MATERIAL INITIATION OF DEBRIS FLOW GENERATION PROCESSES AFTER HILLSIDE FIRES

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.