Flood modelling and risk assessment

Flood hazard relates to the nature or behaviour of flood water, including how fast floodwater flows, how deep the water is and how far out across the land it spreads.

Flood risk is the likelihood of a flood occurring, combined with the potential consequences of flooding for people, property, infrastructure and the environment.

This map is the newest city-wide flood risk map based on our latest modelling. It considers how deep and how fast flood waters could be in different flood scenarios and includes the outer edge of all the city’s floodplains. It captures the probable maximum flood (PMF), which is the worst possible flood that could conceivably occur. The PMF is extremely rare – with a likelihood of 1 in 100,000 or 200,000.

The Flood risk awareness map highlights flood risks and is an important tool that helps property owners to understand their risk. Depending on the situation, the map may prompt property owners to consider a property's resilience (for example, the floor level), review flood insurance policies, and follow the Get Ready steps to reduce potential future flood impacts.

Although it is available for flood awareness, this map is being reviewed by the State Government as part of the proposed City Plan major amendment – Designing for flood. The map may include land you are considering buying, building on or developing in the future.

Important note: The flood risk awareness map is currently non-statutory – it is for information purposes only and does not have any legal or planning obligations at present. We suggest speaking with your builder, engineer or surveyor about how site-specific circumstances may be impacted by the level of risk shown on the map.

The risk categories in our flood risk map do not equate with statistical or design flood events.

Instead, they illustrate flood risk based on the hydraulic risk matrix approach originally used in the Brisbane River Strategic Floodplain Management Plan.

In the past, developments in floodplains were considered against only the extent and the hazard associated with a single statistical design event. However, it is now required that a much broader understanding of flood risk be applied. The Gold Coast risk mapping was developed by considering 6 statistical or design flood events, together with the depth, velocity and hazard associated with each of those events.

The map also includes the outer extent of the probable maximum flood (PMF), generally considered the worst possible conceivable flood. The consideration of extent, hazard and likelihood of multiple events means the flood risk mapping now provides a far more nuanced and detailed spatial understanding of flood risk.

Most flood studies now consider climate change factors.

Most Councils have a defined flood level map, based on a 1 in 100 (1% AEP) chance, including climate change factors such as sea level rise, increased storm tide and rainfall intensity. This is required by the Queensland Government and recommended by the Intergovernmental Panel on Climate Change.

The flood risk map currently doesn't include overland flow path mapping. We plan to make it available for the community as soon as possible.

What is overland flow?

Overland flow describes the movement of water running over the ground after rainfall, before it enters a waterway (for example, a creek, river, canal or lake). In urban areas, overland flows may also occur when the underground drainage system is full, causing stormwaters to rise to the surface and spill out of gutters and drains over the land.

Protecting overland flow paths is crucial for the city’s drainage performance, because local flooding can worsen if overland flow paths have changed. Stormwater or overland flows differ from the flood flows within regional or riverine systems (like rivers and creeks).

Overland flow paths can often be identified by looking at the topography of the land and nearby drainage features like waterways and gullies, as stormwater naturally flows to the lowest point.

The flood insurance map does not include climate change factors because although we build and plan for the future, we only insure for the next 12 months. The flood insurance map shows current risk and may be useful for discussing the appropriate level of insurance for your home or business with an insurer.

We regularly update flood maps and flood information so the community has access to the most up to date, detailed flood risk information. This helps residents make informed decisions and strengthens the city’s resilience to a variety of floods.

Updating flood maps and flood information to align with state planning policy requires that local governments integrate the state interests for natural hazards, risk and resilience into planning schemes to identify and mitigate the risks associated with flooding.

We are preparing for future challenges, including increased rainfall intensity and permanent sea level rises. Updating flood maps and flood information helps us respond to flooding, the effects of climate change and extreme weather in the future and build resilience to a variety of flood events.

Yes, we will undertake future flood studies to ensure the latest and best information is used to predict and manage flooding.

Changing weather patterns suggest that the characteristics of floods will change in the future. As a result, flood studies will need to be updated regularly as new information and improved technologies become available. This will help keep floodplain management plans current and improve our understanding of how floods behave under different conditions.

Council develops flood models to show the most accurate representation of flood risk, using the latest and best information available. However, new software and data may improve the ability to predict and manage flooding in the future.

The defined flood level is the level to which it is reasonably expected flood waters may rise.

Our City Plan outlines the defined flood level as the minimum flood annual exceedance probability (AEP) for different types of land use.

The defined flood level advises the level of flood resilience we require for new buildings. It considers factors like flood hazard, consequence, likelihood and residual risk. To determine the defined flood level, we use flood models and the Defined (Statistical) Flood Event.

We require a minimum area of resilient land, finished floor levels and flood planning levels to be constructed above the defined flood level for buildings constructed in areas vulnerable to flooding.

By setting floor levels for residential and other buildings based on the defined flood level we can ensure that property is resilient against a variety of future floods.

Designated flood level is an outdated term that was used in the City Plan when it commenced in 2016.

Defined flood level is the updated term, that will be used when the Designing for Flood major amendment is adopted.

Both terms mean the same thing, but we will update our terminology to align with Queensland state planning policy and the Queensland Building Act 1975.

The flood event that determines the defined flood level (DFL) is called the defined flood event (DFE). This is the event that we consider offers reasonable protection to people and property from flood risk, while still allowing development.

Most residential buildings need to be built above the DFL set by Council.

The flood level search report provides reliable, engineer-verified, flood information for a property. It’s especially important to obtain one of these if you’re planning to build or renovate and the property is in the defined flood event map (that is, subject to a defined flood level).

The reports include the defined flood level but do not show the flood extent or flood risk category for a property – this information can be seen on our Flood modelling and risk assessment page.

There are several ways to determine the current finished floor level of a building. The most reliable way is to engage a licenced surveyor to survey the property. If the building was built recently, you can alternately request a building search request from us to get a copy of the building approval that includes the finished floor level. Visit our Buildings approvals search page.

Like many coastal and inland cities, the Gold Coast is prone to flooding. Managing flood risks is crucial for our city's long-term sustainability. Due to past settlement patterns, this includes managing both existing and new development in flood-prone areas.

Our City Plan guides how we develop land considering identified flood risks. The City Plan contains a Flood overlay code and map. Any development or building within the Flood overlay map is assessed against the provisions in the Flood overlay code. This might require that a building’s finished floor level be established at a height above the defined flood level.

As technology and the availability of data improves, Councils can better model and map flood likelihood to understand the potential risks to people and property. After completing a city-wide flood risk assessment, we proposed some changes to the City Plan, including updates to the Flood overlay code and map, to ensure we can continue to maintain the city’s existing resilience of people and property to a range of floods into the future.

Development regulation and land use planning is just one part of flood management on the Gold Coast. We also seek to avoid and mitigate flood risk through community awareness and preparedness, infrastructure works, asset upgrades and disaster management measures like flood warnings, improving identification of flood evacuation routes, planning for evacuations, emergency responses and education programs to help communities deal with the residual risk of floods.

Ultimately, in locations that offer lifestyle appeal, state governments and councils need to balance urban growth and housing supply with acceptable flood risk exposure. It is not practical to avoid development in floodplains altogether, as flood fringe areas are only at risk of flooding in very rare events. Development in lower flood risk areas is possible if flood risk measures are adopted as per the City Plan requirements.

The proposed Designing for Flood amendment to the City Plan (including the new flood overlay map and code) has been submitted to the State Government for review. After the State review is complete, public consultation will begin.

Community feedback is then considered and changes made based on submissions. If the changes are significant, further consultation may be needed.

Due to this lengthy process, the proposed amendment may not take effect until late 2026.

Timeline:

State interest review phase:

• We have prepared a response to minor changes requested by the State Government
• Council to endorse the proposed response
• The State Government may require further changes before public consultation
• Approval to proceed to public consultation

Public consultation phase:

• Undertake consultation
• Process submissions, draft responses and determine any necessary changes, then report to Council
• If significant changes are needed, undertake further consultation
• Prepare consultation report and revised amendment (if required)
• Report to Council and seek approval to submit the amendment to the Minister for final approval

Ministerial approval phase:

• Submit to State Government for final review and request approval to adopt
• There may be further information requests and/or conditions which could require Council endorsement of the response

Adoption and commencement:

• Revise the package if required by State conditions
• Report to Council with the final amendment package for adoption and set a date for commencement as part of a new City Plan version
• Commence new City Plan version - supported by internal training and materials and external communications

More information on the Designing for Flood amendment can be found on our City Plan amendments page.

Local planning schemes regulate new development in the community. The best way to influence future development in your area is to get involved when a local planning scheme is being prepared or changed.

As part of the process to amend a planning scheme and in line with legislation, property owners whose land or premises is affected by a proposed planning change will receive a written and personally addressed notification.

An affected property is one that has any part of its land within the current or proposed City Plan flood overlay map.

The statutory consultation phase will start after the State interest review concludes. The proposed amendment package will be available to the public for viewing for at least 20 business days. During that time, submissions can be made on our GC Have Your Say page. Anyone can formally submit comments on the proposed amendment.

This process allows the community to share feedback, support, concerns or suggestions with us. Once submissions have been reviewed, a consultation report will be publicly shared, explaining how each submission has been addressed and any changes made to the proposed amendment.

To receive updates about planning changes and be informed about our planning and decision making, join the City Panel and have your say on current consultations and new ideas.

You can also subscribe to the Gold Coast Weekly newsletter or follow the City of Gold Coast on social media to stay connected.

As part of the process to amend a planning scheme and in line with legislation, property owners whose land or premises are affected by the proposed planning change will receive a written and personally addressed notification.

An affected property is one that has any part of its land within the current or proposed City Plan flood overlay map.

The Minister’s Guidelines and Rules set the minimum period for the public consultation for a major amendment at 20 days.

The proposed amendment package will be available to the public for viewing at that time and submissions can be made at GC Have Your Say. Anyone can formally submit comments on the proposed amendment.

This statutory consultation phase will start after the State interest review concludes.

This process allows the community to share feedback, support, concerns or suggestions with us for consideration. Once all submissions have been reviewed, a consultation report will be publicly shared, explaining how each submission has been addressed and any changes made to the proposed amendment.

Local planning schemes regulate new development in the community. The best way to influence future development in your area is to get involved when a local planning scheme is being prepared or changed.

For updates about planning changes and our planning and decision making, join the City Panel and have your say on current consultations and new ideas.

You can also subscribe to the Gold Coast Weekly newsletter or follow the City of Gold Coast on social media to stay connected.