IPWEA Awards 2011

Water sensitive urban design

Project Summary

A recent community consultation process identified a public desire for more trees in the Smith Street Activity Centre – a busy shopping strip running from Victoria Parade to Alexander Parade, Collingwood. The City of Yarra recruited and coordinated a cross-disciplinary team consisting of landscape architecture and design, water sensitive urban design, arboriculture and innovative engineering. Together, they pioneered an innovative new system that has vastly improved the streetscape aesthetic, harnessed stormwater runoff for fully sustainable tree watering, improved the quality of water entering the stormwater system, and significantly decreased the volume of gross pollutants entering vulnerable waterways.

Project Description

Background

The Smith Street Structure Plan for the Smith Street Activity Centre (adopted by Council in November 2008) involved a community consultation process in which the public were asked what they would like to see in Smith Street. There was a consistent and popular request for more trees and more green in the street. The Smith Street Activity Centre is an inner city street shopping strip with lots of colour, clutter and variety. The local heritage and character is very much evident and alive. Smith Street is heavily used, with a constant flow of vehicles and pedestrians, day and night.

The new street tree planting project included both exciting design opportunities but also numerous constraints. While the character has a contemporary, eclectic feel, it also has a strong heritage character, with the majority of the street covered by a heritage overlay. The road way and footpaths are narrow with many used and de-commissioned underground services . The central section is dominated by small shops with awnings and verandahs making the planting of decent sized street trees virtually impossible. The north and south ends of Smith Street have space for new street trees, with fewer awnings and verandahs and slightly wider footpaths. The new street tree planting was therefore concentrated in these areas.

The north end of the street, from Johnston Street to Alexandra Parade, has a significant slope which created an opportunity to integrate Water Sensitive Urban Design (WSUD) into the street tree planting project. The inclusion of WSUD into the project involved a range of additional constraints and opportunities also, which will be discussed in detail.

The Design: Tree Grates and Guards

The design concept for the new tree guards and grates was to be distinctive, original and functional. They have been inspired by the industrial heritage of many Collingwood and Fitzroy streets, including Smith Street. The design is an attempt to capture, in a stylised and simple form, the idea of the factory and machine mass production. This is reflected in the repeated, circular/curved sections of the tree guards and the use of galvanised steel is used as it is a raw industrial material. The guards are designed to have an unidentifiable, industrial appearance, while not directly replicating a particular industrial form or object. The tree guard and grate design also complement the heritage red brick industrial buildings and general urban feel of the street, while softening the street with their elegant curved (rather than angular) forms.

Smith Street has a strong heritage streetscape character with not only the continuous rows of Victoria shopfronts but also the original (and re-layed in some sections) bluestone kerbs and gutter stone channels. The tree grate design included a lintel (for the WSUD), which is 300mm wide, to match and continue the existing heritage bluestone kerb line. The heritage bluestone pitcher channels were kept intact with only slight changes to the angle of a minimal number of pitchers to direct water into the side entry of the tree pit for the WSUD.

Tree Selection

The eclectic character of Smith Street seemed to need an unconventional approach to the tree selection while ensuring a guaranteed effect – so this is what we attempted to do. A choice of two tree species was selected, both native and exotic. The olives were selected to provide a reliable and regular form, while the Eucalypts (which are a species which had not been used before in the City of Yarra) have bold and bright yellow flowers and unusual large pale pink gum nuts which are highly perfumed and attract birds. The Olive trees and Eucalyptus Lorikeet “summer scensation” interplanted has an element of consistency when moving along or looking down the street, as a whole, as they are of a similar mature size and both species have distinctive blue- grey foliage.

Water Sensitive Urban Design

The City of Yarra has adopted stormwater quality runoff improvement targets. To achieve these objectives the Council is implementing a range of Water Sensitive Urban Design projects such as the bioretention tree pits in Smith Street.

The intent of the WSUD tree pits is to direct stormwater runoff that flows down Smith Street into a “treatment train” of 18 bioretention tree pits. The objective was to treat the polluted stormwater through the tree pits which contain filter media designed to remove contaminants before the water is released back into the stormwater drain and continues on to the local waterways. An additional benefit is that the trees are “passively” irrigated by the stormwater. Due to the fact that a stormwater drain was only present on the west side of Smith Street, WSUD principles could only be included on this side of the street.

The project was modelled with the stormwater quality software tool MUSIC (Model for Urban Stormwater Improvement Conceptualisation) to calculate the number and size of bioretention tree pits required to achieve strong water quality outcomes. Modelling conducted for this project found that the project will see an 83.5% reduction in Total Suspended Solids (kg/per year) and a 40% reduction in Total Nitrogen (kg/per year).

Council contractors open the tree pits every two weeks to clean out litter and accumulated sediment and check on the health of the trees. The entry of the tree pits was designed to allow gross pollutants to fit through the openings to purposely act as a litter trap. Since being implemented maintenance reports show that approximately 240 litres of litter (bottles, food packaging, cigarette butts, etc) is trapped by the tree pits every fortnight. A water quality monitoring program is being developed to study the effectiveness of the tree pits in improving stormwater runoff quality.

The WSUD component of this project was co-funded by the Melbourne Water Living Rivers program.