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Why does water submerge a home in one flood, but not the next?

Why does water submerge a home in one flood, but not the next?

aerial view of brown flood water and built structures

My phone buzzed with text messages from friends last week as the floodwaters rose across Southeast Queensland. We compared the flood to the 2011 worst floods and tried to figure out if our homes would be affected. I live in Ipswich near the now-flooded Bremer river.

I’m also an expert in the history of natural disasters, including Brisbane flooding. I followed with interest the social media updates about flood levels and photos showing rising water. News reports kept referring to the 2011 Brisbane floods.

This time, the Floodwaters have stopped at 3.85 metres at the Brisbane gauge in the central business district – less than the 4.46m peak in 2011. Many homes flooded this year that didn’t flood in 2011 – notably in the northern suburbs of Ashgrove, Windsor and The Gap. Homes in Bellbowrie, a western suburb, were flooded in 2011, but they managed to escape. But why?

Hydrologists will tell you that no two floods are alike. While the water may follow familiar routes, flood behaviour can be altered by natural and man-made factors.

aerial view of brown flood water and built structures
Each flood is unique based on natural and human factors.
Brett’s Drone Photography

How the rain falls

Rainfall intensity is a key factor in determining the extent of “runoff” – water that flows over the ground rather than soaking in. Heavy rain falling in an hour has a greater runoff ratio that if it falls over a period of time.

For example, January 1974 was a good example. 872mm of rainfall fell in Brisbane – including 314mm in a single dayJanuary 26, 2006. The Brisbane gauge recorded a flood of 5.45m.

Last month’s rain was similarly unrelenting, when 611.6mm fellBetween February 25-27. For perspective, Brisbane’s annual rainfall1,149mm. Flooding was inevitable due to the volume and intensity of the rains in one weekend. However, it was difficult to predict.

It matters where the rain falls. 2011 saw heavy rains upstream of Wivenhoe Dam. However, 2022 saw a flood that caused large amounts of water to flow downstream of the dam. This included on the Bremer and Lockyer Creeks, where there are very few flood-mitigation facilities to handle the extra water.

The extent of flooding can also be affected by stormwater infrastructure like drains, gutters and pipes that carry water away.

Many stormwater systems in Southeast Queensland were unable to cope with heavy rains, flooding houses. This problem was especially acute in Some suburbsIt received approximately a metre of rain in three days.

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men ride bikes over flooded road
Stormwater systems couldn’t cope with the influx of water.
Jono Searle/AAP

What’s happening in the river catchment?

People have compared this year’s Brisbane floods with 2011, but they are in fact more similar to the 1974 floods.

The catchment of the Brisbane River is complex. It comprises three rivers – the Stanley, Brisbane and Bremer – and many creeks, the largest of which is Lockyer Creek. Flooding can occur in any or all of these creeks and rivers due to heavy rain.

2011 was a year when floods were most likely to be caused by rivers. However, this year, Ipswich, Brisbane, and other areas were affected by river flooding. Suburban creeks, however, caused the most severe flooding, just like they did in 1974. This was because the rain fell heavily across the entire catchment, filling all watercourses.

Ithaca Creek flooded Ashgrove last month, the first time since 1974. Kedron Brook overflowed Windsor and The Grange which were left dry in 2011.

Rainwater filled the Enoggera Reservoir more than twice its capacity, flooding The Gap and the Fish and Ithaca Creeks.

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bridge over creek submerged by water
The Enoggera Reservoir overflowed, flooding streams and suburbs.
Mark Crocker

Humans affect floods, too

Floods can be significantly affected by humans. Flood risk in your area can be increased when a tree is cut, wetland is drained, or land is developed.

Housing estates are built densely – small subdivisions occupied by large houses. Instead of being elevated, homes are built on slabs to allow water to drain underneath.

Vegetation and soil can absorb water and slow down flooding. Surface runoff is increased by impermeable surfaces, such as roads, footpaths, and parking lots.

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Bridges, ferry terminals, pontoons and pontoons can block waterways. The problem is worsened by debris that gets entangled. Waterways can be blocked by buildings, railway embankments, roads, or other obstructions, which can lead to damming.

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new home beside cleared lot
Reclaiming natural spaces can increase water runoff.
Dan Peled/AAP

History can be a learning tool

History can help us measure the likelihood of flooding and prompt us to prepare, but it’s not that simple. Past experience can also help to reduce or confuse the perception of risk.

Flood risk mitigation requires increasing public awareness and individual responsibility. However, planning authorities must make tough decisions.

Southeast Queensland has allowed developers to increase the urban footprint. This has resulted in more impermeable surfaces, and replaced absorbent green areas, increasing the possibility of flooding.

This is because climate change is predicted that floods will become more frequent and intense, making flood predictions more difficult.

We can’t directly control the rain, but we can change how we respond to future flood hazards.

Margaret Cook is the author A River with a City problem: A History of Brisbane Floods.

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