[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-P04] Seasonal changes in sediment production and sediment discharge after earthquake-induced landsides

*Takashi Koi¹, Akiko Kitamura², Shin’ya Katsura^2,1 (1.Center for Natural Hazards Resarch, Hokkaido University, 2.Research Faculty of Agriculture, Hokkaido University)

Keywords:sediment production, sediment discharge, shallow landslide, volcanic area, the 2018 Hokkaido Eastern Iburi Earthquake

The Hokkaido Eastern Iburi Earthquake (M_w 6.6) that occurred on September 6, 2018 caused shallow landslides in more than 7,000 locations, mainly in Atsuma and Anpei towns. In general, collapsed slopes are expected to recover vegetation and reduce sediment production over time. However, there are few cases of long-term monitoring of sediment dynamics in such areas, and it is not clear how sediment dynamics change after vegetation recovery or landsliding. In Hokkaido, in particular, the freezing and thawing associated with winter snowfall may have affected sediment dynamics in a mountain watershed. In addition, the surface soil in the area where many landslides occurred due to the earthquake is mainly volcanic ejecta, and the constituent material of the produced sediment is relatively fine-grained. Therefore, if the collapsed sediments are discharged into the downstream area, there is a concern that the effects such as stream water turbidity and water quality changes may be prolonged. The purpose of this study is to clarify the characteristics of sediment dynamics in the upper Habiu River catchment of the Atsuma River system (0.37 km²), where shallow landslides were frequently occurred due to the Hokkaido Eastern Iburi Earthquake, for about 4 years after the earthquake, focusing on hydrological information that causes sediment production and sediment transport from the bare failed slopes. For sediment production from the landslide slopes, we used elevation data obtained by Structure from Motion (SfM) using aerial photographs taken regularly by a UAV (DJI Phantom 4 RTK) after the earthquake, and the amount of change in elevation values of the landslide slopes was determined for each collapsed area from difference analysis of elevation data from the two time periods. The obtained changes were characterized in terms of sediment production, assumed to be mainly due to summer rainfall events and freeze-thaw based on winter ground temperature changes, and seasonal changes in sediment discharge, also taking into account differences in topography and slope orientation of the landslides. Changes in sediment discharge were determined by monitoring turbidity at the outlet of the catchment. The differential analysis of elevation data from the two periods showed that sediment production varied according to the shape of the landslide slopes. Gully erosion was predominant on the spoon-type slopes with valley topography during the period including high-intensity rainfall events, confirming the progression of new sediment production. On the other hand, planar-type slopes did not show significant gully erosion during the same period. Changes in sediment discharge from the watershed corresponded well with the magnitude of rainfall events, indicating an increase in turbidity immediately after rainfall. Furthermore, the turbidity increase was also observed during the snowmelt season, suggesting that sediment production originating from the slopes or river channels may be responsible for the sediment discharge during the snowmelt season. The results indicate that even more than four years after the earthquake, sediment production and sediment discharge are still occurring intermittently due to rainfall and freeze-thaw cycles, and that not only sediment produced during the earthquake but also new sediment production is occurring, suggesting that similar conditions will continue for some time in the future.