What are...

• Retarding basins

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

  They will also contain constructed wetlands, and may also provide recreational space for the community when they are dry.

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

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

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

  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 (AR&R).

  Climate change effects to the year 2100 have been considered, with adoption of SSP2-4.5 scenario. For further information refer to Section 11 Climate Change Sensitivity Analysis in the DSS Functional Design Report.

• 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 for 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 Melton East PSP 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 and design.

  The VPA and Melbourne Water intend to use the public consultation period to consider feedback in relation to 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.

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

• Annual Exceedance Probability (AEP) is defined as the likelihood of a flood occurring in any one year. If a flood has an AEP of 1%, it has a 1% 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 Melton East 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 SSP2-4.5 scenario. For further information refer to Section 11 Climate Change Sensitivity Analysis in the DSS Functional Design Report.

• 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 Melton East PSP area, and service the Melton East PSP.

• 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 Rockbank North PSP. As part of the draft concept drainage strategy for the PSP, there will be some drainage outfall connections to existing PSPs with waterways.