The role of wetlands in sustainable drainage systems
Wetlands are important ecosystems for biodiversity and are among the most ecologically abundant places on earth. Freshwater wetlands are a natural habitat in the UK, but their importance goes beyond just the wildlife they support. They also play a key role in natural drainage systems.
Whilst there are many natural wetlands in the UK, these existing wetlands should not be used for surface water management due to the risk of pollution from untreated water. Constructed wetlands can however be created to manage surface water as part of a SuDS (sustainable drainage scheme) and also bring additional benefits to the built environment as green infrastructure.
What is a constructed wetland?
A constructed wetland is an artificial wetland used to treat sewage, greywater, stormwater runoff and industrial wastewater. They can also be used as a mitigation for natural areas lost to land development and can serve as a habitat for native and migratory wildlife. The constructed wetland is used as a secondary treatment for wastewater and is engineered to use natural functions such as vegetation, soil and organisms. They work in a similar way to natural wetlands by acting as a biofilter to remove pollutants including organic matter, pathogens and heavy metals from the water. Water is often detained in wetlands allowing sediments to settle and adhesion and aerobic decomposition of contaminants by vegetation. Permanent water needs to be shallow enough to allow oxygen to reach the bottom of the wetland.
There are two main types of constructed wetlands: subsurface flow and surface flow. Subsurface flow wetlands are designed to have a horizontal or vertical flow of water through a gravel and sand bed and are more effective at removing pathogens than surface flow wetlands. Surface flow constructed wetlands are designed to keep the water level below the top of the rock or gravel. This minimises the human and ecological exposure.
Advantages of constructed wetlands in SuDS
Wetlands for SUDS can include shallow ponds and marshy areas covered in aquatic vegetation, providing both stormwater attenuation and treatment.
Constructed wetlands provide benefits to the community for amenity, and their presence could even enhance property values. They are the richest of the SuDS components for biodiversity, and they are valuable as green infrastructure with additional benefits including the sequestration of carbon and improved air quality. In terms of overall performance, they provide good peak flow reduction, water quality treatment and ecology potential.
Their performance can be enhanced when they are designed by specialist consultants with inlet and outlet sumps. CIRIA published a review of constructed wetlands in 1997. The RSPB and WWT have published a guide to Sustainable drainage systems for local authorities and developers. They explain that the best sustainable drainage schemes incorporate a range of habitats that are good for water management and also good for wildlife.
Although constructed wetlands have many benefits as SuDS and green infrastructure, their use is usually confined to suburban and rural areas. The land take is usually considered too great for urban and high-density areas. They are also other limitations on their use, for example, they are not suitable for steep sided sites and retrofitting of SuDS. It is however, sometimes possible to incorporate wetlands into parks and open spaces as part of Sustainable Drainage Systems (SuDS). They can also be created for urban areas with hard edges to fit into the streetscape or other hard landscaping features but they would need to be of an adequate size for the catchment to allow the water treatment to be supported.