Alluvial fan

An alluvial fan, alluvial fans also, detritus compartments, Sandurs or alluvial fans called, is the accumulation region ( deposition area ) of fluvial sediments at a point where a river on slope loses, for example when crossing the mountains in a plane.

Because of the decrease of the flow rate and thus the transport capacity of a triangular or conical alluvial fans formed. Rivers with active alluvial fans often have multiple flow channels, as the water has to avoid the already alluvial deposits. In addition, the river level is increasingly displaced by alluvial fans. The nature of the transported boulders from the river, the size of the river and the surrounding landscape determine the appearance of the fan.

The larger the gravel parts, the more striking is the alluvial fan. The larger boulders parts can be found at larger height differences, for example, in the Alps. For larger rivers in alluvial form, having large sand and Lehmakkumulationen. Typical alluvial fan, for example, at the end of canyons and wadis.

In the mouth region of lakes or seas go on alluvial fans in the Delta. Alluvial fans are fertile. Examples are the deltas of the Nile or the Ganges. In some valleys, the villages have specifically located on the higher alluvial fans of the side streams to avoid the risk of flooding of a river.

Different types of alluvial fans

Debris to mud flow

A Schutt-/Schlammstrom formed when the sediment / water mixture gives a plastic paste which comes with a corresponding slope of the alluvial fan in motion. The resulting sediment body can be 30 centimeters to several meters thick. They form Praise and dams, some of which sharply differentiate themselves from each other and overlap each other. Such fans ( subjects) have a very steep gradient of 0.1-0.01 %.

The sediment contains rocks ranging in grain size of blocks with several tons of weight up to clays. The particles are surrounded by a Tonhülle and generally very poorly sorted.

The channel filling

Through strong water currents troughs are eroded in a alluvial fan. If the flow energy decreases, these channels are replenished by sands and gravels. The troughs are elongated, lenticular and distinguish themselves sharply from one another. They have a tendency to deposit finer sediment surface downward. The deposited sediment is poorly sorted.

The Intertwined run

The eroded Intertwined run and sedimented on the fan surface gravels, sands and silts. The layers and gutter systems are irregular and usually no deeper than 30 cm. The sediment is poorly sorted and deposited praise and tongue-shaped. The slope of the fan is relatively large in this system. This type of deposition usually occurs in semi-arid areas with irregular water supply.

Meandering systems

In meandering systems, we have a significantly low tendency of the water. This is due to the higher amount of water, which provides not only the lesser gradient for a more spacious spread of the alluvial fan. This also increases the transport distance of sediments, resulting in better sorting and a higher degree of rounding of the components result. In such a system can be found distally coarse components, while the fine fraction is present rather proximally.

Proglacial alluvial

These subjects are characterized by the discharge of melt water from the glacier. This transported sands and silts with it that have been ground at the base of the glacier, or are washed out of moraines. This system has a high water / sediment ratio, which leads to a long transport, good sorting and a small angle of inclination of the slops. The channels are meandering and shift to the fine-grained sediment body ( Sander ) often their gutters.