Wetlands

Wetlands fed by stormwater at Kortright Farm, Vaughan ON

Natural wetlands are ecosystems that have developed over time, providing diverse habitats and naturally filtering water through complex biological and physical processes. Constructed wetlands, on the other hand, are designed and built to mimic these natural functions, targeting specific water quality goals and pollutant removal. They are a cost-effective and efficient method widely used in North America to treat various wastewaters, such as stormwater, sewage, and agricultural runoff. The Ontario Wetland Evaluation System defines wetlands as:

"Lands that are seasonally or permanently flooded by shallow water as well as lands where the water table is close to or at the surface; in either case, the presence of abundant water has caused the formation of hydric soils and has favoured the dominance of either hydrophytic or water-tolerant plants. The four major types of wetlands are swamps, marshes, bogs, and fens."

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). Explore this "image map" with your cursor and click on highlighted labels that appear to go to corresponding pages on the Wiki.

Wetlands can contribute to:

• Enhanced biodiversity
• Enhancing recreational and educational opportunities and aesthetics
• Improving water quality and helping to meet TSS reduction targets
• Storing water and attenuating floods
• Carbon sequestration

Case studies are available for wetlands used in LID systems.

Planning considerations

Constructed 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. Free-water surface flow wetlands are most commonly employed for stormwater treatment and are similar to SWM ponds in function and design. However, ponds and wetlands differ by the extent to which shallow zones for wetland plants are incorporated. A facility is normally characterized as a wet pond if shallow zones (<0.5 m deep) comprise less than 20% of its surface area, while a facility is normally characterized as a wetland if shallow zones (<0.5 m deep) make up more than 70 % of its volume.

Free-water surface flow
Pros Robust Provides excellent water quality treatment Resistant to temporary hydraulic overload Can be beautiful Sludge removal infrequent
Cons Requires larger land area Sludge removal may be more difficult Open water may generate more health and safety concerns.

• 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.

Horizontal sub-surface flow	Vertical sub-surface flow
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 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 recommendations differ between SWM ponds and constructed wetlands. Below are the sizing recommendations for free-water surface flow wetlands.

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

A larger storage volume (up to 140 m³/ha) is recommended for constructed wetlands designed to enhance suspended solid removal and treat runoff from catchments with a high impervious cover. Conversely, constructed wetlands designed for basic treatment and low impervious cover can be have lower storage volumes (minimum of 60 m³/ha). The table below shows recommended storage volumes for constructed wetlands, depending on level of protection and impervious cover. For comparison, the recommended storage volume for wet ponds is 60 - 250 m³/ha.

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.<ref name="JW">

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. A field data sheet for the inspection of constructed wetlands is available on the right. 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.

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

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 falls 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

• 

  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

• Dry ponds
• Plants

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

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

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.

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 upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

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

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

Stormwater Management

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

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.

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.

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.

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