Investigation Of Damage Features And Study Improving To Protection Engineering For Large Debris Flow In The Earthquake Region

After the Wenchuan earthquake,a large number of sources of debris accumulated in the debris flow ditch in the earthquake area.The scale of the post-earthquake debris flow outbreak was huge,and its prevention and control research has not yet matured.Many debris flow ditch control projects were destroyed within a short period of time after completion,and the benefits of disaster prevention and mitigation were greatly reduced.Dengxi ditch is located on the left bank of the Minjiang River in Mianxi Town,Wenchuan County.It is a typical large debris flow ditch in the post-earthquake area.Through field investigation,it was found that the treatment project was full of silt and washed out.Therefore,in order to discuss the damage characteristics and engineering improvement of the debris flow ditch control project after the earthquake,this paper takes the Dengxigou debris flow prevention and control project in Mianxi Town as an example.Through field research and data collection,the characteristics of debris flow after the earthquake are summarized,and the retaining dam protection is briefly analyzed.The damage characteristics of the tank and the blockage of the drain hole are analyzed,and the characteristics of the Dengxi ditch and the damage characteristics of the treatment project are mainly analyzed.Through analysis,it is found that there are many boulders in Dengxi ditch with large particle size.When the blocking dam is full,the boulders roll over the blocking dam and hit the protective tank,which is a great threat to further damage the treatment project.To this end,the FLOW-3D software was used to carry out numerical simulation research on the movement process of debris flow pushing boulders against the protective tank,and to analyze the movement characteristics and impact force characteristics of boulders,so as to provide a reference for the optimization of the protective measures of the Dengxigou debris flow retaining dam foundation.Finally,based on the simulation results,different engineering improvement schemes are proposed to reduce the impact force of boulders,and the numerical simulation methods are used to verify them,so as to provide suggestions for the next step in the prevention and control of debris flow in Dengxi Valley.The main findings of this paper are as follows:(1)The post-earthquake debris flow should be different from ordinary debris flow:Under the influence of the earthquake,the debris flow ditch or non-debris flow ditch will cause a sharp increase in the loose solid matter in the basin,and the micro-topography and hydrological conditions will change.Large-scale,high-group,long-lasting debris flows that remain active after the earthquake are formed.Through statistical research and analysis of the damage characteristics of the debris flow ditch retaining dam in the earthquake area,according to the damage factors of the shield,the damage modes of the shield can be divided into rock impact-fragmentation type,scour-abrasion type and elutriation-suspended type.According to the different blocking parts of the discharge hole of the retaining dam,the blockage types are divided into orifice blockage and hole blockage.It is found that the boulders on the channel can damage the dam body and the protective tank of the dam,and the blockage of the discharge hole can not be ignored.(2)The source of debris flow in Dengxigou is mainly landslide and channel provenance,and the dynamic reserve of channel deposits is the main source of provenance.Comprehensive analysis of the topographic features,provenance features and water source features of the debris flow in Dengxigou shows that the possibility of debris flow outbreaks is high.The protection project of Dengxigou is mainly composed of 3 retaining dams,all of which are blocked by different numbers of sluice holes.The blockages are mainly boulders and driftwood,and the sluice holes of 1# dam are seriously blocked.The shields of dams 1#and 2# are seriously damaged.The main damage modes of the shields are rock impact-fragmentation and scour-abrasion.There are many large-diameter boulders and boulders in the upper and middle reaches of the Dengxigou channel.The maximum particle size is as high as 4m,and the 1# dam protector is very threatened by the impact of boulders falling.(3)The FLOW-3D software is used to simulate the movement process of the boulder hitting the protective tank when the dam is full.The motion of boulders can be divided into five types: start-up acceleration,accelerated motion,uniform motion,falling and continuous acceleration and rolling.The process and the characteristics of each movement process are analyzed in detail.The kinetic energy consumption of boulders when the dam height is 14 m is more prominent than that of dam heights of 12 m and 10 m.The increase in kinetic energy of boulders is mainly provided by the falling motion.Reducing the dam height is the most direct and effective way to reduce the impact of boulders on the protective tank,which provides a reference value for the optimization of the Dengxigou retaining dam.(4)The debris flow movement and accumulation process under different engineering conditions of Dengxigou debris flow was simulated by FLO-2D software.And it turns out that due to the large number of bayonets in the Dengxi ditch and the blocking effect of three retaining dams,the average velocity of debris flow in the circulation accumulation area is small,both as 2.3m/s,the maximum flow velocity at the mouth of the ditch can reach6.22m/s;if the 1# dam is lowered by 2m and the 2# dam is increased by 2m,the simulated debris flow accumulation area and accumulation volume are not much different from the original engineering conditions.(5)Dengxi Valley 1# dam has obvious interception effect under the original engineering conditions,and plays a major preventive and control role among the three dams;by raising the 2# dam by 2m,the silt retention capacity of the 2# dam can be greatly increased,and the2# dam can be improved.The dam is of great significance to the prevention and control of debris flow;the interception effect of the 3# dam is weak.Due to the limited storage space in front of the dam,the increase of the silting volume of the 3# dam is not obvious,and the improvement of the 3# dam has little effect on the accumulation of debris flow,but it can effectively reduce the speed of debris flow out of the ditch,which has a certain significance in weakening the river blocking effect of debris flow.Working condition 2(1# dam 12 m,2#dam 12 m,3# dam 6.5m)is the most effective consideration based on the consideration of reducing the impact kinetic energy of boulders on the 1# dam on the protective tank and preventing debris flow as effectively as possible under the three engineering conditions.However,the hidden danger of debris flow still exists;according to the on-site investigation,there is still room for heightening of the 2# dam,and improving the 2# dam is the main direction for optimizing the Dengxigou retaining dam project.