Frequently asked questions
Councils are required to prepare flood studies in line with the Flood Risk Management Manual. This manual outlines how councils should manage flooding to reduce the risk to people and properties.
On this page, you'll find answers to questions about different aspects and terminology used in a flood study.
Flood events have different definitions based on their size. Some of these definitions are more commonly used than others, for example the '1 in 100 year flood', or design flood.
Some types of floods have not occurred for over a century, so flood levels must be predicted using computer models. These models simulate different flood levels and velocities, overlaid over a terrain model for a variety of different sized floods. This allows us to predict what these larger floods will look like, so we can plan. Emergency services are also in a better position to prepare for these types of flood events.
We have an obligation to maintain current flood information, which means updating flood studies periodically. Reasons for models to be revised include:
- new floods occur, providing extra data to fine-tune the model
- new flood mitigation works carried out may reduce flood levels
- more advanced modelling technology becoming available
- development within the floodplain could change conditions.
A '1 in 100-year flood' is a flood event that has the probability of occurring on average once every 100 years. This means there is a 1% chance of a flood this size occurring at a particular location in any given year. This does not mean that if a location floods one year that it will not flood for the next 99 years, or if it hasn't flooded for 99 years, that it will flood the next year.
Some parts of Australia have experienced more than one ‘1 in 100-year’ floods within a decade of each other.
Within a floodplain risk management study and plan the ‘1 in 100-year’ flood is called the 1% Annual Exceedance Probability (AEP) flood event.
The flood planning area refers to land where development controls are applied to minimise impacts of flooding, for example, a floor level.
The Flood Planning Level (FPL) is a height used to set floor levels for property development in flood prone areas. It is generally defined as the 1% AEP flood level plus an appropriate freeboard (see explanation of ‘freeboard’ on this page). This level may be higher for vulnerable land uses (eg, hospitals or schools).
A freeboard is a height above the 1% AEP flood level that is included in the flood planning level to account for factors such as:
- unforeseen blockages
- other localised hydraulic effects.
Freeboard is usually 0.5m above a flood level.
Council’s flood modelling determines the extent of flooding throughout the catchment and identifies properties partially or fully impacted by flooding.
A flood study provides technical information on the likelihood and characteristics of flooding within a catchment. It provides details about the nature of flood risk including the distribution, extent, depths, levels and velocity of floodwaters for a range of storm events, including a 100-year flood.
A floodplain risk management study and plan analyses flood behaviour. It identifies and prioritises options to help protect people and property through better planning, emergency management and infrastructure works.
If the situation is life threatening you should call 000.
For other assistance during an emergency such as flood, storm or tsunami, please contact the NSW State Emergency Service (SES):
- T: on 132 500
- W: NSW SES website
The State Emergency Service provides advice on how to manage your flood risk on their floodsafe web page.