Reconstructing a millennial‐scale record of flooding in a single valley setting: the 2011 flood‐affected Lockyer Valley, south‐east Queensland, Australia

Abstract

This paper reconstructs past flooding from a range of settings in Lockyer Creek, a key tributary of the mid‐Brisbane River, which experienced extreme flood events in AD 2011 and AD 2013. Optically stimulated luminescence samples (n = 110) were collected from alluvial material preserved in within‐channel benches and floodplains. Age distributions from material in the bedrock reaches confirm an event ∼ 300 years ago which stripped the valley alluvium to bedrock. In the unconfined reaches floodplain deposits indicate lateral stability over the past 6000 years. Marked differences in the inundation patterns of the AD 2011 event highlighted changes in downstream channel geometry. The age distribution of alluvium in reaches not inundated during AD 2011 was older, ∼12 000 years, with no preserved evidence of deposition during the past 1000 years. A relatively continuous record of floodplain deposition in reaches which were inundated in AD 2011 identifies a major peak in flood activity also around 300 years ago (∼AD 1730) with five additional peaks occurring at approximately AD 1962, AD 1897, AD 1300, AD 550 and 5400 BC. The main climatic driver of changes in flooding over this timescale is oscillations in El Niño Southern Oscillation and although proxy records are scarce for this region, some correlations with high‐resolution records of rainfall variability are apparent.

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

“…Extending this record beyond the gauge record requires input from other climate proxy data sets and records. In the millennial scale record of flood events in the Lockyer Valley, a major peak in flood activity was noted in ~1730 (Croke et al, 2016b). This aligns well with dated flood units from the Brisbane, Mary, and Logan-Albert basins suggesting floods during this time may have been regionally synchronous.…”

“…Accurate estimation of the frequency and magnitude of palaeofloods depends, therefore, on our ability to identify discrete flood units in the sampling of SWDs. Croke et al (2016b) used criteria including a notable change in particle size with depth, together with a unit thickness threshold of 0.1 m in the sampling of 'major floods' in the Lockyer Valley, a tributary of the mid-Brisbane, in SEQ. This sampling strategy is acknowledged to potentially underestimate the frequency of smaller floods that deposit thinner sediment layers.…”

“…Like previous studies (e.g., Kale et al, 2010;Zhang et al, 2012, Huang et al, 2013Wasson et al, 2013), there now seems to be general consensus that SWD can move beyond the bedrock gorge, where well-preserved flood units are found in flow expansive and backwater settings, serving as a useful addition to sample settings. Importantly, the macrochannel settings of SEQ fulfil the stable boundary conditions required to provide robust discharge estimates because they have experienced minimal channel adjustments over the past 200 years (Fryirs et al, 2015;Thompson et al, 2016) and even over the longer timescale of the Holocene (Croke et al, 2016a(Croke et al, , 2016bDaley et al, 2016). In a more detailed assessment of how changes in withinchannel sedimentation rates may affect predicted palaeoflood discharges, Lam et al (2017) ACCEPTED MANUSCRIPT…”

“…A detailed reconstruction of cyclone frequency and magnitude has also been presented for tropical northern Australia, which allows better correlation between the occurrences of cyclone-driven floods. In addition, a millennial scale flood record has also been produced for the Lockyer Valley (Croke et al, 2016b). Together these data allow for some preliminary attempts to investigate any possible links between these flood events and key climatic drivers.…”

“…Australia also experienced numerous floods equalling, or exceeding, the largest floods on record, including the 2011 and 2013 floods across southeast Queensland (SEQ) (Croke et al, 2016a(Croke et al, , 2016b. The 2011 flood was the most expensive natural disaster in Australia and cost the economy ~A$30 billion (Australian Government, 2015).…”