WSUD in the City: green infrastructure is thriving in Perth
Given there isn’t too much space for rain to infiltrate into the ground directly in a cityscape, and trees generally have limited space to grow, green infrastructure needs to be incorporated into traditionally hard surfaces to allow a city to become greener and more water sensitive.
While green infrastructure has sometimes been given a bad rap in the past, a recent New WAter Ways bus tour of the Perth CBD went a long way to challenge this, showcasing successful examples of green walls, green roofs and other WSUD infrastructure in our city.
A mix of local and state government officers, academics, consultants, and land developers were stocked up on muffins and juice, as well as on details of how new apartment buildings, offices, and our new shiny sports stadium, are kicking their own environmental goals as part of Perth’s transformation into a water sensitive city.
(In case you’re not sure: A green roof is a roof surface that is planted partially or completely planted with vegetation and a growing medium over a waterproof membrane. They may be have soil over 200mm deep supporting vegetation from groundcover up to the size of trees.)
These buildings were purpose built to incorporate a number of environmental features including green roofs, designed by Deep Green Landscaping. Such green roofs can help to combat urban heat, treat storm water, as well as increase biodiversity and healthy habitats.
Key design features developed by Deep Green Landscaping for these green roofs include:
- specialised lightweight soil mix able to retain rain water just long enough for plants to absorb, but not hold onto for so long as to make it wet and heavy for the slabs underneath. The mix includes Spongelite (silicon dioxide) from Albany which provides ideal microhabitats for microorganisms and effective transport of water and nutrients to plants. At full saturation soil weights ~1.2 tonnes/m3;
- raised garden beds of ~300 mm soil under a slab capacity of 5 kpa;
- very lightweight geofabric lining the inside of cells, directing water to the drainage layer;
- a 50 mm layer of white sand to filter fine particles and sediments;
- plant species selection which balances root growth requirements and the break down of organic matter. Shallow rooted species such as Jacarandas or Melaleucas and other ornamental plants are preferred.
(Did you know green roofs can be installed on any horizontal surface in a building, not just the roof!)
Greywater generated within the building can be used as an irrigation source for green roofs, and thus reduce/remove the use of scheme water to maintain the green roofs that would otherwise occur. Greywater was utilised at the Workzone building using an irrigation system coupling a dripper and spray system together to minimise blockage from fine roots. Greywater is recycled daily using a simple 3-filter system, mixed with 10% scheme water. While some maintenance is needed (including flushing the drippers 3 to 4 times per year to stop salt build up), the resulting water savings have been significant at this site.
Green walls are similar to vertical gardens and may be inside or outside a building. They may be incorporated into the cooling strategy of a building and may even be designed as part of a water treatment system. Green walls can be:
- Green façades — pots with vines on trellises;
- Active — with soil/growing medium running up a wall; or
- Passive — epiphytes.
Benefits of green walls include the reduction of ambient building air temperature, absorption of dust, particulates, and pollution, and increasing available planting area/green space.
The Verdant apartments in Perth’s CBD have a strong focus on energy efficiency and incorporate a 3-story external green wall. Vegetation grows in either a light-weight panel system for internal green walls, and in a plastic bauxite system for external green walls, both designed to retain nutrients and water. Each panel is 1.5 m long and 40 mm deep, and incorporate a water retention mat at the back of the panel into which plants grow. Plants are grown for 3-6 months before installation in order to ensure establishment.
Some challenges in establishing green walls include:
- drying out of plant pores by nearby air conditioning vents;
- over-heating of plants from sunlight allowed in by windows; and
- ensuring appropriate irrigation is achieved.
To tackle these issues, green wall irrigation systems are fully automated and controlled remotely. Four different piped systems were installed at the Verdant apartments forming four different automatic irrigation stations, each adjusted to suit independent sets of conditions affecting the internal and external walls (such as sun/shade/air conditioning vents). Walls are watered for 1 minute daily on average, thus requiring a water use exemption from Water Corporation. Sensors automatically turn off if conditions are too wet.
Other issues requiring consideration around green walls included:
- the use of fibrous root plants for the internal green walls, while woody root plants are used for external walls with larger and deeper panel systems;
- the lifespan of green walls depends on the life of the plants (usually 5-10 years);
- use of liquid fertiliser to minimise wastage; and
- flushing drippers twice per year to remove salt build up and prevent pipe blockage.
There are a number of excellent examples of Water Sensitive Urban Design (WSUD) systems around Perth’s heartland, all serving slightly different functions managing groundwater, preventing flooding, providing irrigation storage, improving water quality, providing habitat/improving biodiversity, and helping to restore the natural local water cycle!
Two significant WSUD sites were visited as part of the bus tour: shiny new Optus Stadium, and the more established Claisebrook Lake.
The primary function of Claisebrook Lake is to provide water storage for irrigation of 10 ha of parkland and streetscape throughout East Perth. It is managed by the parks team at the City of Perth.
Stormwater is the primary water source and is directed into the lake via several local drainage networks. However, sediments, nutrients, hydrocarbons and other debris, have previously caused contamination issues as well as resulting in the growth of aquatic weeds. Leaf litter from surrounding deciduous trees have also contributed to poor water quality. In order to improve stormwater quality and management at the lake, the City of Perth has undertaken a number of practices including:
- installation of a Gross Pollution Trap (GPT) to remove the larger debris;
- installation of an ultraviolet light screening system to reduce bacterial growth;
- installation of aerators to improve oxygenation and mixing of lake water, as well as providing providing aesthetic value;
- implementation of a regular maintenance program to control the growth of the aquatic weed;
- development of a procedure to protect the wildlife during any maintenance operations; and,
- Citizen Science Programs and lakeside signage to help residents understand water quality and maintenance issues.
Perth Stadium is the home of many of Western Australia’s hopes (Fremantle), dreams (Scorchers), failures (Glory) and successes (West Coast). What probably isn’t apparent to the 60,000 that regularly flock there are the water sensitive urban design features around them.
Water management for the stadium and surrounding parklands aim to provide effective drainage and improve water quality of stormwater prior to discharge to the adjacent Swan River and river-fed lake. The stadium’s stormwater management system is designed to direct runoff from all impervious surfaces to underground infiltration tanks, biofilters, swales and drainage basins, sized to retain the first 15 mm of any storm event.
On the northern side of the stadium a rainwater tank collects runoff from the stadium roof. This runoff is used for irrigation of the garden beds in the precinct and toilets in the stadium. Water quality treatment is achieved through vegetation and underlying amended soil profiles in the garden beds, swales and basins.
So next time you’re watching a concert/game, take a look around you and you will notice these seemingly humble WSUD systems and have a better understanding of what they are doing for the nearby parks and Swan River during our rainy days.
Many thanks must go to the New WAter Ways 25th June 2019 bus tour presenters, for taking the time to explain their projects, the design process, challenges faced, and ultimately their success. Specifically, thank you to:
- Julian Rose, Deep Green Landscaping (Capital Square Tower 1, Workzone, & Verdant apartments);
- Shelley Smith, Coordinator Parks Project, City of Perth (Claisebrook Lake); and
- Alan Madigan, Engineer, BG&E (Optus Stadium).
Such projects demonstrate that a considered planning and design process, in combination with a determination to champion and see-through the installation of non-traditional infrastructure, can result in the successful greening of a hard cityscape which is also sensitive to the local water cycle and river system. It also shows us ultimately that Perth truly has the potential to become a fully-fledged water sensitive city.