The role of green roofs in Water Sensitive Urban Design in South Australia M. Razzaghmanesh*, S. Beecham** and F. Kazemi*** * PhD student, Centre for Water Management and Reuse, University of South Australia, E-mail:
[email protected] ** Head of School of Natural and Built Environments, University of South Australia, E-mail:
[email protected] *** Research Fellow, School of Natural and Built Environments, University of South Australia, Email:
[email protected] ABSTRACT The philosophy of all new stormwater management approaches such as Water Sensitive Urban Design (WSUD) is to reduce the environmental effects of urbanization. Urbanization growth produces more impervious areas such as roads and roofs in metropolitan areas. Increasing imperviousness leads to removal of native vegetation in urban areas. It also leads to increased runoff volumes and peak flows which put more pressure on urban drainage systems. Green roofs, as WSUD systems, can cover the already available dense impermeable roof area and provide environmental, economic and social benefits. Despite such benefits, green roofs are relatively new in Australia and there are research gaps and practical barriers to apply the systems more widely. In this review paper, various studies from different climatic regions in Europe, North America, Asia, Australia and New Zealand were compared to understand how green roofs can be adapted to meet WSUD objectives in Australia. It was found that green roofs have been used as an important WSUD infrastructure around the world but that this technology is very much in its infancy in Australia. Furthermore, specific design criteria need to be developed for a range of Australian conditions to develop resilient green roofs.
KEYWORDS Green roof; Stormwater management; Water sensitive urban design
INTRODUCTION Over 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 Zealand environment and conservation council 2000). Australia is one of the most urbanized countries in the world and 84.7% of its inhabitants live in towns or cities (Skinner 2006). The growth rate of urbanisation has led to changes of green spaces with large impervious areas such as roofs, car parks, roads, highways and paving. These have led to changes in the urban hydrologic cycle. Increases in runoff volume and peak flow have also occurred together with reductions in times of concentration. As a response, relatively new stormwater management strategies such as Low Impact Development, Sustainable Urban Drainage Systems (SUDS), Low Impact Urban Design and Development (LIUDD) and Water Sensitive Urban Design (WSUD) have been developed in different countries. The main objectives of these strategies are to attenuate runoff peak flows and to provide water quality control. Stormwater best management practices including LID, SUDS and WSUD often involve applying stormwater management technologies such as sediment basins, bioretention swales, bioretention basins, porous and permeable pavements and green roofs. The concept of WSUD follows certain objectives, as shown in Figure 1. Please cite this article as follows: Razzaghmanesh, M., Beecham, S. & Kazemi, F. (2012). The role of green roofs in WaterSensitive Urban Design in South Australia. 7th international conference on Water Sensitive Urban Design, Melbourne, Australia, 1
Water Quality - Achieve Concentration Targets - Reduce Pollutant Loads - Manage Acute Impacts - Maintain Visual Amenity
Amenity - Protect Sensitive Areas - Preserve Natural Drainage Systems
Function Design for maintenance - Interrogate services - Multiple uses - Adequate life cycle Figure 1-
Water Quantity -Peak Peak Flow Duration -Frequency Frequency -Duration Duration
Water Supply - Demand Reduction Potable Substitution -Recycling Recycling
Water Sensitive Urban Design (WSUD)
Water Sensitive Urban Design (WSUD) objectives (BMT BMT WBM 2007 2007)
Green roofs as a type of vegetated WSUD systems are also known as eco-roofs, roofs, vegetated roofs, living roofs or nature roofs (Berndtsson Berndtsson et al. 2008; Palla et al. 2010) and are types of roofs that have been constructed with different layers. The outer layer is normally vegetated with succulents, sedums (FLL Guideline 2002) or native plant speciess depending on the climatic status of the project site. Usually green roofs have been classified in into three different categories including extensive, intensive and hybrid green roofs,, depending on the depth of planting medium (FLL Guideline 2002). While definitions vary, generally an extensive roof has a media depth less than 150mm while an intensive roof has a depth greater eater than 150 mm. All types of green roofs can provide some degrees of environmental, economic and social benefits (Table 1). The ultimate aim of this paper is to examine international research and practice in order to develop resilient green roofs as ve vegetated WSUD systems in Australia. Table 11 Green roof benefits by (EPA 2010) Environmental
Economic
Social
Increase carbon sequestration
Reduce hard infrastructure construction costs
Establish urban greenways
Improve air quality
Maintain aging infrastructure
Provide pedestrian and bicycle access
Flood protection
Increase land values
Educate the public about their role in stormwater management
Drinking water source protection
Encourage economic development
Replenish groundwater
Reduce energy consumption and costs
Improve watershed health Reduce sewer overflow events Urban heat island mitigation
Please cite this article as follows: Razzaghmanesh, M., Beecham, S. & Kazemi, F. (2012). The role of green roofs in WaterSensitive WaterSensitive Urban Design in South Australia. 7th international conference on Water Sensitive Urban Design, Melbourne, Australia,
In this review paper, various studies from different climatic regions in Europe, North America, Asia, Australia and New Zealand were compared to understand how green roofs can be adapted with WSUD objectives and for a range of climate conditions in South Australia. Water quantity One of the important strategies in sustainable urban drainage systems (SUDS) and Water Sensitive Urban Design (WSUD) systems is source runoff control (Alsup et al. 2010; Palla, Gnecco et al. 2010; Voyde et al. 2010). Installing green roofs is viewed as a best management practice (BMP) to attenuate peak runoff flows in urban areas (Palla, Gnecco et al. 2010). Therefore, one of the most important objectives in green roof studies is determining how green roofs can affect stormwater quantity. This requires an understanding of the hydrology of green roofs. Comparison of runoff from green and conventional roofs As Berndtsson (2010) discussed, the most significant difference between the outflow hydrographs from conventional roofs and green roofs are the peak flow and the time of runoff movement on the surface. As it can be seen from Figure 2, the time of peak flow events in conventional roofs is sooner than the corresponding time in green roofs. Also, in green roofs the time of concentration to the downpipe are greater than the corresponding time in conventional roofs. This delay of peak flow and long travel time are the main benefits of green roofs (Stovin 2010).
Figure 2- A typical runoff hydrograph from a conventional roof compared with that in a green roof (Stovin 2010)
Retention capacity of green roofs Hydrological studies of green roofs, especially studies about water retention of green roofs, began in Germany several decades ago and much of the early literature comes from the German journal "Dach+Grϋn"(Mentens et al. 2006). In addition to the rapid progress of the green roof industry in Germany, North America has also shown significant progress in this area. For example, the study of two Eco-Roofs was undertaken in Portland, Oregon, USA (Hutchinson et al. 2003). In this study after achieving reliable results from small scale green roofs installed on top of a residential site, the Bureau of Environmental Services (BES) constructed and monitored a full-scale green roof on the roof of a Hamilton apartment. The precipitation retention of the green roof in this project was 69% in average and 100% in warm seasons. Palla et al. (2010) conducted a similar study in Europe and the USA and they reported a 42% to 80% retention volume (Figure 3) with an average retention volume of 52% and a peak flow reduction of 83%. Voyde et al. (2010) studied the hydrology of a Please cite this article as follows: Razzaghmanesh, M., Beecham, S. & Kazemi, F. (2012). The role of green roofs in WaterSensitive Urban Design in South Australia. 7th international conference on Water Sensitive Urban Design, Melbourne, Australia, 3
Volume of Retention (%)
green roof under sub-tropical conditions in Auckland, New Zealand. They found that for a green roof at the University of Auckland there was 66% retention of rainfall over a one year period. They concluded that green roofs can significantly reduce runoff and especially the maximum runoff. Their results showed that in some individual events, green roofs could retain the average of 82% of the rainfall and can reduce up to 93% of the peak flow. SWE: Lund University, Sweden (Berndtsson et al. 2005) BLG: KU Leuven, Belgium, (Mentens, Raes et al. 2006) DE 1: Univ. of Neubrandenburg, Germany, (Köhler et al. 2002) DE 2: Technical University of Berlin, Germany, (Centgraf and Schmidt 2005) US 1: Michigan State University, USA, (VanWoert et al. 2005) US 2: Harvard University, USA, (Barth 2001) US 3: NCS University, North Carolina, USA, (Barth 2001) US 4: Washington DC, USA, (Deutsch et al. 2005) US 5: University of Georgia, Athens, USA, (Carter and Jackson 2007) US 6: Bureau of Environment Service, Oregon, USA, (Hutchinson, Abrams et al. 2003) US 7: Columbia University, New York, USA, (Acks et al. 2006) US 8: University of Georgia, Athens, USA, (Prowell 2006)
Figure 3- Annual stormwater retention (%) of different study sites (Palla et al. 2010)
Water quality studies Green roofs have become more widely used in recent years (Emilsson et al. 2006). While much attention has focused on the hydrology of green roofs, several researchers have studied the impact on water quality of outflows. Alsup et al. (2010) in a study in Illinois, USA, measured extraction and leaching of metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) from several potential green roof substrates with and without vegetation. They found different substrates and vegetation types can influence the water quality of the outflow. Some water quality parameters such as SO4, P, COD, Tot-N, Pb, Zn, Cd and turbidity were measured by Bliss et al. (2009) in Pennsylvania, USA. Levels of P and COD were increased but most storm events did not show a first flush phenomenon from the green roof. Both the control and green roofs slightly neutralized acidic rainfall. Carpenter and Kaluvakolanu (2011) compared runoff from asphalt, gravel ballast and extensive green roofs in Michigan, USA. Green roofs had the highest concentration of total suspended solids with no significant differences for nitrates and phosphates. Mean mass values for nitrates and phosphates from the green roof were lower than those from the asphalt roof. Berndtsson et al. (2005) in Sweden measured runoff from extensive sedum-moss roofs and nonvegetated roofs for metals and nutrient loadings (Cd, Cr, Cu, Fe, K, Mn, Pb, Zn, NO3-N, NH4-N, Tot-N, PO4-P and Tot-P). They ascertained whether or not roof age is important in the element loading. Their results showed that with the exception of N which was retained by the vegetation, green roofs are sources of contaminants. Berndtsson et al. (2008) measured first flush runoff during simulated rain events in Sweden. Except for K and dissolved organic carbon (DOC), concentrations and total volumes of the tested chemicals were higher in the initial first flush runoff than what was washed off later. Finally, Berndtsson (2010) from Sweden published a detailed review paper on stormwater quality in which they also outlined knowledge gaps in water quality studies of green roofs. They ultimately recommended long term studies and that green roof monitoring should be taken into consideration. Emilsson (2008) measured nutrient runoff, nutrient storage and plant uptake following fertilization of vegetated sedum mats, shoot-established vegetation and nonvegetated green roof substrates in Sweden. In this study, three rates of either a controlled release fertilizer (CRF) or a combination of CRF and conventional fertilizer were compared. Please cite this article as follows: Razzaghmanesh, M., Beecham, S. & Kazemi, F. (2012). The role of green roofs in WaterSensitive Urban Design in South Australia. 7th international conference on Water Sensitive Urban Design, Melbourne, Australia,
Conventional fertilizers caused high nutrient concentrations in runoff and established vegetation reduced leaching. Hathaway et al. (2008) in a study conducted in North Carolina, USA compared rainfall and runoff from two green roofs and a control roof for TKN, NO3, NO2, NH3, Tot-N, Tot-P and OP. Total N and P concentrations in green roof runoff were greatest compared with the concentrations of these elements in the control roof and in the rainfall. The total P load was also greatest in the green roof runoff. Köhler et al. (2002) studied the water quality of outlet flows from green roofs in Germany. They concluded that green roofs retained 95% Pb, 88% Cd, 80% NO3 and 67% PO4. Retention of PO4 increased from 26% in year one to 80% in year four following green roof installation. Monterusso et al. (2004) from Michigan, USA, compared NO3-N and Tot-P concentration of four commercially available green roof drainage systems planted with sedum and native herbaceous perennials. Nitrate concentrations in the runoff were higher in the sedum planted roofs with a shallower substrate depth. There were no significant differences in P concentration between sedum planted roofs and those planted with herbaceous plants. Steusloff et al. (1998) modeled the retention capability of extensive green roofs for heavy metals in Germany. They reported 97% Cu, 96% Zn, 92% Cd and 99% Pb retention of outflow concentration, during a summer season and 34%, 72%, 62% and 91% during a winter season. Teemusk and Mander (2007) compared runoff from a green roof and a modified bituminous roof for pH, BOD, COD, Tot-P, PO4, Tot-N, NO3, NH4, SO4, Ca and Mg in Estonia. During moderate runoff events, values of COD, BOD and concentrations of total N and P were greater on the bituminous roof. During heavy rain events, these components were less concentrated and more nitrates and phosphates were washed off from the green roof. All components were higher for the green roof during snowmelt. EPA (2009) compared runoff from green and conventional roofs for pH, EC, turbidity, colour, nitrates, P, K, Ca, Fe, Mg, Mn, Na, Zn and S in Pennsylvania, USA. Higher concentrations were reported for most nutrients and ions in the runoff, but the loadings were not always higher. Van Seters et al. (2009) analyzed runoff samples from an extensive green roof for pH, total suspended solids, metals, nutrients, bacteria and PAH (polycyclic aromatic hydrocarbons) in Toronto, Canada. Loads of chemicals from the green roof were lower than the loads from a conventional roof except for Ca, Mg and TP. Gregorie and Clausen (2011) studied water quality from a 248m2 extensive green roof and a control roof in Connecticut, USA, using a paired watershed study. Weekly and individual storm event samples of runoff and precipitation were analyzed for TKN, NO3+NO2, NH3-N, TP, PO4-P, Total and dissolved Cu, Zn, Cd, Cr and Hg. The green roof acted as a sink for NH3–N, Pb and Zn, with minor retention of TN and TKN, which they attributed to the expanded shale and biosolids media. However, the green roof was a source of NO3 +NO2-N, TP, PO4-P, and Cu. The type of fertilizer used on the green roof was a likely source of Cu. Greater than 90% of the Cu, Hg and Zn concentrations in the green roof runoff were in the dissolved form. The combination of vegetation, light-weight expanded shale and composted biosolids reduced the export of TN, TKN, NO3+NO2, NO2-N, dissolved Cu, Pb and Zn through the retention of precipitation and utilization, transformation and/or storage of the pollutants. Overall, the green roof was effective in reducing stormwater runoff and pollutant loadings for most water quality contaminants. Water supply Due to recent global climate change and population growth in Australia, urban water resources are under remarkable stress (Chowdhury and Beecham 2009) and an understanding and research into fit for irrigation water quality of green roof run off is important as it can potentially provide an additional water source for urban landscape irrigation which ultimately improve recreational amenity in urban settings (Beecham 2003). Function and Amenity Please cite this article as follows: Razzaghmanesh, M., Beecham, S. & Kazemi, F. (2012). The role of green roofs in WaterSensitive Urban Design in South Australia. 7th international conference on Water Sensitive Urban Design, Melbourne, Australia, 5
Today there are many significant buildings worldwide that have been constructed with green roofs including the Australian National Parliament building, Chicago Hall in the US, a swirling green roof top in Nanyang Art School in Singapore and other green roofs in the UK and Germany. These all generally mimic several key properties of ground vegetation which are missing from conventional roofs (Oberndorfer et al. 2007). RESULTS AND DISCUSSION Green roofs provide significant benefits to urban environments around the world. In some countries like Germany, Italy, Sweden and Belgium, green roof technologies in research and practice, have been developed for many years. This is to the extent that the outcomes of German research (FLL Guideline 2002) have been developed into a national standard. While the US has already developed a local guideline on green roof construction, Dvoral (2011) shows that further research is still required to enhance these existing guidelines. Such standards can be useful for countries with similar climatic conditions. In Australia, government and policy makers have recently understood the significance and advantages of green roofs. However, there are many problems that impede the rapid development of this technology. Williams et al. (2010) have clearly described opportunities and barriers for green roof development in Australia. These include: 1- Lack of standards 2- High cost of installations 3- Few demonstration examples 4- Lack of relevant and reliable research 5- Difference of Australian climatic conditions compared with the temperate climate conditions of most of Europe and the USA has made using the Northern hemisphere standards less applicable for Australian conditions. 6- Similarly, relying on northern hemisphere research raises some issues because of different rainfall patterns, substrates and types of vegetation. In order to develop resilient green roofs for Adelaide in terms of growing media type and depth, plants, water retention, flood attenuation and outflow water quality, two locations have been selected for this research. One is located on the top of a 22 storey building in the Adelaide CBD and the other one is at the Mawson Lakes Campus of the University of South Australia. CONCLUSIONS Many biotic and abiotic parameters affect green roof performance and this has been documented in a significant number of research studies across the world. Further research is required to select suitable vegetation, growing media or substrates and other design parameters for green roofs, particularly for Australian conditions. There are also gaps in our understanding of how green roofs will respond to stormwater quality and quantity in Australian urban environments. Furthermore, previous researchers have mainly focused on the effect of individual parameters on green roof performance. More research is required to apply an integrated approach and to examine the effects of combined biotic and abiotic parameters on green roof performance. The outcomes of such research will assist urban planners in developing resilient green roof models for Australian cities. ACKNOWLEDGEMENT The authors would like to thank Graeme Hopkins and Christine Goodwin of Fifth Creek Studio and their client, ANZ House. We are also grateful to Tim Golding for technical advice and assistance.
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