Hazard And Risk Assessment For Debris Flows In Small Watershed In Mountainous Area Based On Dynamic Process

The quantitative risk assessment of debris flow in small watershed in mountainous area can not only provide specific and intuitive parameter information for the design and deployment of debris flow engineering protection measures,but also provide reliable technical support for the monitoring and early warning of debris flow disasters,so it has important practical significance.However,due to the lack of efficient and accurate dynamic numerical calculation model and standardized and comprehensive quantitative risk assessment standards,the risk assessment of debris flow mostly stays in the qualitative level,and it is difficult to achieve a technical breakthrough from qualitative to quantitative.Therefore,based on the method of depth integral continuum mechanics,this paper studies the dynamic process of debris flow disaster,calculates the dynamic parameters of the whole movement process of debris flow quantitatively,constructs the two factor evaluation standard of debris flow intensity by coupling the parameters of debris flow silting depth and impact momentum,improves the risk evaluation standard of debris flow with the characteristics of probability distribution,and takes the economic vulnerability degree as an example In order to evaluate the vulnerability quantitatively,a comprehensive risk assessment system of debris flow disaster based on dynamic process is constructed,and it is applied to the risk assessment of actual disaster to provide scientific basis for the prevention and mitigation of debris flow disaster in mountain areas.The achievements of this paper are as follows:(1)Programming to realize the dynamic model of debris flow considering erosionIn view of the current research status of debris flow dynamic model,the dynamic model based on the depth integral continuum theory is adopted.Based on the Massflow numerical simulation platform,Fortran language is used as the programming method,and combined with the empirical erosion formula proposed by Mc Dougall and Hungr,the dynamic model considering the base erosion is realized by programming.At the same time,the reading and calculation of debris flow curve are compiled in the calculation platform,the debris flow dynamics calculationconsidering the base erosion effect is realized,and the flow velocity,flow depth and other parameters of debris flow are provided quantitatively.(2)Construction of quantitative debris flow risk assessment systemBased on the depth and momentum of debris flow,considering the capacity of debris flow silting and impact damage,a two factor quantitative evaluation standard of debris flow intensity is proposed.Combined with the probability distribution characteristics of debris flow,the intensity distribution maps of 20-year return,50-year return and 100-year return are coupled into a unified risk distribution map of debris flow.The potential disaster bearing bodies of debris flow are classified based on the land use type,and the economic value is calculated respectively,and the transformation function is used to standardize,so as to achieve the quantitative assessment of vulnerability with economic vulnerability as the core assessment index.Finally,by using the risk calculation formula of debris flow issued by the United Nations,the risk assessment results and vulnerability assessment results of debris flow are coupled,and a more comprehensive quantitative risk assessment system of debris flow is constructed.(3)Practical case application of risk assessment system: Niwan gully and Jiuzhaigou basinsFor the case of Niwan gully,firstly,the dynamic model verification work is carried out based on the actual disasters in its history,and then the occurrence of debris flow once in 20 years,once in 50 years and once in 100 years is quantitatively predicted,and the risk assessment is completed;the disaster bearing body is divided into towns,farmland and roads for vulnerability assessment,and finally the risk assessment of Niwan gully area is completed.The results show that the unreasonable design of drainage channel and urban layout aggravate the risk of potential debris flow.20% of the areas with high risk of debris flow(towns and roads),39% of the areas with medium risk(near ditch farmland)and 41% of the areas with low risk are the areas with high risk of debris flow.Jiuzhaigou watershed is a watershed with multiple gullies.The accuracy of numerical inversion of Zechawa gully and Xiajijiehaizi gully in the same watershed is 78% respectively.Secondly,the same calculation parameters are used to predict debris flow disasters in different recurrence periods in Shuzheng gully.The disaster bearing bodies are divided into towns,lake and road areas to complete the quantitative risk assessment.The results show that the high-risk area accounts for21%(Shuzheng town),the medium risk area accounts for 49%(Shuzheng town andShuzheng lake),and the low-risk area accounts for 30%(road).Through the risk assessment of Niwan gully and Jiuzhaigou basin,it shows that59% of the high and medium risk areas in the study site are reached,so it is necessary to improve the blocking engineering measures and strengthen the monitoring and early warning work in the watershed.