[H-TT13] HIGH-DEFINITION TOPOGRAPHIC SURFACE DATA FOR CONNECTIVITY IN THE ANTHROPOCENE

[HTT13-05] Assessing the sediment connectivity and mobility with morphological changes in mountainous drainage basins following landsides by the 2018 Hokkaido Eastern Iburi Earthquake

*Tennyson Lap Wing Lo¹, Yuichi S. Hayakawa², Yasutaka Nakata³, Masato Hayamizu⁴, Takuro Ogura⁵ (1.Graduate School of Environmental Science, Hokkaido University, Japan, 2.Faculty of Environmental Earth Science, Hokkaido University, Japan, 3.Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Japan, 4.Forestry Research Institute, Hokkaido Research Organization, Bibai, Japan, 5.Graduate School of Education, Hyogo University of Teacher Education, Japan)

Keywords:Sediment connectivity, Sediment mobility, Morphological changes, Drainage basins , Coseismic landsides.

A strong earthquake with Magnitude 6.7 (focus depth: 37km) happened in Hokkaido Eastern Iburi region on 6th September 2018 with the epicenter at 42.690^oN and 142.007^oE, which caused numerous coseismic landslides (n = 7837) and covered over 700 km². Coseismic landslides within catchments acted as a larger source of sediment supply within channels and hillslope than rainfall induced landslides. Understanding spatial variability of in-channel sediment accumulation is essential for designing disaster risk reduction. This study aims to assess the sediment connectivity and mobility with post-seismic geomorphological changes in small mountainous drainage basins, which may contribute to the identification of potentially reactivated, remobilized and new landslides in earthquake-affected areas. The study sites are 2 drainage basins (approximately 0.1-km²) along the Atsuma River with minor artificial modification after the earthquake. Digital elevation models (DEMs) with a 0.5-m resolution, generated from airborne laser scanning (ALS) data of October 5 in 2012 and September 11 in 2018, were used to represent the pre- and post-seismic drainage basin morphology. For post-seismic periods (from April 2020 to September 2021), topographic data including point clouds, DEMs, and orthorectified mosaic images, were obtained by Unmanned Aviation Vehicles (UAVs) and Structure-from-Motion Multi-View Stereo (SfM-MVS) photogrammetry. Landslide areal density (LAD) was used to examine spatial variability of landslide occurrences in watersheds. Deposit length ratio (DLR) was used to examine in-channel sediment accumulations within watersheds. Landslide mobility index with height, travel distance and area of landslide were used to indicate the characteristics of coseismic landslides. The spatial variability and linkage of coseismic landslides and sediment accumulation could be examined. Besides, various factors in watersheds affecting the locations of sediment accumulations within channels, such as watershed areas and channel gradients, could be analyzed. Further assessments of factors on the near future morphological changes, including freeze and thaw action and characteristics of clay-rich sediment will be necessary.