Brisbane’s next big flood or drought may come sooner than we think

By Dr Jack Coates-Marnane, Research Fellow

Over the last two decades the south-east Queensland region has witnessed its fair share of weather extremes, including the millennium drought and the big floods of 2011 and 2013. But how typical are these events for the region?

Our new study extends current records of South East Queensland’s rainfall climate back by over 1300 years. We found that extreme floods and droughts occurred much more frequently than is currently assumed.

High magnitude floods in South East Queensland result in deaths and cause extensive damage to public and private infrastructure. In addition, high suspended sediment loads in rivers supplying major water treatment facilities have presented significant challenges in guaranteeing the adequate supply of potable water in the short term.

Flood impacts in Brisbane City during the january 2011 flood including a) Flooded industry (a) and sewage treatment plants (b) on the Oxley Creek floodplain. South-Bank (c), the popular river-side pool turning into a muddy torrent (d). Credit: Google earth, DNRM Qld Gov.

The millennium drought (2002-2008) highlighted major deficiencies in the ability of eastern Australia’s water governance and infrastructure to mitigate against widespread water shortages. This prompted major investment in infrastructure and policy innovations aimed at improving the resilience of the potable water supply to future droughts.

Wivenhoe Dam, Brisbane’s major supply of portable water, at the peak of the Millennium drought in SEQ (2005) and following above average rainfall and floods of 2010 and 2011. Credit: Google earth engine

We asked how typical are these events for SEQ and the Australian subtropics? Records of climate in Australia only go back ~200 yrs. This limits our understanding of the frequency of severe floods and droughts and ability to plan effectively. Natural archives of past climate can help us go back beyond the last 200 years and see how climates have varied.

In this study, we used marine sediments in Moreton Bay located adjacent to the Mouth of the Brisbane River to reconstruct the relative discharge of the Brisbane River over the last 1,500 years. High magnitude floods and prolonged periods of aridity are not an uncommon feature of the region’s climate over the last 1,500 years.

In fact, our results suggest that regional rainfall and river discharge has been highly variable over time. During the turn of the last millennium the region was experiencing prolonged aridity, accompanied by anomalously warm conditions.

This period of aridity extended from ~ 600 CE (Common Era) to ~1200 CE. After ~1350 CE the region became gradually wetter with peaks in the frequency of high magnitude floods occurring in the late 1600s and early 1700s.

These broad scale shifts in rainfall and hydrology of the region are linked to the variability of broad scale climate drivers including, hemispheric cooling and warming, the activity of El Nino Southern Oscillation (ENSO), the deflection of the intertropical convergence zone (ITCZ).

Though big floods of the last 1,500 years appear to be of similar magnitude to those of the instrumental record (1893, 1974, and 2011) evidence indicates that the highly erosive nature of the 2011 flood is potentially unprecedented and largely due to ongoing land-use changes in the catchment.

In addition, a growing body of research indicates that periods of even greater aridity to that experienced during the millennium drought are a common and natural feature of the regions climate of the last 1,500 years.

Water resource use in eastern Australia, for the purposes of agriculture, industry and urbanisation has grown rapidly within a relatively short time period (< 200 yrs.). As a result both public infrastructure and governing policies have been inherently tailored to suit a narrow range of natural climate variability in the context of the last 1,500 years.

It is unlikely that current strategies of water management are adapted to suit the full range of natural climate variability in the region.

Mixed water resource use in rural subtropical Queensland (Biloela). Credit: J Coates-Marnane

A proactive approach to water resource management that builds real resilience within catchments to both droughts and floods is now required to meet future water challenges. This approach requires a broad awareness of the inherent high variability of climate of the region, characterized by periods of intense flooding and severe droughts.

The incorporation of this longer term perspective of regional climate and water availability into water governance will ultimately allow for more prudent and adaptable policy and planning.

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