Wetlands are complex natural systems of hydric soils and water-loving plants.
Natural wetlands provide many important functions, including the attenuation of flood flows, water quality treatment, and support of aquatic plants and wildlife. Stormwater wetlands mimic the treatment processes of natural wetlands for detention, fine filtration and biological adsorption, to remove contaminants from stormwater runoff. Further guidance for wetland systems can be found in Auckland Regional Council Technical Publication TP10 Stormwater Management Devices: Design Guidelines Manual (2003).
Stormwater wetlands generally consist of an inlet zone (sedimentation basin or forebay), a planted zone, and a high flow bypass channel. The planted zone generally caters for the water quality volume and the detention of post-development peak flows. Wetlands can also be used for higher detention requirements. There are three main types of constructed wetlands:
- A surface-flow (SF) wetland, including wet swales - Water flows across alternating zones of deep water pools and shelves of wetland plants to perform different stormwater treatment functions.
- A subsurface flow (SSF) wetland - Stormwater passes through the media and rootzones of wetland plants. Stormwater remains below the surface of the substrate at all times. SSF wetlands are best suited to water inputs with relatively low sediment concentrations to prevent clogging, and relatively uniform flow conditions to ensure plant survival (Davis, 1995).
- A floating treatment wetland (FTW) - A FTW is a raft that supports wetland plants, growing in a hydroponic manner within a water basin (Headley & Tanner, 2006). FTWs are still a relatively unproven technology in stormwater ponds but laboratory and mesocosm trials have revealed there is significant potential for these systems to filter fine sediments or contaminants in solution. They are also likely to be highly effective for shading open water areas, and as a curtain to trap heavier sediment, specifically at the exit of forebays. They can be retrofitted to existing ponds.
Treatment wetlands are effective at treating suspended solids, hydrocarbons and dissolved metals. Nutrients such as nitrogen and phosphorus may also be taken up by wetland plants, given sufficient residency time.
Auckland Regional Council Technical Report TR2009/083 Landscape and Ecology Values within Stormwater Management (Lewis et al., 2010) provides specific guidance on enhancing landscape and ecology values within wetlands. If sited within accessible open space, constructed wetlands or ponds can significantly enhance the built environment. Wetland environments provide an urban refuge and a place of tranquility. Plants can form open sedgelands, dense flaxlands and cool and dark forests, providing for diverse and changing experiences for paths and boardwalks within a wetland.
Constructed wetlands are useful areas for outdoor classrooms, as they can demonstrate basic principles of plant succession, food webs and nutrient cycling. Biodiversity is optimised in a wetland through environmental gradients from aquatic to terrestrial zones, and from edge to interior habitats. A constructed wetland can attract a variety of wildlife by incorporating diversity of water depths, landform, substrates and plant communities. Wetlands aid the dispersal of birds, invertebrates and herpetofauna, to spread their home range, to complete their life cycle, and to disperse individuals, populations and genetic material.
Wetlands with connections to existing riparian environments offer excellent opportunities to provide fish passage for increased habitat offline (upstream of the receiving environment), and this is critical where wetland ponds are online (constructed in-stream). Refer to Auckland Regional Council Technical Publication TP131 Fish Passage Guidelines for the Auckland Region (Boubee et al., 2000) and Technical Report TR2009/084 Fish Passage in the Auckland Region – a Synthesis of Current Research (Stevenson et al., 2009) for ARC's fish passage guidelines.
Challenges and solutions
The following table describes some of the common issues and constraints relating to stormwater wetlands. In all circumstances, there is a potential design solution, which must be balanced against other objectives for the project.
Standing water may result in increased frequency of mosquitoes, and stagnant water
Design wetland edges and flows to avoid short circulating.
Combinations of shallow and deep areas provide opportunities for mosquito predators such as dragonflies and wading birds.
Safety around open water areas is cause for concern
Safety is paramount around open water. Access to these areas can be restricted through dense planting around margins.
Safety shelves and shallow slopes allow egress from the water, should anyone find their way in.
Weeds in wetlands can lead to significant maintenance issues
Management of pests is outlined in Auckland Regional Council Technical Report TR2009/083 (Lewis et al., 2010). Stormwater wetlands should only occur where there is effective access and a regular maintenance regime. This allows weeds to be controlled when they are manageable.
Weeds can also be prevented through dense planting, checking the quality of nursery stock to prevent transplants, varying water levels in the wetland, and controlling weed sources upstream.
Maintenance of wetlands can be significant, especially for the control of sediments
Wetlands are relatively self-maintaining once established, with only periodic checks required for inlet and outlet structures.
Forebays are able to capture gross pollutants and coarse sediments and regular maintenance of these systems will extend the design life of wetlands. In other words, an increased rate of preventative maintenance will decrease the occurrence of more costly restorative maintenance.
Waterfowl may be attracted to open water areas and create water quality issues
Large numbers of birds can result in increased faecal material in and around the wetland or pond. Potential solutions include dense planting or rocks at the margins to reduce loafing areas, and planting tall vegetation to limit ‘flight lines’.
Correctly maintained wetlands have a design life exceeding 100 years. If maintenance is not carried out, sedimentation and scour can significantly reduce lifespan.