Formation of a debris flow
Long lasting or heavy rains, hail an/or intense snow melting can set off the debris flow. The debris flow due to the liquefaction of unconsolidated material or the breaking of debris jam in the streambed. This waterlogged mixture flows down into the valley in several surges at the sheer speed of 40 to 60 km/h through the streambed or an old gully.
It occurs in mountain areas, where the grounds are susceptible to erosion due to flysch, Buendnerschiefer or unconsolidated material. In addition to that, a debris flow needs certain amount of water and a slope of at least 25 to 30%. These conditions are very likely to encounter in many mountain torrent areas.
Debris flow deposition
Debris flow deposition is the sedimentation of debris flow material outside the streambed. Very often it consists of very coarse components. Also, debris deposits of mountain streams caused by fluvial processes are counted as debris flow depositions.
A debris flow has a very high erosion potential and is able to transport huge masses of debris and boulder (blocks of several m3 volume, logs, cars etc.). Along the sides of the streambed, dam-like debris flow levees pile up. The debris flow comes to a sudden halt on the debris flow cone, which results in the typical, unsorted deposits, the debris flow depositions with debris flow head and debris flow lobes.