What are...

• Retarding basins

• Retarding basins are low-lying areas of land, set aside to temporarily store stormwater during heavy rain events. When a retarding basin fills, the landscape is designed to hold back stormwater to reduce flood risk to the surrounding area.

  The retarding basins in this PSP will also contain constructed wetlands. They may also provide recreational space for the community when they are dry.

  Visit the Melbourne Water website for more information on retarding basins.

• Constructed wetlands

• Constructed wetlands are a series of shallow, densely planted, man-made ponds that help filter water through physical and biological processes. They provide a natural way to treat and remove pollutants from stormwater before it enters our creeks, rivers and oceans.

  Visit the Melbourne Water website for more information on constructed wetlands.

• Sedimentation ponds

• Sedimentation ponds remove pollutants, by slowing the speed of water flowing through them. This allows particles to sink to the bottom.

• Constructed waterways

• A constructed waterway is an artificial (engineered) waterway. It provides an appropriate drainage and flood protection level of service to new developments.

  Constructed waterways are usually needed to modify a degraded or undersized waterway or drain, to convey larger and more regular flows from an urbanising catchment.

  A constructed waterway also consists of additional land on either side, called the waterway corridor. Together, constructed waterways and their reserves provide for asset maintenance, ecological, social and amenity value.

  Visit the Melbourne Water website more information on constructed waterways.

• Waterway corridors

• Both natural and constructed waterways require waterway corridors. These are the areas of land beyond the wet areas, which are needed to help ensure a resilient waterway system that fulfils flood protection and ecological and social functions.

  For more information on waterway corridors, view our Waterway Corridors - Greenfield development guidelines (PDF, 2.7MB).

• What are the objectives of the draft concept drainage strategy?

• The draft concept drainage strategy has many objectives for new developments. The main ones are:

  • flood protection is to be provided for properties within the PSP,
  • any new developments are not to further exacerbate existing flooding for downstream/neighbouring properties
    
  • new developments are to achieve appropriate Best Practice stormwater quality treatment for flows generated from their property, to mitigate impacts on downstream environments
  • optimise drainage land requirements
  • proposed constructed waterways to provide safe drainage and flood protection and to incorporate environmental, cultural and amenity values
  • climate change effects to be included in the DSS design
    
  • to protect/minimise impacts on existing environmental and waterway values
  • to achieve the principles as set out in the Principles for Provision of Waterway and Drainage Services for Urban Growth (PDF, 962KB).
• Can some of the retarding basins or constructed wetland assets be combined to reduce their number?

• The draft concept strategy has been designed to optimise cost and performance. We have already considered many different alternative arrangements for retarding basin and wetland locations.

  The VPA and Melbourne Water intend to use the public consultation period and any VPA Projects Standing Advisory Committee process to invite proposals for optimising the drainage strategy. Any proposals to alter the size or location of assets will be reviewed against the objectives, requirements and restrictions of the catchment.

• Why are the retarding basins reserves so large?

• The retarding basins have been designed to protect downstream properties from flooding. Their size is largely governed by the volume of stormwater that needs to be temporarily stored during a 1% annual exceedance probability flood (1% AEP) - that is, a flood with a 1% probability of being exceeded in any one year.

  As per the Guidelines for Development in Flood Affected Areas (DELWP, 2019), development should not increase the 1% AEP flood level on any adjoining properties.

  There are also design constraints, such as elevated groundwater tables that limit the excavation depths, thereby increasing retarding basin footprints.

• What is 1% AEP with climate change design for retarding basins?

• Annual Exceedance Probability (AEP) is defined at the likelihood of a flood occurring in any one year. If a flood has an AEP of 1%, it has a 1% (or 1 in 100) chance of occurring in any year. The 1% AEP is the general legal standard for flood protection for properties.

  The retarding basins proposed in the Devon Meadows & Casey Fields South Employment PSP have been designed to mitigate post-development 1% AEP (100 year) storm events with climate change to pre-development 1% AEP storm events with climate change.

  The draft concept drainage strategy considers climate change effects in reference to:

  • Victoria Planning Provisions, 13.01-1S Natural hazards and climate change, and 13.03-1S Floodplain Management.
  • The national engineering standard for flood assessment, Australian Rainfall & Runoff.

  Climate change effects to the year 2100 have been considered, with adoption of the 2100 RCP8.5 climate change scenario.

• Why does the Devon Meadows drainage strategy include waterway diversions?

• Waterway diversions are needed to protect existing residential properties immediately south of the Devon Meadows PSP.

  Urbanisation of the Devon Meadows PSP will increase surface runoff volumes. This is due to impervious surfaces such as roads and roofs replacing more permeable surfaces such as grassland.

  Without the waterway diversions, this increase in surface runoff will intensify flooding and drainage risks to these properties.

  The only alternative is upgrading the existing drainage lines downstream of the Devon Meadow PSP. This option is problematic, because:

  • the current use of the land is not compatible with the upgrades required to the drainage lines
  • the drainage lines are located on private property, often close to private assets, such as houses and sheds
  • significantly more land would need to be set aside for the drainage upgrades needed, and obtaining this land would require compulsory land acquisition.

  Compulsory land acquisition is not preferable due to the potential negative impacts on landowners and tenants, and the higher financial costs to the DSS.

  The preferred option to protect the existing residential properties to the south of the Devon Meadows PSP is to divert waterways to the east into the Casey Fields South Employment PSP.

  Further detail

  Devon Meadows PSP is unusual in that the receiving downstream areas are existing residential properties, rather than greenfield/farming land which is typical of most PSPs.

  These existing residential properties are already impacted by drainage issues such as frequent flooding, nuisance drainage problems and waterlogged soils.

  The causes of these problems are multifaceted. They are partly due to the natural characteristics of the catchment, and partly due to the historic development of the area.

  The natural catchment characteristics include highly permeable soils, which means a large portion of the drainage is via groundwater rather than surface water. This contributes to high groundwater tables.

  The historical development of the area has resulted in surface drainage lines running through residential properties. Unlike modern developments, these drainage lines do not have any easements or reserves that would make it easier for us to access, maintain and upgrade drainage lines. We cannot readily access the drainage lines in the Devon Meadows area, and maintenance and upgrade works will conflict with some of the existing residential use of the land.

  Further exacerbating the problem is the urbanisation of the upstream catchment. Urbanisation leads to increases in surface runoff volumes due to new impermeable surfaces such as roads and roofs. Without intervention, the existing drainage lines would receive greater volumes of surface runoff, leading to increases in flood risk, nuisance drainage problems and water logged soils.

  To manage the problem of increased runoff volumes upon these residential properties, a diversion channel must be constructed. This will redirect surface runoff away from the residential properties, into greenfield farming areas which are more capable of managing the increased volumes.

  The only alternative to a diversion channel would be to upgrade the existing channels through the residential properties. Such upgrades would not be compatible with the existing use of the land. For this reason, compulsory land acquisition would be required. To avoid such an outcome, a diversion channel is proposed.

  It should be noted that the approach typical of most DSSs (i.e. retarding basins and wetlands), would not suffice in this instance. This is because retarding basins do not reduce surface water runoff volumes. They only temporarily hold back volumes, thereby reducing the peak flows. Similarly, wetlands do not reduce water runoff volumes, but temporarily hold them back to remove stormwater pollutants. By contrast, a diversion channel will reduce the volume of water flowing through the existing residential properties.

• Can constructed waterways be piped?

• Open waterways that are shown on the PSP are not to be piped. The waterways are designed to carry high flood flows.

  Piping compromises drainage and flood protection for the development, as it is less reliable and effective for carrying floodwaters. Furthermore, waterways provide for ecological, social and amenity values, which pipes do not.

• Can I reduce the waterway corridor reserve on my property?

• The waterway corridors have been designed to ensure a resilient waterway system for flood protection, ecological and social function.

  Any proposed change to the waterway corridor design must be in accordance with Melbourne Water's:

  • Waterway Corridors: Guidelines for greenfield development within the Port Phillip and Westernport Region
  • Constructed Waterway Design Manual.
• How has the waterway reserve been established?

• The waterway designs, and the required reserve widths, were established in accordance with Melbourne Water's:

  • Waterway Corridors: Guidelines for greenfield development within the Port Phillip and Westernport Region
  • Constructed Waterway Design Manual.

Outside the PSP

• Will properties outside the PSP be affected by flooding as a result of development within the area?

• The concept drainage strategy has been designed so that the development within the PSP will not have any adverse flooding impacts in areas outside of the PSP.

• Can the retarding basins/constructed wetland assets be located outside of the PSP to free up more land for development?

• Retarding basins and constructed wetlands will be located within the adjoining Clyde South PSP area, immediately downstream. A drainage strategy for the Clyde South PSP has yet to be developed as this PSP is in the early stages of development.

  The retarding basins and wetlands within Clyde South PSP will service the Clyde South PSP. In developing the drainage strategy for the Clyde South PSP, we will consider whether there are benefits in combining it with the drainage strategies for the Devon Meadows and Casey Fields South Employment PSPs.

  If there are benefits (notably reduced land take), the drainage strategies for Devon Meadows and Casey Fields South Employment PSPs will be updated.

• Will there be drainage works outside of the PSP?

• There are drainage works already approved and proposed outside of the PSP as part of the existing Ti Tree Creek DSS. As part of the draft concept drainage strategy for the PSP, there will be some drainage outfall works located immediately outside of the PSP to connect the drainage to the existing waterways or drainage asset.

  To enable free draining outfalls, drainage works will be needed immediately downstream of the PSP area, i.e. on the drainage lines immediately east of Clyde Five Ways Road. These will be required in conjunction with retarding basin and wetland works to the west of Clyde Five Ways Road.