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sur le développement des zones arides et semi-arides

Titre : Study on Initiation Mechanism of Mud Flow Accumulation in Gully Desert considering Seepage Effects

Auteur : 何瑶龄

Grade : Master 2022

Université : Lanzhou University of Technology

Résumé
The wind erosion and water erosion alternate in Ningxia-Inner Mongolia reach of the upper Yellow River,and the process of wind,sand,water and sand is intense.There are typical wide valleys of desert,and the areas with the largest intensity of surface wind-water composite erosion in China and even the world are distributed.There are many mountain torrent channels in this region.In winter and spring,the coarse desert sand transported by the wind and the loess produced by the gravity erosion and collapse of the bank slopes on both sides are mixed in the channel,or new accumulation bodies are formed,or the residual mud flow accumulation bodies of historical floods are heightened and thickened to form a desert mud flow accumulation body sufficient to block the channel.When the rainy season comes,the upper reaches of the storm flood start with the accumulation of mud flow,and gradually evolve into high sediment flood and even mud flow disaster.The high sediment-laden floods and even mud flow disasters induced by wind-water composite erosion have seriously affected the health of the Yellow River.This paper focuses on the gully-type desert mud flow under the extreme condition that the accumulation body completely blocks the channel.Starting from the first link of accumulation body starting,this paper uses scientific analysis,model test,fluid-solid coupling numerical simulation and other methods to study the characteristics of accumulation body starting under the condition of rainstorm and flood infiltration,and reveals its disaster mechanism.The following research results are preliminarily obtained :（1）Through field survey and research in the study area,three representative areas（desert area,sand loess transition area,mountain torrents channel area）were selected for scientific research and sampling analysis.The results show that the soil in the channel is mainly composed of loess and desert coarse sand,which has obvious pore structure.Through the comparative analysis of multi-section patterns,the accumulation body with trapezoidal cross-section pattern is selected as the research object.The basic physical parameters of the collected soil samples were measured to provide data support for model test and numerical simulation.（2）Based on the assumption that the microscopic pore flow in the accumulation body is a circular pipe flow and the hydrostatic pressure of the upstream water,the start-up process test of the accumulation body under five working conditions（0%,5%,10%,15%,20%of the coarse sand content in the desert）under seepage effect is carried out.The experimental results show that the volume of wetting body increases with time in a piecewise function relationship.Then,a power function model of the change of pore water pressure in the seepage area with time in the starting process of the accumulation body is established,that is,P=At0.5,and“A”is the coefficient.（3）Based on the Mohr-Coulomb constitutive model in the finite element analysis software ABAQUS 2020,the fluid-solid coupling model of the mud-flow accumulation is established.The micro-starting mechanism of the mud-flow accumulation is revealed when the upstream water level changes by analyzing the changes of parameters such as the accumulation saturation,pore water pressure,plastic strain and displacement under five simulation conditions.The simulation results show that under the fluid-structure interaction,the accumulation body undergoes a series of complex nonlinear changes,including stress field→volumetric strain→porosity→pore water pressure→seepage field→plastic strain→yield failure.When the water level rises,the downstream slope of the accumulation body starts instability first,and the simulation results are qualitatively consistent with the model test results,and the starting of the accumulation body is negatively correlated with the water level rise rate.When the water level drops,the upstream slope of the accumulation body starts instability first.Under this condition,the accumulation body is in the soil state of high water level and the water pressure state of low water level,and the starting of the accumulation body is positively correlated with the water level drop rate.The above research results provide new methods and new ideas for the early warning and prevention of mud flow in gully desert,and also lay a solid theoretical foundation for the prevention and control of mud flow disasters in the future combined with information technolog

Mots clés : the upper Yellow River ;gully desert mudflow ;aeolian-fluvial interaction ;seepage effect ;fluid-solid coupling ;initiation process ;

Présentation (CNKI)