Issue 39 September 2006
Filter tip
Getting the treatment: A raingarden that treats roof run-off in the front yard of a house in Aspendale
Once when stormwater rushed down the gutters into drains, it was seen as a blessing – a natural way of flushing the city’s streets and pavements clean.
Now stormwater has been recognised as the most significant source of pollution in urban rivers and creeks.
Some 500 billion litres of water a year is washed down Melbourne’s stormwater drains into rivers, creeks and the bays. As well as about 250,000 points where litter can enter the stormwater system, there are a myriad of diffuse sources of other contaminants, including heavy metals, hydrocarbons and nutrients from vehicle use.
For some years, wetlands have been the main tool for removing contaminants from stormwater. Melbourne Water has built more than 100 wetlands, and the city has grassy swales that slow and filter stormwater.
Melbourne Water, councils and planners are also using smaller, less expensive water-sensitive design projects such as raingardens to reduce the impact of stormwater pollution on rivers, creeks and Port Phillip Bay.
Raingardens are a simple, inexpensive way of deflecting stormwater pollution from drains.
They resemble regular gardens with one major difference – they are positioned to receive rainwater run-off from a roof, road or paved area. This water filters down through mulch and soil where microorganisms help remove and break down pollutants.
From little things: Students at Richmond Primary School plant one of three small raingardens that filter stormwater from the school roof
Raingardens, a type of biofilter, can be more or less any size or shape, from planter boxes to natural-looking gardens.
They can include native plants, shrubs and grasses that soak up rainfall collected from roofs or driveways, reducing the amount of stormwater that would otherwise wash from hard surfaces into drains and eventually rivers, creeks and bays.
Raingardens can be readily adapted to commercial buildings, private homes and public spaces – from large water catchments to narrow roadside verges and the edges of paved carparks.
Plants benefit from the water and the high levels of nutrients in stormwater, which boost growth.
Matt Francey, Stormwater Quality Team Leader for Melbourne Water, says the individual impact of each raingarden is small, but the collective impact of filtering roof and road run-off has great potential.
“There are huge opportunities here,” he says. “It’s a question of getting enough installed so we can make a real difference.
“For example, each council undertakes between 30 and 50 road and landscape projects each year which could be upgraded to include stormwater treatment.”
Melbourne Water and lower Yarra councils are taking raingardens to schools, with 20 to be built this year.
The Rain Gardens in Schools project is one of several initiatives developed and supported by Melbourne Water in partnership with Boroondara, Maribyrnong, Melbourne, Stonnington and Yarra councils to tackle stormwater pollution as part of the Victorian Government’s Yarra River Action Plan.
“The project is a hands-on way of educating and involving school communities about stormwater, and giving them practical examples of what they can do to improve the health of their local river or creek,” Mr Francey says.
The design of biofilters is continually improving. Melbourne Water is a partner in the Facility for Advancing Water Biofiltration at Monash University, which is investigating and testing the most effective physical, chemical and biological factors that underpin performance.
Experiments have begun to look at how to avoid pipe and subsoil clogging, how plants vary in their capacity to contribute to biofilters, and how biofilters recover from accidents such as oil spills.