Disaster Chain And Risk Quantitative Assessment Of River-blocking Debris Flow That Occurred On August 20,2019 In Wenchuan County

The Wenchuan earthquake not only triggered thousands of landslides,but also caused serious casualties and economic losses.After the earthquake,a large amount of loose solid materials deposited on the hillslopes or channels,which provided a sufficient material source for the activities of debris flows.A large number of debris flows along the Minjiang River and its tributaries frequently occurred.Large-scale solid materials from debris flows often blocked the river,which resulted in floods,threatening the lives of residents.The Wenchuan earthquake increased the activity frequency of landslides and debris flows for a long period,and the debris flows chain disasters has been posing a risk for urban residents and infrastructure construction in the strong earthquake mountainous areas.To prevent the frequent occurrence of large-scale debris flow disasters in strong earthquake areas,quantifiable risk assessment methods are more practical and effective in risk mitigations and managements,and it can also provide more reliable basis and a more intuitive perspective for reconstruction plans and mitigation strategies.However,most disaster risk assessments currently consider a single type of disaster and fail to consider the interaction of multiple disasters.Although the debris flow risk assessment method is constantly improving with continuous research,the risk assessment of the geological disaster chain is still lacking.Therefore,it is significant to explore a more comprehensive and reasonable risk assessment method for improving the accuracy of the risk assessment of the debris flow disaster chain.To solve the above problems,this study firstly used the"8.20"mass debris flow event in the Wenchuan earthquake-affected areas as the research objects to investigate the topographic characteristics,rainfall characteristics and material resource characteristics of the debris flow in the Wenchuan earthquake-affected area.The Xiazhuang debris flow was selected as the research object to analyze the movement process and disaster-causing characteristics of blocking-river debris flow.Then the OpenLISEM numerical model was used to reproduce the entire process of disaster chain,including the run out of debris flow,debris-dam blocking the river and flood inundation.The risk analysis and vulnerability analysis were conducted in the area affected by the debris flow and flood,and a relatively mature system was proposed for the risk assessment of the disaster chain of debris flow blocking river.Finally quantitative risk assessment of the disaster chain for Xiazhuang debris flow blocking the river was conducted.The main findings are as follows:（1）This paper carried out the analysis on the characteristics of the"8.20"mass debris flow event and the typical disaster chain of debris flow blocking the river.First,based on the field investigation,remote sensing images and rainfall data,the topographic characteristics,rainfall characteristics,and material resource characteristics of the debris flow were analyzed;secondly,the movement process and disaster-causing characteristics of the typical disaster chain（landslide-debris flow-blocking river-flood）in the"8.20"debris flow event in Xiazhuang gully were studied.It is of significance to understand the formation characteristics and development trend of the mass debris flow and the typical disaster chain in the strong earthquake-affected areas.（2）Based on the analysis of the characteristics of the“8.20”debris flow blocking the river and their controlling factors,the critical particle size of the debris-dam,the accumulation characteristics of the debris-dam,and the water level rising in the upstream were analyzed to comprehensively evaluate the effect of debris flow blocking river.The results show that the solid particle size and the amount of solid materials of the“8.20”debris flow in Xiazhuang gully are greater than the threshold value required for blocking the river.The blocking mode is that the river was completely blocked,and the probability of blocking the river is 0.726,which is a high probability.The debris flow basin blocking the river coincided with the actual situation of the debris flow event,and the movement mechanism and disaster-causing characteristics of the debris flow blocking the river were more comprehensively reflected,which provided a theoretical basis for the risk assessment of the disaster chain of debris flow blocking the river.（3）Based on OpenLISEM simulation,the movement characteristics of the typical debris flow blocking the river were reproduced and analyzed.This paper selected Xiazhuang gully,a typical watershed in"8.20"mass debris flow event,to obtain the necessary basic data and a large number of parameters through field investigations and experiments.On the basis of calibration,OpenLISEM was used to realize the simulation of disaster chain of debris flow blocking the river.The dynamic process was reproduced to obtain the debris flow velocity,depth,and flood submerged depth,and the movement characteristics of the debris flow blocking the river was analyzed.The verification accuracy is 84.9%,indicating that the simulation effect is good,which shows that the model has good applicability for the simulation of the disaster chain process of landslide-debris flow-flood,especially in the Wenchuan earthquake area where the coseismic deposits fully developed.（4）A large amount of disaster loss data was obtained through field investigation,and we constructed the database of disaster-bearing body in the Xiazhuang watershed.At the same time,the intensity index(I_DF)of the debris flow were calculated based on numerical simulation results,and we calculated the probability of exceedance for each intensity for the current damage state.The fragility curves were developed by using exceedance probability functions for damage in this work,which can quantitatively express the response of the buildings to the debris flow in the study area.（5）The quantitative risk assessment of the typical disaster chain of debris flow blocking the river in the strong earthquake-effected area has been realized.Based on OpenLISEM numerical simulation of the disaster chain of debris flow blocking the river,as well as the analysis of the characteristics of disaster-bearing bodies and disaster intensity parameters in the study area,we quantified the disaster risk by constructing different vulnerability models,and the method was applied in evaluating the quantitative risk of debris flow blocking the river in the Xiazhuang gully.In addition,the flood disaster resulting from the debris-dam is also considered in the debris flow risk assessment,forming a quantitative risk assessment system for the debris flow blocking the river,which improves the rationality and integrity of the risk assessment and the validity of the evaluation results.