In mountainous rivers, sediment transportation occur and flow with high sediment concentration such as debris flow cause huge damage in residential area. There are many studies about sediment transportation in steep mountainous area but most of them focused on large sediment. However, sediment distribution is wide from fine particles to large rocks and flow characteristic seems to be different. Recently, debris flow containing high concentration with fine particles occurred in volcanic regions, and they have been reported to show high flow-ability comparing to stony debris flows. In debris flow simulations, methods considering fine sediment phase change as setting large fluid density have proposed and results have explained the actual event. However, the mechanism and behaviours of debris flows with fine sediment are not clear. In order to predict the run-off process, it is important to reveal the behaviour and effect of fine particle in fluid phase. In this study, we conducted hydraulic experiment with fine sediment and coarse sediment. We presumed that part of the fine particles contribute to increase the fluid phase density when different size of particles exist or when mixture ratio of fine sediment and coarse sediment is different. We found that the density increase is affected by ratio of friction velocity and settling velocity, and fine particle diameter and concentration.

Flow resistance coefficients, such as Manning's roughness coefficient n have been useful, however, property of flow resistance at steep mountain streams are not well understood because of lack of data. We have found that resistance dramatically decrease with increase of discharge but once the water surface was above most of the boulders in the bed, the coefficient then changed little with increasing water depth. If this can be a common phenomenon, prediction of flood flows can be possible using resistance equation that assumed resistance does not change with water depth even at mountain channel. We collected existing data from literature that documenting changes in flow resistance during flood and (1) examined the relationships between relative water depth and channel resistance for each channel, (2) extracted the smallest resistance from each channel and demonstrated the measured ranges of resistance according to channel morphology, and (3) investigated the effect of slope, catchment area and grain size on flood flow resistance. We collected data from 76 channels with bed slope over 1/100. They were classified as cascade, step-pool, plane-bed and pool-riffle morphology. Manning's n decreased with increasing relative water depth, and for most of channels, resistance changed little with increasing water depth on higher water levels. Measured minimum Manning's n, that should represent the resistance during flood, ranged from 0.03 to 0.35. Minimum Manning's n increased with slope and grain size for plane-bed and pool-riffle channels, while no such relationships can be found for cascade and step-pool channels. These results suggested that for the prediction of the large flood flows, constant resistant values can be used also for mountain channels. The prediction accuracy of channel resistance during flood can be improved using slope and grain size distribution for plane-bed and pool-riffle channels but not for cascade and step-pool channels. For better prediction of resistance for cascade and step-pool channels, we may need more information that should contribute to channel resistance, such as bedform.

This study sought to clarify the characteristics of sediment discharge from a forested-mountain watershed where volcanic ash was deposited during the 2011 eruptions of Shinmoe-dake at Kirishima volcano, Kyushu, Japan. Quantitative estimates of sediment discharge were determined by measuring sediment inputs at Sabo dams, bedload transport using hydrophones, and sedimentation at a local reservoir. The Sabo dams were distributed within a 10-km radius from the crater source. The hydrophone stations and the reservoir were located 11 km and 40 km, respectively, downstream from the crater. Post-eruption specific sediment yields (SSY) estimated from the sediment input at the Sabo dams were maximum ten times greater, and the SSY obtained from measurements of reservoir sedimentation were two to six times greater than pre-eruption figures, with an increase of up to 10^３m³ km^－2 yr^－1 order. Bedload transport increased threefold after the eruption. Given that the characteristics of sediment discharge might change before and after the eruption, the high sediment discharge volumes studied were caused not only by intense hydrological events, but also by the removal of volcanic products that originated from the eruption. Processes of sediment discharge including transportation of volcanic products from the mountain watershed to alluvial channels and the downstream movement of sediment waves can be explained by correlating the timing of intense hydrological events with the increase in sediment discharge. Because high sediment discharges corresponded with hydrological events and continued after the eruption, and since the side of the watershed adjacent to the volcano is still covered by large amounts of deposited volcanic products, the continual monitoring of sediment transport remains important in terms of watershed management and future disaster risk mitigation.

The purpose of this research is to improve the evaluation accuracy of sediment transport phenomenon in period after large-scale sediment production by one-dimensional riverbed variation calculation. We carried out a reproduction calculation of sediment transport phenomenon over the three years after large scale sediment production for Harukigawa of the Fujikawa river basin. Hirano's model which can express coarse grain formation of riverbed surface layer was applied to the study model, and, regime theory was applied for flow width evaluation. Furthermore, we considered increase or decrease of α value of the regime theory by due to the occurrence of double-row bar and influence of the stenosed terrain. By comparing calculated value and actual value estimated by differential analysis of elevation values at two time periods, according to the proposed method of setting the α value, it is found that accuracy to reproducibility improves with respect to the river bed variation height and the passing sediment volume. In addition, from the past research, three features of sediment dynamics after large-scale sediment production have been clarified. (1) A period in which sediment discharge is active continues for several years to several ten years after large-scale sediment production. (2) Deposit sediment at the time of large-scale sediment production remains in the basin even after the active sediment discharge period. (3) Deposited sediment coarsen with time, by the time the active sediment discharge is over, it is approaching the average particle size at time before large-scale sediment production. By the proposed method, the features of (1) and (2) were reproduced. Moreover, by this method, we could express the tendency of river bed to coarsen.

Bedload and wash load have been measured using indirect method such as pipe hydrophone and turbidity meter, for the basin monitoring and sediment management in the basin, and those monitoring are conducted at around ninety sites in mountainous basin in Japan. There are few researches for comparative studies of differences of sediment runoff on basin characteristics. In present study, characteristics of sediment runoff and related controlling factors were discussed using monitoring data in representative six basins with different characteristic of water and sediment runoff. Several groups were classified with sediment runoff for flow discharge and sediment transport capacity. Monitoring for sediment and water runoff could be effective tools for basin monitoring through present data analyses.

A landslide occurred in the left-hand stream of the Umi River in Itoigawa, Niigata on October 23, 2017, blocking the main channel of the Umi River. We studied the landslide in order to determine the processes involved, from occurrence to deposition of the landslide. The results showed that the mass of soil that formed the landslide travelled about 400 m from the source area, with the shape of the soil mass remaining almost unchanged, and blocked the main channel of the Umi River. In addition, the landslide blocked the exit of the left-hand stream of the Umi River and muddy sediment was deposited behind the main soil mass of the landslide. Moreover, the sediment deposited behind the mass became muddier because of the continuing supply of stream water.