In 1923 Great Kanto Earthquake attacked the south Kanto region. By the strong shock caused by this quake, in West-Tanzawa mountain area enormous number of hillsides collapsed and huge volume of unstable sediment was yielded. In order to prevent sediment disasters, restorative SABO works such as check dams and hillside works were executed by the Ministry of Home Affairs, the Department of the Imperial Household and local governments. Kitao Moroto, Professor of Tokyo Imperial University, remained many photos and memoirs concerning these SABO works and disasrers. These materials were taken and written on the basis of his field trips around West-Tanzawa mountain area. Through referring to these materials and our field investigations in recent several years, we have confirmed that these many SABO works were implemented by Moroto's guidance and in close coordination with the executive organizations. Hence many check dams remain and have been effectively consolidating foots of hillsides and reducing overabundant sediment runoff to downstream. These check dams have some characteristics in those days when modern SABO technologies extended in all over Japan.

Sediment yield by freezing and thawing is a small-scale phenomenon that occurs only in cold regions. However, the sediment yield is supplied directly and constantly to the mountain stream. Furthermore, a large amount of sediment may cause significant damage to the downstream areas. Therefore, accurate prediction of sediment yield is crucial for consistent sediment management in mountain basins. In this study, we observed sediment yield caused by freezing and thawing. The results showed that the amount of sediment yield depends on the changes in atmospheric temperature and soil temperature, and the amount of soil moisture. Furthermore, we created a model to predict sediment yield by freezing and thawing in cold regions by using freezing and thawing heat. The analysis confirmed that the model could predict sediment yield on the ground surface by freezing and thawing by using the amount of heat transferred at the ground surface.

Early evacuation before sediment disaster occurs is becoming more important especially in hilly and mountainous areas. This study examines factors of the actual early evacuation in hilly and mountainous areas in Nara prefecture by interviews and questionnaires. The results suggest four common important points, namely 1) appropriate information, 2) evacuation calls within a community, 3) opening evacuation shelters early, and 4) safe and non-stressful evacuation shelters. Especially, to promote early evacuation before disaster occurs it is necessary to improve evacuation shelter environments.

Sediment-related disaster alerts are issued based on rainfall cutoffs. Rainfall cutoffs are statistically designed on past rainfall-induced disaster events without noticeable earthquake influence. They are preset to minimize missed-alert rates while containing false-alarm rates for users. Rainfall cutoffs are lowered by 70 or 80% of preset levels after strong earthquakes, for fear of possible missed-alert increases. Lowering rates were revised to current levels after 2011 earthquakes, and formalized by national government in 2013. Provisionally lowered rainfall cutoffs are raised to the preset level, based on prefectural technical studies. As a result, disaster events after earthquakes have been gathered nationally since 2013. In this study, disaster events are compiled and analyzed with characteristics of related earthquakes and of rainfall in relation to the cutoffs. The results indicate that apparent missed-alert events can be explained by repetitive aftershock, elapsed durations, and predominant geological characteristics. Lowering rates are to be applied in focused areas to lessen adversarial impacts of false-alarms.

On July 4, 2020, heavy rainfall was observed in the southern part of Kumamoto prefecture and the northern part of Kagoshima prefecture. Due to the heavy rainfall, floods and sediment disasters such as landslides and debris flows were occurred especially in the Kuma River basin of Kumamoto prefecture. We conducted field investigations at four sites in Ashikita-town and Tsunagi-town, Kumamoto prefecture, where landslides and/or debris flows caused deaths. We also conducted field investigations in the Kawauchi River branch of the Kuma River where huge amounts of sediment discharge caused damage to houses. The objectives of these investigations were to clarify the situation and mechanism of the disasters and to propose procedures to recover from the disasters. This report briefly summarizes the results in these investigations.