| In recent years,due to the impact of global climate change and human activities,especially with the successive construction of cascade reservoirs such as Xiangjiaba and Xiluodu in the lower reaches of Jinsha River in the upper Yangtze River,new characteristics of water and sediment changes have emerged in the upper Yangtze River and the Three Gorges reservoirs,and their characteristics and trends of water and sediment changes have received extensive attention.In this paper,the water and sediment processes at the main stations from1956 to 2016 are taken as the research objects in the upper Yangtze River,and the characteristics of water and sediment changes in the Three Gorges reservoir are studied comprehensively from the perspectives of catastrophe,periodicity,complexity and chaos.On this basis,the causes of the sudden decrease of sediment inflow into the Three Gorges Reservoir are studied from the perspective of climate change(rainfall)and human activities(reservoir sediment interception,soil and water conservation,river sand mining,etc.).In addition,according to the characteristics of river bed-forming in reservoir channel and based on the dynamic sediment transport capacity method,this paper proposes a method to determine the dominant discharge in reservoir channel.Finally,based on the one-dimensional unsteady water and sediment mathematical model of the main and tributary rivers of the Three Gorges Reservoir,the time of sedimentation balance of the Three Gorges Reservoir under new water and sediment conditions is predicted.The main research achievements include the following aspects:1.The rules of water and sediment inflow into the upper Yangtze River and the Three Gorges Reservoir from 1956 to 2016 were systematically studied.The composition,interannual and intraannual variations of water and sediment in the upper Yangtze River and the Three Gorges Reservoir in the past 60 years,as well as the variation law of water and sediment characteristics are systematically studied.The results show that:(1)Since the 1990s,the sediment inflow from the upper Yangtze River has decreased,and the reduction of sediment inflow is much larger than that of the water inflow;(2)Compared with 1991-2002,the annual average incoming water of the Three Gorges Reservoir(Zhutuo+Beibei+Wulong)decreased by 172.1 billion m~3,by 4.44%,and the sediment inflow decreased by 2.01 billion tons,by 56.2%;(3)The size of suspended sediment in the upper Yangtze River is finer,the sediment load below 0.005 mm increases,and the sediment load between 0.05 mm and 2 mm decreases;(4)Except for Pingshan Station,Fushun Station and Wulong Station,there are obvious abrupt changes in the upper Yangtze River,but there are no significant changes in the runoff of the main tributaries and the Three Gorges Reservoir,and there are almost obvious abrupt changes in the sediment discharge.There are periodic changes of water and sediment in Zhutuo,Beibei,Wulong and Three Gorges.The complexity of sediment transport is greater than runoff,and the complexity of runoff and sediment transport in main stream is greater than that in tributary.Both runoff and sediment transport are chaotic,and the chaos of runoff and sediment transport sequence gradually increases from upstream to downstream.2.Regarding the causes of the sudden decrease of sediment inflow into the Three Gorges Reservoir,this paper makes a multi-level study on climate change(rainfall)and human activities(reservoir sediment interception,soil and water conservation,river sand mining,etc.).On this basis,the physical background of the sudden reduction of sediment inflow into the Three Gorges Reservoir is discussed through the analysis of climate change,human activities and other factors.Based on the comprehensive consideration of the sediment trapping effect calculated by the capacity to inflow ratio(C/I)and the capacity to watershed ratio(C/W)in the traditional sediment trapping efficiency model,this paper introduces two factors of particle size and settling velocity,and improves the model by using the logarithmic regression method.At the same time,the trend of runoff and sediment transport in the Three Gorges Reservoir is predicted.The results show that:(1)From 1991 to 2005,climate change(rainfall)is an important factor affecting sediment transport in the upper Yangtze River,especially in the Jinsha River and Jialing River basins.In 2006-2016,climate change has relatively little effect on sediment transport in the upper Yangtze River.Human activities have become the most important factor in the change of sediment transport.At the same time,reservoir sediment interception is the main and direct reason for the significant reduction of sediment inflow into the Three Gorges Reservoir;(2)From 1986 to 2016,the sediment inflow into the Three Gorges reservoir caused by the built reservoir decreased by about1.068 billion t/a in the upper reaches of the Three Gorges;(3)Compared with the conventional empirical models such as Brune,Siyam,Brown and Jothiprakash,the average error between the improved sediment trapping efficiency model and the measured sediment trapping efficiency is the smallest,and the improved model has better applicability and accuracy in the calculation of sediment trapping efficiency of large reservoirs;(4)According to the measured data and current research,the sediment discharge of the Three Gorges reservoir in the future will be about 0.6 billion t/a in2020-2050 with little change in runoff and sediment discharge.3.The characteristics of sediment deposition in the Three Gorges Reservoir area after impoundment are analyzed,and the effects of water and sediment inflow process and reservoir operation mode on sediment deposition in the reservoir area are studied.In addition,based on the characteristics of riverbed construction in reservoir channel,a method for determining the dominant discharge in reservoir channel--dynamic sediment transport capacity method is proposed.The method is applied to the calculation of the dominant discharge in reservoir channel after the Three Gorges impoundment,and compared with the dominant discharge before the Three Gorges impoundment calculated by Makayev sediment transport rate method.The applicability and accuracy of this method in the river course of reservoir area are validated.The results show that:(1)From June 2003 to December 2016,suspended sediment of the Three Gorges reservoir was 21.581 billion tons,suspended sediment of the Huanglingmiao Station was 5.201 billion tons.Without considering the incoming sediment of the Three Gorges reservoir area(the same below),the sediment of the reservoir was 16.38 billion tons,with an average annual sediment of 1.206 billion tons,which was only about 40%of the demonstration stage(using the prediction results of 1961-1970 series of mathematical models),the sediment discharge ratio of reservoir is 24.1%.The sediment deposition of reservoir is mainly concentrated in the perennial backwater area below Qingxichang,which accounts for93%of the total sediment.The sediment deposition of Zhutuo-Cuntan and Cuntan-Qingxichang reservoir sections accounts for 2%and 5%of the total sediment,respectively;(2)From March 2003 to November 2016,the total sediment deposited in the main stream of the reservoir area was 14.68 billion m~3,of which the total sediment deposited in the changeable backwater area(from Jiangjin to Fuling section)was 0.678 billion m~3and that in the perennial backwater area was 15.358 billion m~3.From March 2003 to November 2010,13 main tributaries in the reservoir area accumulated 0.8808 billion m~3,which accounted for 6.9%of the total deposition in the Three Gorges reservoir area during the same period.Most of the sediment deposition was concentrated in the tributaries in the perennial backwater area;(3)Under the same water level in front of the dam of the Three Gorges Reservoir as the basic scheme 2,the basic scheme 1 calculates the accumulative deposition of the Three Gorges Reservoir in 100 years by using the 1961-1970 series,which is 173.897 billion m~3more than that calculated by using the basic scheme 2 of the 1991-2000 series of water and sediment,which is 125.433 billion m~3,48.464 billion m~3more than that calculated by the 1991-2000series of water and sediment.In addition,the water level in front of the dam rises gradually;the upstream Xiluodu,Xiangjiaba and Tingzikou Junction;the sediment deposition of the Three Gorges Reservoir presents different forms under different dispatching modes such as double flood limit and multi-flood limit.The sediment deposition of the Three Gorges Reservoir is affected by both incoming water and sediment and reservoir operation mode;(4)The method for calculating the dominant discharge of reservoir channel based on dynamic sediment transport capacity method not only considers the effect of incoming water and sediment on the bed-forming of reservoir channel,but also considers the influence of reservoir dispatching mode on the shape of reservoir riverbed.The method has clear physical meaning,reasonable calculation results and feasible topographic verification.It is a method for determining reservoir riverbed.The method of high applicability of dominant discharge.4.Based on the different water and sediment combinations that may occur in the Three Gorges Reservoir under the new water and sediment conditions,a one-dimensional unsteady water and sediment mathematical model of the main tributaries of the Three Gorges Reservoir is established,and the main calculation parameters of the model are calibrated.The long-term change of sediment erosion and deposition in the Three Gorges Reservoir area and the balance of reservoir deposition are studied by using the water and sediment.The results show that:(1)the sediment trapping effect of reservoirs is obviously greater than expected in the upper Yangtze River.The average sediment inflow into the Three Gorges reservoir in 2013-2018 is0.725 billion tons,which is only 14.8%of the 1956-1990 sediment and 35.8%of the 2003-2012 sediment inflow.Among them,the sediment inflow into the reservoir in 2017 is only0.344 billion tons,which shows that the measured sediment inflow decreases faster than expected;(2)The variation process of water level,discharge and sediment concentration at each station along the model is basically consistent with the measured data,and the calculated values of total siltation and siltation distribution are basically consistent with the measured results,and the results of model verification are in good agreement with the measured values;(3)Under the 60 water and sediment series conditions and 90 series,the sedimentation equilibrium time of the Three Gorges Reservoir is 320 years and 430 years respectively.With the impact of climate change and human activities,especially the construction of Jinsha River cascade reservoir in the upper Yangtze River,new changes have taken place in the water and sediment conditions of the Three Gorges Reservoir.Under the new water and sediment conditions,the upper and lower limit time of the sedimentation equilibrium of the Three Gorges Reservoir is 560 and 450 years respectively,and the corresponding water and sediment combination conditions are small water and small sediment respectively typical year and typical year of big water and big sand.The study based on the new water and sediment conditions will provide a new reference for the future storage and operation of the Three Gorges Reservoir. |
