Research On Initiation Mechanism And Motion Characteristics Of Longbi Village Debris Flow In Guzhang County

Due to geological environment changes,human activities and heavy rainfall,landslide and debris flow often cause life and property loss,and they are two of the meteorological disasters with the highest frequency,the largest loss and the widest impact range.The Western Hunan area has always been a key area for the prevention and control of geological disasters.On July 17,2016,there was a heavy rainfall in Guzhang county,and a slope debris flow occurred in Longbi village,Guzhang county.This thesis makes a comprehensive analysis and exploration on the initiation mechanism and motion characteristics of debris flow in Longbi village,Guzhang county from the aspects of three-dimensional reality modeling,physical experiment and numerical simulation.In order to show the morphological characteristics of debris flows more intuitively in Guzhang county,the ContextCapture was applied to carry out a 3D reality modeling of debris flows based on the drone tilt photography.The physical and mechanical parameters of the soil in the debris flow source area were obtained through the basic physical test and the direct shear test of the soil samples.The seepage and stability of the debris flow in Longbi village under six different conditions were simulated by analyzing the initiation rain pattern of debris flow and calculating the extreme rainfall that occurs once in every 10 years,50 years and 100 years in the studying area,and considering the actual rainfall that occurs once in more than 100 years.Different erosion depths were set to simulate the motion characteristics of debris flow in Longbi village.And the pre-disaster settlement in the study area was analyzed based on InSAR technique.The main conclusions of this thesis are as follows:(1)2353 drone images were input into ContextCapture to build a 3D reality model of the studying area,and remove the invalid segment and poor tiles.Then the 3D reality model was imported into LocaSpace Viewer platform.The geographical calibration of 3D reality model was conducted.After that,rebuild of ruined houses in the platform.And the body feature of the slope and the building damage caused by Longbi village debris flow can be demonstrated intuitively with the 3D reality model.(2)Based on the description of the rain-type system for debris flow initiation,different combinations of rainfall types are set up to study the influence of early rainfall and effective rainfall on debris flow initiation;According to the rainfall data of Guzhang county in the 30 years from 1987 to 2016,the extreme rainfall that occurs once in every 10 years,50 years and 100 years was obtained using the Gumbel distribution function.Six types of rainfall boundary conditions were set up for seepage simulation in GeoStudio combined the rainfall occurred once in more than 100 years.These rainfall boundary conditions took into account the rainfall intensity,duration and the presence or absence of early rainfall.(3)The basic physical and mechanical parameters of the soil sample in the source area was obtained through physical experiments.In GeoStudio,the initiation mechanism of debris flow in Longbi village was analyzed by analyzing the influence of six different rainfall boundary conditions on the seepage and stability of slope.The stability of debris flow slope decreased with the increase of rainfall intensity and the presence of early rainfall.The high rainfall intensity increases the ability of fine soil to be carried by water flow to the deeper slope body,resulting in pore blockage of the lower soil body,and then the groundwater level goes up sharply and the pore water pressure in the upper part of slope increases sharply,which leads to slope failure.The longer the rainfall lasts,the more the underground water level in the slope will continue to rise along the slope surface.The outflow point of seepage water will increase sharply with the time of rainfall.After the failure of the slope,the soil will quickly turn into debris flow and slide down the slope surface.(4)With the help of Dan3 D,the change of movement characteristics of debris flow under the conditions of simulated erosion thickness of 0m,0.5m and 1m respectively was simulated.The Voellmy model was selected to establish debris flow motion simulation system and input inversion parameters(friction coefficient and turbulence coefficient)into the Dan3 D.The erosion rate is 0.00494 m m-1 when the erosion thickness is not 0.The influence range of debris flow accumulation,movement velocity and slope erosion thickness with time under three erosion conditions was compared in the simulation.With the increase of erosion thickness,the volume of debris flow accumulation increases,the later impact range and the area of building damage increases,and the velocity of debris flow increases as well.(5)Based on InSAR technique,SAR images on May 21,2016 and July 8,2016 was processed in SAR Studio and two rail differential interference was conducted.The surface settlement map of the study area in the period before the disaster was finally obtained.The maximum settlement is 0.058 mm,which is almost no settlement.The result showed that There was no deformation trend in the debris flow source area in Longbi Village before the disaster,and the extreme rainfall condition is the main cause of Longbi Village debris flow.