Train accident in Baden-Württemberg: How do landslides occur?

The cause of the train accident in Baden-Württemberg was likely a landslide triggered by heavy rainfall. "The water triggered a landslide in the embankment area near the tracks, which in turn likely caused the derailment," police said Monday morning.

Severe thunderstorms swept across the region in the early hours of Sunday evening. According to the German Weather Service, 30 to 40 liters of rain per square meter fell within a short period of time. According to the latest information from investigators, three people died and at least 41 were injured in the train accident.

A landslide is a type of mass movement—that is, a mass of rock, debris, or earth moves down a slope under the direct influence of gravity. This movement can occur suddenly or slowly over a longer period of time.

"We always think of a landslide as a large landslide on a mountain," says Ugur Öztürk, postdoctoral researcher at the GFZ Helmholtz Centre for Geosciences and Assistant Professor for Geohazards at the University of Vienna. "But there are different types of mass movements." Mass movements are classified according to the type of movement, speed, and material composition. Rock and debris can, for example, fall, tilt, slide, drift, and flow, among other things.

• Falling : Rock material rolls and bounces downwards – as in a rockfall or landslide.
• Tilt : A block of rock, earth, or rubble rotates forward around a pivot point and moves downward.
• Sliding : A mass of rock moves downwards along a sliding surface, for example a slope – as in landslides and mountain slides.
• Drift : a lateral movement of rock or cohesive soil. The rock slides apart.
• Flow : A mixture of rock material and water flows down a slope—for example, in a debris flow. Debris flows often leave a characteristic, inverted funnel-shaped deposit at the point where the landslide material came to rest.

"Almost every landslide has multiple causes," explains the U.S. Geological Survey (USGS) . "Slope movement occurs when downward forces (primarily due to gravity) exceed the strength of the soil materials that comprise the slope."

Excessive moisture in the form of heavy rain or melting snow can affect the strength of the soil. "If a lot of rain falls in a short period of time, the soil can become saturated with water, making it heavier and more likely to slide," explains Philipp Blum, Professor of Engineering Geology at the Karlsruhe Institute of Technology. The majority of all landslides worldwide would be caused in this way.

Likewise, earthquakes can trigger landslides. Seismic tremors can compromise the stability of the ground and cause it to slide. Earthquakes can also trigger underwater landslides, also known as submarine landslides or underwater landslides. "Underwater landslides sometimes cause tsunamis that damage coastal areas," according to the USGS.

Humans also influence landslides: by cutting down trees whose roots hold the soil together, or by building roads and buildings near slopes, they change the strength of the soil.

Climate change is increasing the risk of landslides. Firstly, it is increasing the risk of extreme weather events such as heavy rainfall. As the air warms, it is able to absorb more water vapor. This water vapor then falls back to Earth as increased rainfall.

Second, global warming is causing snow, glaciers, and even permafrost to melt. Permafrost acts like a kind of natural glue, holding rock material together and thus providing stability. When it melts, the rock can more easily break away and slide. The same applies to glaciers, which also influence the stability of mountain slopes.

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Landslides occur particularly in areas with steep slopes, unstable soils, or heavy rainfall. These include mountainous regions such as the Alps or the Andes. Regions such as Southeast Asia, parts of Africa, and South America are particularly affected by increasing heavy rainfall events.

Landslides can also occur in Germany. "Of course, there are fewer potential landslide areas here," says geoscientist Öztürk. However, such mass movements are also possible in low mountain ranges such as the Harz Mountains, the Black Forest, or the Swabian Alb – and have already been observed, often in conjunction with flooding. With climate change and the associated heavy rainfall events, we will certainly experience more landslides in Central Europe as well.

In Germany, the risk of landslides is "not such an acute issue," says Blum. The events often occur locally and therefore disappear from public awareness after a short time. In addition, landslides in Germany claim only a few lives compared to other countries. This also contributes to the low perception of danger. A study published by Blum together with international colleagues in 2016 found that there were just 15 deaths related to landslides in Germany between 1995 and 2015. In Turkey, the number was 336 during the same period.

"But because the hazard situation is changing due to increasing heavy rainfall, we also need to address the issue differently," demands the engineering geologist. "We should be better prepared." In his view, an important element in prevention is early warning systems that help better assess the risk of landslides. For example, by detecting changes in the slope or ground movement at an early stage.

Weather forecasts can also be used to your advantage. For example, by looking early to see which regions are forecast for heavy rain. "If we have high-resolution precipitation monitoring, like that found in a rain radar app, we can make predictions about the potential landslide risk," says Blum. "The railway then has to consider whether to close tracks or lines in the event of a heavy rain event." Even a few large stones or earth falling during a landslide would be enough to derail a train, as is currently the case in Baden-Württemberg, adds Öztürk.

The big problem is often the reporting chain, Blum continues. Information about potential risks is available, but it's not properly communicated. He therefore recommends using emergency information services like the NINA warning app. "So you can immediately see: Okay, there's a landslide hazard there."

In addition to early warning systems, landslides can also be prevented by greening plants that stabilize the soil with their roots. Structural measures such as retaining walls or ground anchors can also help. Anchoring involves special anchors being placed deep in the slope and connected to a steel structure that holds the earth masses together. "Of course, this method isn't cheap," Öztürk points out. And it only helps with smaller landslides.

"With larger mass movements, we have fewer options," says the geoscientist. "Then we can only observe the changes in the rock to take timely evacuation measures."