Characteristics of burned area and susceptibility assessment of post-fire debris flow of Chongqing wildfire in August, 2022

In August, 2022, forest fires occurred in Jiangjin District, Banan District, and Beibei District in Chongqing, respectively, with a total burned area of approximately 35 km². After the wildfire, an ash layer with a thickness of 1～5 cm was deposited on the slope, and the vegetation and soil structure of the burned area were devastated. Affected by concentrated rainfall, it will pose a serious threat to the infrastructure of nearby villages and towns and residents’ lives and properties once the post-fire debris flow occurs. Based on the remote sensing, field investigation, and in-situ tests, the distribution characteristics of burned areas after the fire were identified. The measured data including the thickness of the ash layer, and the saturated hydraulic conductivity of the soil were derived. The severe, moderate, and low burned areas accounted for 9.6%, 34.0%, and 56.4% of the entire area, respectively, with the corresponding average thickness of ash and soil disturbance layer of 15.10 cm, 3.28 cm, and 1.15 cm. According to the prediction model based on expert experience, a total of 95 potential debris flow gullies in burned areas were evaluated for the susceptibility of post-fire debris flow, of which 1 was of high susceptibility, 43 were of medium susceptibility, 44 were of low susceptibility, and 7 were not prone to occur, contributing to 1.05%, 45.26%, 46.32%, and 7.37% of the total, respectively. Considering that a large number of loose solid sources besides ash and sediment still exist in the moderate and severe burned areas within watersheds under the current conditions, with strong water repellency of burned soil, once the medium and high susceptibility gullies are affected by concentrated rainfall in the rainy season, there is a relatively high probability for the post-fire debris flow occurrence. This study provides basic information for the prevention and risk management of post-fire debris flows.