Mostafa Razzaghmanesh* and Simon Beecham**
*PhD Student, **Professor and Head of School of Natural and Built Environments, University of South
Australia

Abstract

Australia is one of the most urbanized countries in the world and 85% of its inhabitants live in towns or cities.
Urbanization growth increases impervious areas such as roads, roofs, parking lots, highways and pavers in the
metropolitan areas. This leads to removal of remnant native vegetation cover in the urban area. It also increases runoff volume, peak flow and reduces the time of concentration. These effects bring more pressure on urban drainage systems. Green roofs, as one of the vegetated Water Sensitive Urban Design (WSUD) systems can cover the already available dense area and provide environmental, economic and social benefits. Despite such benefits, green technology is relatively new in Australia and there are research gaps and practical barriers to apply the technology widely. To address research gaps in this area and give self-confidence to authorities and organizations, two major experiments have been suggested. The first conducted in trial green roofs (as full scale experiments) built on the roof of the ANZ tower located in the City of Adelaide. The second experiment constructed at the University of South Australia (as small scale experiments). In these experiments, the water quality, quantity and thermal performances of the green roofs were investigated. The outcomes of this study will assist urban planners in developing a resilient green roof model at both the micro and macro scale for the city of Adelaide.

Key words:

Water Sensitive Urban Design, Green Roofs, Adelaide, Urban Heat Island

Introduction

In recent decades the hydrologic cycle of water has changed significantly due to continuous changes in Australian green spaces from forest or other intrinsic vegetation to rural or urban environments (Australian
and New Zeland environment and conservation council 2000). The growth rate of urbanisation has led to changes of green spaces with large impervious areas such as roofs, parking lots, roads, highways and paving. These have led to changes of the hydrologic cycle. Figure 1, shows a conventional catchment before and after urbanization growth which increased impervious surfaces, decreased infiltration rate and then increased stormwater runoff. As a response, relatively new stormwater management strategies such as Low Impact Development (LID) (Voyde et al. 2010), Sustainable Urban Drainage Systems (SUDS) (Stovin 2010) and Water Sensitive Urban Design (WSUD) (Palla et al. 2010) have been developed in different countries. The main objectives of these strategies are to attenuate runoff peak flow and to provide pollution control. Stormwater management practices (BMP), LID, SUDS and WSUD employ two main techniques, namely potable water demand reduction and stormwater management techniques. There are some structural management tools such as sediment basins, bio-filtration swales, bio-retention basins, porous and permeable pavements and green roofs.

Figure 1. Illustrates the conventional urban catchment before and after urbanisation growth

However, Adelaide is the capital city of the driest state in Australia and it currently three major challenges, namely urbanisation growth, water scarcity and climate change. The consequences of these threats put more stress on the urban water cycle and increase metropolitan temperatures through urban heat island effects Error! Reference source not found.. Introducing green infrastructure through water sensitive urban design is one of the possible solutions to reduce the harmful impacts of urbanisation while providing additional amenity and water quality benefits for communities and the environment (SAGovernment 2010). Adelaide’s 30 Year Plan (2010) describes how the capital of South Australia will be one of the most water sensitive cities in the country. This plan also recommends the introduction of green roofs

Figure 3 but research is required to develop this technology to be resilient in South Australia’s harsh climate (Razzaghmanesh et al. 2012). This paper describes the results of a current research project that is investigating the water quantity and quality effects and thermal benefits of two different types of green roofs, namely intensive and extensive

Figure 2. Typical Urban Heat Island effects distribution over a city http://heatisland.lbl.gov/coolscience

 

Figure 3. Water Sensitive Urban Design objectives and relation with Adelaide 30 -year Plan

Methodology

Site 1- ANZ House green roofs storm water quality

The ANZ House green roofs include 4 green roof beds. Of these 2 are intensive (AI and BI) and 2 are extensive beds (BE and AE). The area of each roof bed is 14.4 m2. Figure 4 shows the study plan and sampling points. In each water sampling event, samples from the surface of an existing asphalt roof were also collected as an experimental control.

Figure 4– ANZ House green roof layout and sampling points

To estimate the possible volume of outflow from the systems, Adelaide’s rainfall was investigated for the last 10 years. Figure 5 shows the distribution of wet days (Voyde, Fassman and Simcock 2010) (rainfall with more than 2 mm in a typical year (2010). The focus of the study will be on selected rainfall events in the band of 5-20 mm depth, as shown in Figure 5.

Figure 5. Adelaide rainfall pattern of wet days in 2010 as an example year

Water quality sampling method

The design of the ANZ House Figure 6 green roofs was based on a free drainage system and the designer intended to get rid of excess water from the system as soon as possible after rainfall events. Collecting water samples from the green roof beds was the main challenge in this study. Different methods were considered such as making small scale green roofs at the site, retrofitting beds and adding metal sheet around the beds to collect the water samples. However, none of these methods were feasible due to the building’s operational and maintenance requirements. The only possible way to capture enough water in the porous media was to use half round pipes buried in the soil media. Using the growing media, soil properties and considering the required volume of samples, the diameter and length of the required half round pipe was calculated as 50 mm and 700 mm, respectively. Holes were drilled at both ends of the half round pipe and lengths of hose were attached to facilitate water collection.

Figure 6. ANZ House green roofs trial

The collected water samples were refrigerated at all times at the University of South Australia. Water quality tests were performed for parameters including pH, Turbidity, Electrical Conductivity (EC), TDS, Nitrate, Phosphorous, Potassium and Sodium and Chlorine. To investigate heavy metals, 2 random samples of the last data collection were sent to SA Water’s Australian Water Quality Centre (AWQC) to examine the heavy metals concentrations.

Site 2- Mawson lakes campus experiments

In this study which is associated with the ANZ House green roofs stormwater quality monitoring, the small scaled green roofs were constructed and set up at University of South Australia, Mawson lakes campus. The study site is approximately 15 km away from Adelaide CBD. In this study rainfall and runoff were measured using rain gauge Figure 7 and outflow tipping bucket Figure 8 from the green roofs downspouts. Moreover, thermal sensors were used in different beds and different depths to find out that how green roofs can mitigate the urban heat island effects Figure 9 and how much they have insulation and cooling properties for buildings.

Figure 7. Mawson lakes campus small scale green roofs

Figure 8. Outflow tipping bucket counter with water quality sampler

Figure 9. Shows the method of measuring thermal benefits of green roofs at Mawson lakes

Result and discussion

Water Quality

To investigate the potential for reusing runoff from the ANZ House green roofs in the building, available local, state and national Australian water quality guidelines were reviewed. The qualities of the collected samples with regard to the alternative reuse scenarios such as potable, non-potable and urban irrigation were examined. Results showed with no doubt it is possible to reuse the green roof outflows for urban irrigation and non-potable purposes (such as toilet flushing) but this source of water is not recommended for drinking purposes.

Water Quantity

The rainfall and runoff data were collected each rainfall events and they have been accessible through the available data logger system. The results represent that the green roofs bed could mitigate the 75% of the rainfall

Figure 10 and also the y can attenuate the run off for 400 minutes. Furthermore, the water retention capacity of the green roofs bed were investigated and we concluded that green roof beds could retain the rainfall depend on the rainfall intensity, duration, antecedent dry weather period and initial moisture in the system between 44% to 100%.

Figure 10– Typical Rainfall and runoff of the green roof system of Mawson Lakes Campus

Thermal Benefits

Thermal benefits of the green roofs in terms of mitigating urban heat island effects and also cooling properties have been investigating in cold and warm day’s scenarios. The results so far have shown that green roofs can be cooler in the days from 2-5 degrees depend on the media type and depth and also in the night time are warmer than surrounding area weather between 2.5 to 5.5 degrees.

Conclusion

In this paper, result of the current research on introducing green roofs into the urban and built environments of Adelaide has been discussed briefly. The results confirm that green roof could act as one of the main possible options in Adelaide 30-year plan to make it for Adelaide and South Australia easier to be one of the water sensitive cities in Australia. Due to the different rages of climate in South Australia still more research are required to understand the green roofs behavior. And also, the researchers at university of South Australia are appreciating any research projects in this area and also WSUD area.

Acknowledgments

The authors would like to thank Graeme Hopkins and Christine Goodwin of Fifth Creek Studio for their advice and ongoing support. We also gratefully acknowledge their client, ANZ House, for facilitating this investigation and contributing to this innovative research. We are also grateful to Tim Golding for technical advice and assistance.

References

  • Australian and New Zeland environment and conservation council (2000) “Australian guildelines for urban stormwater management “.
  • Palla A, Gnecco I and Lanza L (2010) “Hydrologic restoration in urban Environmental Using Green Roofs.” Water 2: 140-154.
  • Razzaghmanesh M, Beecham S and Kazemi F (2012) The role of greeen roofs in Water Sensitive Urban Design in South Australia. 7th international conference on Water Sensitive Urban Design, Melbourne, Australia, Center for Water Sensitive Cities, Monash University.
  • SA Government (2010) The 30- Year Plan for Greater Adelaide. Department of Planning and Local Government. South Australian Planning Strategy.
  • Stovin V (2010) “The potential of green roofs to manage Urban Stormwater.” Water and Environment 24(3): 192–199.
  • Voyde E, Fassman E and Simcock R (2010) “Hydrology of an extensive living roof under sub-tropical climate conditions in Auckland New Zeland.” Journal of Hdrology 394: 384-395.