Risk Assessment Of Urban Debris Flow In Wenchuan County Base On GIS

Debris flowis a serious natural disaster,which is widely distributed inmountainous areas of southwestern China. After the5.12earthquake, debris flow’sfrequency and scale were strengthened, which were beyond the engineering designfrequency. Cities and towns are the center of economy and politics, which suffer thehigh risk of this disaster. The debris flow of Nangou gully happened in july3rd1961caused a huge economic loss, and pose a threatto the thousands of death toll,includingAba formal college, on the debouchure. Recently,the risk of the debris flow of theNangou gully still exist,because of that “5.12" earthquake destroyed mountainstructure,created mass loosen solid material on both sides and in of the Nangou gullysuch as collapse,landslides,etc, it provided sufficient material sources for outbreak ofthe debris flow;and because of that the population and house building density on thedebouchureis bigger, in restrictions of mountainous terrainIn order to afford thereasonable design for the disaster prevention and disaster reduction, the assessment ofthe risk of the population and building density was carried out.The paper assess comprehensively the hazard, vulnerability and risk to the debrisflow of Nangou,by investigation and research to parameters of it, such as: topography,geomorphology and tectonics, etc. For hazard assessment: through the right modeland the parameters of the unit weights, runouts, gradient of fan,drainage relative reliefof debris flow underdifferent frequency,we forcast the maximum runout distance andrunout lateral width and the maximum range angle of fan of the Nangou debris flowmodel.then obtain the hazard area base on aforementioned result combined withdebouchure topography;extract and calculate additively to the parameters of gradientof fan and the distance to ditch under the support of the GIS software and1:2000topographic map,then generate hazard assessment map; For vulnerabilityassessment:based on the date occurred from investigation and interpretation to highresolution remote sensing image, analysis and processing to position、type, value and loss rate of objects affected by hazard.and rasterize Study area, calculate maximumpotential loss degree of material and population within each grid base on GISsoftware,and generate vulnerability assessment map according to the size of the lossdegrees. Finally generate risk assessment map in GIS software base on the result ofhazard vulnerability assessment and vulnerability assessment,and applyingthe modleof “the risk=the hazard×vulnerability”.Through the research to debris flow ofNangou gully, we conclude as below:(1) According to the different parameters of unit weights, runouts, depositionalgradients under50a、100a、200a、500a frequency, as well as using the appropriatemodel, we forcast the hazard scope, the predictive hazard areas are67082m~2、77010m~2、88198m~2、103749m~2respectively.(2) The vulnerability assessment for the debris flow of Nangou gully isconcluded as follows: For population vulnerability assessment, high vulnerability is6807m~2,which takes7%of the whole area; middle vulnerability is17591m~2，whichtakes17%of the whole area; low vulnerability is79352m~2, which takes76%of thewholearea.While for material vulnerability assessment, high vulnerability is9530m~2,which takes9%of the whole area; middle vulnerability is26861m~2，whichtakes26%of the whole area; low vulnerability is67359m~2, which takes65%of thewhole area.(3) The risk assessment for the debris flow of Nangou gully is concluded asfollows: high risk area is15775m~2covering15%of the total predictive hazard scope;Middle risk area is38443m~2covering37%of the total predictive hazard scope;Lowrisk area is49529m~2covering48%of the total predictive hazard scope.In which, thehigh risk area mainly focus on sector ditch or housing construction area on thedebouchure, especially the residential area, which is the results of superposition ofvulnerability and risk, and this is coincide with the actual situation, having certainrationality and usability.