Biofiltration systems, such as wetlands and drainage swales, help clean stormwater before it is released into waterways or used for irrigation. Rain gardens operate the same way but they are something we can build in our suburban gardens – to help us and the health of waterways.
A rain garden is a specially designed and constructed garden bed that takes rainwater directly from the roof – the downpipe goes straight into it – or indeed any large, outdoor surface area that produces ‘runoff’ water. If there is an excessive amount of water running into the raingarden bed, an overflow pipe at the top directs water into the stormwater system to prevent water backing up and flooding. At the bottom, aggy pipes slowly take the percolated water away into the stormwater system.
Rain gardens serve two purposes. They help the home gardener by keeping rainwater on site longer, so it’s available to plants and to revitalise the soil. The most important aspect, however, is their effect on the health of urban waterways. Rain gardens slow stormwater entering the stormwater system (which in turn empties into waterways). By the time the rainwater does enter the stormwater system, it has been slowed down dramatically (which reduces flooding events and erosion of banks etc) and the water is also cleaner, because the sand in the rain garden filters out a lot of pollutants (plants and microorganisms also take up some of these pollutants).
Even though rain gardens do help to recharge the soil locally, it’s more about the health of waterways on a larger scale. If there is no rain, the rain garden won’t get any water either, which is why drought tolerant plants that can tolerate temporary inundation (waterlogging) must be planted in a rain garden. If a regular supply of water for your garden is required, a rainwater tank would serve the purpose much better.
Researchers are investigating the efficiency of rain gardens in various areas. Theoretically, any area, regardless of soil type and slope, should be suitable. Appropriate construction is essential though, as rain gardens placed close to a building must not have any potential to undermine foundations etc. They can be built above ground as well as in ground, so those people with heavy clays may find above ground ones easier to install.
It’s important to get good advice on construction. If the rain garden becomes a bog garden, then it’s not working properly. They should be fairly free draining, so even major rain events should drain away within a day.
The Little Stringy Bark Creek project, Mt Evelyn, Victoria
More information and photographs can be found at: www.urbanstreams.unimelb.edu.au
The solution to protecting creeks from the ravages of stormwater runoff is to retain as much of it as possible in the catchment to allow it to filter through the soils, to get to the creek slowly, to provide a more natural, clean base flow. Rain-gardens are the perfect tools for achieving this.
But the large excess volumes of stormwater mean that, if our rain-gardens are to treat stormwater adequately to protect the creek, we also need to harvest and use as much stormwater in the catchment as we possibly can.
In the Little Stringybark Creek catchment, it will be impossible for us to harvest enough stormwater to get the creek back to its pre-urban flows.
The front rain garden is a fully lined system, with an under-drain (top photo, which features Sharyn Ross-Rakesh from Melbourne Water assisting with planting). Approximately 1m deep, this rain garden will filter and use storm water, before directing it, or any overflow, to the rain-garden at the rear of the property.
The rear rain garden is not lined, as there is no danger of damage to buildings.
A demonstration day was held for local residents, and over 50 turned up to learn all about it. Shown here with interested residents is Associate Professor Chris Walsh (far left), principal research fellow, Department of Resource Management and Geography at the University of Melbourne.
Other rain gardens
Many variations on the theme can be constructed, but remember, they must be built correctly to work properly. These images are courtesy of www.urbanstreams.unimelb.edu.au, where more can be viewed.