Wetlands

Wetlands fed by stormwater at Kortright Farm, Vaughan ON

Overview

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

Types of Constructed Wetland

Free-water surface flow	Horizontal sub-surface flow	Vertical sub-surface flow
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

Design parameters for free-water surface flow wetlands

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

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Water volume storage requirements based on catchment type and receiving waters

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

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.

Performance

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. 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.

Gallery

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

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

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  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).

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

See also

• SWM ponds
• Dry ponds

External links

• Ontario's wetland conservation strategy
• Centre for Advancement of Water and Wastewater Technologies at Fleming College

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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.

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.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

Total suspended solids

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