| The 2008 Wenchuan "5.12" earthquake severely disturbed the geological environment,causing drastic changes in the topography of the earthquake area,resulting in unusually active debris flows after the earthquake.Today,debris flow disasters of varying scales are still erupting in the earthquake area during the rainy season each year,causing great losses to local production and life.The rapid and steep channel debris flow is the weak link in the current research on debris flow disasters in strong earthquake areas.One of the difficulties in the early warning and prevention of geological disasters in this area.However,due to the limited data,insufficient attention and insufficient research level,the understanding and research work on the disaster mechanism and process of this type of debris flow are still very insufficient.To this end,this paper focuses on the development and activity characteristics of the steep channel debris flow,the establishment of starting modes and discriminants,the study of erosion mechanisms and erosion models,the study of outburst characteristics,and the calculation model of outburst.Based on the research of debris flow at home and abroad,this paper selects the debris flow gully of the steep gully in the earthquake area as the research object in view of the characteristics of large longitudinal drop and low clay content of the debris flow in the steep gully.Through a large number of field investigations and data collection,Literature investigation,model experiment,numerical simulation,etc.,to explore the dynamic process and mechanism of debris flow initiation-erosion-out under steep channel conditions.The research content and main research results of this article include the following aspects.(1)Summarize the general characteristics of the rapid and steep channel debris flow in the Wenchuan meizoseismal area as follows:(1)The average longitudinal gradient of the gully bed is greater than or equal to 400‰,the drainage area is greater than 1km2 and less than 3.5km~2,and the relative elevation difference is between767-2238 m.Deep "V" shape;(2)The slopes on both sides of the main ditch are the main replenishment areas for debris flow sources,and the unit source volume is large;(3)The gully of the steep channel debris flow is currently in the early stage of strong erosion,and the landform stage is in its infancy.The cutting is lighter,and debris flow is prone to occur,but the scale is small;(4)The formation area is not obvious,the circulation area is longer and the branch ditch is less developed.The longitudinal slope of the main ditch decreases with the increase of the watershed area;(5)The unit source density Large,and mostly large-giant avalanche landslides are developed;(6)The rapid and steep channel debris flow is highly active,mainly rainstorm-viscous-medium-frequency debris flow.(2)Through indoor physical simulation tests,it is revealed that the start-up mode of debris flow under steep trench conditions is the fire pipe effect start-up type;under steep slope conditions,runoff formation time is too fast,resulting in a more permeable effect of runoff on debris flow initiation.The effect is greater.When the slope is the same,the greater the clear water flow,the shorter the pre-saturation time.The eruption time of debris flow under steep channel conditions is much shorter than the general situation.At this time,the larger the runoff,the longer the debris flow duration.It is concluded that under the condition of 30° slope,the initial velocity Vq(m/s)and the impact The relational formula of water flow Vc(ml/s)is: Vs=0.0036Vc-0.1152,R2=0.98.In this paper,through the mechanical analysis of the critical state of the debris flow,a discriminant formula for the initiation of the single particle of the debris flow and the accumulation of the ditch bed is established.(3)In view of field investigations and experimental phenomena,it is found that the erosion characteristics of the steep channel debris flow are between the trench and the gully,and the main erosion is the main effect,which occurs in the peak stage of runoff,and is often accompanied by Side erosion and erosion.Based on the analysis of the erosion characteristics,through the force analysis of the erosion process of the debris flow gully bed in the steep gully,a calculation model of the erosion depth of the debris flow gully bed under the combined action of the debris flow’s dead weight static pressure,flow shear and pore pressure is constructed.While analyzing the erosion characteristics of the steep channel debris flow through field experiments,the three-dimensional laser scanner was used to obtain the terrain erosion characteristics during the debris flow process,which verified the reliability of the erosion model.(4)Based on field investigations and numerical simulations,it is found that the rushing process of the debris flow in the steep channel shows the following characteristics:(1)The debris flow in the steep channel rushes out of the ditch with a fast flow rate,a rapid accumulation process,and the movement time is shorter than that of the conventional debris flow.(2)The debris flow process shows the process of large erosion and large siltation,and the ditch accumulation fan shows the characteristics of small range and large thickness;(3)Because the channel is narrow,the vertical slope is steep,the potential energy of debris fluid movement is high,and the accumulation stays in the channel The material is less,most of the mud and rock fluid is washed out of the ditch,and the material source conversion rate is higher.In order to realize the quantitative evaluation of the threat range of the debris flow in the steep channel,this paper establishes the steep and steep channel by evaluating the single-factor correlation and regression analysis of watershed characteristics,channel characteristics,provenance characteristics,topographic characteristics and other factors of the debris flow.Model of the amount of debris flow out of the channel.In addition,Flow-3d is used to simulate the movement and accumulation process of a typical steep channel debris flow,which increases the understanding of the process of this type of debris flow.Due to the regional limitations of the research data,the conclusions and research results obtained in this paper can be applied to the steep channel debris flow in the Wenchuan strong earthquake area.Whether it is applicable to debris flow outside this range needs to be demonstrated.Due to the inevitable incompleteness of the survey data and the limited research capabilities,the initiation-erosion-outlet characteristics and model results of the steep trench-type debris flow obtained in this article may differ due to various basic conditions in actual engineering applications and scientific research.There are unavoidable errors.However,it is still hoped that this article can further reveal the characteristics and mechanism of the initiation-erosion-out of this type of debris flow,in order to improve the understanding of its disaster mechanism,in order to provide theoretical support for the early warning and prevention of this type of debris flow,and to further improve this type of debris flow.Research provides a certain reference and research basis. |
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