Study Of Risk Assessment Of Debris Flow Disaster Chain Induced By Multi-Causative Factors On The Northern Slope Of Changbai Mountain Based On Multi-model Coupling

As a tourism development zone in Jilin province which integrates various resources(including tourism,ecological environment and mineral resources),Changbai Mountain is of inestimable value.In the meantime,with the booming of China's economy,the regional economic aggregate is rising rapidly,and the social wealth has a substantial growth,as a result of which,the value of the resources in Changbai Mountain tourism development zone has been keeping increasing.Besides,due to the improvement of the national economy,the density of the population in the tourism development zone continues to increase.Therefore,when a debris flow disaster occurs,the degree of risk and the number of hazards it may cause will also significantly increase.By 2018,the occurrence of debris flow disasters in the ten debris flow gullies within Changbai Mountain Tourism Development Zone has added up to25 times,causing an economic loss of 26.35 million yuan,among which the June 2009 debris flow occurring 48 metres south of the waterfall scenic spot on the northern slope of Changbai Mountain destroyed the tourism resources,the ecological environment resources and so forth of this scenic spot,causing an economic loss of about 7 million yuan.Also,in the context of climate change,extreme climatic events(such as extreme precipitation and snowfall)are becoming more as well as more intense.Combined with the fact that the national economy is growing rapidly,the losses caused by debris flow disasters induced by extreme rainfalls are also increasing.Moreover,Changbai Mountain volcano is at the point of once-a-century or once-a-millennium revival.The activities of the volcano will inevitably force the water level of the Tianchi to rise,and if the water breaks or overflow the dam,going down the northern slope,it will cause an extremely serious debris flow disaster.Apart from that,when the risk assessment of debris flow disaster chain is conducted on a regional scale,the insufficient consideration of the relationship between various risk factors will hinder the achievement of quantitative risk assessment,which eventually leaves the results of the final risk assessment on the regional scale in the great controversy.Hence it is an urgent task to study the risk of debris flow disaster chain induced by multiple causative factors in Changbai Mountain tourism development zone and to make a quantitative evaluation.At present,quantitative risk assessment of debris flow disaster on a regional scale is the main direction of the development of debris flow disaster risk research.Taking the northern slope of Changbai Mountain as a research area,this study starts from the formation mechanism of the debris flow and its disaster chain,analyzes the debris flow disaster chains induced by different causative factors within the area,and then determines the debris disaster chain induced by extreme rainfalls and the dam break of Tianchi as a point of penetration to build up the assessment model in combination with the disaster chain model which is based on the relationships of disaster events.Then,the model of steady-state infinite slope stability is optimized based on the random forest model and is made applicable to the study of debris flow disaster assessment on a regional scale.At the same time,this model is used to associate all the disaster events within the disaster chain system,and quantitative assessment of the debris flow disaster chains induced by various causative factors of different intensity or frequency within the northern slope area of Changbai Mountain is made.In addition,on the basis of summarizing predecessors' studies,this study uses the advantages of various models to make up for each other's shortcomings by coupling multiple models,and establishes links between factors in the risk assessment with the coupling of models,enabling the risk factors to relate to each other,and eventually carries out quantitative risk assessment research on a regional scale.The main contents of this paper include:(1)Firstly,through the analysis of the relationship between various geological environment factors and the debris flows within the research area,it is found that there is a linear or nonlinear relationship between the nine geological environment factors—— slope,curvature,source thickness,topographic wetness index,vegetation and other factors—and the occurrence of debris flows.These nine factors are selected as an index system of proneness evaluation.Then,based on the random forest model,the proneness to debris flow disasters on the northern slope of Changbai Mountain is evaluated.The results show that the non-prone area accounts for 51.22% of the total area of the research area,the low-prone22.05%,the moderate-prone area 11.94%,while the high-prone and extremely high-prone areas account for 6.82% and 7.97% respectively.Areas of moderate,high and extremely high proneness generally are distributed in concave slopes within the research area.Meanwhile,the AUC value of the evaluation result reaches 0.9048,indicating that the result of the proneness evaluation based on random forests is highly reliable.(2)The formation mechanism of two types of disaster chains is analyzed in combination with the natural environmental features of the research area and the formation mechanism of disaster chains and based on the disaster chain model of the relationship between disaster events,the risk assessment models of debris flow disaster chain induced by extreme rainfalls and the bank burst of Tianchi are established respectively.The steady-state infinite slope stability model is selected as the model to reflect the relationship between various disaster elements in the disaster chain.Then the model is optimized based on the random forest model,and a steady-state infinite slope stability model based on the random forest model is proposed.Next,the extreme rainfalls are calculated with the Gumbel distribution model,and the once-in-ten-year,once-in-30-year and once-in-50-year extreme rainfalls(85.9415 mm,132.5655 mm and 153.8496 mm,respectively)are selected as an example to assess the risk of debris flow disaster chains induced by extreme rainfalls in combination with the steadystate infinite slope stability model based on random forest model.At the same time,the variation of the risk assessment results of the steady-state infinite slope stability model based on the random forest model which falls in the range over 0.5,is different from those based on the steady-state infinite slope stability model.(3)A three-dimensional simulation study of the Tianchi burst under three hypothetical conditions is designed for the risk assessment of the volume of the debris flow disaster induced by the Tianchi outburst through the Flow3 D model.The results show that the submerged area due to the burst of Tianchi of the three simulated scenarios with the research area accounts for 55.17%,69.97% and 86.32% of the total research area respectively.The areas with submergence depths between 0 and 2m account for 11.84%,9.83% and 12.66%of the total research area.The areas with submergence depths of 2-5m account for 8.07%,8.9% and 8.51% of the total research area.The depths of the rest of the areas are more than5 m.The inundated volume in most of the research areas is more than 10 times their threshold water volume,indicating that the burst of Tianchi under three conditions all can easily induce debris flow disasters.Then,the steady-state infinite slope stability model based on the random forest model is used to assess the risk of debris flow disaster chain induced by the burst of Tianchi under three hypothetical conditions.The results show that the area of extremely high risk expands far more than that of high and moderate risk,which indicates that in different simulation scenarios,with the increase of water level and water volume,more areas are affected by inundation.In the meantime,these areas are more prone to extremely serious debris flow disasters under the condition of extremely abundant water resources.(4)Through the analysis of the existing disaster loss data,the thickness of the material source which may move during the debris flow is determined as the index of the intensity of the disaster causing factors of debris flow disaster.Combined with the historical disaster data of debris flow disaster in Jilin Province,the disaster loss rates of different types of disaster bearing bodies in the research area are calculated and the corresponding vulnerability curves are established.The results of risk assessment are linked to the vulnerability assessment through the formula of the relationship between the dynamic and static reserves of debris flow material sources,and the quantitative risk assessment of the two types of debris flow disaster chain induced by extreme rainfalls and the burst of Tianchi in the research area is carried out.The results show that the loss that debris flow disasters may cause is increasing with the increase of volume of the extreme rainfall and the collapsing water of Tianchi during the return period.The tourist attractions that may suffer the greatest losses in the debris disasters induced by extreme rainfalls are Yaowang temple,Julong spring and others.The forest resource on the northern slope ranks second place.And tourist attractions like Changbai Mountain Waterfall,Julongquan,Xiaotianchi and so on face the greatest threat of debris flow caused by the burst of Tianchi.This study will make up for the deficiencies in the research foundation of quantitative risk assessment of debris flow disasters and solve the key problems in related studies.The research results can be extended to Changbai Mountain scenic area,which is of great significance to the improvement of the scenic area's ability of disaster prevention and mitigation and that of emergency management as well as to the goal of scientific and active disaster resistance.