The Disaster Mechanism And Risk Assessment Of Meilonggou Debris Flow In Danba County

On June 17,2020,a large-scale debris flow broke out in Meilonggou,Danba County,with a one-time rushing out of 4.27 hundred thousand cubic meters of solid substances to form a disaster chain of debris flow,barrier dam,outburst flood,landslide,massive debris flow,causing more than 5,100 households and 21,200 people to be forced to move,with direct economic losses amounting to hundreds of millions of dollars.At present,the provenance in Meilonggou is abundant,and the threshold of the rainfall is reduced,and large-scale debris flow occur in the rainy season.This article takes Meilonggou debris flow as the research object.According to the field investigation,remote sensing interpretation,outdoor test,and numerical simulation,the initiation of Meilonggou debris flow,the development mechanism and the formation of the secondary disaster chain analysis,simulated its movement process and conducted a dangerous evaluation of it.Obtain the following research results:(1)The shape of the Meilonggou Basin is similar to the pneuma,the watershed area is62.79 km~2,the gap valley has a large slope of the trench,the relative height difference is 2640m,and the average longitudinal slope of the main channel is 215‰.Meilong channel is characterized by wide and slow downstream,narrow and steep in the middle and upper reaches,and the valley form is mainly"V"shaped,with ten branch ditches developing in the ditch.The funnel shaped terrain provides sufficient water power conditions for debris flow.The loosening solid material in the basin is abundant.There are abundant loose solid sources in the watershed,mainly gully sources and avalanche sources,with a total volume of about709.49×10~4m~3,and a total volume of about 278.14×10~4m~3 of sources that can participate in debris flow activities.(2)The debris flow capacity of Meilonggou is 1.786 t/m~3,which belongs to cohesive debris flow.Under the rainfall frequency of P=5%and P=2%,the flow rate at the mouth of Meilonggou is 732.76 m~3/s and 950.31 m~3/s,and the flow velocity at the mouth of the gully is4.42 m/s and 4.90 m/s.(3)The"6·17"debris flow formed in 2020 under the combined action of long-term rainfall and heavy rainfall excitation,and the source of the debris flow was the Dashibaogou,and the mode of initiation was"gully bank collapse～debris flow".The strong erosion of the debris flow produced continuous material recharge along the gully and the cascade blocking effect of three large landslides in Dongfengpengzi,Meilong Village and Dayi Village,resulting in the amplification of the flow of the debris flow and a large amount of solid materials washed out of the mouth of the gully.The special environment at the mouth of the Meilonggou has enabled the successful transfer of disaster materials and energy,resulting in the amplification of the spatial and temporal scale of the debris flow disaster,and finally forming a disaster chain of debris flow,barrier dam,outburst flood,Aniangzhai old-landslide revivification,massive debris flow.The mechanism of"6·17"debris flow in Meilonggou can be summarized as initiation mechanism,channel erosion mechanism,cascade blocking and collapse mechanism and secondary disaster chain mechanism.(4)The numerical simulation software of RAMMS was used to simulate the movement of the Meilonggou debris flow under the conditions of P=5%and P=2%.Under the two conditions,the debris flow movement pattern is basically the same,and there are only differences in the maximum movement values.The maximum velocity during the debris flow movement is 11.39 m/s and the maximum depth of flow is 16.32 m in the 50-year event,and the maximum velocity during the debris flow movement is 10.97 m/s and the maximum depth of flow is 14.95 m in the 20-year rainfall condition.The maximum velocity during debris flow movement occurs at the place where the longitudinal slope ratio is the largest,and the maximum depth of flow during debris flow movement occurs at the place where the block-outburst exists.The erosion effect of debris flow on the channel can be understood as increasing the flow velocity and mud depth during debris flow movement,thus increasing the flow rate of debris flow and enhancing the disaster-causing ability of debris flow.(5)Starting from the disaster-causing characteristics and development mechanism of Meilonggou,the longitudinal slope drop of the main ditch,the watershed area,the cross-sectional morphology of the gully in the sand-producing area,the longitudinal slope ratio drop of the main ditch,the storage of loose provenance,the maximum 24-hour rainfall,the human engineering activities and the maximum one-time debris flow outflow were selected as the evaluation indexes,and the weights of the evaluation factors and the affiliation degree to the evaluation level were determined by using the method of fuzzy hierarchical analysis.Finally,the danger of Meilonggou debris flow was obtained as seriously danger.