Green InfrastructureGreen Infrastructure is the network of green spaces and water systems that delivers multiple environmental, social and economic values and services to urban communities. This network includes parks and reserves, backyards and gardens, waterways and wetlands, streets and transport corridors, pathways and greenways, farms and orchards, squares and plazas, roof gardens and living walls, sports fields and cemeteries. Green Infrastructure secures the health, liveability and sustainability of urban environments. It strengthens the resilience of towns and cities to respond to the major current and future challenges of growth, health, climate change and biodiversity loss, as well as water, energy and food security.
‘The way we manage urban water, particularly urban stormwater, influences almost every aspect of our urban environment and quality of life. Water is an essential element of place making, both inmaintaining/enhancing the environmental values of surrounding waterways and in the amenity and cultural connection of the place’ (Wong, 2011).
The ‘nexus’ between sustainable urban water management and the vitality and prosperity of urban environments is only beginning to be recognised and include (Wong, 2011):
- Access to secure and clean water supply
- A clean water environment
- Flood protection
- Efficient and low energy systems
- Urban design strategies
- Mitigating urban heat
- Creating productive landscapes
- Quality of public spaces
Trees and other vegetation can act as Green Infrastructure, providing alternatives to conventional engineering infrastructure in the process of integrated water cycle management and Water Sensitive Urban Design (WSUD). Figure 44 summarizes the water related benefits provided by Green Infrastructure.
Figure 44: Summary of water related benefits of Green Infrastructure
7.2 Hydrological processes
7.2.1 The natural water cycle
Trees and other vegetation play an important role in the natural water cycle (Bernatzky, 1983; MacDonald, 1996; Stovin et al., 2008). According to Day & Dickson (Bartens et al., 2008):
‘Natural forests with their complete canopy cover, large leaf areas, and permeable soils, handle rainwater effectively through interception and infiltration, returning water to groundwater and the atmosphere and protecting water quality in surface waterways’.
Vegetation performs a number of hydrologic functions in the natural water cycle, and has become an important component of Water Sensitive Urban Design strategies which aims to better replicate the natural water cycle in urban areas.
Figure 45: Role of trees in the natural water cycle. Source: (McPherson, 2010).
The Eight Hydrologic Functions of Forests and Trees
There are at least eight hydrologic functions provided by forests and trees.
1) Canopy Interception: The leaves of trees and large shrubs act like umbrellas or shallow cups that collect rainwater and other precipitation; they typically capture (reduce) about 10% of total annual precipitation in a healthy forest; this function varies according to whether deciduous or evergreen trees are dominant.
2) Stem flow: The limbs and trunks of trees and shrubsprovide year-round capture and delays in peak flows; some studies estimate that stem flow in a dense forest captures as much as 15% of annual precipitation.
3) Leaf Litter Absorption: The duff (dead leaf) layer stores and transmits water and protects the underlying soil from erosion; it is estimated to absorb 2 to 4% of annual precipitation.
4) Soil Infiltration: Native soils with ample organic layers are the ‘sponges’ of healthy ecosystems; their pore spaces store water and infiltrate it vertically and laterally, interact with root and fungi systems. 80 to 95% of annual precipitation in United States forests is captured via soil infiltration; it’s the master hydrologic function of forest ecosystems, governing evapotranspiration; hydraulic lift; groundwater recharge; and large storm conveyance.
5) Evapotranspiration: Trees but also shrubs and forbs release large amounts of water vapor through their leaves during photosynthesis. Total runoff is significantly less for a forested watershed compared to an urbanized one. Evapotranspiration also reduces pollutant loadings and cools the air. Nationwide, in the United States, evapotranspiration has been estimated to capture 57% of average annual precipitation.
6) Hydraulic Lift/ Redistribution: First documented in the mid-1990s, tree rootshave been shown to lift groundwater from deeper layers and bring it closer to the surface, where it can be used by other plants as well as the trees themselves.
7) Groundwater Recharge: Recharge refers to replenishment of both groundwater (aquifer) levels and dry weather stream flows. Tree roots in symbiosis with fungi enlarge fissures in bedrock, increasing porosity and groundwater recharge capacities.
8)Conveyance of Large Storms: A well-vegetated catchment conveys runoff from large storms in multiple, distributed channels. Heavy rainstorms, including 100-year storms saturate the soil layer and then cause shallow subsurface flow (interflow); this outcrops as seeps and springheads of temporary rivulets and ephemeral streams.Source: (Cameron, 2011).
7.2.2 Canopy Interception
Tree canopies intercept and store rainfall, thereby modifying stormwater runoff and reducing demands on urban stormwater infrastructure (Xiao et al., 1998; Xiao et al., 2000; Xiao and McPherson, 2002; Xiao et al., 2006). Canopy interception reduces both the actual runoff volumes, and delays the onset of peak flows (Davey Resource Group, 2008). The extent of interception is influenced by a number of factors including tree architecture and it has been estimated that a typical medium-sized canopy tree can intercept as much as 9000 litres of rainfall year. (Crockford and Richardson, 2000). A study of rainfall interception by street and park trees in Santa Monica, California found that interception rates varied by tree species and size, with broadleaf evergreen trees provided the most rainfall interception (Xiao and McPherson, 2002). Rainfall interception was found to range from 15.3% for a small jacaranda (Jacaranda mimosifolia) to 66.5% for a mature brush box (Tristania conferta now known as Lophostemon confertus). Over the city as a whole the trees intercepted 1.6% of annual precipitation and the researchers calculated that the annual value of avoided stormwater treatment and flood control costs associated with this reduced runoff was US$110,890 (US$3.60 per tree).
7.2.3 Soil Infiltration and storage
Trees and other vegetation can increase the rate or volume of water infiltration into the soil, increasing soil and groundwater recharge. Initially a proportion of rainfall held in the canopy will flow down the stem and trunk to the ground (Xiao and McPherson, 2002). Tree root growth and decomposition in the soil also increases the capacity and rate of rainfall infiltration, and reduces surface flows (Bartens et al., 2008). Trees also draw moisture from the soil through the mechanism of transpiration, increasing soil water storage potential (Stovin et al., 2008). Trees can remove large volumes of water from the soil to meet their biological needs, acting as virtual ‘solar pumps’. For example a daily summer water usage of 500 litres has been estimated for each tree in an avenue of seventy English elms (Ulmus procera) at the Adelaide Waite Arboretum (Lawry, 2008).
7.2.4 Flood control
Trees and vegetation also help reduce the risks of downstream flooding by modifying runoff volumes and the timing of peak flows.
7.2.5 Improved water quality
A key WSUD objective is to minimize undesirable impacts on aquatic ecosystems by reducing the volume and timing of runoff, and by improving runoff quality. Vegetation and its associated soil media play an important role in removing nutrients and heavy metals from stormwater runoff (Davis et al., 2001; Henderson et al., 2007; Read et al., 2008). Slowing runoff (for example in vegetated swales) also allows particulate pollutants to settle out before entering waterways or the ocean (Herrera Environmental Consultants, 2008). The root systems of trees and other vegetation can also play a role in the bioremediation of stormwater or contaminated soils, through nutrient uptake and pollutant removal (Hough, 2004).
7.2.6 Erosion control
Trees and vegetation help reduce soil erosion in two main ways. Tree canopies diminish the impacts of raindrops on bare surfaces (Craul, 1992) and root systems reduce erosion by stormwater flows (Lull and Sopper, 1969).
7.3 Water Sensitive Urban Design
7.3.1 Negative impacts of urbanization
Urbanization has created significant changes in the natural water cycle including: increased volumes of stormwater runoff from impervious urban surfaces; and a decline in the quality of runoff from pollutants generated by human activities (Thompson and Sorvig, 2008).Conventional engineering management of urban stormwater runoff has been driven by the attitude thats tormwater has no value as a resource, is environmentally benign, and adds little to urban amenity (Wong, 2006). Urban stormwater management practices have emphasized highly efficient drainage systems that collect and remove stormwater and discharge it downstream to avoid stormwaterponding and flood risk, using an engineered system of above ground channels and underground pipes. Such systemsdriven by public health and flood mitigation concerns, have had significant negative environmental consequences. Impacts on the contributing area or local catchment include the loss of a local water resource. Impacts on the receiving waters downstream include increased runoff and flooding, and the pollution of waterways, resulting in the decline of aquatic ecosystems. In addition the visual connection between human activities and the water cycle has been lost, withthe natural processes being ‘out of sight and out of mind’.
Urbanization has in fact seen the ‘natural water cycle’ replaced with the ‘urban water cycle’. Under the natural water cycle a large proportion of rainfall was retained within its local catchment where it infiltrated into porous soils recharging local groundwater and sustaining vegetation growth. The urban water cycle however operates under a different and less sustainable ‘water balance’ with the three artificial ‘streams’ of potable water, wastewater and stormwater. Water tends to be exported from urban catchments in the form of stormwater runoff and wastewater, while potable water is imported from external catchments.
‘Stormwater is a large but almost entirely untapped water source and is considered a highly valuable resource beyond that of simply an undifferentiated commodity. Stormwater runoff is generated across distributed areas and therefore distributed source control measures typically provide the best opportunities to capture and use urban stormwater, and to derive the other beneficial outcomes related to enhanced liveability of a city.
The most obvious effect of urbanisation on catchment hydrology is the increase in the magnitude of stormwater flow events in urban creeks and the consequent impact on flooding, creek degradation, and public safety. Urban stormwater is predominantly runoff from impervious areas (e.g. roads, roofs, footpaths, car parks, etc.), with runoff from pervious areas (e.g. gardens, lawns, vegetated open spaces, etc.) contributing only during high intensity rainfall events.
Stormwater management has traditionally focused on stormwater drainage, with the principal (and often only) objective of conveying stormwater runoff safely and economically to the receiving waters. Stormwater drainage infrastructure consists predominantly of pipes and drains. Natural waterways in urbanising catchments have become increasingly compromised in their ability to convey the significantly increased quantity and rate of stormwater runoff generated from urban areas. Bank erosion and increased frequency of flooding are the obvious symptoms of this change. Traditional approaches to resolving these problems typically involved increasing the hydraulic capacity of urban waterways by a combination of channelization and partial, or complete, concrete lining.’Source: (Wong, 2011).
7.3.2 What is Water Sensitive Urban Design?
According to (Wong, 2011):
‘The term Water Sensitive Urban Design (WSUD) is commonly used to reflect a new paradigm in the planning and design of urban environments that is ‘sensitive’ to the issues of water sustainability and environmental protection. WSUD, Ecologically Sustainable Development (ESD) and Integrated Water Cycle Management (IWCM) are intrinsically linked.The definitions of WSUD amongst practitioners are often varied, reflecting wide coverage of the applications of the WSUD framework’ p.7.
Water Sensitive Urban Design and Integrated Water Cycle Management aim to better replicate the natural water cycle and natural hydrologic processes in urban areas, reducing runoff to aquatic environments, and reducing the use of potable water through demand reduction and the use of alternative ‘fit for purpose’ water sources. WSUD emerged in the 1990s as a new paradigm for the more sustainable management of the water cycle in the urban landscape (Argue, 2004). WSUD includes the more sustainable management of the three urban water streams: potable water; wastewater; and stormwater (Landcom, 2004). A key focus is on stormwater management, based on a new attitude,that stormwater is a valuable resource; and an emerging set of ‘best management practices’, aimed at better replicating the natural water cycle and integrating it into the planning and design of urban areas at a range of scales (Lloyd et al., 2002; Breen et al., 2004).
‘Conventional approaches to stormwater management are based on a single management objective that considers stormwater as a source of potential hazard to public safety. Stormwater management was essentially that of stormwater drainage using two general methods, i.e. (i) conveyance of stormwater to receiving waters in an hydraulically efficient manner; and (ii) detention and retardation ofstormwater. Many measures designed for stormwater quantity control have inherent water quality management functions while others can be retrofitted to serve the dual functions of stormwater quantity and quality management. Stormwater quality management measures such as roof gardens, bioretention systems, constructed wetlands and ponds can provide effective stormwater detention to varying degrees and therefore can reduce drainage infrastructure requirements. This is particularly relevant within the context of increasing degrees of impervious areas attributed to urban consolidation. A network of green/blue corridors can effectively convey and/or detain flood waters for flood protection of downstream communities’.
Source: (Wong, 2011) p.7.
7.3.3 WSUD Principles
A number of State and local authorities have adopted WSUD strategies or guideline documents (City of Melbourne, 2005).These usually comprise a set of ‘guiding principles’ and a set of ‘Best Management Practices’ (BMP’s) which may address the three ‘streams’ of urban water cyclemanagement: potable mains water; stormwater and wastewater. The following is an example of key WSUD principles as listed by Melbourne Water (Melbourne Water, nd) and are consistent with the Urban Stormwater: Best Practice Environmental Management Guidelines (Victorian Stormwater Committee, 1999):
- Protect natural systems – protect and enhance natural water systems (creeks, rivers, wetlands) within urban developments
- Protect water quality – improve the quality of water draining from urban developments into creeks, rivers and bay environments
- Integrate stormwater treatment into the landscape – use stormwater treatment systems in the landscape by incorporating multiple uses that will provide multiple benefits, such as water quality treatment, wildlife habitat, public open space, recreational and visual amenity for the community
- Reduce runoff and peak flows – reduce peak flows from urban development by on site temporary storage measures (with potential for reuse) and minimise impervious areas
- Add value while minimising development costs – minimise the drainage infrastructure cost of development
- Reduce potable water demand – use stormwater as a resource through capture and reuse for non-potable purposes (e.g. toilet flushing, garden irrigation, laundry).
- Protect and enhance natural water systems in urban developments. Natural systems become assets which are protected rather than exploited, and which are then able to function effectively.
- Integrate stormwater treatment into the landscape. For example by incorporating multiple use drainage corridors that maximize visual and recreational amenity.
- Protect water quality by improving the quality of water draining from urban environments into receiving environments. Water can be treated by filtration and retention, to remove pollutants closer to their source, reducing pollutant impacts on the downstream environment.
- Reduce runoff and peak flows. Local detention, and minimization of impervious surfaces, can provide flood mitigation using numerous small storage points, rather than one large detention basin. This approach also reduces the demand for downstream drainage infrastructure.
- Add value while minimizing drainage infrastructure development costs. Reduced runoff volumes and peak flows reduce the development costs of drainage infrastructure, while enhancing natural features and value-adding to the development.
A key WSUD concept involves matching available water sources with appropriate uses. The four main water sourcesare: potable mains water; stormwater (roof runoff and surface runoff); wastewater (light grey water,grey water and blackwater); and groundwater. The philosophy of ‘fit-for-purpose’ water use wouldsee potable (drinking-quality) mains water replaced with other water sources where appropriate(City of Melbourne, 2005).For example re-used stormwater is a better alternative for landscape irrigation than using drinking quality water.
7.3.4 WSUD Practices
220.127.116.11 Biofiltration systems
Vegetated biofiltration systems are important WSUD treatment tools for improving the quality of urban stormwater runoff, and protecting aquatic ecosystems (Wong, 2006; FAWB, 2009). Small scale ‘at source’ treatment measures have been developed which can be adapted to small catchments and retrofitted into existing streets (Lloyd et al., 2002; Wettenhall, 2006). Vegetation plays an important role in these biofiltration systems, enhancing the pollutant removal function of the soil media in a number of ways, through a combination of physical, chemical and biological processes (Breen et al., 2004; Somes and Crosby, 2007). Two Australian studies have investigated the biofiltration role of a range of plant species, indigenous to south-eastern Australia (Bratiers et al., 2008; Read et al., 2008). Bioretention tree pits are a specialized form of biofiltration systems (Breen et al., 2004; Denman, 2006). One Australian study has investigated the biofiltration performance of a number of common tree species (Breen et al., 2004). The presence of trees resulted in significant reductions of soluble nitrogen and phosphorus in the treated stormwater.
Figure 46: Batman Drive bioretention tree pits, Docklands Melbourne. By author.
In 2011, Brisbane City Council (BCC) used MUSIC V4.00 model (Model for Urban Stormwater Improvement Conceptualisation) to forecast the benefits of integrating Neighbourhood Shadeways and WSUD retrofit sites across the city (Brisbane City, 2013). Neighbourhood Shadeways areBrisbane City Council’s program of street and park tree plantings alongside shade-hungry footpaths and bikeways to support greener and more walkable neighbourhoods’. WSUD (Water Sensitive Urban Design) aims to incorporate water cycle management initiatives into the design of urban landscapes. For 2878 potential street tree planting priority sites of the annual Shadeways schedule, streetscape bioretention was predicted to reduce stormwater flow rates by 5.3% (840ML), suspended solids by 84.1% (2,720 tonnes),phosphorous by 70-72% (4 tonnes) and nitrogen by 43% (11.2 tonnes). These comprise significant potential reductions in stormwater flows and pollutant loads discharging to downstream waterways.
Brisbane City Council has consequently developed WSUD Streetscape Design Guidelines for both developers and Council projects, and promoted partnerships for streetscape retrofit projects between Water Smart Integration and Neighbourhood Shadeways. These partnerships share up front investments in green infrastructure that can deliver both healthier waterways and healthier street trees. In particular, this Brisbane City Council project highlights opportunities to assign dollar values to the stormwater cleansing values of street trees, and incorporating MUSIC modelled stormwater treatment benefits into Australian i-Tree.
18.104.22.168 Stormwater harvesting
Extended drought conditions in most Australian cities since the mid to late 1990’s have focused governments on the emerging challenge of securing reliable water supplies for urban areas. In addition to promoting water conservation and water efficiency, stormwater harvesting is now gaining prominence as an alternative water source, supported by increased government funding for stormwater harvesting schemes (Wong, 2011). According to Wong urban stormwater harvesting represents a rare opportunity to provide a major new water source for Australian cities, while helping to protect valuable waterways from pollution and degradation.Stormwater harvesting can provide an additional and abundant source of water to support the greening of cities. Such Green Infrastructure provides many benefits in creating more liveable and resilient urban environments, including (Wong, 2011):
- Improved human thermal comfort to reduce heat related stress and mortality.
- Decreased total stormwater runoff and improved flow regimes (more natural high-flows and low-flows) for urban waterways.
- Productive vegetation and increased carbon sequestration.
- Improved air quality through deposition.
- Improved amenity of the landscape.
In the recently emerging vision for the ‘Water Sensitive City’, stormwater flows can be conveyed through a network of green/blue corridors of open spaces and productive landscapes that also detain flood water for floodprotection of downstream communities. WSUD elements also help in reducing the need for additional drainage infrastructure to serve increased impervious catchmentsdue to urban densification and consolidation.For effective realisation of these multiple beneficial, it is critical that Green Infrastructure be distributed throughout the urban area, as large scale end-of-pipe systems will have only local impacts.
22.214.171.124 Passive landscape irrigation
In response to issues of drought and landscape irrigation restrictions, a number of authorities have explored techniques for the passive irrigation of street trees by capturing stormwater runoff from kerbs, or from roof runoff (Stein, 2009). The primary objective of such systems is to support tree growth and survival, by increasing stormwater infiltration into the root zone of the tree, and also recharging the surrounding soil water reservoir and groundwater. Pollutant removal and flow control may be the secondary benefits of such systems. In Adelaide David Lawry sees trees planted in verges as a new generation of linear ’wetlands’ for the city. Assuming a tree can transpire 100 kl of water annually, 10,000 trees could take up at least 1 GL of stormwater, reducing polluted stormwater flows to nearby Gulf St. Vincent by the equivalent of the volume diverted to the Parafield Stormwater Harvesting Facility (Lawry, 2008). Such systems can also capture much of the most polluted ‘first flush’ of road runoff (Porch et al., 2003). (Johnson, 2009) however, points out that the ‘capacity of soil to absorb and store water is a limiting factor on the design of infiltration systems’ p.19. A benefit of trees and other vegetation is their ability to enhance the storage capacity of the soil. They may even increase their rate of water use as the availability of water at a site increases (Eamus et al., 2006). Therefore, according to (Johnson, 2009):
‘Incorporating well vegetated stormwater infiltration infrastructure into streetscape design may therefore be an effective means of managing a considerable portion of all stormwater’ p.19.
126.96.36.199 Porous surfaces
The widespread paving of urban surfaces is a relatively recent phenomenon, beginning with the introduction of macadam in the 1880’s. Today, impervious paving covers vast areas of the city,particularly roads and sealed car parks. More recently, increased urban densification has seen a further increase in impervious surfaces (Thompson and Sorvig, 2008). Impermeable paving has been implicated in a range of environmental problems, reversing the natural water cycle in which rainwater infiltrates into the soil, thereby sustaining vegetation and replenishing soil water and aquifers (Hough, 2004). Impervious surfaces reduce infiltration into the soil, reducing groundwater recharge, and increasing stormwater runoff, which can lead to flooding and pollution of downstream waterways, as well as placing greater demands on established stormwater infrastructure.
Thompson and Sorvig (Thompson and Sorvig, 2008) make the case for more porous ‘soft surfaces’ in urban areas, to increase infiltration rates and groundwater recharge, and to reduce pressure on stormwater drainage systems. Seattle landscape architect Richard Haag advocates his ‘Theory of Softness’ which states that 'no ground surface should be harder than is absolutely necessary for its function’, as an alternative to the engineer’s desire to compact and pave every piece of ground in sight (Thompson and Sorvig, 2008) p.181. The same principle can be applied to permeability that ‘no ground should be more impervious than necessary’ (Thompson and Sorvig, 2008).It must be noted that porous (or pervious) surfaces comprise a whole family of different materials and treatments, as described by authors such as (Ferguson, 2005).
Benefits of porous surfaces
Porous paving can provide economic benefits by reducing the volume and timing of runoff, andtherefore the demand for stormwater infrastructure, in terms of the extent of new systems and thecontinued use of existing systems. These benefits can be increased by including a subsurfacereservoir in the designwhich decreases the need for other retentionfacilities.It has also been demonstrated that porous paving can play a role in water quality through pollutantremoval from stormwater runoff, by assisting in biological decomposition of hydrocarbon contaminants (Anon, 2002). By modifying stormwater runoff flows and water quality,porous paving can also help reduce the cost of complying with stormwater regulations. By enhancing infiltration, porous paving can assist in recharging and maintaining natural groundwater andaquifers.Porous paving also creates opportunities to cool asphalt pavements, and thereby reduce the urbanheat island effect through the planting of shade trees and increasing reflectivity (measured by albedo) toreduce heat absorption. Asphalt surfaces can be lightened with coloured stones, aggregate or fines.Cooler pavements may also have benefits for tree root systems (Thompson and Sorvig, 2000).
One of the main benefits of porous paving howeverareits multiple uses which can result in more efficient land use planning. For example stormwater management can be combined with other uses, such as car parking, as distinct from single use engineering installations such as detention ponds. In particular, engineered‘eco-paving’ systems, if properly designed, can have the same structural performance asconventional pavers(Shakel et al., 2008).
Benefits of porous paving to trees
A number of authors refer to the potential benefits of porous paving to trees. According to (Frank, 2009):
‘The benefit to trees of porous paving, lies in its ability to provide a healthy rooting habitat, contributing to tree longevity’ p.2.
In highly urbanized settings, impervious surfaces combined with compacted soils, present trees witha hostile environment with limited groundwater recharge and poor gaseous exchange.
‘Conversely, porous paving that allows moisture infiltration and gaseous exchange to the underlyingsoil, provides an improved rooting environment similar to a natural soil surface. In combination withother ‘tree friendly’ technologies such as load bearing rooting media or structural soils, providingmodified growing environments that includes the application of porous pavement systems will allowmore successful urban landscapes to be developed; a landscape in which increased opportunities fortree planting are provided’ (Frank, 2009) p.2.
And according to (Edwards and Gale, 2004):
‘Porous paving will assist the tree roots so that they need not be dependent on capillary action to drawmoisture from the water table.’ p.124.
(Edwards and Gale, 2004) also conclude that, as the size of a tree pit is related to the water holding capacity of the soil, the porosity of the paving will affect the size of the tree pit required. A recent study by (Morgenroth and Buchan, 2009) however, raised issues regarding the benefits of porous pavements for urban trees. Oriental plane (Platanus orientalis) trees showed improved growth under porous concrete compared with normal concrete. Soil moisture and aeration were similar under both types (wetter and less aerated than the open control), however the soil under the porous concrete was better aerated at depth. They concluded that, while soil moisture dynamics are different between porous and non-porous pavements, the differences are not significant, and if urban trees do benefit it is probably not as a consequence of aeration or soil moisture (Morgenroth and Buchan, 2009). (May, 2009) however, interprets these findings as reinforcing the idea that placing a permeable pavement on ‘normal’ soil may not enhance tree performance due to poor aeration,and the use of well drained soils may be required to maximize the benefits of permeable paving. It has also been shown that urban trees themselves can act as tools for increasing infiltration and groundwater recharge, if their roots can penetrate compacted soils and increase infiltration rates,with root paths acting as conduits for water. It should be noted that vegetation may also be considered as a constraint on permeable paving installations. Decomposing leaves may be beneficial, as they lead to microbial activity and accelerated hydrocarbon removal, but can also lead to clogging. Current advice is not to plant trees close to porous paving (Shakel et al., 2008).
Street trees can be used as water sensitive urban design measures and they have been shown to substantially reduce nitrogen and other pollution loads in stormwater. However, urban planners and local council designers have often been reluctant to include trees as part of their urban street designs in the past due to the susceptibility of pavements to damage by tree roots. Permeable pavements may offer a solution to a number of the common problems associated with incorporating street trees into urban landscapes.
This recent paper by (Beecham, 2012) reports on a new experimental research project to assess and quantify the long‐term performance of permeable pavements in reducing stormwater flows and pollution loads, reducing the incidence of structural damage to pavements by tree roots and in promoting healthier and faster growing trees under typical Australian conditions. Initial experimental results from 18 new street tree‐permeable pavement systems have been very promising. Three separate paving configurations were used in the field trials; two pavements were constructed as permeable pavements and the third was constructed as a typical impermeable pavement. The initial results suggest that trees planted with permeable pavement surrounds generally have a higher growth‐rate than trees planted within the impermeable control pavements. The author notes however that it is still too early to make any long‐term predictions on the effects of the different permeable pavements on the growth rates of the trees, and it is expected that future results from the study should significantly increase knowledge in this area.
7.4 Linking water and liveability
A number of recent initiatives in Australia have expanded the debate on sustainable water management from stormwater treatment to the wider role that water and Green Infrastructure can play in creating more liveable and resilient cities of the future. According to (Wong et al., 2011) water is an essential element in place making, both from maintaining and enhancing the environmental values of surrounding waterways, and in terms of amenity and cultural connections to place.
7.4.1 The Water Sensitive City
Three principles set the foundation for this vision of a Water Sensitive City, developed at Monash University (Wong and Brown, 2009 ):
- Cities as Water Supply Catchments: meaning access to water through a diversity of sources at a diversity of supply scales.
- Cities Providing Ecosystem Services: meaning the built environment functions to supplement and support the function of the natural environment.
- Cities Comprising Water Sensitive Communities: meaning socio-political capital for sustainability exists and citizens’ decision-making and behaviours are water sensitive.
7.4.2 Water and liveability
The Living Victoria Living Melbourne Road Map focuses on the role of water management in urban liveability (Living Victoria Ministerial Advisory Council, 2011). Extended drought, and more recent floods, have shown what a major impact water can have on the liveability of cities and towns. Water management can be seen as playing an important role in underpinning the vitality and prosperity of the city through:
- The provision of safe, secure, affordable water supplies.
- Supporting green landscapes that significantly enhance urban amenity and help to combat the impacts of the urban heat island effect.
- Improving the health of urban waterways and providing opportunities for active and passive recreation.
- Protection from flooding.
A resilient, adaptable and flexible water system is therefore seen as a prerequisite for a liveable city. According to (Wong et al., 2011):
‘The nexus between urban water management and the vitality and prosperity of urban environments is only beginning to be recognised’ p.4.
Some of the key linkages between urban water management and urban liveability are shown schematically in Figure 47.
Figure 47: The influence of urban water management strategies onurban liveability. Source: (Living Victoria Ministerial Advisory Council, 2011).
It is considered that at least eight measures of urban liveability can be influenced by the way urban water management and services are delivered (2011):
- Access to a secured and fit-for-purpose water supply. The Intergovernmental Panel on Climate Change reports have highlighted that, with the exception of temperature, predictions of future trends in climate conditions in Australia, particularly seasonal rainfalls, remain highly uncertain.
- Clean and healthy water environment. Future water sensitive cities are places where waterways are valued as an integral part of cities, and where ecological integrity is actively protected.
- Effective drainage and flood mitigation. In addition to increased flood vulnerability of coastal cities associated with rising sea levels, future climatic scenarios predict higher climate variability including more severe storms.
- Efficient low energy systems. There is a strong nexus between water and energy.
- Urban design strategies. Future developments will feature solutions for mixed-use developments and increased densities while enhancing the quality of public and private spaces.
- Mitigating urban heat island effects. Predictions of higher heat wave conditions and empirical evidence of the heat island effect will drive more climate-responsive urban designs for urban heat mitigation.
- Quality of public spaces. With a progressive shift in emphasis of the importance of public spaces in response to increased development densities, future public realms will serve to anchor developments with the traditional ‘values’ of open spaces and landscape features being bolstered with ‘ecological functioning’ of urban landscapes that capture the essence of sustainable water management, micro-climate influences, facilitation of carbon sinks, and use for food production.
- Productive landscapes. One of the emerging global challenges is that of preserving productive landscapes and improving food productivity.
University of Washington researcher Kathleen Wolf (2014) recently made the case in for trees and green infrastructure to both manage stormwater runoff and also offer a host of health benefits. According to Wolf, “Every small patch of nature in cities and built areas can be ‘hyperfunctional’ and provide co-benefits. While performing the primary purpose of stormwater management, green infrastructure also can be designed to augment park systems and provide places of respite, recreation, and delight.“ The article, ‘Water and Wellness: Green Infrastructure for Health Co-Benefits,’ shows that “with careful design, green spaces can manage runoff and provide a range of co-benefits. Integrated planning of green infrastructure and parks systems helps to cost-effectively provide multiple benefits and contributes to more livable communities.”
7.5 Green roofs and living walls
7.5.1 Green roofs
In a paper Green Roofs for a Wide Brown Land, (Williams et al., 2010) examines green roofs in Australia, including the challenges to increasing their use, and the major information gaps that need to be researched to progress the industry here.Two main types of green roof can be identified; intensive and extensive. Intensive green roofs can support complex vegetation communities in substrate depths greater than 20 cm. They are often designed as roof gardens for human use and usually require irrigation, maintenance and additional structural reinforcement of the roof (Oberndorfer et al., 2007). Extensive green roofs (EGR) sometimes referred to as ‘ecoroofs’ have shallow substrate depths less than 20 cm (2-15 cm), require little or no irrigation and are usually planted with low growing drought resistant and fire retardant vegetation (Dunnett and Kingsbury, 2004a; Oberndorfer et al., 2007). They are underlain with drainage and barrier materials to protect the roof from water and root penetration. These green roofs are specifically designed for limited maintenance.
A number of benefits have been attributed to green roofs including the following cited by (Williams et al., 2010):
Benefits to building occupants:
- Increased roof or roof membrane life (Kosareo and Ries, 2007; Köhler and Poll, 2010).
- Insulating properties that lead to greater energy efficiency through reduced summer cooling and winter heating costs (Sailor, 2008). In temperate North America, a cost-benefit analysis of an EGR on a retail store found small, but significant, reductions in energy consumption (Kosareo and Ries, 2007). In warmer climates, much greater reductions in energy usage are likely to result. (Wong et al., 2007) found that in Singapore over 60% of heat gain by a building could be stopped by an EGR. In subtropical southern China, less than 2% of the heat gained by an EGR during a 24 hour period in summer was retained by the plants and substrate or transferred to the building below. The remainder was lost through evapo-transpiration, re-radiated to the atmosphere, or used in photosynthesis (Feng et al., 2010).
- Reduction of inside and outside noise levels (Van Renterghem and Botteldooren, 2008; Yang et al., 2008; Van Renterghem and Botteldooren, 2009).
- A general sense of enhanced well-being is also gained by virtue of the aesthetic value of plants (Maas et al., 2006).
Benefits to the local environment:
- Biodiversity and habitat provision (Coffman and Davis, 2005; Brenneisen, 2006).
- Reduced stormwater runoff (VanWoert et al., 2005; Carter and Jackson, 2007). A review of research into the hydrological performance of EGRs has shown that they can retain between 34-69% of precipitation (Gregoire and Clausen, 2011). The authors noted that retention capacity is affected by the water holding capacity of the substrate, evapo-transpiration rates, temperature, amount of precipitation, and the number of dry days preceding precipitation. This ability to reduce stormwater volumes is of benefit where expansive areas of impervious surfaces create problems of localised flooding during heavy rainfall events and disturbance of surrounding natural waterways.
- Improved roof water runoff quality (Berndtsson et al., 2006). Research however indicates that this depends on the characteristics of the green roof, particularly the presence of organic material and fertiliser in the substrate (Gregoire and Clausen, 2011).
- It is also noted that, unlike some other WSUD measures, green roofs do not require additional space as they are already part of the building footprint.
Benefits to the wider city:
- Mitigating the urban heat island effect by cooling through evapotranspiration, and subsequent reduction in energy demand and carbon dioxide emissions (Skinner, 2006; Alexandri and Jones, 2008). (Susca et al., 2011) reported an average 2°C temperature difference between areas of New York city that have high and low levels of vegetation. EGRs with their biological activity, high thermal resistance, and low surface albedo (compared with traditional bitumen rooftops) were considered a useful way of combating the UHI effect.
- Carbon sequestration (Getter et al., 2009; Li et al., 2010).
188.8.131.52 Water issues
One of the major drivers for the implementation of green roofs has been to reduce stormwater flows and increase the quality of stormwater runoff from urban areas with extensive impervious catchments, thus reducing impacts on aquatic ecosystems (Walsh et al., 2005). Green roof substrates and plant roots can act as a ‘sponge’, reducing and slowing roof runoff (Mentens et al., 2006; Carter and Jackson, 2007). The hydrological performance of green roofs, however, is dependent on a large number of variables such as roof slope, drainage layer design, substrate depth and composition and plant types (Dunnett and Kingsbury, 2004a; VanWoert et al., 2005; Simmons et al., 2008). Hydrological models have been developed that predict the reduction of runoff achieved by green roofs (Mentens et al., 2006; Carter and Jackson, 2007; Hilten et al., 2008). Adapting these models for the Australian context or constructing local models will provide technical information that will help the uptake of green roofs.
It appears that areas with constant or seasonal hot, dry climates have the most to gain from implementing green roofs as a climate change adaptation measure (Alexandri and Jones, 2008). However, such climatic regions are also subject to water scarcity, which has become a major issue in Australian cities in the last decade(Hensher et al., 2006; Mulley et al., 2007). The use of large volumes of potable water to maintain roof gardens in the Australian climate therefore may not be feasible, however if green roofs are to be used to mitigate urban heat island effects through evapotranspiration in hot weather, some irrigation will be required. ‘This creates an inherent contradiction between one of the objectives of establishing green roofs in Australian cities and the realities of doing so’( Williams et al., 2010). WSUD practices which utilize grey water recycling and stormwater harvesting and reuse may be one solution to this problem.
In the last two decades there has been a substantial expansion of extensive green roofs in the northern hemisphere (Western Europe and North America) mainly through the retrofitting of existing buildings (Oberndorfer et al., 2007) andmore recently in Singapore.There are also a few examples in Australia. Despite increasing government, public and industry interest (due to their potential as a climate change mitigation and adaptation tool) there remain a number of potential barriers to the more widespread adoption of green roofs in Australia (Williams et al., 2010). These include:
- Lack of standards
- High cost of installations
- Few demonstration examples
- Lack of relevant and reliable research
- 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
- Similarly, relying on northern hemisphere research raises other issues due to different rainfall patterns, substrates and types of vegetation
Lack of scientific research data is particularly relevant to Australia, which has a very different climate from the temperate regions of the northern hemisphere where green roofs are more common. Relying on European and North American experience and technology may be problematic due to significant differences in climate, available substrates and plants (Williams et al., 2010).
184.108.40.206 Plant selection
(Williams et al., 2010) specifically highlight the need to identify plant species that can survive and also be aesthetically pleasing in the Australian climate. Plants selected for green roofs must be able to tolerate increased wind velocities, sun exposure, extreme heat, drought conditions and shallow root depths (Durhman et al., 2004). These factors point to water availability being the most limiting factor on green roofs. According to (Williams et al., 2010):
‘If green roofs are to be successful in Australia it is therefore necessary to understand the drought tolerances and associated water requirements of a range of Australian and exotic plant species so that a diverse plant palette is available to green roof designers’
Plant selection is becoming a global issue as green roof monocultures of Sedum (as found in the Northern Hemisphere) restrict the potential for aesthetic diversity and may limit the biodiversity and ecosystem service value of green roofs (Dunnett and Kingsbury, 2004a).Studies into the selection of plants for extensive green roofs have been conducted, but predominantly in cool temperature regions of the northern hemisphere. Sedum species feature recurrently as the primary choice for these regions. Overseas studies, however, are difficult to adapt to Australian conditions, where climate is characterised by low rainfall, high evaporation and high year-to-year rainfall variability (Perkins and Joyce, 2012).
(Williams et al., 2010) investigated the performance of native and exotic species for extensive green roofs in Melbourne. The researchers assessed the suitability of three species of native forbs, three succulents (two native and one exotic) and three native grasses for Australian green roofs by conducting drought trials using species readily available at nurseries and planted in green roof microcosms. They found that many of the species assessed in this study will not be suitable for green roofs in Australian Mediterranean climates due to the extended periods of drought stress experienced.
A recent study by the CSIRO developed a ‘plant selection matrix’ tool, identifying plant species suitable for extensive green roofs and exterior living walls in subtropical Australia, and reported on a workshop aimed at conveying findings to industry and identifying consensus priorities for future work (Perkins and Joyce, 2012). The researchers concluded that:
‘… it is clear that long-term evaluation of a wider range of plant species, substrate formulations and irrigation regimes is required to support increasing confidence in Green Infrastructure for Australia’.
They also concluded that:
‘… climate-specific modelling of environmental benefits such as thermal buffering and mitigation of stormwater flows is vitally important to ensure their accurate representation in building sustainability indicators such as the Green Building Council’s ‘Green Star’ rating. This would offer greater incentive for implementation of Green Infrastructure’.
220.127.116.11 South Australian research
According to (Beecham et al., 2012) Adelaide is the capital city of the driest state in Australia and it currently faces three major challenges:
- Water scarcity
- Climate change
These threats place increased stress on the urban water cycle, and increases metropolitan temperatures through urban heat island effects. Introducing green infrastructure through water sensitive urban design is one of the solutions to reduce harmful impacts of urbanisation while providing additional amenity and water quality benefits for communities and the environment.
Razzaghmanesh (Razzaghmanesh et al., 2012; Razzaghmanesh, 2012a) at the University of South Australia, reviewed a range of studies from different climatic regions in Europe, North America, Asia, Australia and New Zealand to better understand how green roofs can be adapted to meet WSUD objectives in Australia. They concluded that green roofs have been used as an important WSUD infrastructure around the world but the technology is very much in its infancy in Australia. Furthermore, more specific design criteria need to be developed for a range of Australian conditions to develop resilient green roofs. 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 in Adelaide for further 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.
A paper by (Beecham et al., 2012) describes the results of an ongoing research project investigating the water quantity and thermal benefits of two different types of green roofs (intensive and extensive). The study site consists of a series of small scale green roofs located at the University of South Australia’s Mawson Lakes campus. Laboratory and field investigations of rainfall and runoff confirmed that green roofs can retain significant amounts of stormwater and can also mitigate the peak flow and attenuate the time of concentration.
The thermal benefits of green roofs were also investigated for two scenarios (cold and warm days). The results indicated that the thermal variation of the planting media is less than surrounding areas. On cold days the media’s temperature is warmer than outside and on warm days it is cooler. The authors concluded that integrating green roofs into the built environment of Adelaide could act as one of the main potential options for achieving the WSUD objectives outlined in the Adelaide 30-year Plan, and that developing resilient green roofs could also be an effective climate change adaptation tool to mitigate urban heat island effects and to reduce urban temperatures.
Figure 48: Green roof bed layouts at University of South Australia’s Mawson Lakes campus. Source (Beecham et al., 2012).
Several green roof and living wall projects have recently been part funded through the South Australian Government’s Building Innovation Fund (BIF) which aims to demonstrate innovative ways to reduce the carbon footprint of existing commercial buildings.
In one project, four green roof systems were installed in a challenging urban environment on the 22nd level roof of ANZ House Adelaide, and were monitored for environmental benefits including insulation, stormwater quality, water use, cost effectiveness, vegetation and visual amenity (Fifth Creek Studio, 2012). Two proprietary green roof systems commercially available in Australia were used to compare profile depths (an intensive profile depth of 300 mm and the extensive profile depth of 150 mm). Part of the plots were also covered with an open mesh or grating above the plants (aimed at creating a sheltered micro-climate and also facilitating pedestrian access). Key findings of the monitoring included:
- The insulation value of a 125-300 mm thick profile was sufficient to reduce summer heat flow in Adelaide’s climate. The deeper the profile the larger the insulation value and the longer the time delay for peak temperatures. This is based on a dry substrate condition and when water is introduced into the green roof systems the insulation values reduce as temperatures within the profile layers rise, because water is a good thermal conductor.
- Insulation R-Value was difficult to calculate but temperature reduction factors which could be used for the various green roof profiles in Adelaide are: a 300 mm profile can give a 41% reduction in temperature; a 125 mm profile plus grating can give 20.5% reduction; and a 125 mm profile can give an 8% reduction.
- In stormwater quality the performance of the 125 mm extensive green roof was better than the 300 mm intensive system in terms of pollutant removal, which may be related to the reduced volume of soil that can leach pollutants.
- The building energy impact resulting from the 125 mm profile plus grating in summer reduced heat transfer through the roof by 4.2 W/m2 for a NABERS 5 Star rated commercial building.
Table 22: Green roof monitoring summary. Source: (Fifth Creek Studio, 2012).
Green roof factors
300 mm profile
125 mm + grating
125 mm profile
Surface temperature reduction
300 mm profile
125 mm + grating
125 mm profile
Weight/temperature reduction factor
This project has shown that green roof design needs to be carefully considered in a hot, dry climate. The use of a grating to shade the substrate and vegetation shows great potential, and especially when constructed around plant equipment it provides a light weight trafficable surface in addition to the environmental and energy saving benefits of the green roof system.
7.5.2 Living walls
Living walls comprise plants rooted in a vertical structure attached to a building, as opposed to traditional façade greening, in which plants are rooted in the ground and are trained to grow up a wallor trellis. The vertical structure usually comprises rigid modular panels filled with a special lightweight growing medium or a two-layer blanket of synthetic fabricin which ‘pockets’ are filled with plants and growing medium (Hopkins and Goodwin, 2011).Examples of living walls can be found in Europe and North America, but are perhaps most widely adopted in Singapore. The vision of creating a ‘city in a garden’ has led Singapore to focuson research and development of green wall systems that are suited to its tropical climate (Chiang and Tan, 2009). More locally (Hopkins and Goodwin, 2011) have presented five case studies of recently completed living walls in Australia.
The benefits cited for green walls are similar to those cited for green roofs (Perkins and Joyce, 2012).
Some recent studies cite the following benefits:
- In Hong Kong, coverage of a concrete wall with modular vegetated panels reduced exterior wall temperatures by up to 16°C in summer (Cheng et al., 2010). In terms of internal wall temperatures, a difference of more than 2°C was maintained even late at night, indicating that green walls can significantly reduce energy use for building cooling.
- At HortPark in Singapore, a number of green wall systems were assessed for their thermal performance (Wong et al., 2010a,). The researchers reported differences in external wall temperatures of up to 10°C between vegetated and bare concrete walls.
- The acoustic benefits of living walls vary according to the type of construction and level of vegetation cover. (Wong et al., 2010b) showed that green wall substrates effectively reduce sound at the low to middle frequencies. A relatively smaller reduction is achieved at higher frequencies due to the scattering effect of the foliage. The sound absorption properties increase as the level of vegetation increases.
18.104.22.168 Façade Greening
Façade greening is a term used to describe vegetation used on or adjoining a building surface (Dunnett and Kingsbury, 2004a). Traditionally this has involved self-clinging climbing plants grown directly on a building, but more recent methods use plants grown away from the face of the building, often using high tensile steel cables, wire or modular trellis systems(Rayner et al., 2010).
The benefits of façade greening are similar to those provided through green roofs and living walls (Dunnett and Kingsbury, 2004a; Oberndorfer et al., 2007; Currie and Bass, 2008) (Dunnett and Kingsbury, 2004a). The effects on building microclimates can be substantial, and one study reported a 28 % reduction in peak radiation from a west-facing wall covered with Ivy (Hedera helix) during summer (Di and Wang, 1999). Urban greening strategies such as these are also becoming important potential climate change adaptation and mitigation tools, particularly as studies of the life-cycle cost benefit analysis quantify the economic benefits they can provide (Wong et al., 2003; Carter and Keeler, 2008).
Despite the extent of vertical building faces in cities, there are comparatively few studies of façade greening in urban settings. (Rayner et al., 2010) conducted a study of the façade greening at Council House 2 (CH2) building in the Melbourne CBD. An evaluation in March 2008 showed a 61% ‘failure’ (meaning death or poor cover) of all plantings. This ‘failure’ was caused by a number of factors including irrigation system failure, poor plant selection, poor plant quality, container substrate issues and problems in installation and establishment.
22.214.171.124 South Australian research
Another project supported by South Australian Government’s Building Innovation Fund investigated the viability of establishing living wall systems suitable for application on multi-storey buildings in the Adelaide CBD (Fifth Creek Studio, 2012). The project had two main components:
- An initial feasibility study involving a temporary site on the Old Telephone Exchange building in the Adelaide CBD.
- The installation of a prototype hybrid living wall system for Tower 8 in the City Central precinct.
The initial feasibility study had three main aims:
- Identify living wall design criteria appropriate to the extreme conditions of Adelaide’s climate.
- Assess the potential for reduction in Greenhouse Gases, CO2 and the Urban Heat Island effect.
- Address and resolve the hurdles to achieving large scale coverage at height on multi-storey buildings.
Key findings from the post-construction monitoring of the prototype hybrid living wall system included:
- The prototype living wall system with its vegetated layer one metre out from the building facade reduced ambient air temperature at 600 mm from the facade and reduced the surface temperature of the facade by some 80C on an extreme summer day compared to the control wall. This reduces the heat flow through the facade into the building by approximately 2.4 W/m2. Conversely during the winter, the prototype living wall reduced heat loss from the building by approximately 3.6 W/m2.
- The vegetated wall reduced the amount of daylight reaching the building facade. The 100mm cable spacing allows up to 43% of daylight through and the 200 mm cable spacing allows up to 63% of daylight through.
- Solar radiation is reduced 95% by the vegetated wall.
- The living wall can reduce energy and Green House Gas (GHG) emissions when applied to a building. This positive impact would be beneficial to both new and existing buildings.
The data suggests that the living wall also influences the surrounding environment and modifies conditions in the urban canyon.
- The research also showed that the entire prototype living wall removed an average of 11% CO2 per year and sequesters CO2 at a rate of 1.375% (or 187.5 g) per m2, per year, from the atmosphere.
- The surrounding ambient temperature was reduced in comparison with the control wall. This reduces the urban heat island (UHI) effect and creates a more pleasant environment at footpath level.
7.6 Green Streets
‘Green streets’is one label for a growing practice in the United States,where streets are designed or reconfigured to accommodate stormwater runoff management andtreatment, along with other sustainable design practices such as: traffic calming; pedestrian andcycle use; and the creation of attractive streetscapes. Green streets have been described by Thompson and Sorvig (Thompson and Sorvig, 2000) as ‘constructed ecological networks’. Two cities leading the way in green street design are Portland in Oregon and Seattle (Vogel, 2006).
7.6.2 Green street examples
The City of Portland has a long history of comprehensive planning, including urban design, multimodalpublic transport and Green Infrastructure systems. In Portland, a street that uses vegetatedcomponents to manage stormwater runoff at its source is referred to as a Green Street. Portland offers several examples of well-designed green streets. The SW 12th Ave Green Street project in 2005 involved retrofitting a series of stormwater planters into an inner urban street. The retrofit project demonstrates how existing or new streets in highly urbanized areas can be designed toachieve both environmental benefits and be aesthetically and functionally integrated into the urbanstreetscape (ASLA, 2006).
Seattle has probably developed the most innovative green street solutions. Seattle Public Utilitieshas adopted a Natural Drainage System (NDS) strategy. This is based on Street Edge Alternative (SEA) neighbourhood streets, incorporating a variety of low impact development techniques tostore, infiltrate and filter stormwater (City of Seattle, 2008).These techniques were tested in SEAStreet No 1 where a conventional street was redesigned with a narrowed, curvilinear carriageway.A subsequent project, Pinehurst green grid, (covering twelve city blocks), involved redesigned thestreets with an offset template, incorporating drainage swales in the widened side of the street. The next step in Seattle is to adapt NDS to more densely developed areas. A current project,‘Swale on Yale’ applies NDS techniques to the redevelopment of high-density commercial area,incorporating four blocks of interconnected swales in a wide planting strip between street and footpath.
City of Seattle Natural Drainage Systems( NDS) and Street Edge Alternative (SEA) Streets
Today, in several neighbourhoods throughout Seattle - with more to come as funding becomes available - SEA-Streets and their variations have become a much-admired community amenity. Their NDS technologies are being used to provide a variety of community and environmental beneﬁts, including:
New York City
New York City has developed a set of High Performance Infrastructure Guidelines, which providesa roadmap for incorporating sustainable practices into the City’s right-of-way infrastructure capitalprogram (New York City Department of Design and Construction, 2005). In guidelines such asthese, street trees are formally recognized as a form of ‘Green Infrastructure’ delivering tangible benefits.
Chicago has approximately 3057 km of public alleys representing 1417 ha of impervious surfaces (City of Chicago, nd). The Green Alley Program is a strategy for the sustainable redevelopment ofthe city’s alleys based on five techniques: improved drainage through proper grading; permeable pavements; high albedo pavements; recycled construction materials; and dark sky compliant lightfixtures. Four different approaches are adopted in the design of Green Alleys: Green Alley materials with conventional drainage; full alley infiltration using permeable pavements; centre alley infiltration using permeable pavements; and green pavement materials with a subsoil infiltration system.Green Alleys are part of Chicago Department of Transport’s (CDOT) wider ‘Green Infrastructure’program, which includes recycled construction materials, permeable pavements, recycled rubbersidewalks and other efforts. The program began as a pilot in 2006, and at 2008, more than 80 Green Alleys had been installed.
- Streams and drainage corridors are an important component of the urban Green Infrastructure network.
- In some areas the term Green Infrastructure is synonymous with more sustainable stormwater management practices.
- Healthy trees, soils and vegetation also play an important role in the natural water cycle. In Australia the term Water Sensitive Urban Design has been adopted as a new paradigm for the more sustainable management of the urban water cycle.
- Vegetation plays a significant role in Water Sensitive Urban Design through the use of vegetated systems to improve stormwater quality and manage stormwater runoff.
- More recently there has been a broadening of the concept of Water Sensitive Urban Design to that of Water Sensitive Cities and the role of water in urban liveability, backed by a major research effort in Australia.
- One Green Infrastructure initiative being promoted in Australia is that of ‘green roofs and living walls’, popular overseas but with their viability in Australian climatic conditions now being researched locally.
- Recent and ongoing research is investigating the application of green roofs and walls to South Australian climatic conditions.
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