Looking into the past to inform our future

The 2022 floods in South?East Queensland and New South Wales took over 20 lives, caused many more injuries, loss of homes and business and resulted in over $5 billion in damages.

The understanding of rare and extreme floods in Australia is limited by short gauge records, and the problem with many of these gauges is that they become damaged or stop operating during large events.

The design flood used in risk assessment and planning policy is typically derived from traditional Flood Frequency Analysis (FFA), but it is widely acknowledged that this approach has limitations owing to short gauging record length and inadequate representation of high magnitude events.

To advance flood risk management and policy planning there is a need to improve our understanding of the frequency and magnitude of extreme flood events especially in anticipation of more severe storms expected due to the trajectory of climate change.

Whatever view is taken about the relevance of the past being able to inform the future, history will continue to provide the only reliable information about the extremes that nature is capable of. Gauged record lengths are too short to be reliably used for accurate fitting of probability distributions for calculation of extreme events because floods are typically erratic and sometimes clustered in time, with short-term characteristics not necessarily representative of longterm behaviour.

However, there is an established method which can be useful to reduce the risk and uncertainty in flood risk assessments which is termed Palaeoflood Hydrology.

Water Technology's Dr Daryl Lam has been a key part of developing this methodology in Australia. Palaeoflood Hydrology is an interdisciplinary science of reconstructing the timing and magnitude of past large flood events that occurred prior to historical observations and systematic measurements (i.e. gauging).

Palaeological evidence (e.g. fluvial sediments) can extend short gauge records and provide vital information of past extreme events, which are often missing or inaccurately represented. This provides the opportunity to better account for the frequency and magnitude distribution of discharge records.

The integration of palaeoflood records with gauged data can:

? Achieve a more representative discharge estimation of events with larger Annual Recurrence Intervals (ARIs) with greater certainty.

? Significantly reduce uncertainty in FFA.

? Increase the credible limit of extrapolation of the ARIs by more than an order of magnitude. This allows for more robust representation of very rare and extreme events.

Since acquiring a formal name in the early 1980s, Palaeoflood Hydrology has developed significantly and is being increasingly applied. In addition, research continues to demonstrate its potential applications and value. For example, in Australia:

? Research in the Lockyer Valley, Queensland, suggests the January 2011 flood may not be uncommon. In fact, floods of that size have occurred at least five times in the last 1000 years.

? Palaeoflood investigations in the East Alligator River gorge, Northern Territory, provided key information for the design rainfall event in the modelling of the artificial landform at the Ranger uranium mine.

? The City of Bundaberg, Queensland, located on the mouth of the Burnett River, experienced one of the largest recorded floods in 2013. Palaeoflood evidence suggests floods of the same order of magnitude have happened at least ten times in the last millennium.

? The integration of palaeoflood records with the combination of gauge flows and reverse routed inflows provided considerable confidence of the 0.1% Annual Exceedance Probability (AEP) event for the Burdekin Falls Dam, Queensland.

The use of historical and palaeoflood records, in combination with gauge records, is recommended by Australian Rainfall and Runoff (AR&R) for the estimation of design floods.

However, there are limited examples of its use in Australia. This may be attributed to the idea that sites where palaeoflood evidence can be found are limited to bedrock gorges.

A recent study in Southeast Queensland has shown that palaeoflood evidence can be found in semi-alluvium channels. But how and where can you find palaeoflood evidence?

The first step is looking for physical evidence in our “natural archives” of flood stage height such as flood sediments, fluorescent bands in corals, cave deposits, erosional features, flood debris (including boulder deposits), tree rings and tree damage or the absence of vegetation within flood zones.

In terms of assessing flood sediments, state of the art dating techniques are utilised that can measure the last time the sediment was exposed to light or by dating minerals such as quartz or feldspar.

To work out how big the event was, hydraulic modelling is used to estimate the minimum amount of discharge that is required to carry and deposit the sediments where they are located.

The potential applications of Palaeoflood Hydrology include:

? Large infrastructure design and improvements.

? Mine Closure design.

? Flood risk and hazard mapping.

? Deriving extreme flood records where there are few or none available.

In Australia, Water Technology has been working with clients to obtain and use palaeoflood records to better understand extreme floods in their catchments.