A debris flow (also called mudflow or landslide ) is a fast downhill flowing stream of mud and coarse rock material in the mountains, chemically similar to a very coarse suspension. Debris flows have a high solids content and thereby causes a high density (up to 2.6 g/cm3). A debris flow can transport several hundred thousand cubic meters of material. It can cause great devastation caused by its energy. The most clearly pronounced front can reach a speed of up to 60 km / hr.
Formation and flow
A debris flow is produced in the mountains, if poorly consolidated material (gravel, rubble and soil material ) is water supersaturated with steep terrain and usually only by the action of gravity, is set in motion. The wetting is triggered mostly by strong or prolonged rainfall or snowmelt. Debris flows usually follow existing stream beds or gutters and extend it strong, but they can also dig a new channel. Coarse grain sizes (stones, blocks ) focus on the debris flow front, the material can be carried away to meter-sized boulders and tree trunks. Even along the flow path of some of the transported material in peripheral walls ( Levées ) is deposited again. The motion usually ends at the foot of the slope, where the slope decreases. There, the material is deposited tongue-shaped. Repeated debris flow deposition cone to form. Due to the high solids content and the associated strength of the outgoing mixture found in the accumulation sorting instead hardly.
A debris flow has significantly more energy than a flood and depends considerably higher damage. An incident with the full force of debris flow can destroy homes, roads and bridges. Often the roads and the ground floors of houses are meter high shed with mud - gravel mixture. This happens, among other things, when streams in villages are too tight and the channeled debris flow occurs there on the shore. Because of the often long periods between individual debris flows, the population of this danger is often not aware of.
To prevent debris flows and Murgangschäden include:
- Hazard zone planning
- Structural protection measures such as gravel barriers, dams or Ablenkbauwerke
- Widening channelized streams and avoid bottlenecks (especially on bridges ) so that the debris flow does not occur on the " canal bank " ( in Brig for example, was after the catastrophe of 1993, an automatic lift bridge built )
- Cleaning up the mountain streams of loose material ( trees, branches and debris ) that can trigger or nourish a debris flow
- Murgangwarnungen (still in an experimental stage )
Because of global warming is expected for the next decades, with an increase of debris flows. If high alpine permafrost and rock glaciers thaw creates more mobilizable material that can then come off as Mure.
Dating of events
Stay the deposits of debris flows as a Murzungen and Murkegel, they can dated in various ways and so the approximate time of a single debris flow can be determined. The systematic recording of as many debris flow can deliver as information on general Muranfälligkeit and on the climate history of an area. Often, the age of the trees is determined that grow on Murzungen and cones. Are also possible sedimentological investigations. Thus, the relationship between debris flows and the resulting deposits on the lake bed was investigated at Braies. By analysis of the lake sediments, it was possible to create a Mur calendar. Here, to determine significant differences in the frequency of debris flows over the centuries. However, the relation between Mur activity and major climate could not be found, so that anthropogenic causes are suspected.