Floods: How rain turns into a destructive flood | Knowledge & Environment | DW

As if they were houses of cards and toy cars collapsing under brown masses of water or being swept away. Containers that drift like cardboard boxes, basements that become life-threatening traps within minutes. Again and again nature proves us with her powerful elementswho has the upper hand here. At least the people aren’t.

But how can water unleash such power? Exactly this question is answered by Dr. Michael Dietze from the Geomorphology Section at the Helmholtz Center Potsdam on the website of the GFZ German Research Center for Geosciences.

First of all, it is important to know that one cubic meter of water weighs one ton. So that’s not without! And that means: “Water can build up enormous pressure when it hits an obstacle directly. When it is in motion, this results in enormous forces that act on cars or containers and can simply push them along in front of you if they are not anchored very firmly ” says Dietze.

“Then there is also the phenomenon of erosion, which can destroy supposedly stable surfaces: the (earth) surface is eroded by fast-flowing water.”

At the GFZ Potsdam Among other things, it is examined how exactly water mobilizes sediment, how tidal waves move and with what force they make their way through the landscape.

Heavy rain is one of the most underestimated dangers, he warns German Weather Service (DWD). It is difficult to predict and rarely occurs in one location. While meteorologists can predict the region, they cannot predict exactly when or how much it will rain in any particular location.

This can also lead to serious damage in the regions where it is don’t suspect first would – even away from larger rivers or narrow valleys.

“The heavy precipitation brings such large amounts of water to the soil – often already saturated by previous precipitation – that it can no longer seep away there,” explains the geomorphologist Dietze.

Loam, clay, sand, dry, moist: types of soil absorb water differently

It’s not just the amount of water that matters, that too composition of the soil plays a major role, or rather the water flow of the soil. That means: How well can the soil absorb, store or drain water?

Factors such as the pore size of the soil particles or so-called soil colloids play a role here. Soil colloids are particles less than two micrometers (0.002 mm) in diameter.

These particles are so small that they cannot be seen with the naked eye. But because they are so tiny, a large number of soil colloids also have a gigantic surface area – to which water molecules bind.

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Clay and loamy soils contain many such soil colloids on which water is retained as so-called retained water and cannot drain away. These soils contain few pores and therefore, once properly soaked, can hold more water than sand.

Sand, on the other hand, has many large air-filled pores due to its large grain size and contains few colloids. Therefore, the water can hardly be retained as retained water. It drains quickly.

Infographic: water flow of the soil

Also important: what condition was the ground in before the rain?

Healthy, humus-rich soil – that is, it is not sealed, encrusted or compacted – can generally absorb more rainwater, store large amounts of it so that it is later available to plants and soil animals. The rest seeps away cleaned and contributes to the formation of groundwater.

However, if it is after a longer drought period If it suddenly rains heavily, the soil cannot absorb as much water at once. A dried out soil has what is called “wetting inhibition”. As a result, the water does not seep away, but flows off the surface. Plant residues in the soil also contribute to this, since fats and waxy substances are released from them when it is dry.

Water makes its way

If the soil is saturated after long periods of rain, the water has no choice but to drain off the surface. Then it makes its way into streams and rivers. “Once it has arrived in these channels, it can reach very high speeds,” says Dietze. At the Ecological Rhine station at the University of Cologne, for example, the Rhine flows at a speed of 1 to 2 meters per second under normal conditions.

“The higher the speed, the higher the gradient – especially on local steps such as embankments and terrain edges – and the deeper the river, the more power the water can develop on the ground: where it flows along, it pulls with its power, so to speak a weight of several kilograms. That’s enough to tear away sand, stones and rubble,” Dietze continues.

More than just water: A fatal mixture

However, that alone is not enough to sweep away houses and streets. But it is not only the water that plays a role, but also the particles that are carried along. These hit the ground, streets and house walls and develop an enormous erosion capacity.

“Once parts of it are attacked, the underlying material can be carried away much more easily,” explains Dietze. Undermining occurs because roads and houses are often built on unconsolidated ground. Further material can then easily break. “This interaction of carried material and the power to simply carry away further material that is exposed gives the fast-moving water the power to cause such enormous damage in a short time.”

Michael Dietze from the Helmholtz Center Potsdam emphasizes that such floods occur wherever heavy precipitation could occur. Such precipitation events are particularly dangerous in the high mountains, where as a result suddenly failing dams cause entire lakes to leak or landslides melt enormous amounts of ice and thus generate tidal waves in the narrow valleys.

Before the water comes

Yet there is no way before such extreme weather events to warn?

“Warning notices can be derived from weather forecasts,” says Dietze. “For example, weather forecasts can be fed into hydrological models to make predictions about the occurrence and likelihood of flood events.”

The erosion processes, on the other hand, are still problematic. “They are difficult to predict, mainly because these events happen very quickly and their intensity is difficult to assess precisely,” says the geomorphologist.

With the help of satellite images and, above all, seismometers, researchers have been trying to track the tidal waves almost in real time and calculate their intensity for several years. Research on this is still in its infancy, emphasizes Dietze, but has immense potential to warn the population of such floods as quickly as possible.

The article originally appeared on 07/19/2021 and was updated on 05/19/2023.

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