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Dergacheva Irina Viktorovna, senior researcher, Hydrometeorological Research Institute, Uzhydromet, E-mail: Dergacheva_iv@mail.ru
FLASH FLOODS CAUSED BY THE BREAKTHROUGH OF HIGH-MOUNTAIN LAKES ON THE TERRITORY OF UZBEKISTAN
Abstract: The article provides the result of analysis of the spatial and temporal distribution of flash floods caused by the breakthrough of high mountain lakes on the territory of Uzbekistan.
Keywords: high mountain lakes, flash floods, dangerous hydrometeorological phenomena.
In the current conditions of active use of the foothill and mountainous areas of Uzbekistan, the problem of the threat of breakthrough of high-mountains lakes of various genesis is becoming more urgent and still little studied. Analysis of the research showed that in mountainous areas are ubiquitous modern exogenous processes lead to the formation of debris flows, landslides, mudslides, avalanches and mountain lakes breakthrough.
Among the dangerous hydrometeorological phenomena, especially the destructive forces are flash floods formed as a result, of the breakthrough of lakes, as well as intra-glacial and intra-moraine reservoirs.
Flash floods of breakthrough genesis occur much less frequently than rain mudflows, in percentage terms they constitute only 2% of the total recorded number of flash floods over the territory of Uzbekistan, but breakthrough flash floods are the most catastrophic (Figure 1).
Figure 1. Distribution of flash floods by genesis in the territory of Uzbekistan
The mountainous regions of Uzbekistan cover the lower western spurs of the Gissar-Alai and Tien Shan, as well as the greater part of the Fergana Valley. These mountain areas are the most inhabited within the region. A case of the flash floods in the developed areas is accompanied by considerable damage. Big cities of Uzbekistan - Namangan, Andijan, Fergana, as well as settlements, irrigation canals, railroads and highways, irrigated lands are the most endangered of flash floods.
680 lakes have been identified on the territory of the Republic of Uzbekistan and transboundary territories in the course of research. The breakthrough of these lakes threatens the national economy of Uzbekistan. The majority of lakes are located in the territory of neighboring countries, in the
basins of rivers flowing to the territory of Uzbekistan. Such transboundary basins are conditionally assigned to the regions of Uzbekistan to which they carry water, and pose a potential threat if high-mountain lakes break through.
A total, have been identified of 20 river basins, on which there are high-mountainous break-through lakes (Figure 2).
The most concentration of lakes is in the altitude range from 3000 to 4000 m a.s.l, and the largest total volume of lakes at altitudes from 1500 to 2500 m a.s.l. This is explained by the fact that at altitudes up to 2500 m a.s.l. there are large rock-dammed lakes, for example, the largest lake in the surveyed area Sarychelek (493 million m3) is located at an elevation of 1859 m a.s.l.
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Figure 2. Scheme of the location of river basins with break-through lakes
In the altitude range from 3000 m a.s.l. and above there is a pressions, negative forms of relief are associated with the mani-large number of glacial and moraine lakes. Which is because at festation of activity of modern and ancient glaciation, favorable these heights there are powerful geological faults, tectonic de- for the formation ofa large number ofvery small lakes (Figure 3).
INumbcr of laSics. pes. — I ota] areas tfrnnr Lakes, sq.m
Figure 3. Distribution of the number and total volume of lakes on high-altitude zones
Rockdammed lakes caused by landslides of rocks can exist for tens, hundreds and thousands of years, although they may break through. The ice dams, however powerful
they were, are short-lived. The ice is melting, cracking, some of its blocks emerge, and the destruction of such a dam is a matter of time [1].
With the reduction of glaciation, which is observed not only in our region but also around the world, the site of retreating glaciers formed shafts terminal moraines and "dead" ice. Between these ramparts in summertime, with intensive melting of seasonal snow and ice, melt water accumulates, forming temporary lakes. These lakes are very variable, their number and size undergo strong changes from year to year. Such lakes can break through the destruction of ice or moraine dams, which leads to floods or mudflows, sometimes catastrophic. This happened many times in the headwaters of the rivers flowing into the Fergana valley from the northern slopes of the Alai and Turkestan ranges.
The flash flood of 1998 on the Shakhimardan River was formed as a result of the breakthrough of three moraine lakes formed on the terminal moraine and "dead" ice, below the Ar-chabashi glacier (on the territory of Kyrgyzstan), in the upper reaches of the same river - one of the Shakhimardan river flowing to the north from the Alay Range in Fergana Valley [2; 3].
The approximate total volume of water in the erupted lakes was 40,000 m3. The upper lake, the largest, broke through the moraine-ice dams, splashed into the lower lake, which in turn broke and caused flooding. The maximum breakthrough rate was approximately 50-70 m3/s.
When the flood spread down the valley, it seized the ground due to the intensive erosion of the riverbed and the banks, resulting in the formation of a mudflow that, when moving down the river, was replenished by the melt water of
the remaining tributaries of the Shakhimardan River. Of the spreading of the flood, the water flow in the area of the city of Shakhimardan was equal to 150-200 m3/s. As a result, of the catastrophe, more than a hundred people were perished and huge damage was done to the national economy of Uzbekistan.
The intensive melting of extremely large winter snow reserves contributed to the formation of flash flood in early July. By the time the flash flood passed the soils were well "ripened" - saturated water.
In the case of the breakthrough of glacial and moraine lakes, it is almost impossible to predict the time of the beginning of the flood. The only thing that can be done is to identify areas where such floods are possible, and to determine the periods when their occurrence is most likely. Typically, this summer is a hot time, when moraine and glacial lakes are overflowing with water. The time of possible descent of glacial mudslides - the mudflows period - corresponds to the period of maximum intensity of ablation and runoff from glacial basins.
On the basis of a comprehensive study and generalization of the available information on the breakthroughs of the high-mountain lakes of Central Asia, the features of seasonal risk of the threat of mudflows of breakthrough genesis are established. It was revealed that the most number of mudflows of rain genesis occur in May, and breakthrough mudflows are activated in July and in August (Figure 4).
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Hiih 111 I ink fmili hriU' rainfall Kluih fluni H fmili brink! lumi^ii lik(\
Figure 4. Intra-annual distribution of mudflows of different genesis,%
In recent years, there has been a steady trend of global Expert estimates of the response of small lakes in the glacial
warming. According to climatologists, the warming, the maxi- zone indicate an increasing danger of breakthroughs in lakes
mum of which is forecasted for 2020-2040, will primarily af- with unstable dams. This is due, first of all, to the possibility of
fect mountainous and foothill areas [4; 5]. the appearance of new lakes in the preglacial zone due to the
reduction and retreat of the ends of the glaciers as a result of warming. When the glaciers recede, a glacial-moraine complex forms on the lower edge of the glaciers, including dead ice fields, pond lakes, accumulative bodies of moraines and fluvioglacial deposits. This complex is continuously changing, that is, it is in an unstable state, which favors the occurrence of mudflows. The mudflows at the ends of modern glaciers originate mainly due to the breakthrough ofthe preglacial lakes or the breakthroughs of moraine and firn-ice masses. The highest activity of glacial mudflows is characteristic of the stages of the retreat of glaciers. The current degradation of glaciation, associated with global warming, maintains this level of high activity.
The increase in summer temperatures will lead to an increase in the intensity of snowmelt, an increase in the maximum flow ofwater supplying the lakes of watercourses, which will affect the existing equilibrium of the water regime of lakes and the capacity of dams, existing lagoon lakes.
Humanity cannot change or stop the development of dangerous phenomena, so the ability to reduce their destructive effects is the creation and development of systems that allow us to constantly monitor the state of the hydrometeorological environment, predict the occurrence of a dangerous phenomenon and warn the public about the impending danger.
References:
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2. Report on the results of hydrometeorological studies ofbreak-through mountain lakes in Uzbekistan and adjacent territories in 1998 by the hydrographic party of Uzhydromet.- Tashkent, 1998.- 25 p.
3. Chernomorets S. S., Viskhadzhieva K. S., Savernyuk E. A., Petrov M. A., Erokhin S. A. and others. Fouling processes on the Alay and Kirghiz ridges (on the examples of the basins of the Shakhimardan and Aksai rivers) // Sedimentary flows: catastrophes, risk, prognosis, protection: Materials of the IV International Conference Russia, Irkutsk. V. B. Sochava of the SB RAS, 2016.- P. 56-59.
4. Myagkov S. M. Possible changes in the nature of the Central Tien Shan by 2025 // Bulletin of the Moscow State University. Ser. "Geography". 1981.- No. 5.
5. Jamgyrchiev D. C. Landscape-ecological infrastructure of the territory as a basis for sustainable nature management in Kyrgyzstan // Natural Sciences and informational Tehnology. Academic Review - No. 2. 2007.- P. 212-217.
