«Proceedings of the th 14 National Street Tree Symposium 2013 ISBN: 978-0-9806814-1-3 TREENET Proceedings of the 14th National Street Tree Symposium ...»
Introduction 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 158km2, 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 which collects and flows through this network.
Street Trees are a large investment for Council in provision of streetscape amenity, moderate local climate conditions, and frame 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, reduces the soil’s ability to provide soil moisture levels viable for street trees to grow, maintain health and reach maturity. 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 council. These outcomes address issues regarding increased storm water volumes, ponding of storm water, reducing leaf litter debris, improving overall urban permeability, moderating soil moisture levels, passive irrigation of street trees and increased street tree vitality.
City Overview The City is predominantly a residential area but also has substantial industrial, commercial and rural areas.
Total land area is 161 square kilometres, including Adelaide’s second commercial airport and RAAF Edinburgh, many parks, reserves, walking trails and wetlands. Horticultural enterprises (mainly vegetable growing) are located on the western fringes of the 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.
The 14th National Street Tree Symposium 2013 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.
Figure 2. Location within AdelaideFigure 1. Location within Australia
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 rain water to infiltrate the soil are reduced.
These impervious zones shed stormwater at far greater rates than the permeable zones and increase the total storm water volumes within council infrastructure.
Existing older residential area stormwater systems (dating from 1940s to 1980s) tend to be an overland system upon flat grades that have historically had drainage problems. Sub-division and redevelopment of older residential blocks increases 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 hectare within 500m safe walking distance of every house2. However, this open space provision is not protected from becoming the developments stormwater detention basin. In which larger rain events turns the residents playspace, kick and catch area or a reserve 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 5 ARI4 with adjacent native plantings of grasses, shrubs, groundcovers and street trees. The overflow from these rain gardens is plumbed into the storm water network to discharge into the onsite detention basin. A permanent water body surrounded by walking tracks with seating and connected to Councils Green Trail linear path network for passive recreation.
Salisbury Development Plan: Land Division Principles of Development Control 1.i pg 16 Salisbury Development Plan: Public Open Space Principles of Development Control 1.b pg 60 Sustainable Futures: The Living City 3.1 pg 50 Minister’s Specification SA 78AA September 2003 On-Site Retention of Stormwater
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 fans 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 horizons5.
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 deficit begins again in early August6.
City Landscape Plan 2.2.3 Soil Associations. City of Salisbury City Landscape Plan 2.2.4 Weather, Climate and Soil Moisture Budgets. City of Salisbury
The council verge as the next wetland In an average year 160 gigalitres (160 GL) of water flow down Adelaide’s gutters and into Gulf St Vincent. This run off 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 storm water 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 20 GL of storm water reuse by 2025 which is only a 12% reduction on current outputs. Urban sprawl and infill will only add to the torrent of storm water 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 on to 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 storm water 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 localised detailed stormwater data and soil infiltration rates were raised as critical success factors to designing WSUD alternatives. Infiltration can be maximised through designing the sumps with 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 The 14th National Street Tree Symposium 2013 available soil moisture. By maximising contact surface area is to allow the sumps to catch first flush rainfall and once saturation is achieved the infiltration rate is maximised to drain 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 storm water 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 storm water 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.
Figure 6. Street profile using water sensitive design in a verge application
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 no maintenance except for normal street sweeping activities.