Dynamic Mechanism Of Gully-type Debris Flow And Its Numerical Simulation

Gully-type debris flow is not only a common natural disaster,but also an active geomorphological process.Thus,research concerning its dynamic mechanism has important significance in both theory and practice.In order to systematically explain the dynamic characteristics of gully-type debris flow and further understand its formation and evolution mechanism,I conducted field investigation,literature analysis and data collection.Based on bed sediment exchange theory,preferential flow theory,and theoretical achievements in particle flow,two-phase flow and unbalanced sediment transport,the dynamic mechanism and numerical simulation of gully-type debris flow are studied.The main results are as follows.(1)Dynamic characteristics of the gully-type debris flow were described from the aspects of dimensionless eigenvalues,energy dissipation,sediment transport and resistance.The debris flow initiation requires a large critical shear stress,inter-particle collision and friction play an important role on its formation and evolution.During the propagation of debris flows,the energy consumption form of particles changes from friction to collision with the increase of particle size.The water and sediment conditions and sediment transport capacity of debris flows are obviously stronger than those of natural rivers,while the positive correlation between sediment concentration and water and sediment factors will be weakened from river to debris flow.Influencing factors of debris flow resistance can be divided into two types,single effect and double effect.The former includes gully boundary conditions,gully bed water content and coarse particle content,the latter includes fine particle content and solid concentration.Meanwhile,gully erosion also has the characteristics of double effect.(2)Through analysis of the formation conditions and generalization process of gully-type debris flows,a mechanical-stochastic model and dominant channel model were established for debris flows initiated by gully erosion and natural dam break,respectively.The mechanical-stochastic model which can reveal the formation of incipient debris flow due to gully bed erosion,considers the randomness of instantaneous bottom velocity of water flow,sediment particle size and internal friction angle of particles.The results of sample data analysis indicate that the dimensionless starting velocity of dso particles is about 1.0,and the dimensionless sediment transport rate basically intersects with the critical line calculated by the Takahashi formula at the position of critical water depth,which verifies the reliability of the model.The dominant channel model can simultaneously consider the erosion destruction and piping failure of natural dams formed in gullies.The model defines the surface erosion grooves and internal piping bores of natural dams as dominant channels.The stability of natural dam can be analyzed by the seepage field considering dominant channels in real time.(3)In the framework of multi-layer multi-phase media,a debris flow evolution model considering the critical grain size,phase interaction and gully erosion was established.According to the characteristics of sediment transport of debris flows,the method for determining the critical grain size was discussed under different sediment transport modes,and its determination method for the two-phase turbulent mode was established by the concept of sediment transport capacity.To describe the gully erosion process of debris flows,two methods were proposed based on the unbalanced sediment transport formula and the shear stress balance principle,respectively.Based on the collected data and the calculation of simplified form of the model,the three-dimensional structural characteristics of debris flows were analyzed.In the flow direction,debris flow evolution is mainly controlled by resistances,the variation of solid concentration can shape different motion characteristics,thus forming various types of debris flows which may also accompanied with the surge development.In the vertical direction,characteristics of concentration distribution and velocity distribution will directly affect the dynamic process of debris flows,and the calculation of vertical velocity distribution of the two-dimensional constant uniform flow shows that phase interation has a great influence on velocity distribution of the two phases.In the lateral direction,the lateral erosion has an important influence on the formation and evolution of debris flows,and this effect is different from the gully bed erosion.(4)Using the finite volume method of the second-order precision Godunov type,the local Riemann problem was solved by decoupling the velocity and pressure terms,and the numerical simulation of the one-dimensional two-phase evolution of debris flow was carried out.The second-order precision MacCormack-TVD finite difference algorithm was selected to simulate the evolution of two-dimensional debris flow.