Flash Flooding
In urban areas, flash flooding typically occurs when there is intense rainfall over a short period of time, and the underground drainage system capacity is exceeded. Excess water runs downhill - along roads, through gardens, across train lines and streets, and unfortunately, sometimes through houses and shops.
Flash flooding is defined as "...flooding occurring within about six hours of rain, usually the result of intense local rain and characterised by rapid rises in water levels," (Bureau of Meteorology, Weather Services Handbook). In the metropolitan area, it is often storms of two hours or less that cause flash flooding. These large storms are fairly rare, occuring on average every five to ten years. Larger storms that can cause property damage generally occur on average once every 20 to 50 years. But it is possible to have two large storms occurring close together or have very long periods between storms and flash flooding.
Melbourne Water and local councils manage the drainage system. Councils manage the local drainage systems including kerbs and street gutters, and Melbourne Water manages the larger drains as part of its regional drainage functions.
Understanding Flood Risks
Melbourne Water identifies overland flow paths and we work with local councils to include this information in council planning schemes. This enables people considering development to understand that there may be special building requirements to manage the flood risk in this area.
In Melbourne, we work with retail water companies to ensure information about flood risk is available when buying a property. Detailed flood-risk information about individual properties in Melbourne is available on property information statements from retail water companies. Flood level information is also available on this web site by on-line service specialists Anstat and Landata.
When purchasing property, it is recommended that you or your property conveyancer obtain:
- A property information statement from the retail water companies which shows whether your land could be affected by flooding; and
- A planning certificate from the planning authority, usually your local council, or online directly from the Department of Sustainability and Environment to see if planning scheme controls apply.
What you can do to minimise the impact of flash flooding
We can’t prevent flash flooding but as a home or business owner, you can minimise the impact of flooding on your property by following these basic steps:
- Keep the local drainage system clear of litter and debris to avoid blocked drains that can cause flooding. Even if the debris does not block your own drains, it could be swept downhill and cause flooding elsewhere
- Apply for a planning permit for any redevelopment or new building works if your property has been defined as flood prone. In some cases, changes may be required, such as raising floor levels, or reorientating new buildings and driveways in a manner that does not impede flows
- Avoid home improvement practices that exacerbate flooding risks. For example, badly graded paving can direct water into houses. Consider grading paved areas such as paths and patios, as well as other landscaping, so they direct flows around or away from houses or garages rather than straight to the doorway
- Consider that electricity and floodwater are a potentially hazardous mix and talk to a licensed electrician about safety switches or raising power points and switches to higher levels
- Store valuable possessions in a place where they won’t get damaged. This may involve using shelves in garages and not storing valuables under houses where they may be inundated
- Commercial premises may need to consider an individual flood management plan and consider evacuation procedures, possible flood proofing measures and a recovery plan.
You should also place emergency telephone numbers by the phone and insure your home or business against flood damage. For more information contact the Insurance Council of Australia.