Why US floods are getting ‘flashier’

This summer, at least 38 people died in eastern Kentucky after intense and rapid-onset floods

The United States is at risk of a future full of flash floods — and a future full of “flashier” floods.

Areas across the country will experience more frequent and more extreme flash flood events by the end of the century if greenhouse gas emissions rise unabated, according to a study published in the journal Communications Earth and Environment in April.

Recent incidences of flash flooding have proven to be deadly. This summer, rapid onset floodwaters in eastern Kentucky killed at least 38 people, with countless others left needing to rebuild destroyed homes and towns.

In September 2021, over 50 people died in the Northeast after the remnants of Hurricane Ida dropped record-breaking amounts of water in a matter of hours. The deluge came so suddenly that people drowned in basement apartments and were swept away in their cars.

The research “shows in the future that the storms are going to get more intense and we’re going to have bigger impacts from these events,” Jonathan Gourley, a hydrologist at the National Oceanic and Atmospheric Association’s National Severe Storms Laboratory and co-author, told The Independent.

Unlike flooding from rivers or the ocean, flash floods don’t require a body of water to overflow. Instead, flash floods form when there’s simply more water than the ground can absorb, says Dr Gourley.

Gourley and his colleagues had previously defined a flood’s measure of “flashiness” —a combination of how high the waters get and how quickly they get there.

A flood that takes a few hours to rise a few feet? Not so flashy. A flood that rises two feet in twenty minutes? Flashy.

Whether or not a flash flood happens depends on how much rain falls and how the geology and topography of the surface deal with runoff. More permeable soils, for instance, reduce the likelihood of a flash flood because the ground soaks up more water in a shorter period of time.

Steep areas with shallow soils aren’t great at absorbing water, says Dr Gourley, and neither are recently burned areas. Without vegetation, soil in burned areas won’t let in as much water, leading to increased runoff, he says.

To study future flash flood risk, the researchers used a model of the US landscape that considers these factors and overlaid a future end-of-century climate model which assumes greenhouse gas emissions are unabated.

Under such a scenario, the climate is expected to get significantly warmer — which could have two conflicting effects on flash flood risk.

On one hand, warmer temperatures could leave soil drier and more able to soak up water. But warmer temperatures are also likely to lead to more intense storms, dropping even more water.

Researchers found that any benefits from drier soils were entirely overshadowed by these more intense storms, Dr Gourley says.

“The duration of the rainfall from the storm is not longer, but it’s shorter — it’s more compressed and intense,” he says. “And that’s what really triggers these flash floods.”

More rain in less time means higher water levels in less time — aka, flashier flooding.

The study projected that US floods would get an average of 7.9 per cent flashier by the end of the century. Some regions would see an even higher bump in flashiness — they predicted that floods in the southwest US would get 10.5 per cent flashier and floods in the central US would get 8.6 per cent flashier.

The paper notes that the Southwest is already a flash flood hotspot, while the central US hasn’t historically seen as much flash flooding – which could change.

Other regions, such as the West Coast, didn’t see quite as much of an increase, Dr Gourley says. That’s because the regions tend to get more long, steady rainfall coming in from the Pacific, as opposed to short-burst thunderstorms, he notes.

Flash floods can be particularly dangerous because they happen quickly, without ample warning time, says Paul Bates, a hydrologist at the University of Bristol in the UK who was not involved with the study.

“The importance of this study is it’s telling us something about these quite dangerous floods and how they might change from now and into the future,” Dr Bates told The Independent.

The new study only focused on the lower 48 US states and both experts said they would like to see similar studies replicated all over the world. Dr Gourley said that he would particularly like to see research done on existing flash flood hotspots like Puerto Rico.

In addition, the study simulated flash flood likelihood under the worst-case scenario, which assumes near-unrestrained emissions throughout this century. However, if current emissions pledges and targets are met in the next few decades that pathway, known as ‘Representative Concentration Pathway’ (RCP) 8.5, is likely avoidable.

Despite the danger of flash floods, people and local governments have the tools to prepare.

For one, local officials can get better at sending out warnings about flash flood risks, Dr Bates says. Regions can also take long-term measures, such as preventing people from building in flood-prone areas and maintaining storm drains in urban areas, he adds.

Urban areas can be particularly susceptible to flash flooding because they’re often covered in impermeable surfaces like asphalt and concrete, Dr Gourley notes.

Flash floods can also be a long-term result of wildfire. After a fire burns over a landscape, the soil can end up impervious to water, meaning water will form runoff instead of seeping into the ground – leaving areas downhill at higher risk of rapid onset flooding.

This summer, parts of Flagstaff, Arizona have faced multiple days with rapid highwaters downhill of burn scars. And as wildfire risk increases with the climate crisis, these kinds of flood events could become more common, too.

Most people who die in flash floods are out of the house and trying to get somewhere, Dr Gourley says, and end up driving into flooded roads or otherwise putting themselves at risk.

He advised that by staying put during a flash flood warning, people were more likely to stay alive.

“There’s an educational component here that could be used to save lives as we see flash floods increase in the future,” Dr Gourley says.