Beyond the gorge: Palaeoflood reconstruction from slackwater deposits in a range of physiographic settings in subtropical Australia

The application of palaeoflood hydrology in Australia has been limited since its initial introduction > 30 years ago. This study adopts a regional, field-based approach to sampling slackwater deposits in a subtropical setting in southeast Queensland beyond the traditional arid setting. We explore the potential and challenges of using sites outside the traditional physiographical setting of bedrock gorges. Over 30 flood units were identified across different physiographical settings using a range of criteria. Evidence of charcoal-rich layers and palaeosol development assisted in the identification and separation of distinct flood units. The OSL-dated flood units are relatively young with two-thirds of the samples being < 1000 years old. The elevation of all flood units have resulted in estimated minimum discharges greater than the 1% annual exceedance probability. Although these are in the same order of gauged flood magnitudes, > 80% of them classified as 'extreme event'. This study opens up the renewed possibility of applying palaeoflood hydrology to more populated parts of Australia where the need for improved estimation of flood frequency and magnitude is now urgent in light of several extreme flood events. Preliminary contributions to improve the understanding between high magnitude floods and regional climatic drivers are also discussed. Recognised regional extreme floods generally coincide with La Niña and negative IPO phases, while tropical cyclones appear to be a key weather system in generating such large floods.