Development Characteristics And Hazard Assessment Of Debris Flow In Yangjiagou,Beichuan County

As a geological disaster,debris flow occurs all over the world and poses a great threat to human life and property security and urban construction.On June 25,2018,there was a torrential rain in Beichuan County,with a maximum rainfall of 354 mm,which was rare in history.Many areas in Beichuan were seriously affected.In the course of this rainfall,debris flow occurred in Yangjia gully.Based on the field investigation and other background data,this paper analyzes the natural geography and geological environment conditions of the Yangjia gully area and the development conditions and genetic mechanism of the Yangjiagou debris flow.On this basis,Liu Xilin evaluation model and analytic hierarchy process(AHP)are used to evaluate the risk of debris flow in Yangjiagou.Then,the damage process and mechanism of the piled dams after the"6·25"debris flow are studied.Furthermore,based on the FLO-2D model,numerical simulation is carried out under the conditions of no engineering,intact engineering and partial engineering damage,and the three sets of simulation results are compared with the measured values of the field and other theoretical formulas.The accuracy of the numerical simulation results is verified by comparison with the measured values.Finally,the hazard assessment map of the Yangjiagou debris flow under three conditions are established.The main conclusions of this thesis are as follows:(1)The Yangjiagou debris flow is located in Chenjiaba Town,Beichuan County,belonging to the Duba River Basin.It is a secondary tributary of the Minjiang River.The drainage area is about 24.7km~2;the length of the main ditch is about 9.5km;the average ratio of the main ditch is 148‰.It’s worth noting that the landslides are important part of the material source of debris flow in Yangjiagou,and the Weijiashan landslide is the main source.(2)According to the on-site investigation and grey theory,seven evaluation factors(such as the size and frequency)of the debris flow are chosen.The risk of Yangjiagou debris flow is calculated by Liu Xilin evaluation model as high risk.Based on the analytic hierarchy process(AHP),nine factors,such as basin area,length of main ditch and average ratio drop of main ditch,which are closely related to the debris flow,are taken as the scheme layer,and the topographical and geomorphic conditions,provenance conditions,predisposing factors and meteorological conditions are regarded as the criterion layer.Similarly,the risk degree of debris flow in Yangjiagou is also a high risk.The conclusions obtained by the two methods are consistent with the on-site investigation.(3)Based on FLO-2D model,numerical simulation of debris flow movement process under different recurrence cycles(P=10%,P=5%,P=2%,P=1%)was carried out.Under the no-engineering condition,the simulation results show that the outrush volume,accumulation area and average accumulation thickness of Yangjiagou debris flow increase with the increase of recurrence period.And the thickness of the pile basically follows the law of gradually decreasing from the center of the stacking fan to the edge.Furthermore,the hazardous area of the Yangjiagou debris flow is divided into high-hazard area,medium-hazard area and low-hazard area.Finally,the FLO-2D simulation is used to obtain the hazard assessment map of the Yangjiagou debris flow.The simulation results show that the area of high risk is much larger than the area of medium risk and low risk.(4)Combined with the existing controlling engineering of Yangjia gully,the dam body is inserted into the model to simulate the movement of the Yangjiagou debris flow under the condition of intact engineering.The simulation results show that:in the recurrence period of once every 10 years and 20 years,the debris flow solid matter is intercepted by the dams,and the debris flow solid cannot reach the accumulation area;in the 50-year and 100-year recurrence cycles,the debris flow solid matter flushing amount is larger than the dam body storage capacity,and the debris flow fluid crosses the dam and moves to the accumulation area,but the parameters such as the amount of flushing,the area of the accumulation area,and the average depth of the pile are all smaller than the corresponding values under no engineering condition.The debris flow appears back silting phenomenon behind the dam,forming back silting area,and the velocity of fluid movement also decreases.Apart from that,the area of the high-hazard significantly reduces compared to no engineering condition.It shows that the danger of the accumulation area has been controlled after the construction of the disaster reduction engineering.(5)The damage process and mechanism of the piled dams after the"6·25"debris flow are analyzed.The scouring depth of the front gully bed of the pile dam is calculated to be 2.8m,and the scouring process can be divided into three stages.Afterwards,numerical simulation experiments are repeated under partial engineering damage condition.The results show that:under the 20-year recurrence period,the amount of solid matter is less than the storage capacity of the dam,and only a small part of the solid matter reaches the accumulation area through the damaged part of the dam;in the recurrence period of 50 years and 100 years,the amount of solid matter is larger than the storage capacity of the dam and a large amount of debris flow moves to the accumulation area.Moreover,the parameters such as the amount of flushing,the accumulation area,the average depth of accumulation,and the maximum depth of accumulation are all smaller than the corresponding values under no engineering condition but greater than the condition of engineering integrity.This indicates that,under the condition of partial engineering damage,the disaster reduction engineering can still effectively control the debris flow in the recurrence period of 20 years,but has limited prevention and control effect on the debris flow that occurs once in 50years,and cannot play an important role once in 100 years.Compared with the non-engineering condition,the area of the high-hazard significantly decreases,but increases in contrast to the condition of intact engineering.It’s indicated that the hazard of the accumulation area is controlled after the construction of the disaster reduction engineering,but the damage of some dams has adversely affected the effectiveness of the prevention and control measures.