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What are the environmental impacts of flooding?
Along with the human, infrastructure and economic devastation wrought by recent floods, come environmental costs and even benefits.
In the past two years, most Australian states have been affected by severe rainfall and inundation. Some places have been hit repeatedly.
From an environmental perspective, floods can trigger landslides and erosion, pollute waterways, and drown or displace wildlife.
On the positive side, rainfall and flooding is essential to river and floodplain ecosystems, often triggering a boom in plant, insect and animal growth.
What’s in the water?
Flood waters can carry contaminants and pollution from various sources – sewage, petrol stations, industrial and agricultural sites.
In an Australian first, Victoria’s Environmental Protection Authority is partnering with the State Emergency Service and Natural Hazards Research Australia to undertake comprehensive sampling and analysis of bacteria and contaminants in floodwaters and sediments. The long list includes E.coli, trace elements, environmental DNA, hydrocarbons, phthalates, pesticides, pharmaceuticals and more.
Victoria’s chief environmental scientist, Professor Mark Taylor tells Cosmos, “the upshot is, we’re taking this opportunity not only to provide some insight about what’s in this water, and why or why not it may pose a risks to the residents, but also for us to characterize unusual, large flood events.”
“We’re going to collect a really special and really unique data set. And to the best of our knowledge, no one has ever done this in Australia before.”
Taylor expects the final results will be published mid 2023. Although, initial results have allayed some fears.
He hopes the work will help authorities and the public understand, “what’s in the water? Are there any greater risks being dispersed in that water? And what are the sources of those contaminants? And that will help us understand what risks they may pose, both to the natural and human environment.”
Polluted water isn’t just a risk to people. Some ecologists fear parasites and pathogens might cause wildlife deaths when animals drink the water, according to reporting in The Guardian.
There’s also the movement of sediments. Queensland flooding washed some 50 million tonnes of sediment into waterways, including up to five tonnes affecting Moreton Bay according to an independent report by consultant Deloitte.
Rivers and floodplain ecosystems revel in the rain
Associate professor Paul Humphries, an ecologist with Charles Sturt University, says for river and floodplain ecosystems, flooding is a natural and essential process.
“There’s a whole lot of animals out there – plants and fungi and bacteria – which specialise on those areas which are periodically flooded and periodically dry,” he tells Cosmos.
“There’ll be a burst of life of all those types of animals and plants when that water comes along. So really maintaining the biodiversity and the integrity and health of a river system, floods are required.”
Humphries says the way humans seek to manage rivers, with dams for instance, reduces the frequency of small to medium floods. This can lead to a build-up of organic material on the floodplain, which when washed into the water can cause low oxygen, blackwater events and fish kills in the short term.
“But in the medium to long term, these floods are absolutely essential. The health of rivers would be majorly compromised if we didn’t have them. So, I expect to see with these big floods, a boom in productivity on the floodplain and in the river itself. We’re going to get a lot of animals and plants.”
Trees can be both victims and beneficiaries of flooding, writes University of Melbourne botanist Dr Gregory Moore.
Some species like River Red Gums have evolved to depend on cycles of dry and wet periods. Whereas for others, the force of floodwaters and debris, or the water-logged soils can undermine the root systems of some trees, according to Moore.
Dead in the water
Many farm and native animals can’t escape the floods. James Cook University ecologists estimate around 600,000 cows were killed – by drowning, or stranding – in 2019 Queensland floods. Floodwaters can also inundate habitats and trap, displace or drown wildlife, write Deakin University’s Professor Euan Ritchie and Macquarie University’s Chris Jolly.
Waterlogged waste streams
Like the Lismore Library which had to throw out almost 30,000 books, cleaning up after floods generates huge volumes of new landfill waste. Deloitte says a year’s worth of landfill was swept into Queensland waterways, while some 30,000 m3 of extra waste was sent to council tips and recycling centres.
In Queensland, environmental the clean-up such as removing debris is estimated to have cost $42 million according to Deloitte.
Rivers of life
As the massive floodwaters move down the Murray River, towns are desperately sandbagging and building levees to keep the water out, but at the same time are concerned that irrigation pumps will be flooded and switched off.
This “boom-and bust” cycle will occur all along the river. The Coorong wetlands are looking forward to a long drink.
Humphries says, “the reason why humans, have settled around rivers is the same reason why animals and plants do so well. It’s because rivers provide the energy, the productivity, and the biodiversity that we’ve taken advantage of.”
“Floods are required. If they don’t occur, then you you’re basically losing a lot of that the biodiversity of other that makes up a river and its floodplain.”