2019 Volume 56 Issue 3 Pages 104-114
Many sediment movements, such as shallow landslides and sediment flows, occurred in the central part of the Kitakami Mountains in Iwate Prefecture, Japan, due to heavy rainfall during Typhoon Lionrock in August2016. This paper aims to compare and examine the magnitude of sediment transports by this event and those in the early Holocene, focusing on the geomorphological background of this area. Geomorphological settings and scales of sediment production and transportation by the typhoon were studied through field surveys and detailed micro-geomorphological interpretation of the Red 3D Map produced from the LiDAR digital elevation model. The mountain slopes were classified as smooth crest slopes and upper valley head concave slopes of Last Glacial Period origin ; lower valley head concave slopes, talus, and alluvial cones of Holocene origin ; and other slopes. Although the annual probability of hourly rainfall intensity exceeded 300 years, sediment production appeared as shallow landsides with approximately 1m in collapse depth either in the lower valley head concave slopes or around the lines of the convex slope break of Holocene origin. Shallow landslides also occurred on the other slopes and buried valley formed by continuous soil and rock creep on the valley slopes. These sediments were transported mainly by sediment flows and partially by debris flows during the typhoon. Moreover, the old debris flow deposits covered with To―Cu tephra (5―5.5Ka) in the upper tributaries of the Omoto and Hei Rivers were formed by sediment transportation in the early Holocene. These consist of boulders and gravels coarser in size than present river beds and sediment flow deposits by the typhoon. Some of these sediments moved from the lower valley head concave slopes. Gravel size analysis suggested that greater sediment production and transportation occurred in the early Holocene than during Typhoon Lionrock.
