The influence of tropical cyclones on long-term riverine flooding; examples from tropical Australia
Introduction
The link between palaeoflood records and climate is often tenuous despite the fact that phases of enhanced flooding in a record can be suggestive of a wetter climate and an absence of such phases the opposite (Kale and Baker, 2006). In tropical regions palaeoflood deposits are also sometimes suggested to be due to the rainfall and floods generated during tropical cyclones (TCs) and these palaeoflood records are taken as a type of proxy for these storms (Denniston et al., 2015, Rouillard et al., 2016). An assumption is made here however that such high magnitude floods are caused by TCs as opposed to other rain bearing systems such as upper troughs and monsoonal low pressure systems. Historically, TCs do appear to produce many of the largest floods in some catchments such as the Barron River catchment in northeast Queensland (Leonard and Nott, 2016) and also the Kimberley region (Denniston et al., 2015). It would stand to reason therefore that palaeoflood records would also reflect palaeo-TC activity but to date no independent verification has been made to assess whether this is the case. The recent development of high resolution isotope records of TCs within limestone stalagmites and the development of the Cyclone Activity Index (Nott et al., 2007, Haig et al., 2014) offers scope in this regard. Comparisons are made here between these types of long-term high resolution TC records from Western Australia and northeast Queensland and previously published slackwater deposits (SWD) along with two new SWD sequences. Slackwater deposits are defined here as fine-grained sedimentary units deposited during high magnitude floods in areas of relatively calm water away from the zone of higher velocity flow. The two new SWD records presented in this study are from the Broken River in northeast Queensland and the Ord River in northwest Western Australia. These along with existing SWD records from the Burdekin and Herbet Rivers in northeast Queensland and the Margaret, Fitzroy and Leonard Rivers in northwest Western Australia are compared with the high resolution multi-century Cyclone Activity Index (CAI) records from Cape Range, Western Australia and Chillagoe, northeast Queensland. The aim of these comparisons was to examine whether there were any clear associations between periods of heightened palaeoflood activity and TC activity in these tropical regions.
Section snippets
Geological and geographical settings
All of the palaeoflood sites discussed here are influenced by the Austral monsoon between November and April each year. Average annual rainfall however varies substantially between sites. The Burdekin and Herbet River catchments in northeast Queensland receive between 600 and 1500 mm annually. The Broken River catchment receives between 600 and 1000 mm annually. Likewise, the Ord River in northwest Western Australia receives between 600 and 1000 mm annually whereas the Fitzroy, Margaret and
Methods
Two auger holes (JG3 and JG4) were sunk into the SWD deposit at Broken River and two of the SWD/terraces in the Ord River was also augered. The auger holes were drilled to a depth of 4 m in each case. Samples were collected for textural analysis and for optically stimulated luminescence (OSL) dating. The terraces on the Ord River are gullied in places and exposures of the sedimentary unit were evident. These sections were also examined and slumped material near the base removed to obtain as
Results
All the OSL samples showed strong OSL signals and excellent recycling ratios as per Murray and Wintle (2000) with only one sample having a mean recycling ratio more than 1% from unity (X1148 with mean R = 1.024). These characteristics show the samples were close to optimal for obtaining reliable OSL age determinations. However, some of the samples did show signs of incomplete bleaching for there was a relatively high variation between the equivalent dose values (Do) between different aliquots.
Discussion
Recent studies have suggested that the period between approximately 1400 and 1850 CE was a period of relatively wet conditions with an increased frequency of TC landfalls in northeast Queensland (Nott et al., 2007, Haig et al., 2014, Haig and Nott, 2016). Tropical cyclone activity also appears to have been relatively high between approximately 1500 and 1850 CE in the Cape Range region of central coastal Western Australia (Haig et al., 2014, Haig and Nott, 2016). The aim of this present study
Conclusion
Tropical cyclone records in the form of the CAI were compared to a range of SWD palaeoflood records across northeast Queensland and northwest Western Australia. The latter location shows a distinct clustering of palaeoflood events between ∼1400 and 1850 CE which temporally matches a period of enhanced TC activity. The majority of palaeofloods over the past 1000 years also occur during the period ∼1400 to 1800 CE in northeast Queensland although several major floods also occur during the 20th
Acknowledgements
Thanks are extended to Professor Ed Rhodes for supplying the luminescence ages and to Karlina See Kee for help with fieldwork in the Ord River. Thanks are also extended to two anonymous referees for their insights and recommendations.
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