A constructed wetland at Blacktown Showground, Sydney (credit: Blacktown City Council)
Authors: Samiya Tabassum and Michelle Leishman, 16th November, 2021
In natural environments, rainwater infiltrates into the soil, evaporates into the atmosphere or is taken up by plants. However, urban environments often replace natural surfaces with impervious materials such as asphalt and paving, which stops rainwater from infiltrating into the ground. This rainwater accumulates and eventually becomes stormwater runoff, which on its journey to the local creek or river, may collect harmful pollutants and debris from roads, roofs, paths and driveways.
In order to manage the amount and quality of stormwater that flows through our urban landscapes, streetscape designs that allow water to naturally percolate into the ground can be implemented. Water sensitive urban design (WSUD) helps to increase the capacity of urban developments to store, treat and re-use stormwater before it causes detrimental effects to local waterways. WSUD can significantly reduce the chances of flooding in our urban centres during heavy rain by slowing the flow of water into the drainage system, reducing peak flows.
There are many different types of WSUD that can be fitted to suit different urban spaces. Some examples of WSUD include:
- Raingardens
Raingardens are special garden beds that help to slow down and filter stormwater before it reaches our waterways. They are usually found on road corners near drain grills and are often set lower than the road level. Raingardens are generally made using layers of substrate such as sand and recycled crushed glass to help filter pollutants. They are usually planted with waterlogging-resistant grass-like species, which also help to physically trap pollutants.
- Constructed wetlands
Constructed wetlands are vegetated man-made ponds which help to filter stormwater through physical and biological processes. They can be densely planted to help capture fine particles and reduce the flow of water, encouraging particles to settle. Biofilms that grow on the surface of these plants also help to trap pollutants. The regular wetting and drying cycles of constructed wetlands helps to capture and ‘fix’ contaminants into the soil.
- Street-tree bioretention pits (tree pits)
Tree pits are an alternative way to incorporate trees into a streetscape and provide a form of passive irrigation. Not only do the trees in the tree pits provide amenity value, the pits themselves help to reduce and treat stormwater runoff. Usually one large tree is planted into each pit, with species selection favouring species that can handle extremely wet and dry conditions. The tree pit is set lower than the surrounding road surface to allow stormwater to pool and slowly filter through the soil.
- Swales
Swales are depressed channels which help slow the flow of stormwater and promote infiltration into the soil. They are generally planted with grasses or can be more densely vegetated with other species. Swales are relatively inexpensive to build and are often installed along nature strips, median strips and in car parks.
As well as the obvious benefits for stormwater management, WSUD also provides a plethora of environmental and social benefits including:
- Providing habitat for wildlife
The plants used in WSUD provide habitat, shelter and food resources for native fauna. Large WSUDs, such as constructed wetlands, also provide aquatic habitat. To read more about how urban greening can support native fauna, read our blog ‘Why diversity is important for our cities’.
- Providing cooling benefits by reducing the urban heat island effect
WSUD provides cooling benefits through transpiration and shading afforded by plants. To learn more about how plants can reduce the urban heat island effect, read our blog ‘Not all urban green is equally ‘cool!’.
- Improving the aesthetic value of built-up environments
WSUD can enhance the value of urban spaces by ‘breaking up’ continuous hard surfaces and providing pops of colour to often monotonous urban landscapes.
- Providing places of leisure for the community
Large WSUDs, such as constructed wetlands, can function as areas of leisure and recreation for the community, providing space for picnics, bbqs and playgrounds along the banks.