Wetlands

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.

Overview

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 between ponds and wetlands is the extent to which they 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 can contribute to:

• Enhanced biodiversity
• Recreational opportunities and a more aesthetic aquatic landscape
• Improved water quality and achieving TSS targets
• Flood attenuation

Case studies are available for wetlands used in LID systems.

Planning considerations

Wetlands differ based on how water travels through the system. Free-water surface flow wetlands have water exposed on the surface, which provides excellent water quality treatment but may pose health and safety risks. Sub surface flow systems provide generally lower health and safety risks and are sometimes employed to handle stormwater in combination with another wastewater stream. In horizontal sub-surface flow wetlands, water flows horizontally through a media bed, while in vertical sub-surface flow wetlands, water is introduced at the surface and percolates vertically through the media.

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

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

• For planting recommendations, see Wetlands: Plants

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.

Inspections and Maintenance

Constructed wetlands need to be regularly inspected and maintained to ensure that the system is meeting the intended level of treatment. Like all LID systems, construction and assumption inspections should be undertaken during building and handover of constructed wetlands.

Routine operation inspections and preventative maintenance for the lifespan of constructed wetlands should cover:

• Review as-built drawings, maintenance history, and prepare inspection form prior to site visit
• Ease of access for inspections
• Vegetation management - grass mowed to a height greater than 4–6" and clippings collected, invasive species monitoring and control
• Repair of vandalized areas
• Sediment accumulation
• Mechanical equipment check
• Structural component check

A field data sheet for the inspection of constructed wetlands is available below. You can download (downward facing arrow on the top righthand side) and print (Printer emoticon on top right hand side) the Wetland Maintenance and Inspection Form developed by Toronto and Region Conservation Authority (TRCA) and CH2M Hill Canada.

Functional, performance, and environmental effects monitoring can help determine the frequency of routine maintenance and identify the need for more extensive corrective maintenance. Most major corrective maintenance for constructed wetlands fall into:

• major structural repair
• bank stabilization
• sediment removal
• unplugging of inlets and outlets

Gallery

• 

  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

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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).

• 

  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

Articles for review

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).

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

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

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.

Total suspended solids

Reduction of peak flow and increase of the duration of the flow event.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

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.

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

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

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.

Sustainable Technologies Evaluation Program

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

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.

Toronto and Region Conservation Authority

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.

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.