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

Abstract Submission: 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 flood flows in Australia. In September 2024 the guidance on the impacts of climate change on design rainfall updated. As stated in Chapter 6 Climate Change Considerations in Book 1, ARR the “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 mins to 7 days under four climate scenarios as well as guidance on changes in rainfall losses. The Climate Change Calculator is also described. The impact of the climate change guidance on 1% level on the rating curve in 100 catchments in NSW is overviewed. The impacts in two locations in New South Wales are also described. The first location is the Warriewood Valley (a coastal catchment) located in the Northern Beaches of Sydney. The second is the inland rural city of Dubbo which is located on the Macquarie River and which experiences both mainstream flooding (generated by critical storm burst greater than 24 hours) and overland flow flooding (generated by critical storm burst of 1-2 hours). The ramifications for flood planning controls and in particular freeboard are also outlined.

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