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.