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
- Underdrain with clean out and inspection ports
- Filter media, to permit infiltration into the facility (not necessarily to soils below)
Additional components may include:
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|
||Depressed drains: Gallery|
- 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.
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|
|Ditch inlet catch basin||x||x|
Flat metal overflow with stone surround to reduce erosion around the cast concrete structure. Mississauga Road, ON
- 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
All forms of bioretention are complex in their structure, so please follow separate links for the materials.
Streetside swale in Seattle
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.
|No underdrain||Washington||>98 %|
|No underdrain||United Kingdom||>94 %|
|With underdrain||Maryland||46 - 54 %|
|Runoff reduction estimate||85 %|
- 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.