Melbourne Water is upgrading the Ferny Creek Retarding Basin at Upper Ferntree Gully. These works will ensure that the retarding basin continues to reduce the risk of flooding to the community.
Melbourne Water has over 200 retarding basins that reduce the risk of flooding to properties and the community, which are regularly assessed, maintained and upgraded when required. Recently, the Ferny Creek Retarding Basin was assessed by the Australian National Committee on Large Dams (ANCOLD). This assessment highlighted that an upgrade is currently required to ensure the retarding basin continues to reduce the risk of flood to the community and is operating efficiently for years to come.
What is a retarding basin?
A retarding basin is an area of land used to collect stormwater during high rainfall events to prevent the risk of flooding. When a retarding basin fills, the embankment acts as a dam to holdback stormwater, as seen in the image below.
The stored water is slowly released into a downstream drain or waterway, helping to protect homes and businesses. Retarding basins are often used by the public as parks or sporting ovals when they are dried up during low rainfall seasons.
How we upgrade retarding basins
Upgrade works to retarding basins can include embankment hardening and the installation of filter trenches. Hardening the embankment protects the basin from erosion when the water overtops. Embankments can be hardened through reinforcing the top soil, laying concrete or installing geoweb.
Installing a sand filter or filter trench can prevent water destabilising the embankment by flowing through it, as seen below.
Aqua Metro Services (AMS) will be undertaking these works on behalf of Melbourne Water. Works will include:
- removing trees that are at risk
- installing a filter trench on the embankment to increase its strength
- crest capping on the embankment
- constructing a concrete cut off wall
- replacing the spillway
- installing erosion control matting
- installing a concrete wall to protect flora.
Works will start in October 2019 and take approximately five months to complete (weather permitting). Construction hours will on Monday to Friday from 7am to 6pm, and Saturday from 7am to 1pm.
We will do everything possible to minimise disruption during these works, however it is likely that nearby residents will notice:
- large trucks and machinery working around the retarding basin
- worker amenities set up within the construction area
- some noise, dust and vibration from construction works
- removal of trees and vegetation from the retarding basin embankment (trees and vegetation in other parts of the reserve will be retained)
- construction traffic and equipment will enter and exit the site via New Road
- pedestrian access will be maintained where possible.
To ensure the retarding basin continues to operate safely and compiles with ANCOLD guidelines, trees and vegetation on the embankment need to be removed as part of the works. Having trees on the embankment increases the chance that the embankment may fail in the event of a large rain event, increasing the risk of flooding. As pictured below, trees increase the risk of:
- Internal erosion and displacement of soil – tree roots create erosion pathways through the embankment which are worsened when the tree dies and its decaying roots leave voids through the embankment.
- Tree uprooting and taking part of the embankment with them.
- Water speeding up around tree trunks and causing faster erosion.
Come to our Community Pop-up Sessions
The project team will be in the area to answer your questions from Monday 7 October to Wednesday 16 October 2019 (excluding public holidays). Just give us a call and we will ‘pop up’ and see you!
- Call 1800 932 121 (Option 2 – Ferny Creek Retarding Basin)
- Email [email protected]