Dynamic Characteristics And Numerical Modelling For Debris Flows In Natural Channels

Debris flow is a common surficial process of rapid massive sediment and water mixture motion that occurs in many mountainous areas. Associated with this movement are often significant erosion, entrainment and deposition of large amount of sediments, which can dramatically change the morphology along the flow path and bring catastrophic consequences. Numerical modelling and simulation techniques for debris flow represent a valuable tool in risk assessment and mitigation.Based on the field observations of Jiangjia and Zhouqu debris flow disasters, this paper presents a numerical study on the propagation of debris flows in natural erodible open channels, in which the bed erosion-sedimentation processes and the transport of large particles by the deberis flows are important.The content of this dissertation is as follows:1) Based on the investigation in two typical debris flow channels, viz the Jiangjia Gully in Yunnan Province and the Sanyanyu Gully in Gansu Province, the sediments transportation characteristics of debries flows are analysed. By the identification of different dynamic patterns, the debris flow is considered as the combination of the continuum mixture and the granular flow of discrete large particles.2) Based on the Bingham fluid theory, a mathematical model of the two dimensional non-constant debris flow is developed. The governing equations include the continuity and momentum conservation equations of debris flow, the sediment convection-diffusion equation, the bed erosion-deposition equation and the bed-sediment size gradation adjustment equation. The yield stress and shear stress components are also included to describe the dynamic rheological properties.3) Developed the Manning’s roughness coefficient formula for viscous debris flow by means of analyzing the movement mechanism and the field observation data of debris flows. The resistance propertyof the debris flow mainly related to the bed channel, flow depth and rheology of the adhesive layers.4) A deterministic model for predicting the flow of suspensions of particles in viscous debris flow is presented. The Discrete Element Method (DEM) is used to simulate the movement of individual particles by accounting the forces acting on them. The motion of each individual particle is governed by the laws of conservation of linear momentum and angular momentum. Then, based on the B.B.O equations, the motion equations of large particles are presented. 5) The coupling method of the debris flow continuum dynamic model and the descrete large particle model is discussed. Then, the debries flow contiunnm medium-large particle coupling model is developed.6) The upwind control-volume Finite Volume Method (FVM) is applied to discretize the convection terms of the governing equation of the debreis flow continuum. The improved SIMPLE algorithm with velocity-free-surface coupled correction is developed to solve the equations on non-orthogonal, quadrilateral grids.For the transport of discrete particles in viscous debris flow fluid, the synchrony dynamic relaxation method is developed as the numerical algorithm of DEM, which can avoid the numerical error arising from the route correlation of dynamic relaxation method. Simultaneously, the iteration interval can be enlarged to significantly improve the computing speed.7) The model is applied to simulate the debris flow events in Jiangjia Gully, Yunnan Province and in Sanyanyu Gully, Gansu Province to predict the flow pattern and bed erosion-deposition processes. The results show the effectiveness of the proposed numercial model in debris flow simulation and potential hazard analysis. It is proved that the numerical methods developed are simple, reliable as well as stable and accurate.