The DSS 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 conditions for downstream/neighbouring properties
• new developments are to achieve best practice stormwater quality treatment for flows generated from their property to mitigate impacts on downstream environments
• new development proposals are to manage stormwater flow and volume from developments draining into Gum Scrub Creek
• proposed constructed waterways are to provide safe drainage and flood protection and to incorporate environmental, cultural and amenity values
• climate change effects to be included in DSS design
• DSSs are to protect/minimise impacts on existing environmental and waterway values
• DSSs are to meet critical and other agency asset operational requirements to ensure constructability (i.e. gas main crossings).
• to achieve the principles as set out in the Principles for provision of waterway and drainage services for urban growth (PDF).

The DSS contains a number of drainage assets.

Retarding basins are low-lying areas of land, designed to temporarily hold back stormwater during heavy rain, and in doing so reduce flood risk to the surrounding area. The basins in this DSS contain constructed wetlands, and may also provide recreational space for the community while dry. The proposed DSS retarding basins in the Officer South Employment PSP have been designed to 1% Annual Exceedance Probability (100yr) storm events with climate change, as per Melbourne Water standards.

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.

Constructed waterways are artificial waterways created to provide appropriate drainage and flood protection to new developments. They are usually created by modifying an existing, degraded or undersized waterway or drain, in order to service larger and more regular stormwater flows from an urbanising catchment. They and their adjacent land also support asset maintenance, ecological, social and amenity value.

Waterway corridors are the area of land either side of the wet areas of a natural or constructed waterway. They are required to help ensure a resilient waterway system for flood protection and ecological and social functions. For more information, download our Waterway corridor guidelines for greenfield development (PDF).

Previous versions of the DSSs for Officer South Employment PSP were designed many years ago. Since then, design standards and policy have changed along with catchment conditions, and a greater understanding of the catchment.

The current DSS strategy is based on current design standards and informed by the VPA’s proposed urban structure, while considering the risks associated with climate change, sodic soils, and environmental and cultural heritage opportunities.

The PSP is significantly large, at approximately 1069 hectares (equal to 535 MCGs). This means it needs numerous drainage assets to service future development.

Additionally, some of the basins along Gum Scrub Creek are servicing catchments north of the PSP, which are part of other PSPs and DSSs that are already approved.

The size and number of retarding basins and constructed wetland assets have been designed to optimise their cost and performance, and distribute them equitably across the PSP.

Some have also been designed to minimise the amount of stormwater crossing existing major infrastructure such as gas mains. Any proposals will be reviewed against the objectives, requirements and restrictions of the catchment.

The retarding basins service large, local industrial catchments within the PSP, which generate the largest stormwater flow and volumes. Some of the proposed retarding basins along Gum Scrub Creek are servicing large catchments to the north of the PSP, which are part of other PSPs and DSSs that are already approved.

Annual Exceedance Probability (AEP) is a term used for the likelihood of the occurrence of a flood of a given size or larger, occurring in any one year, and is usually expressed as a percentage. If a flood has an AEP of 1%, it has a one in 100 likelihood of occurring in any given year. This is the general legal standard for flood protection for properties, however other higher standards can be set.

Climate Change effects applied into the DSS designs takes into consideration the increase in rainfall intensity that as time continues it is expected that rainfall intensity will increase which will increase rainfall and stormwater flows. This effect has been allowed for in the DSS designs.

Open waterways that are shown on the PSP are not to be piped. They are designed to carry high flood flows. Piping may also compromise drainage and flood protection for the development, and piping does not provide for ecological, social and amenity values.

The waterway corridors have been designed to ensure a resilient waterway system for flood protection, ecological and social functions, and to factor in risks associated with sodic and dispersive soils. Any proposed reduction in the width of these corridors will need to demonstrate no impact to the resiliency of the system.

The waterway reserve has been established using Melbourne Water’s waterway corridor guidelines (PDF), along with numerous background studies such as one that has indicated the need to factor in the risks associated with sodic and dispersive soils.

Not for this PSP. Typically, the required DSS assets are located within the PSP area as they are directly linked to its development. It is generally not deemed equitable or appropriate to move the assets from within the DSS/PSP to areas outside of the urban growth boundary, to the detriment of these downstream properties. Especially when the assets benefit the DSS/PSP properties (i.e. development).

Yes, there will be some drainage outfall works located outside of the DSS/PSP to connect the drainage to existing waterways.

Some areas south of the DSS/PSP already experience flooding, such as in the former swamp areas of the Koo Wee Rup-Longwarry Flood Protection District. The DSS has been designed so that the development within the PSP will not make existing flooding worse in these areas.

Sodic and dispersive soils are types of soils that can be problematic for construction activities. They become unstable when exposed to rainfall and runoff, resulting in extreme erosion risks.

A soil testing study has been completed across the PSP to assist in the concept design of the DSS waterway, wetland and retarding basin assets. The study has identified the constructability issues and potential mitigation activities for developers so they can plan for these risks.