Flashfloods are one cause of river accidents. During a flashflood, the water level rises suddenly. A flashflood can result when a small dam, formed by a landslide or debris flow, bursts during a torrent. When such a dam blocks a river, the flood that follows its collapse can cause a disaster downstream. The time until collapse is short. To prevent damage from the natural collapse of small landslide dams, it is important to understand the characteristics of dam shape and the discharge hydrograph. We studied landslide dams that were smaller than the dams examined in previous studies. We experimented with small landslide dam outbursts in a rectangular flume. The temporal variation in the shape of the landslide dams and the erosion rate around the crown of the landslide dam are examined. In addition, the rapid collapse of a landslide dam was examined to determine whether it would cause a flash flood, based on the hydrograph. The results are summarized as follows : 1) when there is discharge in the flume, the length of the bottom of the landslide dam is increased ; 2) for a small landslide dam when the inflow discharge and volume of sand are both large, the upstream side of the landslide dam slope is not eroded ; however, when the volume of sand is low, it is eroded and moves downstream ; 3) the erosion rate at the crown of a landslide dam is essentially constant, and the rate is related to the inflow discharge ; 4) when a small landslide dam forms, the discharge decreases immediately ; and 5) when the inflow discharge increases, the peak discharge approaches the inflow discharge.

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Sediment transport measurements in mountain torrents are pivotal to comprehend the nature of the process. Long-term continuous measurement in natural river basins with a substantial scale has been circumvented by many obstacles. Direct sampling, therefore, has been augmented by more indirect but stable methods (hereafter “indirect method”) in recent years such as a hydrophone pipe-microphone acoustic sediment discharge measuring system (hereafter “hydrophone system”). A statistical analytical method based on the hydrophone system has been developed, which enables us to calibrate and estimate bedload amounts sampled by a sediment pit within a limited width of river sections. Observation of sediment transport phenomena, however, has not been fully expanded across a river cross section. Crosssectional variability of bedload discharges has to be understood to select appropriate observation points in a section and to estimate full-width sediment discharges out of those at a unit-width.
A bedload discharge observation facility consisting of a pair of sediment trap pits, together with a water level gauge, has been installed in a 100-km² scale river basin, in order to make pair observations at the right and the central point in a cross section. 12 small-to-medium size flood cases were observed and analyzed in order to quantitatively understand the nature of bedload discharges at each point section and their mutual relationship in a sequential time series as well as in aggregated sums over each case. Intensities of bedload discharges underwent fluctuation of an order of magnitude at a different timing for each point over the observed cases. As a result, relative intensities between the two observation points, expressed by their differences, changed from positive to negative across zero, which gave a fluctuation cyclic period of from about 20 to 40 minutes. Further observation, if combined with the indirect method of hydrophone systems, which have a shorter time constant as an observation system, is likely to clarify cross-sectional variability of bedload discharges in a more reliable manner.

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The factors of slope failures triggered by the Noto-Hanto Earthquake in 2007 were researched in this study. The main factor of the study was four of the following. The influence of the distance from epicenter was limited less than 2 km. The influence by the surface earthquake fault was seen in less than 6 km range. The area ratio of slope failure was 0.05% in the study area (97.4 km²). No significant influence was found on the hangingwall of the earthquake fault. Then, we analyzed the factor using area ratio of slope failure limited by the slope angle more than 30 degrees. Area ratio of slope failure in steep slope was high in the low, costal area. Also, the effect by the distance from surface earthquake fault or geological properties evaluated clearly using area ratio of slope failure in steep slope.

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The deluge on July 21 2009 in Hofu-Yamaguchi triggered tremendous damage and resulted in many causalities in the area. Geologically, the area consists of granite and has had many sediment-related disasters such as the one in August 1993. According to an interview with local residents, such a serious disaster had not occurred for more than 200 years. Therefore, it will be very effective for planning disaster-prevention measures counting the regularity of sediment deposition, if we can obtain it accurately.
In this report, we considered the regularity of sediment deposition in Hofu-Yamaguchi, by using the result of carbon dating of carbonized wood gathered at Upper region of Gyokusen Pond and Ishihara region, as well as considering the situation of black soil layers including carbonized wood. Consequently, the sediment disaster as intense as the one on July 21 2009 is estimated to occur once in every 185-370 years in Upper region of Gyokusen Pond and once in every 145-220 years in Ishihara region.

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This paper presents an experimental approach to observe the drift woods capture effect of steel frame structure on the woody debris. The hydraulic model experiments with various parameter of column's span (W) and maximum or average length of drift woods (l _max or l _mean) are carried out. After checking the capture mechanism of drift wood capturing structure to the drift woods, the capture ratio corresponding to the representative parameters, i.e., ratio of column's span to maximum length of drift woods (W / l _max), are discussed. Finally, mutual relationship was sufficiently estimated from the experimental results.

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Debris flows often cause severe and tragic damages to human life and the economy. Sabo dams are effective protection against debris flow. Planning and estimation using numerical simulation are important to determine the type and size of sabo dam, and the installation location required to achieve the most effective solution. However, non-experts involved in sabo work or debris flow planning often find it difficult to conduct simulations to determine the best sabo solution because the existing simulation systems do not have efficient user interfaces. Furthermore, they find it difficult to define and measure damage indices. Therefore, a system that estimates the best sabo solution based on a user-supplied set of initial landform and debris flow conditions would be useful.
We developed a method to estimate the best sabo solution. We selected that moved sediment volume can be used as valid indices of the damage scale. Simulations of several cases of recent disasters showed that sabo dams installed on the lower reaches of river reduced damage more than sabo dams installed on the upper reaches. Higher dams can be more effective, but only up to a certain value. Placing dams on the upper reaches of a river has very little effect in reducing damage. We then combined our method with the KANAKO GUI-equipped debris flow simulator that provides good visualization and simple explanations. This enabled users to simulate and find the best solutions easily.

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