Stormwater management project 2023

Highbury Aqueduct Reserve (adjacent Freshford Avenue)

Background

Following the completion of a Master Plan in 2013, a number of erosion and drainage issues were identified as requiring remediation works in the Highbury Aqueduct Reserve. One of the projects was upstream of the historic drain located at Site 8 (referred to as catchment 10 in the Master Plan).

While previous works at this site have stabilised the watercourse between Wicks Road and the crossing point beneath the decommissioned aqueduct channel, further stormwater remediation work is now required at Site 8b (downstream of the historic drain and adjacent Freshford Aveune).

Commencing in early 2023, the 4-month project will involve replacing an eroded dam wall with a rock chute and stabilising the channel to better manage flows. A security fence will be in place for the safety of park users and detours for pedestrians will be established around the site until the work is completed.

This stormwater project is one of the remaining stormwater projects identified to address drainage and erosion in the Highbury Aqueduct Reserve Master Plan.

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About the project

Exact timing will be communicated ahead of works commencing. People are advised to abide by safety signage, and not enter fenced off areas whilst the project is underway.

Minimal traffic disruption is expected. The access point into the reserve from Filippo Court may be used by project machinery from time to time. Residents of Filippo Court will be advised accordingly to ensure minimal disturbance.

Works will include:

• removing approximately 23 Pinus Halepensis (Aleppo Pine) trees within the stormwater channel where the channel banks will be re-contoured
• removing the eroded earthen dam wall structure and replacing it with rock chute and armouring to better manage flows and provide better structure to the channel
• contouring the banks upstream to re-instate the aligned channel profile to address sedimentation deposits and erosion heads that have developed
• installing an additional rock chute structure upstream of the existing dam wall to slow the flow rate and assist in reducing sediment mobilisation
• revegetating the channel with appropriate sedge and rush species to provide additional erosion control, sediment stabilisation and filtration services and improved habitat for local native fauna
• remediating banks with geofabric, while revegetation becomes established.

Frequently asked questions

1. Why are the works necessary?

After recent heavy rains, a large hole in the dam wall (adjacent Freshford Avenue) has become visible. To ensure the safety of all park users, removal of the unstable, eroded dam wall is necessary to prevent further collapse of the wall and prevent further significant mobilisation of sediment downstream.

2. Can I still use the walking trail?

Yes, but pedestrians using the walking trail will need to take care as there will be heavy machinery in use within the reserve. A security fence will be put in place for the safety of park users and detours for pedestrians will be established around the site until the work is completed.

3. Will traffic in and around Filippo Court be impacted by machinery moving in and out of the reserve?

The access point into Highbury Aqueduct Reserve from Filippo Court is the safest way for machinery to access the works area in the reserve. No lengthy delays to traffic movement are expected. If you are a resident of Filippo Court we ask for your understanding while we move machinery into and out of the site. We also ask that if possible, you do not park on the roadside in or around Filippo Court while works are underway. Residents will be contacted prior to works commencing regarding more specific dates for machinery access.

4. How will noise from machinery be managed?

Machinery will be operated within the City of Tea Tree Gully’s noise control regulations:

Monday to Saturday: 8am to 8pm

Sunday: 9am to 8pm

5. Will trees be removed as part of the project?

A number of Pinus Halepensis (Alleppo Pine) trees will be removed from the banks and from within the stormwater channel to allow the channel to be re-aligned to address sedimentation deposits and erosion heads progressing upstream of the dam wall. The trees to be removed are limited to those in areas where earth disturbing activities would likely damage the existing root structure. Some trees marked for removal were to reduce shading of the works area that may inhibit the establishment of vegetation along the channel. Alleppo Pines are a declared pest plant species that have been earmarked for staged removal from the reserve over a number of years. Their removal is generally followed with revegetation using local native species similar to what would have been found locally pre-European settlement and that provide a more suitable food source for local native fauna such as the Yellow Tailed Black Cockatoos. This project will include revegetation of the re-aligned channel with local native species.

6. Why does the dam wall need to be removed instead of repairing it or replacing it?

The aged and degraded condition of the dam wall is structurally unstable in multiple places due to tree root invasion, hence a repair would not be sufficient to stabilise the dam wall.

The dam is also no longer needed and hasn’t been used for many years. Originally it was built to supply a local orchard, but rainfall over time eroded the earthen dam wall, creating a hole, so for several years, flows have been moving though the dam wall and downstream.

These flows are considered to be contained within the existing channel downstream and the installation of the rock chute structures to replace the dam wall have been designed to effectively manage the modelled flows for 1-in-100 year rainfall events.

7. Has the eroded hole in the dam wall affected water quality downstream?

The continuing erosion of the dam wall contributes to the mobilisation of sediment downstream. Water movement through the dam wall displaces soil particles from the dam - allowing for sediment accumulation in the downstream channel section and River Torrens discharge point. The rock chute structures that will be installed within the channel will minimise further in-channel sediment mobilisation downstream. The channel and banks will also be revegetated with local native species to stabilise the channel and bind the stream banks.

8. Who can I contact if I have any further queries?

Contact the Public Land Stewardship team

Phone: 0437 703 152 or 0407 610 359

Email: NPWS-AMLR-PLS@sa.gov.au