Hazard Assessment Of Debris Flow In Upper Reaches Of Minjiang River Under The Future Climate Change

China is a mountainous country.The characteristics of mountainous area including activate geological tectonic movement,complex terrain condition,frequent geological disasters,relatively backward social and economic conditions and fragile ecological conditions lead to it's weak ability of disaster risk withstanding.Debris flow is a common type of sudden natural disasters in mountainous area,which has great destructive power.Debris flow disaster prevention and mitigation research has always been the priority of geological disaster prevention work in China.The upper reaches of Minjiang river lies in southwest China.Since directly influenced by the Longmen Shan fault zone,the geological tectonic movement in this region is very activate.2008 Wenchuan earthquake has gathered a lot of loose solid material,combined with it's canyon landform and heavy rain conditions,debris flow comes to be the most serious secondary disaster type in future several years.In spite of a gread deal of debris flow research before and after the Wenchuan earthquake,it remains to be necessary to study the debris flow formation in the whole watershed scale,and to take future climate change condition into account for the hazard assessment of debris flow.This paper mainly focused on the solution of the following three scientific problems:the main factors that induced debris flow hazard in study area,weight setting of debris flow disaster risk assessment model,and debris flow disaster risk estimates of study area under future climate change conditions.To achieve these study goals,this paper firstly screened 18 potential hazard formative factors from the aspects of geological background aspects,topography condition,vegetation and soil condition and climatic factors based on the 1285 small watersheds.Then,on the basis of binary logistic regression model to identify key disaster-inducing factors,with reference to the information of 230 watersheds that has been outbreaked debris flow hazards.Thirdly,This paper used rough set theory for assigning the influencing weight of each key disaster-inducing factor,and constructed debris flow hazards assessment model.Finally,on the basis of preprocessing the future climate change data,we assessed the debris flow hazard level in 2030-2060 under the condition of future Climate change.The results were as follows.?1?The results of binary logistic regression analysis show that the selected 18potential hazard formative factors from the aspects of geological background aspects,topography condition,vegetation and soil condition and climatic factors can explain the formation of debris flow in study area,with the ROC curve area of 0.814.The binary logistic regression model identified 7 key disaster-inducing factors,namely the number of?25mm precipitation days?x₁?,multi-year average precipitation?x₂?,the early debris flow density?x₃?,relative height difference?x₄?,soil depth?x₅?,slope?x₆?and the distance from the fault?x₇?.?2?We used rough set theory for assigning the influencing weight of each key disaster-inducing factor.The importance sequence of each are x₃?0.392?>x₄?0.1335?>x₁?0.1202?>x₂?0.1142?>x₇?0.0964?>x₅?0.0964?>x₆?0.0549?,which differentiate to the result of using subjective weighting method,performed as the weight of the number of?25mm precipitation days?x₁?,the distance from the fault?x₇?,slope?x₆?,multi-year average precipitation?x₂?increased 0.0263,0.0764,0.0345,0.1045 respectively,while the weight of soil depth?x₅?,relative height difference?x₄?and early debris flow density?x₃?decreased 0.0238,0.0465,0.1715 respectively.?3?This paper constructed the debris flow hazard assessment model based on subjective weighting method and rough set theory respectively.The accuracy of assessment was testified with the 230 small watersheds that has been outbreaked debris flow hazards.The results show that,on the basis of ensuring the same assessment accuracy,the amount of high risk watersheds recognized by rough set theory is 236 less than using subjective weighting method,the low risk watersheds recognized by rough set theory is 180 more than using subjective weighting method,and the low risk watersheds recognized by rough set theory have never outbreaked debris flow hazards.This shows that the debris flow hazard assessment model based on rough set theory has a higher accuracy than the model based on subjective weighting method.?4?The analysis of climate data of study area in the future shows that the major precipitation center of the this region 2030--2060 located southeast of Wenchuan county and a small part of Dujiangyan city neighboring,the secondary precipitation center located in the Heishui county,The drought center will remain concentrated in Maoxian county.The rain concentrated area of upstream Minjiang will mainly located most regions of Heishui county,western of Songpan county and northwest of Lixian county.The heavy rain days of most regions of Maoxian and northern Songpan county may be the least.?5?The hazard assessment results on the condition of future climate change based on rough set theory show that 65 watersheds that currently are on the level of low risk will changed to be high risk area in the year of 2030-2060.These small watersheds mainly located in the central valley area of Wenchuan county,Zagunao river valley of Lixian county and the southwest mountainous area of Heishui county.The future debris flow disaster prevention and reduction work in upper reaches of Minjiang river needs to be focused on these areas.