Risk Assessment Of Debris Flow In Gasa Town Of Xinping County,Yunnan Province

Gasa Town is located in the middle section of the east foot of Ailao Mountain in Xinping County of Yunnan Province,within the complex geological conditions and debris flow disaster development.There are 5 gully-type debris flows in the town area.These debris-flow gullies have occurred many times in the history of different scale debris flow disasters,which has brought great harm to the town.Since 90's of last century,Yunnan Province has carried out comprehensive prevention and control of debris flow gullies in Gasa Town in stages,and has achieved better control results.Due to the special geological environment and strong human engineering activities,the threat of debris flow to the town area still exists.Therefore,the risk assessment of debris flow in the town area can provide scientific basis for the construction of town area in prevention and control of geological disasters and risk management.Based on the systematic field investigation,this paper analyzes the types,forming conditions,developmental characteristics and influencing factors of debris flow disasters in the area.The risk degree of 5 debris flow gullies in the study area was evaluated by means of combined weighted method and extensology combined with single gully debris flow risk assessment,and the hazard range of debris flow was divided in combination with FLO-2D numerical simulation software.By selecting two kinds of indexes of population vulnerability and material vulnerability,the generalized objective function model is used in the Arcgis software to realize the vulnerability evaluation of disaster-bearing bodies in towns and cities,and combined with the existing prevention and control systems,a certain evaluation system and a certain evaluation system are selected.Finally,the evaluation model of"Risk=Hazard×Vulnerability/Disaster resistance"is selected to analyze the risk evaluation of debris flow in Arcgis software.The main results are as follows:(1)There are 5 rainstorm-type debris flows in the study area,which is Jinchang River,Nanen River,Nanxian River,Manfei River and Nancan River,and the fluid properties are sparse.Among them,Nanen River,Nanxian River debris flow scale is very large,others are large debris flow.The study area is rich in loose accumulation provenance.Through the analysis of provenance,it is found that geological structure plays a controlling role on debris flow provenance,different geomorphology and elevation provenance are different,and fault disaster control effect is obvious.At the same time,human engineering activities also have a great influence on the distribution of provenance.(2)By selecting 10 indexes such as watershed area and adopting the combined weighted extension model to evaluate the risk of debris flow in single gully,the quantitative evaluation results show that Jinchang River,Manfei River is moderate risk,Nanxian River and Nancan River are highly dangerous.The Nanen River is extremely dangerous.Then using FLO-2D numerical simulation software,the mud depth,velocity and accumulation range of debris flow before and after controlling debris flow under different rainfall frequencies are simulated,and the hazard zones are carried out according to certain risk division.The research shows that the control engineering effect is remarkable,and the risk of each rainfall frequency is reduced.When the rainfall frequency is 5%,the accumulation area is reduced by 0.5498 km~2,a total reduction of 518910m~3.When the rainfall frequency is 2%,the accumulated area is reduced 0.8508km~2,and the one-time discharge is reduced 730664m~3.When the rainfall frequency is 1%,the accumulation area is reduced by 1.2607km~2 and the one-off amount of flushing is reduced by 896618m~3.(3)Two indexes of population vulnerability and material vulnerability are selected to evaluate the quantitative vulnerability of Gasa Town area.The results of vulnerability assessment are obtained through more precise indicators and data,and the evaluation results are divided into high and middle.The area of high vulnerable area is 0.8867km~2,the area of middle vulnerable area is 1.32094km~2,and the area of low vulnerable area is 9.0844km~2,accounting for 7.853%,11.697%and 80.450%of the total area respectively.As a result of the establishment of a relatively complete debris flow warning system,when debris flow occurs,residents in the vicinity of each channel will actively avoid disasters,the vulnerability of the population will be reduced,and the area of its high vulnerability area will be reduced.(4)13 small items,such as blocking project and mass policy and mass prevention,were selected to form the disaster resistance evaluation system.The disaster resistance index of Nanen River is 70,the disaster resistance index of Nanxian River is 61,with medium disaster resistance.The disaster resistance index of Manfei River and Nancang river is 53,with medium disaster resistance.Jinchang River disaster resistance index is 35,with low disaster resistance.The corresponding disaster resistance degrees are 6.580,3.459,1.927,1.927and1.517,which is the multiple of disaster risk reduction of each debris flow gully by the existing disaster prevention and mitigation measures.(5)Based on the evaluation of debris flow risk,vulnerability and disaster resistance,the risk evaluation results of Gasa Town area are obtained.Under the rainfall frequency of P=5%,the areas of high,medium,low and risk-free debris flow are 0.0083km~2,0.1302km~2,0.8535km~2 and 10.2999km~2,respectively accounting for0.07%,1.15%,7.56%and 91.21%of the total area of the town.Under the P=2%rainfall frequency,the areas of high,medium,low and risk-free debris flow are0.0525km~2,0.2161km~2,1.3901km~2 and 9.6332km~2,accounting for 0.47%,1.91%,12.31%and 85.31%of the total area of the town area,respectively.Under P=1%rainfall frequency,the areas of high,medium,low and risk-free debris flow are0.0827km~2,0.4049km~2,1.8277km~2 and 8.9767km~2,accounting for 0.73%,3.59%,16.19%and 79.49%of the total area of the town.