| The transport of pebble bed-load in mountainous rivers has impact on river engineerings,such as water intake,channel regulation and flood disaster prevention.The movement characteristics of the bed-load are discontinuous and complex.So,the numerical simulation of its movement process is difficult.With the construction of hydropower projects at the upstream,the supply of the bed-load is restricted.The operation of these projects also introduces complex changes of the flow movement,and the discontinuity of the bed-load is strengthened.Accurate simulation becomes more difficult.For the safety regulation of water intake and the channel regulation,the variation of non-uniform pebble bed-load transport belt under complex flow conditions is one of the key problems.And the difficult in preventing and diminishing flood disasters in mountainous rivers lies in the prominent characteristics of mountainous floods,which is obviously different from traditional floods.Namely,the interaction of flow and sediment can lead to a sharp rise of riverbed and water level,increasing the risk of flood disaster and even causing “low flood discharge with great disaster”.For the steep slope channel in mountain area,the sediment supply intensity from the upstream is high,the flow velocity is high and the riverbed shear stress is large.Therefore,the sediment motion layer with high-speed shear collision is formed near the riverbed,leading to the high-intensity sediment transport rate and the rapid evolution of the riverbed.The above movement process is much complex,so establishing effective numerical simulation methods for bed-load movement process is important.The research on the numerical simulation of the pebble bed-load transport process in mountainous rivers under complex water and sediment conditions has theoretical and engineering practical value.In the thesis,for the numerical simulation of pebble bed-load transport belt in mountainous rivers,the planar two-dimensional mathematical model of bed-load sediment mass is established to simulate the movement process of pebble bed-load transport belt.For the high-intensity pebble bed-load transport and riverbed evolution response in mountainous rivers,the vertical two-dimensional mathematical model of bed-load sediment mass is established to simulate the development process of dynamic hydraulic jump over a steep slope with varying angles.For the complex water-sediment coupling movement after the dambreak,the vertical two-dimensional water-sediment two-phase flow model is established to simulate the break process of accumulated sediment.The research results enrich the numerical simulation method of pebble bed-load movement in mountainous rivers,the main research results are as follows:(1)The continuity of pebble bed-load movement is not synchronous with the continuity of water flow in time and space,and the pebble bed-load transport is random.The thesis extends the particle method of Lagrangian model to the sediment mass method based on solving shallow water equations with Euler-Lagrange method.The sediment mass method is applied to the calculation of unbalanced exchange of non-uniform bed-load.The exchange position and transport amount of bed-load are calculated with this method.Finally,the planar two-dimensional mathematical model of bed-load sediment mass was established,which realized the numerical simulation of non-uniform bed-load transport belt in mountainous rivers.The model was successfully applied to the simulation of pebble bed-load transport belt and sediment diversion process in Dujiangyan,as well as the pebble bed-load transport belt and riverbed erosion and deposition in Dongxikou channel at the upstream of the Yangtze River.The research enriches the simulation methods of pebble bed-load movement and the riverbed erosion and deposition changes in mountainous rivers including the condition with limited supply of sediment.(2)The grid-based models can be cumbersome while handling the fluid-sediment interface problems.A vertical two-dimensional sediment mass model in the framework of the Smoothed Particle Hydrodynamics(SPH)method was presented based on solving water equations with SPH method.In the model,a piece of sediment is represented with a sediment mass.The riverbed erosion and deposition are simulated by moving the riverbed boundary particles up and down.Compared with the traditional particle methods,the number of sediment mass in this model is significantly reduced,which improves the calculation efficiency.The vertical two-dimensional mathematical model of bed-load sediment mass can successfully simulate the generation and development of the dynamic hydraulic jump under the condition of sufficient sediment supply over a steep slope with varying angles.Additionally,the critical triggering conditions of the retrograde sedimentation were also proposed.(3)A vertical two-dimensional water-sediment two-phase flow model based on SPH was established to simulate a large amount of sediment accumulation,such as landslide dam.The water and sediment are treated as two kinds of fluids with different densities and viscosities to accurately simulate the flow structure,sediment transport and water-sediment interaction process.The interaction model acts on the interface of the two phases.The model can simulate the water-sediment coupling process of the accumulated sediment collapse and the dambreak process of the landslide dam.It is helpful to understand the mixing process of water and sediment and the propagation process of the wave when the dambreak happens. |
