Extreme flood events in South East Queensland: evidence and implications

Abstract

The use of flood frequency analysis (FFA) to estimate both the magnitude and frequency of the design flood is severely limited by short gauging records. This thesis seeks to improve our understanding of the frequency and magnitude of extreme flood events, with an aim to advance flood risk management and policy planning through the integration of spatial and temporal extreme flood information. This research undertakes a field-work based regional study, collecting slackwater deposits (SWDs) as extreme flood archives and incorporates these as palaeoflood record of extreme flood events for SouthEast Queensland (SEQ). To do so, this study must first define an extreme flood, which is undertaken through the derivation of an Australian Envelope Curve (AEC). The resultant approach represents a significantly improved method to produce an AEC which comprises data from ~2700 gauges in SEQ. An alternative approach to flood frequency is also investigated in the form of a Probabilistic Regional Envelope Curve (PREC) approach which integrates additional spatial information, from homogenous regions. Results indicate that for gauges with either too few, or too many, extreme flood events, the PREC method shows significant changes to the estimated discharges of low % Annual Exceedance Probability (AEP) flood quantiles. A decision making process is provided to ascertain when this method is preferable for FFA. This thesis also examines historical and palaeoflood records that can provide significant upper-tail flood information, often missing from short gauging records. Palaeoflood studies and records have been spatially limited to bedrock settings, in climatic regions outside the subtropical environs of SouthEast Queensland. Reliable historical records are also temporally limited to the last ~200 years of European settlement. This study produced results on the timing and magnitude of past floods from over 30 SWDs across five catchments in the region, spanning a variety of depositional settings, contributing catchment areas, and discharge magnitude. Sensitivity analyses included tests for stable boundary conditions essential for reliable discharge reconstruction. Results show that cross sectional changes due to either aggradation or incision result in resultant changes in discharge that are within the uncertainty range for most of the concerned % AEP flood quantiles. Additional temporal extreme flood information derived from both historical and palaeoflood records are also integrated with gauge records for FFA. This approach reveals that a significant reduction in uncertainty associated with the estimated discharge of a flood quantile can be achieved. Importantly, the uncertainty associated with the 1% AEP flood discharge is reduced by 50-74%. The reduction in uncertainty, particularly in the estimation of the 1% AEP design flood has important implications for flood risk planning and management. Preliminary contributions to improve our understanding of regional climatic drivers and high magnitude floods are also explored. Preliminary results based on aggregating the timing of known extreme flood events with available climate proxy data indicate that tropical weather systems (e.g. tropical cyclones) have the potential to generate large palaeofloods across a far-reaching spatial scale. The extensive flooding across the eastern seaboard of Australia as a result of Cyclone Debbie in March/April 2017 (weeks before this thesis was submitted) is a most recent example. In addition, interdecadal climate variability such as the El Nino-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillations (IPO) also appear to play a role in generating these high magnitude floods with synchronous periods of regional extreme floods generally coincide with the La Nina and IPO negative phase. This study concludes by recommending that palaeoflood hydrology is rapidly embraced in flood risk management in Australia as other parts of the world now lead in examples of its benefits to improved FFA. Specifically the study illustrates that the creation of a national repository of palaeoflood records, together with an online flood risk assessment tool are some of the outcomes worthy of serious consideration in any revisions to flood risk management in Australia in coming years.