The Sedimentation Characteristics Of Reservoirs In China And The Impact Of Typical Reservoirs On The Variation Of Downstream Runoff And Sediment

Reservoirs are water conservancy projects having multiple functions,such as water supply,flood control,power generation and shipping.Reservoirs play a very important role in optimizing allocation,development and utilization of water resources,flood control and disaster reduction.However,the artificial barriers formed by the reservoirs have changed the runoff and sediment situation of rivers in the natural state.Reservoirs block the movement of sediment,causing sediment deposition in the reservoir area.Sedimentation of reservoir not only affected the function and long-term operation of reservoir,but also has a profound effect on the change of downstream runoff and sediment.There is a lack of studies on the spatial distribution and sedimentation characteristics of reservoirs in China and the influence of reservoir sedimentation on the change of runoff and sediment in the lower reaches of river basins.However,to study on giving full play to the comprehensive function of the reservoirs and obtaining their best benefits are very important.In this study,the number and storage capacity of the reservoirs in various provinces in China,sediment deposition data of 112 reservoirs,and runoff and sediment transport series data of typical hydrological stations in the Yangtze River and the Yellow River were used.This study made a comprehensive analysis of the spatial distribution characteristics of the reservoirs in China and the sedimentation characteristics of different river basins and reservoirs in different sizes.Based on the methods of anomaly accumulation,Mann-K endall and ordered clustering,the trend analysis and mutation test of runoff and sediment change were carried out.The effects of sedimentation of the typical reservoirs in the Yangtze river basin and the Yellow River basin on the variation of runoff and sediment in the lower reaches are also analyzed.The main research contents and conclusions are as follows:(1)Distribution characteristics and influencing factors of reservoirs in ChinaThere are a total of 98,795 reservoirs in china by 2017,of which 0.74%of the large reservoirs accounted for 79.8%of the total reservoir capacity of the country.The number of small reservoirs was large and widely distributed,accounting for 95.28%.The quantity and capacity of reservoirs in China are more in the south than in the north.Provinces with more reservoirs are mainly located in the Yangtze river basin.Hunan province has the largest number of reservoirs,while Tibet has the least.From 2007 to 2017,the number and capacity of national reservoirs increased by 15.67%and 30.49%.According to the analysis of spatial distribution characteristics of reservoirs in China,the factors affecting the number and capacity of reservoirs include:drop height of terrain,degree of water shortage,surface water resources,water demand for agricultural production and water storing conditions.(2)Sedimentation characteristics of major reservoirs in ChinaThe average annual sediment deposition rate of 112 reservoirs ranged from 0.01%tol2.36%,of which 65.19%were below 1%and 8.9%were over 2%.The annual sediment deposition rate of Longtoushi Reservoir in the Yangtze Basin was the highest,which was related to the construction waste residue and debris flow disaster in the upstream area,and the annual sediment deposition rate of Qingcaosha Reservoir is the lowest.Among all types of reservoirs,the sedimentation rate of the small reservoirs is the highest.The annual sediment deposition rate of the small ? reservoies and the small? reservoirs were 1.46%and 1.44%.The average annual sedimentation rates of large ?,large ? and medium-sized reservoirs were 0.44%,1.3%and 0.76%.According to the statistics of 112 reservoirs in different basins,the average annual sediment deposition rate was between 0.04%and 3.73%.Except for catchments with small sample sizes,the sedimentation rates of the reservoirs in the Yellow Basin and the interior drainage area in the northwest were higher,which are 0.8%and 0.77%,and they were related to the characteristics of runoff and sediment of the river basins.(3)Characteristics of runoff and sediment in the Yangtze Basin and the impact of sediment deposition in Three Gorges Reservoir on the changes of downstream runoff and sedimentThe annual runoff of the Yangtze River did not change significantly from 1952 to 2017,which was mainly related to precipitation and less affected by human activities.The annual variation of sediment transport in typical hydrological stations showed a significant decreasing trend.The abrupt change years of sediment transport in the lower Yangtze River region were 1992 and 2003.The abrupt change in 1992 was due to the effects of soil and water conservation in the Yangtze River basin,while the abrupt change in 2003 was due to the use of the Three Gorges Reservoir to trap large amounts of sediment.The impact degree of runoff and sedimentin 2003 was obviously greater than that in 1992,reflecting that the sedimentation of the Three Gorges Reservoir had a faster and more significant effect on the change of downstream runoff and sediment,while the sediment reduction benefits of soil and water conservation were postponed.The runoff-sediment relationship has a strong correlation in the upper reaches of the Yangtze River basin,,but it is not closely related in the middle and lower reaches of the Yangtze River basin..Compared with the average annual sediment transport volume before 2000-2013,the annual sediment transport rate of the four typical hydrological stations in Dongting Lake is greater than 47%,and the annual sediment transport rate of the five typical hydrological stations in Poyang Lake is greater than 25%,and the annual rate of change of the typical hydrological stations in these two lakes is less than 7%,indicating that the sedimentation of the Three Gorges Reservoir has a significant impact on the annual sediment transport of downstream lakes.(4)Runoff and sediment characteristics in the Yellow River basin and the influence of sediment deposition in typical reservoirs on the variation of downstream runoff and sedimentThe spatial distribution of runoff and sediment in the Yellow River Basin is heterogeneou.There is more runoff in the upper reaches and more sidement in the middle reaches.Compared with 1971-1987,the annual runoff decreasing rate of the typical hydrological stations in the Yellow River basin in 2001-2017 was between 15%and 46.1%,and the annual sediment decreasing rate was between 53.6%and 91.3%.It gradually increased along the upstream to the downstream,and the decreasing trend of sediment transport was more obvious.In the middle reaches of the Yellow River,the correlation between runoff and sediment changes at Longmen Station and Sanmenxia Station was not strong.The abrupt changes of the annual sediment transport in Longmen Station were in 1971 and 1979,and the abrupt changes in Sanmenxia Station were 1959 and 1979.The main influencing factors are the sedimentation of large reservoirs and the sediment reduction of water and soil conservation measures in the Yellow River basin.The change of runoff and sediment before and after the operation of Sanmenxia Reservoir showed that the operation of Sanmenxia Reservoir has led to a significant reduction in the amount of sediment transported in the downstream.In the early stage of the reservoir operation,the siltation in the reservoir area was the most serious during 1960-1963.The runoff and sediment changes in the Sanmenxia station during the two periods from 1933-1959 to 1960-1979 showed that the annual sediment reduction rate and the sediment reduction rate during the annual flood season were 29.43%and 29.73%,respectively.The annual runoff reduction rate and the runoff reduction rate during the annual flood season were 13.31%and 21.79%,respectively.