Debris flow

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Edge walls of a Murbahn

A debris flow (also Mure , Gisse or Rüfe mentioned in the media often mudflow ) is a landslide in which a fast downhill flowing stream of mud and coarse rock material in the mountains, physically similar to a very coarse suspension . Debris flows have a high solids content and therefore a high density (up to 2.6 g / cm 3 ). A debris flow can transport several hundred thousand cubic meters of material. He can wreak havoc with his energy. The mostly clearly defined front can reach speeds of up to 60 km / h.

Origin and process

A debris flow occurs when little solidified material ( rubble , rubble and earth material) is saturated with water in steep terrain and is set in motion , mostly by the action of gravity . Soaking is mostly triggered by heavy or long-lasting precipitation or snowmelt , but increasingly also by the melting of glaciers or permafrost soils due to global warming . Debris flows usually follow existing stream beds or gullies and greatly expand them, but they can also erode a new channel. Coarse grain sizes (stones, blocks) are concentrated on the mudflow front, which can carry away material up to meter-sized boulders and tree trunks. Some of the transported material is deposited in edge walls ( Levées ) along the flow path . The movement usually ends at the foot of the slope, where the slope subsides. There the material is deposited in the shape of a tongue. Repeated debris flow events form sediment cones (mud cones ). Because of the high solids content and the associated viscosity of the outgoing mixture, sorting hardly takes place during accumulation .

Hazard potential

Train derailment due to debris flow

A debris flow has significantly more energy than a flood and causes considerably more damage. A debris flow hitting with full force can destroy houses, traffic routes and bridges. Often streets and the ground floors of houses are buried meters high with the mud and rubble mixture. This happens, among other things, when streams in localities are channeled too narrowly and the debris flow overflows their banks. Because of the often long periods of time between individual debris flows, the population is often not aware of this danger.

Preventing debris flows and debris flow damage includes:

  • Danger zone planning
  • Structural protective measures such as rubble barriers , retention dams or diversion structures
  • Widening canalised streams and avoiding bottlenecks (especially with bridges) so that the debris flow does not cross the "canal banks" (in Brig , for example, an automatic lift bridge was built after the disaster of 1993 )
  • Cleansing mountain streams of loose material (trees, branches and debris) that can trigger or feed a debris flow
  • Debris flow warnings (still at an experimental stage)

Due to climate change, an increase in debris flows is expected over the next few decades. When high alpine permafrost soils and rock glaciers thaw, more material that can be mobilized is created, which can then go off as mudslides.

Dating events

If the debris debris deposits are preserved as debris flows and mudslide cones, they can be dated in various ways and thus the approximate time of an individual debris flow can be determined. The systematic recording of as many debris flow events as possible can thus provide information about the general susceptibility to the Mur river as well as about the climatic history of an area. Often the age of the trees that grow on mottoes and cones is determined. Sedimentological investigations are also possible . At the Pragser Wildsee, the relationship between debris flows and the resulting deposits on the lake bed was investigated. By evaluating the lake sediments, it was possible to create a Mur calendar. There are clear differences in the frequency of debris flows over the centuries. However, a connection between Mur activity and large-scale climate could not be established, so that anthropogenic causes are suspected.

Web links

Commons : Debris flow  - collection of images, videos and audio files

Individual evidence

  1. Dieter Rieger: Assessment of the natural framework conditions for the formation of slope debris flows: Possibilities for modeling the debris flow potential . Geobuch-Verlag, Munich 1999, ISBN 3-925308-73-3 ( Munich geographical treatises. Series A, Vol. 51).
  2. Markus Zimmermann: Recognizing and evaluating debris flows . In: Björn Oddson (Ed.): Unstable slopes and other risk-relevant natural processes: Postgraduate course in applied earth sciences . Birkhäuser Verlag, Basel 1996, ISBN 3-7643-5472-0 , p. 183-196 ( online ).
  3. Ralf Irmler: Lake sediments as a natural archive for the creation of a Mur calendar using the example of Lake Braies (Northern Italy). Chemical-Geoscientific Faculty of the Friedrich Schiller University, Jena 2003, dissertation, online as a PDF file (21.1 MB) .