Drainage Questions and Answers
What is Melbourne Water's role in urban development?
Melbourne Water is the drainage authority for the Port Phillip and Westernport region and the Flood Plain Management Authority by delegation from the Minister responsible for the Water Act.
Melbourne Water is a referral authority in the planning system and receives applications for urban subdivisions and other developments from local councils. This allows Melbourne Water to comment on applications and, if necessary, place conditions on town planning permits. These ensure that the drainage system continues to function properly and any new developments are adequately designed to protect occupants.
To undertake these roles Melbourne Water collects drainage information and does flood studies to identify areas associated with the assets and waterways that we manage that may flood following storms.
Who else is involved in drainage issues in urban development?
There are individual, local and regional drainage issues and impacts that need to be considered. Melbourne Water, the Department of Sustainability and Environment, local councils and the development industry are all involved in identifying and addressing issues with respect to drainage and floodplain management.
What is Melbourne Water's role in planning schemes?
Melbourne Water is a town planning referral authority under the Planning and Environment Act 1989 and its associated planning schemes. Melbourne Water is also a referral authority under the Subdivisions Act 1988.
Melbourne Water works with local councils, who are the responsible authorities for most planning schemes, to provide the drainage information for inclusion in these schemes. Planning schemes are presented as maps and the drainage information is shown either as an overlay on a map, or as a separate zoning such as an Urban Floodway Zone.
What is an overlay?
An overlay is a map in a planning scheme that shows the location and extent of a particular development control to provide for things such as natural vegetation, heritage precincts or an overland flow path. Supporting the overlay is a set of provisions that indicate what needs to be considered before development such as buildings and works are permitted to occur in that area.
Overlays and other planning controls are available to be accessed on the internet at the Department of Sustainability and Environment’s website : Planning Schemes Online.
What information has Melbourne Water provided to local councils?
Melbourne Water continues to provide available information to local councils, which is then incorporated into council planning schemes via an amendment process as either a Land Subject to Inundation Overlay for river and creek flooding or a Special Building Overlay for overland flow. The recent inclusion of Special Building Overlays has been a substantial undertaking involving over 30 planning schemes to date, requiring the individual notification of more than100,000 affected landowners and occupiers.
What is the Special Building Overlay?
This overlay shows the extent of the overland flow paths associated with the main drainage system when run-off from heavy rainfall exceeds the capacity of the underground piped drain. As the name suggests, this overlay serves to indicate that there may be special building requirements in this area such as the need to elevate floor levels or ensure additional buildings do not block valleys.
What is an overland flow path?
An overland flow path is an above-ground component of the drainage system. Overland flows occur when underground drainage pipes reach their capacity and cannot cope with more run-off from heavy rainfall. The excess run-off then travels overland, following low-lying, natural drainage paths.
Why has a 1-in-100 year drainage standard been adopted?
This standard has been adopted as it is considered an appropriate balance between the likelihood of occurrence and the consequences of flooding for most developments. In some special cases higher standards may be applied. For example proposed essential services like hospitals, which need to function continuously during emergencies, may need to be protected from 500 year events.
It is a commonly used international drainage standard, and has been applied in Melbourne since the late 1970s. The standard, correctly referred to as the 100-year Average Recurrence Interval (ARI) is a statistical estimate of the average period (in years) between the occurrence of floods of this magnitude or larger. It is also sometimes expressed as a probability, such as "there is a 1% chance in any one year of such an event occurring".
Melbourne Water uses the 100-year ARI standard to define the extent of the overland flow along the drainage system or along rivers or watercourses, to quote flood levels for example in flood warnings and to assess development applications.
Has placing this information in the planning scheme affected property values?
Property values are determined by the market, taking into account a whole range of factors. The information obtained by Melbourne Water through its investigations is used to identify whether there may be special building requirements to be addressed in relation to any development proposed for that property. The information enables informed and responsible development to occur.
It is important to note that identifying the drainage information has not changed the likelihood of a severe storm occurring or the potential of the drainage system to cope with a severe storm.
Will this information stop redevelopment of my property?
Generally not. It will only be used to ensure than any redevelopment is planned in a way that recognises the location of the overland flow path and any public health and safety risks.
Why is this information being provided? What are the benefits?
The drainage information identified by Melbourne Water is used to provide a better planning framework. This will lead to greater awareness of the impact of severe storms upon people and property and, therefore, to better planning and development decisions to minimise these impacts.
Why does this information have to be included in the planning scheme?
By including the information in the planning scheme, owners considering development have immediate access to necessary information about the drainage of a property very early in the development process. Information will also appear on planning certificates.
Why do we need information on the drainage system?
Information on the drainage system is required to enable Melbourne Water to provide local councils with technical advice to ensure responsible decisions are made regarding urban development. This information is increasingly required as redevelopment in existing areas increases to accommodate population growth through dual occupancy and unit/town house development in the suburbs.
Is Melbourne's drainage system adequate?
Melbourne Water has a program in place to regularly monitor the condition of the drainage system to ensure it remains in good condition. We invest $16 million a year on inspections, maintenance and drainage upgrade projects, which are prioritised following surveys and inspections.
However, from time to time, storms occur of such intensity or duration that the stormwater run-off exceeds the capacity of the drainage system, causing overland flows. It would cost the community billions of dollars to upgrade the existing drainage system to cater for these very large but infrequent storms, as well as causing significant disruption and inconvenience. The most practical and equitable solution is to ensure redevelopment in existing areas addresses the overland flow issues. Today, new developments and redevelopments incorporate stormwater management measures that provide greater flood protection. However, when there are recurring local problems with the drainage system, Melbourne Water has a capital works program in place to address these local issues.
Can Melbourne Water upgrade the drainage system to meet the needs of new development?
New subdivisions - Melbourne Water works with developers to design appropriate drainage systems for new subdivisions. This involves an integral approach to drainage to protect housing from flooding during intense rainfall and can include works to improve the quality of stormwater run-off.
Often the design includes open space areas, wetlands and recreational facilities such as bicycle paths. Developers make a contribution towards the cost of the system based on the area being developed.
Redevelopment in existing urban areas - In key redevelopment areas around Melbourne, Melbourne Water is working to establish drainage strategies (redevelopment schemes) to ensure that new development does not affect existing drainage standards.