Riverine flooding, flash floods, overland flow and storm surge — there is a lot to get your head around when it comes to floods.
Key points:
- Flash flooding refers to the speed, within six hours of rain, at which flooding occurs
- Riverine flooding is slower and happens when a water course rises and breaks its banks
- Overland flow occurs when water has not made it to a creek or river and moves across the land
Here’s what you need to know.
The two main terms that fly around during a flood are riverine and flash flooding.
Riverine flooding is basically when regular water courses like creeks and rivers rise and even break their banks.
Flash flooding refers to the speed at which flooding occurs.
The Bureau of Meteorology defines flash floods as those of short duration and relatively high flow that occur within six hours of rain falling.
You can get flash riverine flooding, but big river floods usually occur with a lead time of over six hours.
“Flash flooding is where basically we have heavy rainfall and then, soon after, rivers and creeks can rise within minutes or a couple of hours and cause that flooding,” Natural Hazards Research Australia chief executive officer Andrew Gissing said.
Think small creeks, in hilly areas, filling quickly during heavy rain.
“Whereas riverine flooding occurs with a longer period of warning time,” Dr Gissing said.
Most riverine flooding in Australia occurs along the big inland rivers.
“In Australia a really good example is the floodwaters that take so long to go down the Murray-Darling Basin,” Water Technology principal Neil Dufty said.
“Or another really great example is the water that takes some [time] to get into Lake Eyre.”
The long drawn-out flooding currently happening in New South Wales is also an example.
What is overland flow?
Overland flow is when the water has not made it to a creek or river and is just barrelling across the landscape.
In an urban setting, overland flow often occurs when drains get blocked, and the water ends up streaming down streets or through backyards.
Those in the tropics, South-East Queensland and northern New South Wales would be familiar with such scenarios.
Overland flow usually happens quickly and recedes quickly, but it can still be dangerous and damaging.
Which is the worst?
“All flooding is potentially dangerous and, of course, as you will well know, half of the flood deaths in Australia, and we’ve just seen this sadly again in the last six months, occur from people entering floodwaters,” Mr Dufty said.
“There’s been a fair bit of work to show that even an adult in 30 centimetres of fast-flying water can get knocked off his or her feet.”
Flash flooding in all its forms brings the danger of speed.
“Flash flood environments, they can come up really quickly, be really fast flowing, be quite deep, [and] be really dangerous to enter as well,” Mr Dufty said.
He said people were not going to be isolated all that long by flash floods, “minutes to hours really”, and water generally receded quickly.
Classic riverine flooding happened more slowly, but the trade-off was water levels could stay high for hours, days and even weeks.
With people potentially isolated for extended periods of time, it could also leave people running out of supplies, or having to deal with medical issues, extended periods without power and communications or other emergencies without assistance.
What about storm surge?
Storm surge is another type of flooding you might have heard about in warnings or on council flood maps.
Storm surge occurs along the coast when a strong low-pressure system like a tropical cyclone or an east coast low literally raises the sea level and pushes it onto shore.
The risk is compounded when it combines with high tides and big waves.
Storm surge is often the deadliest part of a tropical cyclone.
Some places will experience flash flooding, overland flow, riverine flooding as well as storm surge.
It is important to know the risk, what kind of flood you could be facing and how long you are likely to have to respond.
Unprepared for tsunamis
Once coastal risk that we are perhaps too willing to ignore is tsunami.
“There is a problem in Australia that we’re not, and there’s a fair bit of research around this, that we’re not prepared for Tsunamis,” warned Mr Dufty.
“We’ve got good warning systems out to sea, but we are close to the Ring of Fire [hundreds of volcanoes stretching around the Pacific Ocean rim] where there’s a lot of earthquakes going on.
“We still have to keep that one in the risk psyche.”
Dam and levee floods
Likewise, something else we don’t like to think about is dam and levee breaches.
Big reservoirs, farm dams, levees and local water detention basins can all slow and divert floods but they are not infallible.
“Those levees can easily fail, and they can be overtopped,” Mr Dufty said.
“There’s no levee in Australia that can’t be overtopped.”
While man-made systems protect us, they can build a false sense of complacency.
These disasters are a reminder that we are not immune to natural processes.
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