Wetlands

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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 wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. plants. A facility is normally characterized as a wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. 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 WetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.[1][2][3]
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
  • 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 [4]
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 DetentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. Suspended solids settling 24 hrs (12 hrs if in conflict with min. orifice size)
ForebayA pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff. Pre-treatment
  • Minimum depth: 1 m;
  • Sized to ensure non-erosive velocities leaving forebayA pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.;
  • Maximum area: 20 % of total permanent pool
Length-to-Width Ratio Maximize flow path and minimize short-circuiting potential
  • Overall: minimum 3:1;
  • ForebayA pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.: 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 controlIncludes the protection of soil from dislocation by water, wind or other agents. water level

.

Water volume storage requirements based on catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. type and receiving watersWatercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.[4]
Performance level Storage volume (m3/Ha) required according to catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. cover
35% 55% 70% 85%
80 % TSSTotal suspended solids removal 80 105 120 140
70 % TSSTotal suspended solids removal 60 70 80 90
60 % TSSTotal suspended solids 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

See Wetlands: Plants

Performance

Relative to a wet pondA body of water smaller than a lake, often artificially formed., a constructed wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. may offer added pollutant removal benefits due to enhanced biological uptake and the filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. effects of the vegetation. Early stage wetlands readily sorb phosphorus onto substrates and sedimentsSoil, 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.. Phosphorus removal in wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. systems is usually carried out by incorporating alum sedimentationDeposition 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. ponds or sand filters as cells of the system, and/or by polishing wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. effluent in an iron-dosed mechanical filter.[5]

Freezing temperatures in winter and early spring can reduce treatment if the wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. either freezes solid or a cover of ice prevents the water from entering the wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.. If under-ice water becomes confined, water velocities may increase, thereby reducing contact times[2]. RunoffThat potion of the water precipitated onto a catchment area, which 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. in excess of maximum design flows should be diverted around the wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. to avoid excessive flows through the wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year..

STEP (under previous name SWAMP) conducted their own research into the performance of stormwater wetlandsShallow, constructed pools that capture stormwater and allow for the growth of characteristic wetland vegetation., the project page and report can be viewed here.

Central Lake Ontario Conservation Authority have been undertaking a coastal wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. monitoring project across Durham region, see here.

Gallery

See also

External links

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 wetlandA vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year. design. EcoSys Bd 8: 51–91. http://www.pixelrauschen.de/wet/design.pdf (accessed 9 May 2018).

  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. Jacques Whitford Consultants, 2008. CONSTRUCTED & ENGINEERED WETLANDS p. 1-21
  4. 4.0 4.1 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.).
  5. Jacques Whitford Consultants, 2008. CONSTRUCTED & ENGINEERED WETLANDS p. 1-21