How Accurate Do Flood Maps Need to Be Useful?

There is a continuing push to flood map all of Australia to help manage natural disaster risk. This has resulted in a common approach where national coverage is given priority over quality mapping in areas of risk. This trade-off between extent and population at risk has seen limited funds spread thinly over a broad area. There is also a move to adopting an all hazards approach where all natural disaster risk is mapped, including floods, fire, storm surge, earthquake, hail, landslide and wind.

Risk mapping is important as it can lead to informed decision making, better planning and the community having a better understanding of risk. It is also important for setting insurance premiums. The public and business’s use of this information is based around a reasonable assumption that this information is a of reasonable quality and can be relied upon for decision making. When some of this information is not reliable or low quality these groups question all the information. This broad brush information is being used for important investment decisions. There are many good reasons to invest in flood mapping as it allows the community to understand their flood risk, allows floodplain managers to make informed planning designs, understand risk and for informed insurance pricing. However, are any of these users served by unreliable maps. It is important that it this information is reliable. The counter argument is that low reliability mapping lead to further detailed assessments.

This paper analyses the advantages and disadvantages of broad brush flood mapping and the arguments used to justify their use. It also discusses the other ways risk can be mapped other than probability including historical floods and soil type based maps.