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

Abstract

The application of palaeoflood hydrology in Australia has been limited since its initial introduction more than 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.

A 1500 year record of river discharge inferred from fluvial-marine sediments in the Australian subtropics

“…High stream discharges over an extended wet season resulted in severe erosion of river banks (Grove et al, 2013;McMahon et al, 2017), and the deposition of extensive muddy deposits in Moreton Bay, the receiving waters of the Brisbane River (Lockington et al, 2017). The severity of this flood has promoted ongoing research into improving the understanding of the frequency of extreme floods in the region (Croke et al, 2016), their geomorphic impacts Kemp et al, 2015), and how such events relate to oscillations in climate (Haines and Olley, 2017;Lam et al, 2017).…”

Incorporating sedimentological data in UK flood frequency estimation

“…Authors have long recognised the challenges around flood record completeness and the reliability of flood magnitude estimates based on epigraphic and documentary sources (e.g., Hosking & Wallis, ; McEwen, ). Likewise, long and complete palaeoflood records are often been restricted to rocky gorge sites (Li & Huang, ), although physiographical settings outside of these areas are increasingly being used for flood risk analysis (Lam, Croke, Thompson, & Sharma, ). Nevertheless, the differential preservation and censoring of potential palaeoflood data (Lewin & Macklin, ) needs to be considered.…”

River flood reconstruction in the Lower Rhine valley and delta

“…Geological methods for reconstructing flood levels predominantly rely on the identification of fine-grained slack-water deposits at high positions along a river (Kochel and Baker, 1982;Zhang et al, 2014;Lam et al, 2017;Machado et al, 2017). The elevation of such a flood bed provides a minimum estimate of the local water level during flooding, as water depth and flow velocity were sufficient to transport suspended sediment load.…”

“…It was an important early medieval trade town, flourishing around 850-1100, and was granted formal city rights in the thirteenth century (the precise date is unknown). The Chronicon Tielense, written and compiled from older sources in the fifteenth and sixteenth centuries, mentions the flood events of 1342 and 1374 (van Leeuwen, 1789;Kuys et al, 1983). For 1342, it reports that only three locations in the Betuwe area (central delta between Waal and Nederrijn rivers) were not flooded, namely the city of Tiel, the village of Drumpt next to Tiel, and part of the nearby village of Wadenoijen.…”

“…In general, geological indicators provide minimum estimates of past flood levels rather than actual maximum water levels reached during peak discharge (Kochel and Baker, 1982;Springer and Kite, 1997;Benito et al, 2004). This is a suboptimal result, as the application of such minimum elevations in palaeoflood model calculations results in underestimating past discharges by up to 20% (Lam et al, 2017). Determining actual flood heights is arguably of even more importance in lowland settings, where the wider floodplain reduces the sensitivity of flood height to discharge, i.e.…”