| The origin and development of human civilization has a close relationship to the river system. In order to make better use of the river resources, people start to build dams on the riverbank to prevent from flood and create a better environmental setting for livelihood. However, as intensifying human activities and utilizating water resources excessively and disorderly, this has resulted in a strong reverse feedback from the river setting, which is contrary to the original intention of human being. Meanwhile, tremendous environmental issues have been raised, such as deteriorations of the river basin environments and intense erosions on the riverbeds downstream of the dams, which endanger the flood control projects. The ecosystem degradation in estuarine areas has also occurred. All of these facts have given out a way for how people can deal with the river setting.In short, the formation and evolution of the fluvial geomorphology can be concluded as a riverbed evolving from stable to unstable status, and then returning to stable again. But during this process it is constrained by a number of natural factors, such as the regional geology, geomorphology and climate in watersheds, as well as the intervension of human activities to the river hydrology and sediment transportation. Intensifying human activities have altered the natural evolution of the fluvial processes, and the change will be much more complex, leading to difficult for future prediction. Strengthening observation, discussion and summarizing the evolution of fluvial geomorphology, especially the responses on the dam construction on the river, is a good way for us to improve understanding of the river system- this self-feedback system, and it is significant to rich theoretical of the fluvial geomorphology, and it is also one of the key contents in the current fluvial geomorphology research.The flows and sediment transportations in the rivers usually have a variety of attributes, which play a vital role in maintaining the structure and function of the fluvial geomorphology system. Flow is the driving force in shaping the riverbed, and sediment is the material basis. Different combinations of flow and sediment characteristics determine the riverbed geomorphology. Flow, sediment and riverbed will effect among each other in the whole system. The dam constructions on the river have an impact on the quantities of flow and seasonal distributions of runoff downstream, but in different rivers and reaches, due to climate, geology, water distribution patterns, and regulation of the reservoirs are not always the same, the degrees of change are different, from little impact to completely change in the season mode. The impact of dams on sediment is manifested in a huge interception, leading to a significant reduction in the amount of sediment downstream. But the flows and sediments released from upstream, and composition of the bed load, the restoration process of the suspended load in the river channel are not always the same in different reaches. As the problem is complex, it still can not get a unified and clear understanding in sediment transport downstream of the dam. The riverbed change is based on the changes in flow and sediment. In different watersheds, the conditions of flow and sediment show a vastly different, this will cause the adjustment in river bed. In order to analysis the riverbed change, it is needed to study the flow and sediment perspectively. The application of dynamic mechanism of water and sediment to the quantitative analysis of the formation mechanism of fluvial geomorphology has become the main content of the modern river geomorphology research.The Yangtze River is the longest river in China, and the third longest river of the world. The monsoon climate and high intensity of human activities create the unique watershed topography environment. Since the 1990s, human activities in the watershed present a strengthening trend, especially the largest hydraulic project-Three Gorges Dam (TGD) building on the river, make people more focused on the geomorphological evolution of the river basin. We select the Yichang-Hankou reach in the Middle Yangtze as our study area. This reach immediately downstream of the TGD, the closure of the dam can make the most rapid and most obvious response on the flow, sediment and riverbed change in this area. Comparison the runoff, sediment and morphological changes between the pre- and post-dam periods, analysis the relationships among these three factors, the dynamic mechanism can be revealed. The SOBEK model is also used to make a prediction on the riverbed changes and sediment supply after TGD closure in 100 years. Through a lot of basic data and information analyze and build the model to make a simulation, the main research work and conclusions can be drawn as follows:1, With the closure of TGD, seasonal distributions of the runoff in the Middle Yangtze have changed. And the sediment supplies have decreased; grain sizes become smaller, for most of the sediments are trapped in the reservoir. While the bed load is coarsening and better sorting in the post-dam period, for the riverbed is scouring. The undercuting on the riverbed is occurring and gradually weakening with the distance away from the dam.The observations on the flow and sediment changes in the middle Yangtze River from Yichang to Hankou, this 626 km long reach, show that after the Three Gorges dam closure, with the reservoir storage, a slight decrease of the runoff in the downstream of the dam. The changes of the seasonal distributions are obvious. At Yichang station, the runoff in flood season has reduced 1/8 to 1/6 in the post-dam period, while in the dry season it has increased 1/4. The differences between seasons have reduced, and duration of the median flow has increased. Effect by sediments trapping in the reservoir, after dam closure the sediment loads also show a decreasing trend from 600 mt/yr in the pre-dam period to less than 100 mt/yr, especially in the July and August, the amount of sediment in the post-dam period is only 1/10 compare to the pre-dam period. With the reducing of suspended sediments in the post-dam period, the grain size has also gradually become smaller, D50 reduced from 0.005mm to 0.001mm. The median diameter of the bed load in the Middle Yangtze River is between 0.2 and 0.3 mm. The grain sizes of bed load from upstream to downstream become thinner. After Three Gorges Dam closure, the river bed in the middle Yangtze occurs erosion, the grain sizes of bed load show a coarsening trend, especially in the immediately downstream of the dam, the riverbed has composed of the pebbles. With fine particulate matter in the bed load is washed away, the election of the bed load is better than before the pre-dam period, the sorting coefficient mainly in the range of 0.5-1.0, as a moderate sorting. The river bed is cutting down in the post-dam period, with different distances away from the dam, from near to far, Yizhi reach-Jingjiang reach-Chenghan reach, the riverbed has cut down 3.7 m,1.5 m and 0.4 m on average;2, In the post-dam period, the duration of the median flow has increased, providing the basic dynamic for the bed erosion. The lateral erosion power in the Jingjiang Reach has caused the lateral migration on the river bed and will continue in the future.In the post-dam period, the runoff in dry season increases and the flood peak cutting by the reservoir regulation and storage, has directly change the basic dynamic factors for riverbed morphology shaping. Therefore at Yizhi reach, the riverbed undercutting is much stronger than else reaches, and the slope gradually reduced, with the distance away from the dam, the runoff change become weaken, the extent of riverbed erosion become smaller. In the Jingjiang River, due to a strong lateral force (0.15N on average), the undercutting of the riverbed is ongoing and the lateral migration has occurred meanwhile;3, After TGD impoundment, it can not supply enough sediments from the Upstream Yangtze, so it need the bed load in the Middle Yangtze to make a compensation, and the bed load can be carried by the flow in the Middle Yangtze.In the post-dam period, a significant reducing of the suspended sediment has occurred at Yichang station, which can only supply 73mt annually, far below the critical value which is 300mt/yr, so in the middle reaches the sediment supply is in shortage, and needs new sediment from the bed load sources to compensate. The sediment incipient velocity in the middle reaches of the Yangtze River in the range of 0.4-0.6 m/s, which means that as long as the flow velocity can reach 2000-4000 m3/s at Yichang station, the bed load can be carried out by the flow. Therefore, after Three Gorges Dam impoundment, even in the dry season, the flow can washed the riverbed. However, due to the coarsening on the riverbed, the sediment incipient velocity increases. At Yizhi reach, the coarsening phenomenon is obvious, the sediment incipient velocity increases rapidly, from 0.55 m/s to 6 m/s, has enlarged more than 10 times, and the pepple on the riverbed can not be driven by flow easily, even in the flood season;4, After TGD impoundment, the increasing on sediment carrying capacity and decreasing on the suspended sediment concentration (SSC) have caused the riverbed erosion during the whole year. The seasonal erosion and siltation on the riverbed which happened in the pre-dam period has weakened, even disappeared.The substance on the riverbed erosion or siltation is whether the sediment carrying capacity can match the sediment concentration in the river. The sediment carrying capacity is calculated by Zhang Ruijin formula. In the pre-dam period, sediment carrying capacity at Yichang station is not saturated if the discharge is less than 20000 m3/s, with the discharge increasing, the capacity is the saturated, so the seasonal erosion has happened at this area. After TGD impoundment, sediment carrying capacity has increased dramatically, sediment concentration can not get saturated to meet the sediment carrying capacity in the whole year. The riverbed is scouring all the time and seasonal erosion has disappeared. While at Hankou station, the sediment concentration and carrying capacity can be matched well, the riverbed erosion is not obvious. The Q-SSC rating curve reveals that at Yizhi and Jingjiang reach, in the pre-dam period, a seasonal erosion and siltation has occurred. In the non-flood season, the riverbed is mainly controlled by the erosion process, while in the flood season, it has transformed to the siltation process on the river bed. With the distance downstream, this seasonal erosion and siltation on the river bed became weakwen, downstream of Chenglingji, this seasonal change is not obvious. In the post-dam period, the whole Middle Yangtze is scouring on the river bed in the whole year, but the erosion rate is weaken with the distance away from dam site, especially in the flood season;5, After TGD impoundment, the erosion on the riverbed has occurred in the Middle Yangtze. So this area has formed a sediment source.Estimation of the erosion amount at Yichang-Hankou reach, in the pre-dam period, this reach deposited 151 mt/yr sediments in the flood season and scoured 46 mt sediments, this area is a sediment "sink", while in the post-dam period, this erosion on the river bed can supply 61 mt sediments to the lower Yangtze and estuarne area, which in the flood season it can supply 29 mt sediments and 32 mt in the non-flood seasons, it has been a new sediment source;6, The prediction on the riverbed erosion and sediment supply which is simulated by SOBEK shows that the riverbed will continuous scouring in the Middle Yangtze with the current sediment supply from the upstream, and this can generate at least 40 mt/yr sediments in the Middle Yangtze. The riverbed will be smooth in the erosion process.The bed elevations and bed load compositions measured in 2002-2003 before TGD closure are used as the initial condition, and the current flows and sediment supplies (2003-2010 on average) to the Middle Yantze River are used as input condition, the one-dimensional model SOBEK is built to simulate Yichang-Hankou river water and sediment movement and estimate the rate cut in the middle reaches after TGD closure in 100 years. If the supply of sediment can still maintain the current level, each year at Yichang station can get 71mt sediments from the upstream, even in 100 years after Three Gorges Dam impoundment, the riverbed will remain in a state of erosion and each can generate new sediments 40-65mt. The erosion in the Middle Yangtze can be divided into three phases. The first of 15 years after closure dam, the river stage will adjustment dramatically, changes in inter-annual sediment load vary largely. The second phase is in the closure dam from 15 years to 45 years, the extent of riverbed adjustment is reduced and the amount of annual sediments supply shows cyclical changes; the third stage is the 45 years to 100 years after the dam impoundmemnt. In this stage, annual sediments supply shows a gradually reduced trend. During the channel adjustment process, the cut rate will vary in different reaches. At Yizhi reach, the riverbeds incised significantly, which the rate can reach 0.4 m/yr then reduced to 0.1 m/yr. At Upper Jingjiang reach, the rate can almost steady at 0.1 m/yr. At Lower Jingjiang reach, the incision rate also decreases year by year, from 0.3 m/yr reduces to 0.05 m/yr; At Chenghan reach, the riverbed will experience a deposition process then transform into an erosion trend with a rate at 0.02 m/yr. Taking into account the coarsen trend on the river bed during the erosion on the river bed, the setting of the coarsening layer is introduced in the SOBEK model, the non-moving layer can play an inhibitory effect to prevent the undercutting on the riverbed. When the coarsening layer buried below the mean riverbed surface 3.5 to 4 m, the cut on the river bed will be inhibited by this layer, the riverbed tends to be smooth, the amount of river erosion will decrease significantly after Three Gorges Dam impoundment in 100 years, the middle reaches can generate the sediment supply less than 30 mt/yr. |
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