Raingardens

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Imagine a way to reduce erosion and pollution in waterways and help water and fertilise your garden too. The solution might be as simple as disconnecting your home’s downpipes

and letting the water flow into an “ephemeral creek” – if you have a large enough block and the right soil.

For the rest of us, rather than risk flooding your garden and those of your neighbours, consider building a simple raingarden to capture and treat run-off before it hits the drains.

But don’t creeks and rivers need more water to stay healthy? The idea of reducing water flows might seem counter-intuitive, but the fast growth of Australia’s cities – and with it increased amounts of hard, impermeable surfaces – has resulted in waterways in urban areas across the country damaged by fast and too frequent stormwater flows and pollutants.

“The problem we have is the quantity and quality of stormwater flowing into our waterways,” says Phil Edwards, Melbourne Water’s team leader for stormwater quality.

“It flows into waterways too frequently, causing erosion; and the run-off from roofs and driveways also carries a degree of pollution and sediment, which is deposited in our waterways.”

Raingardens are designed to collect water from any hard surface – roads, driveways, footpaths or roofs – and to filter and clean it before it hits the nearest river. The filtering is largely done by the soil structure and microbes as well as through absorption by the roots of plants in the raingarden – a process called biofiltration.

AIR POLLUTION

As well as obvious pollutants such as oil, litter and animal faeces that are swept into stormwater drains from the street, dust, leaves and airborne pollution can also flow in from house roofs. Nitrogen and phosphorus are the main culprits, but the good news is that both are essential for plant growth. The goal is to feed raingarden plants rather than encourage algae in waterways.

Surprisingly, tiny amounts add up to make a huge difference even in large bodies of water. According to Phil Edwards, in 1999 Port Phillip Bay received 7000 tonnes of nitrogen – 1000 tonnes more than it is able to process. In Western Australia the Swan River has been hit by algal blooms and in NSW in July there were 11 “red” alerts for algal blooms in various waterways.

South-east Queensland has had an especially bad 12 months, with rains last summer washing high levels of pollutants into waterways.

PUBLIC STORMWATER HARVESTING

Stormwater harvesting programs are being introduced in most major cities as planning departments adopt the principles of Water Sensitive Urban Design (WSUD).

“We’re looking at alternative technologies such as small-scale filters that can fit into urban streets,” says Edwards. “In all major cities you’ll see these appearing, built by local councils or water authorities.”

In Sydney more than 80 stormwater harvesting projects should save about two billion litres of water a year. The city of Orange has devised a scheme to supplement its drinking water supplies with stormwater. The scheme captures some of the high flows in Blackmans Swamp Creek during storms and transfers these into the nearby Suma Park Dam to augment the city’s bulk water supply.

RESIDENTIAL RAINGARDENS

At its simplest, a raingarden can be a swale or depression alongside a driveway or footpath, or even at a low point in your garden, where rainwater can collect and gradually soak into the soil. (Swales created close to buildings should be lined like a pond to avoid damage to the foundations.) We covered this style of raingarden in “Rainwater Gardens” in Sanctuary 4.

If you have installed rainwater tanks plumbed to your laundry, bathroom, or kitchen, you have already reduced the amount of water that’s going to the stormwater drain. Depending on the capacity of your tanks, during wetter months your tank may overflow. By absorbing and filtering the overflow water, raingardens can help ensure all or most of the water that falls on your home and garden remains on-site.

According to Anjali Brown, the Alternative Technology Association’s water projects manager, “Filtering the overflow from your tank into a raingarden will help you get the most out of the rainwater on your roof. Keep in mind, however, that a raingarden which only receives irrigation through overflow will need highly drought resistant plants. One way to provide more water to your raingarden in this situation is to divert your first flush device into the raingarden as well. A first flush device diverts the first flow of water from your roof, which may contain pollutants, away from your tank each time it rains. Usually the first flush water gets sent down the stormwater drain but by diverting it to the raingarden, your filtering plants will get a little extra water every time it rains which will help them survive.”

The best type of raingarden to work in conjunction with a water tank is a plant bed raingarden. Plant bed raingardens are either an above-ground planter box or an in-ground lined trench. Inside the plant beds are layers of gravel and sand that slow water flow, filtering out sediment and pollutants from the water as it passes through. The filtered stormwater is collected via a slotted drainage pipe at the bottom of the filtration bed, and can then be discharged into your garden or the stormwater system.

Raingardens should be planted out with species that are particularly good at removing pollutants, such as Lomandra longifolia or Carex appressa, and hardy, indigenous species that will survive the dry periods between rainfalls such as other Lomandra species and Dianella species.

It is important to remember that any run-off needs to be kept out of neighbouring properties and a registered plumber is needed to connect your raingarden back to the main stormwater drain. Some WSUD principles may clash with local council by-laws, while other councils may have incentive schemes that are worth registering with first, so check before you build.

INSTALLING RAINGARDENS

Melbourne Water is campaigning to get 10,000 raingardens built across the city by 2013. Its website (www.melbournewater.com.au/ raingardens) has a range of information sheets about above- and in-ground raingardens, including the materials you will need to make them yourself, as well as which plants to select, and how to work out the size of raingarden you’ll need for your situation. These information sheets would be applicable to conditions around the country (though it would be best to seek local information about indigenous plantings).

If you would prefer an above-ground raingarden you may need to buy or make a lined raised garden bed or buy a water tank and have a drainage hole installed – see the details on the Melbourne Water site to find out the size of the drainage hole you need.

FOR MORE INFORMATION
Water Sensitive Urban Design
www.wsud.org
Melbourne Water, including links to instruction sheets for building www.melbournewater.com.au/raingardens
Stuart McQuire’s book, Water Not Down the Drain www.notdownthedrain.org.au
Healthy Waterways
www.healthywaterways.org
Alternative Technology Association
www.ata.org.au/sustainability/stormwater- capture/
Some tank suppliers include:
www.tankworks.com.au www.cessnocktankworks.com.au mtwatertanks.com.au/GardenBeds.php

Volcanic awakening

volcanoPublished in Green magazine, July-August 2010

 

One of the positive things to come out of the decade- long drought in Southern Australia has been the development of some innovative water-saving plans. One of the most impressive is that of Melbourne’s Royal Botanic Gardens. Jane Canaway meets its senior designer, Andrew Laidlaw.

After being held in the fierce grip of drought for the best part of a decade, the “wasteful” sound of running water has become a distant memory in much of southern Australia.

In Melbourne’s Royal Botanic Gardens, the Fern Gully has been dry for many years, except after occasional rain, and visitors have grown used to ducks dabbling in muddy puddles where lakes used to be. However, a solution is in sight, and a key to its success lies in the recent re-awakening of a long-dormant volcano in the Gardens’ south-east corner. That landscaping project – completed this March – is stage one of a longer-term plan to create a sustainable water supply for the RBG, starting with approximately 68 megalitres of stormwater harvested from the nearby suburb of South Yarra. This water will enter the gardens through two stormwater pipes that will deliver the water into the Gardens’ lakes where it will be filtered on-site through a series of wetlands. This has the added benefit of removing 68 megalitres of diluted oil, dog faeces, salt and other pollutants washed from nearby streets that would normally run into the Yarra River.

In 1876 William Guilfoyle designed a reservoir in the shape of a bluestone-lined volcano that appeared to rise from the highest point of the Royal Botanic Gardens, with lawns created to mimic the lava flows between massed planting of shrubs, representing outcrops of land. For nearly 70 years it was filled with water from the Yarra River, which was then gravity fed to the rest of the gardens.

“It was possibly inspired by Guilfoyle’s plant collecting trips around the South Pacific,” senior designer Andrew Laidlaw said. “The Volcano was designed as a ‘Garden Folly’, a feature that visitors would come across in their meanderings around the gardens. “It holds 1.4 megalitres of water, which is enough for one cycle of our current irrigation.” However, Guilfoyle’s plan was plagued with many problems – including the loss of fresh water in the lower Yarra after waterfalls at Queens Street were blasted in 1879 to alleviate flooding and allow larger boats upstream. By the 1950s the irrigation system was attached to mains water and the volcano was locked up and slowly forgotten. Hidden by dense stands of olive trees, eucalypts and weeds, it was passed by generations of joggers and walkers without its being noticed – until a 1997 master plan called for its redevelopment.

Then drought hit. Toxic blue-green algae became a regular problem in the Gardens’ lakes, and mud flats were a common appearance in the Ornamental Lake – so a rescue plan seemed more appropriate than a simple redevelopment.

Laidlaw considers himself “very lucky” to be the one who landed the “plum job” of revitalising the feature – and he believes the timing was perfect, too. “If I’d been doing this five years ago we would probably have just done a pretty water feature, but now with the current water crisis it’s become an integral part of the Gardens’ water system,” he said. Along with Richard Barley, Peter Symes and Stephen Paterson, Laidlaw is part of a highly water-smart team that meant the RBG was one of the first Australian Botanic Gardens to develop an award- winning irrigation management plan that has been used overseas as a blueprint for other gardens.

While working on a revamped layout and drought-friendly plantings, Laidlaw consulted with both hydraulic and civil engineers about the possibility of filling the volcano with stormwater. “It was discovered that large amounts of stormwater from nearby suburbs was going straight into the Yarra [River] or to Albert Park and into the sea. Because stormwater hits the river fast it stirs it up and affects the flow rate, as well as carrying road rubbish and pollutants. By redirecting water through the gardens first, it reduces the water flow rate and cleans it. “The extra 68 megalitres is not enough to water the whole gardens but eventually, as part of stage 3, we hope to siphon off 120–150 megalitres of water required through a sewerage–water harvesting scheme,” Laidlaw said.

Becoming totally water-sufficient will be expensive, but Melbourne City Council wants all its parks off potable water-irrigation. Revamping the volcano cost $1.5 million and the next stage is to build the Working Wetlands in the Ornamental, Central and Nymphaea Lakes, which will cost about $2.5 million. Project Manager Stephen Paterson said tenders have been called for and it is hoped the 12-month project will start later this year.

Of the 68 megalitres captured, allowing for evaporation and overflow, “the modelling indicates we will be able to substitute about 40% of potable water with harvested stormwater,” Paterson said. The sewerage-harvesting scheme – if funding can be found for it – will cost at least $10 million. By contrast, water costs the gardens between $100,000 and $120,000 a year – and that is after spending $4 million on a new irrigation system in 1993, which has cut water use by more than half to 130 megalitres a year. “What’s exciting is we’ve got a whole new way of working with water that is environmentally friendly. And we’re actually bringing water to Fern Gully for the first time in years and bringing water to the parts of the garden that need reinvigorating. All based around the volcano.”

HOW IT WILL WORK

Two stormwater pipelines will be diverted to the RBG, carrying about 68 megalitres of water a year into the Ornamental Lake and a receiving wetland in Nymphaea Lake. The water goes through a gross pollutant trap to remove larger pieces of rubbish and sediments before flowing through a series of wetlands where less visible pollution is removed. Wetlands will be constructed in the Ornamental, Central and Nymphaea Lakes. The lake silt will be removed and clay used to line the area and build levees, creating pools of varying depths to hold the stormwater and selected wetland plants, which will grow in different depths of water. A bio-film of bacteria and fungi will grow on the large plant surface area and act as the engine of the wetland, removing nutrients from the water as it slowly flows through the wetland. Plants are also chosen for their ability to take up nutrients and top growth will be harvested and composted to remove the nutrients from the system.

Water from the Nymphaea Lake then flows through the Fern Gully – where waterfalls and mini rapids aerate the water – and into the Central and Ornamental lakes. Water will be circulated through the lakes and wetlands and pumped to the highest point in the Gardens, Guilfoyle’s Volcano. From this reservoir, garden beds on the Volcano slopes will be watered by sub-surface irrigation. The water will then flow back to the Nymphaea Lake and so will circulate through the wetlands; the whole cycle takes about 30 days. By pumping water from the deeper parts of the lake any layers of warm and cool water are mixed, which helps reduce the frequency and duration of Blue-Green algae blooms. Floating garden beds will be installed in the lakes and act as treatment wetlands, as the plants’ bio film-covered roots remove nutrients and improve the water quality.