Towards Long-Lasting Disaster Mitigation Following a Mega-landslide: High-Definition Topographic Measurements of Sediment Production by Debris Flows in a Steep Headwater Channel

Abstract

Mega-landslides usually cause long-lasting subsequent sediment production, and long-term strategies for disaster mitigation are necessary in the case of such extreme events. The Ohya-kuzure landslide in central Japan is typical of sites where hillslope erosion and sediment yield have been continuously active since its formation in 1707. Sediment production is particularly active by debris flows in the headwater channels formed within the landslide. However, the dynamics of such debris flows in steep headwater channels have not been fully examined compared to those in gentler downstream reaches. To investigate the changes in headwater channel bed sediments remobilized mainly by frequent debris flows, repeated high-resolution measurements were carried out using terrestrial laser scanning. Freeze-thaw weathering in the surrounding slopes, which are composed of deformed shale and sandstone layers, delivers quantities of small particles onto the valley floor. Measurements in spring, summer, and autumn conducted over two years provided high-definition (0.1 m resolution) topographic datasets, revealing the seasonal amount of erosion and deposition to be on the order of 1000–5000 m3. Erosion and deposition along the reach also showed contrasting spatial patterns according to the sections bounded by knickpoints and valley narrows. These basic estimates of sediment production in headwater channels can be utilized for further mitigation of possible sediment-related disasters in downstream areas.