[H-GM04] Geomorphology

[HGM04-P04] Landslide-area change and sediment yield on the main scarp of the 1792 Mayuyama Debris Avalanche

*Kanako Fujiwara¹, Tsuyoshi Hattanji², Taku Morimoto³ (1.University of Tsukuba, Graduate School, 2.Institute of Life and Environmental Sciences, University of Tsukuba, 3.The council of Unzen Volcanic Area Geopark)

Keywords:sediment yield, debris avalanche, SfM-MVS, aerial photograph

The change of shallow landslides on the main scarp of the Mayuyama Debris Avalanche, which occurred in 1792, for the recent 45 years was investigated with aerial photographs and airborne LiDAR DEMs. The shallow landslides were mapped with the interpretation of aerial photographs in 1947, 1975, and 2015 and point cloud data from aerial LiDAR survey in 2020. The slope and area of the shallow landslides were measured using the 0.5 m LiDAR DEM. In addition, DSMs in 1975 and 2015 were created from the aerial photographs using the multi-view stereo photogrammetry (SfM-MVS) technique. Using these DSMs and the LiDAR DEM, the sediment discharge from a catchment in the main scarp (the Fourth Stream) was calculated for the two periods from 1975 to 2015 and from 2015 to 2020. The interpretation of microtopography on the LiDAR DEM suggested that old talus slopes and cliffs in the source area of the debris avalanche had been eroded after rockfalls formed the talus slopes immediately after the debris avalanche. More than half of the shallow landslides mapped with interpretation of 1975 aerial photograph occurred after 1947, and the number of landslides also increased in 2020 after decreasing by 2015. The Isahaya Heavy Rainfall in 1957 and the Kumamoto Earthquake in 2016 occurred during these two periods when the number of shallow landslides increased, respectively. These facts indicate that heavy rainfall and earthquakes repeatedly cause new landslides and vegetation recovery on the main scarp of the Mayuyama Debris Avalanche. The amount of sediment discharge at the Fourth Stream from 1975 to 2015 was about a quarter of that from 2015 to 2020. Active sediment discharge occurred after the 2016 Kumamoto Earthquake, sediment discharge also continued due to rainfall events during the period from 1975 to 2015, when the number of shallow landslides was decreasing.