With ever increasing urbanisation, the construction of more roads, and infill development, Australian cities are producing huge amounts of storm water runoff.
This water accumulates in the city drainage systems which are often operating beyond capacity, causing flooding and environmental hazards. When the runoff eventually enters the receiving waters, is also contaminated, resulting in ecological degradation and threats to human health.
Local governments and water authorities in most Australian cities require that new developments manage storm water in some way. This may be by restricting runoff to pre-development flows or by treating storm water on-site.
This can be expensive, particularly in dense developments or on constrained infill sites, and the costs of ‘offsetting’ storm water management off-site can be even greater.
Melbourne Water requires that an offset contribution from developers be paid if storm water runoff cannot be reduced or treated on-site. The requirement is dependent on the type of project, as well as the land area and zoning, but can be considerable – up to $30,000 per hectare depending on the location. Melbourne Water operates a stormwater offset service to calculate this.
In many instances the area required to manage storm water effectively on-site is significant, which can limit the return on developable land.
To avoid the offset contribution, there can be considerable costs involved in the construction of storage and treatment facilities on-site. Traditionally these take the form of rain water tanks, rain gardens, and constructed wetlands.
These facilities may not provide direct benefit to the developer as they may be underground or perceived as ‘lost space.’
The industry uses a sophisticated computer model known as MUSIC (Model for Urban Stormwater Improvement Conceptualisation) to determine exactly how much water will be managed, and the level of contaminant removal required to ensure that the development meets the ‘best practice’ requirements for managing urban storm water (also termed Water Sensitive Urban Design or WSUD).
Rather than paying the offset contribution or building on-ground elements that take up developable space, another way of dealing with this problem and saving money is to include green roofs as a part of the development.
Installing a green roof, or a roof that includes soil and vegetation on top of a building, will reduce storm water runoff and remove contaminants that developers and water authorities spend millions of dollars a year to remove.
A green roof acts like a sponge to slow and reduce the volume of storm water runoff, which means that rain water does not immediately surge into the storm water system, but is held up on the roof garden, slowly trickling down. This water sustains the growth of plants which further take up water into their roots, stems and foliage.
The water retention function of green roofs is well known, but what is less well known is exactly how much water is held, and the quality (or lack of contaminants) of the water that does flow through after it passes through the roof garden.
Recently the Victorian Government commissioned a project to quantify the volumetric and qualitative improvements that a green roof can provide to the urban water cycle.
The proponent of this green roof project – Henrik Ender of the Big Bang Creative Community Hub – believes that a green roof as part of their retro fit of an inner urban warehouse will convert it to a ‘living laboratory’ and demonstrate that there are multiple benefits of ‘living architecture.’
In Ender’s opinion, “not only is the green roof a pivotal component of the creation of this space – it could be the magnet that will invigorate our business and draw attention to our core beliefs which are sustainability in all of its forms be it ecological, urban and social.”
As well as having a potential roof garden, this project includes a vertical garden in the form of Living Louvres that visually connect the building to the adjacent Merri Creek and CERES Environmental Park opposite.
Dr Robyn Overall from Ecoharvest was the environmental scientist who conceived of the Big Bang green roof water quality improvement project. Overall said that “with the data we could gain from this project we will learn much about the way our built form interacts with the hydrology of our urban stream systems. We are confident that green roofs will one day become a conventional part of the urban fabric, reducing impervious surfaces and making our cities less of an impact to our immediate environment.”
Celeste Morgan from E2Designlab agrees.
“A substantial green roof will completely absorb up to around 10 millimetres of rainfall – the kind of rainfall events which occur most frequently,” she said. “By preventing stormwater runoff, green roofs can help to protect our waterways but can also be a great solution for inner urban areas where space is constrained – offering open space, biodiversity and amenity benefits to the local community. We often find ourselves searching for space for water management, when all we need to do is look up.”
Although the Living Louvers of the Big Bang project are nearly completed, the green roof is not yet in place. If new funding can be found this water quality monitoring project would not only set a new bench mark in a green urbanisation, but also provide much needed scientific understanding of how built-form interacts with the urban water cycle.
The eventual outcome of this project will enable the inclusion of a green roof ‘node’ in the MUSIC software that is used to model expected performance of storm water management features to determine whether developments meet the regulatory requirements.
Once this is in place we may see many green roofs sprout over Melbourne and other Australian cities as a more cost effective way of managing the contaminated flood waters that our cities generate.