Study On The Formation Mechanism And Risk Assessment Of Debris Flows In Luanchuan County

Luanchuan County,located in the mountains of Western Henan Province,is characterized by complicated topography,including crisscrossed mountains,precipitous landform,high gully slope and developed unstable slopes.Rock formations within the area are complex and diverse,strongly weathered with well-developed joints and thick loose slope wash.Mining in the county has a long history,and some areas have been seriously reformed,resulting in the deterioration of the environment.In addition,rainfall is heavy and concentrated inLuanchuan County.All these factors have led to the development of most frequent geological hazards in Henan province.Therefore,it is necessary to study the debris flow hazards in Luanchuan County,which will be helpful to the decision-making of disaster reduction of Luanchuan County government,and to reduce the loss of life and property of local residents and the damage to the natural environment.In this paper,the debris flows in Luanchuan County were taken as the research object.By using field investigation,laboratory test,theoretical analysis and particle flow numerical simulation,the formation mechanism of debris flows in Luanchuan County was studied systematically,and the risk of debris flows was evaluated.The main contents and results of this paper are as follows:1.The natural geographical conditions,geological background conditions and historical debris flow records in the study area are briefly introduced.Because of the destruction of geological environment caused by human life,production activities and natural weathering,as well as the good rainfall conditions,debris flows are well developed in Luanchuan County.There have been numerous torrential floods and debris flows in history.The frequency of large scale debris flow activities in the county is relatively low,averaging about 1 time every 10 years.Due to the different rainfall conditions,landforms,vegetation and provenances,debris flows in the study area are not usually occuring in group,but have the characteristics of regional occuring in group.2.The formation conditions,basic types and development characteristics of debris flows in the study area are analyzed.The results show that:(1)The source types of debris flow in the study area are abundant,mainly including materiallithologic weathering and collapse,soil erosion and re-transporting of alluvial-diluvial layer,debris flow deposits in gullies,artificial cultivated land in gullies,residual slope material instability and mining waste.(2)The rainfall in the study area is abundant and varies greatly in years.In flood season,the rainfall is most in July and August.There are obvious regional differences of rainfall in the county.The rainfall in Nanchuan is larger than that in Beichuan,and the daily rainfall has double peaks.There is more rainfall at night and in the afternoon.Debris flow mostly occurs in the period ofshort-term strong rainfall during continuous rainstorm.(3)The debris flow deposits in the study area show the shape of accumulation fans and accumulation cones.The debris flow occured in some valleys are of large scale and high disaster grade,which need to be monitored and controlled.Debris flow mainly distributes in Luanchuan-Shimiao-Taowan area,which is the interlacing zone of low-middle mountain area,high average rainfall area,fault active zone and human active zone.(4)There are various types of debris flow in the study area.According to the soil supply classification,it can be divided into slope erosion-type debris flow,landslide-typedebris flow,slag-type debris flow and composite-type debris flow.Some of the gullies in the county have been seriously altered by human activities,mainly including random piling of slag,farming in steep slope,and river crowding,resulting in the deterioration of the environment and aggravating the damage of debris flow.(5)According to the characteristics of typical debris flows in the study area,several prevention suggestions were proposed: check the existing cultivated land retaining wall and slag retaining dam,put forward the corresponding design and construction guidance scheme;standardize planting activities,promulgate land use planning,rationally utilize the limited land resources,and minimize the environmental impact damage;Strengthen the investigation of the potential landslide-type debris flow sin the study area,promulgate housing construction planning;restore and protectethe vegetation in river basins and plan the resources developmentrationally;for the large-scale and disaster-grade valleys,timely clean up loose deposits in the channel,or build block and drainage engineering to prevent the next debris flow.3.The formation mechanism of typical debris flows in the study area is analyzed.The conclusions are as follows:(1)Formation mechanism of slope erosion-type debris flow(Shishu Gully): The strength of loose material in the gully was weakened by early rainfall.Because of the short-term strong rainfall,the runoff exerted erosion and dragging force on the deposits in the gully,and the loose material on the surface of deposits moved layer by layer with the runoff,and finally the phenomenon of "uncovering the bottom" appeared,then debris flow formed.When the debris flow passed through the old debris flow deposits in the middle and lower reaches of the gully,these materials were punched to supply the debris flow.(2)Formation mechanism of landslide-type debris flow(Taowancun Group 8): Because of short-term strong rainfall,the strength of slope soil decreased,the gravity pressure increased,and the landslide occured along the weak surface.When the landslide material entered the gully and mixed with the rain water,its movementaccelerated,impacted the remaining residual deposit along the way,and drived it to slide downward,forming a debris flow with great impact force.(3)Formation mechanism of Slag-type debris flow(Yudimiao Gully): Because of of short-term heavy rainfall,the mechanical strength of slag decreased.At the same time,the amount of upstreamwater obviously increased,the water level increased rapidly,and the flood pressure increased.Under the action of flood impact force and water pressure,the retaining wall collapsed when its strength exceeded the limit.Slag and flood were mixed together to form debris flow.(4)Formation mechanism of composite-type debris flow: There are many steep ditches and suspended ditches in the first branch ditch of the right branch ditch of the Washiyan gully.Under the condition of heavy rainfall,the water in the steep ditch and suspended ditch merged into the main gully,and produced intense energy exchange with the water in the main gully,resulting in liquefaction of deposit in the gully,which gradually mixed with the water to form debris flow.In addition,the slope debris flow quickly accumulated in the gully and blocked the channel.The upstream water level at the blockage increased,the upstream water pressure increased,and the blockage breaked.The upstream water quickly flowed downstream,and erodedthe deposits on both sides of the gully.Then a composite debris flow formed.4.The numerical simulation of the starting mechanism of erosion-type debris flow(Shishu Gully)and Slag-type debris flow(Yudimiao Gully)is carried out.The main contents and conclusions are as follows:(1)Dimensional effect and loading rate effect exist in the direct shear numerical test of source soil.By increasing the height of the simulated shear box,the size effect can be eliminated while the calculation efficiency can be ensured.The shear rate of the direct shear numerical test is determined.Based on this,the micro-parameters of the source soil were calibrated.(2)The numerical simulation of stratified migration and bottom uncovering of channel deposits and old debris flow deposits in Shishu Gully is realized.The simulation results show that: under the action of scouring and dragging force,the saturated particles on the surface of the deposits migrate overall,and scrape the bottom and leading edge of the deposits,until the bottom is uncovered.(3)The simulation of the collapse of residual and slope deposit in eight groups in Tao Wan village is realized.The simulation results show that: with the decrease of the strength,the residual and slope deposit gradually loos its stability and flushes into the gully at the rate of 1-2 m/s,which provides kinetic energy for the movement of residual and slope deposit in the gully.(4)The simulation of the outburst of the slag deposits in YudimiaoGully under the impact of flood is realized.The simulation results show that: under the impact of upstream flood,the dam first collapses,and then gradually breaks up into the flood with the slag.Then debris flowformed.5.The combinability of subjective and objective weights is studied,and the risk assessment model based on group decision method is established,which is used to evaluate the risk of debris flow in the study area.The main conclusions are as follows:(1)Subjective weight reflects the importance of evaluation factors,and objective weight usually reflects the discrimination of evaluation factors on the evaluation objects.For debris flow risk of single gully,we only need to pay attention to the importance of the evaluation factor,notthe discrimination.It is not necessary to combine the importance degree and the discrimination degree.(2)The reliability of subjective weighting method is affected by the subjective preference and experience of experts.Group decision-making method is suggested to determine the weights of factors.This method can not only improve the reliability of subjective weighting,but also can reflect the importance of each factor.(3)Given that assessment factors should be scientific,representative,comprehensive and easily accessible,and considering the specific conditions of the study area,nine secondary factors of debris flow risk assessment were selected:loose solid matter reserves,length ratio of sediment supply segment,vegetation coverage,watershed area,main gully length,maximum relative elevation,bending coefficient of main gully,24 h maximum rainfall,and annual average rainfall.(4)The results of risk assessment of medium-large scale debris flow in the study area show that the debris flow gullies are in the range of mild to moderate risk.For the gullies with high risk,it is suggested to clean up the material sources in the gullies in time,and build or reinforce the block project to prevent the recurrence of debris flow.