“the review found that the inclusion of very rare and extreme flood events—such as the PMF—in flood mapping was more likely to cause concern than assist most property owners.“
Understand the PMF and What It Means for Property Owners
Recent changes to Logan City Council’s flood mapping have attracted significant community attention after many property owners discovered their land had been identified as being affected by the Probable Maximum Flood (PMF). For many residents, this unfamiliar term raised understandable questions about what it meant for future development, flood planning and whether their property had suddenly become more flood-prone. During consultation on the Draft Logan Plan, Council received thousands of submissions relating to flood mapping, prompting an independent technical review of the Logan and Albert Rivers Flood Study and the way flood information was presented within the planning framework.
The independent review found that the flood study was generally developed in accordance with accepted national standards and industry practice. Rather than questioning the accuracy of the flood modelling itself, the review found that the inclusion of very rare and extreme flood events—such as the PMF—in flood mapping was more likely to cause concern than assist most property owners. The PMF represents the largest flood considered physically possible under the most extreme conditions and is primarily used for emergency management, evacuation planning and the design of critical infrastructure. Following the review, Logan City Council endorsed changes that will remove flood events rarer than the 1% Annual Exceedance Probability (AEP), including the PMF, from key planning and flood awareness mapping, while continuing to retain this information for emergency management purposes.
For homeowners, these changes should provide greater clarity when interpreting flood information and may reduce planning constraints where properties were affected only by these extremely rare flood scenarios. Importantly, the changes do not mean that flood risk has disappeared or that properties previously affected by flooding are no longer at risk. Flood planning controls will continue to apply where appropriate, based on more commonly accepted design flood events such as the 1% AEP (commonly referred to as the “1 in 100 year” flood), while maintaining a strong focus on community safety and flood resilience.
Commercial property owners, developers and design professionals should also be aware that detailed flood assessments remain an essential part of the planning and development process. Whether undertaking a new development, major extension, subdivision or earthworks, hydraulic and stormwater assessments continue to play a critical role in demonstrating compliance with Council requirements and ensuring development is designed to appropriately manage flood risk. The recent changes are intended to improve how flood information is communicated and applied, rather than reduce the importance of sound flood engineering.
Ultimately, flood mapping is one of many tools used to understand and manage flood risk. Different flood scenarios serve different purposes, from guiding everyday planning decisions to assisting emergency services in preparing for the most extreme events imaginable. Understanding which flood scenario applies—and why—is essential to making informed decisions about land use, development and protecting people and property. The recent review reinforces that clear communication is just as important as accurate flood modelling in helping communities better understand their flood risk.
