This study specifies the geographical features of slopes in the basin of the Totsu River in Nara Prefecture, Japan, where many deep-seated catastrophic landslides (DCLs) occurred due to the Typhoon Talas in 2011. Based on LiDAR data acquired before the DCLs, we calculated slope gradients and eigenvalue ratios across entire slope areas and their DCL portions, and compared the DCL-to-slope ratios of these indexes in a 2 x 2 m grid. In comparison to the entire slope area, DCL portions had a relatively large area with a minimum slope gradient of 25°and eigenvalue ratio of < 5.5. Using these values as thresholds, we classified landforms into four major categories, and quantified microtopographic features highly related to DCLs. Our numerical analyses reveals that, 1) steep and smooth slopes where valley landforms had not been formed are widely distributed ; 2) slopes with shallow landslides, gullies and other erosional landforms are in the lower part ; and 3) gentle slopes with depressions are in the upper part. The study results indicate, it is considered that the use of slope gradients and eigenvalue ratios for numerical analysis of microtopographic characteristics is an effective method for predicting potential location and scales of DCLs.

In the mountain stream channels around Nagiso-dake of the Yomikaki area, a debris flow occurred in Nashizawa river on July 9, 2013. Similar disasters were observed in the adjacent streams of Nashizawa river in 1965 and 1966. The characteristics of these disasters indicated that there were no large scale collapsed areas. It was assumed that the debris flow occurred due to the secondary movement of the sediments bed. We considered this fact that the occurrence of debris flow and its cycle are greatly affected by the location and the amount of sediment accumulated in the river bed in the mountain stream. Therefore, we identified the location to monitor the amount and sediment accumulation in the mountain stream and a periodical monitoring to predict the occurrence of debris flow due to the secondary movement of the sediments bed. In the current study, by utilizing these measurement techniques, we analyzed the relationship between the situation of debris flow occurred in the Nashizawa river in 2013 and the distribution of sediments in the four river streams near Nagiso-dake and the Hontani river around Ena mountain. As a result, in order to predict the occurrence of debris flow, we identified the location where sediment deposition should be monitored and estimate the amount of sediment distributed there. In the future, by continuing monitoring, it is possible to predict the occurrence of debris flow due to the secondary movement of sediments bed.

To perform a debris flow simulation, it is necessary to set factors such as the ground surface topography and mountain stream width by confirming on-site conditions. It is difficult to set conditions for multiple mountain streams quickly and objectively. If the “ground surface topography” and “mountain stream width” could be automatically set based on topographical data, it would be possible to perform debris flow simulations of multiple mountain streams easily. Therefore, in this study, a program that automatically abstracts “longitudinal and lateral sections” and “mountain stream widths” from aerial laser measurement data was developed to easily use the debris flow simulation software that has already been developed. The results obtained with this program when it is applied to an actual mountain stream are presented along with an evaluation of the impacts on the results of the analysis of the discharge of debris flows.

A heavy rainfall event in July 2018, attributable to the influence of the baiu front activated by Typhoon Prapiroon, which was named the “Heavy Rain Event of July 2018” by the Japan Meteorological Agency, caused numerous disasters. Therefore, we conducted a field survey of sediment-related disasters around Uwajima City and Seiyo City in Ehime Prefecture and Otoyo-cho in Kochi Prefecture. The total rainfall amounts from July 5-9 observed at the Uwa AMeDAS (Seiyo City) and Uwajima AMeDAS stations were 539.5 mm and 381.5 mm, respectively. In the vicinity of Uwajima City and Seiyo City, many slope failures and debris flows occurred at dawn on July 7, 2018. Many of these slope failures/debris flows occurred on the south side of the mountain range forming the boundary area between Uwajima City and Seiyo City. This boundary area roughly overlaps the Butsuzo tectonic line, which geologically separates the Chichibu zone and Shimanto zone. Although many of the slope failures were shallow landslides with failure depths of about 1-5 m, relatively large slope failures also occurred in the area where the record for cumulative rainfall was updated by a large amount. In Otoyo-cho in Kochi Prefecture, a deep-seated landslide occurred at dawn on July 7, 2018. The total rainfall from July 5-8 observed in the proximity of the Tachikawa rain gauge station was about 1200 mm, and the maximum hourly rainfall, which occurred at 19 : 00 on July 6, was 111 mm/h. This deep-seated landslide in Otoyo-cho was topographically located in a landslide area, and scars of previous landslide movements were confirmed from a topographical survey performed using airborne LiDAR.

On 6^th September 2018, a large earthquake (the 2018 Hokkaido Eastern Iburi Earthquake) with the magnitude of 6.7 struck the eastern Iburi region, Hokkaido and triggered numerous landslides. The landslides were distributed densely over hilly areas (400 km²) in Atsuma, Abira, and Mukawa Town, the total area of which reached 13.4 km². All landslides were shallow landslides except a large-scale deep-seated one generated in midstream area of the Hidaka-Horonai River. The area is covered by thick pyroclastic fall deposits derived from the Tarumae Volcano etc., and the strong seismic shock triggered shallow landsliding of them. Some shallow landslides occurred at a relatively gentle slope (<15°). Shallow landslides moving along a valley topography tended to travel longer than those moving along a non-valley topography. In catchments where shallow landslides densely occurred, the ratio of landslide area to the total catchment area was very large compared with past earthquake events, suggesting that the catchments became highly devastated. Based on these survey results, we propose viewpoints for preventing secondary disasters in the damaged area and for promoting countermeasures against and studies on earthquake-induced landslide disasters in volcanic regions.