Investigations of the distribution of suspended sediments in the upper pool of the reservoir for the Nurek hydroelectric station, which were carried out in [1973][1974][1975], showed that a bottom density current exists [1][2]. It was established that the bottom current transports the bulk of suspended sediments in the reservoir and, consequently, the pattern of the distribution of the density current also determines the deposition process of the suspended particles and the formation of bottom deposits. The purpose of this article is to analyze certain results of investigations conducted in the Nurek reservoir in 1975-1976 and to develop a method for calculating the parameters of the bottom turbidity current and thickness of the bottom deposits on the basis of solvinghydrodynamic equations for a density current. The distributions of the suspended load concentration, temperature, and velocity of the stratified current were measured by a complex of instruments specially developed at the department of physics of the sea and land waters, Physics Faculty, Moscow State University.Problems of the dynamics of density currents as applied to the problem of the formation of bottom deposits are being studied primarily under laboratory conditions. The leading work in this area is by Soviet specialists [7,8]. Modeling does not always permit obtaining quantitative evaluations of the investigated parameters. Therefore, data from full-scale investigations are needed for improving the existing methods of calculating the characteristics of stratified currents and predicting the siltation of reservoirs.The main parameter of a density current is the difference between the densities of the liquid in the bottom current and in the overlying water layers Ap. In a freshwater basin this quantity is determined by thermal and mechanical stratification. The relationship of the indicated types of stratification depends on the seasonal supply regime of the water body and has a substantial effect on the dynamics of the density current. For a bottom current with a high concentration of suspended sediment the density of the liquid is calculated by the formula p-= ( t--S/ps) p= (T) +8,where S is the weight concentration of suspended material in the water; Pw(T) is the density of pure water; PS is the density of the suspended material (PS-2.5 g/cmS).The quantity Ap(S, T) is determined from the expressionwhere Ap(S) = [1--(pw/PS)]AS is the difference in densities due to the increased turbidity of ~he bottom current (at 5=C ~ T~ 20~ the vertical average value of the density of pure water Pw = i g/cm s with an accuracy to 0.2%); ~Pw(T) is the part of the difference of densities determlned by thermal stratification. Figure la shows a graph of the change over the flow width of the ratio of the magnitude of the concentration of suspended sediment averaged over the thickness of the bottom current H ~b= l/h [ s (z),~z //-41
Durlng a study of the dynamics of the bottom stratified current in the Nurek reservoir during 1974Nurek reservoir during -1977 data were obtained on the formation of bottom deposits in the zone of action of the density current. The importance of comprehensive, on-site hydrophysical investigations of siltation, scouring of the bottom, and transport of the suspended load by a density current for the design and operation of reservoirs, ship canals, and other hydraulic structures determined the 8oals of the present work: i) a generallsation of the results of direct measuruments of siltation of the reservoir in the zone of action of the underflow density current during the period from 1974 through 1977; 2) an analysis of certain results of investigations of the main processes of mass exchange in the stratified current influencing the formation of bottom sediments on the basis of data from measurements taken in 1977.
A check of water pollution and alluviation of natural basins and hydraulic structures involves the conduction of on-site measurements of the characteristics of the suspended sediment and currents ~ransporting them. An evaluation of the present state of the problem of silation of the pools of hydroelectric stations due to the accumulation of the sediment load of rivers in them with consideration of the consequences of this process [9] indicates the need for such investigations for improving methods of prediction and prevention of sediment accumulation. For this purpose, a i0-year cycle of work devoted to a detailed study of the bottom sediment-transporting flow was performed on the Nurek reservoir. The creation of new means and methods of recording the main parameters of the thermally stratified current with suspended sediment was required for accomplishing the investigations. The developed complex of equipment of the indicated purpose, methods, and some measurement results are presented here.The bottom flow in the Nurek reservoir is formed at the debouchment of the Vakhsh River, the cold, turbid waters of which form a stratified current with an average thickness h = 20 m, length up to 30 km wlth a maximum velocity U = 60 cm/sec (Fig. i). In different seasons the suspended sediment concentration S in the flow varied from 5 to 300.10 -5 g/cm ~, the differences of the water temperatures AT in the bottom flow and above its upper boundary was plus 0.6-minus 6eC. The rise of the level of the bottom of the axial line of the flow due to deposition of sediment was on average 7 m per year in the first stage of operation of the reservoir [5]. The density of the suspended material Ps = 2.5 g/cm 3, the particle size d did not exceed 70 ~m.The equipment for recording the parameters of the density current was created on the basis of the experience of the 1972-1980 field trips. The results of these investigations showed that the development of measuring instruments and methods should be carried out with consideration of the following natural conditions and technical requirements:i. For a thickness of the density current of 15-30 m at depths up to 300 m it is advisable to use equipment with a cable connection lowered from an anchored ship. Wave action, as a rule, is insignificant, since the effect of the wind isconsiderable only when its direction coincides with the principal axis of the reservoir.2. The density of the water 0 in the current is determined from the data of measuring the suspended sediment concentration S and temperature T as 0 = (i --S/Ps)Pw(T) is the density of water without suspended sediment.3. The axial line of the flow passes through points with maximum depths at the sections. Consideration of this factor facilitates a determination of the current direction.4. The equipment should be suitable for taking measurements from onboard a boat or pontoon without powerful hoisting mechanisms. In this case, it is necessary to provide the possibility of prompt and sufficiently accurate measurements of horizontal and ver...
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