In this study, for the Haruki River basin of the Fuji River system, we investigated motion initiation condition of sediment with high reproducibility using time series data of river bed fluctuation by sand surface gauge and river bed change calculation. Specifically, we confirmed the effects of calculation results by motion initiate evaluation methods and representative particle size for evaluating riverbed erosion, and discussed the reproducibility of floods accompanied by large-scale earth and sand production. Here, the method (A) is an evaluation for individual transport of gravel particles, Egiazaroff formula corrected by Ashida and Michiue (1972) was used in the evaluation formula. The method (B) is an evaluation for collective transportation of riverbed sediment with optional thickness and uses a formula to compare shear stress and shear strength in the river bed with the depth of representative particle diameter by Takahashi (1977). The conclusions obtained through the study are the following two points. First, in the method (A), which use the average particle size of the river bed, the motion initiation water depth of the mountain river was underestimated, and the erosion depth of the flow path and the amount of sediment discharged to the downstream area are overestimated. Secondly, in mountainous rivers, collective transport initiation due to sliding failure is dominant for riverbed erosion than individual transport initiation of gravel particles, by giving motion initiation condition of sediment assuming sliding of representative grain size (average grain size and d₆₀), reproducibility is improved for floods accompanied with large-scale earth and sand production in mountain rivers.

This study elucidates effects of various conservation works on soil erosion at the Dodaira beech forest of the Tanzawa Mountains using USLE, where forest floor vegetation has declined because of deer feeding. We applied soil erosion prevention work of four types: untreated (control), wooden horizontal step, coconut fiber net cover, and bamboo fiber net cover. Then we observed soil erosion amounts for 10-11 years. During the first year of soil erosion prevention work, the soil erosion amount in the treated plots was less than in the control plots. Although the vegetation was restored, litter had accumulated. Soil erosion was reduced at the control plots that had been installed outside of the vegetation protection fence. The effects of soil erosion reduction reached their limit after 10-11 years. The increased forest floor coverage led to reduction of soil erosion inside of the vegetation protection fence, even for the control plots. At the wooden horizontal step works, the soil erosion increased in the final year at the outside of the protection fence despite of the recovery of vegetation. Results show that the soil erosion amount at the wooden horizontal step works can be larger than that expected from the effects of vegetation coverage restoration. Soil erosion amounts during the last year for coconut and bamboo fiber net covering works installed at the steep slope of the outside of the protection fence surpassed those measured during the first year. This fact illustrates the time limitations of the effects of the coconut and bamboo fiber net. It also suggests that the coconut and bamboo fiber cover works installed at the steeper slope of the inside of the protection fence or at the gentle slope have reduced soil erosion for more than 10 years.

Daigentagawa sabo dam No. 1 was built at inlet zone of a gorge of the Daigenta river, which is a tributary of Uono river included in the Shinano river water system. This mortar masonry arch-shaped sabo dam was built of rubble mixed lean concrete and took one and a half years to complete from when its construction was started in 1938. It is one of early arch-shaped sabo dams among other existing ones in Japan. Agency for Cultural Affairs in Japan designated it as a registered tangible cultural property, and moreover, it also attracts many visitors. Whereas, some problems have been found recent years, such as water leakage, because many years have passed since it was constructed. For this reason, a decrepit research of the dam was conducted to examine the condition inside of the dam built of rubble mixed concrete and its strength. Following the result, stability verification by finite element method analysis was conducted. The result revealed it did not keep enough stability. Therefore, a countermeasure for the dam reinforcement was designed with physical property values that had been obtained from the research. In this paper, we report what we examined and learnt from the decrepit research of the dam constructed long time ago but also the study of basis countermeasure design for its reinforcement.

Originating in the Shiga Highlands, which is the remnants of an ancient volcano, the Yomase River has a catchment basin of 117 km² and runs for a total length of 26 km. The downriver area is characterized by a vast alluvial fan measuring 6 km in length and 25 km² in area. Hot spa communities, such as Yudanaka and Shibu, in the midstream area of the Yomase River and the urban area of Nakano city in the downstream area have frequently experienced serious damage due to sediment and flood disasters. The prefectural government of Nagano started work on sabo facilities in 1906, but the sabo facilities constructed then were destroyed by heavy rainfall in 1909 to 1910 and the sabo project was suspended. As such, the topography, geology, and history of sediment disasters along the Yomase River were investigated, and their relationship with sabo projects was studied. The location maps of the sabo facilities constructed from 1906 to 1910 were found in storage at the Nagano Prefectural Museum of History, so they were reviewed in detail. This paper identified the location mentioned in an anecdote by Professor Masao Akagi, who contributed to the sabo works along the Yomase River, where he slipped and fell during a field survey. Based on the relationship between the catchment area and the bed slope, useful knowledge was obtained that will help in reviewing the marginal conditions related to the dry masonry dam work conducted back then and conserving these facilities in the future. A symposium was held in Yamanouchi town in 2018 to commemorate the 100th anniversary of the start of the sabo project for the Yomase River, as a direct project of the central government.

In this paper, we propose the methodology of applying extreme value analysis to the current Japanese early warning system (JEWS). With this method, a return period associated with the probability of sediment disaster occurrence (RP_SL) is obtained by applying extreme value analysis to the distance from the coordinate origin to an x-y coordinate point on the snake line widely used in the JEWS. This distance is defined as the normalized distance, the x-axis of this coordinate is the soil water index divided by the 2-year return period value, and the y-axis is the 1-hour cumulative rainfall divided by the 2-year return period value. The results of applying RP_SLs to the three recorded sediment disasters in the city of Kure, Hiroshima Prefecture, between 1921 and 2018, excepting the 5-year period between 1945 and 1949, are as follows : 1) The RP_SLs of all three sediment disasters were above the 50-year return period and rank in the top three. 2) These sediment disasters occurred at a time when the RP_SL exceeded the past maximum RP_SL that was not associated with sediment disaster. These results reaffirm the effectiveness of the snake line widely used in the current JEWS, demonstrating the validity of the RP_SL. The superiority of the proposed RP_SL is that it is a statistical value (i.e., return period), which leads to the possibility of intuitive recognition of the temporal risk fluctuation associated with sediment disaster occurrences.

The Typhoon Hagibis brought record rainfall and induced many sediment disasters in the Toshin and Hokushin region of Nagano prefecture from October 12 to 13, 2019. In Toshin region, 48 hour accumulated precipitation from October 12 to October 13 analyzed by radar-AMeDAS, ranged from 200 mm to 500 mm. All the sediment disasters in this region occurred within the area with precipitation greater than 200 mm. In the Nukui River basin, 48 hour accumulated precipitation was greater than 400 mm. At the four sediment disaster sites in this basin, debris flows, or sediment gravity flows occurred within a few hours after the peak of precipitation. At each disaster site, only relatively small shallow landslides were found in the upstream area, while stream bed erosion was remarkable. The main cause of these disasters is thought to be a significant increase in the stream flow caused by the large amount of precipitation.