| Qinghai province is one of the provinces seriously affected by geological disasters in China,especially the debris flow disaster.Among these debris flows,there is a unique one whose deposit is made up of the sand-gravel soil only,which is called sand-gravel debris flow.It is mainly developed in the Quaternary sand and gravel strata,which is widely distributed in Qinghai Province.Affected by its deposit,the debris flow has distinct characteristics in its development characteristics and starting mechanism et al.however,few systematic researches in these flied are done by scholars at present.In this paper,three gravel debris flow gullies developed in the Middle and Lower Pleistocene strata on the left bank of the Yellow River in a township of Qinghai Province are taken as the research object.Based on the development background of the three debris flows and detailed field investigation,the necessary conditions which induce the debris flow are analyzed,icluding doposit conditions,topography conditions,water source conditions.The characteristics of the debris flow's gully,erosion and deposition and dynamics are summerised,And the starting mechanism of material sources in different storage modes of this kind of debris flow is also studied;on August in 2016,affected by heavy rains,the debris flows occurred and many infrastructures are destroied in this disaster.So hazard assessment on this gullies is of great importance.Based on the FLO-2D simulation software,the movement and accumulation of the debris flow is studied,And the prediction on the accumulation range of the debris flow under the different rainfall indensity is also studied.At last,the mathematical method is used to quantitatively analyze the hazard of debris flow gully and some suggestions for prevention and control measures are put forward.The main understandings obtained are as follows:(1)The stratum of the sand-gravel debris flow in the study area is the Quaternary mud-calcium weakly cemented gravel layer,which has the characteristics of high gravel content,strong water permeability,hard when dry,weak resistance to weathering,easy to soften and small-scale disintegration on its surface in the presence of water.(2)Taking the three sand-gravel debris flows developed in the study area as examples,the topographic conditions,material source conditions and water source conditions of the debris flows are summarized and analyzed.The results show:despite of the widen vally and small slope of the debris,the strongly cut watershed geomorphology is beneficial to the generation of deposit,and the"funnel"topography developed on the trailing edge of many branch gullies is beneficial to the collection of rainfall;The deposit is is rich in reserves and all composed of loose sand-gravel soil formed by weathering and transportation of sand and gravel layer.3/4 of the doposit is accumulated in the ditch bed,and the dynamic reserves account for about 40%of the total material source.The rainfall induced the debris flow shows the characteristics of concentration,short duration and high intensity.(3)The characteristics of the debris flow's gully,erosion and deposition and dynamics are summerised and analyzed.The formation area and circulation area of sand-gravel debris flow are integrated,which plays the role of source of production,debris flow movement and watershed confluence at the same time.A large number of loose provenances are distributed in the main trench and branch trench,and The surface of the local trench bed accumulation body is cut down seriously to form grooves,and the sulcus brain traceability erosion is serious.Debris flow accumulation fan Formate in many period,among them,the larger debris flow covers the old accumulation body,and smaller debris flows or torrents repeatedly scour the old accumulation bodies to form grooves.Deposits and sand-gravel formations have similar particle and particle size composition,Through the experiment of particle fraction and specific gravity,the bulk density of the three trenches is obtained--1.5 t/m~3.The results also show that sand-gravel debris flow belongs to torrential rain type,gully type,dilute debris flow.The parameters of flow velocity,flow rate and impact force during the debris flow outbreak in August 2016 were obtained through investigation and calculation.The results are consistent with the actual situation,and the scale of the outbreak was determined to be once in 20 years.(4)The initiation and formation mechanism of sand-gravel debris flow are studied.Precipitation increases the water content of gravel soil and decreases its mechanical properties.Source saturation destroyed rapidly on slope under short duration heavy rainfall and accumulated in ditch bed.Floods formed by catchments in the basin scour and uncover the source of deposits in tributaries and main ditches,Debris Flow Formed with Increasing Sediment Concentration finally.Based on the start-up model of Gaoqiao deposit-retaining body and combined with the theory of hydraulics and seepage mechanics,The mechanics mechanism of failure initiation of material source in ditch bed is analyzed.and the theoretical value of the average surface runoff velocity required for the start of accumulation body is obtained.(5)The hazard assessment and hazard range prediction of 1#and 3#debris flow with threat objects in the study area are carried out.Flo2d is used to simulate the movement and accumulation process of different rainfall frequencies in debris flow gullies,The results show that the sand-gravel debris flow has the characteristics of small flow velocity,wide accumulation range and small accumulation depth.and the accumulation range under different rainfall frequencies is obtained.Comparing the simulation results with the actual results,the simulation results are verified to be reasonable.Based on the characteristics and mechanism of sand-gravel debris flow,nine influencing factors,such as basin area and ratio drop of main gully,are selected to evaluate the risk degree by AHP-fuzzy mathematics evaluation.The results showed that both ditches were moderately dangerous,which is consistent with Liu Xilin's evaluation results and Field survey results. |
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