Italy's deadly floods: How can water be so powerful?

Within minutes, catastrophic floodwaters in Italy toppled entire houses, swept away cars as if they were matchboxes and turned basements into death traps. Time and again, nature demonstrates its overwhelming force — and we find ourselves at its mercy.

But how is water so powerful? Michael Dietze of the geomorphology section at the Helmholtz Center Potsdam explains more on the website of the German Research Center for Geosciences.

Dietze said it's important to remember that a cubic meter of water weighs one metric ton, making it very heavy, adding that "water can exert tremendous pressure on an object in its path. And moving water is immensely powerful — powerful enough to sweep away cars or even shipping containers that haven't been anchored down."

But other factors also come into play, including erosion. Degraded surfaces that might appear stable can easily be swept away by fast-moving water.

At the German Research Center for Geosciences Potsdam, researchers are studying exactly how water mobilizes sediments, how flood waves travel and how powerful floodwaters sweep their way through a landscape.

The German Weather Service has said heavy rainfall is an underestimated environmental risk, with torrential downpours difficult to predict and relatively uncommon in most areas. Meteorologists can predict that it will rain, but can't say exactly when or how much rain will fall on a specific area.

As a result, heavy downpours can cause more damage than one might expect, even in places that aren't situated in narrow valleys or near major rivers. As Dietze explained, "torrential rain dumps a massive amount of water on ground that in many cases has already become saturated, which means the soil can't absorb any more water."

Varying soil types absorb water differently

The volume of water is not the sole factor. Soil composition, or rather its ability to absorb, store and release water, also plays a major role.

This is where the pore size of soil particles comes into play. "Colloids" are tiny particles measuring under 2 micrometers wide — too small to be visible to the naked eye. Their tiny dimensions, however, means that in large quantities they form a gigantic surface area for the water molecules to bind to.

Clay and loam soils contain a lot of these colloids, with the "interstitial water" between the pores unable to run off. With few pores, once properly saturated these types of soils can store more water than sand.

Grains of sand are larger, however, and there are many more large air-filled pores and only a small number of colloids in sandy soil. The ground is then barely able to retain the water between the pores, which quickly runs off.

Another crucial question is the soil's condition prior to the rainfall. In the case of a sudden and heavy downpour after a protracted dry period, soil can't soak up the water all at once. Dried-out ground has what's called "water repellency," meaning instead of seeping down the water flows off the surface. Plant residue is also a contributory factor here, due to fats and waxy substances being released during dry conditions.

Water forges its own path

If the soil is saturated after long periods of rain, the water has no choice but to run off along the surface and make its way into streams and rivers.

"Once there, it can reach very high velocities," said Dietze. At the University of Cologne's ecological research station by the Rhine River, for example, the water normally flows at a speed of 1-2 meters (up to 6.5 feet) per second.

"The higher the speed and the steeper the slope — especially at embankments and ridges — and the deeper the river, the more power the water will be able to pick up on the riverbed. Its pull is equivalent to several kilograms, which is enough to sweep away sand, stones and even debris," explained Dietze.

Water and particles: a fatal combination

But that alone isn't enough to wash away houses and streets — it's the particles that get carried along with it. These are driven into the ground, streets and walls of buildings, and develop a hugely erosive power.

"Once parts of these objects start to come under attack, the material underneath is more easily carried away," explained Dietze, adding that streets and buildings on unconsolidated ground can also be undermined, making it easier for more material to break away.

He stressed that such floods can develop everywhere heavy rains occur and that extreme precipitation is especially dangerous in high, mountainous areas where the resulting sudden failure of dams can cause entire lakes to overflow, or where huge amounts of melting ice can trigger landslides and flood waves in the valleys below.

Can floods be predicted?

"Weather advisories can be derived from forecasts," said Dietze. "For instance, weather forecasts can be fed into hydrological models, to be able to make predictions about the probability and development of flooding."

By contrast, the process of erosion is tougher to predict. Because such events happen very quickly, their intensity is difficult to gauge precisely.

With the help of satellite images and, above all, seismometers, researchers have spent the last few years attempting to follow flood waves in real time and calculate their intensity. Their research is still in the early stages, stressed Dietze, but he said it showed great potential when it comes to flood early warnings systems.