Research On The Effect Of Forest Fire On Soil Structure And Hydrological Properties In The Burned Area,in Egu,Yajiang

As a frequent forest fire area in China,Sichuan Province often causes a series of secondary geological disasters after the fire,post-fire debris flow is one of them.As a special category of debris flow disaster,post-fire debris flow has a unique starting mechanism and disaster effect.The paper takes the Egu Township,Yajiang County,Sichuan Province as the research object to divide the fire intensity area and analyze the development characteristics of post-fire debris flow in the burned area.Finally,the relevant change characteristics were revealed through the experimental study on the geological structure and physics and hydrologic properties of the soil in the burned area.The main results are as follows:(1)The paper investigates the geological background of the development of debris flow gully after the fire in Egu Township,Yajiang County.The formation conditions of post-fire debris flow were analyzed in the term of the forest fire,topography,channel and provenance.The development characteristics of the debris flow activity,erosion and deposition and accumulation after the fire were summarized.(2)The paper summarizes the discriminant criteria of the burned area and the regional characteristics of the fire intensity in the burned area of the study area,the physical and mechanical characteristics of the soil in the burned area of Egu Township were tested and analyzed.The results shows that with the increase of fire intensity,the natural density of soil,the particle density,the porosity and the moisture content decreased,the components of clay and sand in soil increase,the content of particles and the cohesion decreased,as well as the angle of internal friction increased slightly.However,the overall shear strength shows a slight downward trend.The influence degree of all indexes decreased with the increase of soil depth.(3)The density of forest fuel load in Sichuan area is much higher than the average value in China,which is a region prone to forest fires.At the same time,it was found according to the investigation of the burned area in the study area that the soil stratum structure changed after the forest fire burns,and the soil structure above the bedrock changed from the original litter layer and humus layer to the ash layer and baking layer after burning.Meanwhile,according to the field survey,it was found that the thickness of the baking layer in the burned area of Egu Township varies from 1-3.5cm.(4)The physics and hydrologic properties of soil also changed after the forest fire burns.The test shows that most of the soil in the burned area of Egu Township will form a "water repellency layer" after burning.At the same time,the results of the WDPT test shows that there is a positive correlation between the water repellency of the soil in the test area and the fire intensity.The results of the disc infiltration test shows that there is a negative correlation between the infiltration rate of the soil in the test area and the fire intensity,which influences are slowed down with the increase of depth.The soil in the test area is basically the same at-3cm regardless of whether it has been burned water repellency or not.(5)Water repellency is a very special property in soil.The analysis shows that The two most important factors affecting the water repellency are the soil permeability coefficient and moisture content in the burned area of the Egu Township.Meanwhile,the splash erosion ability of the soil with strong water repellency is obviously enhanced by using the simulated rainfall experiments,which would seriously affect slope surface runoff.It is an important control factor for the outbreak of debris flow after the first rain in the burned area to enhance the degree of soil erosion.