The images and stories of flash flooding coming out of Victoria’s Great Ocean Road overnight have been described as “terrifying”. It’s easy to see why.
Holidaymakers at Victoria’s Wye River watched as cars were swept out to sea and a caravan park was rapidly inundated. Some had to get new clothes at an op-shop.
One resident at nearby Separation Creek told the ABC:
We missed being killed probably by about ten seconds […] we just ran for our lives.
I’ve been studying floods and flood risks for decades. On the one hand, this was a shocking event, due partly to the bad luck of such a heavy downpour occurring in an area with many geographic risk factors for flash flooding.
On the other hand, it was not surprising. These one-in-hundred chance floods will occur somewhere every year, and with climate change these kinds of events will become more intense and more frequent.
In fact, due to global warming over the last 30 years, there is clear evidence the intensity of such thunderstorm events have increased by 15–20%.
Hard to predict
The challenge with these kinds of floods is that they are caused by thunderstorms; it’s notoriously difficult to predict exactly where they will occur.
You might see a thunderstorm on the radar and know it’s heading in a certain direction, but it’s very hard to know exactly where it will actually impact.
In this case, it just happened to hit the Wye River catchment, which is particularly bad luck for the locals who live near the coast and the visiting holidaymakers. This area features very small and steep catchments draining the southern slopes of the Otways (a forested mountain region in Victoria).
These catchments respond very quickly to thunderstorms; it doesn’t take long for rainfall to produce runoff from all parts of the catchment.
This is a real problem for catchments along the surf coast areas of Victoria as there is little or no opportunity to provide effective flood warning.
At Wye River, the most intense part of the thunderstorm was only a tiny percentage of the whole area of forecast rainfall. That makes things really difficult to predict.
In a catchment like that, flash flooding can happen unbelievably quickly – things can go within minutes from OK to really dangerous.
All that was needed was maybe a few hours of intense rainfall. Most of the rain appeared to fall on the upper reaches of the catchment, so it wouldn’t necessarily have been evident to campers and holidaymakers.
But once it’s in the waterways, the floodwaters can rapidly shoot down the river system. The first anyone might know there is a problem is when the flood suddenly arrives.
What can we do to reduce risk?
The best thing we can do is educate communities about what warnings mean and how to respond when authorities say there’s a storm on the way.
If you’re in a high risk area, you shouldn’t just be saying, “Oh it’s just a bit of rain, we’ll wait it out.”
Look at the effort governments have put into raising awareness about bushfire risk and response. They have worked hard on grading messages so people leave high risk areas when the risks are catastrophic, often before there are any fires in their actual area. We have learned to take such warnings seriously.
I don’t think the community is there yet with floods – but we do need to get better at this.
The camp grounds in such areas are typically on nice flat grassy areas next to the river bank, so you can see why people camp and holiday there. But when floods arrive, that’s the worst place to be.
So, what would I have done, as someone who knows floods?
If I was in a floodplain or on a flat area next to a river, and had a warning severe thunderstorms are predicted to occur in this region, I would probably – at the very least – get the kids up and drive to higher ground. At least, I hope I would.
Even though I am attuned to flood risks in my professional life, I can imagine being caught out in holiday mode and being slow to respond. I can well imagine staying put and hoping for the best.
It can be difficult to think about risk when you’re on holidays; it’s easy to switch off and just think you’ll wait the rain out.
Nature can be capricious, floods especially so, and with climate change we must be more aware of risks.
More common as the climate changes
I spend a lot of time researching the impact of climate change on these events.
In Victoria, we can expect small floods will get smaller (due to drying soils), and large floods will get bigger (due to more intense rainfalls).
As a rough rule of thumb, the risk of flooding will double by the end of this century. The impacts of floods caused by thunderstorms will tend to be larger.
Overall, what hit Wye River was pretty rare. But flash floods like this will always happen somewhere, and unfortunately on this occasion they occurred in an area where a lot of people were on holiday and the steep slopes directed flood waters toward people.
Unfortunately, we will all have to get better at learning to live with floods, even when on holidays.
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