It is believed that when cobbles on the bed of a mountain river are transported downstream while repeatedly rolling, sliding and bouncing, their diameter decreases, they become round and polished, and fine sediment is produced. This phenomenon is important for the integrated management of sediment in a watershed, and a reevaluation of Sabo facilities is expected to take place because of it. Kosuge et al. (2010) clarified the crush and abrasion characteristics of mountain cobbles and gravels ranged at about 200 mm in diameter. Here we limit ourselves to four types of rocks-characteristic mudstone, chert, granite and andesite. We aim to clarify 1) the effect of particle diameter on the crush and abrasion of cobbles and gravels, and 2) the procedure for transforming the experimental process of crush and abrasion with rotation to the natural process of that using the straight experimental channel. Our findings are as follows. 1) Some riverbed cobbles and gravels rock types have a high rate of weight reduction as the rotations increase. Among of these the rate of weight reduction of 100-mm and 50-mm cobbles and gravels becomes lower than that of 200-mm cobbles, and the effect of particle size on crush and abrasion is recognized. However, types of rock for which the rate of weight reduction of cobbles and gravels is low are mainly subject to abrasion, and the effect of particle size is not recognized. 2) The rate of production of silt and clay sediment measuring 0.1 mm and less, which is produced because of abrasion that accompanies an increase in rotations, shows no effect of particle size. 3) The effect of particle size on the production rate of sand particles measuring 0.1-2 mm and cobbles measuring 2 mm and over resulting from crushing is recognized. 4) The shape and change tendency of the sediment grain-size accumulation curve that is produced along with the increase in the number of rotations is characteristic depending on each type of rock, and is influenced more by the rate of weight reduction of the cobbles and gravels than by particle size. 5) A test on the crush and abrasion of 200-mm cobbles using an experimental channel showed the same change tendency for each type of rock when the Los Angeles machine was used, and about the same or a lower rate of weight reduction. 6) A test on the crush and abrasion of cobbles and gravels using an experimental channel was more similar to the flow of cobbles and gravels in an actual river than the test on the crush and abrasion of cobbles and gravels using the Los Angeles machine. 7) We applied Sternberg's law and multiplied the conversion rate by the crush and abrasion coefficient in the case of using the Los Angeles machine, and calculated the crush and abrasion coefficient obtained from an actual river (i.e. when an experimental channel was used) to show that the rate of weight decrease can be estimated.
It is necessary to examine data from a long-term perspective in order to understand the history of large deep-seated landslides because they occur infrequently. Using Kikai-Akahoya tephra (ejected in 7300 cal years BP) as the key bed, we investigated the landslide history of Mount Wanitsukayama, Miyazaki, Japan, an area that has old landslide scars. We dug 42 sites and determined whether the tephra layer was observable in the soil profiles. Three types of soil profiles were observed: type A contained air-carried tephra, demonstrating that neither landslides nor surface erosion had occurred in the past 7300 years; type B contained a mixture of tephra and other soil types, showing that the tephra has been reworked by small shallow landslides in the past 7300 years; and type C lacked tephra, indicating slope denudation in the past 7300 years. Most of the ridges are considered to be long-term stable slopes because type A is widely distributed over them; but some narrow parts of the ridges have been sharpened by old landslides, which destabilized and disturbed regolith of the sharpened ridges, because type B was found on the narrow ridges. Some landslide scars were found on the slopes according to the detailed maps derived from a LiDAR survey. Mosaic distributions of types A, B, and C over the middle area of the landslide were probably attributable to non-uniform movements of slopes in the past 7300 years. Reactivation of this landslide caused a debris-flow-triggering landslide and slope cracking during a rainstorm in the year 2005.
Volcanic disasters take diverse forms and volcanic eruptions are difficult to predict. In order to utilize information to prevent volcanic disasters it is undoubtedly important for inhabitants, local authorities and disaster professionals to have a good understanding of each other. To advance this mutual understanding, volcanic phenomena should be described in a unified manner. The terms “volcanic mudflow”,“mudflow” and “debris flow ” all refer to phenomena in which a flow with densely mixed pyroclastic materials and water runs down on a mountainside. However, these three terms are still controversial in their meanings. In this paper the author abstracts descriptions on “volcanic mudflow”,“mudflow” and “debris flow” from historical literature of volcanology, Sabo engineering, and related fields in order to clarify the history of usage of these terms. Their appropriate usages are discussed from the viewpoint of social demands for volcanic disaster prevention while considering the present efforts actually being made in Japan. As a result, it is proposed that the phenomenon directly caused by volcanic eruptions should be called “volcanic mudflow” and that the phenomenon caused by rainfall after eruptions should be called “debris flow” or “mudflow”, when these terms are used in the field of volcanic disaster prevention. However, the way of defining volcanic eruptions or volcanic activities with tangible expressions still needs further discussion. The phenomena triggered by the direct extrusion of muddy materials from a crater should also be discussed so that they can be categorized depending on the type of volcanic mudflow involved.