| Debris flow is a common geological hazard in nature.At present,there are many studies on debris flow.Most of the external triggering factors of debris flow are rainfall.But at present,most of the relevant studies on debris flow are in the areas with more rainfall data.Xinjiang is a scarcely populated area,the mountainous area is large,there are few monitoring equipments such as rainfall,which can not obtain accurate data when debris flow occurs.In view of this,this paper takes the debris flow in No.1 Gully of Tashna Village,Shache County,Xinjiang Autonomous Region,which broke out on July 9,2016 as the research object,On the basis of field investigation,the mechanism of debris flow initiation and flooding areas are studied,and draws the following conclusions:(1)Through on-site investigation and data collection,the area of the debris flow basin in Tashina village is 1.1 km~2,the length of the gully is 2.9 km,the vertical gradient ratio of the gully is 163‰,the altitude is about 1690~2162 m.No perennial snow.It belongs to rainstorm debris flow according to the induced factors.According to the geomorphological characteristics of catchment area,it belongs to Valley-type debris flow.According to the classification table of debris flow outbreak scale,the results are small and dilute debris flow.(2)The debris flow gully in Tashna village is surrounded by mountains on three sides and opens to the south.The slope of the bank is steep,which is conducive to the gathering of rainwater and provides favorable topographic conditions for the occurrence of debris flow.The surface vegetation is sparse and the ecological environment is fragile in the basin.In addition,there are weak cemented conglomerates on both sides of the gully.The temperature difference between day and night in southern Xinjiang is large,the physical weathering ability is strong,and there are more slope deposits in the gully,which provides abundant source conditions for the development of debris flow.Through on-site trench exploration,the maximum thickness of loose deposits in the ditches is estimated to reach 6.5m,and the thickness of loose deposits is about 26.02×104 m~3.Rainfall in the basin is mostly concentrated in May to September,accounting for more than 65%of the annual precipitation.Rainstorm concentration,easy to form rainstorm flood of steep rise and fall,high peak flow and low total flood,has become a favorable triggering factor of debris flow.(3)The main channel of debris flow is divided into three areas:clear water area,formation-circulation area and accumulation area.The structure type of bank slope in the basin is found to be reverse slope.The compressive strength of conglomerate outcropped on the bank slope is high,but its shear strength is low.(4)The volume weight of debris flow in Tashina village is 1494 kg/m~3,which belongs to the category of sparse debris flow.The accumulated weight is 2369 kg/m~3.The laboratory particle analysis experiments show that the main type of debris flow is gravel soil,part of which is sandy soil.The clay content of the accumulated body is relatively small,the average clay content is about 2.25%.Five of the specimens were poorly graded,The soil with poor gradation is dispersed in the process of debris flow,and the flow pattern is generally turbulent or semi-turbulent.The way of moving stones is rolling and jumping,which has obvious effect on the downcutting of river bed.The downcutting depth of debris flow is 0.5~0.8m after field investigation.The natural water content and saturated water content of debris flow deposits in Tashina village are about 4%and 20.3%.With the increase of water content,the shear strength of debris flow deposits in Tashina village decreases sharply at first,and then decreases slowly at about 10%water content.(5)Based on the data of Kaqun hydrological station nearest to Tashina village debris flow basin,this paper uses the line fitting method to deduce that the frequency of debris flow in Tashina village is once in fifty years.It's maximum 24-hour rainfall is66.5mm,and the average flow velocity is 4.14m/s.Among them,the flow velocity of big rocks is 2.19m/s,the confluence time is about 0.054h,the rainfall is about 3.1mm,the peak flow is 15.46m~3/s,The total amount of a debris flow is about 6295.8 m~3,the total amount of solid material is about 2271.82 m~3,the overall impact pressure is 3.38Pa,and the maximum impact height is 0.85m.(6)The debris flow in Tashina village is induced by rainfall.Based on rainfall,the debris flow initiation can be divided into three processes:rainfall infiltration process,surface thin layer runoff process and channel accumulation initiation process.Based on this,mechanical models of channel accumulation body in different stages are established.According to the limit equilibrium principle,the critical average velocity of debris flow starting is 0.71m/s,which is much less than the calculated average velocity of 4.14m/s.(7)Through the fluid simulation software ANSYS CFX to simulate the once-in-fifty-year mudslide flow in Tashna village,it is found that the maximum area of the flooded area is approximately consistent with the actual investigation results,the coincidence degree is 80.6%,the average velocity is 4.6 m/s,and the error of the calculated average velocity is only 11.1%.The effect is good.The average flow velocity of debris flow with design frequency of 1%(once in a century)is 8.5 m/s,the confluence time is 0.032 h,the rainfall is 2.36 mm,the design peak flow is 22.5 m~3/s,and the average flow velocity is 7.9 m/s.The error between the simulation and the calculation is 7%.The maximum flow velocity can reach 10.9 m/s.Compared with the simulation results of debris flow once in 50 years,the maximum flow velocity increases by 13%.The maximum distance from the debris flow flooding area to the right is about10.77 m,and the left is 3.8 m.The flow range increases to both sides except the bend.The flow in the channel changes little.The trend of the flow expanding to the right after the outflow of the channel is obvious,which can play a certain role in predicting and early warning the threat range of debris flow outbreak for the residents of the ditch. |
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