Characteristics Of Debris Flow Movement In The Bend And Risk Assessment

During the movement of debris flow in the bend,it damages the engineering facilities near the bend,threatens the property and personal safety of residential areas,and also poses challenges to the design of disaster prevention and reduction projects.This article focuses on the motion characteristics of debris flow bends and theoretically deduces the relationship between bend superelevation and impact force.The continuous uniform medium method was used to simulate the movement of debris flow in curved channels under different conditions of debris flow characteristics and bend characteristics.The effects of different characteristics on the depth,velocity,bend superelevation,impact force,and sedimentation range of the bend were explored,and the accuracy of the total superelevation equation for the bend was verified.The undetermined coefficient values of the bend impact force were determined,and a sedimentation range function model was established.Based on the investigation results of the hoe ditch,the equation for the total superelevation of the bend was validated again,and numerical simulations of the motion process were conducted under different scales and conditions.Finally,combined with the analysis of the motion characteristics of the bend,a risk assessment of the siltation range and the risk assessment of building resistance to impact damage were established.The main achievements are as follows:(1)A new curve superelevation equation is derived by considering the channel geometry and combining the method of capacity conservation: under the assumption of continuous homogeneous medium and rectangular channel section,the debris flow velocity in the curve is decomposed into the tangential velocity that leads to superelevation and the normal velocity that leads to climb.Based on the displacement of the center of gravity of the flow section during debris flow movement,the calculation formulas for the superelevation,climb,and total superelevation of the debris flow in the curve are derived by substituting them into the conservation of capacity equation.The proposed equation can determine and describe the evolution of the total superelevation value of the curve,which helps engineers to carry out targeted design in debris flow prevention and control engineering.At the same time,42 sets of curve simulations were conducted using the total superelevation equation and empirical equation derived in this article,as well as the equation that also considers the climbing effect,and compared with the calculation of 5 curve points in hoe ditch.The results show that the fitting linear error(0.34%)between the calculated results of this formula and the simulation results is better than the commonly used empirical formula(22.21%)in engineering and the formula that also considers climbing(-29.32%).The average superelevation error of the five curve points in the hoe ditch is also the smallest,and the fitting coefficient of the calculated superelevation evolution process curve is consistent with the simulated superelevation evolution process curve.(2)The impact force of debris flow is an important criterion for measuring the degree of damage during the process of curve movement.In this paper,based on the existing impact pressure model,considering the geometric conditions of the bend,a new formula for the impact force of the bend is derived through the dimensional analysis method.The proposed bend impact force model was fitted with the Levenberg Marquardt optimization algorithm and simulation results to obtain the value of the undetermined coefficient.The coefficient value after linear fitting was 0.852,which is close to 1,which is more in line.The proportion of the geometric conditions of the 42 simulated curves was only 0.842%,indicating that the direct impact of the geometric conditions on the impact force is relatively small.(3)Based on the simulation results of 10 sets of working conditions with different bend wall heights,the formulas of the farthest moving distance and superelevation,siltation range and superelevation are established respectively,which can be used to calculate the optimal bend wall height and determine whether there is siltation range.Segmented siltation range curves were fitted based on the heights exceeded by different superelevation,and summarized into a siltation range function model table.(4)Based on the simulation results of 42 sets of bend grooves,the influence of different debris flow characteristics and bend characteristics on mud depth,flow velocity,superelevation,and impact force was analyzed.The comparison of the absolute values of the power term of the fitting function between different characteristics and the depth of debris flow shows that the flow rate has the greatest impact on the depth of debris flow,while the width has the smallest impact.By comparing the absolute values of the power term of the fitting function between different characteristics and debris flow velocity,the slope has the greatest impact on the velocity,while the curvature radius has the smallest impact.By comparing the maximum and minimum difference ranges of the superelevation of debris flow curves under different characteristic operating conditions,the slope has the greatest impact on the superelevation of debris flow curves,and the curvature radius has the smallest.The influence of roughness coefficient on superelevation is relatively complex,and changes in mud depth and flow velocity need to be considered.By comparing the differences in the impact pressure of debris flow curves under different characteristic operating conditions,the slope has the greatest impact on the impact pressure of the curve,while the curvature radius has the smallest impact.The influence of roughness coefficient on the impact force is also relatively complex,mainly depending on the changes in flow velocity.(5)Based on the simulation results,a siltation range function curve model was established,and three different risk levels of siltation range regions were established: low,medium,and high.Introducing the siltation range of the hoetou gully into the risk assessment of this siltation range indicates that the siltation range of the hoetou gully debris flow bend is of medium to high risk.In the classification of impact risk for M-W(brick,wood,and stone)building structures,the area of high risk areas accounts for 44.83%,the area of moderate risk areas is17.74%,and the area of low risk areas is 37.43%.In the classification of impact risk for B-C(brick concrete)building structures,the area of high risk areas accounts for 38.16%,the area of moderate risk areas is 17.89%,and the area of low risk areas is 43.95%.Both classification methods indicate that the impact damage of the hoe gully debris flow is of medium to high risk level.