| On April 7,2019,a forest fire broke out in the mountains of Lawo Township,Mianning County,Sichuan Province,lasting for 9 days.Due to concentrated heavy rainfall,a debris flow occurred in the Huayanzi gully in the burned area on July 4,2019.The material composition mainly consisted of ash and rock blocks,which entrained along the channel,and then destroyed the drinking water source of local residents,damaged a large area of arable land and crops,and broke the only village road in the downstream village,directly threating the pervious Lawo Township Government and Lawo Central School.Exemplifying the post-fire debris flow that occurred in Huayanzi gully as a typical example.Using on-site geological survey,rainfall simulation test and remote sensing interpretation,this paper is devoted to ascertaining the topographic characteristics,fire behaviors,source materials,runoff and sand production features related to the formation of post-fire debris flow,as well as the corresponding ash layer,water repellency,and permeability characteristics.Meanwhile,following studied the disaster mechanism of debris flow in Huayanzi gully and calculated kinematics parameters,based on field surveys,utilizing an improved post-fire debris flow risk assessment model based on Liu Xilin evaluation model,we selected key indicators that affect the risk of post-fire debris flow,and performed the ANSYS CFX software to predict the movement process and the region of the dangerous zone of the debris flow in the 50-year returned period.And we evaluated the post-fire debris-flow risk in the Huayanzi gully.The results reveal the evolution process of the post-fire debris flow,and provide theoretical support for the monitoring and early warning and prevention projects of the post-fire debris flow.The main conclusions of this paper are follows:(1)The main channel of Huayanzi gully is 3.61 km in length,and 4 branches on both banks are developed.The catchment has an area of about 3.69 km~2,with the maximum elevation difference of 910 m,and the downstream mud level of about 3?7 m.The burned areas in the study area are divided into severely,moderately,slightly burned and unburned areas.The Huayanzi gully has a total burned area of 3.69 km~2,of which the area of severely burned area is 1.19 km~2,accounting for 33%;the area of moderately burned area is 0.56 km~2,accounting for 15%;the area of slightly burned area is 1.01 km~2,accounting for 27%;and the unburned area is 0.92 km~2,accounting for 25%.(2)Forest fires are closely related to the outbreak of post-fire debris flows.Fires can burn vegetation and produce a large amount of ash,which becomes the first particular source material of debris flow in the burned area.Moreover,the roasting effect destroys the surface soil structure,which evolves into loose and fragile.The forest fire can greatly weaken the ability of plant roots to consolidate the soil.Furthermore,the occurrence of forest fires can enhance the water repellency of the surface soil,and the water repellent materials will increase slope runoff and reduce the rainfall threshold for debris flow occurrence.Due to the production of water-repellent materials in the soil of the burned area,rainwater is difficult to infiltrate,resulting in the surface runoff increasing,and consequently the erosion source material formed by the intensified erosion of the slope runoff will become an important part of the source materials of the secondary debris flow.(3)Through the rainfall simulation test and disc water seepage experiment of the slope erosion in different fire severity areas on the burned area,it is found that the soil permeability of the unburned area is much greater than that of the burned area,and the productive discharge of the burned area is significantly greater than that of the unburned area.The amount of sand production is basically positively correlated with fire intensity.The amount of sand production in severely burned areas was significantly higher than that of moderately and slightly burned areas.The difference in sand production in slightly and moderately burned areas was not obvious,about 1/4 of that in severely burned areas,and the unburned areas had the least amount of sand production.(4)The initiation modes of the loose source material of post-fire debris flow often include shallow surface landslide failure triggered by rainfall infiltration and surface runoff erosion.The post-fire debris flow in Lawo Township was investigated by field investigation,and most of it was formed by surface runoff erosion.The loose rock and soil on both sides of the channel would be unstable and disintegrated due to the channel scouring,supplying for the subsequent occurrence of debris flow.(5)Using an improved risk assessment model for post-fire debris flow to assess that the debris flow in the Huayanzi gully after fire is highly dangerous.The debris flow simulated by CFX software has a maximum accumulation area of 0.15 km~2 at the gully mouth,which accounts for 75.83%of the residential area of Lawo Village,indicating that once the debris flow occurred after the fire,it would pose a great threat to the original Lawo Township Government and surrounding villagers.It is recommended that relevant government departments take corresponding prevention and control engineering measures in time. |
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