The FRMS&P found that for a one-in-one-hundred-year flood or a 1 % AEP flood (a flood with a 1% probability of occurring in any given year).

• Approx 7000 lots affected by 1% AEP
• Approx 18,500 lots affected by PMF

As stipulated by Council’s Development Control Plan, as these properties are located in the Flood Planning Area they are identified as Flood Control Lots (FCL) and as such would be subject to special development controls.

What has Council done to minimise the impact?

NSW Flood Prone Land Policy requires Council to be responsible for managing flood prone land within the Shire. To do this, we have produced this plan for Hornsby.

The plan looks at ways to reduce flood damage by suggesting preventive measures and responses.

The plan highlights our priority areas for drainage improvement and outlines key areas for drainage upgrades. Council has already started work and will continue to roll out improvements over the coming years.

The plan also enables Council to source NSW government support to implement recommended solutions within the plan.

A Flood Control Lot (FCL) in Hornsby Shire is a property where flood models show that the property is lower than the level of a 1% AEP flood. If your property has been identified as a Flood Control Lot (FCL), your property is either partially or fully below the 1% AEP flood level in the flood model.

The overland flow paths identified in the FRMSP mapping will not usually change with more accurate ground surveys. Detailed surveys can provide a clearer picture of the flow path, including flow depths and accurate water surface levels.

However, a review is unlikely to change the flood notation. The flood affectation can change if there is change in the NSW State policy or methodology of the flood map or if there is significant change in the landform due to development activities. Council will notify you if such change occurs and your property’s flood affectation is altered.

If you still believe you have more current land survey information that can potentially change your properties flood notation, please provide the information in the comment box on the main page.

We encourage you to read all the information on the Councils Your Say page including the Draft Flood Risk Management Study & Plan (FRMS&P) document.

If you still think your issue is unique and not addressed within our responses, please submit your specific issue in the comment box on the main page for Council to review. You may be invited for a one-to-one meeting with a relevant Council officer to discuss further.

Please note one-to-one meetings will only be offered if your issue is unique and not addressed by any information Council has already provided.

It's unlikely that development on all the properties identified as Flood Control Lot (FCL) are prohibited. But the proposed development may need to meet specific conditions to keep people and property safe.

For instance, you might have to build on a specific area of your land, raise your building to a certain height, or avoid building where flood risk is high.

Please check the specific Hornsby Development Controls page for more details.

Council can help you understand these considerations if you are planning to develop a land identified as Flood Control Lot (FCL).

Annual Exceedance Probability (AEP) flood event means there is a chance of a flood event of a certain size or larger happening in any one year. For example, a 1% AEP means there's a 1 in 100 chance it could occur in any given year.

This is the largest flood event that could conceivably happen in a certain area at any time. It’s estimated using the most extreme rainfall that could occur, along with the worst weather conditions that make flooding more likely.

Overland Flooding is when the water has not made it to a creek or river and is just barrelling across the landscape.

Flood Frequency is how often a flood might occur based on how many times similar floods have happened in the past.

The Flood Control Level (FCL) is determined by combining the flood level from the Design Flood Event with a safety margin known as 'freeboard'.

Freeboard is an additional height added to account for uncertainties in flood modelling and risk. The NSW Government's Flood Risk Management Manual (2023) recommends adding up to 500mm of freeboard to the 1% Annual Exceedance Probability (AEP) flood level. However, Hornsby Council does not apply any freeboard, meaning the Council’s FCL is set at the 1% AEP flood level.

Design Flood Event

Design flood event is a particular flood event considered for the design of housing or infrastructure to make it flood proof. For the purpose of land use planning or setting Flood Control Lot (FCL) a Design Flood Event is a 1% AEP event.

Riverine flooding is when regular water courses like creeks and rivers rise and even break their banks.