Gipps Street median

A high water table is present along the sides of Gipps Street, but not under the centre of the road. The basic design concept was to redirect excess water from the edge of the road to irrigate trees in a centre median.

Several lengths of underground pipe were installed at intervals along the street. They direct excess groundwater to the median. The gradient of the pipes is set so that the water moves under gravity. An underground pipe also runs the length of the median, to convey water that is not absorbed into the soil along the median. This excess water is directed back into the stormwater drainage system.

Business case development
2012 – 2013, submitted March 2013

Concept plans developed
2012

Community engagement
April 2013

Revised design
May 2013

Construction
June 2013 to November 2013

City of Melbourne staff and contractors completed this project. Funding support was provided by the State Government of Victoria.

Cost/benefit assessment
A number of solutions were considered in relation to the waterlogging issue in Gipps Street. These included installing a new stormwater drain or replacing failing trees with water-tolerant species. The new median was deemed to be best option.

Identify problem
To identify the source of the excess water under the kerb, City West Water was engaged to inspect the function of the existing water main. Although the water main was old, it was not found to be leaking and eliminated as the water source.

Geotechnical analysis study 
A geotechnical study was commissioned to try and work out the cause of the water pooling in the soil at the edge of Gipps Street. The study identified that a high water table was the cause.

Traffic study
A traffic study was carried out to determine the impact of the median on traffic in and around Gipps Street.

Community consultation
The local community was asked to provide feedback on the initial design concepts. This feedback was used to develop the final plans.

Access
Gipps Street is used for delivery access to the Mercy Hospital and a number of other medical facilities in surrounding streets. We had to make allowances in the design for truck movement and turning.

In addition, we had to maintain car park access for a large residential building. The initial design was altered to allow easy access for residents.

Street width
At 20 metres wide, Gipps Street is ten metres narrower than standard streets in the area. This is likely the reason that a median was not originally installed. A narrow median was designed, 2.5 metres wide rather than the usual six metres. On a standard median, trees would be planted in individual wells. In the narrower design trees are planted in a trench that runs the length of the median, providing room for the root system to grow.

Gipps Street residents were engaged very early in the design process because they had requested that the median be constructed.

We sent concept designs to all local residents, property owners and impacted businesses and received over 100 responses. 75 per cent of respondents asked for changes. The designs were revised, shared with the community and were received favourably.

Under the Melbourne Planning Scheme, projects carried out by the City of Melbourne costing less than $1 million do not require a planning permit. The Gipps Street median project fell into this category.

Drainage pipe

Type:
Slotted plastic drainage pipe

Size:
100 mm diameter

Location:
Underground, running from the edge of the road to the centre median at intervals along the street. Another length of pipe runs the length of the median, carrying excess water to the stormwater drains.

Purpose:
The drainage pipe transports groundwater from the edge of the road to the roots of the trees in the centre median.

Centre median

Composition:
concrete kerb edging surrounding tree-growing medium

Dimensions:
the median is 2.5 metres wide. Tree plots within the median are 3 m x 1.8 m and 0.8 m deep. Tree plots are spaced at 10 metre intervals along the median.

Soil type:
coarse sandy loam.

We pulled up the road base along Gipps Street and excavated the soil along the new centre median area. Specified tree growing soil was installed to provide a large root zone for the new trees. In the plots where trees were planted, the excavation was slightly deeper.

We used sandy soil rather the highly organic landscape or planter mixes, because it has the inherent capacity to resist compaction. This makes it more resilient during the construction process than a loam or clay loam soil would be.

Highly organic mixes were avoided, because they lose volume and undergo changes in porosity as the organic matter decomposes. This can cause problems for plant growth.  Large amounts of organic matter buried deeper than 200 mm can also undergo anaerobic decomposition. This leads to poor plant growth and even plant death.

We planted Quercus coccinea (scarlet oak) in the median. It is a water tolerant tree, so it will thrive in the new median where the excess water is directed. Scarlet oak is a deciduous tree. It will provide canopy shade in summer and let plenty of sunlight through in winter after it drops its leaves.

New trees were also planted along the sides of Gipps Street to replace the ones that had failed due to waterlogging. We chose Ulmus parvifolia (chinese elm) in accordance with the East Melbourne Urban Forest Precinct Plan.

The total cost of the new centre median for the entire length of Gipps Street was $600,000. The passive irrigation component of this was $60,000.

The Gipps Street passive irrigation system is very low maintenance. Filters prevent the pipes from becoming blocked. The only work required is standard maintenance of the turf and trees in the median, such as pruning.

We monitor the effectiveness of the system by observing the trees in the footpaths and centre median. Since we completed the project in 2013, the trees have shown good signs of healthy growth.