Wetlands

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Wetlands fed by stormwater at Kortright Farm, Vaughan ON

Overview[edit]

Constructed wetlands are similar to SWM ponds in function and design, with the most significant difference being that 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:

  • Text A
  • Text B

Planning considerations[edit]

Types of Constructed Wetland[1]
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
Pros
  • Robust
  • Provides excellent water quality treatment
  • Resistant to temporary hydraulic overload
  • Can be beautiful
  • Sludge removal infrequent
Pros
  • Well established technology
  • Low cost
  • May be natural looking
  • 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
Pros
  • High levels of treatment possible
  • DIY possible (to lower cost)
  • May be run without power if significant gradient is avaialble
  • Can be attractively designed to generate interest in the technology.
  • Maintenance is technically simple
  • Biologically complex and robust
  • Failure tends to be gradual
  • Will function prior to establishment of vegetation
Cons
  • Requires larger land area
  • Sludge removal may be more difficult
  • Open water may generate more health and safety concerns.
Cons
  • Requires more land
Cons
  • Requires fall of at least 1.5 m to provide sufficient treatment
  • May be high cost
  • Sensitive to hydraulic overloading

Design[edit]

Design parameters for free-water surface flow wetlands [2]
Element Design Objective Criteria
Drainage Area Sustaining vegetation, volumetric turnover 5 Ha (≥10 Ha preferred)
Treatment Volume Provision of appropriate level of protection (Table 3.2) See Table 3.2
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;

8Minimum 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

Performance[edit]

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.

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

Modeling[edit]

SubWet 2.0 is a modeling tool for subsurface 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.

Gallery[edit]

Construction[edit]

See also[edit]

External links[edit]

Ontario's wetland conservation strategy 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. https://www.epa.gov/sites/production/files/2015-10/documents/constructed-wetlands-handbook.pdf

  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. 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.).