Only 11 years on from the devastating 2011 flood, Brisbane was again inundated, but this time it was different. In a new edition of 'A River with a City Problem', Dr Margaret Cook, from Griffith University's Australian Rivers Institute, shows that while the quick-moving water followed familiar riverine flood paths on the cities southside, northside residents were left unprepared as waters inundated areas untouched since 1974.
Although forecasts predicted that the weather system would move south, the rain remained over Brisbane for days, dumping phenomenal volumes of rain. Premier Annastacia Palaszczuk expressed the thoughts of many:
"We never expected this rain, this rain bomb is just unrelenting. It's not a waterfall, it's like waves of water. The 'intense weather' was like an unpredictable cyclone."
Between 23 and 28 February, the Brisbane City Council area received between 400 millimetres and 1,100 millimetres (on average 795 millimetres), much of which fell between Friday 25 and Sunday 27 February.
"Not since the 1893 floods had Brisbane experienced these volumes in one month," Dr Cook said. "The three-day record exceeded that of the 1974 flood, with the northside suburb of Alderley receiving more than metre of rain over those three days."
"In 72 hours, Brisbane received around 80 per cent of the city's average annual rainfall, that's almost all the rainfall that London gets in an entire year. Upstream, Wivenhoe Dam, the city's main flood mitigation strategy, received about three Sydney Harbours worth of water in under three days."
Fortunately, at the start of the 2022 flood event Wivenhoe Dam was only 56 percent full and was able to hold back 2.2 million megalitres before water releases were necessary. The problem was that the rainfall downstream of Moggill was more than three times the rainfall in 2011.
Although a moderate flood alert was issued on 26 February, constantly changing weather scenarios caused predicted river flood heights to be revised five times in the next 11 hours, with alerts being out of date soon after (or before) their release.
"People might be surprised to hear the peak flood height in 2022 at the Brisbane City Gauge in Edward Street was 3.85m, substantially lower that the 4.46m recorded in 2011," Dr Cook said.
"The 2011 event was primarily a riverine flood, with the heaviest flooding in the main river itself, which is vastly different to what we saw in 2022. In last year's event flooding occurred in the Brisbane River and creeks and through overland flow, all at once."
"Flooding in creeks, especially on Brisbane's north side, surpassing all previously height records heights, including those in 1974, which were much more like the 2022 flood."
Kedron Brook, in Brisbane's north, broke records with 893 millimetres, compared with 661 millimetres in 1974 and 315 millimetres in 2011. The swollen creeks soon reclaimed their floodplains, flooding streets and homes. After two La Niña years of frequent rain, the soaked ground increased run-off and overland flow, which when combined with creek flooding meant areas left dry in 2011 flooded in 2022.
Jamica Santos had lived near Enoggera Creek in Acacia Drive, Ashgrove for 25 years. In 2011, floodwaters reached her driveway. in 2022, the creek rose less than two metres outside her home.
"It was crazy," She told The Courier-Mail, "it just kept rising, it wouldn't stop. I was scared because of how quickly it was rising."
She wasn't alone. Images of flooded streets, submerged homes and dramatic rescues on social media showed residents were caught off-guard by the rapidly rising floodwaters. People and pets were rescued by watercraft as 2,770 Brisbane streets flooded and the Bruce, Warrego and Ipswich highways were all closed for days. Thirteen people in total lost their lives in the 2022 floods, with 23,400 properties flood-affected in all but 11 of Brisbane's 188 suburbs.
Pontoon debris was found as far away as K'gari (Fraser Island) and Noosa, and Moreton Bay was shrouded in a plume of mud for weeks. Once again, Brisbane faced a massive recovery effort and a huge financial liability, while city residents again confronted property loss, homelessness and heartache.
With two major floods in quick succession (2011 and 2022), that were so vastly different in mechanism and outcome, we have been given a sharp reminder of the region's sub-tropical climate and propensity to flood.
"Despite the region's complex network of four rivers and 22 creeks, we tend to focus only on the Brisbane River when it comes to flooding," - Dr Margaret Cook
"But as 2022 showed, floods can occur via the river, Brisbane's creeks or from overland flow, or any in combination of these. While history offers insights into flood patterns, every flood will be different, and we need to be prepared for all scenarios."
With climate change and growing levels of urban density, the risk of extreme flooding is predicted to increase, making proactive changes essential to reduce risk rather to avoid being caught by surprise the inevitable next flood. But as Dr Cook points out, Brisbane's current reliance on a dam, the Wivenhoe, to save it from floods is heavily dependent on where that rain falls.
"Dams have a finite storage capacity and while they can hold back upstream floodwaters, heavy rainfall downstream will fill Brisbane's creeks and stormwater systems and inundate he city as it did in 2022. To avoid this, any flood mitigation strategy must consider that Brisbane creeks, overland flow and stormwater systems, can all flood independently of the river. This complex hydrology needs to be front of mind when planning and redeveloping the city."
After the 2011 floods measures were introduced to raise and retrofit homes to make them more flood resilient and since 2022 a buy-back scheme was implemented to move 500 homes from the floodplain. "These are all proactive steps in the right direction as we develop adaptive strategies to living with the region's complex flood hazard," Dr Cook concludes. "But more needs to be done in terms of re-zoning, preventing increased development in flood-prone areas and removing more properties from these areas."
"We need to go beyond dams for flood mitigation and embrace strategies like revegetation and creating flood soaks on the floodplain, better stormwater management, new building designs and materials and lastly public education to adapt to living in a sub-tropical city like Brisbane, situated on river that floods."
The second Edition of 'A River with a City Problem' by Margaret Cook is available in book shops or from University of Queensland Press from 23 May 2023.
