Heritage sewer transformed into parkland with a rain garden

The Main Outfall Sewer was built in the 1890s and was the largest civil engineering project ever undertaken in Victoria at the time.
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Part of the historic Main Outfall Sewer (MOS) in Williams Landing, Melbourne, has been turned into a parkland with a rain garden and water storage.

Part of the historic Main Outfall Sewer (MOS) in Williams Landing, Melbourne, has been turned into a parkland with a rain garden and water storage.

The MOS was built in the 1890s, and was the largest civil engineering project ever undertaken in Victoria at the time. It was decommissioned in 1993 and heritage listed in 2001.

A few years ago, Melbourne Water began considering how the structure and the space around it could be used for the community.

“The drivers around that were really stemming from the water industry as a whole starting to look at what its contribution could be in the liveability space, and Melbourne Water wanting to take that thinking forward for itself,” said Michelle Ezzy, principal, Liveability – Liveability and Environmental Stewardship at Melbourne Water.

With a heavy focus on stakeholder engagement and using the New York Highline as inspiration, the idea of a linear parkland emerged and the Greening the Pipeline initiative began.

Challenges

The heritage aspect of the MOS on the 100-m-long Pilot Park project posed numerous challenges.

Throughout the project, the team had to ensure they had approval from Heritage Victoria while they developed the design. This included a number of permits to penetrate the MOS to drill holes inside it for pipes to allow water to come and go. Any changes to the design also required heritage approval.

Working inside a 3.3 m diameter concrete open sewer also had its challenges, and rock was present in the surrounding land, so the team had to work with difficult ground conditions on the site.

“The key focus of our design was to design a finished surface that enabled us to showcase the MOS in terms of providing viewing portals down the central spine,” said David Howard, team leader, water strategy at GHD.

“We did that by designing a biofiltration system that runs along the central spine, which allowed us to then continue that viewing portal down the MOS with the design of some very smart balustrades.”

The team also designed for water to flow into and out of the structure, installing a liner over the MOS and filling it with gravel, and slotted concrete pipes that provided space for water to be stored in.

Two storages within the MOS also harvest stormwater from the neighbouring catchments. Stormwater is diverted from these catchments via the existing underground pipe drainage system into a 12 kL primary storage tank.

The stormwater from this tank is then pumped to the surface and treated through the biofiltration system, which consists of sand filter media and vegetation.

The treated stormwater then filters down into a 28 kL secondary storage tank, which is constructed in a similar fashion to the primary storage tank with a liner, gravel, slotted concrete pipe and a pump station that pumps water to the surrounding landscape for irrigation and greening.

The original approach was to have the storage located outside the MOS in the surrounding landscape. But Howard said from a constructability point-of-view, there was a lot of risk with basalt rock in the ground, which would have posed a significant burden on the cost of the project.

“So we decided to use the air space – the void – that was already present within the MOS, and fortunately, due to the depth of the MOS relative to the underground drainage system, there was an ability to harvest water under gravity into the MOS,” Howard said.

The Pilot Park project biofiltration system consists of an off-the-shelf biofilta product, with concrete cells cast off-site. This system was installed at the top of the MOS to provide a clean finish and ensure the surrounding exposed aggregate open areas and paths also drain into the biofiltration system.

“Those biofiltration systems line up down the spine of the MOS, and they effectively have a sand media at the top with a gravel media at the bottom. Stormwater is pumped into those biofilta cells from the primary storage,” Howard said.

“They’re filled quite regularly, so that the plants go through a wetting and drying cycle, and we remove as much nitrogen and phosphorous as possible. They also free-drain back into the MOS under gravity into the secondary storage, so they provide a means for both nutrient and metal removal.”

Meanwhile, the heritage aspect of the MOS also meant the team had to be strategic about how they used machinery on the site to ensure the structure wasn’t damaged.

One way it did that was to take machinery over the structure in stages. It also put mechanisms in place to protect the structure from heavy machinery.

For example, machinery of a certain size was prohibited from driving over the structure. Where larger machinery was required, the MOS was protected by using a clay bund over the top of the structure so that there was adequate protection in place to carry heavier loads.

Further development

Moving forward, project partners Melbourne Water, Wyndham City Council, VicRoads and City West Water hope to develop all 27 km of the MOS into community-based parklands over time.

Master planning is being undertaken for a 5-km section in Wyndham between Laurie Emmins Reserve and Skeleton Creek. Further alternative open space and stream naturalisation design options for re-purposing additional sections of the MOS are also being explored.

The project partners are also working with Heritage Victoria to determine which parts of the overall 27 km have the most heritage value.

With the Pilot Park project part-funded by the Victorian Government, the project partners are now seeking funding to develop further sections of the Greening the Pipeline initiative.

“The vision is to create a really vibrant space in Melbourne’s west and to use it as a linear parkland that can connect communities and enhance the health and wellbeing of local communities,” Ezzy said.