The flood disaster of the last few days poses a danger in two respects: On the one hand, the masses of water are a direct threat to people because they can drown in it - both outdoors and in buildings, where cellars in particular can become deadly traps. On the other hand, the flowing water sweeps away things that we think are standing firmly on the ground - not only cars or containers, but even entire houses and streets.
How can the water develop such a force?
First of all, one cubic metre of water weighs one tonne. That means water can build up enormous pressure when it hits an obstacle directly. Set in motion, this results in enormous forces that can act on cars or containers and simply push them ahead of themselves if they are not very firmly anchored. Then there is the phenomenon of erosion, which can destroy supposedly stable surfaces: (Earth) surfaces are eroded by fast-flowing water.
Can you explain this in more detail?
The heavy precipitation brings such large amounts of water onto the soils - often already saturated by previous rainfall - that they can no longer percolate there. (Clay soils, for example, can absorb and drain much less water than sandy soils).
The water then runs off superficially: it makes its way into streams and rivers. Once in these channels, it can reach very high velocities (in rivers already in the order of one metre per second under normal conditions).
The higher the speed, the higher the gradient - especially at local steps such as embankments and terrain edges - and the deeper the river, the more force the water can develop on the ground: Where it flows along, it pulls with the force of a weight of several kilograms. That is enough to sweep away sand, stones and even rubble.
But still no houses and roads...?
No. As a result, it is not only the water itself that has an effect, but also the particles it carries. They smash into the ground, roads and house walls, unleashing an enormous erosive force. Once parts of it are affected, the underlying material can be carried away much more easily. Undercutting occurs because roads and houses are often built on unconsolidated ground, and further material can easily cave in. This interplay of entrained material and the power to simply carry away exposed further material gives the fast-flowing water the power to cause such enormous damage in a short time.
Are you researching such processes yourself?
At GFZ Potsdam, we study, among other things, how exactly water mobilises sediment, how tidal waves move, and with what force they make their way through the landscape. Such floods occur not only in Germany, but everywhere where heavy precipitation can occur.
Such precipitation events are particularly dangerous in the high mountains, where suddenly failing dams cause entire lakes to overflow or landslides melt huge amounts of ice, creating flood waves in the narrow valleys.
Is there a way to warn of such events?
There are two approaches to this. Warnings can be derived from weather forecasts. For example, weather forecasts can be fed into hydrological models to make predictions about the occurrence and probability of flood events.
Erosion processes are always problematic. Predicting them is difficult, especially because these events occur very quickly and their intensity is difficult to assess accurately. With the help of satellite images and, above all, seismometers, we researchers have been trying for some years to track these flood waves almost in real time and to calculate their intensity. Research on this is still in its infancy, but it has immense potential to warn the population of such floods as quickly as possible, not only here in Germany, but in many endangered areas around the world.
Comment on the photo by Michael Dietze:
The picture is from the community of Loch, a little below the Steinbach dam. The gentle slope without a channel had turned into a river the day before, which shot over a bend in the terrain and formed this amphitheatre (almost like Niagara Falls, only smaller). On the field above, there were metre-long tree trunks (30 cm in diameter), so there must have been a lot of water flowing. Down in the village, the slope drained into the river and swept away many houses and streets.