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 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	Pros 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
Cons Requires larger land area Sludge removal may be more difficult Open water may generate more health and safety concerns.	Cons Requires more land Multiple substrate layers will promote stratification and channelization	Cons Requires fall of at least 1.5 m to provide sufficient treatment May be high cost Sensitive to hydraulic overloading

Design

Sizing free-water

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 sub-surface

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.

Materials

Planting

STEP stars These are species which have demonstrated good performance in projects designed, installed and monitored by the Sustainable Technologies Evaluation Program.

These are not the plants recommended for bioretention cells, rain gardens etc.
Bioretention cells, bioswales and similar structures normally have dry to moderate 'soil' conditions.
If you're looking for bioretention plants, you may choose from all of these grasses, perennials, shrubs, and trees.

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The most common types of emergent vegetation used in North American wetlands are:

• cattails (Typha spp.),
• bulrushes (Scirpus spp.), and
• reeds (Phragmites spp.),

although a variety of other wetland vegetation can be used as well. In many cases, planting a monoculture may be recommended rather than attempting to support biodiverse vegetation, since the more “aggressive”, stress-resistant wetland plants (e.g., cattails) will quickly displace others if they are present. In surface flow wetlands, cattails and bulrushes are the most common types of emergent vegetation used. The most common type of plant used in sub-surface flow wetlands is reeds, but cattails, bulrushes, reed canary grass (Pharis arundinacea) and managrass (Glyceria maxima) have also been used.

ALL of the plants listed above have issues associated with invasive species and/or hybridization between native and invasive species in Ontario.
If you are uncertain in making the correct selection, see the external links below for Provincial advice and specialist organisations that can help.

Sortable table, click on header you wish to select wetland plants by:

Scientific Name	Common Name	Native	Partial shade tolerance	Drought Tolerance 1=Low 2=Med 3=High	Salt Tolerance 1=Low 2=Med 3=High	Pollution tolerance	Compaction tolerance	STEP star!	Illustration
Schoenoplectus tabernaemontani (Scirpus validus)	Soft-stem Bulrush	Y		1	2-3	Y
Scirpus atrovirens	Dark-green Bulrush	Y		1	2	Y
Scirpus cyperinus	Cottongrass Bulrush	Y		2	2
Typha latifolia	Broad-leaf Cattail	Y		1	2-3	Y

Links

• Plants

External links

• List of invasive aquatic plants Ontario MNR (includes Glyceria maxima)
• Phalaris arundinacea BMP Ontario MNR
• Phragmites BMP Ontario MNR
• Ontario Phragmites Working Group

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. 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.

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
• Plants

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

Sustainable Technologies Evaluation Program

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

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

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

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

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.

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