Water Sensitive Urban Design – opportunities within private andpublic greenspace

Mellissa Bradley, Water Sensitive SA

Introduction

State government priorities and strategies seek healthier neighbourhoods with greater walkability, safe communities, urban microclimate management (particularly under a future warmer climate), and the creation of community spaces that are engaging and create a sense of place for the people who live there.

It comes as no surprise that green spaces are integral to the achievement of all of these objectives, and we need urban design that better integrates water – water sensitive urban design – to sustain these green spaces. The challenge then is how to translate these broad objectives into practical solutions that add value to private and community spaces.

The proposed urban design code under the Planning Reform could, if a truly holistic approach is taken, set the benchmark for more liveability, resilient, sustainable and productive communities, under both urban renewal and greenfield growth scenarios. This will, however, need a commitment to incorporate design outcomes to deliver across all lifestyle objectives.

With private green spaces diminishing, we are increasingly turning to the public realm to provide our connection with nature, shade for microclimate benefits, and passive and active recreation. With such high expectations, our existing and emergent dedicated greenspaces will have to work harder to provide multiple benefits that also include flood mitigation and stormwater treatment.

In established suburbs targeted for urban infill, the streetscape and existing paved areas such as community car parks will need to be modified to offset the impacts of additional runoff, in terms of both pollution (quality) and flood risk (quantity). A burden perhaps – or an opportunity to redefine how we revitalise our suburbs?

We should not be resigned to forgoing opportunities on private land to contribute to quality greenspaces that are able to provide ecosystem services to cleanse and store stormwater runoff and better sustain vegetation.

Background

There is wide consensus that urban infill represents a sound planning policy measure to protect both quality agricultural areas and biodiverse tracks of land at Adelaide’s urban fringe, while delivering savings in infrastructure costs. The 30 Year Plan for Greater Adelaide, currently under review, aims to deliver 70% of new housing stock from  infill development and the remaining 30% from  greenfield sites (Government of South Australia, 2010). This translates to an estimated 258,000 net additional dwellings in Greater Adelaide (including Murray Bridge) to provide for a population increase of 560,000 by 2040.

Green space in private allotments has decreased in recent years due to a downward trend in median allotment sizes in both infill and greenfield developments: 375 m2 in 2011-13 compared with 520m2 2002-03 (UDIA, 2013)

Councils within the greater metropolitan area have dedicated significant resources in recent years to investigate the opportunities this growth offers to their communities and the challenges that this targeted growth presents for infrastructure management, and in particular how we manage urban runoff.

The state government issues paper, Transitioning Adelaide to a water sensitive city , Towards an Urban Water Plan for Greater Adelaide and the South Australian Water Sensitive Urban Design Policy (2013) set a framework and principles for SA to set a pathway towards a water sensitive city (or community), and with recognition that the achievement of this goal is through the adoption of water sensitive urban design, the role of green space to provide ecosystem services for stormwater treatment and storage is now evident.

Projections for changes in imperviousness (paved areas)

Considering the case study of the City of Marion and City of Holdfast Bay, 1347 hectares of Council’s land is located within the areas targeted for urban growth. Investigations into the future development potential indicate that 89% (Jensen Planning + Design 2011) of this area will potentially be covered in impervious surfaces (including road reserves). As a means of comparison, the existing suburbs with traditional sized residential blocks (>600m2) consist of up to 65% impervious surfaces.

Council has developed a stormwater management plan to address the increased stormwater run-off from the estimated increase in impervious surfaces and will seek amendment to their development plan to give effect to the stormwater management plan with a key driver being flood risk.

Designing our suburbs

When designing our suburbs, the ability to deliver multiple benefits from both private and public green space will be enhanced by adopting a few basic principles.

The hierarchy of the roadway will influence the opportunities as wider collector roads provide opportunities for stormwater treatment within the centre medians of collector roads. Within the streetscape variation of verge widths, by providing one narrow verge and one wide, this design can afford sufficient space within the wider verge for bioretention systems and other WSUD treatments. Randolph Avenue, Fullarton, while a retrofit, demonstrates how a wider verge has not only enabled the inclusion of a series of 10 large raingardens and 33 street trees, once established the project will provide unique shade refuges with spaces for the community to interact.

Councils are recognising the traditional traffic calming devices that increase the width of the road verge in kerb blisters to constrict traffic can provide multiple benefits through the introduction of WSUD features such as a raingardens. Examples of this include Tarragon Street, Mile End within the City of West Torrens and Hindley Street, Adelaide within the City of Adelaide.

Streetscape bioretention systems will become increasingly popular as a means to provide greening of the landscape, however, a shift in practice will be required from the current approach to species selection of low lying sedges, shrubs and groundcovers to the incorporation of trees, to deliver greater microclimate benefits.

Maude Street, Unley, has a narrow two-way carriageway with no parking lanes that would typically almost double the impervious area of the road reserve. Parking is, however, provided within the road verge as part of a shared pedestrian/parking scheme. Vehicle parking bays are delineated with permeable paving.

The allotment configuration for the Lochiel Park urban infill development by Renewal SA within the City of Campbelltown provides narrow allotments with vehicular access from a rear lane. The front streetscape is then free of driveway cross overs, providing space for an array of bio-retention systems. Entry avenues into the sub-division provide biofiltration swales within centre medians where stormwater runoff flows uniformly into the kerb-less swales, reducing the likelihood of erosion as flow concentration points are limited.

Retrofit

In the future, we will need to increasingly look to road space for the creation of open space and stormwater treatment options within existing suburbs. Both Albert Park Secondary School, Foote Street Park (VIC and Unley Primary School (SA) have gained more open space via road closures, facilitating greater stormwater infiltration, to create a shared park for the school and the community.

Solutions at allotment scale

Limited space on private allotments mean that any water sensitive urban design measure needs to provide multiple benefits. On-site detention has been the stormwater management (runoff discharge rates) solution for single dwellings and large developments historically. While they are effective at restricting stormwater discharge rates to the required pre-development levels, assuming they are maintained, e.g. pumps, they provide no direct benefits to the individual household. Providing a solution that delivers multiple benefits is more likely to be embraced by the community.

Table 1.0 Allotment scale solutions for low impact development (adapted from Water by Design 2014)

For those areas specified as being subject to the Residential (Full) Development Code, a roof area of 60% of the site is permitted (Jensen Planning + Design 2011). Minimum requirements for private open space for allotments less than 300m2 is 24m2 (8%). For narrow allotments, any green space in the front of the property is generally not of a sufficient size to be used for recreating, nor would a landowner generally plant a lawn as it would be hard to justify owning a lawnmower to services the space (nor would there be space on the allotment to store a lawnmower). This space, which may be as small as 4-8 square metres, could readily be dedicated as a raingarden to treat and store overflow from a rainwater tank or direct from a downpipe.

A number of proprietary products exist that enable the storage of rainwater aboveground in a garden setting and also add value as both a storage facility and a planter box.

Water Sensitive SA is working with our partners to develop a tool for on-line stormwater assessment that will enable a developer to optimise their stormwater management solution to retain discharge rates to predevelopment levels. The tool will give the applicant flexibility in providing a solution that suits their needs and any site constraints. The contribution of impervious areas to maintaining the stormwater discharge rates from the site will be recognised within the tool, together with solutions including onsite detention, onsite retention for indoor use (plumbed to toilets, laundry and hotwater services) and raingardens that treat and store runoff from paved surfaces and overflow from rainwater tanks.

Maximising the impervious areas may include simple measures such as using permeable pavements for carparks, driveways and pathways around the allotment; however, a low impact designs can include reducing the overall footprint of the home. A design that configures the building floor area over two storeys rather than one can provide more green space and less impervious areas (Water by Design 2014).

The target dwelling density for the proposed transit oriented developments with the City of Holdfast Bay and City of Marion is 60-120 dwellings per hectare (Jensen Planning + Design 2011), compared with the current density of 15 dwellings per hectare represents a genuine opportunity to develop high quality multi-functional open space. High-density development will not only achieve targets for maximising use of public transportation, but represents an opportunity to provide greater areas of open space for stormwater management contributing to healthier vegetation, if a water sensitive approach to design is taken. Infill, at all scales, should be encourage to go up instead of out and the areas that would otherwise have consisted of impervious roofs, should be reserved for dedicated greenspace and other pervious surfaces.

Opportunities for the future

Raising awareness amongst decision makers and the community of the values of both private and public green space beyond passive or active recreation is key – for example, the importance of impervious spaces as a means to facilitate infiltration to groundwater and address the imbalance in the urban water cycle.

Raingardens as streetscapes solutions to stormwater management and treatment requirement are appearing in the urban landscape with greater frequency. Guidance for practitioners regarding raingarden species selection is an area for future peer-to-peer learning to ensure that plantings perform the required stormwater treatment function, are resilient and provide high amenity values.

Integration of street trees in combination or in association with raingardens in the streetscape would extend the community benefits beyond environment protection and increase overall site amenity.

At the allotment scale the new urban design code could provide greater flexibility than the current Residential Code to allow for building floor areas to be configured over two storeys in order to provide more yard and less impervious area. Stormwater management performance requirements need to be included in the criteria for assessment within the urban design code, featuring one or a combination of the following solutions: Rainwater tanks, overflow from rainwater tanks, runoff from impervious surfaces to raingardens and/or maximisation of impervious areas. The proposed on-line stormwater assessment tool will assist applicants to optimise solutions given the site constraints.

A key driver for these changes will be the adoption of the SA WSUD policy within Development Plan of greater metropolitan Adelaide Councils and the regions.

References

• DEWNR, 2013, Water sensitive urban design Creating more liveable and water sensitive cities in South Australiareferred to as the SA Water Sensitive Urban Design Policy.
• DEWNR, 2014, Transitioning Adelaide to a water sensitive city, Towards an Urban Water Plan for Greater Adelaide. Issues paper.
• Government of South Australia, 2010, The 30-Year Plan for Greater Adelaide.
• Jensen Planning + Design, 2011, Stormwater Management Plan: Cities of Holdfast Bay and Marion, Discussion Paper: Development Potential within the Catchment.
• Water by Design, 2014, Off-site Stormwater Quality Solutions Discussion Paper (version 1), Healthy Waterways Ltd. Brisbane.
• Urban Development Institute of Australia, 2013, State of the Land Report 2013.