Simulation Of Fine Sediment Deposition In The Three Gorges Reservoir

Fine sediment flocculation exists in the Three Gorges Reservoir.The medium diameter of deposition particles is mostly less than 0.01 mm.Estuaries and coastal areas often are subjected to the deposition of these fine sediments.The deposition processes include settling of flocculative sediment,suspended load transport,movement as fluid mud and consolidation.However,previous studies seldom have simulated the above mentioned processes of fine sediment deposition in the reservoir.In this thesis,based on a 2-D numerical model,a mathematical model referring to the above propcesses has been estabilished.According to the simulation results of the Huanghuacheng sharp meandering reach and the near dam mildly meandering reach,the model is applicable for further research.In addition,due to the dependence of model parameters on the measured data,the effects of some key parameters on the deposition distribution have been analyzed extensively.The main results are as flows.(1)In order to establish a secondary flow model applicable to flow simulation in sharp meandering rivers,three typical models have been tested via laboratory sharp meandering channels firstly.The results show that the linear model is available after adjustment of some parameters.Secondly,to extend the application of nonlinear model to the whole width of channel,the effects of different terms in momentum equations have been examined,which suggested that the lateral gradient terms in lateral momentum equations play an important role in near wall region.Finally,theoretical analyses point out that 3d equations are recommended when coming to flow simulation in sharp meandering channel exactly.Howerver,considering the complex of the 3d equations and the simulation results of Huanghuacheng reach,the linear secondary flow model can be applied to the natural sharp meandering river for 2d simulation.(2)The equivalent deposition time method has been proposed for the process of fine sediment deposits.This method can achive the nearly same deposition thickness by a constant discharge and sediment load as the calculation conditions with field measured data.Based on this method,three methods of fine sediment deposits have been applied to simulate sediment deposits in Huanghuacheng reach.According to the results,the effects of some key parameters for shear stress and capacity method have been analyzed in detail.Results show that as for the shear stress method settling velocity of flocculation can increase the amounts of deposits while the critical shear stress can enlarge the areas of deposits;with the increase of roughness the amount and area of deposits reduce obviously;the influences of velocity and boundary conditions on the adaptive coefficient should be considered if the capacity method has been taken.(3)A model which can simulate the movement of fine sediment deposits has been established based on the properties of deposits and the theory of fluid mud movement.It has been checked by the deposits in the near dam areas.It can reflect the uplifting of deposits surface horizontally and considers the physical mechanisms of deposits movement.The effects of key parameters on the uplifting of deposits surface have been analyzed.The results show that,yield stress plays an important role and takes a negative relationship with the thickness of deposits,while the falling velocity takes a positive relationship with it.In addition,the yield stress can work only when the the bed slope surpasses a critical value.