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

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.