The ideal is alive and kicking at the City of Melville. Sitting and working side by side in its local government offices are the City’s civil engineers and landscape architects*. And while this is not new in itself, Melville’s projects are changing form thanks to the two disciplines bouncing ideas off each other. One in particular - water sensitive urban design - is a growing accepted practice across many professions, producing results using ‘soft engineering’, aka plants. Landscape architect, Zoe Williamson, explains how landscape and engineering work together at Melville, using the recent Esplanade works as an example...
“Landscape Architects have been using ditches and swales - land forming - to manage run-off for as long as the profession’s existed. However, the terminology is now changing: they’re known as biofilters and rain gardens and the science and design behind them is growing rapidly. So when our civil engineers were recently faced with designing a riverside drainage upgrade along the Esplanade in Mt Pleasant, we unanimously decided on a rain garden design to capture a large proportion of the road run-off.” The two teams joined forces – their aim was to engineer a biofilter.
To appreciate the challenge that this engineering-architecture team were facing, it’s good to know a little more about the setting. Melville is just 12 kilometres south of Perth’s CBD, and like most of the area along the coastal plane, the soil is a shocker. “The base is limestone with nutrient poor, alkaline soils on top – essentially variations of sand – with perfect drainage.” Unfortunately, what you’d think was the native soil’s one asset, its ability to drain freely, is actually a problem given Melville’s total annual rainfall arrives almost entirely during winter. “We needed to find ways to hold that water for a while.” Oh, and there’s one more aspect that the team had to factor into their design solutions: the need to slow the water enough to allow it to infiltrate the beds and benefit the plants rather than undermining them.
The classic post-war civil engineering configuration is to collect the water off a road with a gutter then pipe it out into rivers and lakes, sometimes through a litter trap. It’s an ‘efficient’ system in that it gets the water out of sight and out of mind rapidly, often piping it impressively long distances. However, there is a growing realisation that this method concentrates pollutants in our water bodies and deprives the local landscape of the groundwater needed for healthy vegetation and aquifer recharge. Water isn’t slowed or allowed to infiltrate naturally; litter traps need regular maintenance and they don’t protect water-bodies from algal bloom or fine pollutants.
The Esplanade biofilter design, put into place late last year along a narrow strip of residential road riverfront, is a trial aimed at tackling these challenges. “In the design, the rain runs off the road surface and over a flush kerb before being slowed by a half metre wide inclined strip of laterite spall. Debbie Goodwin from the civil design team made sure all the levels worked. Since using flush kerbing to allow water into the landscape, we’ve had some issues with erosion here and there. Our Civil Construction Supervisor, Mike Powell, had a lot of valuable design and construction ideas to contribute. With years’ worth of firsthand knowledge of the roads, soils and rain patterns around the area, Mike knew how the water would behave and how we might try to slow it down and help it infiltrate.”
The existing soil was in a compacted, hydrophobic condition. So the soil was removed from the beds to 400mm depth and back filled with a local soil mix to 300mm then finished with 100mm white quartz sand mulch. The soil mix included TerraCottem at the trial rate of two kilos per cubic metre. It was planted in late spring and truck-watered to establish. The plants and soil need to remain moderately hydrated throughout the summer for the winter filtration function to be effective. “It’s the matrix of plant roots and soil microbes that do the filtration work, so we needed them to establish quickly and well. The plants chosen were all local natives. Paperbarks (Melaleuca rhaphiophylla), underplanted with a high density of sedges (Ficinia, Lepidosperma) with some shrubs for diversity (Scaevola, Hibbertia).” The entire distance is approximately 360 linear meters; incorporating approximately 470m² surface area of biofilter beds.
So how does this all this soft engineering work? The hard landscaping directs and slows the water before capturing it in the TerraCottem laced planting zone, and then it’s the plant roots and living soil which filter pollutants from the run-off before it reaches the river. It’s early days yet but it’s looking like a success story. The run-off is slowed. Tick. Natural infiltration has been boosted. Tick. The plants are establishing well. Tick. And cleaner water is entering the Canning River and ultimately the Swan River and the ocean. Tick.
If all continues along this happy path, this system may end up being cheaper to maintain given the occasional truck watering during the height of summer as opposed to routine litter trap clearing and the remedial measures required when dealing with polluted, eroded waterways. It’s certainly a lot more pleasing to look at! And if you take a step even further back, Zoe’s ultimate goal is also looking likely. She joined the team at Melville to do work at a large scale for the public good. “It’s about creating beautiful, accessible spaces for all people, that function as habitat as well as providing environmental services - in this case filtration.”
* The team: Zoe Williamson (Landscape Design), Debbie Goodwin (Civil Engineering) and Mike Powell (Civil Construction)
