Typhoon Talas (T 1112) induced many rapid, deep-seated landslides in the Kii Peninsula. Previous studies have examined the role that geology and geological structure play within the occurrence of these rapid, deep-seated landslides. However, there is no single, widely used physical model to describe deep-seated rapid landslides. Thus, we propose a physically based model that describes the widening process in water channels that accompanies overtopping erosion during landslide dams. In the model, we assumed that channel widening was induced by the collapse of side slopes due to channel bed degradation. We used slope stability analysis to describe side-slope collapse. Additionally, we considered whether our model could describe valley development processes during deep-seated landslides due to overtopping erosion of landslide dams. This was included because both side-slope collapse and deep-seated landslides could be affected by a decrease in slope stability with slope height and angle. Thus, we examined whether our proposed model could be used to model overtopping erosion of landslide dams, to determine the location and area of deep-seated landslides. This study focused on slopes where Typhoon Talas caused deep-seated landslides ; we used detailed topological data (LiDAR) before and after Typhoon Talas to measure slope gradients and relative heights, to evaluate the applicability of our model.

The advancement in terrain survey technologies and sophistication of terrain data are gradually enabling swifter estimation of sediment movement from landslide disasters. However, volume estimation of colluvial deposits in a collapsed area is impossible via simple differential analysis of precise terrain data comparing before and after the collapse alone, as this does not take into account the rupture surface below the colluvial layer. That estimation is crucial for avoiding secondary disasters during search and rescue operations. This paper presents the trial of the simulation of the rupture surface followed by swift estimation of the volume of colluvial deposits in the collapsed area from the slope failure at Yabakei on April 11, 2018. The trial used datasets obtained only from aerial surveys without conducting any field surveys in order to prioritize swiftness over accuracy. The trial was completed in just three days, and the hypothetical rupture surface was mostly consistent with the later reports provided after conducting field surveys.

Heavy rains in July 2018 caused several slope failures and debris flows in Okayama Prefecture, Japan ; this was the largest rainfall event to have occurred in Okayama within the past fifty years. As a result, rather than a single remarkable sediment-related disaster in a concentrated area, disasters occurred widely throughout the prefecture during this event.Many local governments on July 6 announced information on sediment-related disasters. Despite this effort, many people were affected by these disasters within their own homes. Thus, although there was sufficient time for evacuation, many people chose not to evacuate, demonstrating that there are substantial problems nonstructural measures for sediment-related disasters.In this report, we describe five sediment-related disasters and they related to regional characteristics of Okayama Prefecture, as well as evacuation efforts during these disasters and their problem.

From 5^th to 7^th July 2018 in various parts of western Japan, continuous heavy rain occurred due to Typhoon Prapiroon supplying warm moist air towards the baiu front, and caused numerous disasters. The Japan Meteorological Agency named it “the Heavy Rain Event of July 2018” and the damaged area extended widely from western Japan to Hokkaido. In Kansai region, serious sediment disasters occurred including human damages. Therefore, Japan Society of Erosion Control Engineering Kansai branch members conducted field surveys in Hyogo and Kyoto Prefecture. In Ayabe City, Kyoto Prefecture, slope failure occurred on a thick soil layer containing a lot of fine sediment and high water retentivity. It was presumed that the rather large scale slope failure occurred due to the great increase groundwater level resulting from 30-50 mm/hr rainfall causing just after 170 mm/3 days. In Tanigo River, Fukuchiyama City, Kyoto Prefecture, landslide dam have formed due to the slope failure from right bank side. Although the landslide dam upstream water level monitoring has been conducted, the risk of landslide dam outburst need to be considered. In Kobe City, Hyogo Prefecture, many landslides and debris flows occurred and caused flooding/deposition damage in wide area. Fortunately there were no human damages, but some of the sediment have extended to the outside of sediment disasters designated areas. Although it might be difficult to predict and realize the sediment disaster risk in residential area because torrents width become rather narrow and landform changing due to land development, we need to consider effective countermeasures.

In July 2018, heavy rain due to Typhoon Prapiroon affected western Japan and caused numerous sediment disasters such as landslides and debris flows in Hiroshima Prefecture. In a southern part of Hiroshima, approx. 8,500 slope failures occurred, and total number of sediment disasters were reported as approx. 1,250. Therefore, members of Japan Society of Erosion Control Engineering and Chuushikoku branch conducted field surveys in Hiroshima City, Aki Gun, Kure City, and Higashi-hirosima City. In Kawasumi area, Aki Gun, large rock which seemed to be core stone and diameter approx. 10 m moved down from the torrent, and at downstream side 6 m diameter rock seemed to hit the house with destructive power. In Aki-Gun, Saka-Cho, Koyaura area, one old stone masonry sabo dam was destroyed. We estimated the flow discharge from the investigation at the upstream of dam and considered the flow process from the flow traces around the dam. The results showed that the dam destroying process was as following. Firstly, the large rocks accumulated at the frontal part of debris flow collided and destroyed the right bank side wing, and then stone masonry product peeled off continuously. Furthermore, debris flows occurred from several streams in Koyaura and 1-1.5 m sediment deposition occurred at downstream residential area. In Higashihiroshima City Kurose-cho, many collapses and debris flows occurred around Hiroshima International University and there were no casualties fortunately. In Kurose-cho, most of the collapses seemed to occur from the top and ridge of the mountains with gentle slope around 15 degrees.