| The problem of reservoir siltation is a key issue that restricts the reservoir's continuous use of its engineering benefits.The research on the characteristics of influent water and sediment changes,influencing factors,and the effects of sedimentation on various functions of the reservoir is the premise and basis for formulating a plan for reservoir sediment reduction and silt reduction.The Kezir Reservoir is located in the upper reaches of the Weigan River Basin and is a mountainous reservoir built on sandy rivers in the mountains.The reservoir has been operating with severe siltation problems so far.In order to explore the causes of siltation in the Kezir reservoir and the effects of reservoir siltation,a targeted proposal for siltation and silt reduction suitable for reservoir siltation was proposed.In this paper,the evolution law and driving factors of sediment discharge from reservoirs,simulation and prediction of water and sediment,the evaluation of the influence of the reservoir silt deposition,the cause of sediment deposition and the scheme of sediment discharge are studied.The conclusions are as follows:(1)During the period from 1959 to 2015,the annual amount of water and sediment discharged from the Kezir Reservoir showed an increasing trend,the runoff showed three phases,namely,continuous reduction(1959-1976),alternating fluctuation(1977-1994)and significant increase(1995-2015),and the sediment intake showed four phases,namely,continuous reduction(1959-1991),slow increase(1992-1996),significant increase(1997-2003)and alternating fluctuation(2004-2015);runoff and sediment transport occurred from less than a year of mutation to 1987 and 1979 respectively;The scale of runoff and sediment transport is consistent,and there are 3 time scales,which are 7,12,28 years and 6,11 and 26 years respectively,the length of the main period is 28 years and 26 years,respectively.The increase of incoming water and sediment is one of the main reasons leading to the increase of siltation degree of reservoirs;(2)Climate change and human activities are the main driving factors leading to changes in water and sediment input to the reservoir.The impact of climate change on the runoff from the Kezir Reservoir is greater than the impact of human activities,and human activities have a greater impact on sediment input than climate.The results of attenuation analysis showed that the effects of precipitation and human activities on runoff were 62.4%and 37.6%respectively,and the impact on sediment discharge was 35.2%and 64.8%respectively.The results of the cumulative slope change rate comparison show that the effects of climate change and human activity on runoff are 377.79%and-277.79%respectively,and the effects on sediment transport are 11.33%and 88.67%,respectively;(3)Based on the characteristics of NAR neural network and BP neural network,a NAR-BP combined prediction model based on BP neural network modified NAR neural network prediction results was established;the average relative error of combined forecasting model for annual runoff forecast was 1.28%,the average relative error of the annual sediment transport forecast is 2.55%,and the model type has better prediction effect on the sediment sequence.The average annual runoff volume of the Kezir Reservoir from2018 to 2067 reached 30.49×10~8m~3,and the average annual sediment input reached 1486.31×10~4t.Compared with the current stage,the total runoff and sediment load showed an increasing trend;In 2038,annual runoff and annual sediment discharge reached peaks at 38.72×10~8m~3 and 3095.21×10~4t,respectively;Under the condition that the existing sand removal measures are unchanged,in 2027,2040,and 2048,the initial reservoir water level,flood control high water level,and initial design flood level will all be filled with silt and the reservoir will lose its flood control function;In 2052 and 2059,the reservoir will completely lose its storage capacity and total storage capacity.The development of reservoir siltation is grim;(4)By establishing a multi-level fuzzy comprehensive evaluation model based on analytic hierarchy process,fuzzy mathematics theory and cloud model theory for reservoir sedimentation,the problem of inaccurate evaluation results caused by expert's personal experience in the evaluation process of reservoir sedimentation is effectively avoided;The model was applied to the evaluation of the impact of sedimentation in the Kezir reservoir.The result shows that the expected degree of sedimentation in the Kezir reservoir is 0.7002,indicating that the impact of reservoir sedimentation is between the heavy and extreme effects,Entropy and hyperentropy were 0.0306 and 0.0143,respectively,indicating that the evaluation results were less volatile;(5)Because the parameters in the current run-time formula for concret gravity flow are not easily obtained,the sediment concentration,single-wide flow rate,and slope at the bottom of the reservoir are used as the calculation parameters to establish the relationship between the flow rate and the running time of the uniform gravity flow;After controlling the sluice gate by the calculation formula of the operation time of the gravity flow,the average sediment discharge ratio of the gravity flow is increased from172.34%to 301.75%,which provides a scientific basis for the sediment discharge work of the reservoir.According to the combined gravity flow scheme of the tributary of the tributaries of the tributaries of the tributaries,a comprehensive analysis of the effects of sediment discharge at different water levels during the flood season was carried out.It was found that when the water level is controlled below 1140.0m,the effect of tributary flow in the tributary flow is best,At the same time,trace sediment flushing occurred in the tributaries of the tributaries,and the sediment discharge ratio was greater than 100%;(6)Through the analysis of reservoir sedimentation,sand draining project layout and sediment deposition characteristics of the river,it has been found that the sediment deposits increasing year by year,the low sand discharge ratio during the flood season,the unreasonable layout of flood discharge and sediment draining holes,and the limited operation mode of reservoir dispatching,the obstruction of the sand barrier at the junction of the branches and branches of dry rivers and sediments is the cause of serious siltation in the Kezir Reservoir.Considering the current status of sedimentation and sediment discharge conditions in the reservoir,it is proposed that the sediment discharge in the near future should be dominated by Density current;Through the joint adjustment of flood season reservoirs and mechanical dredging,the conditions for reservoir low-level sedimentation are gradually realized to alleviate the problem of reservoir siltation.The long-term reservoir remediation program includes increasing upstream water and soil conservation work and the creation of new reservoirs upstream of the main stream of the reservoir to form a virtuous cycle of sediment discharge from the Kezir Reservoir. |
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