Study On The Disaster Pregnancy Mode And Starting Mechanism Of Narrow And Steep Channel Debris Flow In The Upper Minjiang River

After the "5.12" Wenchuan earthquake in 2008,a large-scale debris flow broke out in the polar earthquake area for many years in a row,showing the characteristics of group trench outbreak,large scale,wide range,long duration,and heavy damage.One of the main reasons is the insufficient understanding of the disaster mechanism of channel type and high-level landslide-type debris flows in strong earthquake areas.The treatment plan does not distinguish between the characteristics of wide,narrow and steep channel-type debris flows and the corresponding prevention and control The measures lack a reasonable optimized combination.Traditional debris flow prevention theory and technology cannot fully meet the special needs of debris flow control in strong earthquake areas.According to the requirements of the “Complete Public Safety System” and “Improving the Ability of Disaster Prevention,Mitigation and Disaster Relief” of the 19 th National Congress of the Communist Party of China,the research on the narrow and steep channel debris flows in the upper reaches of the Minjiang River is of great significance for the prevention and control of debris flow disasters in strong earthquake areas.This paper takes narrow and steep channel debris flow as the main research object.Based on the multi-temporal remote sensing survey and field investigation,this paper studies the narrow and steep channel debris flow hazard model and summarizes the main source supply methods and Starting position,etc.,to establish the main disaster-preventing model of typical narrow and steep channel-type debris flow after the earthquake;the method of combining remote sensing and field investigation with indoor physical simulation experiment to study the starting mechanism of narrow and steep channel-type debris flow,and to establish the main Start mode,achieved the following main research results:(1)The debris flow in Shaofanggou belongs to the typical narrow-ditch trench debris flow with concentrated supply of landslide accumulation,the main source is H1 landslide,and its disaster-pregnancy model can be divided into: early upstream rainfall formed debris flow accumulated above the H1 landslide,which caused the stability of the landslide to decrease;continuous rainfall caused the weir formed by the H1 landslide The plug body broke out,forming large-scale debris flow disaster.(2)The debris flow in Zhangjiaping Gully is a typical channel erosion type debris flow.Its provenance starting mode is scouring-etching-slipping-blockingdestruction.Its disaster-pregnancy mode is mainly manifested as: the earthquake causes loose solid sources to accumulate in the trench Beds and hillsides;the effect of later surface runoff increases sediment,accompanied by strong erosion and erosion;undercutting causes the bank to collapse,causing clogging and clogging;the kinetic energy of the debris flow progressively increases,further erosion and side erosion ditch beds and loose materials along the coast,Eventually a mudslide will form.(3)The laboratory physical model test results show that under the conditions of median particle size of 12.3mm and channel slopes of 25 °,30 ° and 35 °,the debris flow is all started by landslide fluidization,and its starting process is mainly as follows: The initial water flow continued to infiltrate,and the upper surface particles of the accumulation body were gradually eroded and produced short-term slip;when the accumulation body was saturated,the surface runoff of the slope caused the surface particles to appear short-term slippage again;the continuous increase of water content caused the upper accumulation The body eventually becomes unstable overall and starts to form a debris flow in a fluidized state.(4)The laboratory physical model test results show that: under the conditions of median diameter of 8.5mm and trench slopes of 25 °,30 ° and 35 °,the debris flow is all started by erosion of the trench bed,and the starting process is mainly as follows: The water flow is infiltrating continuously,the first runoff is generated in the lower part of the slope body,and the surface particles in the lower part are gradually eroded and washed out,and the debris flow is gradually generated,and then the runoff continues to expand from bottom to top,which eventually leads to the bottom of the accumulation body above.(5)Indoor physical model test results show that under the conditions of median diameter of 4.4mm and trench slopes of 25 °,30 ° and 35 °,the debris flow is started by the fire tube effect method,and the starting process is mainly: the beginning The water flow produces small beams in the upper part of the slope and continues to expand,causing a small amount of particles to start,which in turn causes strong erosion and erosion of the accumulation body,and then triggers a large amount of particles in the upper part to start and generates siltation in the middle;the runoff continues to expand and wrap along The accumulative deposits in the middle reach the high point in the middle silt;as the bulk density of the accumulating body continues to increase,when the flow reaches the starting value,the middle and upper accumulating bodies are completely washed away,and the debris flow starts.(6)According to the laboratory physical model experiment process and data,under the same median particle size condition,the change of channel slope basically does not affect the starting method of debris flow,and the slope only plays a role in accelerating the process of debris flow.Under the same slope conditions,the larger the median particle size,the greater the pore water pressure required for the corresponding debris flow start;under the same median particle size conditions,the steeper the slope,the smaller the pore water pressure required for the corresponding debris flow start.