Flash floods - How do they happen and do we get them in New Zealand?

Explainer - Hundreds are dead or missing in Texas flash floods. The Tasman District has been hit hard in recent weeks. What is flash flooding, and why is it so hard to predict?

The terrible power of water has been in the news lately, between the Tasman region's massive "100-year floods" and the deadly flash flooding in Texas that has claimed 130 lives and left nearly 200 people missing.

There's been a lot of talk about "flash floods" and the carnage they can cause, but how is a flash flood different from more common flooding caused by heavy rain?

Here's what you need to know.

What exactly is a flash flood? How is it different from a normal flood?

A flash flood is a flood - just far faster, and often, deadlier than other flooding, which typically comes with just a bit more warning.

"The main difference between flash flood and regular flooding is speed on onset of flood," said hydrologist Shailesh Singh from Earth Sciences New Zealand (formerly NIWA).

"As [in] the word flash, flash flood onset very fast can happen within few minutes to few hours as compared to regular flood [that] occurs gradually."

Location matters, as being close to a river, creek or in a narrow canyon can create more of a risk.

"There are lots of different factors that can lead to flash flooding, including elements such as the topography of an area and how a river is flowing," said MetService meteorologist John Law.

In Texas, the devastating flash flood that swept through the hill country along the Guadalupe River on the weekend of 4 July left hundreds of people dead or missing, many of them children.

The river rose 26 feet - nearly 8 metres - in mere minutes, stunning residents.

Watch: Timelapse video shows Texas river rising

Rivers flooding can create staggering death tolls. For instance, the flooding of the Yellow River (Huang-He) in China in 1887 is estimated to have killed between one and two million people.

Does New Zealand get flash floods? Could the same thing happen here?

The recent flooding in the Tasman District was catastrophic and has caused immense damage, but most of it hasn't met the definition of a flash flood.

"It was forecasted a few days ahead, which doesn't really fit the sudden nature of a flash flood," said 'Earth Sciences New Zealand (formerly NIWA) principal scientist for groundwater and hydrological modelling Channa Rajanayaka.

Of course, isolated incidents of flash flooding could still be seen in Tasman. Shiloh Hobi saw what was once a small creek at the back of his Motueka Valley property explode into a massive flood of water, silt and forestry slash in Friday's rain.

"It's so wild when a flood like this comes and so many logs together - it's absolutely unpredictable," he told RNZ.

Flash floods are "short but can be really destructive," Rajanayaka said. "Regular floods tend to build up more slowly and they usually give you more warning."

In Texas, the Guadalupe River has been prone to flash flooding for many years, a long and narrow river basin where "the steep topography produces rapidly rising river stages during storm periods, leaving residents with little warning time," the US Geological Survey says. There have been multiple fatal floods there in the past, too.

There have also been questions about the locations of homes within flood zones and close to the river in Texas, including a camp where dozens of children were staying. The deadly floods also came in the middle of the night, catching many off guard.

But there is also a vulnerability in New Zealand, "especially in steep hill country or in cities/towns where water can build up quickly", Rajanayaka said.

Historically, several deadly events in Aotearoa have been attributed to flash floods, such as when 21 people were killed in the Kōpuawhara flash flood of 1938 which struck workers at a railway building site.

How can we pinpoint what areas might flash flood?

New Zealand is particularly prone to flooding, with 425,000 km of rivers and streams, according to the Ministry for the Environment.

"In terms of meteorology, understanding the intensity and location of rainfall is key to forecasting areas likely to be impacted by flooding," Law said.

He said Severe Weather Warnings highlight areas that are likely to see heavy rainfall and these are used by hydrologists to model how rivers and streams will respond.

"In addition to the broad areas of rainfall covered by Heavy Rain Warnings, some of our heaviest rainfall can be in the form of small scale but intense thunderstorms and downpours of rain," he said.

"These can produce extremely large amounts of rainfall in a very short space of time, leading to localised flooding."

Are they possible to accurately predict?

Pinpointing exactly where downpours will develop is a harder forecast, Law said.

"Severe Thunderstorm Watches are issued to highlight areas where all the ingredients come together and there is a potential for intense rainfall.

"Once a severe thunderstorm has developed, it is tracked using radar and Severe Thunderstorm Warnings are issued which show the predicted movement of the storm over the next 60 minutes."

A variety of local government authority hydrological forecasting systems are in place across the country.

Earth Sciences New Zealand (formerly NIWA) is developing a national river flow forecasting system as well, with a model prototype that has been put forth as a case study.

Can people plan ahead for possible flash floods in their area?

For flash flood and flood risks in general it's good to be prepared.

"We don't have a separate set of advice for flash flooding, as it is essentially flooding that happens rapidly," said National Emergency Management Agency communications manager Anthony Frith.

"The general advice stands; pay very close attention to weather watches and warnings, and - super important - evacuate to higher ground immediately if you encounter rising floodwaters.

"Don't wait for an official warning for a phone alert. Or as our campaign slogan says - get the flood out."

NEMA's Civil Defence flood preparation website warns people to find out what the flood risk is in their area.

"Practice your emergency plan and your evacuation route to higher ground," it recommends.

"Take measures to reduce potential flood damage and make sure your insurance policy covers you for flood damage."

Singh said property owners can mitigate some possible impacts as well.

"Maintaining vegetation cover and reducing soil erosion on hill slopes to reduce runoff can be adopted to minimise the risk."

Another big risk during flooding is attempting to drive through flood waters. Even a relatively small amount of water can damage cars or strand them. In America, it's been reported by government agencies that over half of all flood-related drownings occur when a vehicle is driven into hazardous flood water.

Are emergency services and weather agencies doing enough?

Flash floods can move incredibly fast, and even the best planning can be caught off guard.

There have been questions about the response in Texas by emergency services, and whether emergency alerts were issued in time.

Texas' Republican governor has pushed back at any attempts to cast blame, while Homeland Security Secretary Kristi Noem has denied reports recent policy changes under the Trump administration have slowed disaster aid.

In New Zealand, the emergency mobile alert system can be activated during extreme weather.

Mobile alerts were issued during the recent flooding in the Nelson region.

In the end, the terrible truth about flash floods is that they are inherently difficult to see coming.

"It's pretty hard to forecast exactly, because it happens so fast and in such small areas," Rajanayaka said.

"The scale makes it tough. But better warning systems, smarter land use, and good drainage can definitely help reduce the risk."

Sign up for Ngā Pitopito Kōrero, a daily newsletter curated by our editors and delivered straight to your inbox every weekday.