Explainer: What is a 1 in 100-year flood event?

A ‘1 in 100-year flood’ means that there is a 1 per cent chance each year that water will rise above a certain level.  Sometimes, it is technically described as a flood with a 1 per cent annual exceedance probability (AEP). It doesn’t mean the flood will only happen once every 100 years; in fact, several floods of this kind can occur in a short period. This standard helps planners, engineers, insurers, emergency managers, and the community understand and compare flood risks.

Why the term matters for Melbourne Water and our work

Using AEP standards helps with land-use planning, flood maps, designing infrastructure, and creating flood warnings and emergency plans. Mapping areas that could flood at a 1 per cent chance level highlights places likely to be affected. These maps guide building regulations, set height requirements for important structures, help with flood prevention projects, and prepare the community. By providing clear information, we reduce misunderstandings and support better choices.

Melbourne Water reviews flood maps every five years and makes complete updates every ten years. We also re-evaluate maps after significant flood events. This thorough method ensures the maps are based on the best data and any recent changes to the environment or climate.

Recent examples in Victoria

• The major floods in Greater Melbourne, especially in Maribyrnong, in 2022 were classified as a 1 per cent AEP event, showing how large storms can cause severe flooding in both urban and regional areas. 
• In 2022, another significant flood caused the Yarra River at Heidelberg to breach its banks, inundating Warringal Park and surrounding low-lying areas in Melbourne’s northeastern suburbs. This event matched the intensity of a 10 per cent AEP event.
• In 2016, a flood at Elster Creek near Head Street in Elsternwick was estimated to be a 3-5 per cent AEP event, which means it had a chance of occurring every 20-30 years.

These examples show that 1 per cent AEP floods are real and can cause serious disruptions, highlighting the need for current flood maps and preparedness. Understanding what a 1 in 100-year flood means can help the community get ready for potential risks and lessen the impact of such events.

The science behind the 1 per cent AEP flood

Experts use historical data on streamflow and rainfall to determine what a 1 per cent AEP flood could look like. They also use models to simulate how water flows through rivers and over floodplains. By studying the land’s shape and current usage, they can determine how water moves and where hazards may be.

Plus, to ensure the maps are as accurate as possible, they validate their models by comparing with past flood marks and information from the community. It’s all about building confidence in these maps while acknowledging the uncertainties.

How Melbourne Water creates new flood maps

Melbourne Water has developed Australia’s first city-wide flood mapping program called the Greater Melbourne Flood Information Program. This program replaces over 1,000 old local models with 250 new, consistent regional simulations so that people can easily access flood information for their property. The updated flood maps have been created according to the latest Australian Rainfall and Runoff Guidelines for consistency.

What the map shows and what it does not

The 1 per cent AEP flood map displays areas that might flood and typical water levels for that risk. It serves as a tool for planning, designing, and preparing. However, the map does not:

• Predict when the next flood will happen
• Guarantee that every property will flood every time
• Account for local issues like temporary blockages, debris, or sudden changes in the landscape.

Maps are the best scientific estimates available at the time and should be used together with local knowledge and official warnings.

Practical actions for households and planners

• Households: check whether your property is in the 1 per cent AEP area, create a flood plan, store important items at safe heights, and follow local warnings.
• Planners and asset owners: use the map to set finished-floor levels, design drainage and transport assets, and prioritise risk reduction projects.
• Emergency managers: integrate the maps into evacuation routes, response plans and community communications.

Mapping and modelling flood risk takes careful work. Melbourne Water’s ongoing efforts provide a solid foundation for reducing flood risks across Greater Melbourne, making our communities safer and better prepared.