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Masses in motion: debris flows and bedload

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In steep mountainous regions, mountain torrents represent a significant source of danger: when it rains heavily, the torrents sweep away destructive quantities of rock and earth. Using special measuring systems, we analyse these processes in order to develop models (simulations) and warning systems.

 

As part of a particular study, we were able to show that bedload transport processes cause damage costing over CHF 100 million each year. Debris flows, also commonly known as mudflows, are especially feared. During severe storms, a slurry-like mixture of water, fine matter and rock intermittently pours into villages and onto roads and railway lines, often at high speed.

The largest debris flows in the Alps have swept away up to half a million cubic metres of material, transported it down the valley and deposited it there. Such a vast quantity would require over 40,000 trucks to remove it. Quantities that would fit into around 1,000 trucks are not uncommon.

In order to minimise risks and take protective measures, it is essential to know exactly how a debris flow behaves. This is why we conduct experiments and take long-term measurements. For this purpose, we have developed what are, in some respects, all-new types of measuring system, such as a debris flow scale for determining the mass of the debris flow running over it.

Thanks to the unique data from our systems, we have been able to enhance the avalanche modelling software RAMMS with a debris flow module. The simulation calculates the potential flow and breakout paths of a debris flow depending on rainfall, terrain and slope gradient. (RAMMS::DEBRISFLOW)

"Fishing" for bedload

The link between rainfall, vegetation on mountain slopes, bedload volume and flood events was observed early on. We have been studying bedload transport in the Erlenbach stream in Alptal (Schwyz) since the 1980s. This is one of the longest observation periods in this field of research worldwide, which makes it especially valuable for studies of sediment transport processes.

New types of measurement method have since been added, such as automatic basket samplers, which catch bedload from the stream for a specific period of time during flooding in order to estimate bedload volume and grain composition.

Our research not only provides a scientific basis, but is helpful for assessing risks and developing protective measures. Given that it is a question of weighing up significant costs on the one hand and risk on the other, we also look into how the public address the risk and what their opinion is of the countermeasures.

 

Topics

Debris flow

To minimise the dangers of debris flows and improve forecasting, we research processes in the field, in the laboratory and via computer simulation.

Mountain torrents

Mountain torrents often sweep away large quantities of sediment. We conduct research to better understand these processes and calculate the risks.

 

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