Peter Young & Peter Levett, Landscape Design, City of Salisbury
This paper details water sensitive urban design (WSUD) in practice in the City of Salisbury including the TREENET inlet system.
In collaboration with Uni SA and TREENET, the City of Salisbury is planning and implementing a range of research projects to provide the evidence required to support the adoption of the technologies as standard practice for city engineers and arborists.
The demands for onsite stormwater retention, the need to defer capital expenditure on kerb and channel replacement and the much-needed regeneration of an aging urban forest provided the incentive for Salisbury Council to investigate putting WSUD to the test. It is anticipated that there will be significant savings in expenditure on repairing uplifted kerbs and footpaths as the root systems are naturally redirected away from this infrastructure in response to the relocation of water resources to the driest and thirstiest zone currently in the urban environment, the curiously named universal “nature strip”.
The City of Salisbury (CoS) is located in Adelaide’s northern suburbs about 25 kilometres from the Adelaide GPO and extending from the shores of Gulf St Vincent to the Para escarpment and the foothills of the Mt Lofty Ranges. With an estimated population of 130,000 people and encompassing an area of 158 km2, the region enjoys a typical Mediterranean climate, having cool, wet winters and warm to hot dry summers.
City of Salisbury’s largest asset is its road network. The requirement to maintain this important network is more than asphalt and kerbing, it is the management of water that collects and flows through this network. Street Trees is a large investment for the Council in the provision of streetscape amenities, moderate local climate conditions, and framing the presentation of residential homes.
Within older residential areas the existing stormwater systems tend to be overland systems that are beginning to show signs of stress. Concerns regarding elevated predicted and current climate conditions have motivated alternate investment to reduce stresses upon aging infrastructure.
As urban density increases through Greenfield and Brownfield developments the available area for street trees to be planted reduces. Urban densification further reduces permeable surfaces and reduces the soil’s ability to provide soil moisture levels viable for street trees to grow, maintain health and reach maturity. The service provider’s infrastructure further reduces areas in which street tree placements can be considered viable.
Council is actively working towards future-proofing these systems with simple solutions to create sustainable outcomes for the benefit of residents and the council. These outcomes address issues regarding increased stormwater volumes, ponding of stormwater, reducing leaf litter debris, improving overall urban permeability, moderating soil moisture levels, passive irrigation of street trees and increased street tree vitality.
The City is predominantly a residential area but also has substantial industrial, commercial and rural areas. Too tall and the area is 161 square kilometres, including Adelaide’s second commercial airport and RAAFE dinburgh, many parks, reserves, walking trails and wetlands. Horticultural enterprises (mainly vegetable growing) are located on the western fringes of urban development. Significant growth over the previous two decades has been fuelled by factors including major urban development projects, industrial investment, a burgeoning defence sector and strong growth in tertiary education. General Motors Holden’s Elizabeth vehicle assembly plant is located in an adjoining suburb.
Salisbury has gained international recognition for its integrated water management practices, and particularly stormwater harvesting and aquifer storage and recovery. It has eight stormwater harvesting sites supported by 150km of purple pipe network.
The estimated resident population of the City of Salisbury was 132,500 in 2011, increasing steadily over the previous two census periods.
The 30-year plan for greater Adelaide identifies a key target of 169,000 extra residents for greater northern suburbs; whilst a large portion of this is achievable via green field development the remainder implies densification of existing urban areas. Greater urban density reduces private garden spaces in size and impervious areas are greatly increased, thus permeable areas for rainwater to infiltrate the soil are reduced. These impervious zones shed stormwater at far greater rates than the permeable zones and increase the total stormwater volumes within council infrastructure.
Existing older residential area stormwater systems (dating from1940s to the 1980s) tend to be over land systems upon flat grades that have historically had drainage problems. Sub-division and redevelopment of older residential blocks increase local catchment volumes; stressing these overland systems. High-intensity rain events have shown that localised flooding occurs in these areas; causing the need for emergency relief work, property damage in extreme events and ultimately discontent with residents. Concerns regarding elevated predicted and current climate conditions have motivated alternate investment to reduce stresses upon aging infrastructure.
New housing developments are required to accommodate predicted stormwater requirements without impacting downstream stormwater networks1. The Development Plan requires an open space provision of 0.4 hectares within 500m safe walking distance of every house2. However, this open space provision is not protected from becoming the development’s stormwater detention basin. In which larger rain events turn the resident’s play space, kick and catch area or serve turned into a temporary pond with tree plantings.
CoS has embarked upon housing development projects including 15% affordable housing3. Incorporated into the streetscape design are rain garden bays designed to capture rain events up to 1 in 5ARI4 with adjacent native plantings of grasses, shrubs, ground covers and street trees. The overflow from these rain gardens is plumbed into the stormwater network to discharge into the onsite detention basin. A permanent water body surrounded by walking tracks with seating and connected to Council’s Green Trail linear path network for passive recreation.
Soils and Climate
Soil types across the Salisbury district, referred to as The Lower Alluvial Plain, are characterised by Red-Brown Earths(RB). These soils are considered to be the most productive within the Adelaide Plains which allows for a wide-ranging street tree palette. There are a number of variations of Red Brown Earths which are found within the lower alluvial plains of Salisbury including RB3, RB6 and RB7.
RB3 comprises a sandy or silty grey to red-brown A horizon over a well-developed red clay B horizon, with varying lime content into the C horizon. The RB3 soils are generally deeper and contain finer textured sediments than other RB soils. The RB6 and RB7 show little variation between horizons. RB6 soils form towards the lower reaches of the outwash fan sand and are often affected by the high water tables and salinity levels. RB7 soils form closer to streams and creeks resulting in larger granular material within the A and B horizons 5.
Rainfall varies significantly across the Salisbury area, decreasing significantly closer to the coast. While the average annual precipitation for the Salisbury region is 460.50 mm, some areas of the upper alluvial plain will receive as much as 550 mm, while areas along the coast will receive much less. South Australia experiences droughts during which time average rainfall can be reduced significantly, and available rainfall and soil moisture become important factors in determining plant survival. The budget for the Salisbury region shows a long period of deficit where evaporation exceeds precipitation, followed by a short period of recharge (where moisture is retained within the soil) during June and July. During this time, the soil does not become saturated (saturation point is the amount of water that the soil can hold before run-off begins). The use period (the period when the available soil moisture from the recharge period is being used through evaporation and transpiration) is very short, less than one month, before the deficit begins again in early August 6.
The council verge as the next wetland
In an average year, 160 gigalitres (160 GL) of water flow down Adelaide’s gutter sand into Gulf St Vincent. This runoff carries a high load of nutrients and sediment which are destroying the marine environment. The recommendations from the Adelaide Coastal Waters Study final report (ACWS Nov 2007) are for an urgent reduction in the volumes of stormwater discharge to bring about a 75% reduction in nitrates, a 50% load reduction in particulate matter, as well as reduced flows of organic and mineral toxicants to coastal waters. The State Government in its 2004 Waterproofing Adelaide blueprint set a target of 20GL of stormwater reuse by 2025 which is only a 12% reduction on current outputs. Urban sprawl and infill will only add to the torrent of stormwater so there is little chance that any improvement in the marine environment as envisaged in the ACWS can be achieved. No doubt there will be increasing adoption of “wetland” technologies to clean some of this polluted rainwater before sending it onto the ocean or preferably to the aquifer. However, this is a very capital-intensive end-of-pipe strategy suited to a few locations remote from the source where land is available. What Adelaide needs are new, at-source, low-cost, readily implemented systems that deliver multiple benefits to the community and the environment. Taking water from the gutters and putting it into the subsoil adjacent to street trees is an option currently using WSUD.
The demands for onsite stormwater retention, the need to defer capital expenditure on kerb replacement and the much-needed regeneration of an aging urban forest provided the incentive for Council to put localised WSUD to the test. It is anticipated that there will be significant savings in expenditure on repairing uplifted kerbs and footpaths as tree roots are naturally redirected away from this infrastructure in response to the relocation of water resources to the driest and thirstiest zone currently in the urban environment, the Council verge.
Considering soil and climate conditions localized detailed stormwater data and soil infiltration rates were raised as critical success factors in designing WSUD alternatives. Infiltration can be maximized by designing the sumps with the greatest surface contact area with surrounding soils. The floor area of the sump is the most likely to allow infiltration due to gravity and soil porosity, whilst the side walls of the sump allow horizontal infiltration. Yet the amount of horizontal infiltration is reliant upon the local soil hydraulic conductivity. The side wall is however the most likely point where adjacent tree roots are able to access available soil moisture. By maximizing contact surface area is to allow the sumps to catch first flush rainfall and once saturation is achieved the infiltration rate is maximized to drain the remaining ponded water within the kerb. This increased amount of moisture taken up by surrounding soils increases the amount of soil moisture available to street trees thus recharging soil moisture volumes.
Trees provide additional significant benefits other than simply providing an alternative for stormwater and pollutant discharge to the marine environment. The direct influence of trees on climate and hydrology are standout advantages. Trees are very efficient solar-powered pumps, capable of returning hundreds of litres of water a day to the atmosphere. In transpiring all that water, trees are giant evaporative coolers and combine this with shading and controlling air movement to reduce the temperature of the city and suburbs conservatively by 4 deg C.
Trees and infrastructure have been in conflict in the urban environment for centuries and the competition is over space and water. Impermeable pavements, kerbs and gutters conspire to deny trees these vital resources. In response, tree roots follow moisture gradients produced at the interface between soil and concrete often damaging these same elements of infrastructure in the process. Another factor is that the primary source of water for many street trees are well-watered front gardens which are accessed by shallow roots, lifting the footpath and producing trip hazards requiring expensive remediation. The placement of an inlet in the kerb midway between street trees can divert stormwater from the gutter to a trench or cistern in the nature strip. This sets up a moisture gradient in the verge creating a preferential root pathway running parallel to the roadway and away from the infrastructure.
The problem with most systems designed to take water from the gutter is that they also direct sediment, leaves, and other gross pollutants into the system eventually clogging them and reducing the infiltration capacity of the soil in the verge. The TREENET inlet has been successful in separating the majority of these components from the first flush stream which carries the soluble heavy metals and nutrients into a trench or cistern at the back of the kerb. The oldest installation (September 2010 Oxford Street Unley) is flowing freely after 3 years and has no maintenance except for normal street sweeping activities.
However, the design of the “back end”, the hole in the ground that receives the water from the inlet, is very much the subject of current research. The principal requirements are that it has sufficient capacity to accept all of the most polluted first flush components and that it is accessible to tree roots for extraction of the captured water. A hole or trench of at least half a cubic meter is backfilled with a no-fines aggregate and a standard 90 mm stormwater pipe conveys the stormwater from the back of the TREENET inlet in the kerb to this simple “cistern”. The open nature of the aggregate allows rapid uptake of the first flush water and retains it until it has infiltrated into the surrounding subsoil, principally at depth. Infiltration rates into the surrounding soil are improved as the roots decompact the verge and biopores are created following the senescence and replacement of fine absorption roots. One simple system installed at the Waite Institute in 2011 is illustrated below. It is called the “black hole” because it has been backfilled with approximately. 75 m3 of recycled water filtration residue from Happy Valley reservoir which contains a high proportion of pollutant-absorbing activated carbon. This waste material has a high water uptake of 40%and so this “blackhole” can take 300 litres of first flush stormwater, lock up the soluble pollutants, and then make available the nutrient component for later uptake by the tree root system.
Water Sensitive Urban Design Development
Common street trees throughout Salisbury are E.leucoxylon (var.), E.torquata, and E.sideroxylon7. When planted within narrow verges and in close proximity to kerbing (within 600mm from the back of kerb), the larger species of E.sideroxylon has presented lifted and broken sections of kerb. In turn, ponding of stormwater after rain events is evident. Ponding within driveway crossovers and adjacent properties are regularly reported complaints to the council, raising issues regarding standing water, build-up of leaf-litter and debris. As part of maintenance works, Councils Road Reseal and Kerb Replacement Programs aim to rectify lifted and damaged kerb, correcting the grade for storm water flows. Locations are identified by visual inspection following asset audits that rank streets according to road failures. In some cases to address ponding, twenty metres of kerb replacement is usually required to achieve the appropriate grade. The average cost of replacing a standard section of kerb is $130 per lineal metre with the council spending $700,000 in 2012/13 and with increased road resealing in 2013/14 $1M for kerb replacement will be expected to be invested for the 2013/14.In many locations, kerbing has been replaced numerous times to prevent such ponding. This ongoing work and repetitive replacement signified a change in thinking.
An investigation into alternate methods ensued. With the following points in mind, six differing methods of implementing localised infiltration/ bio-filtration sumps ensued.
- Reducing ponded water within kerbing
- Improving soil moisture recharge
- Maximizing contact surface area with the surrounding soil
- Varied methods of distributing captured water within the verge
- Depth of sumps relative to infrastructure and services
- Maximizing volume capacity within sump
- Reduced spatial capacity for restricted verge spaces
- Reducing leaf litter and debris buildup
- Minimize maintenance
- Public safety included trip hazards, saturated/ boggy areas,
- Vehicle damage
- Proximity to infrastructure
- Root damage during excavation
As a launching point Council required an initial test location for the six options; Orlyk Street Para Hills West was chosen as the test site. This street was selected due to the amount of locations presenting standing water within the kerb and the presence of mature street trees(E sideroxylon).
Initial installation of the infiltration sumps was undertaken by existing contractors for kerb and gutter replacement. The training was required for the contract to understand the differing method of installing the TREENET Inlet and infiltration sumps. Continued implementation of these infiltration sumps relies upon effective monitoring and asset take-up by Council maintenance staff.
|Net Sump Volume
(Pore Space cum)
|ContactSurface Area (sqm)
Figure 9. Salisbury Council Infiltration Sump Designs
Leaf litter within kerbing is generally cleared from kerbing as part of Council’s Verge Cutting Program. The program is programmed on a 5–8 week cycle of grass reduction (May to December) and accompanied removal of clippings from footpaths and driveways. Street sweeping follows within 48 hours of cutting activities 8 collecting leaf litter and debris from kerbing simultaneously. Autumn signifies the flowering of E side roxylon and the migration of the Cacatuagalerita (Sulphur Crested Cockatoo), Glossopsittaconcinna (Musk Lorikeet) and Platycercuselegans (Crimson Rosella) to the district. Large numbers of these birds foraging amongst the street trees shed leaf litter and trimmed branch-lets onto the roadway. Consequently, autumn rains flush this leaf litter along the kerb. Due to the over-landstorm water system upon flat grades, leaf litter and debris exacerbates localised ponding to occur within kerbing. During high-intensity rain events side entry pits (SEP) can become choked with leaf litter and debris, slowing down the stormwater flows. This causes the SEP to back up and local flooding ensues. Council has programmed vacuum clearing of SEPs to prevent blockages; it is unrealistic to rely on this program to offset the total volume of litter during certain times of the year.
To counter the build-up of leaf litter and debris in front of the Tree Inlets face plates and SEPs leaf litter collection/ infiltration bay shave been developed. Infiltration bays or Rain Gardens are anticipated to reduce roadway debris with select street tree species. The design of these relies upon gravity, as its precedent the slotted kerb and swale approach does. As stormwater discharges along the kerb, it flows past the infiltration bays; the open kerb causes the stormwater to fall into the Rain Garden carrying with it any leaf litter or debris. The Rain Garden is a trench of two meters long filled with 20 mm aggregate screenings and the edges are battered with local spoil and planted with native grasses. Stormwater discharges down through the profile of the Rain Garden, leaving leaf litter and debris upon the surface of the aggregate. Upon saturation of the infiltration bay and immediate surrounding soil, the Rain Garden at full capacity allows the stormwater to continue its path along the kerb; as it would had the infiltration bay not have been there. During high-intensity flows, leaf litter and debris are anticipated to be continually deposited and removed to within the Rain Garden due to a naturally occurring eddy 9.
The methods explored thus far include nine designs using the TREENET Inlet and two Rain Garden designs consisting of various-sized unlined sumps filled with 20mm aggregate, with three different ways of dispersing the inflow around the sump. The variation in size has been designed to accommodate different stormwater volumes, available space within the verge area including the placement of sumps below footpaths and driveway crossovers; and maximize contact surface area with the surrounding soil.
Implementation of infiltration and bio-filtration sumps has required the cooperation of Council Tree Services staff identifying in-appropriately planted street trees for removal and replacement plantings adjacent TREENET Inlet sumps. This cooperative approach has expedited the process and timing of work. Using the TREENET Inlet sumps in proximity to replanting’s has raised the opportunity to test the benefits of street tree establishment. Exploring further has led to Tree Services staff actively pursuing problematic trees for removal, replanting and placement of TREENET Inlet. Previously where staff displayed apprehension in removing problematic trees due to re-establishment concerns; the TREENET Inlet has empowered staff to take positive steps towards replanting locations.
Trees and Infrastructure and the Heat Island Effect
Pavement stresses can be attributed to many factors and heat being one that can be reduced by shading using the trees canopy. Salisbury Council is currently using Micro surfacing treatments, and polymer road stabilisation in conjunction with WSUD.
Randrup et al.(2003) suggested that certain pavement construction methods may even promote damage to pavements by tree roots. It was explained that soil moisture loss by evaporation can be blocked by a barrier such as a concrete or asphalt pathway. Due to the evaporation barrier effect, there are differences in temperature between the soil and pavement and this causes the soil moisture to condense on the underside of the pavement. Damage is caused therefore to the pavement surface by the root growth being attracted to this condensation moisture at the soil/concrete interface. Randrup et al.(2003)proposed that pavements constructed from porous materials that limit condensation and lower the temperature under concrete labs may reduce the incidence of rooting at the interface and the subsequent damage this can cause.
The negative environmental impacts of treeless streets (this includes enhanced heat island effects) have been researched significantly, plus continued concern about the impacts of climate change has helped sway a change in attitude toward urban street trees by town planners and designers; (Shashua-Bara and Hoffman, 2000)- it is duly acknowledged that urban street trees provided positive economic and environmental benefits to the community 10.
Salisbury recognises the important role that street trees play in urban environments and has launched research into developing alternate methods of passively irrigating street trees that may lower maintenance costs and promote healthier and faster-growing trees.
Opportunities for using Water Sensitive Urban Design
By implementing TREENET Inlets/rain gardens throughout the council area it is envisaged improvements in the community’s amenities may, include but not limited to the following:
Mitigate Infrastructure damage by reducing kerb replacement works and pavement deterioration
- Minimize impact on established trees
- Improve the establishment of street tree planting using water-sensitive urban design
- Catchment of nitrates, particulates, pollutants and organics and improve soil moisture for street tree
- Reduce first flush pulses
- Reduce silt build-up in Council Wetlands
- Reduce stormwater volumes discharging out to sea
- Reduced maintenance practices by redirecting leaf fall and diverting water prior to the CoS pipe network. Work Health and Safety improvement by lessening the requirement to clean side entry pits.
- Reduced impact of the heat island effect from heat pavement reflection.
Peter B Young B.Des.St.,M.Larch
Landscape Design Officer, City Of Salisbury,SA
Peter’s early career was focused on skate park design and promotion of this form of recreation culminating in 2009 with a redevelopment masterplan for Canberra’s Belconnen Skatepark to become Australia’s largest.
During the University of Adelaide’s studies in landscape architecture, he worked with Taylor CullityLithe and credits a rigorous design ethos with this experience.
Since 2008 Peter has been employed at City of Salisbury working on a range of landscape and biodiversity projects.
Peter is a strong advocate for water-sensitive urban design in local government.
Capital Works Officer,City of Salisbury SA
Peter Levett is a highly experienced practitioner in civil and landscape construction and maintenance in local government public works with major metropolitan Adelaide councils.
In his current role at City of Salisbury Peter administers a significant road surfacing program and other civil works. He has lead initiatives to achieve carbon reduction strategies and is active in cross-council forums seeking improved industry standards in road surfacing.
- Trees as essential infrastructure: Engineering and design considerations. Beecham S.School of Natural and Built Environments and Centre for Water Management and Reuse.University of South Australia
- Plant available moisture in stone-soil media for use under pavement while allowing urban tree root growth. Grabosky J, HaffnerE and Bassuk Nin Arboriculture & Urban Forestry, 35(5), 271-2(2009)
- A review of tree root conflicts with sidewalks, curbs and roads. Randrup TB, McPherson E G and Costello L R in Urban Ecosystems,5, 209-225 (2001).
- Vegetation as a climatic component in the design of an urban street: An empirical model for predicting the cooling effect of urban green areas with trees. Shashua-Bara L and Hoffman ME, in Energy and Buildings, 31(3), 221-235 (2000)
- From The Gutter To The Stomata By The Closest “Root” David Lawry
- The benefits of Adelaide’s street trees revisited Professor Randy Stringer
- Are street trees and their soils an effective stormwater treatment measure? Liz Denman
- Salisbury Development Plan
- Sustainable Futures
- The Prosperous City 1.2 Support the development of a work force possessing the skills required to adapt to industry restructuring and meet the needs of growth orientated industry sectors.
- The Prosperous City 2.2 Deliver high quality urban development incorporating sustainability, connectivity, diversity and integrated urban design principles.
- The Prosperous City 3.5 Build on Council’s investment in water reuse projects to attract investment, contribute to urban amenity and support local firms.
- The Sustainable City 4.1 Further maximize re-use opportunities and mitigate the impacts of stormwater inundation and flooding.
- The Sustainable City 5.2 Ensure that existing and future urban environments are able to withstand and adopt to future deman
- Achieving Excellence 6.3 Use expertise, knowledge and technology to improve and develop alternative modes of service delivery.