Alluvial cone

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Small alluvial cone
An extensive alluvial cone stretches across the wasteland between the Kunlun and Altun mountain ranges that form the southern edge of the Takla Makan desert . (China, Xinjiang ). The left side is the currently active part of the alluvial cone.
In Europe, for example, extensive alluvial cones cover around 70 percent of the Vinschgau .

An alluvial cone , also called alluvial fan , scree fan , sandur or alluvial fan , is a subaeric fluvial ( alluvial ) sediment body that arises where a flowing water abruptly loses its gradient, typically when it exits a high area into lower-lying terrain with less relief. It is not to be confused with a delta , the sedimentation of which mostly occurs under relatively high and permanent water cover (subaquatic). If, on the other hand, a cone forms from rubble without the influence of water, one speaks of a rubble cone .

Because of the abrupt decrease in the gradient, there is an equally abrupt decrease in the flow velocity. At the same time, the transport capacity for the coarsest fractions of the rock material carried by the water at this point in time decreases (see →  Hjulström diagram ). Thus, a conical or fan-shaped sediment body is formed at the corresponding points.

Rivers with active alluvial fans often have several arms there, as the water has to avoid the deposits that have already been washed up. In addition, if the conditions remain unchanged, the alluvial cone builds itself further and further into the lower terrain ( progressation ). The type of material transported by the water, the maximum flow of the water and the strength of the relief contrast at the outlet of the water from the high area determine the properties of the fan.

Alluvial fans with a particularly large amount of coarse material can be found in high mountain regions on torrents , as the gradient is particularly large there. Alluvial fans at the end of gorges or wadis are typical .

An alluvial fan of a confluent tributary can force the main river towards the opposite side of the valley. An example of this is the mouth of the Sill in Innsbruck .

Importance to humans

In some valleys, villages have been specifically created on the higher alluvial cones of the side streams in order to avoid the risk of flooding from the main river flowing on the valley floor. However, the situation on such an alluvial cone is also risky, as the sedimentation events that largely created these cones are accompanied by the deposition of enormous amounts of material in a relatively short time, so that a small town can be completely buried. There remains the risk that the torrent that forms the alluvial cone will commute and change its course.

Recognizable on maps

Alluvial cones can be easily recognized on maps with hillshades and / or contour lines , especially from the typical semicircular course of the contour lines. One example is the display of the alluvial cone between Domat / Ems and Chur in OpenTopoMap.

Different types of alluvial fans

Alluvial cone between Domat / Ems and Chur

Debris to mud flow

A debris / sludge flow forms when the sediment / water mixture results in a plastic slurry that starts to move when the alluvial fan is tilted accordingly . The resulting sediment body can be 30 centimeters to a few meters thick. Praises and dams are formed, some of which are sharply demarcated from one another and overlap one another. Such subjects have a very steep gradient of 0.01–0.1 (1% –10%).

The sediment contains rocks with grain sizes ranging from blocks weighing several tons to clays . The particles are surrounded by a clay shell and are generally very poorly sorted.

The gutter filling

Strong water currents erode channels in an alluvial fan. When the flow energy decreases, these channels are filled up again with sand and gravel. The grooves are elongated, lens-shaped and sharply delimited from one another. They have a tendency to deposit finer sediment downstream. The deposited sediment is poorly sorted.

The entwined run

The interwoven course erodes and sediments gravel , sand and silt on the fan surface . The layers and gutter systems are irregular and usually no deeper than 30 cm. The sediment is poorly sorted and is deposited in the shape of a praise and tongue. The slope of the fan is relatively large in this system. This type of deposit usually occurs in semi-arid areas with irregular water flow.

Meandering systems

In the case of meandering systems, we have a significantly lower slope of the water. This is due to the higher amount of water, which, in addition to the lower gradient, also ensures that the alluvial fan spreads over a larger area. This also increases the transport distance of the sediments, which results in better sorting and an increased degree of rounding of the components. In such a system, coarse components are found proximally , while the fine fraction is more present distally .

Proglacial alluvial fans

These compartments are characterized by the emergence of meltwater from the glacier gate. This transports sand and silt that have been ground at the bottom of the glacier or washed out of moraines. This system has a high water / sediment ratio, which leads to a long transport route, good sorting and a low angle of inclination of the slope. The channels are meandering and often shift their channels on the fine-grained sediment body ( sander ).

Individual evidence

  1. a b alluvial fan. In: Lexicon of Geosciences. Spectrum der Wissenschaft Verlagsgesellschaft mbH, accessed on August 24, 2020 .
  2. ↑ Alluvial cone. In: Lexicon of Geosciences. Spectrum der Wissenschaft Verlagsgesellschaft mbH, accessed on August 24, 2020 .
  3. a b Dangers in the Alpine Region - Alluvial Cone. Bavarian State Office for the Environment, accessed on August 24, 2020 .
  4. Estuary. In: Lexicon of Geosciences. Spectrum der Wissenschaft Verlagsgesellschaft mbH, accessed on August 24, 2020 .
  5. OpenTopoMap. Retrieved August 24, 2020 .

Web links

Commons : Alluvial cone  - collection of images