Why does water flood a home but not the next time?

My phone buzzed with messages from friends as the floodwaters rose in Southeast Queensland last Wednesday. We compared the flood to the 2011 worst floods and tried to figure out if our homes would be affected. I live in Ipswich which is not far from the now-flooded Bremer.

I’m also an expert in the history of natural disasters, including Brisbane flooding. I watched with interest how social media struggled with constant updates on flood levels, and photos of rapidly rising waters. News reports kept referring to the 2011 Brisbane floods.

This time, the Floodwaters were 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. Although the water may follow familiar patterns, natural and human factors can alter flood behavior each time.

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 within an hour will result in a higher runoff ratio than if it falls over a week.

For example, in January 1974 872mm of rain fell in Brisbane – including 314mm in a single dayJanuary 26, 1996 At the Brisbane gauge, the floodwater reached 5.45m.

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

It matters where the rain falls. 2011 saw heavy rains that fell upstream from Wivenhoe Dam. However, in 2022, large amounts of water fell downstream of the dam on the Bremer River, Lockyer Creek and other areas where there are not many flood-mitigation structures to handle the extra water.

The extent of flooding is also affected by the state of stormwater infrastructure such as drains, gutters, and pipes that carry water away.

Many stormwater systems in Southeast Queensland were unable to cope with heavy rains, flooding houses. The problem was particularly severe in Some suburbsThe area received about a metre rain in three days.

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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. While Brisbane and Ipswich experienced flooding this year due to rivers, the worst flooding was caused by suburban creeks, just as in 1974. This was because the rain fell heavily across the entire catchment, filling all watercourses.

Ithaca Creek, for example, flooded Ashgrove last month for the first time since 1974. Kedron Brook flooded Windsor, The Grange, and they 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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Humans affect floods, too

Floods can be significantly affected by humans. Flood risk can be increased by the destruction of trees, land development, and draining of wetland.

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.

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

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History is a source of inspiration

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 granted developers permission to increase the urban density and 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.