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.
This study introduces a method to detect active deep-seated landslides with eigenvalue ratio calculated from airborne LiDAR survey data acquired in different survey years. Applying it to the area prone to deep-seated landslides in Biratori Town, Hokkaido, where LiDAR data for 2009, 2015, and 2016 are available, field survey confirmed that the emergence of eigenvalue ratio in the range of 2.5-5.0 between the years agreed with slope activity on the spots. Optimized hotspot analysis was then applied for the spatial distribution of the cells. Hotspots suggested from the analysis successfully spotted active landslide mass.
A heavy rain event in July 2018 caused many natural disasters such as landslides and floods in western Japan. Many shallow landslides were induced by heavy rain between the night of July 6 and the morning of July 7 in Gogoshima Island, which is located offshore of Matsuyama City, Ehime Prefecture. The main industry of the island is the cultivation of oranges. However, the orange farming infrastructure was devastated by shallow landslides across a large portion of the island. We created a spatial distribution map of the shallow landslides, based on interpretation of the SPOT－7 satellite imagery, to understand their characteristics. Moreover, the mass movement processes were clarified using the results of geomorphological and geological field surveys. The analysis revealed the following : 1) The shallow landslides induced by the heavy rain event were distributed across the island, and the total number of landslides was 207. 2) Relatively elongate, shallow landslides are distributed on the slopes of Mt. Kofuji. The shallow landslide materials changed to debris flows. 3) The field survey revealed that the heads of shallow landslides are located on the geological boundary between granodiorite and andesite. The landslide material was formed of loose, highly weathered granodiorite, called “masa” in Japanese, and thermally metamorphosed granodiorite. 4) The previous debris flow deposits that filled the gullies were eroded by the new debris flows and flash floods.