Debris flow dangers of the 21st century in the northern caucasus (russia)
Keywords:degradation of glaciers, dammed lake neoformations, outburst processes, disastrous debris flows, moraineice complex
The negative impact of debris flow processes on vital facilities is expected to increase as a result of the forecasted degradation of mountain glaciation, related to the changes in the climate. In the Caucasus region of Russia, where debris flows stand out for the frequency of occurrence in space, time and the power of their energetic manifestation and are mostly of glacial origin, that follows from the analysis of the developing situation, which relies on the knowledge of facts of the last 30 years (events on the Kullumkol-Su, Kaya-Arty-Su, Genaldon, Gizeldon, Birdzhaly-Su, Bulungu-Su and Sylyk-Suu rivers). As glacier tongues retreat to higher true altitudes (~300 m for the last 50 years), massifs of friable single-grained, primarily moraine, formation become exposed, periglacial water bodies rapidly (comparatively speaking) appear, gradients of tributaries, in which water torrents may become saturated with friable detritus, increase. Threats and risks of initiation of regimes of debris flows grow. Most considerable debris flows, in terms of their destructive effect, are connected with outbursts of lakes, lying close to glaciers. Discharge of intraglacial hollows more and more frequently acts as a triggering mechanism for the start and further development of a debris flow process. In every potentially dangerous debris flow basin of the region, due to peculiarities of the Quaternary period, there are various ways of realisation of disaster scenarios for mouth parts of mountain rivers. Even in the territorially close basins there may develop various flows of processes of hard material transport by water torrents, [various] distances and volumes of deposit transport, which depend on morphologic conditions along the course of movement. The length of the period of high temperatures in July- August, which causes heightened ablation of ice masses against the background of a lowered general and (or) filtering stability of natural retaining dams, is most significant. At the same time not every close-to-a-glacier hollow or glacial hollow with the realisation of the outburst scenario thereupon is able to initiate a debris flow process – for that to happen outer slopes of a natural dam (usually of most recent moraine) should have 30- 35° steepness (~20° steepness is insuf cient whereas within the 20-30° steepness range the possibility of an avalanche-like increase of conditions for transformation of a water flood into a debris flow may be realised only with certain water mass volumes (~ ≥ 80-100 thousand cubic metres) and relative heights of an eroding slope (~ ≥ 80-100m). Herewith every occurred significant debris flow changes geomorphological preconditions for the next one. Landslides towards river mouths, wherewith short-lived blockage-caused water bodies are formed, are sometimes responsible for secondary waves of debris flows, which have originated from glacial sites (the Buzulgan landslide on the Gerhozhan-Su river and a landslide in the Rakyt river valley).
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