Reducing uncertainty with flood frequency analysis: The contribution of paleoflood and historical flood information

Abstract

Using a combination of stream gauge, historical, and paleoflood records to extend extreme flood records has proven to be useful in improving flood frequency analysis (FFA). The approach has typically been applied in localities with long historical records and/or suitable river settings for paleoflood reconstruction from slack‐water deposits (SWDs). However, many regions around the world have neither extensive historical information nor bedrock gorges suitable for SWDs preservation and paleoflood reconstruction. This study from subtropical Australia demonstrates that confined, semialluvial channels such as macrochannels provide relatively stable boundaries over the 1000–2000 year time period and the preserved SWDs enabled paleoflood reconstruction and their incorporation into FFA. FFA for three sites in subtropical Australia with the integration of historical and paleoflood data using Bayesian Inference methods showed a significant reduction in uncertainty associated with the estimated discharge of a flood quantile. Uncertainty associated with estimated discharge for the 1% Annual Exceedance Probability (AEP) flood is reduced by more than 50%. In addition, sensitivity analysis of possible within‐channel boundary changes shows that FFA is not significantly affected by any associated changes in channel capacity. Therefore, a greater range of channel types may be used for reliable paleoflood reconstruction by evaluating the stability of inset alluvial units, thereby increasing the quantity of temporal data available for FFA. The reduction in uncertainty, particularly in the prediction of the ≤1% AEP design flood, will improve flood risk planning and management in regions with limited temporal flood data.

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

“…They have been shown to have large channel capacities, with bank top capacities approaching a 50-year annual recurrence interval (ARI), and are laterally stable because of adjacent, highly resistant, clay-rich Pleistocene alluvium. Results over a range of timescales from post-European to Holocene indicate that lateral migration is limited and changes in cross-sectional area, even postextreme flood events are relatively minor (Croke et al, 2013;Fryirs et al, 2015;Thompson et al, 2016;Lam et al, 2017).…”

“…Whilst the issue of gauge record length is readily appreciated in the application of historical and palaeoflood data sets, it is generally agreed that providing additional evidence for extreme events that can better populate the upper tail of the flood frequency distribution is vital flood information. For example, using just 1-3 palaeoflood dates was found to significantly reduce uncertainty in the estimated AEP, and the integration of these limited palaeoflood dates into traditional at-site FFA resulted in >50% reduction in uncertainty associated with the 1% AEP flood quantile (Lam et al, 2017). As uncertainty increases with estimation of rarer floods (smaller AEP A C C E P T E D M A N U S C R I P T flood), partly owing to the lack of sufficient upper-tail distribution flood, palaeoflood information becomes vital.…”

“…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…”

“…While the increase in the application of SWDs across Asia has been rapid, studies in Australia remain very limited and geographically biased (Lam et al, 2017). Initial application of SWDs in arid and semiarid areas used clear contrasts in flood unit stratigraphy to help delineate individual flood units.…”

“…For example, the OSL age of 82 ±21 year for the top flood unit in the Mary basin approximates the timing of the 1955 flood that occurred extensively throughout Queensland and the second largest flood recorded by the gauge (Lam et al, 2017).…”