火后泥石流形成过程的物源启动模式研究

MATERIAL INITIATION OF DEBRIS FLOW GENERATION PROCESSES AFTER HILLSIDE FIRES

火后泥石流是火烧迹地最为严重的次生地质灾害,相对于传统泥石流和震后泥石流,其物源启动模式及致灾机理呈现出特殊性。通过对四川省乡城县仁额拥沟火烧迹地沟道不同时间尺度下的累积侵蚀量统计分析,将火烧迹地物源启动分为3个阶段:坡面侵蚀阶段、高含沙水流沟道侵蚀阶段和泥石流沟道刨蚀阶段,其中面蚀到沟蚀转变所需的汇流面积与斜坡倾斜度和火烈度呈负相关,高含沙水流转变为泥石流后常常造成沟道侵蚀率的激增;火烧后2 a的坡面侵蚀量相当于火烧前10~30 a的侵蚀总量,且主要发生在中度及严重火烧区;火烧区的滑坡发育率远高于未火烧区,但未发现火烈度对滑坡体积有明显影响,其主要受临空面高度影响,并呈幂函数正相关,滑坡物源启动模式为坡脚切坡触发的逐级牵引后退式补给。

After occurrence of fires on slope ground,debris flow is the most hazardous secondary disaster in the burned area and closely linked to fire activity. Compared with traditional debris flow and post-earthquake debris flow,the post hill-fire debris flow features in its material initiation and hazard formation. This research conducts a detailed investigation in Ren’e Yong gully,so as to get better understanding of the post hill-fire debris flow. Ren’e Yong gully burned with area of 6. 9 km2. Six tributary channels were affected by the forest fire,five of them generated post hill-fire debris flows during the following four years. Debris flows destroyed the country road and buried the houses. According to statistic analyze of cumulative channel entrainment,the generation of post hill-fire debris flows experienced three phases,including surface erosion,hyper-concentrated flow erosion and debris flow erosion. Enhanced overland flow after fire incised into cohesive sub-soil in critical condition,and subsequently started channel scour as hyper-concentrated flow. When hyper-concentrated flow transformed to debris flow,the yield rate of channel erosion increased dramatically with a magnification of 1. 5 ~ 5. 7. The collapse of log dams in channel and abrupt changes in channel geometry could promote the translation from hyper-concentrated flow to debris flow. Fire severity derived from remote sensing image of Landsat 8 and field survey was used to evaluate the effect of fire behavior on debris flow generation. As a consequence,significant surface erosion was only found in moderately and severely burned area,slightly burned and unburned area experienced little surface erosion. The erosion thickness in area with moderate and high fire severity equals to that during 20 to 30 years before fire. Fire severity also positively linked to the contribution area needed for the translation from surface erosion to channel incision,in which the angle of slope surface also matters positively. Statistical data shown the distribution density of landslides in burned area was obviously higher than that in unburned area,but little influence of fire intensity on landslides volume. Landslides volume was positively linked to height of free face following the law of meditation function. Multi-stage retrogressive landslide was found to be the most common mode of material initiation.