Why Flash Floods Are So Dangerous

By: Mark Mancini  | 
flash floods China
Aerial view of submerged cars and bicycles in a flooded street July 21, 2021, in Zhengzhou, Henan Province of China. The heavy rain across Henan Province began on July 16, with Zhengzhou being one of the hardest-hit areas. Visual China Group via Getty Images

Key Takeaways

  • Flash floods are particularly dangerous due to their rapid onset and powerful force, often catching people off guard.
  • Their unpredictability and ability to quickly overwhelm areas make flash floods one of the deadliest natural disasters.

They don't call them "flash" floods for nothing. On July 20, 2021, panicked subway riders in the Chinese city of Zhengzhou pleaded for help as flood waters filled up their subway cars, trapping them inside. Many commuters were swamped up to their necks as they sent texts and cellphone videos out.

At least 12 commuters died and five were injured, according to the BBC. In another rescue operation in Henan province, about 500 people were rescued from subway tunnels filled with floodwaters. The flash floods occurred after days of torrential rains in China caused several dams and reservoirs to breach, requiring more than 200,000 emergency rescues.



Flash Floods and Speed

Flash flooding problems like this are hardly limited to China. At the same time flash floods were occurring in China, major flooding was also happening in Western Europe, Colorado and Arizona. Every year, flooding in general is responsible for more deaths around the world than any other type of natural disaster. But flash floods are particularly dangerous because of their signature trait: speed.

The U.S. National Weather Service defines a flash flood as "a rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above the predetermined flood level." (That's the point at which the water spills out over the banks and covers dry terrain.) While certain types of flood can take days to unfold, the NWS notes that most flash floods break out in the span of six hours or less.


Dam failures and ice jams can both generate flash floods. However, these are usually the result of heavy rainfall that's sustained over a long period and doesn't get absorbed into the soil. Floods are common during China's rainy season, but they have been getting worse, and scientists blame that on climate change and urbanization, according to ABC News.


The Soil's Job

Soaking up water is one of the main services provided by soil. Rainwater that doesn't get absorbed into the ground and instead flows over it is called "runoff." Let's say your hometown gets a sudden downpour. If the soil is already oversaturated with water when the rain starts falling, it won't be able to soak up much more of the liquid. As a result, there could be an awful lot of runoff — and that greatly increases the odds of a flash flood breaking out.

Yet if the soil is too dry, that can also be a problem. Dry dirt tends to be compact, which limits its ability to absorb water, as well. Another point worth mentioning is the fact that not all substrates are created equal: Loose, sand-based dirt has a much easier time soaking up rainwater than clay-heavy soils do. And then there's the added problem of urbanization. Manmade surfaces like concrete and asphalt are (for the most part) quite bad at absorbing even small amounts of rainwater.


With this information in mind, let's review the Zhengzhou incident, which killed at least 25 people and displaced hundreds of thousands of others. The city received more than its annual average rainfall in just four days — and nearly 24 inches (624 millimeters) in just one hour on July 20, according to the World Meteorological Organization. An abundance of roads and waterlogged soil led to significant runoff, which turned streets into rivers that poured into the region's tunnels and subways.

Monitoring and Prevention

Alright, so what can be done about flash floods like these? Some communities utilize artificial ponds that capture runoff before it hits populated areas. Officials in Southern China (home of Zhengzhou) have spent decades rethinking flooding management, particularly how to manage the Yangtze River, China's longest. The focus since 1998 has been on nature-based solutions, including planting billions of trees and restoration of floodplains along the Yangtze.

But perhaps more drastic action is needed: one that includes planning for a new normal of rain and floods because climate change will make flash floods like these more common in the future.


Frequently Asked Questions

Can flash floods occur in urban areas with extensive infrastructure?
Yes, flash floods can occur in urban areas with extensive infrastructure due to factors such as impermeable surfaces, inadequate drainage systems and rapid runoff from paved surfaces.
How can individuals prepare for flash floods and minimize their risk?
Individuals can prepare for flash floods by staying informed about weather forecasts, creating emergency evacuation plans and avoiding low-lying areas and flood-prone regions during heavy rainfall. Additionally, maintaining flood insurance coverage can help mitigate financial losses in the event of flood damage.