The formation of debris flow must have the following three conditions:
topography and geomorphology that is precipitous and easy to
collect water; rich in loose matter; a lot of water resources
in a short time.
Topographical and geomorphic conditions
There are mountains and deep valleys, steep terrain, steep slope and
large drainage basin, which is easy for water collection.
In geomorphology, the landform of debris flow can be divided into three parts:
formation area, circulation area and accumulation area. Terrain in upper
reach of rivers is surrounded by mountains on three sides and one exit is
like gourd ladle or funnel, which is open. In addition, there are many
high and steep mountains around there and some mountains crush with
poor vegetation failing to grow well. This kind of terrain is conducive to
concentration of water and detrital material; the middle reaches of the
topography is mostly narrow and steep canyons, which can make the mudslide flow
down swiftly. The topography of the downstream accumulation area is the
open and flat plain or valley terrace, which is the accumulation place.
Loose material source conditions
Debris flow often occurs in areas of complex geological structure,
fracture and fold development, strong new tectonic activity,
and high earthquake. Surface rock fragmentation, collapse, dislocation,
landslide and other undesirable geological phenomena development provide
a rich source of solid material for the formation of debris flow;
In addition, the rock structure is loose, weak, proning to weathering,
joint development, or areas with soft and hard layers, so the area is easily
destroyed and can also provide abundant detrital sources for debris flows.
Some human engineering activities, such as deforestation, soil erosion,
mining of the mountains and slag of rocks have also provided a large number of
material sources for the mudslide.
Water is not only an important part of debris flow but also the excitation
condition and handling medium (power source), such as water resorce of
debris flow, heavy rain, ice and snow melting water and reservoir
dam (pool) water, etc. The main water source of debris flow in our country
is rainstorm, long-continued rainfall and so on.