Flood Management
In this section:
- An introduction to floods
- Flash floods – what are they?
- Flood and Climate Change
- Monitoring Melbourne’s drainage system
- Flood warning system
- Flood risks
- Retarding basins
- Preparing for floods
An introduction to floods
Storms and floods, like drought, are a normal and inevitable part of climate variability that must be managed. We cannot always control floods. Therefore, we must learn how we can live with them while minimising risks to public health and safety, property and infrastructure.
Melbourne’s drainage system copes well with most storms experienced across Melbourne. However, from time to time, storms can occur of such intensity that not all the stormwater can get into the drains at once and floods occur.
The Port Phillip and Westernport region is known to have more than 100,000 properties that are at risk from floods, of which 82,000 properties are at risk of flooding from overland flows. More than 40,000 of these properties contain buildings that are at risk of flooding above floor level. The average damage caused by flooding each year is estimated at $245 million.
Flash floods – what are they?
Flash floods happen when a large amount of rain falls in a localised area over a very short period of time. These localised storms are generally beyond the capacity of the underground drainage system, so excess water runs along the low points of the area which can include roads, gardens, train lines and streets, and sometimes through homes and businesses.
In urban areas, even storms of two hours or less can cause flash floods. These storms are fairly rare, occurring on average every five to ten years. Larger storms that can cause property damage generally occur on average once every 20 to 50 years. Because these events are so rare it would not be economically feasible to design drains to cater for these very large, but infrequent storms.
Also see:
- Melbourne Water’s animated Floods Explorer. This animated tool shows you what happens when it rains in urban areas, how houses and parks are affected, and the consequences of blocked gutters and stormwater pollution.
- Flood risks
- Flood level information
Flood and Climate Change
Climate change projections suggest we can expect more extreme weather events, including heavy storms that deliver a large amount of rain in a short period (2-24 hours). This is despite the fact that annual rainfall volumes are expected to decrease.
A 2010 paper co-authored by Melbourne Water evaluated the impact of these changing rainfall patterns that show most areas of Melbourne could expect an increase in rainfall intensity (as distinct from more rainfall overall) of up to 30% by 2030 under higher emission scenarios.
Melbourne already has a massive network of stormwater drains, retarding basins and wetlands that help reduce the risk of flash flooding. If climate change is likely to mean more extreme storms but less rain overall, developing ways to capture and reuse stormwater (e.g. rainwater tanks) is becoming more important. This helps protect against flash flooding when it’s wet but also reduces demand on our dams for non-drinking purposes when it’s dry.
More information:
- Paper: Impacts of Climate Change on Urban Flooding in the Melbourne Area (2010) (PDF, 443kb)
- What we’re doing: Flood Management and Drainage Strategy
Monitoring Melbourne’s drainage system
We have a program in place to regularly monitor the condition of Melbourne’s drainage system to ensure it remains in good condition and continues to provide essential flood protection.
Around $17 million a year is invested in drainage inspections and maintenance of drains. In recent years we have made significant progress in upgrading Melbourne’s drainage system.
After floods, we inspect our drains to ensure they were working to full capacity and to see what impact these extreme storms have on our drainage system.
Climate change may also mean we may need to do things differently, especially in coastal and flood-prone areas.
More information:
Flood warning system
We operate an on-line, 24-hour a day rainfall and river level monitoring network.
The Bureau of Meteorology is responsible for issuing flood warnings throughout Australia. We provide the Bureau with predicted times and areas of flood, so that formal warnings can be issued to responsible agencies and the community.
Our flood warning system relates to the following rivers and creeks:
- Yarra River
- Maribyrnong River
- Bunyip River
- Dandenong Creek
- Diamond Creek
- Plenty River
- Kororoit Creek
- Merri Creek
Early predictions and warnings can be made for these rivers and creeks because of their size. Early warning allows an effective emergency response. Locally determined flood levels are set to classify flows as minor, moderate or major. Flood effects above each height reflect the category of flooding. Councils that have flooding issues are working with us to develop and review their Municipal Emergency Management Plans so that information from flood studies is included.
More information:
Downloads
You will need Adobe Acrobat to access the above PDF documents. 