What’s in floodwater?

Parts of Queensland and New South Wales are currently dealing with a tremendous amount of water as they battle floods.

What’s actually in floodwater? We know to avoid it because of the risk of drowning, but should we also be careful about the water itself?

The short answer is: yes.

What’s in floodwater?

Floodwaters aren’t just brown from dirt. They can also contain sewage, pesticides, and liquid waste from agricultural, industrial, medical and household sources.

These pollutants won’t be obvious in the floodwater, beyond a possible change in colour or smell. Turbid (cloudy) water can also hold bigger hazards, like downed power lines, debris, and snakes or other animals.

Can we tell exactly what’s in a flood when it’s happening?

Floods in different areas will have different contents. Is it possible to guess whether there will be higher or lower levels of sewage and other things?

“It’s really hard to predict,” says Professor Jamie Pittock, from the Australian National University’s Fenner School of Environment and Society.

There are some broad conclusions you can draw.

“If water’s coming off soils that are easily eroded, then there’s likely to be more sediment,” says Pittock. “If it’s coming off cropping land that’s been ploughed, it’s definitely likely to have more sediments. Dirt roads, in particular, are big sources of sediment.

“And obviously, floodwaters go into towns. That is very problematic. Many rural areas rely on septic systems that are easily flooded. If sewage treatment plants get flooded, obviously that’s a problem.”

Beyond these guesses, it’s not easy (or in our interests) to figure out the exact content of floodwater. Unlike wastewater testing, which relies on fixed devices and schedules, floods are too unpredictable.

“It’s not easy to precisely determine what’s in the floodwater without testing, and that often takes too long,” says Pittock. “And it’s hard to access the floodwaters safely to do it.”

This is why the blanket approach to floodwaters is to assume they’re contaminated, and avoid going near them. If you do come into contact with floodwaters, through accident or clean-up, you should wash your hands and clothes afterwards. The US CDC also recommends cleaning and keeping an eye on any wounds, and seeking medical attention if they look infected.

Do different floodwaters cause different effects to houses?

Associate Professor Iftekhar Ahmed, from the University of Newcastle’s School of Architecture and Built Environment, says that floods with high levels of corrosive chemicals (from agricultural or industrial waste) can be more damaging to buildings than regular floodwaters.

“It can stain them, and if underwater for a prolonged duration, it can eat into the materials and weaken them.”

But the effects also vary depending on the building, and what it’s made of. Wood is high risk. “Timber materials are weakened by exposure unless they’re specifically treated,” he says.

Sewage, on the other hand, doesn’t cause structural damage beyond what the water is doing already. “It’s not harmful [to buildings] by itself, but the clean-up work is going to be yucky,” says Ahmed.

The water itself is the major cause of destruction to buildings. Wet timber attracts insects and rots. And flash floods in particular can degrade the stability of a building by removing sediment around the base, and causing settlement.

Ahmed adds that the height the water reaches, and the time it stays around, are two other key factors when assessing flood damage.

Is there anything we can do to clean floodwaters?

Floods and heavy rainfall are often, unfortunately, going to knock out water treatment plants – like the two which have recently been closed in South-East Queensland. If we’re not able to provide safe drinking water, it’s unlikely that we’ll be able to sanitise floods.

But there are things we can do that will lower the risk of floodwaters becoming contaminated.

“These floods really highlight the importance of restoring riverside forests,” says Pittock.

“Riverside forests are very important for binding the soil so that you’re reducing the amount of runoff. And in limited circumstances, it can also slow the flow of water. If water is moving more slowly, it’s less likely to erode soil and sediment,” he adds.

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Read more: Slow the flow: Rehabilitating Australian rivers

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Ahmed says that buildings should be designed with flood avoidance in mind – whether that’s steel frames instead of timber structures, making buildings higher, or moving settlements to higher ground.

“There are different approaches. But I think we need to take that on board, not just keep on responding to disasters,” says Ahmed.

“Policymakers and communities in Queensland have to be prepared for and undertake more intensive planning for the growing future risk.”

Given we can expect more flooding in a warming climate, this seems to be salient advice.