2.1.3 Ground failures floods (Mudflow, Mud-rock flow, Mud stream, Alluvial fan floods. Volcanic ashes and Mudflow).

The above mentioned floods appear mostly at the rivers or dry river valleys they are included in the group of the river flooding.

Alluvial fan floods is a temporary spring formed in the beds of the mountain rivers, characterized in the sharp increase in the water level, which has a high content /10 to 75%/ hard particles /soil products and rock decomposition products /.

This kind of flood originate as a result of intensive and continuous heavy rainfall, swift melting of the seasonal winter cover or of glaciers, as well as due to the landslide of loose, cracked rock material /at terrain inclination of no less than 8 – 10%/.

Depending on the content of the torrent mass we can specify the following torrents: stone-muddy, water-stone and water-gravel. Besides that as a physical phenomenon they can be free or latent. The transporting medium of the free torrents is water with hard admixtures /Mud-steam/, and for the latent ones the medium is the soil mixture where the main water mass is connected with finely dispersed particles (Mud-flow, Debris flow) Mud-rock flow and Alluvial fan floods – the contents of the mass is mostly soil and rocks/stone –mud/.

In contrast to the regular flows, flows of the mass by default do not move permanently, but in separate waves /pushes/. The volume of the one-time mixture, transported with high velocity, may vary from hundreds of thousands to millions of cubic liters. The front of the moving mixture can have a diameter of 3-4 meters, and mass of 100 – 200 tons. With their large mass and movement speed this mass destroy roads, equipment, agricultural land and others.

The causes for the origination of this kind of flood are mainly the heavy rainfall, the accumulation of masses of crumbly and cracked material, the increase of the intensity of the surface water outflow.

The practice has demonstrated that the centre of this kind of flood/accumulated soil and rock masses/ starts to move after overcoming the resistance of the terrain and other obstacles, that are destroyed by the rain waters, and the rock and soil mass finds a way under the form of separate flows, which join the main flow, moving with high velocity.

This kind of flood within the river basin can be local or common. The local torrents originate in the beds of the river tributaries or in separate gullies.

The common torrents pass through the main river bed and most of them have stone-muddy structure. They are particularly dangerous since they often pass through urbanized regions. Their most dangerous characteristics are the suddenness, the straightforwardness of movement and the transportation of large quantities of hard materials, which pile up on, or destroy along their way settlements, roads, railway tracks, engineering equipment and agricultural lands.

The destructive forces of the torrent are defined by the increased capability to transport hard materials, the interruptibility of the movement, the high velocity of up to 15km per hour.

The human activity, related to the intensive use of mountain slopes leads to the increase in the volume and frequency of this type of floods, as well as to the increase in the number of locations they can occur.

Alluvial fan floods. Their name/fan/ corresponds to the shape of the spread mass. In substance they are the same as the above described but the solid mass prevails and they are more similar to the landslide.

Alluvial fan floods can cause greater damage than typical riverine flooding.

Volcanic ashes and Mudflows. They arise as a result of the Pyroclastic flow – this is a flow of heat gasses over the volcano slopes /about 1 000 C/ which quickly melt the snow into worm water running with high speed over the steep volcano slopes and carrying with itself solid materials. This phenomena is extremely dangerous for the people living near by.
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