Logan’s controversial flood maps may be headed for a rethink, after an independent review backed residents’ concerns the city’s rarest flood scenarios were too extreme for everyday development decisions.
The Restore Blue review found the Logan and Albert Rivers Flood Study was broadly developed using accepted industry methods, but raised significant questions about how rare and extreme floods — including the Probable Maximum Flood — are used in planning controls.
The report also found earlier historic floods from 1887 and 1947 were not included in modelling, which preliminary analysis suggests may have led to some present-day flood levels being overestimated, although updated climate change data could partly offset that reduction.
Logan mayor Jon Raven said the review confirmed concerns raised by residents during consultation on the draft Logan Plan.
“This report confirms what the community told us through the Logan Plan consultation: that the inclusion of rare and extreme events in flood mapping creates alarm and isn’t practical,” Cr Raven said.
“Councillors wanted this issue examined closely and independently because the community had raised concerns about how the most extreme flood scenarios are used to determine what can be done on their land.
“These findings validate those concerns.”
The review did not find Logan’s flood study was fundamentally flawed, saying its general approach was in line with industry practice at the time.
However, it identified several issues the council should consider before relying on the flood levels for planning and development controls.
A key issue was the exclusion of older floods from 1887 and 1947, which occurred before systematic streamflow records began.
The reviewers said preliminary analysis suggested the one per cent Annual Exceedance Probability (AEP) flood estimate at the Yarrahappini gauge could be about 25 per cent lower if those earlier events were included.
A one per cent AEP flood is often called a one-in-100-year flood.
The report warned any possible reduction would need to be weighed against updated climate change guidance, which was released after much of the technical modelling work was completed.
It said accounting for warming that has already occurred could reduce the estimated overstatement from about 25 per cent to about 20 per cent, while future climate change could increase flood risk over time.
The review also recommended planning scheme maps focus on flood risks relevant to development controls, typically around one per cent AEP events.
It said rarer scenarios, including the Probable Maximum Flood, were more useful for emergency, dam safety and life-safety planning than ordinary residential planning controls.
Cr Raven said the council would work with the Queensland Government to advocate for a more practical approach.
“When the flood maps were commissioned, the inclusion of extreme flood scenarios was required under State Planning Policy,” he said.
“We will now work with the Queensland Government to advocate for a more practical approach on how flood mapping is used in planning for the benefit of Logan residents and all Queenslanders.”
He said any potential changes would go through a transparent process and be clearly communicated to the community.
Councillors will consider the review at a Special Council meeting on Wednesday 27 May, with a second meeting on Thursday 4 June to consider possible flood mapping and policy changes.
Key points
- The review did not reject the flood study; it found broadly sound methods, but planning-use issues
- Older 1887 and 1947 floods were excluded, possibly overstating current flood levels
- Any map changes would require further detailed modelling
- Updated climate science may partly offset lower estimates by accounting for warming already experienced
- Future flood risk is still expected to rise, especially by 2090
- Storm surge and river flooding interactions need stronger assessment, especially for lower floodplain areas
- Floodplain risk remains real, even if maps change or some levels are revised
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