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The following is a labelled schematic of a constructed "Free-water surface flow wetland". These types of wetlands are generally used as a BMP for stormwater treatment and are most similar to stormwater management ponds, both in function and design, the major difference being that they are shallower to allow for wetland species (i.e. macrophytes to effectively grow and biologically treat incoming flows of pollutant and suspended sediments). A note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.


Wetlands fed by stormwater at Kortright Farm, Vaughan ON

Free-water surface flow wetlands are most commonly employed for stormwater treatment and are similar to SWM ponds in function and design The most significant difference is the extent to which they are designed to incorporate shallow zones for wetland plants. A facility is normally characterized as a wetland if shallow zones (<0.5 m deep) make up more than 70 % of its volume.

Wetlands are an ideal technology for:

  • Enhancing biodiversity
  • Providing a more aesthetic aquatic landscape

Sub surface flow systems provide generally lower health and safety risks and are sometimes employed to handle stormwater in combination with another wastewater stream.

Planning considerations[edit]

Types of Constructed Wetland[1][2][3]
Free-water surface flow Horizontal sub-surface flow Vertical sub-surface flow
Schematic of the Free Water Surface Constructed Wetland.jpg Tilley et al 2014 Schematic of the Horizontal Subsurface Flow Constructed Wetland.jpg Tilley et al 2014 Schematic of the Vertical Flow Constructed Wetland.jpg
  • Robust
  • Provides excellent water quality treatment
  • Resistant to temporary hydraulic overload
  • Can be beautiful
  • Sludge removal infrequent
  • Well established technology
  • May be natural looking, although often rectilinear in plan
  • Need little to no gradient
  • Provides buffer to discharge
  • Good pathogen removal from die off and predation
  • Minimal maintenance
  • Wide range of plants suitable
  • Robust
  • High levels of treatment possible
  • May be run without power if significant gradient is available
  • Can be attractively designed to generate interest in the technology, may be any shape.
  • Maintenance is technically simple. Sludge easily removed
  • Biologically complex and robust
  • Failure tends to be gradual
  • Will function prior to establishment of vegetation
  • Requires larger land area
  • Sludge removal may be more difficult
  • Open water may generate more health and safety concerns.
  • Requires more land
  • Multiple substrate layers will promote stratification and channelization
  • Requires fall of at least 1.5 m to provide sufficient treatment
  • May be high cost
  • Sensitive to hydraulic overloading


Sizing free-water[edit]

Design parameters for free-water surface flow wetlands [4]
Element Design Objective Criteria
Drainage Area Sustaining vegetation, volumetric turnover 5 Ha (≥10 Ha preferred)
Treatment Volume Provision of appropriate level of protection See below
Active Storage Detention Suspended solids settling 24 hrs (12 hrs if in conflict with min. orifice size)
Forebay Pre-treatment
  • Minimum depth: 1 m;
  • Sized to ensure non-erosive velocities leaving forebay;
  • Maximum area: 20 % of total permanent pool
Length-to-Width Ratio Maximize flow path and minimize short-circuiting potential
  • Overall: minimum 3:1;
  • Forebay: minimum 2:1
Permanent pool depth Vegetation requirements, rapid settling The average permanent pool depth should range from 150 mm to 300 mm
Active storage depth Storage/flow control, sustaining vegetation Maximum 1.0 m for storms < 10 year event
Side slopes (See also berms) Safety
  • 5:1 For 3 m above and below permanent pool;
  • Maximum 3:1 elsewhere
Inlet Avoid clogging/freezing
  • Minimum 450 mm;
  • Preferred pipe slope: > 1 %;
  • If submerged, obvert 150 mm below expected maximum ice depth
Outlet (See also flow control) Avoid clogging/freezing
  • Minimum: 450 mm outlet pipe;
  • Preferred pipe slope: > 1 %;
  • If orifice control used, 75 mm diameter minimum;
  • Minimum 100 mm orifice preferable
Maintenance access Access for backhoes or dredging equipment
  • Provided to approval of Municipality;
  • Provision of maintenance drawdown pipe preferred
Buffer Safety Minimum 7.5 m above maximum water quality/erosion control water level


Water volume storage requirements based on catchment type and receiving waters[4]
Performance level Storage volume (m3/Ha) required according to catchment impervious cover
35% 55% 70% 85%
80 % TSS removal 80 105 120 140
70 % TSS removal 60 70 80 90
60 % TSS removal 60 60 60 60

Modeling sub-surface[edit]

SubWet 2.0 is a modeling tool for sub-surface flow wetlands (both 100% constructed and naturalized/adapted). It can be used to simulate removal of nitrogen (including nitrogen in ammonia, nitrate and organic matter), phosphorus and BOD5 in mg/l and the corresponding removal efficiencies (in %). Although the model has been calibrated already with data from cold and warm climates, users can further calibrate and validate it using local data observations.



See Wetlands: Plants


Relative to a wet pond, a constructed wetland may offer added pollutant removal benefits due to enhanced biological uptake and the filtration effects of the vegetation. Early stage wetlands readily sorb phosphorus onto substrates and sediments. Phosphorus removal in wetland systems is usually carried out by incorporating alum sedimentation ponds or sand filters as cells of the system, and/or by polishing wetland effluent in an iron-dosed mechanical filter.[5]

Freezing temperatures in winter and early spring can reduce treatment if the wetland either freezes solid or a cover of ice prevents the water from entering the wetland. If under-ice water becomes confined, water velocities may increase, thereby reducing contact times[2]. Runoff in excess of maximum design flows should be diverted around the wetland to avoid excessive flows through the wetland.

STEP (under previous name SWAMP) conducted their own research into the performance of stormwater wetlands, the project page and report can be viewed here.

Central Lake Ontario Conservation Authority have been undertaking a coastal wetland monitoring project across Durham region, see here.


See also[edit]

External links[edit]

Articles for review[edit]

  1. Kennedy, G., and T. Mayer. 2002. Natural and Constructed Wetlands in Canada: An Overview. Water Qual. Res. J. Canada 37(2): 295–325. doi: 10.2166/wqrj.2002.020.
  2. Bendoricchio, G., L. Dal Cin, and J. Persson. 2000. Guidelines for free water surface wetland design. EcoSys Bd 8: 51–91. http://www.pixelrauschen.de/wet/design.pdf (accessed 9 May 2018).

  1. Grant, N., M. Moodie, and C. Weedon. 2000. Sewage Treatment Solutions. p. 35–67. In Sewage Solutions: Answering the Call of Nature. Centre for Alternative Technology Publications.
  2. 2.0 2.1 United States Environmental Protection Agency. 1995. A HANDBOOK OF CONSTRUCTED WETLANDS: A guide to creating wetlands for agricultural wastewater, domestic wastewater, coal mine drainage and stormwater.
  3. Jacques Whitford Consultants, 2008. CONSTRUCTED & ENGINEERED WETLANDS p. 1-21
  4. 4.0 4.1 Toronto and Region Conservation Authority (TRCA), and CH2M Hill Canada. 2018. Inspection and Maintenance Guide for Stormwater Management Ponds and Constructed Wetlands (T van Seters, L Rocha, and K Delidjakovva, Eds.).
  5. Jacques Whitford Consultants, 2008. CONSTRUCTED & ENGINEERED WETLANDS p. 1-21