| The river system connects the land to the sea,and the process of sediment transport a foundamental issue to many subjects.With rapid societal development,human interference on the river becomes more and more intense.Especially the dam construction,which directly changes the process of water and sediment transport in natural river systems,usually comes with great impact on the livelihood of modern society.The natural river system,its discharge and sediment are likely to change due to the influence of the dam,and the original balance will be broken,which promote to adjustment of the river bed,and re-adapt to the new balance.Clear water due to damming works up the riverbed scouring,then the fine sediment is taken away,and coarse sediment preserved in situ,which lead to the riverbed coarsening,and reduce the sediment carrying capacity gradually,so as to realize a new equilibrium state.In the process of river bed topography adjustment,the formation of river bed coarsening and the development of sand wave reflect the evolution of the river dynamic landscape from the unbalance to the balance,and it has important influence on the estimation of riverbed roughness coefficient and sediment yield,closely related to the actual amount of sediment erosion.The Yangtze River,one of the largest rivers in the world,transports abundant materials to affect the evolution of the Yangtze delta,which is densely populated with highli developed economy.Thus,the material transport of the Yangtze River is becoming a hot topic for both academic community and government.Since the Three Gorges Dam(TGD)started to operate in 2003,it had reduced the sediment flux from 490 million tons/year to 130 million tons/year from the Yangtze River into the sea.This inevitably caused changes in hydrological,geomorphological,and ecological conditions of the middle-to-lower Yangtze.Sediment flux from Yangtze River into the sea includes five major sources,such as the drainage of the TGD,the Dongting Lake basin,the Han river basin,the Poyang Lake basin and the erosion of the riverbed below the TGD.In general,the sediment flux from upstream is directly controlled by the TGD,the sediment flux from the Han River and Poyang Lake flow directly into the Yangtze River,which all tends to be stable at present.However,the sediment flux from the Dongting Lake has both input and output with the Yangtze river,which is more complicated.So the sediment flux from the riverbed erosion below the dam and the Dongting Lake basin are the top two from the list.What are the evolutionary processes and mechanisms of the sediment dynamic behavior of the riverbed below the dam after the construction of the TGD,and when the future sedimentation of the Yangtze River reaches the minimum?Those problems depend largely on the amount and time of the erosion below the dam,that is,the riverbed erosion reaches a new equilibrium.Based on collected data and field survey,this study 1)probes into the annual trend of the runoff and sediment of the Dongting Lake,and analyze the feature of sediment transport and its future trend from the Dongting Lake into the Yangtze River,clarify the influence and function of the Dongting Lake as the boundary conditions on the middle reaches of the Yangtze river;2)Moreover,this study analyzes the sediment dynamic by using the sediment carrying capacity in the pre-and post-dam to discuss the river channel deposition and erosion in the middle reaches of the Yangtze River.3)Finally,on these basis,the perfect boundary parameters are set,including the water-sediment flux of the Dongting Lake,the roughness of the main channel of the middle reaches,the thickness of the mixed layer(using the sand wave form method),the bed load gradation composition,etc.Then this study tries to reveal the trend of the sediment flux from the Yangtze River into the sea in the future by using the SOBEK model.Some conclusions can be drawn as below:1.The runoff and sediment flux of the Dongting Lake both decline,but the decline in sediment flux is much significant.The runoff is mainly affected by rainfall,nevertheless,the sediment flux is affected by various factors and can be divided into three distinctive periods.Period 1(before 1970),the sediment flux is dominated by precipitation;Period 2(1970-2003),influence from human activities increases and the influence of precipitation decreases;Period 3(after the TGD,2003)human activities dominates the sediment flux.The sediment flux outflowing the Dongting Lake maintains around 20 million tons per year into the mainstream of the Yangtze River.Meanwhile,the Dongting Lake has changed from the "sink" to "source",from sedimentation to erosion after the completion of the TGD.Since the construction of the TGD,the net sediment transport from the Dongting Lake to the Yangtze River is average of 8 million tons per year,but the net sediment transport will be raised to 20 million tons per year if the three tributaries of Jinjiang River further decrease.2.After completion of the TGD,90%of sediment has been intercepted in the reservoir area,contributing only 50 million tons per year to the river channel below the TGD.Thus,sediment flux decreases significantly in the middle and lower reaches of the Yangtze River,as the annual sediment flux into the sea reduces from 440 million tons per year to 130 million tons per year.The TGD breaks the balance in natural river system,converting the seasonal scouring and silting in middle reaches into all-year-round erosion,outcroping 65 million tons of sediment per year at the river channel between Yichang and Hankou.As all year round erosion occurrs at the river channel near Yichang,the riverbed becomes coarsened and the sediment carrying capacity decreases.When the bed load sediment matches with the suspended material from upper reach,it comes to a new dynamic balance.Obviously,the riverbed has coarsened at Yichang and Shashi,the riverbed coarsening even reached 20 mm at Yichang,0.5mm at Shashi.At present the 90%the riverbed sediment(<2 mm)has been washed at Yichang and the sediment carrying capacity has been reaching critical number,so Yichang has no potential can be washed.But the median grain size of bed load has no obvious change at Jianli,Luoshan and Hankou,which reflects strong erosion is completed at Yichang station,and riverbed erosion takes place mainly above Jianli.The sediment carrying capacity in the Jingjiang River is far from the limit value,and it is foreseeable that the channel erosion would be concentrated on this reache.The sediment carrying capacity and unit stream power of the reache below Chenglingji is low,due to the multiple interference of the slope,bedrock,Dongting Lake etc,which lead to the channel erosion will cease at the Chenglingji.3.As a result of riverbed erosion,the sandy riverbed will undergo a distinct transformation of "coarsening-sediment transportation-bedform".Because the hydro-power of the upper Jingjiang reach is stronger than the lower Jingjiang reach,the waveform of the upper Jiangjiang reach is larger than the lower Jingjiang reach,so as the wave length.The median grain size of the bed load is coarser in the upperstream,which has a good correlation with the length of sand wave.But wave height and water depth have no obvious difference between the upper and lower Jingjiang reach.4.The SOBEK model is validated by the actual erosion of the middle reaches of the Yangtze River.The model is reliable for the interannual sediment transport problem in the reach of the Yichang to Chenglingji in the middle Yangtze River.In this paper,five schemes are designed.The maximum scouring amount is about 1.46 billion tons and the scouring time is 46 years in the Scenario 1;the maximum scouring amount is about 1.53 billion tons and the scouring time is 41 years in the Scenario 2;the maximum scouring amount is about 1.47 billion tons and the scouring time is 49 years in the Scenario 3;the maximum scouring amount is about 2.34 billion tons and the scouring time is 32 years in the Scenario 4;the maximum scouring amount is about 2.43 billion tons and the scouring time is 36 years in the Scenario 5.Considering with the actual situation,we choose the Scenario 1 as the basic scheme,then deeply discuss the change of the middle Yangtze River and the rationality of the results under Scenario 1.5.The erosion of the riverbed below the dam,the process is going from the near to the distant,the erosion intensity at the reach between Yichang to Shashi is strong in the 10 years after the construction of TGD,and the erosion will stop in the 19th year,and accumulated 390 million tons of sediment,the erosion at the reach from Shashi to Jianli will been basically completed in the 32nd year,and accumulate over 490 million tons of sediment.As the erosion stop in the upper reaches,the erosion center will gradually down,the reach from Jianli to Chenglingji begin to strong erosion in the 20 years after the dam,continued until 46th year,and the sediment yield of 600 million tons.In the first 10 years,it is mainly concentrated in the first 100 km downstream of the dam,the average incision depth is 3.65 m,and then converts to the area between 100 and 200 km,2.75 m incision during 10?20 year.In the 20?30 year after damming,the erosion of the reach above Shashi was completely,and the erosion center gradually shifted to the lower reach of the river,1 m incision at 150?200 km,and 2.1 m incision below 200 km.There is no erosion above Jianli station,and the erosion center go downstream to the reach from Jianli to Chenglingji,average 2.5 m incision during 30?40 year.At this stage(40?50 year),only the reach from jianli to Chenglingji keep a slight erosion,and the average erosion depth is 1 m.6.The current pattern of sediment flux from the Yangtze River into the sea is mainly composed of the outbound of the TGD,Dongting Lake,Hanjiang River,Poyang Lake,as well as the erosion below the dam.Among the five main sources of sediment,most of them contributed steadily to the Yangtze mainstream.The sediment flux is approximately 50 mt/yr from the TGD;8 Mt/yr from the Dongting Lake,but the net sediment transport will be raised to 20 million tons per year if the three tributaries of Jinjiang River further decrease;8 Mt/yr from Hanjiang catchment and 12 Mt/yr from the Poyang Lake,the only variable is the erosion below the dam.According to the simulation by SOBEK,the annual sediment flux from the Yangtze River into the sea will maintain at 130 Mt before 2025,and then it will gradually decline to 90 Mt/yr in 2049. |
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