Preliminary Study Of Non-uniform Sediment Transport And Simulation Technologies Downstream Reservoir

As an important form of rivers function development for human, reservoir plays a tremendous benefit in the fields of flood control, power generation, navigation, irrigation, and so on. However, it also brings a far-reaching impact to the evolution of the downstream river reach of the reservoir, due to the changes in the water and sediment conditions after impoundment. History of the river development and lessons all over the world indicates that the inadequate knowledge of sediment transport downstream of the reservoir will result in a number of works hard to fulfill their purpose, or even negative benefits. Therefore, in-depth analysis of the sediment transport in non-uniform characteristics downstream reservoir, definitude of its internal mechanism and the elevation of its simulation technology will provide benefits of great importance to take early measures to against possible trouble and get the maximum effectiveness of projects.This paper first summarized the general laws of sediment transport and river adjustment in forms of erosion or deposition downstream the reservoir, deepened the understanding of their internal mechanism, and then explored the key simulation technologies and calibrated the model parameters by using the measured data combined with the current research results. On this basis, combined with specific issues, a one-dimensional river network mathematical model of unsteady water and sediment transport for the middle and lower reaches of the Yangtze River downstream form the Three Gorges Reservoir was established and tested by using the latest actual measured data, and then the water and sediment transportation, river erosion or deposition amount, and the changes of hydraulic properties were predicted under different water and sediment condition corresponding to different impoundment progress.Some key issues are shown as following:(1) Characteristics and mechanism of non-uniform sediment recovery downstream reservoir:the two aspects affecting the river bed erosion and deposition were summarized, and then based on the analysis of general characteristics of sediment recovery downstream reservoir, according to the characteristics of sediment exchange posed by deposition of the sediment in water and entrainment of the sediment on river bed, a clear elaboration of the internal mechanism for non-uniform sediment recovery downstream the reservoir was raised. And a journal expression for sediment recovery at different stage of erosion period was constructed. It can well reflect the journal law of sediment recovery downstream from reservoirs. At the same time, based on the measured data, the recovery coefficient in the sediment transport equations was analyzed. The results showed that its magnitude can reaches about form 10-3 to 10-1, generally increasing as the particle size decreasing, and it also showed a descending trend with the increasing of the duration of the erosion state. Meanwhile, the non-uniform sediment transport properties and recovering features after a reduction of input sediment amount in middle and lower Yangtze River reaches were also analyzed. It was pointed out that there is a closely relationship between annual non-uniform sediment amount of erosion and input in the middle reach of the Yangtze River. Generally with the sediment particle size increasing, the relevance is improved, and the erosion amount in the lower reach is raised due to the unit sediment input amount change, and the input amount, corresponding to the balance of erosion and deposition, is smaller. Analysis also showed that, although there are two reductions in the input sediment amount before and after 90 years and the Three Gorges Reservoir, but the sediment recovery phenomenon of the later period is more obvious, because the later sediment reduction is larger, and the reach has been in the state of erosion, combined with the impact of hydraulic conditions.(2) The river adjustment in the process tending to the balance of erosion and deposition downstream reservoirs:Based on the measured data downstream reservoirs under natural conditions and the tank experimental data, according to the difference of river bed composition, the phenomena of the adjustment in the process towards to the balance, both for sand-gravel and sand river beds, was summarized and summed up, and their roles in the process were also analyzed. It is pointed out that the ultimate result of the adjustment of various factors is to change two aspects which affect the river evolution:the river's ability to anti erosion and the flow's ability to shape the riverbed. When the two aspects come to an equal level, the river will enter into the state of equilibrium, which is ultimately determined by the sediment start-moving condition. In the process trending to the balance, the role of vertical gradient adjustment is inevitable, but not the only, and the role of the increasing resistance cannot be ignored.(3) Discussion and exploration on the key technologies in one-dimensional numerical simulation of non-uniform sediment:Based on the current study, further discussion on the key technologies in non-uniform sediment mathematical model was proposed:â‘ Using entrainment and deposition fluxes based on statistical theory of sediment movement, expression of non-uniform sediment carrying capacity was derived, and contrasted with the earlier research, the inadequacy and improvement was also proposed;â‘¡According to the diffusion theory of sediment movement, through the introduction of the adjustment factor, formula of the vertical sediment concentration distribution under the non-equilibrium sediment transport condition was derived and its parameter was fitted, and it can be used to calculate the recovery coefficient based on the statistical theory of sediment movement;â‘¢It is pointed out that the impact of the time step on the values of the mixed layer thickness must be considered in unsteady flow mathematical model, and the method of calculating the thickness of the mixed layer based on the sand wave movement was proposed;â‘£Velocity distribution formula along the cross-section direction was derived and fitted, which can be used in the method of allocating the erosion and deposition area along the cross-section direction based on the flow unsaturated level in one-dimensional sediment mathematical model.(4) Research on the one-dimensional numerical simulation and prediction of water and sediment in the middle and lower reaches of the Yangtze River after the impoundment of Three Gorges Reservoir:a one-dimensional unsteady flow and non-uniform sediment river network mathematical model for middle and lower reaches of the Yangtze River downstream the Three Gorges reservoir was established, and it was tested and verified by using the latest actual measured data. After that, the predictive calculation of the river erosion or deposition amount and its development process, and the water level changes at major sites, under the different impoundment program condition, was carried out. The results were better fitted with the general laws of the sediment transport downstream reservoir than the earlier research, and it was in good agreement with the relevant prediction of the limit erosion amount and the latest actual measured data.