Flood Studies

A Flood Study provides technical information on the likelihood and characteristics of flooding within a catchment. It provides detailed information 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 and identifies and prioritises options to help protect people and property through better planning, emergency management and infrastructure works.  

A 1 in 100 year flood is a flood event that has the probability of occurring on average once every 100 years, i.e. there is a 1% chance of a flood of 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. Nor, if it has not flooded for 99 years that it will necessarily 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 the Floodplain Risk Management Study and Plan the ‘1 in 100 year’ flood is referred to as the 1% AEP (Annual Exceedance Probability) flood event.

The Flood Planning Area (FPA) is the area within which developments may be conditioned with flood related development controls. The FPA is calculated as the area below the Flood Planning Level.

The Flood Planning Level 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’ below). This level may be higher for vulnerable land uses (e.g. hospitals or schools).

A freeboard is a height above the 1% AEP flood level that is included in the FPL to account for factors such as wind, waves, unforeseen blockages, other localised hydraulic effects. Freeboard is usually 0.5m above a flood level.

The original Flood Study included a preliminary assessment of the impacts of sea level rise and changes in rainfall. The Floodplain Risk Management Study (linked below) will assess the impacts of climate change on flooding in more detail.

Council’s flood modelling determines the extent of flooding throughout the catchment and the identification of properties partially or fully impacted by flooding.

Residents may contact Council for further information. 

Council frequently publishes the adopted findings of its completed flood study and floodplain risk management study and plan projects can viewed on our Managing Floodplains webpage.

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) on 132 500 or visit their website at www.ses.nsw.gov.au

It is best to be prepared for any flood. The State Emergency Service provides advice on how to manage your flood risk at floodsafe.com.au

NSW Councils have responsibility for identifying and managing flooding, and informing the community. Councils follow the NSW Flood Prone Land Policy, which outlines how Councils should manage flooding to reduce the risk to people and properties.

Councils prepare Flood Studies and Plans according to the NSW Government's Floodplain Development Manual (2005), and implement associated recommendations with the technical and financial assistance of NSW Government and key stakeholders through the Flood Prone Land Policy.

There are three main ways of managing flood risk:

• emergency management
• property modification
• flood modification

Emergency management measures seek to help people at risk during a flood. This includes sharing information with the State Emergency Service, flood warning systems, increasing community awareness of flooding, and identifying evacuation routes and locations of emergency refuge centres.  

Property modification measures consider the way in which land is used, the buildings that are on it and the extent of flooding on a property. These measures are typically addressed in the Local Environmental Plans and Development Control Plans.

Flood modification measures change the behaviour of flood waters with structural methods. This includes levees, creek management, estuary entrance management and improved stormwater infrastructure. 

The information from the adopted study outcomes will be used to inform future floodplain risk management activities to manage the identified flood risk. Some of the study outputs and/or future activities will better inform land-use planning outcomes, emergency response and strategic asset planning. Where a house may be in an identified flood zone, certain land-use planning or development controls may apply. The relevant land-use planning or development controls are typically noted on a section 10.7(2) certificate that list the relevant controls for a parcel of land. The study outcomes will be used to inform and review existing section 10.7(2) certificates, emergency response and strategic asset plans for the relevant catchment area.

There is a chance that floods of any size will occur in the future. As the size of a flood increases, the chances of it occurring becomes smaller. Because some rare types of floods have not occurred for over a century, the height of future floods is predicted using computer-based models. These models simulate different flood levels, speeds and direction for different sized floods.

Given the importance of accurately predicting flood levels and information, Council engages experts to establish and operate these computer-based models.

From time to time, these models are reviewed, and predicted flood levels may change slightly. The reason why the models are revised can include:

• New floods occurring, providing additional data to fine-tune the model
• Flood mitigation works undertaken may change flood levels
• More advanced computer-based models becoming available
• Development within the floodplain (which may be outside our control)
• Changes in technical guidelines and requirements
• Changes in State or Local policies.

For the Wolumla region, no models have been developed. The development of computer-based models to predict flood behaviour is a key outcome of this study.