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][2]
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 [3]
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.

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.

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]

  • Wetlands on the Rouge River, Toronto ON. Photo credit: Vlad Litvinov (flickr)

  • Emergent wetland vegetation supported by stormwater runoff at Kino Environmental Restoration Project. Photo by Matthew Grabau, US Fish and Wildlife Service

  • Azalea Park, Charlottesville VA - "This side of the park, formerly located along a runoff channel that led into Moore's Creek, has been converted into a wetland which supports a surprising amount of insect and amphibian life." -Credit and Photo: Scott Clark (certhia on Flickr).

  • Walberswick Mill sluice, part of the water level control and coastal defence infrastructure within the marshes, UK

See also[edit]

External links[edit]

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.

  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. ↑ 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.).

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

A pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

A pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

Includes the protection of soil from dislocation by water, wind or other agents.

A body of water smaller than a lake, often artificially formed.

The technique of removing pollutants from runoff as it infiltrates through the soil.

The portion of water precipitated onto a catchment area, which then 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.

Sustainable Technologies Evaluation Program

Shallow, constructed pools that capture stormwater and allow for the growth of characteristic wetland vegetation.

The portion of water precipitated onto a catchment area, which then 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.

The liquid waste from domestic, commercial and industrial establishments. City of Toronto Wet Weather Flow Management Guidelines November 2006 49

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Toronto and Region Conservation Authority

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