| On March 30,2020,a forest fire broke out in Jingjiu Township,Xichang City,and Xiangshui gully vegetation was burned in a large area by forest fire.In the rainy season of the same year,debris flow broke out in many gullies in Xiangshui gully basin,of which 1#,2#,3# gullies were adjacent to residential houses and cultivated land,and the impact of the debris flow was more serious.In order to find out the disaster mechanism of post-fire debris flow in Xiangshui gully,taking Xiangshui gully 1#,2#,3# gully as the research object,based on detailed field investigation,the landform,geological structure,formation lithology and meteorological and hydrological conditions of the study area are analyzed,the topographic and provenance characteristics of the study area are summarized,and the disaster mechanism of post-fire debris flow in the study area is analyzed by means of data statistical analysis,remote sensing interpretation,outdoor test and so on.At the same time,the risk evaluation formula of single gully debris flow and grey correlation fuzzy mathematics method are used to quantitatively evaluate the three gullies in the study area.Finally,the post-fire debris flow in the study area is numerically simulated based on FLO-2D software.Gain the following main knowledge:(1)After forest fire disturbance,not all gullies will break out of debris flow,but forest fire is the fuse of debris flow.If the adjacent gully with similar topographic conditions is not disturbed by forest fire,debris flow will not break out under the same rainfall conditions,because the rainfall threshold of post-fire debris flow is significantly lower than that of conventional debris flow.Forest fire will reduce the soil permeability coefficient of 0-2cm on the surface and 2-4cm below the slope.With the increase of forest fire intensity,the decline degree of soil permeability coefficient is higher.The decrease of permeability coefficient indicates the decrease of rainwater infiltration and the increase of slope runoff,which leads to debris flow.(2)The slope soil with different forest fire intensities will be eroded by slope runoff.With the increase of rainfall times,the degree of slope erosion will deepen.The erosion depth of slope soil with moderate and severe forest fire intensity is significantly higher than that of mild slope soil.There is little difference in the degree of slope erosion between moderate and severe areas,indicating that when the forest fire intensity reaches moderate,there will be a large degree of slope erosion.A large area of slope erosion on the upstream slope of the study area gradually transited to rills.Subsequently,the rill gradually becomes a deep and narrow gully and transits to the gully eroded into bedrock.The more serious the forest fire intensity is,the stronger the erosion capacity of debris flow is,and the more obvious the gully erosion and lateral erosion are.Gully erosion increases the width and depth of gully,provides more material sources for debris flow and intensifies the risk of debris flow.(3)The risk evaluation formula of single gully debris flow and grey correlation fuzzy mathematics method are used to evaluate the risk of debris flow in the study area.The results of single gully debris flow risk evaluation formula show that the three gullies in the study area are moderately dangerous.For the grey correlation fuzzy mathematical method,firstly,the maximum amount of debris flow,the frequency of debris flow,the basin area,the length of the main gully,the maximum relative height difference of the basin,the cutting density of the basin,the proportion of unstable gully bed,the proportion of moderate and severe fire intensity,the average slope of the formation area,the maximum rainfall in 24 hours and human engineering activities are selected as the evaluation factors.Secondly,the weight of evaluation factors is determined by grey correlation method,and the subordinate degree of each evaluation factor in each risk level is determined by fuzzy mathematics method.Finally,the risk evaluation set of the study area is obtained according to the fuzzy transformation.Considering the limitation of the maximum membership principle,the fuzzy comprehensive index method is used to transform the evaluation set.The results show that Xiangshui gully1# ditch is moderately dangerous,and 2#,3# ditch is highly dangerous.Therefore,in the future monitoring and control of debris flow,we should focus on 2# gully and 3# gully.(4)The rainfall threshold of Xiangshui gully post-fire debris flow is lower than the fiveyear rainfall.Therefore,FLO-2D simulation can be carried out based on the five-year rainfall.The results show that the debris flows in the three gullies in the study area are accumulated in the reservoir,and the accumulation depth is not deep.The flow velocity of debris flow is faster in the upstream steep slope section and slower in the downstream gentle slope section.In terms of risk,1# ditch should pay attention to whether the reservoir will collapse,2# ditch should pay attention to the two branches on the right and in the middle of the basin,and 3# ditch should pay attention to the branch in the middle of the basin.The risk assessment of post-fire debris flow based on FLO-2D can visualize the risk assessment results,which is conducive to focusing on some gully sections with high risk and improving the pertinence of post-fire debris flow prevention and control. |
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