西昌市经久乡森林火灾火烧区特点及火后泥石流易发性评价

THE CHARACTERISTICS OF FOREST FIRE BURNED AREA AND SUSCEPTIBILITY ASSESSMENT OF POST-FIRE DEBRIS FLOW IN JINGJIU TOWNSHIP,XICHANG CITY

2020年3月30日四川省凉山彝族自治州西昌市经久乡发生特大森林火灾,大火持续3 d,过火面积超30 km2。山火后在坡面堆积了厚度1~5 cm不等的灰烬层,同时火烧迹地本身结构被扰动,在强降雨条件下极易诱发火后泥石流灾害,对附近学校、居民集中居住点,尤其是泸山—邛海风景区等重要基础设施及人民生命财产构成了严重威胁。通过遥感解译、野外调查及勘查,查明了不同火烈度及对应火烧迹地灰烬层和结构扰动特点,表明重度、中度和轻度火烧区分别占比46.8%、37.9%和15.3%,且火烧区火烧迹地坡面灰烬及土壤结构扰动层平均厚度与火烈度成正比,其中轻度、中度和重度火烧区平均厚度分别为1.20 cm,3.23 cm和5.04 cm。提出了基于中度及重度火烈度区面积占比、火烧迹地坡面灰烬及结构扰动层厚度、产沙区平均坡度、主沟平均纵比降、沟道流域面积5个影响因子,采用专家经验法对火烧区共63条潜在泥石流沟道进行火后泥石流易发性评价,结果表明:火烧区所有沟道中,有29条为高易发,占总数的46.0%;23条为中等易发,占总数的36.5%;8条为低等易发,占总数的12.7%;3条为不易发性,占总数的4.8%。考虑火烧区超过50%以上的沟道流域面积不足0.2 km2,且这些沟道平均纵比降大,坡面灰烬、泥沙等松散物源堆积较厚,使得中等及高等易发性泥石流沟道数量占比高达82.5%,一遇集中降雨,发生火后泥石流可能性极大。

On March 30,2020,a huge forest fire broke out in Jingjiu Township,Xichang City,Liangshan Yi Autonomous Prefecture,Sichuan Province.The fire lasted 3 days and the fire area exceeded 30 km2.After the mountain fire,an ash layer with a thickness of 1~5 cm was deposited on the slope.At the same time,the structure of the burned area was disturbed.Under heavy rainfall conditions,it is very easy to induce post-fire debris flow disasters,which poses a serious threat to nearby schools,residents’concentrated residential areas,especially Lushan-Qionghai Scenic Area and other important infrastructure and people’s lives and property.Through remote sensing interpretation,field investigations and surveys,the characteristics of ash layers and structural disturbances in different fire intensity and corresponding burned areas were identified.The results suggest that the severe,moderate and low burned areas account for 46.8%,37.9%and 15.3%,respectively,and the average thickness of the ash and soil structure disturbance layer in the burned area is proportional to the fire intensity,of which the low,moderate and severe burned areas are 1.20 cm,3.23 cm and 5.04 cm respectively.Five influencing factors were proposed based on the proportion of moderate and severe fire intensity area,the thickness of ash and structural disturbance layer on the burned ground,the average slope of the sand-producing area,the average vertical ratio drop of the main ditch,and the channel drainage area,the expert experience method was used to assess the susceptibility of a total of 63 potential debris flow gullies in the fire zone.The results showed that of the total channels in the fire zone,29 were of highly susceptible,accounting for 46.0%of the total.23 were of medium susceptibility,accounting for 36.5%of the total.8 were of low susceptibility,accounting for 12.7%of the total.Three were not easy to occur,accounting for 4.8%of the total.Considering that catchment area of more than 50%of the burned area is less than 0.2 km2,the average longitudinal slopes of these gullies are large,and the loose source materials such as ash and silt on the slope are thick,the number of medium and high-prone debris flow gullies is as high as 82.5%.In the event of concentrated rainfall,the probability of post-fire debris flow occurrence is extremely high.