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.
Originally Published: Jun 14, 2018