| The debris flow that occurred in the Sejiao Gully of Sanyanlong Township,Jiulong County in 2016 is highly relevant to the forest fire in Sejiao on March 15,2016.It is a typical post-fire disaster.In order to clarify the developmental characteristics and disaster mechanism of debris flow after the Fire,this paper analyzed the formation conditions of the post-fire debris flow in the Sejiao watershed,including topographic conditions,formationlithologyconditions,structuralgeologicalconditions,meteorological and hydrological conditions,vegetation conditions and fire behavior conditions.And the characteristics of developmental background of the debris flow after the forest fire,such as topographic features,provenance characteristics,and dynamic characteristics,are summarized.On this basis,through on-site investigation,remote sensing interpretation,indoor and outdoor experiments,etc.,the developmental characteristics and disaster-causing mechanisms of post-fire debris flow were studied in depth.At the same time,the potential factors affecting the formation of post-fire debris flow in the study area were analyzed,and the indicators suitable for risk assessment of post-fire debris flows were selected,and a risk assessment model for post-fire debris flows was established using the Fuzzy-AHP method,for the purpose of assessing the risk of the post-fire debris flow gullies in the Sejiaogou watershed.The main findings of this article are as follows:?1?Remote sensing interpretation combined with on-site survey results show that the total fire area of the forest fire in the Sejiao Group,Sanyanlong Township,Jiulong County,Sichuan Province,which occurred on March 15,2016,was approximately 160 hectares.According to the fire intensity standard,the burned area can be divided into severe,moderate,mild and unfired fire zones.After the fire,the debris flow all occurred in the area that the fire was excessive and the vegetation was burned seriously.The post-fire debris flow was highly correlated with the fire intensity.?2?the ash layer after fire is a unique source of post-fire debris flow,the average permeability coefficient of disturbed soil in burned area is 4.31×10-5m·s-1,while the average permeability coefficient of soil in unburned area is 7.52×10-5m·s-1.Different permeability lead to the difference of slope-runoff and then change the catchment conditions in the gully,and reduce the rainfall threshold ofstarting debris flow,so as to affect the trigger mechanism of the debris flow in the burned area.the developmental characteristics and disaster mechanism of post-fire debris flow are totally different from those conventional debris flow and post-earthquke debris flow.?3?Combining the characteristics of the post-fire debris flow in Sejiao and its disaster mechanism,selecting a maximum flow rate of debris flow?possibly?,the frequency of debris flow,the drainage area,the maximum relative elevation of the gully basin,the average gradient of slopes in the formation area,the area proportion of severe and moderate fire area in the basin,sediment supply length ratio,24-hour rainfall,the population density in the basin,a total of nine indicators to assess the risk of Sejiaogou post-fire debris flow.The index of area proportion of severe and moderate fire area in the basin is used to quantify the influence of fire behavior conditions on the degree of post-fire debris flow,and a risk assessment model for post-fire debris flows is established based on the Fuzzy-AHP method.?4?The risk assessment result of the 1#gully in the Sejiao is 2.112,and the danger level was moderate.The risk assessment result of 2#gully is 2.556,and the danger level is moderate to high risk,which is biased towards high risk.The risk assessment of 3#gully is 2.737 and the danger level is high risk.The danger assessment result of 4#gully is 2.401,and the danger level is moderate to high risk,which is biased towards to moderate risk.the 2#and 3#gully are more dangerous in Sejiao basin,more focus should be put on 2#and 3#gully in the process of monitoring and treatment of post-fire debris flow in the basin of the Sejiao. |
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