The Impact of National Climate Change Guidance on Flood Estimation in NSW, Australia

Since 1958, when it was first released, Australian Rainfall and Runoff (ARR) has remained one of the most influential and widely used guidelines for estimation of design flood flows in Australia. In August 2024 the guidance on the impacts of climate change on design rainfall updated. As stated in Chapter 6, “Climate Change Considerations” in Book 1, the ARR “guidance reflects the contemporary science, is applicable across the range of design flood approaches, and facilitates consistent application of climate change in design flood estimation.” Key features include the provision of guidance on increases in rainfall intensities from 1 year up to and including the probable maximum precipitation (PMP) across storm burst durations from 5 min to 7 days under four climate scenarios as well as guidance on changes in rainfall losses. The Climate Change Calculator is also described. Two case studies are overviewed. The first case study is the Warriewood Valley (a coastal catchment undergoing urban intensification) located in the Northern Beaches of Sydney, Australia. It is concluded that the evolving climate change guidance is significantly reducing the timeframe in which a significant increase (30%) in rainfall volume is expected to occur in the Warriewood Valley. The second case study is 155 rural catchments across New South Wales. The impacts of climate change on 1% AEP peak flows and 10% AEP and 1% AEP flood levels are also outlined.