Constructed wetlands

Typical design

Constructed stormwater wetlands are relatively large end-of-pipe facilities that aim to use multiple treatment methods (for example sedimentation, biological and chemical processes) to remove pollutants from stormwater. The different processes have different design requirements so wetlands contain multiple zones.

Typically, these include:

• A pre-sedimentation to remove coarse sediments where the collected sediments can easily be removed.
• A wetland with deep water zones for additional sedimentation and zones with shallower water and vegetation for further treatment processes (including filtration, adhesion and biological uptake).

The combination of various treatment methods means wetlands have the potential to remove a wider range of pollutants, including fine particles and dissolved pollutants.

As with other stormwater quality facilities, high flow rates could damage the facility unless a bypass is included in the design.

Aims in stormwater management

Wetlands can remove both particulate and dissolved pollutants. As with other stormwater quality facilities, they are designed primarily to treat runoff from small rainfall events (return intervals >1 year or design rainfall depths, e.g. 10 or 20 mm). For these events they can contribute to runoff reduction, but for larger events they will not have an effect. Instead, it is often necessary to build a bypass for higher flows to avoid damage to the facility or re-suspension of sediments.

Implementation in catchments

Compared to sedimentation ponds, wetlands require even more space and are therefore only suited as end-of-pipe solutions.

Additional benefits

• Biodiversity
• Recreational values
• Aesthetic values