The Run-out Scale Prediction Of Therapid-steep Channel Debris-flow Based On FLO-2D

The geological environment became more vulnerable after the earthquake in Wenchuan county,and the activity of geological disasters such as collapse,landslides and debris flows were intenser.The debris flows also became one of the major potential disaster threatening the safety of people's lives and property.The activity characteristics of post-earthquake debris flow mainly are the sharp increase in quantity,the higher frequency,the increase of the scale,the decrease of the critical rainfall,and most of it is viscous debris flow.Among the many debris flow disasters,there is one type of debris flow with typical and special activity and scale,namely,the debris flow basin area is small,and the topography of its source zone,transport zone and accumulation zone is relatively steep.The terrain features of these debris flows are similar to those of the gully debris flow with the good condition of collecting water.The steep terrain of the channel makes the movement characteristics of the debris flow similar to the slope-type debris flow with the characteristics of pouring out and rapid accumulation.For the first time,our team defined such debris flows as rapid-steep gully debris flows.In this paper,three typical rapid-steep gully debris flows were selected as the research objects,and forecasted and researched the scale of debris flow under different rainfall frequencies.It provides a basis for evaluating the risk of such debris flows,and provides reference for controlling such debris flows.The paper takes the three typical debris flow basins of Shaofanggou,Futangou,and Wayaogou as research objects,and combined with the existing basic data,the survey data,field surveys,and remote sensing images,the debris flow was simulated without engineering prevention measures and the run-out scale of debris flow were further simulated with engineering prevention measures under different rainfall frequencies.The run-out scales of the debris flow at different rainfall frequencies were obtained.Simultaneously,the comparative analysis was conducted on the run-out scale of debris flow under natural conditions and with the preventive measures.The influence of debris flow prevention measures on the run-out scale of debris flow was obtained,and the effect of engineering prevention measures was verified.The main research results are as follows:1.The simulation results are analyzed under natural conditions.The summary and analysis are as follows: The terrain of rapid-steep gully debris flows shows a "funnel beam flow",namely,the width of the channel in the upstream zone is large and the width of the channel in the downstream zone is relatively small,such as thefunnel.Therefore,both the scale of erosion and deposition are larger.Compared with general debris flow,the different characteristics of the debris flow in the steep channel are as follow: 1)running out of the debris flow with fast speed,high kinetic energy in the vertical direction,and rapidly impacting the accumulation zone.2)under the same basin area,the amount of transported material from rapid-steep gullies is larger than that of the general debris flows.3)because of the larger longitudinal gradient and the energy conversion relation,the gravity potential energy mainly converted to the kinetic energy of the erosion of debris flow and the flow energy of debris flow,so it can transport the numerous solid material.However,due to the channel presents“funnel beam” with wide upstream zone and narrow in the downstream zone,some of them would remain in the transport zone and caused blockage.With the continuous impact of the debris flow,eventually some of the large amount of solid material will be taken out of the channel.2.Analyze the simulation results of the channel with engineering measures.It is concluded that the prevention and control of debris flow mainly through the way of blocking and draining,which makes the volume of solid material,the distance and the width of the run-out of debris flow can be controlled effectively.In the treatment of gullies with barrier dam projects,the scale of debris flow is controlled mainly from three aspects:1)The debris flow is blocked by the dams,and the volume of run-out solid material can be reduced to achieve the effect of controlling the scale of debris flow.2)the barrier dam in the transport zone can slow down the "snowball effect" and "fire pipe effect" of debris flow,and effectively control the erosion of debris flow.3)the silt dam of accumulation zone effectively controls the accumulation of debris flow,and protects the human construction works.3.The effectiveness of the debris flow channel engineering protection measures is verified and analyzed.Through the analysis of two cases of engineering treatment and natural conditions,the project management measures can effectively control the scale of debris flow.According to the characteristics of rapid-steep channel debris flow,the engineering treatment measures can achieve the effect of treatment,such as the increase of the accumulation depth behind the dams,which can reduce the longitudinal gradient of the channel,reduce the kinetic energy of the debris flow,and thus reduce the scale of the debris flow.From the simulation results of the Shaofanggou,Futanggou and Wayaogou,it can be concluded that under the condition of engineering protection measures,the run-out scale of debris flow is effectively controlled.