DEBRIS FLOWS

Debris flows, or swift mountainous torrents carrying fragmental products, are characteristic for many areas of Tajikistan. They rush at a great speed and possess a huge destructive capacity, bringing a lot of damage to the country economy (destroying irrigational facilities, bridges, roads, villages, and covering the agricultural fields with debris).
The frequency of debris flows is caused by the specific mountainous topography of steep slopes with poor vegetation and badly developed erosive soils; peculiarities of the geological structure and the intensity of the eolation processes of the mountainous rocks; flash flood character of atmospheric precipitation and intensive melting of snow cover; peculiarities of the condition and regime of the mountainous glaciers; high seismicity of the territory.

With regard to the debris flow danger, the territory of Tajikistan is divided in accordance with the elevation zoning which defines the occurrence conditions and the character of debris flows.
Foothill area occupies lower parts of slopes represented by the so-called adyrys - small hills with a gentle-convex or almost flat top. The slopes are covered by a layer of loess or rock-waste of low capacity soils; vegetation is ephemeral and desert type.
Debris flows occur during flash floods and can be observed from March till June, seldom - in September, October. They take the many existing ditches and dry routes. Some of the routes encounter debris flows annually, and sometimes several times a year. They can be characterized 
as liquid turbulent flows. They are not overloaded by detritus which usually consists of sandy-clay particles with an addition of rock-waster and gravel. The speed is rather high - 3-4 m/sec.
The midlands are characterized by a steeper and more dissected topography. Occurrence of debris flows here is also caused by flash floods. Debris flows are observed here mostly from April to June and usually take the dry routes and permanent channels. By character - they also represent turbulent streams yet the detritus and the size of detritus increase here. They occur within a 3-5 years cycle.
The flows emerging in the foothill and midland zones often die out before coming from the mountains. The most powerful of them come out into the valleys. Passing through the areas with an intensive economic activity, these debris flows cause a great economic loss. 
Most of the debris flows occur in the highlands of Tajikistan. The topography here is rather dissected, huge territories are covered by rocks, talus and moraines. The river beds with high falls are overloaded with the detrital material. The occurrence conditions and the debris flows characters differ within the highlands of the western part of the country, approximately to the 71 meridian (Gissaro-Darvaz), and also to the east of this meridian (Western Pamirs).

Debris flows at the mountainous ridges of Gissaro-Darvaz are caused by excessive precipitation and also by an intensive snow melt. Sometimes debris flows are caused by different reasons. It is well known that powerful debris flows occurred after landslides and earthquakes. Most of the debris flows happen in May and June. In some low precipitation years, occasional debris flows take place in July and August, following the rains. These are flows of huge capacity. The most dangerous in this respect is the southern slope of the Gissar Ridge.
Wester Pamirs is characterized by the intensive development of modern glaciation. The level of precipitation in the highlands is high, and the precipitation takes a hard form. The main reason for the debris flows occurrence is an intensive melting of snow and ice, relatively fast fault of the melt-waters from the glaciers, moraines washing by these waters, breakthrough of reservoirs in glacier areas. Debris flows occur in the warmest summer months - July and August. The most dangerous in this respect are the Vanch River valley, the lower stream of the River Gunt, and the upper stream of the River Obikhingou.
The debris flows observed in Gissaro-Darvaz and the Western Pamirs are mostly mud and rock flows with a high density of detritus, which sometimes reaches the concentration of a mud-avalanche. This kind of flow is often accompanied by jams, the breakthrough of which increases the speed of the flow. This kind of debris flows occur once in 50 and more years.
Wide intra-mountainous valleys and basins, where debris flows do not form, sometimes also suffer their invasion. This happens when the flows originated in the mountains cross such valleys using dry routes.
In the Eastern Pamirs, conditions for the debris flows formation practically do not exist; the level of precipitation is poor (70-120 mm/year); and low temperatures prevail through most of the year.