Potential Debris Flow Hazard Assessment In Lushan City Based On Watershed Scale

China is one of the countries which have most severe geological disastersaround the world. The earthquake happened on4·20in Lushan of Ya·an haveinspired a lot of the geological disasters. It has brought the serious security hiddendanger to the selection of sites for residents, restoration and reconstruction and cityplanning. It would be a serious threat to people s lives and property safety of thecountry, which bring them the huge economic loss if these disasters occur. Therefore,using new technology and new method to quickly and roughly pinpoint thedistribution areas of landslide hazard, and division the risk zoning is very necessary.It not only provides the basis for geological hazards assessment, disaster preventionor reduction and early warning or forecasting, but provides reference for the futureresearch on the debris flow in strong earthquake area.The paper based on the earthquake area of Lushan, choose the image maps datasuch as the geological map, the topographic map, the remote sensing image afterearthquake and the zoning map and so on to investigation and analysis the influencefactors(including11factors such as geological structure, stratum lithology, seismicintensity, morphological information entropy, elevation, gradient, aspect, vegetationcoverage, rivers, rainfall and human activitiesˇon different basin scale of debris flow,to establish the risk assessment model of Debris flow, then calculate the riskquantitative values of each basin according to the model, finally get a result thepotential debris flow risk degree is divided into five grades. The main work includesthe following aspects:(1)The paper will make DEM(from the topographic map) as the main datasource, extracting the river network the water threshold value are50001000015000and20000, and compared to real rivers, choose the better fitting riverbasin(threshold value are10000and15000) to division basin. When the water threshold value is10000, the study area was divided into64watersheds. When thewater threshold value is15000, the study area was divided into41watersheds.(2)Choose the OLI image after earthquake from the Landsat8satellite (launchon February11,2013) as the main remote sensing data source, data is novel. Choosethe image On December7,2013(winter), increase the reliability of the vegetationfactor NDVI (the sensitivity of NDVI will be decreased in the study area where becovered abundant vegetation).(3)Based on the defined sub basin to calculate the morphological informationentropy:(a the area of the basin above each contour h the elevation differencebetween each contour and the lowest point in the basin A total area of the basin H the biggest elevation difference in the basin), do a scatter plot of X, Y andsimulation polynomial curve(the Strahler’s area-elevation function has the highestfitting by making the cubic polynomial curve to simulation by experiment research,curve fitting are all above0.95, the result of calculation is reliable), the Strahlervalue of the fitting curve be obtained by integral, and then calculate themorphological information entropy, finally delimit the degree of risk in the studyarea according to the calculation result. The results showed: the geomorphicEvolution Stage in most area of the Lushan is the stage of maturity. Regionaltectonic activity belongs to medium level.(4)In the Debris flow risk assessment based on the watershed scale, the studyadopt these sub basins the threshold value of catchment is10000and15000forpotential debris flow risk assessment, according to the different scales dividing thesub basin and calculating the morphological information entropy in the study area.Taking sub basin as the evaluation unitˊchoose the theory of analytic hierarchyprocess (ahp) to determine the weights, use11evaluation factors include geologicalstructure, stratum lithology, seismic intensity, morphological information entropy,elevation, gradient, aspect, vegetation coverage, rivers, rainfall and human activitiesand so on to evaluate the risk of potential debris flow in the study area. It takes thedebris flow risk of Lushan divided into five levels: high, higher, medium, lower andlow.(5)By the results of the study, the number of sub basin the water thresholdvalue is10000are64, among them there are4debris flows which their potential riskis high. In addition, higher risk has16, medium risk has28, lower risk has12, andlow risk has4. Data is normally distributed on the whole, the result is more reliable.The number of sub basin the water threshold value is15000are41, among them there are2debris flows which their potential risk is high. In addition, higher risk has10, medium risk has21, lower risk has6, and low risk has2. Data is normallydistributed on the whole, the result is more reliable. From the point of spatialdistribution, on the whole, the northeast is higher than the southwest, and the east ishigher than the west on the debris flow potential risk. Low risk area is roughlydistribution in some county in central and southern regions. From the Angle ofadministrative divisions, Taiping, Dachuan, Shuangshi and Baosheng belong tohigher potential risk area of debris flow. But lower potential risk areas of debris floware Longmen, Qingren, Luyang, Siyan and Feixian Guan. Few sub basins belongs tohigh risk areas in the country.