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
• protect/minimise impacts on existing environmental and waterway values
• achieve the principles as set out in the Principles for Provision of Waterway and Drainage Services for Urban Growth (PDF, 962KB).

The draft concept strategy has been designed to scheme principles which includes optimising cost and performance. In arriving at the final design, many different alternative arrangements for retarding basin and wetland locations and designs have been considered

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

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.

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.

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 (PDF, 2.71MB)
• Constructed Waterway Design Manual.

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 (PDF, 2.71MB)
• Constructed Waterway Design Manual.

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.

Retarding basins and constructed wetlands will be located within the PSP area, and service the PSP.

There are drainage works required outside the PSP in order to provide drainage outfalls to service this PSP.

The Public Acquisition Overlay (PAO) is proposed to be applied to identified land to reserve it for drainage outfall construction and indicates that this land could be compulsorily acquired in the future. The acquiring authority for this land is Melbourne Water.

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.

The retarding basins proposed in the 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.

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.

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.

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

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.

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).