Dynamics of morphometric and hydraulic parameters of the Amudarya River channel downstream the Tuyamuyun hydroscheme

. The construction of large water reservoirs on level rivers has dramatically changed the nature and dynamics of channel processes in the upstream and downstream rivers. The purpose of the research is to study the relationship between the morphometry of the riverbed and the parameters of the waterworks rigged by the reservoir. To analyze the dynamics of the flow's channel morphometry and hydraulic elements, the lower course of the Amu Darya River, which is characterized by intense channel processes, was chosen. The study was carried out using field data on the Tuyamuyun gauge, located 1.8 km below the hydroelectric complex. The period for the last 25 years is covered, and 6 multi-year periods are selected from these. As a result of data analysis from long-term field studies and hydrometric measurements on the Tuyamuyun hydraulic section of the Amu Darya River, functional relationships have been established between the morphometric parameters of the channel and the hydraulic parameters of the flow. The relationship between the roughness of the channel and water flow is ambiguous. This is explained by the fact that the target is in the zone of the general spreading of the channel, and the bottom of the channel was observed to sink. When establishing the hydraulic resistance of the downstream, i.e., the relationship between the morphometric parameters of the channel and the hydraulic parameters of the flow, it is possible to predict changes in the water level and the state of water intake facilities. According to forecasts, it is possible to determine the type of event that allows for the supply of estimated flow rates through water intake facilities located in general erosion of the Tuyamuyun water reservoir.


Introduction
Intensive national and agricultural sectors' intensive development generates increased water resource demand.The region is characterized by a warm climate, especially global warming has exacerbated this.As a result, it leads to a sharp increase in water consumption in all sectors of the economy and mainly in the country's agriculture [1][2][3][4].Uzbekistan is a vivid example of the region mentioned above.About 2.3 million croplands in Uzbekistan is irrigated by the Amu Darya River, one of the largest rivers in Central Asia, which runs mainly on easily erodible soil.Typically, in the lower reaches of the Amu Darya River (even in the general erosion section), the unbreakable ratio of the average velocity (formula 1), so the channel is subject to intensive channel processes [5][6][7][8].0 5.0 6.0 v v ( 1 ) Where: v is unbreakable ratio; v 0 is average velocity As a result of the construction of large facilities for irrigation and energy in the riverbeds and their plains, river flow dynamics were sharply affected, which led to changes in river morphometry [9][10][11].To ensure the safety of the facilities under construction, it is necessary to carry out forecast calculations of canal processes in the river channel, considering the impact of hydraulic and hydropower facilities on the flow dynamics [12][13][14][15].

Materials and Methods
The purpose of the research work is to study the dynamics of channel morphometry and hydraulic flow parameters influencing the course and direction of channel processes in the conditions of river flow regulation has been identified as the main goal of this research [16][17][18][19][20].
To achieve the purpose of the study, we have set the following tasks: 1) to select these crowded observation periods from a series of 25 years of observations; 2) according to the data of the selected years, to build graphs of the relationship between the width and depth of the channel and the water flow; 3) to establish relationships between the roughness coefficient and water flow; 4) to establish the nature of the change in the Chezy coefficient [21,22].The Tuyamuyun gage, located 1.8 km downstream of the Tuyamuyun water reservoir hydroelectric complex, was chosen as the object of study.
The study subjects are the Amu Darya Riverbed's morphometric and hydraulic parameters under regulated flow conditions.
In addition, the results of field observations downstream of the Tuyamuyun water reservoir were used.In field observations, within two years, the course of the general erosion was studied in 12 sections located at a distance of 20.4 km.The Tuyamuyun gauging station was the initial target, Tashsak gauging station was the final gauging station.Every two months, transverse profiles were taken in 12 sections, and the stream and channel hydraulic characteristics were determined.Empirical formulas for the calculation of flow capacity have been tested, considering the influence of lateral walls of the riverbed and hydraulic elements of the flow, and comparing their values with the measured ones.To reveal functional regularities between channel morphometry and hydrodynamic flow characteristic, the data of hydrometric measurements were processed using standard computer programs.

Results and Discussion
The Tuyamuyun gauging station located 1.8 km downstream of the Tuyamuyun water reservoir was chosen for the study.River channel morphometry and hydraulic flow parameters in this site are influenced by the operation regime of the Tuyamuyun water  1).In the gauging station, the left bank up to 10 -12 m high is steep and precipitous, composed of dense red clay, and devoid of vegetation.The right bank is 3 -4 m high and floodplain.The riverbed is sandy-silty, strongly deformed.The riverbed at the section of the post is slightly curved.The average diameter of bottom sediments is -d cr = 0.15 mm.The width of the river on this gully is up to 1300 m, and the slope of the water surface -is i=0.00015.
For analysis of the interrelation of morphological parameters of the river channel and hydraulic characteristics of the stream in the Tuyamuyun gauging station, the data of highwater periods of observed years were selected.Such years for the last 25 years were 6 observation years.As shown by the results of many researchers, channel processes and high-water years most fully reflect interrelations and dynamics between morphological elements of the stream and hydraulic parameters [16,23].Based on the data of the selected observation years, graphs of the relationship between channel width (B) and channel width (H), and water consumption (Q) were plotted (formulas 2 and 3).
Where: B is channel width; H is channel width; Q is water consumption; f is function Since there are so many graphs, we analyzed some of them (Fig. 2).For the other graphs, let us give their equation with correlation coefficient (Table 1).Determination of relations between formula 2 and formula 3 was performed by the static method of correlation analysis for each year of observations separately  Figure 2 shows a graph of the relation between the average depth of the stream and discharge in the river for high-water 1992, as well as the relation H =f(Q) (Table 1) for 1988, 1998, 2010, and 2012 after commissioning of the Tuyamuyun hydroscheme in 1980.
The correlation between average depth and water flow in Year I (Table 1, 1-line, 1988, r hu = 0.31) was very weak.But in the next second year of observation (Fig. 2, Table 1, 2line, 1992, r hu = 0.96), the relationship between mean depth and flow improved, and the correlation coefficient was as high as 0.96.The lowest flow depth was observed at the lowest water flow, and the average flow depth began to increase as the water flow increased.
As the plots of the relationship between flow depth and discharge in a given water body show, the consistent relationship between average flow depth and discharge was maintained in the following high-water years third, fourth, and sixth of the observation period.Because of the intensity of deformation processes at the site, the flow depth was variable, and the sharp variability of the river hydrograph led to the establishment of different functional relationships between these flow parameters.In the last high-water sixth year of observation, the relationship between mean depth and flow decreased dramatically (Table 1, 6-line, 2012, r hu = 0.40).
To establish the relationship between the morphological element-channel width-(B) and water flow-(Q), graphs were plotted for the sixth year of observation.
Since the graphs were built for the fourth year of observation, for convenience, we will present the graph only for the second year of observation (Fig. 3); the data of other graphs will be recorded in Tab 2. As can be seen from the plot in Fig. 3 and data from other graphs in Table 2, the smallest channel width is observed when the discharge decreases in the I-year of observation and the channel width increases with increasing water discharge in the river.The highest channel width is observed at the highest discharge.The relationship of channel width from the 1988 discharge (Table 2, 1-line, 1988, r hu =0.94) is normal, and the correlation coefficient reaches 0.94.
In the second year of observations, the channel width increased as a function of increasing streamflow (Fig. 3, Table 2, 2-row, 1992, r hu =0.93), equal to 2000 m 3 /s.At a flow rate of 2000 m 3 /s, the width of the channel was 1100 m.The width remained constant when the flow rate increased further.In this year of observation, as well as in the first highwater year of observation, the relationship between channel width and discharge up to 2000 m 3 /s was seen, which corresponded to the quasi-stationary-equal flow regime (The highest channel width in the fourth year of the observation was 1400 m, and the channel width began to decrease and was 1100 m in 1992 (second year), i.e., 300 m less.The channel width decreased further as well.In the high-water third year, when the maximum discharge was 4500 m 3 /s, the channel width was 950 m, and the channel width decreased to 750 m in 2005 (fourth year).The observation with a maximum flow of 3.200 m 3 /s, the channel width was 850 m in 2005 (fourth year), and it decreased to 750 m in the fourth year.In total, during the whole 25-year period with six high-water years of observations, the channel width in this section was reduced to 650 m.
Overall, the average depth at the maximum discharge in year VI began to increase compared to the first year.On the contrary, river channel width in the given site, which is in the zone of Tuyamuyun water reservoir influence, tended to reduce in the sixth year of observation compared with the first year of observation.
The Tuyamuyun gauging station is located is characterized by general erosion (Fig. 1).Figures 4 and 5 show the relationship between roughness coefficient and water discharge for I and 2 years of observation at the Tuyamuyun gauging station.As can be seen from the graph for the 1st year of observation (Fig. 4), there is no connection between "n" and "Q" the roughness variation fluctuations are from 0.015 to 0.043, and the flow rate is from 200 to 3000 m 3 /s.Tuyamuyun outlet is downstream of the reservoir, and there is general erosion and deepening of the riverbed, i.e., intensive channel process, which has its effect on preservation without change of channel roughness regardless of water discharge growth in the river [24].The plot of the relationship (formula 4) for I-(1988) is shown in Fig. 5.Here roughness coefficient "n" changes from 0.016 to 0.03 at low-water flow and from 0.03 to 0.04 at the flood, i.e., there is a growth of roughness coefficient with the growth of water flow in the river.
( ) Where: n is roughness coefficient; Q is water consumption Data of observation years (I, II, and IV in Fig. 4, 5, and 6) and observations years (I, II and IV) are recorded in table 3.In third which is a high-water year, the roughness coefficient correlation with river discharge is low (Table 3. 3-row.1998.r hu = 0.67).This year, the roughness coefficient varies from 0.018 to 0.058 at low-water flow and from 0.02 to 0.04 at the maximum flow in the river.Roughness coefficient fluctuations are greater in low-water periods compared to flood periods.
The relationship n=f(Q) fourth year of observation (Fig. 6).turned out satisfactory (Table 3. 4-line.2005.r hu = 0.69).This year, we find a relationship that shows a decrease in the roughness coefficient with an increase in water flow rate.The roughness coefficient varies from 0.026 to 0.04 at low-water flow and from 0.025 to 0.03 when the maximum flow is passed.
There is no correlation between roughness coefficient and water discharge fifth year of observation (Table 3. line 5. 2010.r hu = 0.0164).Here, the roughness coefficient varies from 0.023 to 0.040 at low-water flow and from 0.025 to 0.035 at the maximum flow in the river.
The relationship n=f(Q) sixth year of observation improved than the first and fifth years, and it has stronger relationships (Table 3. 6-line.2012.r hu = 0.48).This year, the roughness coefficient varies from 0.028 to 0.042 at low-water flow and from 0.022 to 0.03 when passing the maximum flow.
The relationship n=f(Q) of observation years (II, III, and IV) shows that the roughness coefficient decreases with increasing discharge, and all three of these high-water years are indicative of the relationship (Fig. 4 and 6, Table 3).
The unsatisfactory correlation of roughness coefficient with water discharge in observation years (I, V, and VI) are explained by the intensive channel process taking place in this section, which leads to a general erosion of the channel and a decrease in average bottom elevation.
The satisfactory connection of years (II, III, and IV) of observation shows that in these years, the intensity of erosion began to decrease and the beginning of stabilization of the channel process in this section.
The results of multivariate calculations have shown the Manning formula.Considering the braking effect of the riverbed bank gives a good correlation with the data of field studies in general scour zone.The braking effect of the riverbank is considered by a correction factor, which in the absence of floodplains is determined by a modified formula (5) of I.F.Karasev [25,26].
Where: g is force of gravity referred to unit mass, m/s 2 , χ is wetted perimeter of the channel, m., R is hydraulic radius, m., C is Chezy coefficient determined using the following formula (6) for the uniform flow movement according, φ is the coefficient that considers the ratio of the preserved longitudinal exchange masses between flows in the transition zone and the laminar layer to the average flow velocity for the Amu Darya Riverbed, φ = 0.002.In the first observation, year calculated and measured values of average flow velocity give good convergence (Table 4).The dynamics of calculated and measured velocities depending on flow rate were from 0.73 m/s to 0.61 m/s, with the change of Chezy coefficient value from 62.177 to 47.35 m 0.5/s.
The average error between measured and calculated values of average flow velocities and Chezy coefficient in the section of the general erosion of the Amu Darya River channel was 0.25% and 0.31, respectively.At the same time, the value of the hydraulic resistance coefficient changed from 0.035 to 0.0203.
It should be noted that the dynamics of all these channel parameters at different discharges corresponded to values of channel roughness coefficient in the general scour zone, which had dynamics from 0.026 to 0.0275.
In the second and third high-water years of observations, calculated and measured values of average flow velocity also gave a good convergence; calculated and measured values of velocities ranged from 0.67 m/s to 0.79 m/s, with a change in the value of the Chezy coefficient from 6.177 to 47.35 m0.5/s.
The average error between measured and calculated values of average flow velocities and Chezy coefficient in the section of the general erosion of the Amu Darya River channel during the second year of observation was 0.05% and 1.7%, respectively.At the same time, the value of the hydraulic resistance coefficient decreased from 0.025 to 0.001 It is necessary to note that those dynamics of all these parameters of a channel at various discharges corresponded to values of the coefficient of the roughness of a channel in a zone of general erosion, which had dynamics from 0.026 to 0.039.
In classical channel hydraulics, the energy reserves of the water flow are mainly spent on friction along the length of the channel and local hydraulic resistance [27,28].Friction losses in the case of developed turbulent flow, which is typical for open flows, mainly depend on the roughness of the channel and the hydraulic radius [29].The strength of the riverbed that opposes the movement of the flow -the hydraulic resistance of the channel and the hydraulic radius, in many hydraulic calculations of the engineering practice of hydraulic engineering and hydropower design, is considered with the Chezy coefficient in the Chezy formula for the average flow rate [5,[30][31][32].Since the depth of the stream and the width of the channel along the flow of the stream vary greatly, frequently, in engineering calculations, the movement is assumed to be uniform or quasi-stationary, which will allow engineers to calculate the average flow velocity to determine the value of the Chezy coefficient using the classical empirical formulas of Manning, N.N.Pavlovsky, and I.Agroskin and many others [18,[33][34][35].In fact, the movement in the riverbeds is unsteady, and the value of the Chezy coefficient depends on the type of liquid, the roughness coefficient of the channel, and the dynamics of the shape of the channel along the flow of the stream.In addition, the value of this parameter is strongly influenced by the shape of the channel in the plan, the presence of floodplains of the channel, the coverage of the channel and its floodplain by various vegetation, and many natural and artificial factors, etc. [25,28,36,37].
An analysis of n= f (Q) relation graphs for the Tuyamuyun gauging station showed the following three types of below-mentioned changes of roughness coefficient with water discharge growth are observed at the station.
The second year (1992) of observation refers to the first type, i.e., the roughness coefficient grows with water discharge growth (Fig. 5).
First (1988.Fig. 4) and fourth (2010) years of observation refer to the second type, i.e., the roughness coefficient remains unchanged with an increase of water discharge in the river.
In the fourth, fifth, and sixth-high-water years of observations, the calculated values of hydraulic flow parameters determining the character of the process in the channel and Chezy coefficient, hydraulic resistance changed in interrelation with flow dynamics and channel roughness.
In the fourth, fifth, and sixth -high-water years of observations, calculated and measured values of average flow velocity also gave good convergence; calculated and measured values of velocity changed in the range in IV observation year, from o.77 m/s to 1.14 m/s by changing the Chezy coefficient value from 45.30 to 31.20 m0.5/s, in speed from 0.58 m/s to 1.08 m/s, when changing the value of Chezy coefficient from 31.08 to 40.20 m/s, the hydraulic resistance of the channel changed in the range 0.066 to 0.046, in the VI -year of observation from 0.77 m/s to 1.14 m/s, when changing the value of Chezy coefficient from 44.18 to 31.65 m/s observed an increase of forces of the channel counteracting the flow -the hydraulic resistance of the channel in the range 0.05 -0.097.
The average error between measured and calculated values of average flow velocities and Chezy coefficient at the section of the general erosion of the Amu Darya River channel during the years of high-water observation was 1.25% and 1.7%, respectively.
The difference between measured and calculated flow rates and their percentages are shown in Table 4 (10-pillar)

Conclusion
Based on the discussion of the results of monitoring the dynamics of the morphometry of the riverbed and the hydraulic parameters of the water flow located in the influence of the Tuyamuyun water reservoir, the following conclusions can be drawn: The analysis shows that the connection with the hydraulic parameters of the flow and morphometry is established for the conditions of the natural regime of the Amu Darya River.The reliable and safe operation of the constructed structures directly depends on the forecast of channel processes in the conditions of regulated river flow.
The studies used field data on the Tuyamuyun gauging station 1.8 km below the reservoir hydroelectric complex.
With an increase in flow, an increase in the depth of the flow occurs; however, there is no single dependence; therefore, 6 graphs and dependences ‫ܪ‬ = ݂ (ܳ) with correlation coefficients of 0.3 to 0.96 are proposed The channel width increases with an increase in discharge for high-water rivers, with correlation coefficients from 0.8 to 0.93 An ambiguous relationship between the roughness of the channel and the flow of water is due to the continuation of the reshaping of the channel under the influence of the reservoir hydroelectric complex.
Calculating the channel's capacity to determine the value of the Chezy coefficient, the Manning formula with a correction factor is recommended, considering the inhibitory effect of the bank of the river channel according to I.F.Karasev [25].
In general, the analysis of the channel roughness coefficient under regulated water flow downstream of the Tuyamuyun water reservoir, in the zone of general erosion, showed that the channel process had not yet stabilized.Due to the influence of the operation mode of the Tuyamuyun water reservoir, it was not possible to identify a functional relationship between the integral characteristic of the channel forces opposing the movement of the flow rivers -by the hydraulic resistance of the channel and the flow rate of water moving in it -n=f(Q).
At present, the Tuyamuyun water reservoir is almost completely silted up.The water enters the downstream pool with the daily saturation of suspended sediments.At the same time, the length of the total erosion fluctuates depending on the flow rate, and the saturation with suspended sediments is insignificant, and on average, it can be taken constantly.Thus, the task of further research is to determine the hydraulic resistance of the channel downstream with a completely silted upstream, i.e., reservoirs.

Table 4 .
Calculation of flow capacity (10)B H V(10)Where: Q is Actual value of water flow rate; H is Stream depth; n is Actual value of roughness coefficient; λ is Hydraulic resistance; g is Acceleration due to gravity; C is Chezy coefficient; Q p is Estimated flow rate; B is channel width; H is channel width; V is Average flow velocity