Risk Assessment Of Debris Flow Based On Watershed In Panxi Area

With the increasing in global climate change, tropical cyclone, rainstorm, flood and other extreme weather and climate events occurred frequently. The debris flow disaster is more and more frequent, causing a huge impact and serious loss to human society, economy and life. Risk assessment of debris flow is the basis of disaster prediction, prevention and reduction work. Debris flow disaster is restricted and influenced by various factors, the geographical conditions in watershed is one of the most important factors. Panxi area of Sichuan Province where the debris flow occurred frequently is chosen as the study area. Using RS and GIS spatial analyst methods to study the spatial distribution characteristics and distribution law of actual debris flow point in study area, statistical analysis for the relationship of debris flow point and the geographical environment where the debris flow point located, spatial analysis of the spatial relationship between debris flow point and the geographical environment where the debris flow point located. Using cumulative watershed effect as internal relations to get assessment factors related to geographical conditions in watershed including average slope of sub-watershed, elevation difference of sub-watershed, pour point of watershed, the distance to sink point of river, the distance to river and other factors like formation lithology, the distance to fault, rainstorm frequency, vegetation coverage, river network level, land use type. According to the relationship debris flow point distribution and influential factors and their cumulative curve to do the quantification and classification of every factor. Then regard grid cell as the assessment unit, and use information method to calculate the information quantity of each factor to different categories and the entropy function to calculate the weight of every factor, construct the debris flow risk assessment index system. Finally do the debris flow risk assessment of settlement points in Panxi area based on the specialization of population data. Our results show that:(1) By analyzing the relationship between debris flow point and different factors, we find that the distance to river, the distance to sink point of river, the distance to fault have negative relationship to debris flow, and among them the distance to sink point of river has the maximum correlation value, while the distance to fault has the minimum. But river network level, vegetation coverage have positive correlation, and the correlation value of river network level is bigger than vegetation coverage. The pour points of watershed, elevation difference of sub-watershed, average slope of sub-watershed, formation lithology, and rainstorm frequency are uncorrelated, but it doesn’t mean they have nothing to do with debris flow. Debris flow only happened in some specific scope.(2) Among 53 categories of 11 factors, the maximum value of information quantity is the distance to sink point of river located in 0-2.4km.The minimum value of information quantity is vegetation coverage located in0-0.5&0.9-1. With the help of entropy function, we get that the maximum weight value is vegetation coverage, which is 10.59% followed by the distance to river and average slope of sub-watershed, which are 9.68% and 9.67%.the minimum weight value is river network level, which is only 7.7%.(3) According to the results of risk assessment of debris flow in Panxi area, the very high risk zones are 7813.5km2 and mainly near the Jinsha river, Yalong river and their tributaries, which account for 11.65% of the total study area, the high risk zones are 11658.5km2 mainly near the very high risk zone in the mid-eastern and along the coast of Litang river and Shuiluo river in the west, which account for 17.38%, the middle risk zones are 20045.5km2 and mainly near the high risk zone in the central and eastern, which account for 29.88%, the low risk zones are11850.5km2 and mainly scattered in the east where far from the river, which account for 17.66%, the very low risk zones are 15720km2 and distributed in the mid-western of our study area, which account for 23.43%.Through the test of validity, the results of risk assessment are consistent with the actual distribution of debris flow point, which means the risk assessment index system could do the debris flow risk assessment effectively in Panxi area.(4) There are 2.717 million people in the very high risk zones accounting for 48.51% of the total population,1.594million people in the high risk zones accounting for 28.46%,1.008 million people in the middle risk zones accounting for 18.00%,138 thousand people in the low risk zones accounting for 2.47%,144 thousand people in the very low risk zones accounting for 2.56%.(5) By combination information method and entropy function, and get assessment factors related to watershed, this study avoid the influence of subjective factors in the risk assessment of debris flow. Through the test of validity, the results of risk assessment are consistent with the actual distribution of debris flow point, which means the risk assessment index system could do the debris flow risk assessment effectively in Panxi area. This study could provide scientific basis for forecasting the debris flow and reference for disaster prevention and disaster reduction work in Panxi area.