equivalent dimension would be the depth between the invert of the overflow and the lip of the planter (150 mm minimum)
  • Where the stormwater planter is configured more like a lined/non-infiltrating bioretention system, the inlet will be the depth to which this is measured, as above (150 mm minimum).


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



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


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.


Starting after TRIECA (end March) members of STEP will be undertaking a literature review on the performance of our most popular BMPs. The results will be combined with the information we have to date from the development of the Treatment Train Tool and agreed performance metrics established. Until then, please feel free to continue to ask questions via email or the feedback box below.

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[1] >98 %
No underdrain United Kingdom >94 %
With underdrain Maryland[2] 46 - 54 %
Runoff reduction estimate 85 %