Study On The Threshold Change Rule Of Debris Flow Induced By Rainfall In Wenchuan Area

In the years after the earthquake in Wenchuan,many debris flow disasters have occurred successively in the earthquake area,which caused serious harm to the people's life and property safety,and also brought many difficult problems to the reconstruction work after the disaster.Taking the debris flow after Wenchuan earthquake as the research object,this paper establishes a database of debris flow after earthquake in the earthquake area by means of literature review,data collection and field investigation.The database contains 696 debris flow events and rainfall data in the vicinity of each debris flow.Based on this database,using GIS based spatial statistics and rainfall threshold model(rainfall intensity I-D model and rainfall intensity cumulative rainfall I-R model),4 regions(Beichuan,Qingping,Dujiangyan and Yingxiu)in the post earthquake debris flow(Beichuan,Qingping,Dujiangyan and Yingxiu),the debris flow events from 2008 to 2017 The rainfall threshold is analyzed statistically.By comparing the threshold relationship between the starting rainfall threshold of the debris flow in different regions and different years in the Wenchuan earthquake area,the temporal and spatial evolution of the starting rainfall threshold of the debris flow after the earthquake in Wenchuan is studied,and the results of this study are compared with the other scholars' research results.Based on the above macroscopic scale,the statistical model of the spatio-temporal evolution of the rainfall threshold is studied.From the watershed scale,the debris flow of Hong Chun gully in Yingxiu town of August 14,2010 is selected as the research object,and the debris flow numerical simulation program based on the PCRaster GIS platform is used to make use of the actual rainfall,the same earthquake landslide and the actual three dimensions.The topography is used as input data to reconstruct the debris flow events in the tochun valley.The simulation results are consistent with the actual debris flow rate,the accumulation range and the occurrence time,and the physical and mechanical parameters of the rock and soil,including the erosion rate coefficient and the soil infiltration amount,are properly checked.Then,by using the physical and mechanical parameters of the checked rock and soil,the debris flow events are simulated by changing the rainfall parameters(rainfall intensity I and rainfall duration D)to determine whether there is any debris flow in the basin under the given rainfall parameters.The rainfall threshold of the debris flow in the Tokai Valley can be obtained by distinguishing the rainfall events from debris flow and debris flow.The thresholds obtained by macroscopical statistical method are compared with those obtained by watershed scale numerical simulation.Finally,the numerical simulation method is used to study the controlling factors of the dynamic evolution of rainfall threshold.The effects of effective source reduction,particle size change(fine grain reduction)and vegetation restoration on the rainfall threshold of debris flow are considered.Effective source reduction was achieved through continuous simulation of debris flow events.To keep the other parameters unchanged,the simulated output layer and uncorroded material are used as the source data of the next simulation,and many debris flow events are simulated until the resulting runoff does not corrode enough solid material to the outlet of the basin,and the particle diameter changes through the change of the average particle diameter d50 and permeation in the process.The coefficient ks is studied,and vegetation restoration is simulated by increasing the critical erosion shear stress ?c and reducing the erodibility coefficient kd,so as to study the key factors for controlling the evolution of rainfall threshold.The following results and recognition are made in this paper.(1)Through literature review and data collection,the database of debris flow after the Wenchuan earthquake was established,including 696 debris flow events,including the scale parameters and rainfall information of each debris flow event,which laid a good data base for the follow-up study.(2)Based on the debris flow database,the threshold of rainfall threshold for debris flow is studied.The support vector machine(SVM)is used to distinguish between the occurrence of debris flow and the rainfall events that are not occurring,and the changes in the rainfall threshold of four regions(Beichuan,Qingping,Dujiangyan and Yingxiu)have been studied since the 2008 earthquake.(3)The rainfall threshold model of 4 regions(Beichuan,Qingping,Dujiangyan and Yingxiu)from 2008 to 2017 was studied by using the rainfall intensity rainfall duration(I-D)threshold model and the threshold model of rainfall intensity cumulative rainfall(I-R)threshold,and the results were compared with those of other researchers.(4)A numerical simulation method based on physical modeling method is used to quantitatively analyze the rainfall threshold of debris flow at watershed scale.The debris flow numerical simulation program based on PCRaster GIS platform is used to determine whether the debris flow in the basin occurs by changing the input rainfall parameters(including rainfall intensity I and rainfall duration D),and divides the rainfall conditions that induce the debris flow and the non lure stone flow according to the I-D threshold model method.The rainfall threshold of debris flow based on physical model is obtained.The rainfall threshold obtained from the physical model is compared with the I-D rainfall threshold obtained by statistical analysis.(5)The key factors affecting the dynamic evolution of rainfall threshold of debris flow are further studied by means of numerical simulation.Three factors including effective source reduction,particle size change(fine grain reduction)and vegetation restoration were analyzed.The results show that the reduction of the effective source source will cause the increase of the threshold of the debris flow,but the effect is not obvious in the short term.The change of particle size has the greatest influence on the conditions of debris flow,the decrease of the fine particles makes the threshold change of the debris flow most obvious;the vegetation recovery also affects the threshold conditions of the debris flow,and the more vegetation coverage is.The higher the threshold,the higher the rainfall threshold.