As cobble gravel travels downstream, its radius reduces in size. In most cases, this is thought to be a consequence of the way it is carried along by the water flow. To date, the crush and abrasion of cobble gravel has received little attention in the field of Sabo and river studies. However, monitoring indicates that this phenomenon, which occurs with the downstream movement of gravel, is prominent in mountain rivers. The authors therefore carried out a study into the crush and abrasion of gravel using the Los Angeles machine, focusing on cobble gravel from a representative sample of rocks distributed across Japan's mountain rivers. This was done with the aim of identifying the characteristics of the phenomenon and obtaining basic data for further dynamics and hydrology research on gravel crush and abrasion.
The results of the study reveal new information about the characteristics of crush and abrasion of cobble gravel : 1) The ratio of the weight reduction that occurs (=gravel weight after rotation/gravel weight before rotation) indicates the curved shape that gradually degrades along with the increase in number of rotations, and the characteristics of the curved shape vary according to the type of stone. 2) The rate of increase in the production(=sediment weight produced by crush and abrasion after rotation/cobble gravel weight before rotation)of sediment particles produced by crush and abrasion of cobble gravel as it rotates varies, depending on whether the particles are smaller than 2 mm or larger than 2 mm. 3) The production rate of suspended load particles smaller than 0.1 mm rises in a linear fashion as rotations increase. The same tendency for the production rate to rise can be seen for particles between 0.1 mm and 2 mm in size. 4) The production rate of gravel particles larger than 2 mm is high during the initial period of rotation, and subsequently decreases. 5) Rock types with a high rate of cobble gravel weight reduction (shale, mudstone) produce sediment particles of a roughly equivalent size and quantity, which become finer as they rotate. 6) Types of rock with a low rate of cobble gravel weight reduction (andesite, sandstone, granite)mainly produce sediment, and silt and clay particles that are smaller than 2 mm. 7) Types of stone with a medium rate of cobble gravel weight reduction(chert, limestone, greenstone)produce forms of sediment that fall between the other two categories.

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Flash floods are floods characterized by a very rapid rise of the water stage and they are known to be frequently generated in mountainous torrents. Many deaths are caused by the flash floods that occur almost every year. In Japan, a flash flood is called “teppou-mizu". The origin of this term is associated with silvicultural technologies for timber floating. For example, “teppou-zeki" is an artificial dam to flush timbers downstream even during mountainous torrents. “Teppou-mizu" is not a scientific term although it has been used as common term. After investigating the past usage of these terms in media such as newspapers and dictionaries, it turned out that “teppou-mizu" is generally recognized as a term referring to a phenomenon which generally differs from a debris flow or flood.

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Hydrophone has been used to measure bedload transport intensity by counting the number of pulse made by hitting of sand and gravels to a steel pipe. However, this method has some problems, for example, when the sediment rate is high, the sound level is continuously high, and thus, the number of pulses decreases or becomes zero. This problem can be eliminated by recording and integrating the sound pressure. Results of flume experiments carried out using glass beads have confirmed the existence of a relationship between the sediment rate and sound pressure. When the number of beads is small, the sound pressure is proportional to the sediment rate. As the number of beads increases, the detection rate, which is the ratio of the experimental value of the sound pressure to the anticipated value calculated with the proportional relation, decreases due to the influence of the interference of sound waves. These results agree well with those of a numerical simulation carried out using the wave data of the individual collisions. These results suggest that the transformed data whose sediment rate is equal to the integral multiple of that of the original data can be calculated by dividing and superposing the original data. In this study, we propose a method involving the use of the ratio between the detection rate corresponding to the transformed data and original data to calculate the detection rate and number of beads. We thus compute the sediment rate and average grain diameter.

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Landslide dams induce catastrophic outburst floods or debris flows mostly by overtopping. However, adequate information about sediment transport of landslide dam and changes of grain size distribution due to overtopping are still lucking. The 2008 Iwate and Miyagi inland earthquake triggered 15 landslide dams. Here we showed new data about the sediment transport of two landslide dams occurred by The 2008 Iwate and Miyagi inland earthquake due to overtopping erosion by the LiDAR data. We measured at 3 times, before overtopping erosion (just after the earthquake), after the overtopping erosion, around 4 mouths after the overtopping erosion). We found that about only 1/2～2/3 was eroded by the overtopping among the height of a landslide dam, and the water channel is formed in overtopping erosion ; it had indicated that the water channel is wide in the downstream more than upstream. One side a part of the sediment from a landslide dam has deposited between landslide dam and small-scale landslide dam in the downstream, and the longitudinal riverbed gradient approaches to initial gradient. In addition, after overtopping erosion once, the substantial change was not seen in the shape of a landslide dam though there was a rainfall of the total about 120 mm.

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