Melbourne Water is the waterway, drainage and floodplain authority for the Port Phillip and Westernport region. We work with local councils and government to ensure that appropriate drainage and water management infrastructure is in place to support increased demand, to protect people and property from flooding, and to protect our waterways.

The Victorian Planning Authority (VPA) prepares PSPs, which are land use and infrastructure plans that guide the development of an area over time. A PSP sets out the preferred locations of residential and employment land and infrastructure, and provides guidance for transport and parking, urban design, heritage and character, open spaces and integrated water management. As part of the VPA’s planning process, Melbourne Water prepares concept drainage strategies for future urban development.

A draft concept drainage strategy is a conceptual design (as opposed to a functional or detailed design) for the required precinct scale drainage assets.

A draft concept drainage strategy:

• is a drainage engineering plan for Melbourne Water’s preferred conceptual drainage option that ensures that the environmental values of waterways are protected and that new developments meet current standards for drainage and flood protection;
• informs the areas of the PSP that are required for waterways and drainage infrastructure; and,
• does not include financial contributions which will be determined at a later stage in the development process.

Once the PSP planning phase has concluded (PSP has been gazetted and incorporated into the Planning Scheme), the concept drainage strategy is finalised into a Development Services Scheme (DSS).

A DSS:

• is a plan of proposed concept drainage infrastructure (including stormwater quality treatment facilities) that is required to service future urban development; and,
• determines what financial contribution developers need to make to Melbourne Water to fund the required drainage and stormwater quality treatment works.

For more information on DSS, view Drainage schemes explained.

The concept drainage strategy is an engineering plan that informs the drainage and waterways land requirements for the PSP. The DSS consists of the concept drainage strategy and the financial contributions required to fund it.

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 and may also provide recreational space for the community while dry.

The draft concept drainage strategy retarding basins in the Croskell Employment PSP have been designed to mitigate post-development 1% AEP (100yr) storm events with Climate Change to pre-development 1% AEP storm events under current climate.

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

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.

The draft concept drainage strategy design takes account Climate Change effects as recommended by AR&R guidelines. This includes progressive increases in rainfall intensity through time, resulting in increased stormwater flows. Melbourne Water adopted 2100 RCP8.5 climate change scenario which translates to an 18.5% increase in rainfall intensity in the year 2100.

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.

A constructed waterway is an artificial (engineered) waterway created to provide an appropriate drainage and flood protection level of service to new developments. It is usually required to modify an existing degraded or undersized waterway or drain to service larger and more regular flows conveyed from an urbanising catchment. Constructed waterways and their reserves provide for asset maintenance, ecological, social and amenity value. A constructed waterway also consists of additional land on either side called the waterway corridor.

Visit the Melbourne Water website more information on constructed waterways.

A sedimentation pond removes pollutants through gravity settling at reduced flow speeds causing particles to settle.

Swales are small earth channels that remove excess water without more sophisticated alternatives such as piped systems. They also lessen flow and remove larger sediments from the water.

Both natural and constructed waterways require waterway corridors. A waterway corridor is the area of land beyond the wet areas that are required 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).

Sodic soils have high sodium content which can swell when wet. In dispersive soils the clay particles can separate when wet and cause the soil to slump. Sodic and dispersive soils can be problematic for construction activities. When these soils are exposed to rainfall and runoff, erosion can occur as the soil becomes unstable. A geotechnical study was completed in August 2023 across the Croskell Employment PSP site to help design the waterway, wetland and retarding basin assets. The study showed traces of dispersive soils and has outlined constructability issues and potential mitigation activities for developers so they can plan for these risks.

The Victorian Desalination Pipeline (VDP) is an underground power cable that runs east-west through the Croskell Employment PSP in its own easement and is co-located within the high-voltage (500kv overhead conductors) electrical transmission easement and existing fibre optic cables. The cable provides power to the desalination plant at Wonthaggi. The concept drainage strategy design considers the strict requirements to ensure that drainage does not impact the integrity, maintenance and performance of the state-specific infrastructure.

The size and number of retarding basins/constructed wetland assets have been designed to optimise their cost and performance as well as being distributed equitably across the PSP. Some have also been designed to temporarily store stormwater to minimise the amount of stormwater crossing existing major infrastructure such as the VDP cable and water supply mains.

The VPA and Melbourne Water intend to use the public consultation period and any Victorian Planning Authority Projects Standing Advisory Committee process to consult regarding whether the Drainage Strategy Concept Design can be optimised and the drainage & land use planning consequences of any optimisations.

Any proposals to alter the size/location of assets will be reviewed against the objectives, requirements and restrictions of the catchment.

The retarding basins size is mainly governed by the stormwater volume that they need to temporarily hold to mitigate the 1% AEP stormwater under climate change scenarios. However, this size may increase due to other technical constraints such as groundwater which limits the depth of retarding basins. Therefore, based on the preliminary groundwater investigations at the site, the draft concept drainage design considers a 20% contingency for groundwater issues. Another constraint is the capacity of the existing culverts at Thompsons Road and Berwick-Cranbourne Road and ensuring the safe conveyance of flows across these culverts.

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 on the resiliency of the system.

The waterway reserve was established using Melbourne Water’s waterway corridor guidelines, along with many background studies such as a study that indicated the need to factor in the risks associated with sodic and dispersive soils.

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

Previous versions of the Ti Tree Creek DSS were designed many years ago. Since then, design standards and policies have changed along with catchment conditions, and a greater understanding of the catchment. The current draft concept drainage strategy for the PSP is based on current design standards and informed by the VPA’s proposed urban structure (which has a different land use mix and in turn different imperviousness outcome to that assumed by the Ti Tree Creek DSS), while considering the risks associated with climate change, sodic soils, and environmental and cultural heritage opportunities.

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.

Not for this PSP. Typically, the required drainage assets are located within the PSP area because the assets are linked to the development of the PSP and will be to the benefit of the PSP properties.

It is not deemed equitable and appropriate to locate retarding basins/constructed wetland assets outside the PSP, to the detriment of downstream properties.

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.

A contribution rate covers the cost of building required drainage infrastructure within the DSS. All funds from the charge are used to pay for drainage works in the scheme area. Land not zoned for urban development will not attract any drainage contributions. Developer contributions only apply to developers of land within a DSS.

There are two main purposes for the developer contribution:

• Provide a guide to the expected cost of implementing sufficient drainage infrastructure for the Ti Treek Creek DSS area.
• Provide a fair way of sharing the costs of drainage infrastructure required for urban development within the Ti Treek Creek DSS.

No, Melbourne Water will not make money from the developer contributions. The developer contributions will fund the works required to implement the necessary drainage infrastructure. Each DSS is designed to be cost neutral over the expected life of the scheme with all contributions received to pay for the cost of drainage infrastructure to service the scheme. The contribution amount is based on the area of the development and the rate, quantity and quality of stormwater run-off.

All water bills across Melbourne include a drainage rate. This funds Melbourne Water’s work to maintain and improve existing main drains and waterways throughout the greater Melbourne area. This rate does not relate to the DSS and developer contributions. It is a necessary rate to ensure that we continue to maintain and enhance Melbourne’s waterways and drainage assets.

Yes. Wherever there are planning scheme amendments or rezoning of land in growth areas, a need is generated for Melbourne Water to develop a DSS. We work with local councils and the state government to ensure that adequate infrastructure is in place to support the community and environment’s needs. Currently there are around 150 active DSSs all over Melbourne.

DSSs exist in most growth areas across Melbourne that are planned for development. The information obtained by Melbourne Water during the development of the DSS identifies whether there may be drainage requirements to be addressed in relation to any development proposed for that area. It enables informed and responsible development to occur. It is unlikely to affect property values as drainage costs are a required consideration for any new areas of development.