Dynamics of the hydraulic and alluvial regime of the lower reaches of the Amudarya after the commissioning of the Takhiatash and Tuyamuyun hydrosystems

Abstract

Abstract The article presents data on changes in the main hydraulic parameters of the flow in the hydrological sections of the Amudarya river located in the zone of influence of the Takhiatash and Tuyamuyun reservoirs. It has been established that in recent years an acute water shortage has been observed in the lower reaches of the river during the growing season. In dry years for the Amudarya River, the Takhiatash hydroelectric complex becomes the last section, in some periods there are even no sanitary passes through the hydroelectric complex in the prescribed manner, and as a result, a tense situation is created in the Amudarya delta below the Takhiatash hydroelectric complex. Analysis of the hydrological regime of the river showed that in the lower reaches of the Amudarya on the territory of the Kharezm region and Karakalpakstan, the water regime of the river is very different from the domestic one. The low-water period on the river practically begins in September, as there is a large flow of water in September and October. The waters coming from the upper reaches of the river are retained to fill the Tuyamuyun reservoir. At the same time, the end of the low-water period is actually shifted to the beginning of March, since in March, the work of the water accumulated in the reservoir begins and its supply for flushing irrigation begins. The flood waters entering the Tuyamuyun reservoir are either fully accumulated (if the reservoirs are not filled up to the normal retention level of the NRL) or are very strongly transformed (if the reservoir is filled up to the NRL). Analysis of the dynamics of sediment runoff showed that the amplitudes of fluctuations in sediment runoff over periods have a very wide range of changes. In the initial period of operation of the hydroelectric complex, the sediment runoff mode almost repeats the household mode, that is, the largest runoff occurs during the dry season, but with the lowest solid runoff values on average 3 and 8 (at the Samanbai station) times. For 7 years from 1975 to 1981, the annual flow at the Kipchak gauging station amounted to 41.722 thousand tons, that is, 32% of the annual household regime. The largest annual flow at the Kipchak and Samanbai gauging stations took place in the high-water year 1978 and amounted to 66329 and 57971 thousand tons соответственно. The smallest amount of annual runoff at gauging stations was observed in dry 1981 and amounted to 25074 and 3943 thousand tons. The study of the sediment runoff regime of the Amudarya river during the period of joint operation of the Takhiatash and Tyuyamuyun hydrosystems showed that the amount of solid runoff entering the zone of influence of the Takhiatash hydroelectric complex significantly decreased due to a sharp decrease in water discharge under the influence of regulation and low water. At the same time, as the analysis of field studies has shown, despite their relatively small number, accumulation of sediments is observed in the pays of the hydroelectric complex, that is, silting occurs. The amount of sediment siltation in the area of the Takhiatash hydroelectric complex varies depending on the water content of the year and the operating mode of the Takhiatash hydroelectric complex. Analysis of the alluvial regime shows that during the period of operation of the hydroelectric complex, depending on the hydrological regime of the river and the regime of the water level in front of the dam, the clarification of the flow occurs in April, May, June and August. The seasonally average clarification of the flow within the upper pool ranges from 10 to 40%, and in some months, at levels close to the NSP, reaches 90%, that is, the upper pool of the node turns into a kind of settling tank. The composition of suspended sediments of the Amudarya varies both in time and along the length of the river, but particles smaller than 0.25 mm remain predominant. Particles larger than 0.25 mm are found only in the upper areas and in an amount of no more than 3 - 4%. During the flood period, along the entire length of the river, the percentage of fines content increases in comparison with the low-water period.