Research On Erosion Depth And Change Of Water Level In Lower Reach Of Hydro-junction

After the construction of reservoir in alluvial rivers, the sediment discharge in downstream reach of reservoir will fall sharply. The changing of boundary condition will result in river reformation, such as riverbed broadening and downcutting, falling of water level, as well as the bed material adjustment. If the prediction of these issues is not accurate, it will lead to impact on the navigation condition of the ship lock and the downstream channel.About the riverbed erosion and falling of water level in lower reach of hydro-junction, many research achievements are presented. However, the difference between the forecasting result and the field data is great, because of the complexity of the mechanism of erosion, and it is difficult to solve the practical problems. With the development of river hydrodynamics as well as the technology of river channel regulation, and with the increasing construction of hydro-junction, the new calculation methods of riverbed erosion depth and falling of water level in lower reach of hydro-junction should be further investigated, in order to preferably resolve practical project problem. Thus, it is significant to carry out the research on riverbed erosion depth and the falling of water level.The key issues discussed in this paper are shown as follows:(1) The riverbed adjustment (including the adjustment of longitudinal section, river width, bed material composition and the maximum erosion depth) and falling of water level in constructed hydro-junctions are analyzed. The key factors which initiated falling of water level are put forward, and its regularity is summarized.(2) Through the flume experiments by clear water, the adjustment of longitudinal bed profile, the sand wave height, the bed material composition and the coarse layer thickness of river bed are studied. The mechanism of riverbed erosion and falling of water level was analyzed.(3) The characteristics of bed load transport rate are analyzed. According to the results of flume experiment, the changing law of bed load transport rate is investigated, and the formula of transport rate is presented. The computed results were in better agreement with the experimental ones.(4) By macro-analysis, the estimated formulas with simple structure and easy computation for the erosion depth of riverbed and the falling of water level are presented. The computed results were in better agreement with the experimental ones. The formula can be used to estimate erosion depth at any time and in any reach, and to predict the longitudinal deformation of riverbed during the erosion process.(5) The model of non-homogeneous suspended load transport is established. The exchange forms between the suspended load and riverbed are divided into three types: sole depositions, sole erosion, and deposition of the coarser ones and erosion of the finer ones. Different types result from different sources of sediment and result in different formulas of carrying capacity. The non-equilibrium sediment transport model of non-homogeneous bed load is established. Same as the suspended load, the exchange forms between the bed load and riverbed are also divided into three types:sole depositions, sole erosion, and deposition of the coarser one and erosion of the finer ones. Different types result from different sources of sediment and result in different formulas of effective transport rate of bed load.(6) A plane 2-D full load mathematical model for unsteady flow is established under the body-fitted coordinate system based on staggered grid. In this model, the finite volume method is adopted to disperse the basic governing equations. The SIMPLEC algorithm is adopted for resolving the flow equation together with non-equilibrium transportation equations of suspended load and bed load, which can improve the convergence and stability of computation. The mathematical model established in this paper is verified in a simplified suddenly widened channel. The result shows that the model can be preferably used in the calculation of the movement of flow and sediment transportation, as well as the riverbed deformation.(7) Finally, the estimated formulas and the full load mathematical model established in this paper are adopted in lower reach of Danjiangkou Reservoir. The results show that both methods can preferably simulate erosion depth and the water level. Thus, the estimated formulas with simple structure and easy computation for the erosion depth of riverbed and the falling of water level established in this paper can be applied to natural practical engineering.